Deep basements & cut & cover - 4&5

8/19/2019 Deep basements & cut & cover - 4&5

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National University of SingaporeDepartment of Civil Engineering

CE 5112

Structural design and construction of

deep basements &cut & cover structures

Lecture 4/5

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Words of wisdom

1. The state of mind which enables a man to do work of this kind ... is akin tothat of the religious worshipper or the lover; the daily effort comes from nodeliberate intention or program, but straight from the heart. "Principles of Research"

2. The ordinary adult never gives a thought to space-time problems.... I, on the

contrary, developed so slowly that I did not begin to wonder about space andtime until I was an adult. I then delved more deeply into the problem thanany other adult or child would have done.

3. The important thing is not to stop questioning. Curiosity has its own reasonfor existing. One cannot help but be in awe when he contemplates themysteries of eternity, of life, of the marvelous structure of reality. It is

enough if one tries merely to comprehend a little of this mystery every day.

4. My interest in science was always essentially limited to the study of principles.... That I have published so little is due to this same circumstance,as the great need to grasp principles has caused me to spend most of my timeon fruitless pursuits.

5. One thing I have learned in a long life: that all our science, measured againstreality, is primitive and childlike—and yet it is the most precious thing wehave.

Albert Einstein

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Words of wisdom

1. A hundred times every day I remind myself that my inner and outer life depend on the labors of other men, living and dead, and that Imust exert myself in order to give in the same measure as I havereceived.

2. There are two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.

3. Before God we are all equally wise - equally foolish

4. Everything should be made as simple as possible, but not simpler.

5. The search for truth is more precious than its possession.

6. I have never belonged wholeheartedly to a country, a state, nor to a circle of friends, nor even to my own family. When I was still a rather

precocious young man, I already realized most vividly the futility of

the hopes and aspirations that most men pursue throughout their lives. Well-being and happiness never appeared to me as an absoluteaim. I am even inclined to compare such moral aims to the ambitionsof a pig. (Written in old age?)

Albert Einstein

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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 communications4

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Design and analysis of retaining system

Shortcoming of Current (UK) PracticeThere is a lack of clear authoritative guidance on appropriatedesign standards or code of practice for the design temporaryretaining system. Consequently there is the absence of an

industry-wide approach.

The design of the temporary retaining system within a limitstate framework need to set up to meet both geotechnical andstructural considerations. In the limit state approach ultimate

failure (ULS) & failure caused by loss of serviceability (SLS)(e.g. excessive deformations) are treated separately & differentfactor of safety apply to each.

The absence of a standard approach to design has led engineers

to apply design guidance for permanent works (e.g. BD42/94)to the design of temporary retaining systems. It is never therequirement that the temporary works be designed to the samestandards as the permanent works and this misuse led to over-conservative design.

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DESIGN STANDARDS (UK)

Two design standards in use in the UK that cover the derivation of loads for thedesign of retaining systems for deep excavations:

• BS8002 (1994): Code of practice for earth retaining structures

• Eurocode 7 (EC7) (1995): Geotechnical design (A pre-standard to replaceBS8002 by 2010).

BS8002 aims to be a limit state code, but its approach is unclear and so not beenwidely adopted. For singly propped walls, the code is clear about the serviceabilitylimit state (SLS) but unclear about the partial safety factor for the ultimate limitstate (ULS).

For multi-propped walls, BS8002 recommends the use of the Peck envelopes to

obtain a prop load, but it does not provide guidance on how this load should be usedin SLS and ULS calculations. This leaves some gaps at the interface where proploads derived from the geotechnical design are used in the structural design.

Only method that gives characteristic prop and waling loads can there be a proper interface between the geotechnical and structural designs. These characteristic loadscan be used with any of the limit states codes (e.g. steel, concrete) and be factored

appropriately to give SLS and ULS prop loads.CIRIA Report 104 (1984) was adopted as an unofficial design standard before the

publication of BS8002 and is still widely used, due to familiarity and also concernsabout BS8002. C104 did not address multi-propped walls, but its principle of factoring soil strength has been used in analyses of such walls by deformationmethods. In a similar way, Eurocode 7 includes the principle of factoring soilstrength as ULS Design Case C. 6

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EUROCODE 7: GEOTECHNICAL DESIGN

EC7 philosophy applies to the design of temporarysupport systems for deep excavations in general,requires a design be verified for three separate cases

A, B and C. In each case both ultimate andserviceability limit states are specified.

Case A is not relevant to the design of temporary

propping systems.In Case B the actions (i.e. loads and imposeddisplacements) that act on the retaining system are

increased and characteristic values are taken for the properties of the soil.

