Deep basements & cut & cover - 1

8/19/2019 Deep basements & cut & cover - 1

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National University of SingaporeDepartment of Civil Engineering

CE 5112

Structural design and construction of

deep basements &cut & cover structures

Lecture 1

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Words of wisdom

8All things are wearisome,

more than one can say.

The eye never has enough of seeing,

nor the ear its fill of hearing.

9What has been will be again,

what has been done will be done again;there is nothing new under the sun.

10Is there anything of which one can say,"Look! This is something new"?

It was here already, long ago;

it was here before our time. Eccl 1:8-10 (NIV)2


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Practical Design Considerations

1) Introduction – sharing of structural engineer perspectives

2) General requirements – clients, builders & designers

3) Ground, soil profile & gases

4) Concept of effective stress vis-à-vis total stress5) Groundwater control

6) Movements caused by excavation activities

7) Methods of construction8) Types of earth retaining system

9) Influence of foundations type adopted

10) Site Investigation

11) Geotechnical & structural analysis, soil-structure interaction

12) Protective measures

13) Durability and waterproofing

14) Safety, legal and contractual issues & risk communications3


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Topics of Interest

In the next 4-5 lectures, we should spend some

time on topics relating to Temporary Earth

Retaining (TER) structure that you would liketo know in depth. Please email these topics to

me or Prof. Liew and we will try our best to

look for books, papers or source from others toaddress them.

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Introduction – sharing of structural engineer perspectives

1) A deep excavation is one for which the depth, structuralarrangement and loads, surrounding structures & utilities, soil &groundwater conditions are such that due diligence is requiredon geotechnical & structural aspects and their interaction.Normally an excavation > 5-6m, i.e. more than 1 basement, can

be much less – soft marine or fluvial clay stratum, 3m.

2) Ground-structure interaction requires many engineering skillsincluding reliance on observation and monitoring; clear understanding of geotechnical and construction materials;

appreciation on the effects of groundwater & seepage;development of proper conceptual and analytical models; & judgment based on a knowledge of case histories andconstruction methods, with properly evaluated past experience.

3) The next few lectures are intended to develop overall conceptual understanding. Drawing attention to key aspects of deepexcavation from a structural engineer perspective, with somecase histories. This is a complex and wide-ranging subject,where in-depth understanding of some subjects is needed at

times. So engage “real” specialists whenever necessary.

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Basic technical considerations:

1) Excavation will cause displacement to the

surrounding ground. Need to determinelikely & acceptable max. ground movements

– Alert & Work Suspension Levels.

2) Construction method adopted is intertwined

with the final underground structures –

creativity to balance buildability, safety &economy.

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Other considerations1) Professional responsibilities & liabilities – public

& client interests. Temporary works are mainly thedomain of builder QP (TER) but BCA requires QP(Supervision) to review builder’s temporary workssubmission. Some temporary structures become

permanent after completion.

2) Construction methods must be fully discussed atthe preliminary design stage with the contractor and/or architects (if they are interested). This is toascertain construction and related design

approaches.3) Construction must finished within specified time

and price. Simplicity of concept which allowsdesign and construction expediency may be the

key.7


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Other considerations

1) The need for better consultant selection,

improved tendering arrangements and clearer

regulatory & client guidance in order to achieve

better working practices for the construction

industry.

2) The client and his relevant professional advisors

are responsible for the permanent works in the

permanent condition.

3) The contractor and his advisors are responsible

for the temporary condition of the permanent

works and for temporary works.

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PERMANENT AND TEMPORARY DESIGN

Designers of props for a temporary retaining system, need to ensure thatthe performance requirements for the wall are met and site operations arenot unduly constrained. He should take account of the methods of constructing the permanent works & preferred method of prop removal(Contractor inputs is necessary).

Where the retaining wall also form part of the permanent works thedesigner of the temporary props may need to consider aspects of the

permanent works design. To minimize delays and inefficiencies the tender documents for such projects should include one of the followinginformation:

1.The assumptions made about the temporary works for the design of permanent works (propping levels and spacing, construction sequence,support system, stiffness, prop removal sequence, etc); If this approach ischosen, the permanent works QP is likely to attract some liability for the

performance of the wall in the temporary case. The contractor may not besolely responsible if the temporary works scheme complies with theassumptions made in the design of the permanent works.

