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DFI-IGS-IITB-APS Conference 2013, Mumbai

DEEP FOUNDATION TECHNOLOGIES FOR INFRASTRUCTURE DEVELOPMENT IN INDIA

EARTH RETAINING SCHEMES FOR BASEMENTS AND DEEP EXCAVATIONS

Keynote Lecture delivered

on 16th November, 2013by

D. V. KARANDIKARB.Tech., M.Tech., FIE, FIGS, FIIBE

D. V. Karandikar & Associates Consulting Engineers

INTRODUCTION

• With rapid growth in Infrastructure, deep excavations are requiredfor : Construction of underground metro stations,

Basements for parking facilities and

Accommodating various services

The required open space is seldom available for excavations with flatterslopes or benches and the excavations are more often vertical.

• Designing and installing a suitable retaining scheme becomes aprimary responsibility of the project consultants and contractors toensure:

Safety of persons working in the excavated pits,

Avoid / minimize subsidence in the vicinity plots

Ensure traffickability of adjoining roads

Protection of trees near the plot

• What is important is to remember that deep excavations needprotection in all types of soils, sands, clays and even in rock.

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TOPICS PRESENTED

1) Brief outline of appropriate Geotechnical Investigation for deep

excavations

2) Gathering information on foundations of buildings nearby

3) Alternative Retaining Schemes

4) Estimating lateral pressures on retaining walls and design methods to

ensure wall stability. Use of typical software for design and parametric

study

5) Case Studies of Retaining Schemes

6) General Guidelines for Design of Excavation Protection Schemes

7) Conclusions

TOPIC 1-PLANNING OF GEOTECHNICAL INVESTIGATION

• Planning of appropriate geotechnical investigation is veryessential to obtain relevant geotechnical parameters fordesign of retaining scheme. Various aspects to beconsidered are:

• Regional Geology Igneous and Metamorphic Rocks covered over by Residual Soils

Sedimentary Rocks and Deep Alluvial Deposits

Laterites

Marine Clays

• Topography and Locality Hilly, Coastal, River plains, Urban

• Ground Water Table and its Variation Presence of Nalla, Tides, Rains

• Seismic Zone Stability during earthquakes, Possibility of liquefaction

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Planning of Geotechnical Investigation Contd.

• Chemical aggressiveness of soil and ground water

• Proposed Plan Area of Excavation: one borehole on periphery

of every side is desirable

• Maximum Depth: Boreholes should go up to twice the depth of

excavation or 5 to 10m in rock if met with earlier.

• Tests in Boreholes: SPT, UDS, Pressure meter Tests in Weathered

Rock, Packer Permeability Tests in rock, Vane Shear Tests in soft soils

• Retaining Scheme to be Temporary or Permanent

7 Islands of Mumbai and Mumbai Geology

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Soil Profile at Walkeshwar, Malabar Hill

-

Soil Profile near Sea Beach-Mahim

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Soil Profiles at Juhu Vile Parle and Andheri Development

Topic 2-FOUNDATIONS OF NEARBY STRUCTURES

a) Foundation plan, footing or pile layout of nearby buildings

b) Distance of nearby buildings from plot boundary

c) Structural condition and age of buildings

d) Drainage and Service lines routing details and manhole locations

e) Presence of Trees near Excavation Boundary

f) Presence of Nalla nearby

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Presence of Nalla Nearby

Presence of Service Lines and Cables

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Subsidence in Adjoining Plot due to Excavation

Assessment of Ground Subsidence near Excavations

( After Peck, Ref. Fig. 14-8, Foundation Analysis and Design, J.E.Bowles, ELBS 4th ed.)

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Checking Foundations of Adjoining Building

Typical Pile Foundation Exposed

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Topic 3 - ALTERNATIVE RETAINING SCHEMES

• Diaphragm Wall

• Contiguous Bored Piles

• Secant Piles

• Metre Panels

• Intermittent Shore Piles

• Micro piles

• Sheet Piles

• Peripheral Grouting

• Closely Spaced Prestressed Concrete Piles

Retaining Scheme for a Deep Excavation with a Bench on Weathered Rock

G.L.

