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YORKSHIRE GEOTECHNICAL GROUP
Evening Meeting
The Geotechnics of the Selby Bypass
Hugo Wood (High-Point Rendel)
Dr Martin Pedley (Cementation)
Keith Sleightholme (Skanska)
Outline of Presentation
Hugo Wood
• History and background to scheme
• Description of scheme and ground conditions
• Summary of geotechnical challenges and solutions adopted
for design of embankments, cutting and bridge foundations
Martin Pedley
• Discussion of design approach for piles to supported embankments
Keith Sleightholme
• Construction and Environmental Issues
Scheme History
1929 Bypass first proposed for Selby
1938 Scheme entered roads programme of Ministry
of Transport
1950-70’s Numerous routes considered and scheme
entered and removed from various roads
programmes
1980’s Scheme resurrected and NYCC appointed
design agent for the Highways Agency
1991-95 Public Enquiries into line orders and CPO
1997-98 Roads review announced that the A63 Selby
Bypass would be part of targeted programme
of investment
Scheme Benefits
The A63, A19 and A1041 all converge on the centre of
the market town and the existing single carriageway
through Selby carries 20,000 vehicles per day
After Construction of the bypass:
• 40% reduction in traffic flows through town
• Estimated 250 fewer accidents over next 30 years
• Release sites for development currently constrained
by existing congestion
Background to current scheme
• Tenders invited Autumn 2000
• Design and Build contract
• Quality submission covering technical issues, safety,
public liaison and traffic management
• Financial submission opened only after consideration
of quality submission
• Contract awarded Summer 2001
• Contractor: Skanska Construction UK
• Designer: High-Point Rendel
• Construction cost approximately £44M
Details of Scheme
• 10km single carriageway highway, from Thorpe
Willoughby in the west to Barlby in the east
• Road passes over Selby Canal, Selby-Doncaster and
Selby-Hull railway lines and the River Ouse
• Roundabouts at each end of the scheme and at
junctions with the A19 and A1041
• Main construction started in January 2002
• Road due to open Spring 2004.
Plan of Alignment
Vertical Alignment
Ground Conditions
Main Geotechnical Challenges
• Poor ground conditions
• High embankments (>9m)
• Rigid settlement criteria
• Existing services
• Integral bridge design for high skew bridges
• Integral bridge foundations
• Swing bridge foundations
Embankment Stability
• Stability of embankments:
–Side slopes 1V:2H fixed by land take constraints
–High embankments
–Low undrained shear strength of foundations
(Su as low as 20kN/m2)
–Development of excess pore water pressures,
including potential for pore pressure spread in
laminated clays
Geotechnical Solutions (Stability)
• Lightweight PFA fill used for high embankments
(g = 15kN/m3)
• Limits imposed in Specification on rate of
construction of high embankments
• Basal reinforcement
• Monitoring
Basal Reinforcement
Monitoring
Monitoring Results
Excess PWP vs Time Ch 5460
0
10
20
30
40
50
60
70
80
90
100
21-09-02 28-09-02 05-10-02 12-10-02 19-10-02 26-10-02 02-11-02 09-11-02 16-11-02 23-11-02 30-11-02
Date
Excess P
WP
(kP
a)
0
3
6
9
12
15
Em
ban
km
en
t C
on
str
ucti
on
(m
OD
)
PNP2
PNP3
PNP4
Embankment Construction
Monitoring Results
PWP Ratio (excess PWP:s'v) Ch 5460
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
21-09-02 28-09-02 05-10-02 12-10-02 19-10-02 26-10-02 02-11-02 09-11-02 16-11-02 23-11-02 30-11-02
Date
PW
P R
ati
o
0.0
3.0
6.0
9.0
12.0
15.0
Em
ban
km
en
t C
on
str
ucti
on
(m
OD
)
PNP2
PNP3
PNP4
Embankment Construction
Embankment Settlement
• Settlement Criteria (measured at the end of the 5
year maintenance period relative to design levels):
–Maximum allowable settlement 75mm
–Maximum differential settlement gradient 1 in 500
(along the carriageway)
–Maximum differential settlement 25mm (across the
carriageway)
• Settlement:
–Greater than 1m for embankments on alluvium
–Up to 250mm for embankments on laminated clay
–Time for 95% consolidation up to 100 weeks
Temporary Surcharge
A19 Barlby Roundabout
• Low height embankment (1-3m)
• 2m of peat and organic clay in foundation soils
• up to 400mm of settlement anticipated
• Surcharge and drainage solution adopted
• Band drains at 2.7m c/c installed on a triangular grid
Band Drain Locations
Typical Section through Band Drains
Design Options for Ouse Flood PlainCOMPARISON OF PILE SUPPORTED AND UNSUPPORTED EMBANKMENTS ACROSS OUSE FLOOD PLAIN
Issue Pile Supported Embankment Unsupported Embankment
PRO CON PRO CON
Overall settlement (post construction)+
Negligible-
Significant (up to 2m). Impacts on structures adjacent toembankment (including existing drainage network etc.) as well asstructures associated with the embankment (culverts, toe drainageetc.). Settlement (absolute and differential) cannot be accuratelypredicted at this stage and even as a result of monitoring duringconstruction long term behaviour may not be defined sufficientlywell.
