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GEOTECHNICAL ENGINEERING GEOTECHNICAL ENGINEERING CHALLENGES FOR HIGHWAY CHALLENGES FOR HIGHWAY

DESIGN AND CONSTRUCTION ON DESIGN AND CONSTRUCTION ON SOFT GROUNDSOFT GROUND

By Engr. Dr. Gue See Sew (P.Eng)Engr. Dr. Wong Shiao Yun (G.Eng )

CHALLENGES FOR CHALLENGES FOR GEOTECHNICAL ENGINEERS GEOTECHNICAL ENGINEERS

ON SOFT GROUNDON SOFT GROUND

Consolidation Settlement

Bearing Capacity and

Consolidation Settlement

FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA

SABAH

FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA

SIBU

FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA

FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA

FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA

FAILURE EVENT IN SINGAPOREFAILURE EVENT IN SINGAPORE

IDENTITY OF SOFT IDENTITY OF SOFT GROUNDGROUND

IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND

Su < 10kPa

Su > 10kPa

IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND

IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND

VEGETATIONVEGETATION

VEGETATIONVEGETATION

Alluvial Deposits

Soft Ground in Malaysia

Alluvial

Deposits

Embankment FailureEmbankment Failure

Embankment Treated with Vacuum Embankment Treated with Vacuum Preloading with Vertical DrainsPreloading with Vertical Drains

Very Loose Clayey SAND

Medium to Stiff Silty CLAY and Clayey SILT

Embankment Fill(Without Vacuum Preloading)

Embankment Fill (Failed Area)(Vacuum Preloading with Vertical Drains)

Vertical Drains

Liner and Sand Layerfor Vacuum System

Scale (m)

0 5 10

Soft Sandy CLAY

Very Soft Silty CLAY

(After Gue et al. 2001)

Embankment FailureEmbankment Failure• Embankment failed = Fill height of 5.5m• After Failure of Vacuum Preloading ���� Remedial

with Stone Columns.• Embankment Failed Again at 3.2m

Sheer Drop and Cracks

Heave Up

Undrained Shear Strength ProfileUndrained Shear Strength Profile

16

14

12

10

8

6

4

2

0D

epth

(m

)0 10 20 30

Sensitivity, St

Su-Undisturbed from VS-A

Su-Remolded from VS-A

Su-Undisturbed from VS-B

Su-Remolded from VS-B

In-Situ Vane Shear TestVS-A

VS-B

Su = 10 kPa

Su = 8 kPa

Su = 13 kPa

Su = 17 kPa

Su = 19 kPa

0 10 20 30 40 50 60Undrained Shear Strength, Su (kPa)

Monitored Pore Water PressuresMonitored Pore Water Pressures

Fist Crack Observed on Day 162

Excess Pore Water Pressure generated at PZ-A3, U = + ve

Excess Pore Water Pressure generated at PZ-A2, U = + ve

Stage BStage C

Stage D

Stage EStage F

∆

-2

0

2

4

6

8

10

12

Pie

zom

eter

Hea

d (

m)

Piezometers at Location Aat 3.0m depthat 6.0m depthat 8.0m depth

0 50 100 150 200 250 300 350Days0

2

4

6

Fill

Hei

gh

t (m

)

Designed Water Head is 3m at PZ-A1

Designed Water Headis 6m at PZ-A2.

Designed Water Headis 8m at PZ-A3 ∆

Figure 4 – Construction Sequence and

Failure of Embankment treated with Failure of Embankment treated with Stone ColumnsStone Columns

• Only Priebe’s Method was used

• Bulging & General Shear Failures not checked

• Independent review shows inadequate General Shear Capacity

Methods of Estimating Ultimate Methods of Estimating Ultimate Bearing CapacityBearing Capacity

• Large range of possible Ultimate Bearing Capacity• Attention when using stone columns in very soft

ground (e.g. su < 15kPa)

Lessons LearnedLessons Learned

• Vacuum Preloading Method shall be closely monitored

• Remedial design for failed embankment shall used “disturbed” soil strength

• Stone columns design shall check for all modes of failure+ Observational Method (recommended)

• Understand the Limitation of Software used ���� It may not check all the required modes of failures

Stone ColumnStone Column

The Principle� Stone Columns

= Granular Pile = Vibro Replacement

� Involves partial replacement of unsuitable subsoil with compacted column of stones or aggregates