2.Vertical and horizontal bending moment and shear capacities of the wall(horizontal bending can affect prop removal) and any other details

pertinent to the temporary works design.

3.Put out the excavation works as a D&B contract with performancerequirement including that of the permanent works if relevant.

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Uses & Consideration of Underground Structure

Ever increasing land cost is making underground

structures more economical - car-park, storage,

commercial, utilities, transportation tunnel &station, etc.

BCA and FSSD’s approval for adequate

ventilation, provision of fire-fighting lobby &area of refuge, locations of fire-lifts, protected

escape staircases & passages; fire fighting

appliances: sprinklers, smoke & heat detectorsincluding dry and wet risers; means of access for

fire personnel & engine. M&E rooms.

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Uses & Consideration of Underground Structure

Fire spread risk is addressed by compartmentalization

and full isolation of high fire risk zone, e.g., ventilation

ducts, effective smoke extraction – performance based

design e.g. by CFD analysis. E&M provisions affecthead room thus excavation depth.

Planned construction access and hauling of spoil.

4 hours fire rating for underground structure.

Ramps for car park & skylight (architecture) - openings.

Minor changes in layout or use may result in extensiveredesign and redetailing, so get your view heard early.

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Get your view heard on Underground Structure

E a r t h P r e s s u r e

Earth Pressure

Slab as beam in plane

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The concept of effective stress

Effective stress principle is essential to the understanding of mechanical behavior of the ground.

Saturated soil consists of discrete solid

particles in mechanical contact forming a skeletal structure with voids (pores) filledwith water (& gas).

Deformation or failure of soil is mainly

result from slip at contact points rather than crushing of the solid particles.

Change in total vertical load, Δ, will beresulted by additional load on the soilskeleton and/or change of porewater

pressure.

Chemical bonding is a generalized term for 1) coldwelding of mineral contact points between particles. 2)exchange of cat-ions, and 3) cementation.

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The concept of effective stress

Any plane through an element of soil has acting on it a

resultant normal stress and a shear stress . In

addition, the water in the pores will be under a

pressure, , porewater pressure. By definition, the

effective normal pressure ’ acting across the plane is

the difference between the resultant or total normal

pressure and the porewater pressure. Thus:

As water cannot take shear, will be an effective

stress:

'

'

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Effective stress principleAn effective stress may be thought of as that part of the totalstress transmitted through the soil skeleton. This refers to thecomplex state of stress at particles’ contact points.

Effective stress principle: all measurable effects of a change instress, such as compression, distortion or shearing resistance,are due exclusively to changes in effective stress.

The strength of a soil in terms of effective stress is defined by

Coulomb’s equation:

where f is the shear strength, c’ is the effective cohesion & ’ is

the effective angle of shearing resistance. Both of these parameters refer to the soil in its undisturbed state of stress andstress history – drained condition.

Effective stress soil properties are denoted by a prime ’.

' ' tan 'f c

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Effective stress principle

The classical equation of Coulomb

derives from experiments sliding

blocks of material with different

normal loads.

When combined with the Mohr circles representing individual soil

tests, parameters c and can be used

to describe a failure line. This

allows simple mathematics to predict one principal stress at

failure given the other principal

stress.

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Effective stress parameters

Typical results from undrained triaxial tests with porewater pressure measurement (a), or from drained triaxial tests (b), on

good quality undisturbed samples of a uniform overconsolidated

clay):

Expressed in

terms of

Angle of shearing

resistance

Cohesion

intercept

,

’ ø’ c ’

t , s ’ ψ' t '

'sintan' 1

'tan

'tan''

ct Where &

If ’ = 30

’ = 26.57

t ’ = 0.866 c ’

If ’ = 20

’ = 18.88

t ’ = 0.940 c ’

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Soil shear strength

When sheared, loose or slightlyover-consolidated soil willgradually compress until isreaches a critical state of constant shear stress , normaleffective stress

’ and specific

volume v .Dense or heavily over-consolidated soil (jet grout) will

initially compress & then dilateto reach similar critical state.

L= Loose Sample (disturbed)

D = Dense Sample

L= Loose Sample (disturbed)

D = Dense Sample

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Soil shear strength

For a loose soil, the criticalstate is relatively easy toidentify.