While in Case C lower factors apply to the actions

but the soil properties are reduced (factored). 7

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The relevant features of the EC7 philosophy are:1. EC7 states that Case B is often critical when

determining the strength of the structural elements

of retaining walls while Case C is generally criticalin cases where the strength of structural elements is

not involved. The partial factors for ULS for these

two cases are CIRIA 517:

Table EC7 partial factors for ultimate limit states in permanent and transient situations

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Partial factors for ultimate limit states Geo/Str DA1Footing, Walls and Slopes

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Plaxis and EuroCode 7 - issue 16 / Oct 2004Considering the safety of slopes and excavations, distinction is made in EC7 in three different

design approaches: DA1, DA2 and DA3, whereas in DA1 two sets of partial factors have to be

considered (DA1/1 and DA1/2). Moreover, distinction is made between Actions, Soil

Properties and Resistances.

With the current option of Ø-C reduction in Plaxis it is, to a certain extend, possible to prove

that situations comply with DA1 or DA3. DA2 involves an increase of unfavourable

permanent action. This means for a situation of an excavation that the active soil pressure behind a wall (= unfavourable permanent action) needs to be increased by a factor 1.35.

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Partial factors for ultimate limit states for persistentand transient situations (Japan)

* Partial factors not relevant, and hence not provided, for Case A.11

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Partial factors for ultimate limit states for persistentand transient situations.

Case

Actions Ground Properties

Permanent Variable

Unfavourable Favourable Unfavourable tan Ø c ' c u q u#

Case A 1.1 0.90 1.50 1.1 1.3 1.2 1.2Case B 1.35 1.00 1.50 1.0 1.0 1.0 1.0

Case C 1.00 1.00 1.30 1.25 1.6 1.4 1.4

# Compressive strength of soil or rock.

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2. Permanent actions include pressures caused by ground,groundwater and free water. Variable actions may alter with time and include surcharges and temperature effectson the prop loads.

3. Design pressure due to ground and groundwater may bederived using the partial factors in Table or by other methods. The partial factors in the Table indicate the levelof safety appropriate for conventional design in mostcircumstances and are to be used as a guide to the required

level of safety when the method of partial factors is not used.

Where design values for ultimate limit state calculationsare assessed directly, they are selected such that a more

adverse value is extremely unlikely to govern theoccurrence of the limit state.

4. The characteristic value of a parameter is one that is a cautious estimate of the value governing the occurrence of limit state. If statistical methods are used the probability of a worse value is not greater than 5%. 13

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5. The Table indicates that for the method of partial factorsall permanent characteristic earth pressures on both sides of the wall are multiplied by 1.35 if the total resulting actionis unfavorable, and by 1 if the total resulting action effect isfavorable. Variable characteristic earth pressures are

multiplied by 1.50. However, it also permits the partialfactors to be applied to the action effects derived from thecharacteristic earth pressures (i.e. multiply prop loads from permanent actions by 1.35 and from variable action by1.50). EC7 states that this latter method should be used for the design of the structural elements of a retaining wallsystem.

6. For ULS, the design water pressures should be the most unfavorable values occur in extreme circumstances. For SLS the design water pressures should be the most unfavorable which could occur in normal circumstances.

7. For ULS calculations, the excavation depth should beincreased by 10% of the height beneath the lowest support, up to a maximum of 0.5 m. 14

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Design water pressures are affected by Tide and Rainfall

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& Distributed Prop Loads (DPL)Analyses of propped excavations in soft clay, stiff clay and drysand have been undertaken to establish that Case B is likely togive the higher ultimate prop load in most situations (Case C

only gave significantly higher loads for 3 out of the 20 propsfor excavations in dry sand). Case C aims to address uncertainty in the ground. Where Case C gives the higher propload, the distributed prop load method will usually account for this because it is based on actual field data. The recommended

characteristic DPL diagrams have been assessed conservativelyand it is reasonable for designers in conventional situations toconclude that only Case B has to be considered.

The distributed prop loads (DPL) are the action effects of

ground and water pressures. Following EC7 Case B philosophy, characteristic values of the DPL can be multiplied by 1 to give the serviceability limit state (SLS) design values,and by 1.35 (permanent) or 1.50 (variable) to give the ultimatelimit state (ULS) design values.

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& Distributed Prop Loads (DPL)METHOD OF DESIGN

The method of establishing the SLS and ULS prop

and waling loads from the DPL diagrams isstraightforward. The SLS prop and waling loads arecalculated from the characteristic distributed propload diagram recommended for the relevant soil class

with the partial factor of 1.0. The ULS prop andwaling loads are obtained by multiplying thecharacteristic prop loads by a factor of at least 1.35,except for the load contributed by variable actions

such as surcharges that should be multiplied by 1.50.

