Hearn - Summary Slope Engineering for Low Cost Mountain Roads (1)

8/10/2019 Hearn - Summary Slope Engineering for Low Cost Mountain Roads (1)

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Slope Engineering for Low Cost

Mountain Roads

G J Hearn

with significant contributions from T Hunt

NB This summary is taken from the full document currently in press by the Geological Society of London. No information or illustrations can be

reproduced without prior agreement from the Geological Society of London and Scott Wilson Ltd. For further information on the document

please contact Gareth Hearn at [email protected]


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Slope Engineering for Low Cost

Mountain Roads

G J Hearn

Table of Contents

Table of Contents ...................................................................................... i

List of Figures.......................................................................................... vi

List of Tables ........................................................................................... xi

List of Text Boxes................................................................................... xii

Acknowledgements ............................................................................... xiiiHow to use this document ................................................................... xxii

Part A Landslides and Low Cost Roads ..............................................1A1 Introduction .......................................................................................................1

A1.1 Purpose ......................................................... ........................................................... ..........1A1.2 Low volume and low cost roads ........................................................... ............................. 1A1.3 Geographical coverage ..................................................... ................................................. 2A1.4 Landslide hazards and low cost mountain roads................................................................6

A2 Project phasing and procurement in relation to slope management............8A2.1 Project phasing ....................................................... ........................................................... 8A2.2 Construction procurement .......................................................... ..................................... 11

A2.2.1 Price-based..............................................................................................................11Price-based: lump sum .................................................... ..................................... 11

Price-based: remeasurement ..................................................... ........................... 12A2.2.2 Cost-based...............................................................................................................12

Cost-based: target cost .................................................... ..................................... 12Cost-based: cost reimbursable (cost plus) ......................................................... 12

A2.2.3 Rates-only ...................................................... ......................................................... 14A2.2.4 Directly-employed labour ............................................................ ........................... 14

A2.3 Labour-based and local resource-based approaches ........................................................ 14A3 Slope materials, landslide causes and landslide mechanisms......................16

A3.1 Common soil types and their influence on slope stability ............................................... 16A3.1.1. In situ weathered soils............................................................................................16A3.1.2 Transported soils.....................................................................................................24

A3.2 Common rock types and their influence on slope stability..............................................26A3.3 Outline of landslide causes ......................................................... ..................................... 31A3.4 Landslide types and characteristics..................................................................................34

A3.4.1 Slides.......................................................................................................................39Planar failures......................................................................................................39

Debris slides .......................................................... ............................................... 40Mudslides............. ............................................................ ..................................... 40Rockslides .................................................... ......................................................... 41Wedge failures ....................................................... ............................................... 42Circular failures .................................................... ............................................... 42

Mudflows ................................................................................... ........................... 44A3.4.2 Debris flows............................................................................................................44A3.4.3 Avalanches..............................................................................................................46

A3.4.4 Falls and topples ...................................................... ............................................... 48A3.5 Landslide displacement .................................................... ............................................... 50


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A3.6 Landslide impacts on the road network ......................................................... ..................51A3.7 Case studies .................................................. ........................................................... ........54

A3.7.1 Ethiopia...................................................................................................................54A3.7.2 Philippines ..................................................... ......................................................... 55A3.7.3 Himalayas ...................................................... ......................................................... 55A3.7.4 China.......................................................................................................................57

A4. Landslide risk management for low cost roads...........................................59A4.1 Introduction................................................................................................................59 A4.2 Landslide susceptibility, hazard and risk ........................................................... ........59A4.3 Risk management ................................................... ......................................................... 62

A4.3.1 New road construction ....................................................... ..................................... 62A4.3.2 Improving existing roads ................................................... ..................................... 64A4.3.3 Landslide stabilisation on existing roads ........................................................ ........65

Part B Site Investigation......................................................................68B1. Introduction ....................................................................................................68

B1.1. Range of techniques........................................................................................................68B1.2 Programming of techniques ............................................................ ........................... 69

B1.2.1 New road construction ....................................................... ..................................... 69

B1.2.2 Road improvement................................................... ............................................... 70B1.2.3 Landslide damage reinstatement ........................................................... ..................70B1.2.4 Road operation and maintenance .......................................................... ..................70

B2 Desk studies ................................................................................................72B2.1 Introduction................................................................................................................72 B2.2 Aerial photograph interpretation......................................................................................73B2.3 Satellite image interpretation ...................................................... ..................................... 79

B2.3.1 Potential applications and choice of imagery..........................................................79B2.3.2 Satellite-derived topographical data........................................................................85B2.3.3 Airborne imagery ..................................................... ............................................... 85

B2.4 Terrain models ........................................................ ......................................................... 87B2.5 Terrain classification........................................................................................................89B2.6 Landslide susceptibility, hazard and risk maps................................................................95

B2.7 Geographical Information Systems (GIS) applications ................................................. 101B2.7.1 Introduction...........................................................................................................101B2.7.2 Using GIS..............................................................................................................101

B3. Field mapping ...............................................................................................104B3.1 Reconnaissance surveys.................................................................................................104B3.2 Reference condition mapping ..................................................... ................................... 104B3.3 Landslide hazard mapping .......................................................... ................................... 104B3.4 Engineering geological mapping .......................................................... .........................107

B3.4.1 Recognition of landslide features .......................................................... ................107B3.4.2 Landslide mapping................................................... ............................................. 107

B4 Ground investigation.....................................................................................117B4.1 Introduction....................................................................................................................117B4.2 Scope of investigation....................................................................................................117

B4.2.1 Access .................................................. ........................................................... ......118B4.2.2 Depth of investigation........................................................ ................................... 118B4.2.3 Sampling and testing .......................................................... ................................... 118

B4.3 Investigation methods ....................................................... ............................................. 118B4.3.1 Trial pits and trenches ........................................................ ................................... 119B4.3.2 Augering................................................................................................................121B4.3.3 Boreholes and drillholes........................................................................................121

Cable percussion boring (boreholes)..................................................................121Rotary drilling (drillholes) .............................................. ................................... 121Wash boring........................................................................................................122

B4.3.4 Probing..................................................................................................................122B4.3.5 Sampling and testing in soil and rock ................................................... ................122