Silty Clay

Weathered rock

Anchor

Dowels

Shotcrete/ RCC Wall

Raft

Waling Beam

Shore Piles or Diaphragm Wall

Layer 1

Layer 2

Rock

Layer 3

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Typical Secant Piles Shoring Scheme

Intermittent Bored Piles, RCC Tie Beams, Prestressed Anchors, Brick Filler Panels

with Drainage Pipes behind

Schemes have been successfully used in Mumbai to retain sides of deep excavations upto 14m in Cohesive strata.

Prestressed Anchor

Bored cast-in-situ RCC pileRCC Capping Beam pile

Brick Wall

Perforated Drainage Pipe

RCC Tie Beam

Sandy Draining Fill

Access Road

Fill

Ground water Table

Soft to Firm Silty Clay

Stiff to Very Stiff SiltyClay

Anchor taken into Rock

Weathered Rock

Water Proofing Layer

RaftBottom of Exc.

GL

Bored Piles

SECTION PLAN

Anchor

Anchor

Water proofing layer

Brick Wall

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Retaining Scheme with Micro PilesIntermittent Micro Piles with Internal Props and M.S.Plate Filler

Panels used to retain Sandy fill upto 5m. Filler panels were progressively installed ahead of excavation.

Micro Pile with M.S. Liner

Capping Beam

Internal Prop Truss

Ground LevelM.S.Filler Plates

M.S.Tie Beam

Micro Piles

Raft

Bottom of Exc.

Prop

M.S.Plate

Weathered Rock

Ground water Table

Loose to Medium Sand

SECTION PLAN

Fill

Alternative Retaining Scheme with Prestressed Piles

(Fig. 14-2, Foundation Analysis and Design, J.E.Bowles, ELBS 4th ed.)

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Diaphragm Wall

• Generally the diaphragm wall is the most desirable option

for providing positive retention.

• The wall however needs to be supported by prestressed

anchors.

• The wall generally goes below the raft level to permit

keying and for reduction of seepage.

• In restricted or congested locality however, it becomes

difficult to install the diaphragm wall since working space

is not available.

Alternative Retaining Schemes Contd…

• Secant Piles:

The installation of primary piles and then cutting the secondarypiles into them requires close tolerance and careful pile installation.

Anchors are required for deep excavation stability and they have tobe carefully positioned in primary piles.

• Contiguous Piles:

These require careful installation and filling of gaps between piles.

At anchor locations, pile spacings have to be suitably adjusted.

• Metre Panels

Special chisel tool is used to form adjoining bores of 1m wide x0.6m thick RCC panels and they are concreted in-situ to form aretaining wall. Depth of panel is limited.

• Micro piles:

Micro piles require M.S.liners and the retaining scheme requiresmulti layer waling beams and anchors.

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Intermittent Shore Piles with Anchors

• Bored piles with tie beams and anchors is one of the safeest optionsin restricted area. An innovative modification consist of installingbored cast in situ piles at say 2 pile dia. spacing along the face ofexcavation and providing tie beams at various levels as per design.Anchors, either prestressed (active) type or passive (tor-rod) type,can be installed through the tie beams.

• The arching of the soil in-between the piles is relied upon.Continuous brick wall with mullions can be constructed on the tiebeams to give a plane inside face, which could be used forsupporting water proofing layer. The gap between the brick wall andexcavated face is filled up with graded gravel. A perforated HDPEdrainage pipe is placed to collect the seepage water and relieve thewater pressure on the brick wall.

Alternative Retaining Schemes Contd…• Sheet Piles:

Not always possible since difficult to drive due to presence ofboulders,

Space restrictions for operation of the crane and hammer,

Maximum length restricted to12 metres,

Joints are cumbersome,

Withdrawal of sheet piles very difficult,

Sheet piles are not readily available

• Peripheral Grouting:

In silty clayey soils, experience shows that sufficient spread ofgrout can not be achieved. Time required is large.

Grout holes can not be punctured in adjoining road and If oldmasonry walls exist on the boundary, then grout pressure is difficultto develop since wall foundation might give way.

Prestressed Piles: Piles are prestressed to take lateral loads andanchors are avoided.

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Topic 4 -Various methods for Retaining Wall Design ( Ref. CIRIA Report 104)

1) Factor on Embedment, (Fd), Method: (Fixed earth Method): For acantilever wall decide the depth of embedment,’do’, based on soiland water pressures,(Ref Fig.5.25), and increase the same by 20%.

For propped walls, compute ‘do’, as above but no further increaseis necessary.

