GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE 26.pdfGEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of Civil Engineering, IIT Bombay, Powai , Mumbai

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of Civil Engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Module - 6LECTURE - 26

Geosynthetics for reinforced soil retaining walls


OUTLINE

Part - I Mechanically stabilized reinforced soil retaining walls with modular blocks or panel facings Introductions Geosynthetic reinforced soil wall system Different precast concrete modular blocks or panel

facings and connections Analysis and design procedures for geosynthetics

reinforced soil retaining wall Cost considerations Construction procedure for precast concrete faced walls Submission of material and test report by manufacturer Design critique Failures of structures Tolerances


Part – II Geotextile or geogrid wrap-around-facedmechanically stabilized earth (MSE) walls General Design of geotextile wrap-around-faced wallWraparound face construction details

Part – III Gabion walls General Gravity gabion wall design Reinforced soil gabion wall design Feasibility Study on Fly Ash as a Backfill Material Geocell walls


Part – I

Mechanically stabilized segmental reinforced soil retaining wall


Basic concepts

Embankments

Interaction Polymer properties

Applications

Steep slops

Soil mechanics

Reinforced fill applicationsSoft soil applications

Unpaved roads Retaining walls

(Short term reinforcement strength required) (Long term reinforcement strength required)


Different types of conventional rigid retaining structuresmade up of masonry and concrete are available to resistthe lateral pressures:

Gravity retaining walls, Semi-gravity type retaining wall Cantilever retaining walls, Counter fort retaining walls and Bridge and abutment. Anchored Sheet Pile Soil Nailing Braced Excavation


Gravity wall Semi-gravity wall

Cantilever wall Counter-fort wallProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Braced ExcavationSoil Nailing

Anchored Sheet pileBridge and abutment


Inclusion of reinforcements in soil is not new. It has beenused since biblical age. The concept of reinforced earthsystem is well established.

Components parts and key dimensions of reinforced earth wall

(Lee et al., 1973)Vidal (1966)


The traditional concrete and masonry gravity walls orcantilever retaining walls are almost obsolete due to highercost of construction.

Reinforced soil wall is the best cost effective solution.Metallic strips or geosynthetics can be used asreinforcement. Geosynthetic is an emerging bona-fideengineering construction material around the world.

The mild steel degrades due to electro-chemicalcorrosion whereas, the polymer materials suffer from creepproblem causing reduction in the ultimate tensile strength.

Therefore, adequate factor of safety should beconsidered to meet the serviceability limits.


There are many disadvantages of using metallic strips inthe mechanically stabilized reinforced earth wall,

High Cost

Long term susceptibility to corrosion. Protective coatingcan reduce corrosion, but it is uncertain in the field due toground water or electric current.

Sustainability depends on the correct choice of Backfillmaterial ( i.e. gradation, chemical properties etc.)

It cannot be used with many indigenous materials.

Back fill material cost is about 85% of the total cost of theReinforced Soil Wall.


Advantages:

Polymer do not corrode Economical Used with many indigenous materials More deformable than the metal reinforcement Long term durability The geosynthetic is flexible Unskilled labour can place it Minimum excavation Good drainage Heavy equipment is not needed

Geosynthetic Reinforced Soil Wall System




Superimposed (Tiered) walls

(After FHWA-NHI-10-024,2009)

Uneven reinforcement wall


Back-to-back walls


- Overall base width is large

- Overlapping ofreinforcement

SM ≈ 0.5 H

L0 > 0.3 H

LR/H = LL/H ≈ 0.6



Stable feature walls


(After Simac, 1990)

Influence of surcharge for tiered walls


Water front structureProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

In the past 40 years, a tremendous number ofgeosynthetic reinforced soil walls have economically beenconstructed around the world. The geosyntheticsreinforcements are placed horizontally in the retaining wallbackfill.

Geosynthetics reinforced soil mass are basically gravitystructures resisting the earth pressure developed behindthe reinforced soil zone. The facia resists the mass ofreinforced soil, retained soil and the surcharge loads.

Geosynthetics reinforced soil walls are flexible.Therefore, it can tolerate larger settlements and earthquakeloading than the conventional retaining walls. The groundimprovement can also be avoided.