Disturbed sampling...................................... ....................................................... 122Undisturbed sampling.........................................................................................122In-situ testing ......................................................... ............................................. 123


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Laboratory testing .............................................................................. ................123B4.3.6 Geophysical investigations....................................................................................125B4.3.7 Groundwater observations.....................................................................................125

B4.4 Supervising a ground investigation................................................................................126B5 Slope movement monitoring.........................................................................127

B5.1 Introduction....................................................................................................................127

B5.2 Monitoring methods.......................................................................................................127B5.2.1 Observational methods..........................................................................................127

Remote sensing ...................................................... ............................................. 127Field inspections.................................................................................................128

Line-of-sight monuments .................................................................... ................128B5.2.2 Measurement methods ....................................................... ................................... 129

Surface monuments.............................................................................................129Tension crack measurements..............................................................................129

B5.3 Interpretation of monitoring data .......................................................... .........................130B5.4 Assessing depth of slope movement from monitoring data ........................................... 131

Part C Design and Construction ......................................................134C1. Route corridor selection and alignment design .........................................134

C1.1 Introduction....................................................................................................................134C1.2 Route corridor identification and selection....................................................................134C1.2.1 Topography .................................................... ....................................................... 134C1.2.2 River crossings......................................................................................................136C1.2.3 Slope stability........................................................................................................136C1.2.4 Land use ......................................................... ....................................................... 139C1.2.5 Comparison of alternatives....................................................................................140

C1.3 Alignment and carriageway design................................................................................140C1.3.1 Alignment..............................................................................................................140C1.3.2 Camber and crossfall.............................................................................................142C1.3.3 Superelevation.......................................................................................................142

C1.4 Case history .................................................. ........................................................... ......142C2 Design and construction of earthworks.......................................................147

C2.1 Choice of cross-section..................................................................................................147C2.2.1 Knowledge-based approach ......................................................... .........................153C2.2.2 Empirical approach ............................................................ ................................... 154C2.2.3 Analytical approach ........................................................... ................................... 156

C2.3 Choice of cut slope profile.............................................................................................158C2.3.1 Benched profiles....................................................................................................158C2.3.2 Continuous slope profiles......................................................................................160C2.3.3 Compound slope profiles ................................................... ................................... 160

C2.4 Fill slopes.......................................................................................................................162C2.5 Earthworks balance........................................................................................................163C2.6 Spoil disposal.................................................................................................................165

C2.6.1 New roads and road improvement schemes.......................................................... 165C3 Soil slope stabilisation ...................................................................................170

C3.1 Introduction..............................................................................................................170 C3.2 Soil slope stability assessment.................................................... ................................... 170

C3.2.1 Ground model .......................................................... ............................................. 170C3.2.2 Stability analysis ...................................................... ............................................. 172

Basic concepts .............................................................................................. ......172C3.2.3 Design factors of safety.........................................................................................176

C3.3 Soil slope stabilisation ...................................................... ............................................. 179Requirement......................................................................................................................182

C3.4 Slope failure type 1: cut slopes................................................... ................................... 182C3.4.1 Case history...........................................................................................................183

C3.5 Slope failure type 2: fill slope failures ........................................................... ................186C3.5.1 Case history...........................................................................................................186

C3.6 Slope failure type 3: above-road slope failures..............................................................188C3.6.1 Case history...........................................................................................................189

C3.7 Slope failure type 4: below-road slope failures..............................................................191


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C3.7.1 Case history...........................................................................................................193C3.8 Slope failure type 5: failure of the entire slope........................................................ ......195

C3.8.1 Case history...........................................................................................................196C4 Rock slope stabilisation.................................................................................201

C4.1 Rock slope stability assessment ............................................................ .........................201C4.1.1 Controls on rock slope stability.............................................................................201

Failure controlled by the intact rock strength .................................................... 201Failure controlled by discontinuities..................................................................203Persistent discontinuity-controlled stability (assessment method 1) ..................203

Multiple, closely-spaced discontinuity-controlled stability (assessment method 2)

...........................................................................................................................................204 C4.2 Managing rock slope stability along mountain roads .................................................... 206C4.3 Stabilisation measures....................................................................................................209

C4.3.1 Reinforcement and support .......................................................... .........................209Dowels ......................................................... ....................................................... 209Bolts................................................... ........................................................... ......211Anchors........................................................ ....................................................... 211Tied-back walls and similar structures...............................................................211Shotcrete ...................................................... ....................................................... 212

Restraining mesh ................................................................................ ................212Underpinning buttresses.....................................................................................213Toe support ............................................................ ............................................. 215

C4.3.2 Drainage......................................................... ....................................................... 215C4.3.3 Removal (scaling) .................................................... ............................................. 216

C4.4 Protection measures .......................................................... ............................................. 216C4.4.1 Containment................................................... ....................................................... 216

Hanging mesh nets................................................. ............................................. 216Catch ditches and buffer zones ........................................................... ................217Catch walls and rigid barriers............................................................................217Catch fences........................................................................................................219

C4.4.2 Shelters and tunnels...............................................................................................219C4.4.3 Case history...........................................................................................................220

C5 Retaining structures ......................................................................................222C5.1 Introduction....................................................................................................................222C5.2 Types of earth retaining structure ......................................................... .........................222

C5.2.1 Masonry retaining walls........................................................................................225C5.2.2 Concrete retaining walls........................................................................................225C5.2.3 Gabion walls ............................................................ ............................................. 226C5.2.4 Crib walls ....................................................... ....................................................... 226C5.2.5 Soil nails................................................................................................................228C5.2.6 Reinforced fill structures.......................................................................................229

C5.3 Design of retaining walls ............................................................ ................................... 230C5.3.1 Equilibrium calculations for retaining walls ................................................... ......230C5.3.2 Passive pressures...................................................................................................234C5.3.3 Water pressures ........................................................ ............................................. 234C5.3.4 Surcharges.............................................................................................................234C5.3.5 Seismic loads.........................................................................................................234C5.3.6 Factors of safety ....................................................... ............................................. 234C5.3.7 Case history...........................................................................................................235

C5.4 Selection of wall cross-section.......................................................................................238C5.5 Wall backfill and drainage .......................................................... ................................... 239