2) Factor of Safety on Shear Strength (Fs) Method: Same as abovebut active pressure and passive resistance are factored to increaseactive pressure and decrease passive resistance and ‘do’ isdecided and increased for cantilever wall but not for propped wall.

3) Factor of Safety on Moments (Fp) Method, (as in BS 8002):Depthof embedment , ‘do’ is decided for ensuring chosen Fp based onmoments about the toe and ‘ do ’ is increased by 20% forcantilevered walls but not for propped walls.

Various methods for Retaining Wall Design Contd.. ( Ref. CIRIA Report 104)

• Factor of Safety on Moments (Fr), as per assumed net availablepassive pressures given by Burland and Potts (Ref. Fig. 5.27)

• Design Approach A with moderately conservative soil parameters,with c ’ , ’ or Cu or as per Approach B with worst credibleparameters or drained soil parameters with c’=0 and ’

• Approximate values of drained ’ based on Plasticity Index aregiven as follows: (Ref. Padfield and Mair)

PI ’

15 30

20 28

25 27

30 25

40 22

50 20

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Factors of safety for Stability Analysis( Ref.CIRIA 104 and BS 8002)

Method Design Approach

A(Moderately Conservative

Design Approach

A(Moderately Conservative

Design Approach B(worst

parameters)

Design Approach B(worst

parameters)

C’, ’ or Cu C’, ’ or Cu C’=0, ’ C’=0, ’

Temporary Permanent Temporary Permanent

1)Fd: Effe. Stress 1.2 1.5 1.2

Fd: Total Stress 2.0

2)Fs: Effe. Stress 1.2 1.5 1.0 1.2

Fs: Total Stress 1.5

3)Fp: Effe.Stress 1.2 to 1.5 1.5 to 2.0 1.0 1.2 to 1.5

Fp: Total Stress 2.0

4) Fr: Effe.Stress 1.5 2.0 1.0 1.5

Fr: Total Stress 2.0

Soil and Water Pressures on the Retaining Wall

Effective Soil Pressure

‘d0’

Pa

La

LpR

Pp

Cantilevered Wall

Find ‘d0’ based on Following Eq. & increase by 20%Pp*Lp + Pwa*Lwp = Pa*La +Pwa* Lwa

p'a = Ka ( γz-u ) - 2 c' *√Ka

p'p = Kp ( γz-u ) + 2 c' * √Kp

WaterPressure

‘d0’

Pwa

Lwa

LwpR

Pwp

PRESSURE DISTRIBUTION ALONG THE RETAINING WALL

Propped Wall

Effective Soil Pressure

‘d0’

Pa

La Lp

Pp

PropW

Find ‘d0’ based on Following Eq. but no increasePp*Lp + Pwa*Lwp = Pa*La +Pwa* Lwa

FACTOR ON EMBEDMENT METHODRef: Fig. 5.25, Foundation Design & Construction, ELBS, 6th ed.,M.J.Tomlinson

WaterPressure

‘d0’Pwa

Lwa Lwp

Pwp

W

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Soil and water Pressures-Burland-Potts Method

Effective Soil Pressure

‘d0’

Pa

La

LpR

Pp

Cantilevered Wall

Find ‘d0’ based on Following Eq. & increase by 20%Pp*Lp + Pwa*Lwp = Pa*La +Pwa* Lwa

p'a = Ka ( γz-u ) - 2 c' *√Ka

p'p = Kp ( γz-u ) + 2 c' * √Kp

Water Pressure

‘d0’

Pwa

Lwa

LwpR

Pwp

Propped Wall

Net Water Pressure(No Seepage)

‘d0’

Pw1Lw1

Lw2

Pw2

T

Net Activating Soil Pressure Net Resisting Pressure

‘d0’

Pa1La2

Pn

Ln

Pa2

La1

T

FACTOR OF SAFETY ON MOMENTS METHOD

Fr = Pn*Ln / ( Pa1*La1 + Pa2*La2 + Pw1*Lw1 + Pw2*Lw2)‘do’ obtained from above is not further increased

Ref: Fig. 5.27, Foundation Design & Construction, ELBS, 6th ed.,M.J.Tomlinson

Stagewise Pressures on Anchored Retaining Walls

Stress due to Anchor Stressing

1st Stage Anchor Cantilever

Stage 2

Cantilever

Wall Movement

Beam

Anchor

Stage 3

Cantilever

Wall Movement

Stage 1

Active Soil Pressure

PRESSURE DISTRIBUTION ON ANCHORED RETAINING WALL

Ref: Fig. 5.28, Foundation Design & Construction, ELBS, 6th ed.,M.J.Tomlinson

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Stagewise Pressures on Anchored Retaining Walls-Contd.

