Ce5112 Chap1

National University of SingaporeDepartment of Civil Engineering

CE 5112Structural design and construction of

deep basements &cut & cover structures

Lecture 1

Words of wisdom8All 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)

Practical Design Considerations

1) Introduction sharing of structural engineer perspectives2) General requirements clients, builders & designers3) Ground, soil profile & gases4) Concept of effective stress vis--vis total stress5) Groundwater control6) Movements caused by excavation activities7) Methods of construction8) Types of earth retaining system9) Influence of foundations type adopted10) Site Investigation11) Geotechnical & structural analysis, soil-structure interaction12) Protective measures13) Durability and waterproofing14) Safety, legal and contractual issues & risk communications

Topics of Interest

In the next 4-5 lectures, we should spend sometime on topics relating to Temporary EarthRetaining (TER) structure that you would liketo know in depth. Please email these topics tome or Prof. Liew and we will try our best tolook for books, papers or source from others toaddress them.

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

2) Ground-structure interaction requires many engineering skillsincluding reliance on observation and monitoring; clearunderstanding 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 conceptualunderstanding. 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 attimes. So engage real specialists whenever necessary.


Basic technical considerations:

1) Excavation will cause displacement to thesurrounding ground. Need to determinelikely & acceptable max. ground movements Alert & Work Suspension Levels.

2) Construction method adopted is intertwinedwith the final underground structures creativity to balance buildability, safety &economy.


Other considerations1) Professional responsibilities & liabilities public

& client interests. Temporary works are mainly thedomain of builder QP (TER) but BCA requires QP(Supervision) to review builders temporary workssubmission. Some temporary structures becomepermanent after completion.

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

3) Construction must finished within specified timeand price. Simplicity of concept which allowsdesign and construction expediency may be thekey.


Other considerations

1) The need for better consultant selection,improved tendering arrangements and clearerregulatory & client guidance in order to achievebetter working practices for the constructionindustry.

2) The client and his relevant professional advisorsare responsible for the permanent works in thepermanent condition.

3) The contractor and his advisors are responsiblefor the temporary condition of the permanentworks and for temporary works.

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 ofconstructing 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 thepermanent works design. To minimize delays and inefficiencies the tenderdocuments for such projects should include one of the followinginformation:

1.The assumptions made about the temporary works for the design ofpermanent 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 theperformance 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 detailspertinent 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.

Uses & Consideration of Underground Structure

Ever increasing land cost is making undergroundstructures more economical - car-park, storage,commercial, utilities, transportation tunnel &station, etc.

BCA and FSSDs approval for adequateventilation, provision of fire-fighting lobby &area of refuge, locations of fire-lifts, protectedescape staircases & passages; fire fightingappliances: sprinklers, smoke & heat detectorsincluding dry and wet risers; means of access forfire personnel & engine. M&E rooms.

Uses & Consideration of Underground Structure

Fire spread risk is addressed by compartmentalizationand full isolation of high fire risk zone, e.g., ventilationducts, effective smoke extraction performance baseddesign 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.

Get your view heard on Underground Structure

Earth PressureEarth Pressure

Slab as beam in planeFigure 1

The concept of effective stressEffective stress principle is essential to theunderstanding of mechanical behavior ofthe ground.Saturated soil consists of discrete solidparticles in mechanical contact forming askeletal structure with voids (pores) filledwith water (& gas).Deformation or failure of soil is mainlyresult from slip at contact points ratherthan crushing of the solid particles.Change in total vertical load, , will beresulted by additional load on the soilskeleton and/or change of porewaterpressure.

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

The concept of effective stressAny plane through an element of soil has acting on it aresultant normal stress and a shear stress . Inaddition, the water in the pores will be under apressure, , porewater pressure. By definition, theeffective normal pressure acting across the plane isthe difference between the resultant or total normalpressure and the porewater pressure. Thus:

As water cannot take shear, will be an effectivestress:

Effective stress principle An 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 byCoulombs equation:

where f is the shear strength, c is the effective cohesion & isthe effective angle of shearing resistance. Both of theseparameters refer to the soil in its undisturbed state of stress andstress history drained condition.Effective stress soil properties are denoted by a prime .

Effective stress principle The classical equation of Coulombderives from experiments slidingblocks of material with differentnormal loads.

When combined with the Mohrcircles representing individual soiltests, parameters c and can be usedto describe a failure line. Thisallows simple mathematics topredict one principal stress atfailure given the other principalstress.

