GEOTECHNICS ON IRISH ROADS, 2000 - 2010 A Decade of Achievement

CPD Conference Thursday 11th October 2012

PILED EMBANKMENTS

RECENT EXPERIENCES

Geotechnics on Irish Roads,

2000 – 2010- A Decade of Achievement

Wyatt Orsmond

October 2012

Overview of Piled Embankments

• First used in 1960 in Finland

– (no basal reinforcement)

• Act as load transfer mechanisms

– Partial or full load transfer

– Rely on soil arching or basal reinforcement tension or

structural slab or a combination.

• Piled Embankments and Load Transfer

platforms defined

• Various design methods for piled embankments

– mainly focus of basal reinforcement design.

The elements of Piled Embankment Design

From bottom to top

• the pile supporting condition (end bearing or floating),

• the pile itself (type, joints, installation)

• pile caps

• Basal reinforcement for taking vertical load

• Basal reinforcement for taking horizontal loads

• Edge conditions (horizontal tieback)

• Embankment height

• Road Pavement

• Live loading (regular and irregular)

• But you have to build it first!

Working Platform Design

• How do you

get a 60 ton

pile rig onto

a peat bog

which you

can barely

walk on?

Working Platforms

Geotextile with two layers of geogrid sandwiched between sandy rockfill.

BRE (2004) guidance document ‘Working platforms for tracked plant’.

NOT SUITABLE for subgrade conditions where the cu < 20 kPa

Embankment Design Considerations

Pile design

Basal Reinforcement design

Pile Design – what to consider

• Dead load

• Negative skin friction

• Live loads

• Soil arching

• Load share (subsoil)

• Shaft support

• Toe and Head Conditions

• Induced moments

• Pile Cap

Pile Design – Dead and Live loads

• Uneven loading?

– Floods

– Road camber

– Spoil infill / landscape fill

Pile Design – Load Share and Arching

• Soil arching

– Embankment height

– Clear span length

• Load share (subsoil)

Where the subsoil is weak or where its strength is uncertain, load share should not be considered.

Pile Design – Support & Boundary Conditions

• In weak peat, piles should be designed as slender columns – Can the horizontal support be relied on?

• Pile cap interaction (pin or fixed) – (temporary and permanent conditions)

• Top and toe boundary conditions

Basal Reinforcement Design

Dead and live loads

Soil Arching, load share,

Lateral sliding and anchoring

BS 8006

Piled Embankments in Ireland

• First recorded piled embankment

– 1985 : built as part of the New Shannon

Bridge near Athlone.

• Information on

– 12 basal reinforced platforms

– 4 load transfer platforms (concrete slab)

Enniskillen Housing development (1999)

• precast piles on a 2.75 triangular grid

• 3 layers of Tensar geogrid (2xSS30, 1 x SS20).

• Failed due to settlement of the ground away

from the fill material below the lowest geogrid

causing the LTP to sag to such an extent that

there was a failure of the arching mechanism

upon which the design depended.

– (Court Citation NIQB 68, ref COGC5319)

N5 Scramoge (2002)

• precast piles on a 2.5m grid and

embankment height of about 3.5m.

– (no other information available)

• Had initial settlement issues but has since

performed well.

Gortinty N4 Drumsna to Jamestown Bypass

(2002).

• 200 mm square precast piles on a 1.6 to 1.45 m square grid

• 800mm square pile caps.

• 1.3m to 1.8m high embankment.

• Basal reinforcement comprised two layers of 150 kN/m Paragrid.

• Some issues with positional and verticality tolerances of piles during driving

• Lateral movement of soft ground during driving the outer row of piles.

• Since construction has performed well.

N4 Drumsna

Shannon (2002) (Trial embankment)

• Vibro concrete columns,

• Three layers of geogrid (Tensar SS30 and

SS20) and one layer of Basetex 200/50.

• Performed well.

– (Quigley et al, 2003)

Corravokeen – North Mayo (2003) Timber piled

embankment for Coillte

• 250mm diameter Sitka spruce poles on a 2.1m and 1.2m grid

• Basal reinforcement of A252 steel mesh and later with 2 layers of Stabilenka 200.

• Performed well

• as a temp solution. – (Ryan et al, 2004)

Glen of the Downs (2002),

• Eastern Embankment and Western Embankment

• 350mm square precast piles on a 2m grid

• Two layers of Stabelenka supporting a bridge embankment 8m high.