C5.5.1 Backfill..................................................................................................................239C5.5.2 Drainage......................................................... ....................................................... 239

C5.6 Retaining wall construction ........................................................ ................................... 242C5.6.1 Access for wall construction ........................................................ .........................242C5.6.2 Wall heights, founding levels and foundation stability ......................................... 242C5.6.3 Wall length............................................................................................................245

C5.6.4 Excavation stability...............................................................................................246C5.6.5 Construction quality control..................................................................................248Backfill........................... ............................................................ .........................248


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Mortared masonry walls............................................................ .........................248Gabion walls.......................................................................................................249

Reinforced concrete walls.......................................................... .........................250C5.6.6 Improvements to the stability of existing walls.....................................................250

C6 Drainage .........................................................................................................252C6.1 Introduction....................................................................................................................252

C6.2 Slope drainage ........................................................ ....................................................... 252C6.2.1 Routine slope drainage....................................................... ................................... 254C6.2.2 Drainage as a slope stabilisation measure ....................................................... ......256

Surface drains.....................................................................................................256Herringbone drains ........................................................................... ................257Counterfort drains ........................................................... ................................... 257Sub-horizontal (drilled) drains ........................................................... ................257Toe drains...........................................................................................................258

C6.3 Road drainage ......................................................... ....................................................... 258C6.3.1 Design criteria .......................................................... ............................................. 258C6.3.2 Field inspection........................................................ ............................................. 260C6.3.3 Road drainage structures .................................................... ................................... 261

Roadside drains ..................................................... ............................................. 261

Turnouts..............................................................................................................261 C6.3.4 Types of cross-road drainage structures .......................................................... ......265C6.3.5 Design of culverts .................................................... ............................................. 265

C7 Erosion control ..............................................................................................268C7.1 Introduction....................................................................................................................268C7.2 Slope erosion .......................................................... ....................................................... 268

C7.2.1 Options..................................................................................................................268C7.2.2 Revetments............................................................................................................268C7.2.3 Surface coverings ..................................................... ............................................. 271

Shotcrete ...................................................... ....................................................... 271Chunam...............................................................................................................273

Mattresses and matting.............................................................. .........................273C7.2.4 Bio-engineering measures............................................................ .........................273

Using vegetation in engineering......................................................... ................277Bio-engineering technique selection.......................................... .........................278Slope preparation ............................................................ ................................... 284Selection of the appropriate plant species... ....................................................... 284Timing of bio-engineering works........................................................................285

C7.3 Stream erosion ........................................................ ....................................................... 285C7.3.1 Checkdams..................................................... ....................................................... 285C7.3.2 Cascades................................................................................................................287C7.3.3 Channel linings ........................................................ ............................................. 290

C7.4 Culvert outlets................................................................................................................290C7.5 Toe protection and river training .......................................................... .........................297

Part D - Slope Management.................................................................302

D1 Slope Management ........................................................................................302D1.1 Introduction .................................................. ........................................................... ......302D1.2 Planning slope maintenance ....................................................... ................................... 302D1.3 Categories of slope maintenance .......................................................... .........................303

D1.3.1 Routine slope maintenance .......................................................... .........................304D1.3.2 Preventative slope maintenance................................................... .........................306D1.3.3 Emergency slope maintenance..............................................................................307D1.3.4 Remedial slope maintenance.................................................................................308

D1.4 Inspection and prioritisation ....................................................... ................................... 308D1.4.1 Routine inspection ................................................... ............................................. 309D1.4.2 Detailed inspection................................................................................................310

D1.5 Prioritisation ........................................................... ....................................................... 313D1.5.1 Preventative maintenance ............................................................ .........................313D1.5.2 Emergency and remedial maintenance..................................................................315

D1.6 Risk management ................................................... ....................................................... 316


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D1.7 Maintenance procurement .......................................................... ................................... 317

References..............................................................................................320

Index.......................................................................................................330

List of FiguresFigure A1.1 Broad distribution of climate and topography in the humid tropics and sub-tropics............5

Figure A1.2 Typical landslide hazards affecting mountain roads .................................................. ..........7

Figure A2.1 Local resource-based approaches to slope and drainage management...............................15

Figure A3.1 Rock weathering grade classification.................................................................................18

Figure A3.2 Case-hardening on the surface of a residual soil slope formed on granite ......................... 19

Figure A3.3 Some typical departures from the standard weathering profile and their engineering

implications ..................................................... ............................................................ ........................... 20

Figure A3.4 Core stone within weathering grade V granite gneiss ........................................................ 21

Figure A3.5 Weathering grade VVI soil developed on folded metamorphic rock with weathering

grade III rock adjacent............................................................................................................................22

Figure A3.6 Shear strength along relict joints is often the most important control on stability .............23

Figure A3.7 Black cotton soil, overlying weathered tuff........................................................................24

Figure A3.8 Volcanic ash overlying silt/clay residual soil ................................................... ..................24

Figure A3.9 Typical transported soils found in hilly and mountainous regions ..................................... 25

Figure A3.10 Core stone erratics within a weathered colluvial soil ....................................................... 26

Figure A3.11 Folded geological structure creates complex outcrop patterns.........................................27

Figure A3.12 Tight folding causes fracturing of the rock mass prone to ravelling failure..................... 28

Figure A3.13 Cleavage orientated out of the slope, adverse to stability ................................................ 29

Figure A3.14 Failure along adverse jointing leading to loss of road edge ............................................. 29

Figure A3.15 Typical rock structure variations................... ........................................................... ........30

Figure A3.16 Joint infill with fine-grained weathering products ................................................... ........31

Figure A3.17 Some factors controlling the stability of soil and rock slopes .......................................... 33

Figure A3.18 River scour on bend opposite an aggrading tributary fan triggers slope failure and road

damage .................................................. ............................................................ ..................................... 34

Figure A3.19 Common forms of slope failure........................................................................................37

Figure A3.20 Common failure mechanisms in rock...............................................................................37

Figure A3.21 Common failure mechanisms in soil/debris ................................................... ..................39

Figure A3.22 Subtle planar failure in weathering profile overlying rockhead ....................................... 40

Figure A3.23 Deep-seated planar failure through rock blocks valley .................................................... 42