Anchor

Stress due to Anchor Stressing

Anchor

Cantilever

Stage 4

Beam

Beam

BeamAnchor

Stage 6

Beam

Vertical Force due to Anchors

Base Slab/raft

Floor Slab

Floor Slab

Cantilever

Beam

Anchor

Stage 5

Beam

Vertical Force due to Anchors

2nd Stage Anchor

Simplified Lateral Pressures on Braced Excavation

Lateral Earth Pressure Dia. on Braced Excavation Walls as per PeckRef. Fig. 14-4 Foundation Analysis and Design, J.E.Bowles, ELBS 4th ed.Ref. Fig. 23, FHWA-IF-99-015, Geotech. Engg. Circular No. 4

Lateral Earth Pressure Dia. on Braced Excavation Walls as per TschebotarioffRef. Fig. 14-5 Foundation Analysis and Design, J.E.Bowles, ELBS 4th ed.

H

0.25H

0.75H

0.65 H Ka

0.25H

0.5H

0.25H

0.1H

0.7H

0.2H

SandSoft to Medium Clay

Stiff Fissured Clay

H Ka 0.2H to 0.4H

0.6H

0.4H

0.2H to 0.4H

0.3H 0.2H

0.75H

0.25H

0.375H 0.5H

Sand

Temporary Support in Stiff clay

Permanent Support in Medium clay

Ka = 1- (4 Su/ H)Ka = tan^2 ( 45 - /2 )

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Pressures due to Prestressed Anchors

• The pressures exerted on the retaining wall due to installation of

prestressed anchors depend on the deflection of retaining wall, its

stiffness and consequent soil-wall interaction.

• Various reported studies on instrumented excavations however

confirm that use of simplified pressure diagrams are acceptable for

practical purposes. (Refer FHWA -IF-99-015 , Section 4.4.4 and

FHWA-RD-98-067,1998)

• It is necessary to keep in mind that for basement walls, earth

pressure ‘at rest’ needs to be considered in the design, which would

be much higher than active earth pressure.

Analysis and Design of Retaining Walls using softwares

• For analysis and design of embedded retaining walls with or withoutprops, a typical software such as CADS PWS can be used.

• It is based on widely accepted CIRIA 104, net available passiveresistance method proposed by Burland and Potts.

• Construction Stage wise Analysis is available along with envelope ofmaximum moments, shears and prop forces.

• If active pressure tends to become negative(below zero) in cohesivesoils above water table, then software works out minimum positivepressure based on minimum equivalent fluid density and alsochecks possible positive pressure with tension crack filled withwater.

• Structural design is also available for various retaining wall systemssuch as diaphragm wall, bored pile walls, sheet piles etc. Typicaloutput obtained for one of the case studies is given below.

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Typical Section of Retaining Wall with Props / Anchors Locations (Walkeshwar , Mumbai site)

Analysis of Piled wall using software, input geotechnical properties

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Analysis of Piled wall using software-define stages in excavation and prop levels

Typical Bending Moments and Shear Force Diagrams

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Typical output of moments, shears and prop forces

Walkeshwar Sea Front Site 1– Intermittant Shore Piles for Retaining Deep Excavation

• The location: Malabar Hill slope at Walkeshwar Seafront in Mumbai.

• Plot 70m * 40m as shown in figure.

• The structure: 7 storeye building with 4 storeyes, including parkingfloors with car lifts, going 14m below the existing road level.

• Excavation 5.5 m below max. tide level and 1 m below sea bed level.

• 10 boreholes, (2 in the sea), drilled from 4 different elevations as perexisting benches on the plot.

• UDS, SPT in soil.

• In rock, rotary core drilling with double tube core barrels fitted withdiamond drill bits.

• Cyclic packers permeability tests in rock strata of varying recoveries,the data considered useful during anchor grouting.

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Case Study 1-Malabar Hill-Walkeshwar Sea Front Site Layout and Borehole Data

Topic 5-Case Study 1-Borehole data and stratification

Subsurface Stratification

• Layer I : G.L. to 7.5m depth: Stiff yellowish brown silty clay withgravels. Classification ‘MH’. SPT ‘N’ typically 10.