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Characteristic Distributed Prop Load diagrams for

Class A, B & C soils

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Characteristic DPL diagrams for Class A soils

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Apparent Pressure Diagram for braced excavations in

soft clay with diaphragm wall

Simplified soil profile andstrutting/excavation sequence

of the “Swiss Tower” project,

Taipei. (Chang & Wong)

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Soil profile andgeometry adopted for

parametric study using

computer program

EXCAV95

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soft clay with diaphragm wallWhen the Ei/c u ratio falls between 200 to 500, except for the lowest strut, the

reference APD underestimates strut forces by as much as 100%. As the Ei/c uratio increases to 1000, the reference APD becomes more applicable.

(a) Strut force intensity vs. soil’s initial

tangent modulus (Ei/cu×10-1)

(b) strut force exceedance ratio vs.

soil’s initial tangent modulus.22

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soft clay with diaphragm wallStrut forces is dependent on diaphragm wall stiffness. The greater the wall

stiffness, the larger are the strut forces. This phenomenon is believed to be

linked to the arching effect induced by wall displacement. Stronger arching

effect associating the larger displacement of thinner walls reduces the

corresponding strut forces.

(a) Strut force intensity vs. wallstiffness

(b) Strut force exceedance ratio vs.wall stiffness

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Effect of wall penetration (D)

As the normalized wall penetration, D/T, increases beyond a ratio of 0.5, the

strut forces do not vary significantly.

But when D/T drops below 0.5, the effect of wail penetration becomes

noticeable with an increase in the exceedance ratio. An exceedance ratio of 2

seems to be sufficient in enveloping the observed strut force variations.

Effect of excavation width (B)Varying excavation widths appear to have no significant effect on the strut

force as long the excavation width is about 3 times the excavation depth for a

constant B/T ratio of 1.

For a narrower excavation, more passive resistance below the excavation levelis mobilized due to the interaction from the side walls, which results in

reduced strut forces, especially at the lower strut levels. It appears that, when

the B/H ratio is below 1, the reference APD is able to compute strut forces

satisfactorily. 25

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soft clay with diaphragm wallEffect of thickness to hard stratum (T)

The thickness of soft clay below the excavation level is seen to impose a

strong effect on the strut force. When the T/B ratio drops below 1, the

restraining effect from the presence of hard stratum at shallow depth reducesthe strut force. A maximum of 80% reduction is noted at the lowest strut level

as the T/B ratio scales down from 1 to 0.25.

Conversely, as the T/B ratio increases above 1, the effect of clay thickness

becomes negligible. It is, therefore, postulated that, for a braced excavationwith a T/B ratio more than 1, the layer of soil below a depth of l.0 B from

the excavation level could be neglected from the strut force analysis.

Effect of number of strut levels

Regardless of the number of struts, the shape and the magnitude of theapparent pressure diagram tend to remain the same, provided that no

buckling develops in any of the strut. A strut force exceedance ratio of 2 is

sufficient in encompassing all variations of the strut forces.

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soft clay with diaphragm wallBy adopting the amended APD and a factor of safety of 1.5 for temporary

work, the factored strut forces are sufficient in encompassing the maximum

strut force exceedance ratio, regardless of any variation to the configuration.

The amended APD is derived from cases with T/B ratio greater than 1. Whenthe T/B ratio is less than 1, the strong restraining effect from the underlying

hard stratum reduces the strut force.

Proposed amended

Apparent Pressure

Diagram is the strut forceexceedance ratio

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Correlation among strut force exceedance ratio, Ei/cu value and cu*/H value.

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soft clay with diaphragm wall (Chang & Wong)

CONCLUSION

1. For braced excavation in soft clay with diaphragm wall, the Terzaghi-Peck

Apparent Pressure Diagram tends to underestimate the strut forces when

the ratios of Ei/c u and c u*/H fall below 500 and 1.5 respectively.

2. The forces at the top and the lowest strut levels tend to be near or below

the value computed from the reference APD, regardless of the excavation

configuration and shear strength variations.

3. The shape and the magnitude of the Apparent Pressure Diagram are not

affected by the number of strut levels.

4. For T/B ratio > 1, a value of 2 appears sufficient in enveloping all possible

variations in the strut forces.5. For T/B ratio < 1, the restraining effect imposed by the underlying hard

stratum reduces the strut force. The shallower the soft clay deposit, the

smaller are the strut forces.