Figure A3.24 Circular (rotational) failures in fine-grained soil and weathered rock..............................43

Figure A3.25 Mudflow, probably triggered by seismicity .................................................... .................44Figure A3.26 Levees deposited either side of a debris flow channel ..................................................... 45

Figure A3.27 Typical debris flow in mountain terrain at bridge outlet .................................................. 45

Figure A3.28 Debris avalanches arising from the 2008 Sichuan earthquake, China..............................46

Figure A3.29 Rock avalanche flow track .......................................................... ..................................... 47

Figure A3.30 Landslide dam formed by a rock avalanche................................................... ..................47

Figure A3.31 Catastrophic rock fall along vertical joints in sandstone... ............................................... 48

Figure A3.32 Incipient toppling failure ................................................... ............................................... 49

Figure A3.33 Columnar jointing in basalt promotes small-scale planar, topple and wedge failures on

steep slopes and in excavations ..................................................... ......................................................... 49

Figure A3.34 Talus slopes formed beneath retreating cliff faces ................................................... ........50

Figure A3.35 Typical range of slope failures in relation to a road constructed across side-long ground

................................................................................................................................................................51


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Figure A3.36 Mudslide and mudflow impacts on mountain roads.........................................................52

Figure A3.37 Fill slope erosion due to lack of road runoff control ........................................................ 53

Figure A3.38 Typical low-angle slope failures developed on residual clay soils bordering the RiftValley in Ethiopia...................................................................................................................................54

Figure A3.39 Extreme slope conditions along the Halsema Highway, Philippines ............................... 55

Figure A3.40 Slope failures and excavations make access difficult for traffic ...................................... 56Figure A3.41 Circular (rotational) failure in weathered Murree Formation mudstone, Pakistan,

triggered by stream erosion ........................................................... ......................................................... 56

Figure A3.42 Mudslide in the Murree Formation, Pakistan ........................................................... ........57

Figure A3.43 Rock avalanche in limestone (with flow tracks indicated) following the 2008 Sichuan

earthquake, China...................................................... ........................................................... ..................58

Figure A4.1 Illustration of risk matrix application...................................................... ........................... 62

Figure A4.2 Failed section of road and viaduct requiring total road realignment .................................. 64

Figure B 2.1 Features commonly observed in stereo aerial photography of landslide areas.......... ........73

Figure B2.2 Engineering geomorphological map for route corridor assessment in west Nepal, derived

primarily from aerial photograph interpretation .......................................................... ........................... 76

Figure B2.3 Stereo aerial photographs for an area of east Nepal ................................................... ........77Figure B2.4 Terrain and landslide interpretation of air photography in Figure B2.3 ............................. 78

Figure B2.5 Illustration of some satellite image interpretation from Nepal ........................................... 86

Figure B2.6 Terrain model for the Blue Nile Gorge, Ethiopia ....................................................... ........88

Figure B2.7 Sub-model for a problematic alignment section ......................................................... ........88

Figure B2.8 Landscape evolution model for a route corridor feasibility study, West Africa.................90

Figure B2.9 Geological model developed for a specific alignment corridor in West Africa..................91

Figure B2.10 Terrain classification mapping .................................................... ..................................... 92

Figure B2.11 Terrain classification for route selection, Nepal...............................................................94

Figure B2.12 Illustration of landslide susceptibility mapping for route corridor assessment, Nepal .....97

Figure B2.13 Post-construction landslide that occurred in one of the least stable areas shown on FigureB2.12 ..................................................... ............................................................ ..................................... 98

Figure B2.14 Landslide susceptibility, hazard and risk mapping for route corridors, Nepal .................99

Figure B2.15 Observed landslide runout distances according to failure mechanism and volume........100

Figure B2.16 Flow chart of GIS activities for terrain and landslide assessment .................................. 103

Figure B3.1 Perspective map showing reference condition distribution along the Blue Nile gorge

alignment, Ethiopia...............................................................................................................................105

Figure B3.2 Landslide hazard and runout mapping, Papua New Guinea............................................. 108

Figure B3.3 Hazard matrix applied to mine access road field investigations, Papua New Guinea ......109

Figure B3.4 Failure surface daylights in cut slope ...................................................... .........................111

Figure B3.5 Geomorphological map of deep-seated landslide.............................................................112

Figure B3.6 Mapped slope showing subtle landslide features (dotted lines are back scarps, dashed linesare deposits, solid line is road. Photo taken in direction of arrow on Figure B3.5)............................. 113

Figure B3.7 Engineering geological investigations for bridge abutment stability assessment....... ......114

Figure B3.8 Structural geological rock slope stability mapping...........................................................115

Figure B3.9 Engineering geological and geomorphological map for road widening and slope protection

on a hairpin stack in Sri Lanka...................................................... ....................................................... 116

Figure B5.1 Common methods of surface movement monitoring........................................................129

Figure B5.2 Typical layout of crack extensometer...............................................................................130

Figure C1.1 Choice between gentle, unstable ground and steep, rocky, though largely stable ground for

route corridors ........................................................... ........................................................... ................135

Figure C1.2 Bridge and bridge approach buried by tributary fan deposition ....................................... 136

Figure C1.3 Hairpin stack construction abandoned due to slope instability.........................................138 Figure C1.4 Hairpin stack constructed on stable slope with minimal earthworks disturbance.............139


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Figure C1.5 Blue Nile and Difarsa River confluence showing Option 1 ............................................. 143

Figure C1.6 Summary geomorphology of alignment options ........................................................ ......144

Figure C1.7 Option 2 (bend out of sight) .......................................................... ................................... 145

Figure C2.1 Use of retaining structures in steep mountain terrain ....................................................... 149

Figure C2.2 Piered structures can be used where full cut and retaining walls are impracticable.........150

Figure C2.3 Choice of section in difficult and potentially unstable ground ......................................... 151Figure C2.4 Slope cut to 3:1 in weathering grade V material remains largely intact forty years after

construction ..................................................... ............................................................ .........................154

Figure C2.5 Slope height angle stability relationships for rock slopes, Papua New Guinea..........156

Figure C2.6 Mass haul diagram............................................................................................................164

Figure C2.7 Failure of end-tipped spoil into watercourse .................................................... ................165