To define fully the state of a soil, 3 variables arerequired: specific volume v ,

shear stress & normaleffective stress ’. Criticalstates are combinations of

these three variables atwhich steady, continuedshear deformation take place. v = 1+ e (voids ratio)

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Soil shear strength

Undrained state paths for clay samples having the samespecific volume: (a) v vs ln’; (b) vs ’. Sample A -heavily overconsolidated; sample B - lightlyoverconsolidated.

Undrained shear failure at constant v must follow a horizontal path on the graph of v vs ln’ from initialcondition to the critical state (a). The position on the

critical state line is fixed by v of the sample beingsheared: defines the shear stress at undrained failure (b).

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Soil shear strength

For a dense or heavily overconsolidated soil, the stress-strain behavior is more complex. The shear stress risesto a peak, at or near which a rupture surface develops.

The shear stress then falls rapidly, and failure is brittles.Once the rupture has formed, it governs the overall behavior of the soil element.

Compression between the ends of a triaxial test sample

is due to relative sliding along the rupture surface rather than a uniform, continuous axial strain. The axial loadthat the sample can sustain depends on the stress state of the soil in a thin rupture zone, which is likely to soften

and swell preferentially and differ markedly from theremainder of the sample.

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Groundwater conditions play a vital role in

ground engineering problems. Porewater

pressures in soil can change because of seepage,water-table fluctuations, increases of applied

total stress (consolidation) and decreases of

applied total stress (swelling).

Any process that results in a decrease in effective

stress is potentially dangerous, since it results inswelling and reduction in strength.

Effective stress & soil strength

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Fine-grained soils (cohesive clay) are relatively impermeable, andso volume change will be gradual and related to the length of

time taken for porewater to dissipate - undrained to drained

state.

Short term strength of a clay will be controlled by the initial

effective stresses, giving what is called the undrained strength, c u- apparent cohesion. ( u=0)

c u is dependent on the water content. High water content giveslow undrained strength and low water content gives high

strength. If identical clay samples are tested without allowing

any change in water content, then no matter what confining

pressure is applied they will all fail at the same shear stress.

Undrained strength

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Assessment of soil properties such as unit weight,strength and stiffness, etc. should be based on a

comprehensive site investigation with high quality

laboratory testing, to derive relevant total andeffective stress parameters. The investigation should

establish the properties of all soil layers for foundation

and retaining structures design. The current methods

of assessing the soil properties are:

SOIL PROPERTIES

1. CIRIA Report 104 type A - moderately conservative

2. CIRIA Report 104 type B - Worst credible3. BS8002 Representative values of either peak

strength or critical state strength

4. Eurocode 7 (EC7) - characteristic values.25


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SOIL PROPERTIES

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ROCK PROPERTIES

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SINGAPORE ROCK PROPERTIES

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1. The industry is moving towards the adoption of the

Eurocode system, both in the UK and Europe.

2. It is broadly comparable with the CIRIA Report 104 type Amoderately conservative method. The Eurocode system

uses partial factors to achieve an overall margin of safetysimilar to that given by the global safety factors of CIRIA104.

3. BS8002 introduces a new set of approximations, e.g. that a

constant percentage of the representative strength ismobilized throughout the soil mass in the service conditionof the wall, This may not be conservative in somesituations, but there are cases in which prop loads predicted

in this way are higher than those experienced in practice.

SOIL PROPERTIESEC7 characteristic value method is adopted in CIRIA Report517 for the following reasons:

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Typical granular soil, c’=0 &

strength defined by ’, the

Mohr diagram gives ‘active’and ‘passive’ earth pressure

coefficients K a and K p

according to the horizontalstress vis-à-vis the vertical

stress.

Rankine theory with noallowance for wall friction

which reduces active pressure &

increases passive resistance.

Earth Pressure – Granular Soil

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Undrained strength allowsearth pressures due to clays to

be assessed in the short term,

before moisture contentschange and when actual porewater pressures are unknown.

Pressure coefficients

K ac=K pc=2, have been assessed by wedge theory to allow foradhesion. In this caseK a =K p=1.

It takes no account of theeffects of wall adhesion whichreduces active pressure andincreases passive pressure.