Effective stress parameters Typical results from undrained triaxial tests with porewaterpressure measurement (a), or from drained triaxial tests (b), ongood quality undisturbed samples of a uniform overconsolidatedclay):

Expressed in terms of

Angle of shearing resistance

Cohesion intercept

, ct, s ' t'

Where &

If = 30 = 26.57t = 0.866 c

If = 20 = 18.88t = 0.940 c

Effective stress & soil strengthAn increase in effective stress () results incompression of soil and an increase in strength.This increase in effective stress could resulteither from an increase in the total stress () ordecrease of porewater pressure ().

Decrease in results in swelling of soil and adecrease in strength. This result either from adecrease in the or increase of .

Soil shear strength

When sheared, loose or slightlyover-consolidated soil willgradually compress until isreaches a critical state ofconstant shear stress , normaleffective stress and specificvolume v.Dense or heavily over-consolidated soil (jet grout) willinitially compress & then dilateto reach similar critical state.

L= Loose Sample (disturbed)D = Dense SampleL= Loose Sample (disturbed)D = Dense Sample

CREEP RATE BEHAVIOUR OF Ko CONSOLIDATED UNDISTURBED HANEY CLAY UNDER AXIALLY SYMMETRIC LOADING.

q is the principal stress difference normalized with respect to the vertical effective stress during consolidation

RELATIONSHIP BETWEEN RUPTURE LIFE AND MINIMUM CREEP RATE

Illustration : primary, secondary and tertiary phases of creep

Wall movements showing phases of creep

Soil shear strength

For a loose soil, the criticalstate is relatively easy toidentify.To define fully the state of asoil, 3 variables arerequired: specific volume v,shear stress & normaleffective stress . Criticalstates are combinations ofthese three variables atwhich steady, continuedshear deformation takeplace. v = 1+ e (voids ratio)

Soil shear strengthUndrained 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 ahorizontal path on the graph of v vs ln from initialcondition to the critical state (a). The position on thecritical state line is fixed by v of the sample beingsheared: defines the shear stress at undrained failure (b).

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 overallbehavior of the soil element.Compression between the ends of a triaxial test sampleis due to relative sliding along the rupture surface ratherthan a uniform, continuous axial strain. The axial loadthat the sample can sustain depends on the stress state ofthe soil in a thin rupture zone, which is likely to softenand swell preferentially and differ markedly from theremainder of the sample.

Groundwater conditions play a vital role inground engineering problems. Porewaterpressures in soil can change because of seepage,water-table fluctuations, increases of appliedtotal stress (consolidation) and decreases ofapplied total stress (swelling).

Any process that results in a decrease in effectivestress is potentially dangerous, since it results inswelling and reduction in strength.

Effective stress & soil strength

Fine-grained soils (cohesive clay) are relatively impermeable, andso volume change will be gradual and related to the length oftime taken for porewater to dissipate - undrained to drainedstate.

Short term strength of a clay will be controlled by the initialeffective stresses, giving what is called the undrained strength, cu- apparent cohesion. (u=0)cu is dependent on the water content. High water content giveslow undrained strength and low water content gives highstrength. If identical clay samples are tested without allowingany change in water content, then no matter what confiningpressure is applied they will all fail at the same shear stress.

Undrained strength

Assessment of soil properties such as unit weight,strength and stiffness, etc. should be based on acomprehensive site investigation with high qualitylaboratory testing, to derive relevant total andeffective stress parameters. The investigation shouldestablish the properties of all soil layers for foundationand retaining structures design. The current methodsof assessing the soil properties are:

SOIL PROPERTIES

1. CIRIA Report 104 type A - moderately conservative

2. CIRIA Report 104 type B - Worst credible

3. BS8002 Representative values of either peakstrength or critical state strength

4. Eurocode 7 (EC7) - characteristic values.

SOIL PROPERTIES

ROCK PROPERTIES

SINGAPORE ROCK PROPERTIES

1. The industry is moving towards the adoption of theEurocode system, both in the UK and Europe.

2. It is broadly comparable with the CIRIA Report 104 type Amoderately conservative method. The Eurocode systemuses 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 aconstant 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 predictedin this way are higher than those experienced in practice.

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

Typical granular soil, c=0 &strength defined by , theMohr diagram gives activeand passive earth pressurecoefficients Ka and Kpaccording to the horizontalstress vis--vis the verticalstress.

Rankine theory with noallowance for wall frictionwhich reduces active pressure &increases passive resistance.

Earth Pressure Granular Soil

Only a small wall movementaway from a soil face isrequired to reduce the at restearth pressure (Ko) to the activepressure (Ka). A very muchlarger movement towards thesoil face is needed to mobilizethe full passive resistancecoefficient Kp. (This applies tonormally consolidated clays andto sands and gravels but not tostiff overconsolidated clayswhich have high Ko values.)

