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

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60

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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

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Em

ban

km

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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

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(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.