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Condition For the Use of DPL Method

The distributed prop load method is based onempirical relationships and its selection for use on

any project should be considered carefully. It is

advisable to use one or more alternative methods as

well and to compare the results obtained. When

considering whether the DPL method is appropriate

the engineer should consider:

1. Is the specific site stratigraphy covered by the data set?

2. If the answer to (1) is “No”, do the site specific soils

behave in a similar way to the soils in the data set, i.e. dothe specific soils behave differently from the general Class

A, B or C soils? Is it reasonable to apply DPL

recommendations to the site?30

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Condition For the Use of DPL Method3. Is the geometry of the excavation and propping system within the

range represented by the data set? This should be considered

particularly in regard to:

• width of excavation

• depth of excavation

• number of props and their horizontal and vertical spacing

• duration of propping

• installing props before excavating below the prop level

4. Do the limitations stated for each soil class apply, e.g. T/B < 0.5 &

T/H < 0.8 for excavations in soft clay where the wall enhances base

stability?5. Are there any other unusual features of the project, e.g. very high

surface surcharges, which might make the DPL recommendations

inapplicable?

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Sufficient Toe Embedment

The construction of the DPL diagram for each case

history assumes that the bottom of the excavation is a

prop. The soil in front of the toe of the wall isassumed to support the wall between the base of the

excavation and halfway towards the lowest prop, as

well as the earth pressures from the retained groundover the embedment length. The engineer should

check that the wall embedment is sufficient to satisfy

this assumption, with a factor of safety on the passive pressures or soil strength appropriate to the allowable

movement of the toe of the wall.

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SurchargesThe characteristic DPL diagrams include for thenominal surcharges associated with generalconstruction activities and adjoining roads. Thesewill not generally exceed a distributed surcharge of 10kPa. Identifiable additional loads such as tower cranes, mobile cranes, material storage and loads

from adjacent buildings should be treated separately.These extra surcharges should be multiplied by theRankine active earth pressure coefficient and added to

the characteristic DPL diagram, provided they are a second order contribution to the load diagram. If thesurcharge effect contributes to a significant portion of the earth pressure, the resulting prop loads should be

corroborated by another method. 3333


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Preload

The characteristic DPL diagram can be applied to

situations where preloading is used to remove slack in

the support system. Preload applied to remove slack should not exceed 15% of the characteristic prop load.

Higher values are not usually required and may result

in prop loads that exceed the characteristic value.

Some authority asked for 50% preload!

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C di i F h U f DPL M h d


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TEMPERATURE EFFECTS (Not thermal)The characteristic DPL diagrams do not allow for changes intemperature but it is not necessary to increase the calculatedloads to allow for temperature increases. It is insteadrecommended that the temporary support system should bedesigned for the characteristic prop loads. The resultingstructural members should then be checked using the simpleserviceability and ultimate limit state criteria. These criteria will often be satisfied.

Prop removalAvailable data indicate prop removal can increase the propload by up to 30%, but may equally have very little effectdepending on the specific circumstances of the site.

The characteristic DPL diagrams do not include for propremoval. Prop loads should be established from DPL diagramsfor both excavation to final level & the subsequent sequence of removal of props. However, the higher loads so calculatedneed not control the structural design, as it may be possible toadopt lower partial safety factors. 35

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C diti F th U f DPL M th d


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Condition For the Use of DPL MethodMixed support systems

The case histories on which the method is based do not permit

an assessment to be made of the influence of ground anchors

on prop load distributions. The potential for the stiffer

propping system to attract a disproportionate amount of theload relative to the prestressed, but less stiff ground

anchorages. The high prop loads is the result of lower than

expected stiffnesses for the ground anchorages.

Where props are used as part of a combined support system of

props and anchors the DPL method of calculating prop loads is

not applicable. (FEM)

Frost effects

Case history in Norway showed frost effects on the ground can

increase the prop loads dramatically (e.g. 800%). Such frost

effects are not included in the characteristic DPL diagrams. 3636

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Structural considerations

Eccentric Axial LoadingNo eccentricity of axial loading should be assumed in thedesign if the end plate is grouted/concreted to a concretewaling or the connection to a steel waling has been designed to

eliminate eccentric loading e.g. by spherical bearings.For other situations, CIRIA Special Publication 95 gives thefollowing advice on the eccentricity of axial load to be used for the prop design:

• for walings made from a single section (UC or UB), theeccentricity should be approximately 10% of the overalldimension of the prop in the vertical plane

• where the walings are constructed from twin beams, theeccentricity in the vertical plane should be ½ the distance between the webs of the two beams.

