Deep basements & cut & cover - 1
Transcript of 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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Introduction – sharing of structural engineer perspectives
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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Introduction – sharing of structural engineer perspectives
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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Introduction – sharing of structural engineer perspectives
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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Movements caused by excavation activities
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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Movements caused by excavation activities
Ground movements caused by vertical loadingand unloading of an excavation:
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Movements caused by excavation activities
Movement in the props supporting a wall (e.g. because of temperature changes, shrinkage or
loss of support:
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Movements caused by excavation activities
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Movements caused by excavation activities
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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Movements caused by excavation activities
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Movements caused by excavation activities
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
Movements caused by excavation activities
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Movements caused by excavation activities
Control of water table for different permeability of soil
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Movements caused by excavation activities
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Movements caused by excavation activities
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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Jet Grout Pile Properties
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Jet Grout Pile Properties
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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Jet Grout Pile Properties
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Cement Content & Stress-Strain Behaviour of Treated Clay(Is jet grout a soil replacement or mixing technique?)
Jet Grout Pile Properties
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Jet Grout Pile Properties
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Unconfined compressive strength and cement content relationship at
different curing periods.
Jet Grout Pile Properties
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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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Jet Grout Pile Properties
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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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Movements caused by excavation activities
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y
Effect of deflection of wall toe on groundmovements:
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Movements caused by excavation activities
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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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Movements caused by excavation activities
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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:
63
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
65
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
69
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
70
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
71
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
72
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
74
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
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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
76
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
Base Heave Stability
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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
Base Heave Stability
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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
Base Heave Stability
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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
Base Heave Stability
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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
Movements of stiff clay
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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
Movements of stiff clay
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
Movements of stiff clay
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
Movements of granular soils
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
Movements of granular soils
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
Movements of granular soils
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
Instrumentation and Monitoring
Daily Instrumentation Review Table
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95
Instrumentation and Monitoring
Daily Instrumentation Review Table
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Instrumentation and Monitoring
97