Earth Pressure – Cohesive Soil

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When calculate earth pressures on walls we must beclear about what type of analysis (long or short term) is

to be applied to each layer and type of soil.

In the long term, pore water pressures in clays willstabilized (excess porewater pressures dissipation) to a

steady state controlled by external conditions. These

long term water pressures can be estimated just as for sands and gravels, and long term effective stress

pressure calculations should be made for clays using

effective stress parameters c’ and ’.

Earth Pressure – Cohesive Soil

Long term = Drained = Effective stress = c’ ’

Short term = Undrained = Total stress = c u u

Note: usually c’ = 0 & always u = 0

Long term = Drained = Effective stress = c’ ’

Short term = Undrained = Total stress = c u u

Note: usually c’ = 0 & always u = 033


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Basis of calculation of soil pressures

Note: For temporary cofferdams c’ is normally taken as zero

for clays as well as for sands and gravels.34


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Movements caused by excavation activities

1) Ground movement caused by excavation activities maydamage surrounding structures, roads & services,depending on their sensitivity, magnitude & types of movement. Detailed instrumentation and monitoring of ground movements are often required – also a precautionagainst frivolous claims.

2) The amount & extent of movement can be controlled bymethod of construction and good control & standard of workmanship. Cost increases with more stringent

movement limits – balanced by knowledge & insurancecost.

3) The main causes of damage to adjacent buildings aregenerally wall installation and problems associated with

groundwater lowering.4) Ground movements computation is a complex problem &

much experience is required to make sensible use of complex FEM analyses when warranted, best applied with

precedent.35


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C i f il


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Computation of soil movements

Calculations based on soil strength can be used toassess stability, but not to estimate wall and soil

movements under working conditions. A stress-strain

relationship for the soil is needed.Stiffness of clay is defined either as tangent stiffness,

d/d, or as secant stiffness Δ/Δ where Δ & Δ

represent changes of generalized stress & strain froma defined starting point.

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C i f d


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Computation of ground movements

The maximum shear strain increment in the soil around anembedded retaining wall with small deflections is ≈ 0.1%. Thiscan be used to estimate a suitable soil stiffness profile for use inanalysis.

Usually, the soil stiffness must be allowed to vary with depth toaccount for the effects of increasing average effective stress anddecreasing over-consolidation ratio.

With judicious choice of stiffness parameters, numericalanalyses (e.g. finite element or difference) using a linear elastic- plastic soil model can lead to reasonable estimates of wallmovements and bending moments.

Computation of realistic ground movements requires the use of a more complex soil model that better represents thedegradation of stiffness with strain. It is important to cheek,that the computed stresses do not take the soil beyond the strainrange for which the stiffness parameters are chosen.

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M d b i i i i


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Ground disturbance during installation of in-

situ walls: due to vibration (driving &

retrieving), loss of ground (boring & retrieving)or heave (driving of pile).

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M t d b ti ti iti


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Ground movements caused by vertical loadingand unloading of an excavation:

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Movement in the props supporting a wall (e.g. because of temperature changes, shrinkage or

loss of support:

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Movement due to changes in groundwater conditions, i.e., water table drawdown can be

far reaching and time-dependent for low

permeability clay: (dragdown & consolidation)

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Most wall movement tends to occur before the insertion of anytemporary support, because the walls deflect as cantilevers untila prop is installed. To reduce ground movements fromexcavation the designer may raise the level of the top prop,

decrease the spacing between prop levels and increase thestiffness of the wall. It is comparatively less effective to increasethe prop stiffness; for example by preloading. Preloading doesnot affect movements caused by flexure of the wall or overall

movements due to the unloading effect of excavations.With many of the deformation methods of analysis in current use, it is possible to obtain smaller calculated wall movements by assuming high prop loads or prop stiffness. This is to comply

with the specified wall movement criteria, these calculatedresults are of little practical relevance. The measuredmovements described above show that these factors are of secondary importance, It is not efficient to provide stiffer props

in an attempt to restrict movements.43



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Control of water table for different permeability of soil

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Control of water table - layout of grout injection holesIn coarse granular materials or rocks, the excavation is surrounded by a grout curtain

consisting of one to two rows of primary injection holes at 3-6m centres in both

directions, with secondary holes and possible tertiary holes to ascertain effectiveness.

stop

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Control of water table - layout of grout injection holes


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Control of water table layout of grout injection holesLake Mathews outlet facility, Southern California

Grout mixes with water/cement ratio ranging from 4:1 to 1.5:1, injection refusal wasreached when < 28 liters of grout was injected in 10 mins. For w/c ratios of > 1.5:1,

refusal was when there was no intake in 5 mins.

