introduction (16 aug 2011)

CE5101 AUG 2010 Prof Harry Tan

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CE5101 Lecture 1Introduction

16 Aug 2011

1

Prof Harry Tan

Outline

• Scope and Objectives

• Seepage– FEM analysis of Seepage

• Consolidation– FEM analysis of Consolidation

S

2

• Summary


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CE5101 SEEPAGE AND CONSOLIDATION OF SOILS 1. Basic Concepts Pore pressure and effective stress, continuity equation, Darcy’s law and its

limitations, seepage forces and general flow equations 2. Steady State Ground Water Flow through Soils Seepage theory, flow net, flow to wells, Dupuits’s assumption, idealized solutions

and determination of permeability of soils in laboratory and field pumping tests 3 Seepage and Stability Analysis3. Seepage and Stability Analysis Use of FEM in seepage modelling, slope stability including seepage analysis EC7 on Hydraulic Issues – Uplift, heave, erosion and piping 4. Consolidation of Soils I- One dimensional Review of Terzaghi’s theory, laboratory tests for compression and consolidation

parameters, application to settlement analysis 5. Consolidation of Soils II- Two and three dimensional Biot’s consolidation theory, Cryer-Mandel effects, secondary consolidation 6. Numerical Modelling of Consolidation

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Consolidation analysis in FEM, embankment loading, excavations 7. Methods of Accelerating Consolidation Preloading, surcharge, vertical drains, influence of method of installation, smear

well resistance, FEM modelling of vertical drains, hyperbolic and Asaoka method of field consolidation monitoring

8. Transient Seepage Analysis Concepts of partially saturated soils, soil characteristic water content and

permeability curves, Van Genuthen soil characteristic functions

Literature• Cedergren, H.R., "Seepage, Drainage and Flow Nets",

3rd Ed., John Wiley & Sons, 1989.• Craig RF, “Craig’s Soil Mechanics”, 7th Edition, Spoon

Press 2004.Press 2004.• Fredlund, D.G., and Rahardjo, H., "Soil Mechanics for

Unsaturated Soils", John Wiley & Sons, 1993.• Hausmann, MR. “Engineering Principles of Ground

Modification”, McGraw Hill, 1990.• PLAXIS Version 8 Users Manual, by PLAXIS BV,

2002.• Plaxis Course Notes on Seepage and Consolidation

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Plaxis Course Notes on Seepage and Consolidation• Whitlow R, “Basic Soil Mechanics”, 3rd Edition,

Longman 1996.• Yong, R.N., and Towsend, F.C.,

"Sedimentation/Consolidation Models, Prediction and Validation ", ASCE, 1984.


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Module Learning Objectives

• Attain correct understanding of seepage d lid ti i iland consolidation processes in soils

• Apply concepts to practical geotechnical problems

• Able to do some basic FEM analysis of common seepage and consolidation

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common seepage and consolidation problems

Part 1 - SEEPAGE of SOILS1. 1D and 2D Seepage Analysis2 Steady State Seepage (FEM)2. Steady State Seepage (FEM)

• PLAXIS and PLAXFLOW• Combined SEEP/W with SLOPE/W or• Slope Stability

3. Transient Seepage• PLAXFLOW• SEEP/W

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SEEP/W


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Part 2 – CONSOLIDATION of SOILS

1D Consolidation -

Terzaghi theory (Plaxis simulation)Finite Strain theory

2D, 3D and Radial Consolidation (Plaxis) -

Pseudo 2D and 3D - Uncoupled theory of

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Pseudo 2D and 3D - Uncoupled theory of Terzaghi-RendulicBiot’s theory of coupled consolidationBarron's radial theory (for PVD)

SETTLEMENTS AND CONSOLIDATION

Foundation Requirements

Elastic Stress Distribution Methods

Concept of Effective Stress

Settlements of Soils - Immediate, Delayed, and Creep Compression

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and Creep CompressionHand CalculationsSPREADSHEET Calculations (UNISETTLE)GGU-SettleFinite Element Analysis (PLAXIS)


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Determination of Compression Properties -Determination of Compression Properties

Laboratory tests and interpretationStandard Oedometer

Insitu tests correlationsSPT

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CPTu


Measurements and Interpretation

Asaoka’s method

Tan’s Hyperbolic method

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When is Settlement Analysis Required?