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Accidental Loading

The provision in the design for accidental loading, & possible

loss, of a prop depends on the risk and consequences of failure.

These are matters of judgement for the designer and the project

team, which should always be given thorough consideration

and evaluation.

It is recommended that this loading condition should beconsidered in the design unless positive steps are taken in the

management and operation of the site to eliminate effectively

the risk of accidental loading or loss of a prop.

CIRIA Special Publication 95 suggests accidental loading be

considered as a load of 10 to 50 kN applied normal to the prop

at any point in any direction.

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WALINGSTue design and construction details of walings are covered in CIRIA SpcialPublication 95, to which reference should be made. Some of the salient

points are mentioned here.

While the waling will be designed for a uniform loading, the actual loadwill vary considerably depending on the variation of the ground and itsmovement, any arching effect, the construction methods, quality of

packings between the wall and the waling, etc. It is therefore normal to usea simplified approach to design.

Goldberg et al (1976) recommended using 80% of the design prop loaddetermined from the Peck envelope for design of the waling to theAmerican permissible stress code (AISC). The reduction was an allowancefor arching of the soil resulting from deflection of the waling between the

props.

The waling deflection depends on the stiffness of the wall and waling, andthe spacing of the props. It is likely to be small for stiff walls, especially instiff ground conditions, e.g. Class B and C soil profiles. Consequently, it isrecommended that the waling is designed for 100% of the prop design load

unless the effects of arching are assessed.39

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The walings should be continuous over two or more supports and bedesigned for a max. bending moment of wL²/l0, where w is the walingload per unit length and L is the horizontal prop spacing. If continuity isnot possible, it should be designed for wL²/8. Similarly the end of a continuous waling acts as cantilever about the last support and should be

designed for wL²/2.The waling design should consider the effects of load increases fromtemperature rise in the same way as for props.

Where a waling acts as a prop to another waling or the arrangementinvolves diagonal props, the waling has to resist both the axial (in-plane)load and the bending moments and shears due to the out-of-plane load. If there is an imbalance in the axial force in the complete waling system, theload is transferred into the ground via the wall. Sufficient shear connection

between wall and waling is needed and the wall must provide bending &shear capacity for these in-plane forces in combination with those out-of-

plane. It is also necessary as part of the wall design to consider how thewall will act to transfer the in-plane loads into the ground (e.g. as a diaphragm or as individual elements).

Where raking props are used, the waling and wall should be designed to

support the vertical component of load with minimal deflection. 40

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Structural considerations PROGRESSIVE COLLAPSE


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Structural considerations - PROGRESSIVE COLLAPSE

The design of individual props should be robust but in addition the designer

should also consider the implications of the accidental loss of a prop. Thismay be done in one of two ways:

1. Incorporating the loss of a prop into the design of the support system.

This design case could be combined with reduced partial safetyfactors, reflecting the accidental nature of the loading. Collapse of theexcavation would be prevented, but there could be large wall andground movements close to an ultimate limit state. These movementscould damage adjoining property and impair the watertightness of theretaining wall and its subsequent serviceability.

2. A risk assessment and management strategy to eliminate the risk of

accidentally damaging/removing a prop.This aspect of the design of the temporary propping system is of interest to the client and the engineer as well as to the maincontractor, and is often interpreted differently. Widely differingviewpoints were expressed. Any requirements of the client/engineer

should be specified in the tender documents.Some of the engineers contacted during the study considered it good

practice to design the props to have greater capacity than the waling. Thedifference in capacities is chosen such that the waling exhibits signs of overloading before the props become overloaded and hence provides an

early warning of impending prop failure. 41

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PROP REMOVALProp removal is often the most critical stage of construction for both the prop and walls. It can be the

worst design case and is easily overlooked. Theremoval of props may cause the largest prop loads andwaling spans.

It is important that the general principles of the propremoval sequence are agreed between the temporaryworks designer and the site staff (and that anyconstraints arising from the permanent works are

identified). A situation in which the constructedsupport system is not sufficient to permit the sitestaffs preferred method and sequence of removalshould be avoided. 42

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It may be appropriate to adopt reduced partial safety factors for theelements of the support system during prop removal. This will depend primarily on:

the load increase from removal of other props

duration of increased loading

whether the increased loading is combined with the maximumtemperature rises

the amount of support offered by the constructed permanent works,and hence the consequences of potentially excessive prop

deformations.Where walings carry axial (in-plane) loads, the props will be acting asintermediate supports, so reducing the effective length of the waling. It isnecessary to check that the waling will not buckle when the props areremoved.