Numerous large grout takes were experienced and many additional holes (tertiary and

quaternary) were needed in order to achieve curtain closure. The total water inflow

into the completed excavation was 115 l/min.

The grout curtain performed well during excavation and blasting immediately adjacent

to the curtain had no measured or observed effects.

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J t G t Pil P ti


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Jet Grout Pile Properties

Stabilization of soft ground by deep cement mixing and jet grouting

methods have been used in Singapore for stability and deformation control

in many deep excavation projects involving soft marine clay.

Jet grouting and deep cement mixing are two different approaches of introducing cement into the ground, which are carried out before the start

of any excavation work.

The resulting material formed is called Cement Treated Soil. The treated

soil layer helps in control the movement of soil mass below the finalexcavation level.

The unconfined compressive strength of cement treated clay increases with

the increase of cement content and curing time.

Strength and Stiffness Characteristics of Cement Treated Singapore Marine Clay,

A.H.M. Kamruzzaman, F.H. Lee & S.H.Chew and T.S.Ong

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Sampling techniques must be compatible to the grout strength achieved

and when coring is use, without QC, sample strength > 1.3 MPa is

desirable to achieve > 90% sample recovery. With good sampling

technique, sample strength of 400 kPa can be obtained (Roybi Kiso).

From experience, for water-cement ratio of between 0.65-0.75 and

withdrawal rate during jetting of 15-20 cm/minute to form a 1.2m Ø

column, water and grout flow rates of around 110-130 litres/min

respectively is required. Average Jet grout unconfined compressivestrength (UCS) is expected to exceed 1.4 MPa based on 63.5mm Ø core

samples taken at the intersection between columns. Core recovery will

be between 90-100%.

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Jet Grout Pile Strength Chart


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Jet Grout Pile – Strength Chart

Grout column Ø is largely a

function of the time that the jet and binder is kept at one

fixed level.

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Cement Content & Stress-Strain Behaviour of Treated Clay(Is jet grout a soil replacement or mixing technique?)


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Unconfined compressive strength and cement content relationship at

different curing periods.


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Effect of strain measurement

on stress-strain behaviour oftreated clay

Is jet grout a soil replacement or

mixing technique?

Comparison of stiffness

measured by Hall's effect

transducer and conventionalLVDT

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Stress-Strain Behaviour of Concrete

p

0.1% 0.2 0.3 0.4 0.5 0.6% 0.2 0.25%

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E 100 300


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E50 = 100q u – 300 q u

= 150q u – 400 q u S’pore Marine clay

S’pore Marine Clay

E50 = 125q u (LVDT) x 3 => or 375q u (Local strain transducer)

Ei = 135q u (LVDT) or 430q u (Local strain transducer)

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Jet Grout Pile Installation


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Jet Grout Pile Installation

Stages of conducting jet-grouting method & the equipment used: 1 – cement

silo, 2 – cement-inject plant, 3 – high-pressure pump, 4 – high-pressure conduit,5 – rotary drilling rig, 6 – casing head, 7 – beginning of the jet injection after

having driven a drilling rod until the designed depth, 8 – jet petrification of the

first pile, 9 – next pile forming

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y

Effect of deflection of wall toe on groundmovements:

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Relative wall and ground movements of cantilever and propped walls:

For soft clay, V = 2%H

Thus, 2%H = 0.6-0.8 H2or

H2=2.5%-3.3%H

For loose sand or gravel,V = 0.5%H

Thus, H2 = 0.625%-0.833%H

For stiff clay, V < 0.15%H

Thus, H2 < 0.188%-0.25%H

Comparative wall and ground movements of

cantilever and propped walls (after Burland et al,

1979)

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Distance from excavation/maximum depth of excavation [%]


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Zones defined by Peck (1969): (The data used by Peck to derive the three zones were taken fromstrutted excavations supported by soldier pile or sheetpile walls)