•Land Reclamation

•Large Storage Tanks

•Shallow Foundations

•Highway and Airfield Pavements

•Large Fills

L E b k t

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•Large Embankments

•Houses Damage by Settlements

Changi South Bay Reclamation

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Changi South Bay Reclamation

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Extent of Soft Marine Clay/Silty Clay

Marine CLAYMarine CLAYTRUE PLANT

N

BH BH --1 1 BH BH --22

BH BH --66

BH BH --99

BH BH --44

BH BH --88

T-1BH BH --33

BH BH --1616

BH BH --55BH BH --77

TRUENORTH

PLANT NORTH

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Silty CLAYSilty CLAY

BH BH --1010 BH BH --1111

BH BH --1515

BH BH --1212

BH BH --1313 T-4T-3 T-2

BH BH --1414


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Layout of Tanks and Instruments

Settlement Profiler ( SPT - )

Inclinometer ( I - )

LEGEND:

TRUENORTH

PLANT NORTH

N

S-1I-3

S-1 I-2 T-1I-1S-5

S-1

S-13

S 1P-1

s

Inclinometer ( I - )

Piezometer ( P - )

Settlement Point ( s - )

NORTH NORTH

15

S-9

S-5S-13T-3

S-5T-2S-13

S-9

S-9

S-7

S-5

S-3

S-1

T-4

P 1

I-4

SPT-

2

T ank N o . 3

15

20

25

Le

vel (

m)

Time-Settlement (edge) Curves of Tank No. 3

0

5

10

Wa

ter

0

50

100nt

(mm

)

16

150

200

250

0 10 20 30 40 50 60 70

Time (Day)

Se

ttle

me

S-1 S-5 S-9


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ATRUE

NORTH

PLANTNORTH

N

Tank Pad

A B

TANK NO. 3Settlement Profiler

Profiles of Soil Settlement recorded beneath Tank PadProfiles of Soil Settlement recorded beneath Tank Pad

B

0

50

100

150

em

ent,

mm

17

200

250

300

-30-20-100102030Distance, m

Se

ttle

Tank Pad Shell Erection Water Level = 5m Water Level = 10m Water Level = 15m Water Level = 20m Water Level = 10m Water Level = 0m

PLANTNORTH15

20

25

Unit : mm

Settlement Contours of Tank Base Plate measured after Hydro-test

NORTH

10

-5

0

5

10

15

Y(m

)

190

170

170

18-25 -20 -15 -10 -5 0 5 10 15 20 25

X (m)

-25

-20

-15

-10

70

17090

Tank No. 3


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Houses damage by settlement

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Houses damage by settlement

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Site Plan of Sembawang Runway

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Sembawang airfield fill site

22


12


23


24


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Settlement Failure in Condo

25


26


14


27

Causes of Settlement Failure

28


15

Causes of Settlement Failure

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Can we predict and prevent problem?

• Predict YES; prevent may be difficult and costlycostly

• Need Consolidation Tests

• Need to understand Stress History of Site

• Need to predict how much and how long settlements will occur

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• Need Ground Improvement to accelerate consolidation before Condo is built


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Table 2 Summary of New Lab Tests on Peaty Clay

BH Peaty clay range Sample SPT N Liquid Plastic Water LiquidityUndrained

shear strength, CompressionBH No.

Peaty clay range bgl Thk (m)

Sample No.

SPT N values

Liquid Limit (LL)

Plastic Limit (PL)

Water content (%)

Liquidity Index (LI)

shear strength, Cu (kPa)

Compression Index (Cc) Permeability, k (m/s)

BH-1 -2.5mRL~4.0mRL 1.5 m UD1 N = 1 51% 35% 66% 1.9 21 kPa 0.65 5.5E-11~1.4E-9 m/s

BH-2 -1.5mRL~4.5mRL 3.0 m UD2 N = 1 139% 75% 127% 0.8 16 kPa 1.28 2.2E-11~8.0E-10 m/s

UD2 N = 2 - - 172% - 18 kPa 1.62 3.4E-11~5.5E-10 m/s

UD3 N = 2 197% 113% 162% 0.6 25 kPa - -

UD4 N = 2 - - 175% - 19 kPa 1.48 2.9E-11~6.8E-10 m/s

BH-3 -1.5mL~4.5mRL 3.0 m

3D FEM mesh is based on the idealized 4 boreholes to create and interpolate the subsurface soil profiles in 3D