• Layer II :7.5m to12m :Very Stiff yellowish brown silty clay with finesand and kankers. ‘MH’, SPT ‘N’ 15.

• Layer III :12m to16m:Dense yellowish brown medium to fine clayeysand with gravels. ‘SC’. SPT ‘N’ 37 to 62

• Layer IV:16m to 23m:Yellowish brown medium to fine grainedWeathered Basalt rock, C.R. <35%, RQD Nil

• Layer V: Moderately hard, dark and bluish grey medium to finegrained Basalt rock, C.R. >50 %, RQD upto 25%.

• The soaked UCS about 175 Kg/sq cm.

• The ground water table around 9.5 m depth, i.e. about 3.5 m abovesea bed level.

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Geotechnical parameters and shore pile details

• Cohesion, C = 10 T/sq m

• Angle of internal resistance = 20 degrees.

• Bulk density = 1.8 T/cu m

• Retaining Scheme: 600mm dia bored cast-in-situ piles wereinstalled at 1.2m c/c . They were tied with RCC waling beams andthree level anchors. Brick wall facing was constructed to receivewater proofing and back side was filled with draining fill. Details areshown in the drawings.

• During construction raft stability had to be ensured against uplift dueto high sea level. 10 T anchors were provided by installing 25 mmdia. tor-below raft.

Anchored Shore Piles

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Seafront (South) side Protection with Shore Piles with Grouting of Existing Rubble wall

Case Study 2: Combination of Diaphragm Wall and Intermittant Shore Piles

• The location: Walkeshwar Seafront.

• The plot size: 20 m*17.5 m with one side (south) right on seafront.

• The structure: 6 storeyed building with 4 storeys going below theexisting road level upto 10.35m depth with car lift base further downupto13 m depth.

• Excavation had to be vertical with side protection on east, west andnorth (road) sides. On the sea front side, an existing rubble wall wasto be backed up by R.C.C. retaining wall.

• Excavation was going to be about 4 m below the max. tide level.Besides dewatering arrangements ,base instability due to uplift alsoneeeded to be prevented.

• Four boreholes at four corners of the plot, going upto 25m depth,revealed following stratification.

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Case Study 2-Seafront Site

Geotechnical Data-Case Study 2

• Layer I: Yellowish brown very stiff silty clay (residulal soil), thicknessvarying from 2.75 m to 6.5 m. SPT ‘N’, 14 to 34, higher N due tokanker and gravels.

• Classification ‘ MH / CH, sand, silt and clay 20%,60%and20%respectively, LL 51 to 64% ,PI 19 to 38%,

• Cohesion 2.4T/m2, 26 degrees, saturated density 1.89 g/cc,

• Layer II: Brownish grey Weathered Basalt, about 10 m thick. C.R.upto 50%, RQD nil to 20%

• Layer III: Greyish moderately strong Amygdaloidal Basalt with blacktachylitic bands with C.R. 50 to 100% , RQD about 50%

• Soaked UCS about 150 kg/cm2.

• The ground water table 9 m below G.L.

• Based on above properties retaining schemes were designed withdetails as below. CADS PWS software was also used.

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Case Study 2-Retaining Scheme Details

• Road (North) Side: 600mm thick, 2 level anchored diaphragm wall,cast in 4m wide panels, founded at about 8m depth in weatheredrock.

• 10 T/m2 uniform surcharge considered to allow for traffic andconstruction equipment loading.

• Terzaghi-Peck simplified soil pressure diagram for anchored walls insoft to medium stiff silty clay adopted for design.

• 60T and 100T anchors @2m c/c spacing were installed.

• Incidentally, to ensure proper founding of diaphragm wall inweathered rock, a minimum Chisel response Energy parameter of50 T/m2/cm was specified, which was confirmed in pilot bores. Thisis based on the approach suggested by eminent geotechnicalconsultant, late Mr K.R.Datye.