• Some issues with pile installation (load capacity) but otherwise performed well.

– (Orsmond, 2004)

N11 Glen of the Downs ( 2 embankments)

A1N1 Flurry Bog (2006).

• 300mm square precast piles

• 0.8m caps on a 2.5m grid

• Two layers of Polyfelt 600 - 800kN/m.

• Embankment of 3.5m high.

• Issues with pile verticality and positional tolerances but post construction has performed well.

– (Orsmond, 2008)

A1N1 Flurry Bog

N7 Annaholty Bog (2009).

• 355mm precast square piles with 1.85 to 2.15 spacing

• 0.8 -1m pile caps

• Stabilenka basal reinforcement in two to three layers ranging from 150 to 1000kN/m.

• Embankment height from 2m to 6m.

• Failure of outer piles on a short section post construction – required reconstruction

– done with a concrete slab.

• Some post construction deflection near repair occurred but otherwise performing well.

N7 – Annaholty

N7 Drominboy Bog (2009).

• 355mm precast piles on 1.8 to 1.9m

square grid

• 2.5m to 6m high embankment.

• Designed as a basal reinforcement system

but finally built as a concrete slab due to

pile vertical and positional tolerance

issues.

Limerick Tunnel bridge approach

embankments (2009).

• At two bridge locations

• Steel H piles 400x122

• installed on a 4x4m grid supporting a

concrete slab.

• Performed well.

– (Buggy & Curran (2011)

• Limerick Tunnel Bridge approach

N25/N27 Kinsale Road Interchange

• 650 and 750 dia bored piles

• 2m x 2m pile caps

• 2 layers of Fortrax 800

A4/A5 Dungannon to Balleygawley (2010).

• 275mm and 350mm precast concrete piles on grids of 2.5m to 3m

• 0.75 to 0.9m square pile caps – (one area interlinked with beams)

• Two layers of basal reinforcement ranging from 700 to 800kPa.

• Embankment failed during construction due to excessive uneven loading with landscape fill adjacent to embankment. – (info courtesy of AGEC Ltd.)

Lessons Learnt

Working Platforms

Piling

Basal Reinforcement design

Embankment Fill

Monitoring

Working Platforms

• Keep it thin (light) to limit long term settlement

– It could even be removed – eg A650 Bigley

• Cant simply base design on in-situ and lab tests – need to do trials

• Consider alternative routes for haul trucks

• Strict control of loading

– A1N1 : 60 ton crane when platform designed for 30 ton pile rig

– A4/A5 : pile test kentledge placed on platform

Piling – Construction Issues

• Verticality

• Positional Tolerance

• Pile caps

• Pile joints

Piling – Design Issues,

Basal Reinforcement Design - Layering

• Layers together or separated with fill?? – Can it be done without causing excessive damage to

the reinforcement layers?

– Can the layers be placed taught over the surface and covered in an even manner to get uniform strain?

– How does the placing of the basals’ affect the pile caps?

Basal Reinforcement

• Overlap between rolls

• Edge detail

Embankment Fill

• Placement of first layers

– Excessive/uneven loading of pile caps

– No or little compaction, increase with height

• Outer edge fill – non structural, lightweight

• Avoid infilling adjacent to platform

Monitoring

Ignorance is bliss but knowledge is power!

• Monuments and survey plates

– Limited for basal monitoring, vehicle damage

• Horizontal inclinometers

– Problems with large deformation

• Fibre optic strain gauges

– Easily damaged, costly

• Vertical inclinometers

– Very seldom done but should be mandatory

A1N1 Flurry Bog

Horizontal Inclinometer Displacement A1 and A2

Mid span compared to pile span

-200

-150

-100

-50

0

50

0 2 4 6 8 10 12 14 16 18 20

Embankment width (m)

Dis

pla

cem

en

t (m

m)

10-Oct 13-Nov 21-Nov 28-Nov12-Dec 19-Dec 10-Jan 16-Jan26-Jan 01-Feb 07-Feb 15-Feb23-Feb 03-Mar 24-Mar 05-Apr19-Apr 17-May A2-13 Nov

Acknowledgements

• RPS Consulting Engineers

• AGL, Eric Farrell

• Roughan & O’Donovan, Fintan Buggy

• AGEC, Turlough Johnston, Paul Jennings

• ARUP, Paul Quigley

• Geotechnical Society Of Ireland

THANK YOU