Stone Column

DD

Firm strata

Stone Stone columncolumn

Soft clay

Sand platform

� Usually completely penetrates the weak strata

Stone Column

`

EmbankmentSand Platform

Compressible Layer

Firm Strata

FunctionStone Column

� Provide bearing capacity / strengthening immediately upon installation

� Reduce settlement� Increase the rate of consolidation� Facilitate subsoil drainage

� Diameter: 0.6m - 1.2m

Other Available Option for Other Available Option for Ground ImprovementGround Improvement

SurchargingSurcharging• Temporarily compress the subsoil with

higher pressure than permanent load

• Achieve higher initial rate of settlement + reduce long term settlement

• Larger portion of fill left behind

• If fill material is available

Embankment

Soft SoilSoft Soil

Finished Level

SurchargingSurcharging

Surcharge

Fill

Thi

ckne

ss

Rest Period

Set

tlem

ent

Filling

Time

Time

With Surcharge

b a

FASTERFASTER

Without Surcharge

SurchargeSurcharge

Settlement

Settlement

Service Life of Service Life of

embankmentembankment

Permanent Permanent

LoadingLoading

Log Log

TimeTime

Log Log

TimeTime

Permanent Permanent

Loading OnlyLoading Only

Primary Primary

ConsolidationConsolidationSecondary Secondary

ConsolidationConsolidation

Permanent & Permanent &

Surcharge Surcharge

LoadingLoading

Service Service

Life Life

Settlement Settlement

without without

SurchargeSurcharge

Vertical Pressure from

Vertical Pressure from

Embankment Loading

Embankment Loading

Service Life of Service Life of

EmbankmentEmbankmentSurcharge Surcharge

DurationDurationConstructionConstruction

Temporary SurchargeTemporary Surcharge

Earthwork Surcharge in Progress

VERTICAL DRAINSVERTICAL DRAINS

FunctionsFunctions

• Provide shorterdrainage path

• Accelerate dissipation of excess pore water pressure

HD

Drainage Path for ConsolidationDrainage Path for Consolidation

Consolidation TheoryConsolidation Theory

ccvv = T= Tvv HHDD2 2 / t/ t

Where c v= coefficient of consolidation in vertical direction (m 2/year)

Tv = Time factor (dimensionless)

HD = Drainage path length (m)

t = Time application of loading (year)

RearrangeRearrange ……

t = Tt = Tvv HHDD22 / / ccvv

ThereforeTherefore

100 100

times times

faster!faster!

1t 100

10m1mHD

t t ∝∝∝∝∝∝∝∝ HHDD22

INSTALLED PVDINSTALLED PVD

PVD INSTALLATION VIDEOPVD INSTALLATION VIDEO

Failure of Bridge Failure of Bridge Foundations and Approach Foundations and Approach

EmbankmentEmbankment

OverviewOverview

Abutment II

Pier II

Abutment I

Pier I

OverviewOverview

Abutment I Abutment II

Pier I

Pier II

Subsoil ConditionSubsoil Condition

25m coastal & alluvium CLAY

Sheer DropSheer Drop

Sheer Drop

Pilecaps

Slip FailureSlip Failure

Tilted Abutment & Tilted Abutment & Gap between Bridge DecksGap between Bridge Decks

Opening between bridge

Tilt from Vertical

Pier IIPier II

Tilted Pilecap

Slip Failure of EmbankmentSlip Failure of Embankment

• At 25m behind Abutment II

• Abutment II :- Tilted 550mm on top- Angular distortion of 1/6

• 300mm gap between bridge decks

Geotechnical InvestigationGeotechnical Investigation

• Hfailure @ 3m • HDesign @ 5.5m

���� NOT SAFE

HOW TO CHECK?

What Is The Critical Height?What Is The Critical Height?