Methods of prop removal can increase the prop and waling loads. Propsmay be unloaded by jackmg the wall back but the movement required to dothis can give rise to very large increases in load. The structural capacity of the prop, connection, waling and wall may be exceeded unless such jacking

was allowed for in the original design of the support system. 43

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CIRIA 580 Geotechnical characterization of


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CIRIA 580 – Geotechnical characterization of

retaining walls

CIRIA 580 establishes design requirements

for geotechnical categories 1, 2 and 3. Prior to the geotechnical investigations, the

designer should assign a geotechnical

category to the earth retaining structure. Thecategory indicates the degree of effort

required for site investigation & design. This

should be reviewed and changed (if necessary)at each stage of the design and construction

process.44

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Geotechnical categorization (Simpson and Driscoll, 1998)


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Small & relatively simple?

Unusual & exceptionally

difficult ground?

Structure very large &

unusual?

Ground conditions known from

comparable experience to be

straightforward, routine design &

construction methods?

Excavation below water table &

comparable experience indicate

straightforward solution?

Site free of abnormal risks e.g.

unusual loading, seismic risk?

Negligible risk to life &

property?

CATEGORY 1

Small & relatively simple

Earth retaining system less

than 2m in depth

CATEGORY 2

Conventional

Retaining system supporting

soil & water Bridge piers & abutment

CATEGORY 3

All other earth retaining

systems

High seismic area?

Loading conditionsunusual or abnormal?

Abnormal risks?

No No

No

No

No

No Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

No

No

Yes

Geotechnical categorization (Simpson and Driscoll, 1998)

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CIRIA 580 Geotechnical categories EC7 (1995)


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CIRIA 580 – Geotechnical categories EC7 (1995)

Category 1Walls are small and relatively simple structures with thefollowing characteristics:

retained height does not exceed 2 m

ground condition are known from comparable experienceto be straightforward enough to allow routine methods of design and construction to be used

previous experience indicates that a site-specificgeotechnical investigation will not be required

there is negligible risk to property or life.

Comparable experience is defined as:

documented or other clearly established information relatedto the ground being considered in design, involving the sametype of soil & for which similar geotechnical behavior isexpected, & involving similar structures. Information gained

locally is considered to be particularly relevant. 46

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CIRIA 580 Geotechnical categories EC7 (1995)


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CIRIA 580 – Geotechnical categories EC7 (1995)

Category 2walls comprise conventional structures with no abnormalrisks or unusual or exceptionally difficult ground or loadingconditions. These walls require site-specific geotechnical

data (e.g. a desk study and ground investigation) to beobtained and analyses to be carried out.

The majority of embedded retaining walls fall intogeotechnical category 2.

Category 3

walls are structures or parts thereof that do not fall withinthe limits of geotechnical categories 1 & 2. These include

large or unusual structures, structures involving abnormalrisks, or unusual or exceptionally difficult ground or loadingconditions & structures in highly seismic areas.

Specialist advice should be sought to deal with special circumstances adequately.47

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CIRIA 580 – ON ANALYSIS


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CIRIA 580 ON ANALYSIS5 major elements of geotechnical design:

understanding the geological and

hydrogeological setting of the situ and its

environs & the historical development of

the site

determination of ground stratigraphy &groundwater conditions

understanding soil behavior

undertaking calculations & analyses

applying empiricism based on sound

judgment & experience.

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CIRIA 580 ON ANALYSIS

Retaining walls with a stabilizing base

In some circumstances, a wall with a

stabilizing base (i.e. a platform extending a short distance in front of the wall with a rigid

connection at formation level) can represent a

more economic solution than either a rigidly propped wall or an unpropped wall of deeper

embedment.

Finite element analyses by Powrie & Chandler

(1998) suggest an optimum stabilizing base

width of about ½ the retained height 49

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Forces acting on a stabilizing base retaining wall

Relief Platform

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Retaining walls with a stress-relieving platform

If some excavation and/or fill is needed on the retained side of

the wall, there maybe an advantage in constructing a stress

relieving platform, attached rigidly to the wall stem somedistance below the top and protruding horizontally into the

retained soil. The relieving platform will reduce bending

moments in the wall by:

(a) applying a reverse moment at platform level, due to theweight of the soil on top of it, &

(b) reducing vertical stresses in the retained soil below

platform level. For maximum efficiency, the platformshould extend far enough into the retained soil to reducevertical stresses adjacent to the wall, & there may need to be a void below it.

51

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Pressure redistribution - arching

Local variations in wall movement and rotation can, for

propped or anchored walls, lead to non-linearities in lateral

stress distribution. This redistribution of stress away from the

linear-with-depth variations assumed in simple limitequilibrium analyses can be exploited to reduce design bending

moments and wall depth if a soil-structure interaction analysis

is carried out.