Earth Pressure Granular Soil

=30Ka = 0.333Kp = 3

Undrained strength allows earth pressures due to clays to be assessed in the short term, before moisture contents change and when actual pore water pressures are unknown.Pressure coefficients Kac=Kpc=2, have been assessed by wedge theory to allow for adhesion. In this case Ka=Kp=1.It takes no account of the effects of wall adhesion which reduces active pressure and increases passive pressure.

Earth Pressure Cohesive Soil

When calculate earth pressures on walls we must beclear about what type of analysis (long or short term) isto be applied to each layer and type of soil.

In the long term, pore water pressures in clays willstabilized (excess porewater pressures dissipation) to asteady state controlled by external conditions. Theselong term water pressures can be estimated just as forsands and gravels, and long term effective stresspressure calculations should be made for clays usingeffective stress parameters c and .

Earth Pressure Cohesive Soil

Long term = Drained = Effective stress = c Short term = Undrained = Total stress = cu uNote: usually c = 0 & always u = 0

Long term = Drained = Effective stress = c Short term = Undrained = Total stress = cu uNote: usually c = 0 & always u = 0

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.

Movements caused by excavation activities1) Ground movement caused by excavation activities may

damage surrounding structures, roads & services,depending on their sensitivity, magnitude & types ofmovement. Detailed instrumentation and monitoring ofground 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 ofworkmanship. Cost increases with more stringentmovement limits balanced by knowledge & insurancecost.

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

4) Ground movements computation is a complex problem &much experience is required to make sensible use ofcomplex FEM analyses when warranted, best applied withprecedent.

Movements caused by excavation activities198 South Bridge Road Building Settlement

Trend plots for building settlement are stable generally.Start to slow down & turn off recharge wells. Then monitor movement for another week before props removal work.

199 South Bridge Road Building Settlement

Computation of soil movementsCalculations based on soil strength can be used toassess stability, but not to estimate wall and soilmovements under working conditions. A stress-strainrelationship for the soil is needed.

Stiffness of clay is defined either as tangent stiffness,d/d, or as secant stiffness / where & represent changes of generalized stress & strain froma defined starting point.

Computation of ground movementsThe 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 ofa 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.

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).


Ground movements caused by vertical loadingand unloading of an excavation:


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


Movement due to changes in groundwaterconditions, i.e., water table drawdown can befar reaching and time-dependent for lowpermeability clay: (dragdown & consolidation)

Movements caused by excavation activitiesMost 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 overallmovements due to the unloading effect of excavations.

With many of the deformation methods of analysis in currentuse, it is possible to obtain smaller calculated wall movementsby assuming high prop loads or prop stiffness. This is to complywith the specified wall movement criteria, these calculatedresults are of little practical relevance. The measuredmovements described above show that these factors are ofsecondary importance, It is not efficient to provide stiffer propsin an attempt to restrict movements.


Control of water table for different permeability of soil


Control of water table - layout of grout injection holesIn coarse granular materials or rocks, the excavation is surrounded by a grout curtainconsisting of one to two rows of primary injection holes at 3-6m centres in bothdirections, with secondary holes and possible tertiary holes to ascertain effectiveness.

stop

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 andquaternary) were needed in order to achieve curtain closure. The total water inflowinto the completed excavation was 115 l/min.

The grout curtain performed well during excavation and blasting immediately adjacentto the curtain had no measured or observed effects.

Jet Grout Pile Properties

Stabilization of soft ground by deep cement mixing and jet groutingmethods have been used in Singapore for stability and deformation controlin many deep excavation projects involving soft marine clay.

Jet grouting and deep cement mixing are two different approaches ofintroducing cement into the ground, which are carried out before the startof any excavation work.

The resulting material formed is called Cement Treated Soil. The treatedsoil layer helps in control the movement of soil mass below the finalexcavation level.

The unconfined compressive strength of cement treated clay increases withthe 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


Sampling techniques must be compatible to the grout strength achievedand when coring is use, without QC, sample strength > 1.3 MPa isdesirable to achieve > 90% sample recovery. With good samplingtechnique, sample strength of 400 kPa can be obtained (Roybi Kiso).

From experience, for water-cement ratio of between 0.65-0.75 andwithdrawal rate during jetting of 15-20 cm/minute to form a 1.2m column, water and grout flow rates of around 110-130 litres/minrespectively is required. Average Jet grout unconfined compressivestrength (UCS) is expected to exceed 1.4 MPa based on 63.5mm coresamples taken at the intersection between columns. Core recovery willbe between 90-100%.

Jet Grout Pile Strength Chart

Grout column is largely afunction of the time that thejet and binder is kept at onefixed level.