Figure C2.8 Failure of natural ground beneath spoil dump due, at least in part, to loading.................166

Figure C2.9 Rapid vegetation regrowth within twelve months on side-cast material .......................... 168

Figure C2.10 Use of bio-engineering (principally grass and shrub planting) to protect a spoil slope from

erosion (Section C7.2) ......................................................... ........................................................... ......169

Figure C3.1 Driving and resisting forces acting on an existing or potential failure surface.................173

Figure C3.2 Typical soil slope failure mechanisms..............................................................................174

Figure C3.3 Five common configurations of soil slope failure along mountain roads.........................179

Figure C3.4 Type 1 slope before and after implementation of remedial measures .............................. 184

Figure C3.5 Geomorphological map and plan of remedial measures...................................................185

Figure C3.6 Type 2 fill slope failure ....................................................... ............................................. 187

Figure C3.7 Below-road masonry wall constructed at crest of fill slope with grass planting on.... ......187

Figure C3.8 Type 3 slope stabilisation in Hong Kong ......................................................... ................189

Figure C3.9 Taluvium exposed in access track excavation ................................................... ...............190

Figure C3.10 Cross-section showing analysed reinforced concrete and gabion retaining walls ..........191

Figure C3.11 Road located across the head of a type 4 regressive landslide (realignment in progress

into hillside towards right)....................................................................................................................193Figure C3.12 Plan showing road across landslide head........................................................................194

Figure C3.13 Cross-section through landslide at A-A1 ........................................................................194

Figure C3.14 Road access maintained across a type 5 landslide..........................................................195

Figure C3.15 Road realignment to avoid a deep-seated landslide........................................................196

Figure C3.16 Analysed sections .................................................... ....................................................... 197

Figure C3.17 General layout and design details for the selected option...............................................199

Figure C3.18 Gabion toe wall and fill slope shortly after construction................................................200

Figure C3.19 Previous type 5 landslide slope with gabion toe wall and full growth of planting to the

lower slope (September 2009)..............................................................................................................200

Figure C4.1 Incipient type 1 failure through the rock mass brought about by blasting effects ............202

Figure C4.2 Discontinuity-controlled rock slope stability....................................................................203

Figure C4.3 Planar, wedge and toppling discontinuity failure mechanisms.........................................204

Figure C4.4 Simple classification of rock slope materials, failure mechanisms1and outline prescriptive

remedial measures ..................................................... ........................................................... ................208

Figure C4.5 Rock stabilisation and protection commonly applied to mountain roads .........................209

Figure C4.6 Typical details for dowels, bolts and anchors (modified from GEO 1997) ......................210

Figure C4.7 A tied-back surface retention structure under construction on a type 3 slope ..................212

Figure C4.8 Wire mesh dowelled to rock face (modified from GEO 2003a).......................................213

Figure C4.9 Typical detail for a concrete rock buttress (modified from GEO 2003a) .........................214

Figure C4.10 Masonry dentition as cavity infill to cut slopes ........................................................ ......215

Figure C4.11 Wire mesh applied to a rock slope..................................................................................217Figure C4.12 Gabion catch wall with access for machine clearance....................................................218


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Figure C4.13 Reinforced concrete catch wall.......................................................................................218

Figure C4.14 Shelter to protect against rock fall ......................................................... .........................220

Figure C4.15 Large type 3 rock slope failure.................................................... ................................... 221

Figure C5.1 Retaining wall descriptors ................................................... ............................................. 222

Figure C5.2 Typical retaining structures ........................................................... ................................... 223

Figure C5.3 Use of mortared masonry retaining walls for road reinstatement ..................................... 225Figure C5.4 Composite masonry retaining wall .......................................................... .........................226

Figure C5.5 Reinforced concrete grid wall...........................................................................................226

Figure C5.6 Concrete crib wall.............................................................................................................227

Figure C5.7 Timber crib wall ........................................................ ....................................................... 227

Figure C5.8 Typical detail of soil nail incorporating shotcrete surface protection (modified from CEDD

Standard Drawing C2106/3F, 1991).....................................................................................................228

Figure C5.9 Installation of soil nails in a cut slope ..................................................... .........................229

Figure C5.10 Reinforced fill wall.........................................................................................................230

Figure C5.11 Active and passive forces acting on a retaining wall......................................................231

Figure C5.12 Failure mechanisms for soil retaining walls .................................................... ...............232Figure C5.13 Options for below-road retaining walls and road reinstatement for a road project inEthiopia ........................................................... ............................................................ .........................236

Figure C5.14 Illustrations of the three main wall types constructed following selection and design from

Figure C5.13.........................................................................................................................................237

Figure C5.15 Typical drainage details for retaining walls....................................................................240

Figure C5.16 Interceptor drain detail....................................................................................................241

Figure C5.17 Drainage grip detail ........................................................... ............................................. 242

Figure C5.18 Dynamic Cone Penetrometer (DCP) ..................................................... .........................244

Figure C5.19 Wall constructed too short..............................................................................................246

Figure C5.20 Angled return into hillside..............................................................................................246

Figure C5.21 Unsupported temporary excavation in taluvium for an above-road mid slope retainingwall ........................................................ ............................................................ ................................... 247

Figure C5.22 Steep temporary excavation for a below-road retaining wall showing the made

ground/original ground interface..........................................................................................................247

Figure C5.23In situdensity testing of backfill ........................................................... .........................248

Figure C5.24 Examples of good (on the left) and poor (on the right) mortared masonry ....................249

Figure C5.25 Dry stone retaining wall in excellent condition after 40 years ....................................... 249

Figure C5.26 Examples of good (on the left) and poor (on the right) stone filling in gabions.............250

Figure C5.27 Masonry buttressing to an above-road gabion wall supporting a failed slope................251

Figure C5.28 Masonry buttressing to a below-road fill retaining wall.................................................251

Figure C6.1 Typical slope drainage measures......................................................................................252

Figure C6.2 Typical slope drainage details ....................................................... ................................... 253

Figure C6.3 Typical benched cut slope erosion and drain blockage problems.....................................255

Figure C6.4 Slope failure on fill embankment with berm drain at immediate risk...............................256

Figure C6.5 Construction of an ..................................................... ....................................................... 257

articulated drain across an unstable slope.............................................................................................257