FEM meshDriveway

Carpark slab on il

Open-cut trench

Buildingpiles

Top fill

Soft peaty clay

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Existing canal

Underlying hard soil (N>30)

Firm soil (N=10~30)


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The trench was retained by soldier piles (UC300x300x84.5kg/m) at 5m c/c spacing and steel plate to retain the soil in-between. Excavate to 1.2~1.5m bgl and install the top strut (300x300x84.5kg/m) before excavating to formation level of about 4.1m~4.6m bgl

33

It should be noted that the excavation and laying of pipelines are conducted in segments. However, in the present 3D FEM analysis, a whole stretch of trench excavation was conducted in one shot. Thus, the analysis results will maximize its impact on the adjacent ground and is thus on the conservative side.

Similar to the observation of the water drawdown adjacent to the launching shaft as revealed by the water standpipe data, the trench excavation work is expected to cause certain water drawdown which will cause increase of effective stress on the very soft peaty clay layers and cause additional ground settlements.

In the 3D FEM analysis, the General Water Table is set to 3m below the Tradehub21 ground surface, while the water elevation was set to the base of excavated trench, and Ground Water Flow analysis was selected to derive the steady-state ground water condition (worst case of GW drawdown possible)water condition (worst case of GW drawdown possible).

However, it should be noted that actual trench excavation work with duration of about 3 months will not cause the ground water condition to reach steady state condition. As such, the current analysis is thus on the conservative side.

34


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As expected, the trench excavation coupled with ground water drawdown cause quite some ground settlement both adjacent to the trench and along the driveway, with a Max value of about 80mm.

35

Along the drive way: Initial water condition set at 3m below ground surface

Initial water table at 3m bgl

36


19

Along the drive way: water drawdown of about 1m after trench excavation

Initial water table at 3m bgl

water table at about 4m~4.5m

bgl, with a water level

drawdown of about 1~1.5m

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Cut a cross section A-A cut along the centerline of the driveway


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The induced ground settlement along the driveway at the Tradehub21 side of about 30~70mm:

70mm

30mm

45mm

Without water drawdown, the induced ground settlement will be very small due to trench excavation (Max = 15mm)

70mm

30mm

45mm


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Another 3D FEM mesh for 1 segment of excavation with a excavation length of 8m only as shown (Cross section along A-A).

70mm

30mm

45mm

The induced ground settlement along the driveway will be mainly concentrated at the opposite side of the segmental excavation with comparable but slightly smaller magnitude.

6565mm

15mm

25mm


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Concluding remarks for effect of trench excavation and the accompanied water drawdown on the driveway settlement:

Using the Ground Flow analysis in 3D FEM the calculatedUsing the Ground Flow analysis in 3D FEM, the calculated water drawdown at the driveway is estimated to be about 1m~1.5m with accompanied increase of vertical effective stress. The caused ground settlement along the driveway at the Tradehub21 side of is calculated to be about 30~70mm.

Seepage Induced Slope Failures

• Cut Slopes• Long term FS governs use Drained Analysis• Long-term FS governs, use Drained Analysis• Seepage condition is critical• Need FEM Seepage analysis coupled with

Stress analysis (PLAXIS)• Or combined with Stability analysis eg

SEEP/W with SLOPE/W or

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SEEP/W with SLOPE/W or


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Performance of Repaired Slope using a GEONET p gDrain to lower Ground-Water Table under Very

Heavy Rainfall Condition

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Tan S.A., Chew S.H.,

G P Karunaratne, Wong S.F.,

The National University of Singapore

Order of Presentation

• Introduction

• Possible causes of failure

• Site investigation of failed slope

• Failure analysis

46

Failure analysis


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Order of Presentation (cont’d)

• Design of permanent stable slopes

• Parametric study of influence of GEONET installation depth

• Construction of repaired slope

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Construction of repaired slope

• Conclusions

Introduction

• 70m long slope with gradient of 1(V):2(H) was cut in medium stiff ( ) ( )residual soil

• After period of intense rainfall, slip failure

… slip about 1 to 1.5m deep over slope of 30m length

48

of 30m length

• Slope repaired using dry cut fill soil obtained from same site failed again without use of subsurface drains


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Possible causes of failure

• Large overburden stress relief due to

slope cut

• Rise in ground water table level

• Inadequate sub-soil drainage

– water absorption in residual soil

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water absorption in residual soil