Reduction of lateral stress

in the retained soil due toarching on to a rigid prop

52

52



53/136

For stiff wall, where the deflection at the level of the excavated soil surface

was of the same order an the deflection at the toe, the stress distribution infront of the wall under working conditions is approx. triangular. Measured

bending moment were in agreement with those from a limit equilibrium

calculation based on a fully active triangular stress distribution behind the

wall and a smaller-than-passive (is factored) triangular stress distribution in

front.

For flexible wall, so that the deflection at excavation level was significantly

greater than at the toe, the centroid of the stress distribution in front of the

wall under working conditions was raised. This led to smaller anchor loads

and bending moments than those given by the factored limit equilibriumcalculation.

Components of walldisplacement & definition

of a stiff wall

A stiff wall has e ≤ t

Deflection @

excavated soil

surface eDeflection @ toe, t

53

53



54/136

Changing wall EI to allow for cracking, creep of concrete54

54



55/136

The high short-term stiffness on OA is required to drop to the

lower long-term stiffness on line OBC. Consider an element of

structure that in the short-term baa been stressed to point A. In

the course of time, its state will move to be somewhere on line

BC. If it is in a situation in which there is no change of strainduring this change, stresses will simply relax and it will move to

point B. If, on the other hand, the load on the clement cannot

change, it wilt creep and move to point C.

If an element is at point A and the only change made is to

change the Young’s modulus in the data, further behavior will

proceed along tine AD. This does not represent creep or

relaxation. The soil-structure interaction analysis should ensurethat even if nothing moves, stresses will change from point A to

point B. If these new stresses are no longer in equilibrium, the

analysts should then indicate further strains such that the stress

state will move up line BC. 55

55



56/136

Wall flexural stiffness

Appropriate values of the flexural stiffness of the wall, EI,

should be used at each stage of the analysis to model wall

stiffness during construction & in the long term. Where E, is

the uncracked short-term Young’s modulus of concrete(typically, E0 = 28 GPa) & I is the 2

nd moment of area of the

section.

The calculated load effects & wall deflection will depend upon

the magnitude of the wall flexural stiffness adopted inanalysis. The value of EI assumed should be appropriate for

each construction & long term stage. For reinforced concrete

walls, this should allow for the effects of flexural crack &

concrete creep.

In subgrade reaction & pseudo-finite element analyses, it is

necessary to input explicitly the wall flexural stiffness EI for

all stages. 56

56



57/136

Reinforced concrete wallsFor reinforced concrete, the value of EI should strictly be

determined for the section with the value changes overtime &

with long term creep, equal to 50% of the short-term uncracked

value at infinity. EI should therefore be calculated at each

construction & long term stages.

It is appropriate to adopt 0.7EI & 0.5EI during the construction

& long-term stages respectively. The way in which the reductionin EI is applied in the analysis should be considered carefully in

a soil-structure interaction analysis. This approach is required

in moat available computer programs in which stiffness

represents response to load increments only. The same approach

may be used to model corrosion of steel sheet piles, in which I

reduces with time.57

57

CIRIA 580 – on steel sheet pile walls


58/136

Values of I for steel Larssen (U-profile), Frodingham (Z- profile), box and high-modulus piles are given by

manufacturers. The development of full section modulus in a

sheet pile wall is based on the assumption that any 2 adjacent

flanges are able to work together in bending (composite).

Z-profile steel sheet piles have their interlocks in the flanges to

develop the full section modulus of the combined wall

(BS8002). It should be noted that with Z-profile piles, theeffective section modulus will be reduced if the piles are

allowed to rotate about a vertical axis during driving: as a

rough guide, 5º of rotation will result in a 15% reduction in

the combined sectional modulus. The construction tolerances

compatible with the design assumptions must be specified in

this respect.58

58



59/136

CIRIA 580 on steel sheet pile walls

U-profile steel sheet piles incorporates an interlock

which is located on the centre line or neutral axis of

the wall. If the two piles are able to displace relative to

one another along the interlock, then the full modulusof the combined sections will not be realized. These

piles rely on the transfer of longitudinal shear stress

between adjacent piles through friction at theinterlocks or clutches. It is likely that shear will be

generated by surface irregularities, rusting, lack of

initial straightness & soil particle migration into theinterlocks during driving.