Figure C6.6 Herringbone sub-soil and surface water drainage of a landslide ...................................... 257

Figure C6.7 Installation of sub-horizontal drains ........................................................ .........................258

Figure C6.8 Installation of a subsoil drain beneath a mass concrete U drain ....................................... 259

Figure C6.9 Side drain erosion leads to failure of adjacent cut slope...................................................261

Figure C6.10 Use of scour checks in areas vulnerable to erosion and failure ...................................... 262

Figure C6.11 Slope failure exacerbated by water discharge from a roadside drain turnout .................262Figure C6.12 Erosion beneath side drain turnout ........................................................ .........................263


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Figure C6.13 Continuous outlet protection below side drain turnout from a hairpin stack..................264

Figure C6.14 Severe scour at lower end of retaining wall....................................................................265

Figure C6.15 Chute inlet ..................................................... ........................................................... ......266

Figure C6.16 Cross drainage configuration and its influence on inlet blockage and outlet scour potential

..............................................................................................................................................................267

Figure C7.1 Masonry revetments ............................................................ ............................................. 269Figure C7.2 Mortared masonry revetment used to protect the lower portion of a cut slope against

erosion and shallow failure...................................................................................................................269

Figure C7.3 Combined concrete and mortared masonry revetment ..................................................... 270

Figure C7.4 Composite masonry revetment used in combination with planting..................................270

Figure C7.5 Reinforced concrete slope revetment................................................................................271

Figure C7.6 Use of fibre-reinforced shotcrete for rock slope protection..............................................272

Figure C7.7 Hand-applied mesh-reinforced concrete...........................................................................272

Figure C7.8 Natural root development of a tree in shallow soil (about 200-300mm depth) overlying

laterite...................................................................................................................................................274

Figure C7.9 Differences of root development in two shrubs used for bio-engineering in Nepal .........275

Figure C7.10 Differences of rooting in two grass species .................................................... ................276Figure C7.11 Dense surface protection provided by the large grassImperata cylindrica...................276

Figure C7.12 Typical bio-engineering details ................................................... ................................... 282

Figure C7.13 Some illustrated planting applications............................................................................283

Figure C7.14 Typical gabion checkdam details....................................................................................286

Figure C7.15 Typical scour check detail ........................................................... ................................... 286

Figure C7.16 Dry stone cascade where flow rates are low...................................................................287

Figure C7.17 Gabion cascade where flow rates are high......................................................................287

Figure C7.18 Masonry and gabion cascades ..................................................... ................................... 288

Figure C.19 Gabion cascade constructed on an unstable slope ...................................................... ......288

Figure C7.20 Typical details for a gabion cascade...............................................................................289Figure C7.21 Rigid channel lining dislocated by ground movement ................................................... 290

Figure C7.22 Example of typical pipe culvert outlet scour protection for a low cost road ..................292

Figure C7.24 Example of culvert outlet protection works, Laos..........................................................293

Figure C7.25 Details of culvert outlet protection shown in Figure C7.24............................................294

Figure C7.26 Stilling basin and cascade combination .......................................................... ................295

Figure C7.27 Damaged outlet protection..............................................................................................296

Figure C7.28 Undermining of roadside structures due to river scour...................................................297

Figure C7.29 Flood damage in Nepal...................................................................................................298

Figure C7.30 Riverside scour protection in a highly erosive environment...........................................299

Figure C7.31 Typical protection in a riverside location of moderate scour potential...........................300Figure D1.1 Indicative schedules of routine slope maintenance activities for two geographical areas(wet season shown as hatching)............................................................................................................306

Figure D1.2 Landslide debris tipped on a newly-stabilised slope during emergency maintenance .....308

Figure D1.3 Slope, wall and culvert inventory.....................................................................................309

Figure D1.4 Instability Report..............................................................................................................311

Figure D1.5 Decision-making process for slope inspection and maintenance ..................................... 317


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List of TablesTable A1.1 Countries most relevant to this book ........................................................ ............................. 4

Table A2.1 Common split of activities between project stages..............................................................10

Table A2.2 Common forms of construction contract and their principal advantages and disadvantages

................................................................................................................................................................13 Table A3.1 Common soil types found in the humid tropics and sub-tropics..........................................17

Table A3.2 Main factors controlling the stability of rock and soil slopes .............................................. 32

Table A3.3 Summary characteristics of landslide types: slides..............................................................35

Table A3.4 Summary characteristics of landslide types: flows, avalanches and falls.......... ..................36

Table A4.1 Risk assessment matrix........................................................................................................60

Table B1.1 Relative importance of desk studies and field investigations according to project stage.....69

Table B1.2 Use of site investigation techniques for low cost road projects ........................................... 70

Table B1.3 Typical information provided by site investigation techniques at each project stage..........71

Table B2.1 Data typically derived from traditional desk study sources ................................................. 72

Table B2.2 Landslide and related features commonly identifiable on stereo aerial photographs........... 74Table B2.3 Range of government and commercial satellite data currently available (correct as of July

2009).......................................................................................................................................................80

Table B2.5 Potential applications of satellite imagery for low cost road projects..................................83

Table B2.6 Imagery used in the Nepal and Bhutan study.......................................................................84

Table B2.7 Digital mapping data from the common sensors..................................................................85

Table B2.8 Land facet designations ........................................................ ............................................... 92

Table B2.9 Extract from land facet table detailing engineering attributes ............................................. 93

Table B2.10 Typical uses of GIS..........................................................................................................102

Table B3.1 Reference condition summary for in siturock and transported soil units in the Blue Nile

gorge, Ethiopia .......................................................... ........................................................... ................106

Table B3.2 Common features indicative of landslides and potential landslide locations.....................110Table B4.1 Typical range of sub-surface techniques for the investigation of slopes along low cost roads

..............................................................................................................................................................119

Table B4.2 Range of laboratory tests on soils for slopes affecting low cost roads...............................124

Table C1.1 Landform and land use features, slope stability and construction difficulty......................137

Table C1.2 Typical national standards for low volume roads in mountainous terrain .........................142

Table C1.3 Comparison of alignment alternatives for a section of new road, Ethiopia .......................146

Table C2.1 Comparison of road sections..............................................................................................149

