Geotechnical Challenges on Soft Ground
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G&P Professional Sdn Bhd (www.gnpgroup.com.my)
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
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CHALLENGES FOR CHALLENGES FOR GEOTECHNICAL ENGINEERS GEOTECHNICAL ENGINEERS
ON SOFT GROUNDON SOFT GROUND
Consolidation Settlement
Bearing Capacity
and Consolidatio
n Settlement
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FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA
SABAH
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FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA
SIBU
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FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA
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FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA
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FAILURE EVENT IN MALAYSIAFAILURE EVENT IN MALAYSIA
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FAILURE EVENT IN SINGAPOREFAILURE EVENT IN SINGAPORE
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IDENTITY OF SOFT IDENTITY OF SOFT GROUNDGROUND
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IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND
Su < 10kPa
Su > 10kPa
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IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND
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IDENTITY OF SOFT GROUNDIDENTITY OF SOFT GROUND
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VEGETATIONVEGETATION
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VEGETATIONVEGETATION
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Alluvial Deposits
Soft Ground in Malaysia
Alluvial Deposits
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Embankment FailureEmbankment Failure
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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)
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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
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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)
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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.
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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
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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)
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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
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Stone ColumnStone Column
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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
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Stone Column
`
EmbankmentSand Platform
Compressible Layer
Firm Strata
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FunctionStone Column
Provide bearing capacity / strengthening immediately upon installation
Reduce settlement Increase the rate of consolidation Facilitate subsoil drainage
Diameter: 0.6m - 1.2m
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Other Available Option for Other Available Option for Ground ImprovementGround Improvement
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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
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Embankment
Soft SoilSoft Soil
Finished Level
SurchargingSurcharging
Surcharge
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Fill
Thic
knes
sRest Period
Settl
emen
tFilling
Time
Time
With Surcharge
b a
FASTERFASTER
Without Surcharge
SurchargeSurcharge
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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
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Temporary SurchargeTemporary Surcharge
Earthwork Surcharge in Progress
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VERTICAL DRAINSVERTICAL DRAINS
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FunctionsFunctions
• Provide shorter drainage path
• Accelerate dissipation of excess pore water pressure
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HD
Drainage Path for ConsolidationDrainage Path for Consolidation
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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)
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RearrangeRearrange……
t = Tt = Tvv H HDD22 / c / cv v
ThereforeTherefore
100 100 times times faster!faster!
1t 10010m1mHD
t t H HDD22
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INSTALLED PVDINSTALLED PVD
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PVD INSTALLATION VIDEOPVD INSTALLATION VIDEO
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Failure of Bridge Failure of Bridge Foundations and Approach Foundations and Approach
EmbankmentEmbankment
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OverviewOverview
Abutment II
Pier II
Abutment I
Pier I
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OverviewOverview
Abutment I Abutment II
Pier I
Pier II
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Subsoil ConditionSubsoil Condition
25m coastal & alluvium CLAY
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Sheer DropSheer Drop
Sheer Drop
Pilecaps
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Slip FailureSlip Failure
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Tilted Abutment & Tilted Abutment & Gap between Bridge DecksGap between Bridge Decks
Opening between bridge
Tilt from Vertical
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Pier IIPier II
Tilted Pilecap
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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
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Geotechnical InvestigationGeotechnical Investigation
• Hfailure @ 3m • HDesign @ 5.5m
NOT SAFEHOW TO CHECK?
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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
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• Failures (temporary works)
- Inadequate geotechnicaldesign
- Subsoil Condition (Lack of understanding)
- Lack of constructioncontrol & supervision
Lessons LearnedLessons Learned
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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)
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SETTLEMENT OF SETTLEMENT OF APPROACHES APPROACHES
BRIDGESBRIDGES
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Pile
O.G.L.
Final Profile
Long Term Profile
Abutment
Typical Cross-SectionTypical Cross-Section
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SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM
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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
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USE OF TRANSITION EMBANKMENT PILESUSE OF TRANSITION EMBANKMENT PILES
Transition EmbankmentPiles
Pile
Abutment
O.G.L.
Final Profile
Long Term Profile
Approach Slab
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EXAMPLE (BERNAM JAYA)Transition Piles + Surcharging = Fewer Piles + Cost Saving
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SETTLEMENT OF SETTLEMENT OF APPROACHES TO APPROACHES TO
CULVERTSCULVERTS
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PILED CULVERTPILED CULVERT
Final Profile
Pile
Long Term ProfileO.G.L
.
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SOME SOLUTIONS TO SOME SOLUTIONS TO THE PROBLEMTHE PROBLEM
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ENLARGED CULVERTENLARGED CULVERT
Final Profile
Long Term Profile
Silt
O.G.L.
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TRANSITION PILESTRANSITION PILES
Pile
Final Profile
Long Term Profile
Transition Embankment
Piles
Transition Embankment
Piles
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Guidance Notes on Subsoil Guidance Notes on Subsoil InvestigationInvestigation
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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
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Localised Weak ZoneLocalised Weak Zone
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Localised Weak ZoneLocalised Weak Zone
• Generalised moderately conservative design line (MCL)
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Localised Weak ZoneLocalised Weak Zone
• If not identifies, likely to cause failure
• Surcharge + PVD Piled Embankment
• Further verify by Vane Shear Tests, Piezocones & MPs
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ConclusionConclusion
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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
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CONCLUSIONCONCLUSION
• DO NOT Abuse geotechnical design, detailed
analysis
Overlook localised weak zones
Overlook structural detailing
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SOFT GROUND CONSTRUCTION
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