Soil Type Silt / Clay Sand Gravel

UCS (MPa) 5 10 20

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


Unconfined compressive strength and cement content relationship at different curing periods. (Roybi Kiso)


Development of Jet Grouted Soil Strength with time (Keller)


Effect of strain measurement on stress-strain behaviour of treated clay

Is jet grout a soil replacement or mixing technique?

Comparison of stiffness measured by Hall's effect transducer and conventional LVDT

Stress-Strain Behaviour of Concrete


0.1% 0.2 0.3 0.4 0.5 0.6% 0.2 0.25%

E50 = 100qu 300 qu= 150qu 400 qu Spore Marine clay

Spore Marine ClayE50 = 125qu (LVDT) x 3 => or 375qu (Local strain transducer)Ei = 135qu (LVDT) or 430qu (Local strain transducer)

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


Effect of deflection of wall toe on groundmovements:


Relative wall and ground movements ofcantilever and propped walls:

For soft clay, V = 2%HThus, 2%H = 0.6-0.8H2

orH2=2.5%-3.3%H

For loose sand or gravel, V = 0.5%HThus, H2 = 0.625%-0.833%H

For stiff clay, V < 0.15%HThus, H2 < 0.188%-0.25%H

Comparative wall and ground movements ofcantilever and propped walls (after Burland et al,1979)

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 workmanshipZone (II) very soft to soft clay, a) limited depth of clay layer beneath excavation bottom

b) greater depth of clay, but Nb7Further experiences:

Excavations in Chicago (ORourke 1976)Medium-dense to dense sand (ORourke 1981)

Distance from excavation/maximum depth of excavation [%]

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Movements of low permeability clay

For low permeability clay, movements will betime-dependent. Initially, the clay will respondin an undrained state with no volume change.With time, water will drains, causing a generalvolumetric expansion when clay has beenunloaded or compression when loaded.Eventually, when excess porewater pressureshas dissipated, i.e. reached a fully drained state,movements will cease, except perhaps creepmovement.

Movements of low permeability clay

During drainage, the strength of the clay changes. Thisis because, in the case of expansion, water is drawninto 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 softenfollowing the relief of the overburden pressure. Theconsequent loss of resistance may dominate the wallduring this drainage stage, especially with cantileverwalls, The relative magnitudes of undrained anddrained movements, and the rate at which the latterdevelop, depend on the nature of the clay and can besignificantly affected by the presence of high-permeability layers within the soil.


Movement resulting from reduction of lateralpressure from the inner face of the retainingstructure, due to bulk excavation or theinstallation of large bored piles within theexcavation:

Movements of low permeability soft clayWhen excavation takes place within soft clays,the reduction of vertical pressure inside theexcavation decreases the ability of the soilbelow the level of excavation to sustain thevertical pressure applied by the soil outside, i.e.an undrained bearing capacity failure (baseheave) can take place:

Movements of low permeability soft clay

In soft clay, the depth to which excavation canreach before base heave failure starts may besmall. This will generally start when the basestability number, N=H/cu > 3 - 4 &Uncontrolled deformation is likely for N = 6 - 7:

Movements of low permeability soft clay

Base heave occurs below excavation level,horizontal props alone cannot eliminate it. Ithas to be controlled by ensuring that:

1) the retaining wall is sufficientlystiff,

2) is adequately embedded below thedeforming zone by keying into astronger stratum, or

3) in-situ props are cast belowexcavation level, e.g. using jetgrouting, diaphragm cross-wallingtechniques or tunnel struts.

Base Heave Failure Prevention

a) Extend walls to strong stratum

b) Excavate under water or bentonitemud

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

(b) Excavate under water or bentonite

(a) Extend walls to strong stratum

3-D FEM Analysis of Long Retaining Wall Construction

X-section of road corridorInitial and final Ground profiles

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

Model: concreting of diaphragm wall trench: (a) pressure simulating wet concrete; (b) replacement of pressure by elastic concrete elements

Berm geometry

Model: showing diaphragm wall panels

Model: excavate to berm profile

Model: excavate primary bermsection from the central bay Model: construction of primary prop

slab section in the central bay

Finite element mesh showing completed carriageway section

Calculated and observed displacements of the central panel

Comparison of wall displacements calculated using 2- & 3-D analyses

3-D FEM Analysis of Long Retaining Wall Construction

Base Heave StabilityCommon 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 base heave, Fbase ,was proposed by Bjerrum & Bide (1956):

Where:

su = undrained shear strength of the soil beneath the excavation

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 retained side.

This approach does not account for the reinforcing effects of wall penetration below the base of excavation.