Table C2.2 Recommended cross-sections for unstable ground...................................................... ......151

Table C2.3 Typical cut slope angles1

...................................................................................................153

Table C2.4 Maximum observed angles (in degrees) for natural slopes (based on data from Nepal) ...155Table C2.5 Indicative cut slope angles in rock1....................................................................................157

Table C2.6 Indicative cut angles for structurally-controlled rock slopes ............................................ 158

Table C2.7 Advantages and disadvantages of benched cut slopes ....................................................... 159

Table C3.1 Level of importance of information for soil slope stability assessment and analysis ........172

Table C3.2 Suggested factors of safety for the analysis of new slopes ................................................ 177

Table C3.3 Suggested absolute minimum factors of safety for the analysis of existing slopes and for

remedial or preventive works ........................................................ ....................................................... 177

Table C3.4 Recommended Partial Factors for use with Approach 1 Combination 2 of Eurocode 7 (BS

EN 2004) ......................................................... ............................................................ .........................178

Table C3.5 Acceptable annual probability of failure according to consequence in Norway................179

Table C3.6 Typical engineering management options for the failure types shown in Figure C3.3......181Table C3.7 Options for soil slope stabilisation.....................................................................................182


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Table C3.8 Calculated factors of safety................................................................................................198

Table C4.1 Level of importance of information for rock slope stability assessment and analysis.......201

Table C4.2 Rock type groups ........................................................ ....................................................... 202

Table C4.3 Options for rock slope stabilisation and protection............................................................207

Table C5.1 Advantages and disadvantages of various wall types .................................................. ......224

Table C5.2 Factors affecting active earth pressures .................................................... .........................233Table C5.4 Wall shape advantages and disadvantages.........................................................................238

Table C5.5 Typical presumed/allowable bearing pressures1 ................................................................244

Table C5.6 Allowable bearing pressures and the equivalent DCP blow count .................................... 245

Table C6.1 Types of slope drainage, their function and limitations..................................................... 254

Table C6.2 General guidance on the design of drainage structures for low cost roads ........................260

Table C7.1 The main plant classes and their anticipated effective depths of rooting...........................275

Table C7.2. Bio-engineering techniques ........................................................... ................................... 279

Table C7.3. Selection of bio-engineering technique according to site characteristics..........................284

Table D1.1 Typical routine maintenance works for slopes and retaining walls.........................................304

Table D1.2 Checklist for completing the Instability Report (Figure D1.4)..........................................312Table D1.3 Risk-based prioritisation matrix for preventative works....................................................314

Table D1.4 Possible vulnerability class definitions for use in Table D1.3...........................................315

Table D1.5 Risk-based prioritisation matrix for emergency and remedial works ................................ 315

Table D1.6 Road category....................................................................................................................316

List of Text BoxesBox A3.2 Fully-developed residual soil (weathering grade VI).............................................................21

Box A3.3 Relict discontinuities in highly and completely weathered residual soils (weathering grades

IV and V)................................................................................................................................................22

Box A3.4 Earthquake effects on slope stability......................................................................................32

Box A4.1 Feasibility study for slope stability management in Laos ...................................................... 63

Box A4.2 Flood damage to the Naubise to Mugling road, Nepal, following road improvement...........65

Box A4.3 Landslide damage to the Hirna to Kulubi road, Ethiopia, following road improvement .......65

Box B2.1 Aerial photographs ........................................................ ......................................................... 75

Box B4.1 Trial pitting investigation of a landslide in Ethiopia ...................................................... ......120

Box B5.1 Mapping landslide movements from aerial photographs......................................................128

Box B5.2 Inclinometers and slip indicators..........................................................................................131

Box B5.3 The use of short-term inclinometers to help determine movement depth in conditions of

complex failure.....................................................................................................................................132

Box C1.1 Recommended geometric standards for low volume roads..................................................141Box C2.1 Choice of cross-section ........................................................... ............................................. 148

Box C2.2 Variable soil and rock sequences encountered in Laos and Ethiopia................................... 160

Box C2.3 Importance of the accurate assessment of the soil/rock interface.........................................161

Box C3.1 Stabilisation of a type 3 landslide in Bhutan ........................................................ ................188

Box C3.2 Type 4 regressive landsliding leading to significant loss of road formation ........................192

Box C4.1 Rock slope stability analysis for slope design......................................................................205

Box C6.1 Experience with cut-off drains in Nepal...............................................................................255

Box D1.1 Performance-based contracts ............................................................ ................................... 318


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This book has been prepared by Scott Wilson Ltd for its Client, the Department for

International Development (DFID) UK. The Authors and Scott Wilson Ltd would like to

thank the following for the opportunity to carry out the work illustrated in this book:

Department for International Development (DFID) UK Asian Development Bank

World Bank (IDA) International Finance Corporation (IFC)

United Nations Environment Programme (UNEP) Islamic Development BankDepartment of Roads, Nepal Department of Roads, Bhutan

Department of Public Works and Highways,

Philippines

Ministry of Works and Transport, Laos

Ministry of Transport, Tajikistan Road Development Authority, Sri Lanka

Geotechnical Engineering Office, CEDD, Hong Kong Ethiopian Roads Authority

Department of Roads and Railways, Libya IKRAM, Malaysia

SNV Netherlands Rio Tinto Iron Ore

Broken Hill Proprietary (BHP) Arcelor Mittal


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How to use this bookThis book covers the design, construction and maintenance of low cost, low volume mountain

roads in the humid tropics and sub-tropics and focuses on slope stability aspects. During the

early stages of its preparation the intended use of the book was as a manual. The manual was

aimed principally at road engineers who are required to build and maintain roads in hilly and

mountainous regions with limited resources. As the document developed it became expandedto include reference to, advice on and illustration of, a range of desk study and field

investigation techniques, some of which are routinely employed on low cost roads, others of

which are of a more specialist nature but have the potential to be of value to low cost road

applications. The book should therefore be of interest to a range of practitioners with

experience in engineering geology and road design, construction and maintenance.

The book is split into four parts.

Part A: Landslides and low cost roads (background)

Part B: Site investigation (desk study, field mapping and ground investigation)

Part C: Design and construction (alignments, cross-sections, earthworks, soil and rock slope

stabilisation, retaining structures, drainage control and erosion protection)

Part D: Slope maintenance management (inspections, works prioritisation, routine andemergency management).

