geotechnical challenges on soft ground

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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 Dato’ Ir. Dr. Gue See Sew

Ir. Dr. Wong Shiao Yun

CHALLENGES FOR CHALLENGES FOR GEOTECHNICAL ENGINEERS GEOTECHNICAL ENGINEERS

ON SOFT GROUNDON SOFT GROUND

Consolidation Settlement

Bearing Capacity

and Consolidatio

n 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

M edium to Stiff Silty CLAY and Clayey SILT

Embankment Fill(W ithout Vacuum Preloading)

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

Vertical Drains

L iner and San d Layerfo r Vacuu m 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

0

Dept

h (m

)0 10 20 30

Sensitivity, St

S u -U nd is tu rb ed fro m V S -AS u -R em o ld ed fro m V S -AS u -U nd is tu rb ed fro m V S -BS u -R em o ld ed fro m V S -B

In-S itu Vane Shea r Tes tVS-AVS-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-A 3, U = + ve

Excess Pore Water Pressure generated at PZ-A 2, U = + ve

Stage BStage C

Stage DStage E

Stage F

-2

0

2

4

6

8

10

12

Piez

omet

er H

ead

(m)

P iezo m eters a t L o ca t ion Aat 3.0m depthat 6.0m depthat 8.0m depth

0 50 100 150 200 250 300 350Days0

2

4

6

Fill

Heig

ht (m

)

Designed Water Head is 3m at PZ-A1

Designed W ater Headis 6m at PZ-A2.

Designed Water Headis 8m at PZ-A3

Figure 4 – Construction Sequence and Monitored Pore Water Pressure Changes.

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

Thic

knes

sRest Period

Settl

emen

tFilling

Time

Time

With Surcharge

b a

FASTERFASTER

Without Surcharge

SurchargeSurcharge

Sett

lem

ent

Sett

lem

ent

Service Life Service Life of of embankmentembankment

Permanent Permanent LoadingLoading

Log Log TimeTime

Log Log TimeTime

Permanent Permanent Loading Loading OnlyOnly

Primary Primary ConsolidatiConsolidationon

Secondary Secondary ConsolidatioConsolidationn Permanent & Permanent &

Surcharge Surcharge LoadingLoading

Service Service Life Life SettlemeSettlement nt without without SurchargSurchargee

Vert

ical

Pre

ssur

e fr

om

Vert

ical

Pre

ssur

e fr

om

Emba

nkm

ent

Load

ing

Emba

nkm

ent

Load

ing

Service Life Service Life of of EmbankmentEmbankment

SurchargSurcharge e DurationDuration

ConstructionConstruction

Temporary SurchargeTemporary Surcharge

Earthwork Surcharge in Progress

VERTICAL DRAINSVERTICAL DRAINS

FunctionsFunctions

• Provide shorter drainage path

• Accelerate dissipation of excess pore water pressure

HD

Drainage Path for ConsolidationDrainage Path for Consolidation

Consolidation TheoryConsolidation Theory

ccvv = T = Tvv H HDD2 2 / t/ t

Where cv= coefficient of consolidation in vertical direction (m2/year)

Tv = Time factor (dimensionless)

HD = Drainage path length (m)

t = Time application of loading (year)

RearrangeRearrange……

t = Tt = Tvv H HDD22 / c / cv v

ThereforeTherefore

100 100 times times faster!faster!

1t 10010m1mHD

t t H HDD22

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 SAFEHOW TO CHECK?

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

HHfailurefailure = (N = (Ncc x Su) / x Su) / fillfill

NNc c 5 5HHfailurefailure = (5 x Su) / = (5 x Su) / fillfill

e.g. :e.g. :When When Su = 10 kPa ; Su = 10 kPa ; fill fill = 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

Pile

O.G.L.

Final Profile

Long Term Profile

Abutment

Typical Cross-SectionTypical Cross-Section

SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM

Expanded Polystyrene (EPS)Pile

Abutment

O.G.L.

Final Profile

Long Term Profile

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 EmbankmentPiles

Pile

Abutment

O.G.L.

Final Profile

Long Term Profile

Approach Slab

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

SETTLEMENT OF SETTLEMENT OF APPROACHES TO APPROACHES TO

CULVERTSCULVERTS

PILED CULVERTPILED CULVERT

Final Profile

Pile

Long Term ProfileO.G.L

.

SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM

ENLARGED CULVERTENLARGED CULVERT

Final Profile

Long Term Profile

Silt

O.G.L.

TRANSITION PILESTRANSITION PILES

Pile

Final Profile

Long Term Profile

Transition Embankment

Piles

Transition Embankment

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

Localised Weak ZoneLocalised Weak Zone

Localised Weak ZoneLocalised Weak Zone

• Generalised moderately conservative design line (MCL)

Localised Weak ZoneLocalised Weak 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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