Zone (I) sand and soft to hard clay, average workmanship

Zone (II) very soft to soft clay, a) limited depth of clay layer beneath excavation bottom b) greater depth of clay, but N b7

Further experiences:

Excavations in Chicago (O’Rourke 1976)

Medium-dense to dense sand (O’Rourke 1981)

S e t t l e m e n t / m a x

i m u m d

e p t h o

f

e x c a v a t i o n [ % ]

bu N C

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Movements of low permeability clay


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For low permeability clay, movements will be

time-dependent. Initially, the clay will respond

in an undrained state with no volume change.With time, water will drains, causing a general

volumetric expansion when clay has been

unloaded or compression when loaded.Eventually, when excess porewater pressures

has dissipated, i.e. reached a fully drained state,

movements will cease, except perhaps creep

movement.

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Movements of low permeability clay


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During drainage, the strength of the clay changes. Thisis because, in the case of expansion, water is drawn

into the clay, softening it and reducing its strength.

For example, in front of a wall in stiff clay, followingexcavation, the clay will gradually expand and soften

following the relief of the overburden pressure. The

consequent loss of resistance may dominate the wallduring this ‘drainage’ stage, especially with cantilever

walls, The relative magnitudes of undrained and

drained movements, and the rate at which the latter develop, depend on the nature of the clay and can be

significantly affected by the presence of high-

permeability layers within the soil.

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Movement resulting from reduction of lateral pressure from the inner face of the retaining

structure, due to bulk excavation or the

installation of large bored piles within theexcavation:

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Movements of low permeability soft clay


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In soft clay, the depth to which excavation canreach before base heave failure starts may be

small. This will generally start when the base

stability number, N=H/c u > 3 - 4 &Uncontrolled deformation is likely for N = 6 - 7:

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Movements of low permeability soft clay


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Base heave occurs below excavation level,horizontal props alone cannot eliminate it. It

has to be controlled by ensuring that:

1) the retaining wall is sufficiently

stiff,

2) is adequately embedded below thedeforming zone by keying into a

stronger stratum, or

3) in-situ props are cast belowexcavation level, e.g. using jet

grouting, diaphragm cross-walling

techniques or tunnel struts.64

Base Heave Failure Prevention (b) Excavate underwater or bentonite(a) Extend walls tostrong stratum


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a) Extend walls to strong stratum

b) Excavate under water or bentonite

mud

c) Unload soil adjacent to excavation

d) Construct in a series of excavations

with reduced plan area – compartmentalization (3-D effect)

e) Artificially increased soil strength

– jet grouting

(e) Increase soil strength

(d) Reduce plan area

of excavation

(c) Unload retained soil

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3-D FEM Analysis of Long Retaining Wall Construction


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X-section of road corridor Initial and final Ground profiles

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3-D finite element mesh 2-D finite element mesh

Model: diaphragm wall panel

trench excavation with bentoniteModel: excavate to pre-diaphragm

installation ground profile

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Model: showing diaphragm wall

panels

Model: excavate to berm profile


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Model: excavate primary berm

section from the central bay Model: construction of primary propslab section in the central bay

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Calculated and observed

3-D FEM Analysis of Long Retaining Wall Construction


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Finite element mesh showingcompleted carriageway section

Calculated and observed

displacements of the central panel

Comparison of wall displacements calculated

using 2- & 3-D analyses

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Base Heave StabilityCommon problems of base failure are only likely in soft clays One widely used


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c ubase

N s Factor of safety F

D p

Common problems of base failure are only likely in soft clays. One widely used

method of determining the critical depth D, or the factor of safety against baseheave, F base ,was proposed by Bjerrum & Bide (1956):

Where:

s u = undrained shear strength of the soil beneath theexcavation

Nc = the bearing capacity factor (as for footings) which

depends on the shape and depth of the excavation.

P = surcharge applied at the ground surface on the retainedside.

This approach does not account for the reinforcing effects

of wall penetration below the base of excavation.