HHfailurefailure = (= (NNcc x Su) / x Su) / γγγγγγγγfillfill

NNcc ≈≈≈≈≈≈≈≈ 55

HHfailurefailure = (5 x Su) / = (5 x Su) / γγγγγγγγfillfill

e.g. :e.g. :When When Su = 10 kPa ; Su = 10 kPa ; γγγγγγγγfillfill = 18 kN/m= 18 kN/m33

HHfailurefailure = (5 x 10)/ 18 = 2.8 m= (5 x 10)/ 18 = 2.8 m

• Failures ���� (temporary works)

- Inadequate geotechnicaldesign

- Subsoil Condition (Lack of understanding)

- Lack of constructioncontrol & supervision

Lessons LearnedLessons Learned

Preventive MeasuresPreventive Measures• Proper design and review

• Stability check of embankment & abutment

• Most critical :-During construction(must check temporary works)

• Proper full-time supervision(with relevant experience & understand design assumptions)

SETTLEMENT OF SETTLEMENT OF APPROACHES APPROACHES

BRIDGESBRIDGES

PilePilePilePile

O.G.L.O.G.L.O.G.L.O.G.L.

Final ProfileFinal ProfileFinal ProfileFinal Profile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfile

AbutmentAbutmentAbutmentAbutment

Typical CrossTypical Cross --SectionSection

SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM

Expanded Polystyrene Expanded Polystyrene Expanded Polystyrene Expanded Polystyrene (EPS)(EPS)(EPS)(EPS)PilePilePilePile

AbutmentAbutmentAbutmentAbutment

O.G.L.O.G.L.O.G.L.O.G.L.

Final ProfileFinal ProfileFinal ProfileFinal Profile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfile

USE OF LIGHT WEIGHT MATERIALUSE OF LIGHT WEIGHT MATERIAL

http://www.nhi.fhwa.dot.gov

USE OF TRANSITION EMBANKMENT PILESUSE OF TRANSITION EMBANKMENT PILES

Transition EmbankmentTransition EmbankmentTransition EmbankmentTransition EmbankmentPilesPilesPilesPiles

PilePilePilePile

AbutmentAbutmentAbutmentAbutment

O.G.L.O.G.L.O.G.L.O.G.L.

Final ProfileFinal ProfileFinal ProfileFinal Profile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfile

Approach Approach Approach Approach SlabSlabSlabSlab

EXAMPLE (BERNAM JAYA)Transition Piles + Surcharging = Fewer Piles + Cost Saving

SETTLEMENT OF SETTLEMENT OF APPROACHES TO APPROACHES TO

CULVERTSCULVERTS

PILED CULVERTPILED CULVERT

Final Final Final Final ProfileProfileProfileProfile

PilePilePilePile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfileO.G.L.O.G.L.O.G.L.O.G.L.

SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM

ENLARGED CULVERTENLARGED CULVERT

Final Final Final Final ProfileProfileProfileProfile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfile

SiltSiltSiltSilt

O.G.L.O.G.L.O.G.L.O.G.L.

TRANSITION PILESTRANSITION PILES

PilePilePilePile

Final ProfileFinal ProfileFinal ProfileFinal Profile

Long Term Long Term Long Term Long Term ProfileProfileProfileProfile

Transition Transition Transition Transition Embankment PilesEmbankment PilesEmbankment PilesEmbankment Piles

Transition Transition Transition Transition Embankment PilesEmbankment PilesEmbankment PilesEmbankment Piles

Guidance Notes on Subsoil Guidance Notes on Subsoil InvestigationInvestigation

Guidance Notes on Subsoil InvestigationGuidance Notes on Subsoil Investigation

• Collect UD from BH

• Laboratory Test: UCT & 1-D Consolidation Test

• Piezocone : – To detect presence of sand lenses– qu

– Especially for surcharge design with or without PVD

LocalisedLocalised Weak ZoneWeak Zone

LocalisedLocalised Weak ZoneWeak Zone

• Generalisedmoderately conservative design line (MCL)

LocalisedLocalised Weak ZoneWeak Zone

• If not identifies, likely to cause failure

• Surcharge + PVD⇒⇒⇒⇒ Piled Embankment

• Further verify by Vane Shear Tests, Piezocones & MPs

ConclusionConclusion

CONCLUSIONCONCLUSION• Important:

Bearing Capacity assessment by CRUDE checkSYSTEMATIC check & review process(review by experienced engineers)STRUCTURED training programmes(enhance technical knowledge & share lessons learned)Full-time SUPERVISION with team of suitable experienceExtra Care on TEMPORARY WORKS

CONCLUSIONCONCLUSION

• DO NOT� Abuse geotechnical design , detailed

analysis

� Overlook localised weak zones

� Overlook structural detailing

SOFT GROUND CONSTRUCTION

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