– increased seepage force from infiltration

– rise of water table within slope mass

Site investigation

108

110

P3Slope Failure Profile and GWT Data

102

104

106

Ele

vatio

n (m

RL)

Observed Slip Plane

Probable Ground Water TableP1

P2

104.5103.8

1V:2H104.6

106.3

50Ground water has risen close to failed ground surface

0 2 4 6 8 10 12Distance (m)

98

100


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Failure analysis

FSc'

H

( ( h)/( H)) 'w

sin cos

tan

tan

1

GWT

Parallel Seepage

Hh

51

= slope angle (degrees)

H = depth to slip surface (m)

h = height of GWT from slip surface (m)

Failure analysis (cont’d)

Table 1: Results of infinite slope stability analysisCase c’

kPa’

deg

kN/m3

degHm

hm

FS State of Soil

1 10 22 18 26.5 1.5 0 1.74 Dry2 5 21 18 26.5 1.5 0 1.23 Softened3 3 20 18 26.5 1.5 0 1.01 Soaked

4 5 21 18 26 5 1 5 01 1 20 S

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4 5 21 18 26.5 1.5 0.1 1.20 Seepage5 5 21 18 26.5 1.5 0.2 1.18 Seepage6 5 21 18 26.5 1.5 0.4 1.12 Seepage7 5 21 18 26.5 1.5 0.6 1.06 Seepage8 5 21 18 26.5 1.5 0.8 1.00 Seepage9 5 21 18 26.5 1.5 0.9 0.98 Seepage


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Design for permanent stable slopep

• Seepage analysis by SEEP/W

• Flow rate of about 1.89 x10-3 m3/s per m expected to be conducted safely out re-compacted soil on

53

repaired slope

Design for permanent stable slope (cont’d)

NO DRAIN 0.5m Sand Track 150 m m /h Rain fa ll

3110

112

114

Pond leve l a t 104.6 m RL

GWT

Co ncrete L iner

Recom p acte d R es idual Soil 1.8945e-008

3.3004e-008 3.3003e-008

1.2023e-011

3.2 60 1e -004

96

98

100

102

104

106

108

110

54Steady seepage without internal drain

Distance (m)0 5 10 15 20 25 30 35 40

90

92

94


28


3

1.5

9

2.5

1.1

1.3

1.9

0 .9 23

D e sc r i p t io n: P o n d W a t e rU n it W e ig h t : 9 . 8 0 7

D e s c r ip t io n : R ec o m p a c te d R e s id u a l S o ilU n it W eig ht: 1 8C o h e s io n : 3Ph i: 2 0

D e s c r ip t io n : In s itu R e s id u a l S o ilU n it W e ig h t : 1 8

0 .5 m S a n d - tra c k

1 5 0 m m /h R a in fa l l

N O D R A IN

G W T

100

102

104

106

108

110

112

114

55Slope analysis without internal drain

U n it W e ig h t : 1 8C o h es i on : 1 0Ph i: 2 7

D istanc e (m)0 5 10 15 20 25 30 35 40

90

92

94

96

98


GEONET 4m Depth 15 0 mm /h R a in fa ll112

114

Pon d le vel a t 104 .6 m R L

GEON ET

C oncre te L ine r

R ecom pacted R es idua l So il 0 .5 m Sand TrackGWT

2.1829e-012

5.2122e-008 5.2122e-008

1.4009e-011

3.0994e-004

96

98

100

102

104

106

108

110

56Slope analysis with 4m deep GEONET

Distance (m)0 5 10 15 20 25 30 35 40

90

92

94

96


29


1.8

2

2.4

G E O N ET 4 m D e p th

1.8

2

.2

1.269

D e s c r ip t io n : P o n d W a t e rU n i t W e igh t : 9 . 8 0 7

D e s c r ip t io n : R e c o m p a c te d R e s id u a l S o ilU n it W e ig h t : 1 8C o he s io n : 3P h i: 2 0

D e s c r ip t io n : In s itu R e s id u a l S o il

G W T

G E O N E T

0 .5 m S a nd Tr ac k

1 5 0 m m /h R ain fa l l

100

102

104

106

108

110

112

114


U n it W e ig h t : 1 8C o h e s io n : 1 0Ph i: 2 7

D is tanc e (m )0 5 10 15 20 25 30 35 40

90

92

94

96

98


GEONET 8m DepthGW T

150 m m /h R ain fa ll112

114

Pon d le vel a t 104.6 m R L

GEON ET

C oncre te L ine r

R ecom pacted R es idua l So ilGW T

0.5m Sand track

7.0160e-008 7.0158e-008

7.6699e-012

3.1082e-004

96

98

100

102

104

106

108

110


Distance (m)0 5 10 15 20 25 30 35 40

90

92

94

96


30


1.7

1.8

2

2.