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

If T 0.7B, B1 = 0.7B

If T < 0.7B, B1 = T

Or modified (Nc = 5.7)

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

Basis & Application Limits:

Narrow Excavation

Ignore effect of clay thickness

Ignore effect of wall stiffness

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

Where H (D) is the retained height; B is the breadth and L the length of the excavation; and is the rigid layer correction 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.

General Bearing Capacity FactorsGeneral Bearing Capacity Factors

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26o

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

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

General Bearing Capacity FactorsGeneral Bearing Capacity Factors

Nc rectangular = (0.84 + 0.16 B/L) Nc squareDiagram for the determination of bearing pressure coefficient, Nc (Skempton)

p p

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

Base Heave Stability

Base Heave StabilityORourke (1992) proposed a method to account for the flexural capacity ofthe wall extending below the excavation. He used plasticity principles andconservation of energy to show that the flexural effects of the wall may beused to evaluate factor of safety against base failure. Factors of safetydetermined by this method were in better agreement with the observedperformance of excavations at or about base failure. The method uses adimensionless stability number NOR for three different end conditions ofthe wall are given below:

1. Wall installed to some depth in clay below the excavation, but not within an underlying firm stratum (free-end wall):

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 end wall):



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.

sub = representative undrained shear strength of the basal clay


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 variousgeometries of wide excavation in soft clay, by using the nodal displacement method offinite element analysis. He proposed the following expression for base stability:

Where

= unit weight of the soft clayH = depth of excavation

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

B = width of excavation

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

T = thickness of soft clay beneath the base of the excavation

d = multiplying factor which is function of De/T

De = depth of embedment of the wall

w = multiplying factor, which is a function of De/T, wall stiffness and T/B.


Gohs charts of Nh, t, d, & w shows the following trends:

GOH, A T C (1994) Estimating Basal-Heave Stability for Braced Excavations in Soft ClayJournal of Geotechnical Engineering, American Society of Civil Engineers, Vol. 120, No. 8

1. The presence of a rigid stratum close to theexcavation (T/B < l) increases the factor ofsafety. The rigid stratum reduces the size of theyielding zone by restraining the displacement ofthe soil beneath and around the excavation.

2. The two conventional methods of calculatingbase stability (Terzaghi, 1943; Bjerrum andEide, 1956) may give overly conservative factorsof safety for T/B less than unity.

3. Factor of safety increases with increasing De/T(i.e. increasing embedment), but the effectbecomes insignificant for values of T/B greaterthan about 1.5.

For condition of Infinitely long excavation:

For condition of Rectangular excavation:

Blowout Failure relieve wells

Movements of stiff clay

Stiff clays are generally good materials to workwith provided the effects of drainage arelimited. (turn soft when wet)Stiff clays (rock) may possess high locked-inlateral stresses. The process of excavation mayreleases large stresses, building up large supportloads. Adopting a soft support system, e.g.flexible props and flexible walls, may reduce theloads and stresses in the structural elementswith a consequent increase in movementsoutside the excavation. Preloading may not benecessary.


Movement of unsupported (cantilever) wallsdue to drainage of soil in front of the wall. Thiscan occur rapidly if the ground is not protectedfrom water ingress:


Movement of the toe of propped wails duringconstruction. The clay in front of the toe of aretaining wall may drain rapidly. Need toensure that the toe area is not left exposed forlong. One common method is to leave soilberm, removed and replaced later withpermanent support:


If left unloaded, stiff clay under an excavationmay expand causing structures supported on itto lift:

Movements of granular soils

The process of basement construction in high-permeability soils, e.g. sands, will result in analmost instantaneous response to changes inloads and groundwater conditions, i.e. fullydrained conditions.

For granular soils, principal concerns are thecontrol of groundwater to avoid loss of groundand movements during the installation of walls.


Settlement occurring during wall installation byloss of ground during drilling or the compactionof loose sands/silts due to vibration:


Water seeping through a wall during excavationgives rise to local lowering of water tableoutside the excavation and loss of fines throughthe wall, causing settlement:


Insufficient penetration of the wall orinsufficient dewatering within the excavationleading to high hydraulic gradients, piping ofthe basement floor or large scale heave. Seepageflows also reduce the passive pressurerestraining the toe of the wall:

Instrumentation and Monitoring

Monitoring Array Type B

Rod extensometer & tip location

Inclinometer

Vibrating wire piezometer

Inclinometer / extensometer in soil

Ground settlement Marker

Casagrande Standpipe Piezometer

MHWN RL 100.448

MLWN RL 99.548

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

Instrumentation and MonitoringDaily Instrumentation Review Table

Ce5112 Chap1

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