& 1, c ubase c N s p

when F D D

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Base Heave StabilityThe factor of safety against base heave, Fb , as proposed by Terzaghi (1943):


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The factor of safety against base heave, F base , as proposed by Terzaghi (1943):

If T ≥ 0.7B, B1 = 0.7B

If T < 0.7B, B1 = T

Or modified (Nc = 5.7)

1

1

5.7

ub

base

uh

C B Factor of safety F

B C H

1

1

c ub uh

base

N C B C H Factor of safety F

HB q

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Base Heave StabilityThe factor of safety against base heave, Fbase , as proposed by Eide et al.’s (1972):


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The factor of safety against base heave, F base , as proposed by Eide et al. s (1972):

1 12

c u a

base

N C C D L

Factor of safety F

H q

Basis & Application Limits:

• Narrow Excavation

• Ignore effect of clay thickness

• Ignore effect of wall stiffness

73

Base Heave StabilityThe stability number, Nc, as given by Program ReWaRD:


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y , , g y g

Where H (D) is the retained height; B is the breadth and L

the length of the excavation; and is the rigid layercorrection derived from the bearing capacity factors given

by Button (1953):

where T is the depth below excavation level to the top of

the first rigid layer and B is the breadth of the excavation.

1.4

1 0.008 T

B

1 0.2 1 0.29 2.5

1.2 1.5

1 0.29 2.5

1.2

c

c

B H H L B N for B

B H L N for B

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General Bearing Capacity FactorsGeneral Bearing Capacity Factors


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1000

100

10

1

0.10 10 20 30 40 50

B e a r i n g

C a p a c i t y F

a c t o r

Friction Angle (deg)

Nc

14o

N c = 10

Nq

35

o

N q = 33N

26 o

N

= 8

75


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For the condition of H < B (wide, shallow excavations) (Terzaghi)

For the condition of H > B (trench type excavations) (Skempton)

(5.7 )

2

uc

u

S q D

S

c uc

N s p D

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General Bearing Capacity FactorsGeneral Bearing Capacity Factors


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2

a

b u

C d

D N C B

b u D N C

2 tantan 452

1

1.8 1 tan

q

c q

q

N e

N N Cot

N N

Nc rectangular = (0.84 + 0.16 B/L) Nc square

Diagram for the determination of bearing pressure

coefficient, Nc (Skempton)

2 a b u

C d D p N C

B

p p

77

Base Heave Stability


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Ø 0 5 10 15 20 25 30 34 35 40 45 48 50

Nc 5.7 7.3 9.6 12.9 17.7 25.1 37.2 52.6 57.8 95.7 172.3 258.3 347.6

Nq 1 1.6 2.7 4.4 7.4 12.7 22.5 36.5 41.4 81.3 173.3 287.9 415.1

Ng 0 0.5 1.2 2.5 5 9.7 19.7 35 42.4 100.4 297.5 780.1 1153.2

N'c 5.7 6.7 8 9.7 11.8 14.8 19 23.7 25.2 34.9 51.2 66.8 81.3

N'q 1 1.4 1.9 2.7 3.9 5.6 8.3 11.7 12.6 20.5 35.1 50.5 65.6

N'g 0 0.2 0.5 0.9 1.7 3.2 5.7 9 10.1 18.8 37.7 60.4 87.1

78

Base Heave StabilityO’Rourke (1992) proposed a method to account for the flexural capacity of


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the wall extending below the excavation. He used plasticity principles andconservation of energy to show that the flexural effects of the wall may be

used to evaluate factor of safety against base failure. Factors of safety

determined by this method were in better agreement with the observed

performance of excavations at or about base failure. The method uses a dimensionless stability number NOR for three different end conditions of

the wall are given below:

1. Wall installed to some depth in clay below the

excavation, but not within an underlying firmstratum (free-end wall):

2

8

y

OR cw ub

M N N

L s

79

2. The wall has been installed into an underlying firm



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2. The wall has been installed into an underlying firm

stratum with sufficient penetration to result in full

moment restraint (fixed-end wall):

3. The wall is driven to rock, but tends to slide along the

interface without full moment restraint (sliding endwall):

2

2 y

OR c

w ub

M N N L s

29

32

y

OR cw ub

M N N

R L s

80



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Where

My = yield moment per metre of wall

R = B/√2 or thickness of soft clay beneath the base (T), whichever is the

smaller and B = width of excavation.Lw = wall length beneath the lowest, or next to lowest, level of propping

depending on depth to firm stratum.