G E O NE T 8 m D e p th

1.9

2

2.2

.2

1.617

D e s c r i p t io n : P o n d W a t e rU n it W e ig h t : 9 . 8 07

D e s c r ip t io n : R ec o m p a c te d R e s id u a l S o ilU n it W eig ht : 1 8C o h e s io n : 5Ph i: 2 1De s c ri p t io n : In s itu R e s id u a l S o il

U n it W e ig h t : 1 8

G W T

0 .5 m S a n d T r a c k

1 50 m m /h R a in f a ll

G E O N E T

98

100

102

104

106

108

110

112

114


U n it W e ig h t : 1 8C o h e s io n : 1 0Ph i: 2 7

D is tanc e (m )0 5 10 15 20 25 30 35 40

90

92

94

96

98

Parametric Study of influence of GEONET installation depth

Table 2: Influence of GEONET depth on GWT and FS of repaired slope

GEONET Depth 0 1 2 4 8 12 15GEONET Depth(m)

0 1 2 4 8 12 15

GWT at SlopeCrest (m RL)

108.1 108.0 107.9 107.6 106.8 104.7 104.7

GWT at Mid-Slope (m RL)

107.1 106.9 106.4 105.7 104.7 104.7 104.7

Seepage intoSlope (m3/s /m)

1.89x 10-8

1.72x 10-9

9.80x 10-12

2.18x 10-12

< 1.0x 10-12

< 1.0x 10-12

< 1.0x 10-12

S il S i Sl F ll F ll F ll F ll S f C C

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Soil State in Slope FullySoak

FullySoak

FullySoak

FullySoak

Soften Comp-acted

Comp-acted

Drained cohesionc’ (kPa)

3 3 3 3 5 10 10

Drained frictionangle, ’ deg

20 20 20 20 21 22 22

Drained FS 0.923 0.968 1.137 1.269 1.617 1.780 1.808


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Parametric Study of influence of GEONET installation depth (cont’d)

Modified Manning’s Eqn. for discharge of equivalent pipe drain in place of GEONETequivalent pipe drain in-place of GEONET drain

Q = 1.137A RH0.66 S0.5 (m3/s)

A=flow cross-section area (sq-m)

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A flow cross section area (sq m)

RH=hydraulic radius (m)

=R/2 for full flow

S=slope (m/m)

Construction of repair slope• Repair job to be done panel by panel

• Fully soaked residual soil that has slid y

was removed completely

• Exposed soil was re-compacted to

produce firm stable base for GEONET

or pipe drain to be installed after

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or pipe drain to be installed after

compaction

• Residual soil fill re-compacted to

achieve slope height


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Conclusions

• Installation of geosynthetic internal

drain proved be to cost effective

• GEONET or equivalent longitudinal

geopipe provide effective interceptor

63

g p p p p

drain to high GWT and conduct water

safely out of slope below re-compacted

soil zone

Excavation for Effluent Pond

64


33

Tension crack forming on slope

65

Close-up on tension crack zone

66


34

Collapse of kaolinitic soil formation

67

Collapse of residual soil and high GWT exit

68


35

Initial repair without drains and failure again

69

Install geotextile-wrap 15m long, 75-mm diameter pipe drains at 1.5 m intervals

70


36

GW discharge from internal geopipe subsurface pipe drains

71

Use of geotextile seperator/filter layer for subgrade soil protection

72


37

Use of 150-mm geopipes as interceptor subsurface drains

73

Filter details at outlet discharge point

74


38

Subsurface drain failure without geotextile filters

75

Clear discharge water from geopipe drain

76


39

Clear discharge water from weepholes

77

Long-term Settlements on Soft Clays

78


40

Long-term Settlements on Soft Clays

79

Fundamental Knowledge Quiz

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introduction (16 aug 2011)

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