Components of geosynthetic reinforced soil wallsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Major components of reinforced soil system:

Foundation soilIt is required to improve the foundation soil by introducingreinforcement layers, geocells, prefabricated vertical banddrains or encased stone columns. Check the factor of safetyagainst bearing capacity failure.

Reinforced soilThe reinforced soil is the combination of soil and thehorizontal layers of geotextiles or geogrids. It is preferable touse CEG < 30 mm mol/ kg and molecular weight > 25,000gm/mol for good quality PET resin.

BackfillThe backfill soil is located behind the reinforced soil zone.


Drainage fillFace drain behind the wall facia.

Blanket drain beneath the reinforced soil zone,

Back (chimney) drain behind the reinforced soil zone

To prevent build up of hydrostatic pressure. The drainageoutlet must be connected to the collection pipe.

Polymeric geogrids or geotextiles Polymer geogrids and polyester strips, both flexible andstiff, are usually used as horizontal layers.

Geocomposite reinforcement or hybrid reinforcement

Geotextiles (woven and nonwoven) are also used inwrap-around faced mechanically stabilized earth walls.


FaciaThe facings have aesthetic views and can be of anyshape and colours.

Wrap-around facings Segmental precast concrete panels Full-height concrete panels Modular block wall Gabion facings Timber facingWelded wire meshes facing Gunny bag facing Brick facing


Vertical spacing of reinforcements = 0.3 m - 0.5 m

It is required to protect the geotextile against ultravioletlight, degradation, vandalism and damage due to fire. Insuch case, shotcrete should be applied to the wall facing.

Warp- around facing


Segmental precast concrete panels

HDPE geogrids are casted into the panels duringmanufacturing process in the field. The main geogrid isthen connected to the HDPE geogrid (bodkin joint) about30 cm away from the facing panel.

The flexible polyester geogrid should not be casted dueto high alkalinity in presence of wet concrete.


Three types of Precast concrete face panels:

Hexagonal shaped panel:

1.5 m height, 1.75 m width and 0.165 m thick

Rectangular panel:

3.81 m long, 0.61 m height and 0.2 m thick

T –shaped panel:

3.2 m area and 0.16 m thick


Bodkin connection details

A rigid PVC pipe is used as bodkin. There should not beany slack in the connection.


The full height concrete panels are 12.5 cm - 30 cm thick,240 cm - 300 cm wide and 750 cm high.

Stiff polyethylene geogrids are casted into the panelsimilar to segmental precast concrete panels.

The minimum compressive strength of concrete at 28days is 27.56 Mpa.

Full-height concrete panels


Modular concrete block wall (MCBW)

Length = 200 mm - 600 mm

Height = 100 mm - 200 mm

Width = 200 mm - 0.6 m.

The weight of dry casting MBW = 15 kg to 50 kgProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Gabion facing wall

The gabion is a kind of basket made up of galvanizedmild steel wire mesh and polymer geogrids filled withrocks/stones.


Timber facing Welded wire mesh facing

Gunny bag facing Brick facing wallProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

DIFFERENT PRECAST CONCRETE MODULAR BLOCKS OR PANEL FACINGS AND CONNECTIONS

Modular concrete blocks for segmental retaining walls (After Bathurst and Simac, 1994)


Shear Pin Shear Key

Leading shear lipGeogrids connected with modular blocks either mechanically

or by friction (After Simac et al. 1993)


(a) Frictional connection and (b) Mechanical connectionProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Details of frictional connection between geogrid and segmental panel


Construction Details


Wall Construction


General view on Wall During Construction


Placing Facing Blocks


Wall Ties Fixing False Facing


ANALYSIS AND DESIGN PROCEDURES FOR GEOSYNTHETICS REINFORCED SOIL RETAINING WALL

Geosynthetic reinforced soil wall with inclined surcharge loadProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Schematic view of segmental reinforced soil retaining wall

HM = Mechanical height, HF = Facing height,HD = Design height


Step 1: Physical characteristics of mechanicallystabilized soil walls.

