Geotechnical Investigation 1225510757282453 9

7/30/2019 Geotechnical Investigation 1225510757282453 9

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Importance of Proper Geotechnical

Investigation in Engineering Project:

Some case studyJ.N.Jha*, K.S.Gill* & A.K.Chaudhary**

*Department of Civil Engineering, Guru NanakDev Engineering College, Ludhiana

** Department of Civil Engineering, NIT,Jamshedpur


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Introduction

Construction activities increasedmanifold (development of economicactivities)

Different types of complex structuresare coming up (to meet the growingdemand)

Attempt being made to make soilsuitable to project and not theproject to soil.


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Geotechnical Engineer-

very important role to play in thischallenging task.

Geotechnical Engineering Practice-Atpar with the best in the world.

Range of Geotechnical practice varywidely in India.


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Field investigation-Most primitiveequipment are in use

Laboratory testing-Practice varywidely with little standardization andaccreditation.


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Quality of Investigation

India World Standard

Generally Poor quality of theEquipment

Highly sophisticated andmechanized equipment

Calyx Drilling Technique Continuous core sampling (insoils as well)

SPT Equipment unchangedover the years (unreliable)

SPT Equipment with BlowEnergy Directly on top of thesampler

Conventional Static ConePenetration Equipment

Static Cone Test with ElectricCone

Very recently few companies

have electric cone


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Result (Substandard practice)

Substantial difference between actualsoil profiles and available soil profiles(at the time of design as part of

tender specifications)

Variation can be minimized ifstandard practices are followedduring the soil investigation


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Unfortunately this is not the case quiteoften

Who is responsible?

Responsibility squarely rests onGeotechnical community of the country

and is a major failure on our part.


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General and Standard Practice

Tender for a project (informationsupplied)

Subsoil profile and soil characteristicsis of general information only

Owner is not responsible for thecorrectness of this information


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Contractor if desired should satisfythe correctness of information beforesubmitting his offer

To safeguard the owner to avoid anydispute


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Contractor (point of view)

Time interval between issue oftender document and submission oftechnical bid is very short

Soil investigation is expensive

Impossible to carry out soilinvestigation

Bidder accepts the stipulation givenin tender


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Case study

Road over Bridge (ROB)

Bridge :

5 span of 10.7 m with certainembankment on either side

As per tender SPT value 12 to 16 fortop two layer extending up to 7 m.


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Recommend allowable bearingpressure=150 kN/m2 at depth 2 m belowGL for Pier foundation.

Accordingly Piers were constructed onshallow foundation

4 Pier constructed and 5th was underconstruction approach earth embankmentsettled by 2 m and corresponding heavingup of soil 1.5 m


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EMBANKMENT

9m2m SETT LEM ENT BRIDGE

PIER

RAIL LINE

AB UT M ENT

SOIL HEAVE

SOFT CLAY

- 6m

SAND

10.7 m

ROTAT IONAL FAILURE

G.L.

.


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Confirming soil investigation wascarried out

Soil Profile:

Top 1-1.5 m : Sandy Clay

1.5-8 m : Soft marine clay


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10

8

6

4

2

0

12 SAND

14

16

12

29

38

SILT

CLAY

BH-I SPT

(N)

(a) As Per tender

10

8

6

4

2

0

0.1

BH-I SPT

(N)

(b) As Per confirmatory

bore Hole

0.0

0.0

0.0

15

110.3

C=40KN/m2

C=24KN/m2

C=31KN/m2

C=40KN/m2


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Rehabilitation Measure

Piles installed around shallowfoundation and integrated withfoundation

Delay in completion of project,additional cost & dispute


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Petro Chemical Complex

As per Tender

Recommended depth of Pile = 25 m

Test pile failed to take design load

Confirmatory (Bore hole) test 12 such confirmatory bone hole

consistently showed that SPT valuereported in original soil report are higher

Pile depth after confirmatory test =20m Confirmatory soil investigation saved a

major disaster.


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3 2

2 4

1 6

8

0

B H - 3 B H - F

T O P F I L L

S I L T

R O C K

C L A Y

S A N DB H - F ( c o n f ir m a t o r y )

B H - 3 ( t e n d e r)

4

1 2

2 0

2 8

8 16 24 32

S P T (N ) V a lu e s

Dep

th( m )


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Choice of Appropriate foundation

and execution

Optimum foundation design shouldensure

Technical adequacy

Cost effectiveness

Ease of execution


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Reasons

Insufficient and inaccurateinformation at the time of designvariation in strata

Changes in project requirementduring execution.