An Index, Glossary of Terms and Reference List are also provided. Definitions and

explanations or clarifications are also provided as footnotes where required.

Part A describes and illustrates the background to landslide and slope instability problems

affecting roads in hilly and mountainous areas of the humid sub-tropics. Simple

classifications of soils and rocks are provided, along with weathering grades and the

description of soil types and rock structures that prove problematic for the stability of roads

and roadside slopes. Mechanisms of landslides in both soil and rock are also described in

simple terms together with a review of landslide causes and the impact that landslides and

earthworks failures have on mountain roads. Basic considerations of hazard and risk are

discussed because these have important implications for road planning, design, construction

and maintenance.

Road engineers already working in the tropics and humid sub-tropics will gain most benefit

from Parts C and D. However, even though they will be familiar with the ground conditions

and site investigation techniques available to them, it is recommended that they consult Parts

A and B as well in order to provide context to their own experience, and to draw their

attention to issues and considerations that they might not have otherwise been aware of.

Part C is by far the largest section of the book, and its aim has been to provide practical

advice on a range of issues that relate to the design and construction of alignments, slopes,retaining structures, drainage and erosion protection works. All these have important bearings

on slope stability.

Part D focuses on slope maintenance and related activities. In terms of engineering, there is

much overlap between the construction and maintenance of roads in unstable terrain, and

frequent reference is made to Part C. The principal reason for this is that mountain roads in

the tropics and humid sub-tropics are located in a dynamic environment where slopes,

retaining structures and drainage works need to be maintained, repaired and sometimes

redesigned and reconstructed due to the changing patterns and effects of rainfall, runoff, river

scour, land use, seismicity and, of course, slope instability.


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Part A Landslides and Low Cost Roads

A1 Introduction

A1.1 PurposeThis book deals with landslides, earthworks (cut and fill slopes), retaining structures and

erosion protection on low cost roads and embraces feasibility study, investigation, design,

construction, improvement and maintenance. The focus is on hilly and mountainous areas in

the humid tropics and sub-tropics where heavy seasonal rainfall is responsible for a high

incidence of slope instability. Nevertheless, large parts of this book will also be of interest to

practitioners working in higher latitudes.

A1.2 Low volume and low cost roadsThe term low volume roads is used to describe roads with traffic volumes of up to 400

AADT1(Keller and Sherar, 2003). These authors also suggest a maximum design speed of 80

kph as another defining parameter, but this would not apply in hilly or mountainous areas

where design speeds associated with steep terrain and difficult alignment geometry might beexpected to be lower (Section C1).

A low volume, low cost road would normally be considered to apply to rural roads and

feeder roads, and not to the more heavily trafficked roads that form important links in a

countrys transport network. However:

due to limited budgets the trunk road network in many countries within the humid

tropics and sub-tropics (Section A1.3) is also constructed and managed within a low cost

framework, despite the fact that some of these roads have AADTs in excess of 1000, or even

as much as 3000 in some cases. While the budgets for the construction and maintenance of

these trunk roads are usually significantly higher than for low volume roads per se, they still

remain essentially low cost by comparison with some other parts of the world; low volume, low cost roads are common in hilly and mountain areas throughout the

world. These areas are often prone to landslides and earthworks failures arising from steep

terrain, high rainfall or snow melt, earthquakes in some areas, and changing land use and

drainage patterns (the latter potentially being affected significantly by deforestation, irrigation

and road construction effects). Even in the advanced economies of the world available

budgets may be insufficient to counter these hazards and temporary road closures and road

damage are frequent outcomes (for example Winter et al 2009).

The term low costis therefore preferred to low volumein the context of this book because it

encapsulates, to varying degrees, low volume/low cost as well as high volume/limited budget

situations.

Although there have been several key texts published in recent decades concerning landslide

hazard and risk assessment for engineering projects (for example Fookes et al 1985, Turner

and Schuster 1996, Fookes 1997a, Griffiths et al 2001, Fookes et al 2005, Glade et al 2005

and Waltham 2009), there have been few that focus on the assessment and management of

landslides in the context of road design, construction and maintenance. The publication by

Fookes et al (1985) was probably the first detailed account of the use of engineering geology

and geomorphology in the design and construction of low cost mountain roads to appear in

the international literature2. The FHWA

3 in the United States (FHWA 1988) published a

1AADT ~ Average Annual Daily Traffic (motorised vehicles)2

The five-fold terrain model, for example, developed by these authors has become one of the mostcopied of illustrations found anywhere on the subject.3FHWA ~ Federal Highway Authority, US Department of Transportation


17/21

manual of landslide management on federal highways in six states, but this focussed primarily

on embankment failures. The publications by TRL4 (1997) and Keller and Sherar (2003)

provide greater engineering detail and advice for mountain road construction and maintenance

than Fookes et al (1985), however the former focuses to a large extent on Nepal, while the

latter does not cover landslide management and slope engineering in any great detail. Finally,

the Department of Roads, Nepal (2003) has published a useful guide to slope protection on

low cost mountain roads but this guide is aimed principally at management level practitioners

and provides only limited engineering detail.

Given the rate at which mountain road construction is taking place in many countries, and the

need to maintain existing infrastructure for strategic, economic and community access

purposes, an update and expansion of the work of Fookes et al (1985) and TRL (1997) is

required for geologists, geotechnical engineers and civil engineers responsible for the design,

construction and maintenance of mountain roads. Accordingly, this bookdescribes, illustrates

and advises on:

the geological, geomorphological and engineering context of landslide and slope

instability impacts on mountain roads, with particular reference to the humid tropics and sub-

tropics (Part A); the techniques available to identify, define and assess landslides and unstable slope

conditions for design purposes (Part B);

the design, construction and improvement of mountain roads with regard to

topography and ground conditions and slope stability considerations in particular (Part C);

the maintenance of mountain roads with regard to slope stability (Part D).

A1.3 Geographical coverageThis book is aimed principally at those regions of the world where many or all of the

following factors combine to create serious sl

Documents

Hearn - Summary Slope Engineering for Low Cost Mountain Roads (1)