Differential settlement (long.)+
Negligible-
Potentially unacceptable within unsupported embankment if groundconditions vary locally (e.g. buried channels), Will require treatmentbetween piled sections adjacent to bridge structures and mainunsupported embankment.
Differential settlement (lat.)+
Negligible-
Difficult to accurately calculate, potentially unacceptable,
Stability+ -
Basal reinforcement required to preventlateral spreading (loading on piles) toBS8006, but conservative design and failureis very unlikely. Reinforcement required tospan between piles.
-Significant reinforcement required to prevent lateral spreading, andfoundation extrusion, particularly given the requirement to steepenthe sideslopes to allow placement of surcharge. Difficult toefficiently design without the results of a trial embankment. Stagedconstruction may still be required to prevent excessive movementof foundation soils.
Instrumentation+
Minimal required-
Significant instrumentation required, to be regularly monitored andresults interpreted to determine progress of construction/contingentmeasures etc.
Programme+
Little uncertainty, shorter programme-
Longer programme, unknown at start of construction, greateruncertainty in meeting overall deadline
Cost-
Greater cost (but little uncertainty)+
Probably lesser estimated cost (but greater uncertainty andpotentially greater long term costs)
Pile Supported Embankments
Typical Plan of Pile Locations
Reinforcement Layout
Protection of Existing Services
Earthworks
• Use of waste products from other industries (PFA and
minestone)
• Testing of materials prior to construction to confirm
appropriate design parameters
• Detailed testing regime during construction to confirm
properties of materials
Cutting
• Overall 700m long, up to 13m maximum depth
• Sherwood Sandstone
• Variable weathering profile
• Landscaping of cutting important consideration
• Horseshoe Bridge taking bridleway across cutting
Plan of Cutting
Typical Section through Cutting
General Arrangement - Horseshoe Bridge
General Arrangement - Oakney Bridge
Integral Bridges
• All Highway Bridges <60m and <30° skew to be
integral with their abutments
• Thermal cycling of bridge decks leads to the
development of high earth pressures on abutment
• BA 42/96 provides derivation of design earth
pressure coefficient k*
Earth Pressures on Integral Abutments
Fig 3.1 from BA 42/96
Design Approach for Integral Bridges
• Full 3 dimensional frame analysis model of bridge
and foundations
• Piles modelled with springs defined as secant p-y
curves for lateral load resistance of soil
• p-y mutipliers applied to spring stiffnesses to model
interaction of pile rows
Frame Model for Integral Bridges
Foundations to Integral Bridges
General Arrangement - William Jessop Bridge
Supported Bankseat Abutment
Reinforced Soil Wingwalls
• Designed to BS 8006 (as implemented by BD 70/97
for Highway Structures) on coherent gravity method
• 1.8mx2m pre-cast concrete panels with galvanised
steel reinforcing strips
• Standardisation of panel types to minimise variety of
panels to be constructed
Reinforced Soil Wingwalls
Panel Layout
River Ouse Swing Bridge
Ouse Bridge Foundations
Summary of Geotechnical Challenges
• Poor ground conditions on eastern half of alignment
• High embankment construction leading to concerns
over settlement and stability of embankments
• Integral bridge form adopted for high skew bridges
• Differential settlement between bridge structures and
embankments
• Complex loading on swing bridge foundations
Summary of Geotechnical Solutions
• Different forms of embankment construction including
surcharging, pile supported embankments, basal
reinforcement and drainage
• Monitoring of embankment stability and settlement
• Supported foundations to integral bridges
• Arrangement of vertical and raked tubular steel piles
to swing bridge foundation.