s ub = representative undrained shear strength of the basal clay

81



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The effect of wall stiffness, depth of embedment and thickness of clay layer on basestability by Goh (1994). He evaluated the factor of safety on base stability for various

geometries of wide excavation in soft clay, by using the nodal displacement method of

finite element analysis. He proposed the following expression for base stability:

h u

base t d w

N s

Factor of safety F H

Where

= unit weight of the soft clay

H = depth of excavation

Nh = bearing capacity factor and is a function of H/B

B = width of excavation

μt = multiplying factor which is a function of T/B

T = thickness of soft clay beneath the base of the excavationμd = multiplying factor which is function of De/T

De = depth of embedment of the wall

μw = multiplying factor, which is a function of De/T, wall

stiffness and T/B.82



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Goh’s charts of Nh, μt, μd, & μw shows the following trends:

GOH, A T C (1994) Estimating Basal-Heave Stability for Braced Excavations in Soft Clay

Journal of Geotechnical Engineering, American Society of Civil Engineers, Vol. 120, No. 8

1. The presence of a rigid stratum close to the

excavation (T/B < l) increases the factor of

safety. The rigid stratum reduces the size of the

yielding zone by restraining the displacement of the soil beneath and around the excavation.

2. The two conventional methods of calculating

base stability (Terzaghi, 1943; Bjerrum and

Eide, 1956) may give overly conservative factors

of safety for T/B less than unity.

3. Factor of safety increases with increasing De/T

(i.e. increasing embedment), but the effect

becomes insignificant for values of T/B greater

than about 1.5.

83

Blowout Failure – relieve wells


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For condition of “Infinitely long”

excavation:

For condition of Rectangular

excavation:

2T ublow w

Bd c d Safety Factor F hB

2 ( )T ublow

w

dBL d c B LSafety Factor F

hBL

84

Movements of stiff clay


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Stiff clays are generally good materials to work with provided the effects of drainage are

limited. (turn soft when wet)

Stiff clays (rock) may possess high locked-inlateral stresses. The process of excavation may

releases large stresses, building up large support

loads. Adopting a ‘soft’ support system, e.g.flexible props and flexible walls, may reduce the

loads and stresses in the structural elements

with a consequent increase in movements

outside the excavation. Preloading may not be

necessary. 85



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Movement of unsupported (cantilever) wallsdue to drainage of soil in front of the wall. This

can occur rapidly if the ground is not protected

from water ingress:

86


M f h f d il d i


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Movement of the toe of propped wails duringconstruction. The clay in front of the toe of a

retaining wall may drain rapidly. Need to

ensure that the toe area is not left exposed for long. One common method is to leave soil

berm, removed and replaced later with

permanent support:

87


If l f l d d iff l d i


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If left unloaded, stiff clay under an excavationmay expand causing structures supported on it

to lift:

88

Movements of granular soils

Th f b t t ti i hi h


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The process of basement construction in high- permeability soils, e.g. sands, will result in an

almost instantaneous response to changes in

loads and groundwater conditions, i.e. fully

drained conditions.

For granular soils, principal concerns are thecontrol of groundwater to avoid loss of ground

and movements during the installation of walls.

89


S ttl t i d i ll i t ll ti b


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Settlement occurring during wall installation byloss of ground during drilling or the compaction

of loose sands/silts due to vibration:

90


W t i th h ll d i ti


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Water seeping through a wall during excavationgives rise to local lowering of water table

outside the excavation and loss of fines through

the wall, causing settlement:

91


I ffi i t t ti f th ll


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Insufficient penetration of the wall or insufficient dewatering within the excavation

leading to high hydraulic gradients, piping of

the basement floor or large scale heave. Seepageflows also reduce the passive pressure

restraining the toe of the wall:

92


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Monitoring Array Type A

Instrumentation and Monitoring


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Monitoring Array Type A

Rod extensometer & tip location

Inclinometer

Vibrating wire piezometer

Inclinometer /

extensometer in soil

Heave Stake

Ground settlement Marker

Casagrande Standpipe Piezometer

MHWN RL 100.448

MLWN RL 99.548

Piezometer for Kallang Formation

94


Daily Instrumentation Review Table


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95


Daily Instrumentation Review Table


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Documents

Deep basements & cut & cover - 1