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Achieving this is easily said thandone-needs engineering judgement

Engineering Judgement comesfrom experience.

Experience comes from badengineering judgement


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Case study

Fertilizer plant in Gangetic belt-possibility of optimum design

Phase-I

Soil strata (Site)

N20 For a depth upto 10-20m


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Type of Soil

Silty sand with high water table

Threat of liquefaction duringearthquake

Foundation Design


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Foundation Design

(Recommended)

Provide RCC cast in situ piles(diameter 400 mm) with pilecapacity

Axial vertical load 50 Tonnes

Uplift - 5 Tonnes

Horizontal capacity=2.5 Tonnes


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To overcome the problem ofliquefaction during earthquake

Provide sand compaction pile 2 to 3rows around RCC piles


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Total requirement

As per design No. of RCC piles 16000 No. of sand compaction piles 32,000

Time required for installation of RCC pilesand sand compaction piles=6 months more than what was originallyplanned

This prompted for the review of foundationdesign


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Sand Compaction Pile

Original design

Spacing of compaction pile 3D and5D with triangular pattern

Spacing 3D (desired improvementin N-values)

Spacing-5D (desired improvement inN values not adequates)


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Additional Recommendation

Spacing of sand compaction pile-4D

Result-Adequate to obtain requireddensification (N-values)

No. of piles (now required)=16000instead of 32000


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Pile capacity (Revised)

Vertical downward-65 tonnes insteadof 50 tonnes original

Uplift capacity=25 tonnes instead of5 tonnes original

Lateral capacity-3.5 tonnes asagainst original 2.5 tonnes


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Requirement of no. of RCC piles(based on revision)=9400 piles

Reduction in no. of piles =40%

Observation:

Performance of the foundation-fullyadequate and satisfactory.


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Phase-II (To double the capacity of

the plant)

Ground improvement Vibro stonecolumn in place of RCC piles and sandcompaction piles

Vibro stone column diameter- 960 mm Load test carried on single column and

group of columns

Footing test conducted for confirmationduring execution.


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Trial Test

Test plot 10 m x 10 m

Vibro stone column

11 m (length), c/c spacing 15 m, 2.15m & 1.8 m

(Triangular pattern


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Standard Penetration Test

Area Depth Layer N Aftertreatment %increase inN Value

Prill Tower 0-2.5

25-11

Silty Clay

Silty Clay

13

20-36

30

17-120

BenefieldArea

0-2.3

2.3-11

Silty Clay

Silty Clay

20

16-34

81

36-123

CompressorHouse

0-3.5

3.5-11

Silty Clay

Silty Clay

11

24-46

01

60-400


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Static Cone Penetration Test

(SCPT)

Depth Pre-treatment

(ConeResistance)

Post-treatment

(Cone

Resistance)2-8m 50-80 kg/cm2 130-300

kg/cm2


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Dynamic Cone Penetration Test

(DCPT)

Depth Pre-treatment(No. of blowsper ft.)

Past-treatment(No. of blows

per ft.)2-10 m 10-40 22-95


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DESCRPTION

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

10 20 30 40 50 60

SOILPROF.

DEPTH

(m)

SPT (N) VALUE

BROWN

CLAYSILT

2.50

5.25

BROWNSILT FINE

SAND

GREY

FINE SAND

GREY SILTY

MEDIUM

TO FINESAND

9.55

X

PRE COMPACTION

POST COMPACTION

POST COMPACTION

PRE COMPACTION


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Vibro Stone Column of 960mm withspacing 2D, 2.25D and 2.5D whereadopted depending on loading

intensity

Substantial saving in time and cost

Subsequently observation during the

operation of Phase-II confirmed asatisfactory behaviour of foundation


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Concluding remarks

Commitment to excellence fromGeotechnical Engineers

Positive attitude to continuouslylearn and to accept change for better

Partnership and team work among allconcerned i.e owner, consultant and

contractor


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Thank you..

Documents

Geotechnical Investigation 1225510757282453 9