Case Study 2-Excavation with Short Shore Piles and Bench

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Diaphragm Wall on Road (North) Side

Typical Details of Shore Pile,RCC Waling Beam and Anchor

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Case Study 2-After Installing Shore Piles with Anchors, excavation and brick wall construction

Case Study 2- Dowels and 225 mm thick RCC Wall constructed in 2m stretches below Anchored

Diaphragm Wall

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Case Study 2 - Shore Piles on either side of Sea Front protected by RCC Wall

Case Study3 - Contiguous Piles and Secant Piles, with Anchors through capping Beam

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Juhu Vile Parle Site-Salient Details of Shore Piles and Anchors

• Plot: 80m*35m

• Site Constraints: Nalla nearby, Planning too tight to permit

benches, Existing 7 storeyed building resting on old footings very

close to excavation

• Stratification: 0 to1m Fill, 1m to 5m Stiff Silty Clay (Residual soil),

5m onwards weathered Rhyolite rock

• Excavation Depth: 14m to15m, surcharge load 10 to 30 T/m2

• Shore Piles: 900mm dia @1000mm c/c, 16m long with 115 T

anchors @4.2mc/c thro’ capping beam

• Secant Piles :Near 7 storeyed building both primary and secondary

piles, 900 mm dia. 16m long, with secondary RCC piles cutting

200mm on either side of PCC primary piles, with 115Tanchors

@2.1m c/c thro’ capping beam

Shore Piles and Secant Pile layout

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115 T Anchors @4.2m c/c, inclined 45 degrees, through Capping Beam

900mm dia Shore Piles with Anchors Through Capping Beam (Nalla Side)

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900mm dia.Secant Piles near Existing Building

Excavation with ground water in part area

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Mahim, Mumbai Site Shore Pile Scheme-Combination of 750 and 1000mm dia piles

Case Study 4-Mahim Site-BH1-Typical Data

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Mahim Site - Salient Details of Shore Piles, Anchors and Micropiles

• Plot: 65m*20m, Excavation Depth: 10.5m

• Stratification: 0 to 1.5m Murrum Fill, 1.5m to 7.5m Loose to

medium Sand, 7.5m to 10.5m Marine Clay and weathered rock

beyond 10.5m depth onwards, GWT at 3m depth

• Shore Piles: On 3 sides,750mm dia @800mm c/c, 12m long

ensuring rock socket min. 2pile dia., M.S. liner upto 8m depth

(penetrating 0.5m in marine clay), and 1000mm dia @1050mm c/c on

4th side to avoid anchors.

• Prestressed Anchors: 80 T capacity @ 3.35 m c/c, inclined 45

degrees to horizontal, located at -2.5m depth

• Passive Uplift Anchors in Raft: 200mm dia micropiles@3.25m

c/c both ways, 4m in rock

• Access Road Slab Supported on 300mm dia Micropiles @6m c/c,

going 1 m in rock on one side and shore piles on the other side.

Details of Shore Piles

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Access Road Structural RCC Slab resting on Shore Piles and Micropiles

Uplift Resisting Passive Anchors in the Raft

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Mahim Site - Excavation in Progress

Excavation Completed

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Dewatering and Excavation

Laying of PCC and Installing Passive Anchors

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Site 5-Charni Road, Mumbai-Sandy strarta, Intermittant Micropiles, M.S.Sheeting and Internal

Props used

Case Study 5 - Salient Details

• Plot: 12m*30m

• Constraints: Existing Multi Storeyed Buildings on sides

• Excavation Depth: 5m

• Stratification: Loose Beach Sand with GWT within 1 to 1.5m

• Retaining Scheme: 200mm dia M.S. pipe micro piles @600mm

c/c,10m deep, reinforced with 2*ISMC100 box and grouted, gaps

between micro piles welded with M.S. plates, progressively lowered

ahead of excavation.

• Internal Props: 12m long lattice girders @ 4m c/c which also

supported the working platforms at G.L.

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Micropiles with Multilevel Structural Steel Waling Beams, Internal Props and M.S. Sheeting

Site 6-Excavation 5m deep in 50m*30m,Open space permitting Rubble Retaining Wall on one side but due to

less space,Shore Piles provided on other 3 sides

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Site 6 - Andheri, Mumbai-Typical Borelog

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600mm dia., 8m deep Shore Piles without Anchors

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Rubble wall on one side and Shore Piles on the other sides

Caution-Check the Design -Site where Shore Piles Without Anchors Tilted

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Topic 6-GENERAL GUIDELINES FOR DESIGN OF INTERMITTANT PILE / MICROPILE EXCAVATION RETAINING SCHEME

• Plan Geotechnical Investigation with at least one borehole per sideof excavation, conduct SPT, UDS, PMT, VST as necessary anddecide the stratification and geotechnical design parameters.