Wall geometry:The height of wall = H,The length of wall = L,Wall face batter angle = ,The wall requires a nominal batter of 3° to 10°Slope angle of the soil surface = i,

Loading:Surcharge loads: Live load = qLDead load = qDTotal surcharge (q) = qL + qD


Type of facing Full-height concrete panels,Wrapped facings, Modular or Segmental concrete blocks. Gabion Vertical spacing of reinforcements (Sv)

Wrapping:Maximum spacing (Sv) is 0.5 m to 0.6 m for geotextile(woven and non-woven) or geogrid wrapped face walls.

Precast concrete face panels:The spacing of the geogrid reinforcement may be keptfrom 0.5 m to 1 m. However it is recommended to keepthe vertical spacing of reinforcement as 0.6 m - 0.8 m.


Modular block:- For the modular concrete block of height 200 mm to 250mm, the spacing of the reinforcement may be 200 mm,400 mm, 500 mm, 600 mm, 750mm, 800 mm and 1 m.

- For segmental concrete block, if the spacing is more,use secondary reinforcement.

Vertical spacing of the reinforcement depends on thestrength of the reinforcement, facing connection and typesof panels or blocks used for construction.


Establish preliminary wall dimensions

Case Minimum L/H ratioStatic loading without or with

traffic surcharge 0.7

Sloping backfill surcharge 0.8Seismic loading 0.8 to 1.1

a) Minimum length of reinforcement (FHWA –NHI-10-024, 2009)

b) For walls founded on slopes, a minimum horizontalbench of 1.2 m wide should be given in front of wall.Minimum embedment depth should be 0.5 m.

Minimum 1 m embedment length is recommendedbeyond Rankine failure wedge for pullout resistance.


Step 2: Evaluate engineering properties of thefoundation soil.

Detailed soil exploration has to be carried out alongthe alignment of the reinforced soil wall at every 25 minterval.

Evaluate grain size distribution, moisture content,liquid limit, plastic limit, shrinkage limit and plasticityindex of soil.

Calculate the shear strength and consolidationparameters of foundation soil. Check the location ofground water table.


Step 3: Evaluate reinforced fill and retained backfill soil.

Check the grain size distribution and plasticity index.

Plasticity index should not exceed 6 (AASHTO T-90)

Coefficient of uniformity of reinforced fill ≥ 2.

Organic content should be limited to 5 %.

Determine optimum moisture content (OMC), maximumdry density or relative density with the aid of standardproctor test.

The minimum compaction of backfill soil should be 90%of maximum proctor density.


Internal friction angle (Φr) of the soil in reinforced zonecan be determined from the drained direct shear test.

For retained backfill, the internal friction angle (Φb) canbe determined by drained triaxial compression test ordirect shear test.

Generally, angle of internal friction ≤ 34º.

Coefficient of permeability should be ≥ 1 x 10-2 cm/sec

No cohesion should be considered, i.e. fine silts and clay should not be used for reinforced fill.

Appropriate drainage system is required at the back,base and front of reinforced soil retaining walls.

If the quality of backfill is poor, the adequate drainagecan not be achieved (Saidin, 2007).


For polyester geosynthetic, pH value of soil should liebetween 3 and 9 (Elias and Christopher, 1997)

For polyethylene and polypropylene, pH of soil > 3(AASHTO T-289-91).

Minimum aperture size of geogrid > 3.5 times theparticle size of the backfill soil (Sarsby, 1985)

In many cases, we use the minimum average rollvalues (MARV) obtained from the manufacturer’scertificate.

For good design, it is recommended to verify the testresults of geosynthetic materials from the third party.


Sieve SizeNumber

ParticleSize

PercentPassing

# 4 4.76 mm 10010 2.0 90-10040 0.42 0-60

100 0.15 0-5

200 0.075 0

Notes: FHWA adopts15% passing #200 sieveNCMA adopts 35% passing #200 sieve

Gradation of backfill soil for reinforced soil zone(Walls and slopes)(After Koerner et al.1993,GSI/GRI)


Polymer type Creep reduction factors

Polyester (PET) 2.5 to 1.6Polypropylene (PP) 5.0 to 4.0

High Density Polyethlene (HDPE) 5.0 to2.6

Creep reduction factor for polymer(FHWA –NHI-10-024, 2009)


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


Documents

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE 26.pdfGEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of Civil Engineering, IIT Bombay, Powai , Mumbai