• Work out the Lateral Pressures on the retaining system consideringgeotechnical parameters, depth of excavation, surcharge loads dueto material stacking near the edge of excavation, movement ofconstruction equipment, variation in ground water table.

• Based on site constraints, accessibility for movement of pilingequipment, decide whether to go for bored piles or micropiles.

• Decide the termination depths of piles or micropiles, ensuring thatthey go well below the bottom of excavation and provide thenecessary fixity to pile and also have required lateral and verticalload and bending moment carrying capacity.

Guidelines contd.

• If piles are terminated in rock above the bottom of excavation, thenan adequate ledge of rock should be left between pile tips and faceof deeper excavation. The exposed rock face should be dowelledand shotcreted at the earliest.

• Spacing of piles should be decided based on soil stratification, shearstrength of weakest strata to develop arching action and location ofground water table during construction period.

• Decide whether to go for brick filler panels or M.S. sheet panels andwhether RCC waling beams are to be provided or structural steelgirders to be used. Structural design of panels and beams should bemade based on loading and their connection details properly workedout. Beam design should consider anchor loads and bendingmoments and RCC details modified at anchor locations.

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Guidelines Contd..

• Study the site constraints to decide whether prestressed anchorscould be adopted or not and what could be their permissibleinclination with horizontal. Also firm up whether the anchors are tobe designed as `temporary’ or `permanent’ type. In case they are`temporary‘ and destressed later, then the basement walls will needto be designed for eventual full lateral loading coming on theretaining wall system.

• Decide optimum spacing of anchors and their capacity based ontheir cost and time for execution.

• If anchors are not possible then design suitable internal props andconnect them temporarily to main building columns or speciallyconstructed supports.

• A close coordination and technical discussions between the projectgeotechnical consultant and the structural design consultant arevery essential for ensuring appropriate design of the retainingscheme and the safety of the excavations.

Conclusions

• Retaining schemes with bored cast-in-situ piles / micropiles with waling beams, anchors and filler panels, is asafe and economic method of retaining deepexcavations in urban environment.

• Various options of retaining schemes should be exploredbefore finalizing a scheme for a particular project.

• Effect of deep excavations on foundations of nearbystructures should be studied and proper precautionstaken.

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Acknowledgements

• The author is thankful to M/s Talati & Panthaky Associated Pvt. Ltd.the architects and M/s Ghadiali and Raval, Structural Consultants forinvolving him in the projects covering the first 2 Case Studies.

• He also wishes to thank Mr. Pravin Gala Structural Consultant, DrKelkar Design Consultants, Mr. Kohojkar of Design Excellence andDr. Nori and Mr. Agugia of Shirish Patel Associates Consultants forinvolving him in the projects covering the other case studies andsimilar projects.

• He thanks Mr. Kedar Birid for assisting with the preparation of thispresentation.

References

1) Brandl H. (1976) “Stabilization of high cuts in slide areas of

weathered soils”, Proceedings of the Sixth European Conference on

Soil mechanics and Foundation Engineering, Vienna.

2) North-Lewis J.P. and Lyons G.H.A. (1975) “Contiguous Bored

Piles”, Proceedings of the Conference on Diaphragm walls and

Anchorages, Institution of Civil Engineers, London, P.189 - 194.

3) Datye K.R., Karandikar D.V. (1988) “Bored Piling in Bombay

Region” 1st International Geotechnical Seminar on Bored and Auger

Piles, Van Impe (ed.)/Ghent/ 1988 Balkema.

4) Karandikar D.V. (1996) “Innovative use of piles for Retaining

Sides of Deep Excavation” , 6th International Conference and

Exhibition on Piling and Deep Foundations, 1996, Mumbai, India.

46

References Contd..

5) FHWA-IF-99-015, Ground anchors and Anchored Systems.

6) Foundation Analysis and Design : J.E.Bowles, McGraw Hill

International ,4th ed.

7) Foundation Design and Construction: M.J. Tomlinson, ELBS, 6th

ed.

8) Soil Mechanics in Engineering Practice, Second Edition, Karl

Terzaghi, Ralph B. Peck, Wiley International Edition, 1967

Be Wise and Discriminative

47

Thank You