59

59



60/136

For U-profile sheet pile walls, it is common for toassume the full combined modulus, except in

circumstances where shear transfer may not be fully

effective, e.g.:• piles forming cantilever walls

• piles cantilevering a significant distance above or

below walings• piles driven into and supporting silts and/or soft clay

• piles retaining free water over a part of their length

• piles that are prevented (e.g. by rock or obstructions)

from penetrating to their required toe level.

60

60



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For the earlier circumstances, it is common by

welding, pressing or other means, to connect the U-

profile sheet pile sections together to develop the

necessary shear resistance so that the full combinedsection modulus can be relied upon in design.

Friction between the interlocks probably contributeat least 40% of the full section modulus

development.

Little is known about the effect of clutch slippage insheet pile walls; significant further research is

required in this area to improve understanding.61

61

CIRIA 580 – on axial stiffness of supports


62/136

In subgrade reaction & pseudo-finite element analyses, it isnecessary to input explicitly the axial stiffness, k (in kN/m

/m run) of any temporary or permanent props calculated as

follows:

k =AE(cos 2 )/Ls

Where

F = Young’s modulus of the material comprising the prop A = cross-sectional area of the prop

L = effective length of the prop (typically the half-width of

the excavation that the prop spans) s = prop spacing

= angle of inclination of the prop from the horizontal62

62

CIRIA 580 – on axial stiffness of supports


63/136

If concrete slabs are used to support the wall (e.g. ina top-down construction sequence), the calculated

axial stiffness of the slab should be reduced to allow

for any openings. For concrete stabs and props, theYoung’s modulus should be reduced to allow for the

effects of creep as described earlier.

63

63

CIRIA 580 – EFFECT OF METHOD OF ANALYSIS


64/136

Four generic retaining wall design & commercially

available software:

• limit equilibrium methods: STAWAL; ReWaRD

• subgrade reaction & pseudo-finite element

methods: FREW; WALLAP

• finite element & finite difference methods:

SAFE; PLAC

The problems analysed are defined in the followingfigures.

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Cantilever Wall - effective stress analysis

65

65



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Propped Wall - effective stress analysis

66

66



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Cantilever Wall - total stress analysis 67

67



68/136

Propped Wall - total stress analysis

68

68



69/136

The main conclusions:• in circumstances where there is little or no stress

redistribution, e.g. cantilever walls, simple limit

equilibrium calculations & soil-structure interaction

analyses (subgrade reaction or pseudo-finite element

methods & finite element or finite difference methods) are

likely to give similar wall embedment depth & wall

bending moments• for propped or anchored walls where stress redistribution

will occur, design by limit equilibrium calculations will

result in deeper walls with higher wall bending moments

compared with those obtained from soil-structureinteraction analyses. Use of soil-structure interaction

analyses may result in significant savings in wall material

costs, depending upon project & site-specific details 69

69


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71/136

• for walls embedded in soils where the total

horizontal pressures near the base of the wall on

the retained side are similar in magnitude to those

on the restraining side (soft clay), the results of

calculations will be very sensitive to relatively

small changes in pressures around the wall. The

results of such calculations will also beinfluenced by node spacing in beam spring &

pseudo-finite element models, & mesh details in

finite element & finite difference models. Thedesigner should carry out sensitivity checks on

the effects of such variations in the models

adopted. 71

71


72/136

CIRIA 580 – DESIGN PARAMETERS

Soil parameters required for various design approach


73/136

Soil parameters required for various design approach

Note: 1. Special input parameters required depending upon analytical medal adopted

73

73


Knowledge of the soil density (unit weight) & shear strength is


74/136

Knowledge of the soil density (unit weight) & shear strength is

essential in the design of an embedded retaining wall & also general

appreciation of the following soil properties:

classification and index properties, e.g. particle size

distribution, moisture content, plasticity indices (for fine-grained soils)

soil permeability.

Knowledge of in situ stress conditions, particularly the value of thein situ earth pressure coefficient K o, & soil stiffness is essential in

soil-structure interaction analyses.

Stiff over-consolidated soils have several soil strengths: peak,

critical state, residual, and drained or undrained. There is also a range of soil stiffnesses, depending on shear strain.

Backfill materials may require parameters for the determination of

compaction and swelling pressures.

74

74


st


75/136

The 1

st

step is to decide which soil parameters are appropriatefor a particular analysis. Then to consider other issues such as

reliability, selection of values for design & factors of safety.

It may be appropriate to adopt different selected values for a

parameter in different limit states & design situations. E.g., in

total stress analysis, the selected value of the undrained shear

strength of the clay should consider the mechanisms or modes

of deformation being considered for the wall. Differentstrengths will be required for a shear failure in fissured

material depending upon whether the shear surface in free to

follow the fissures or is constrained to intersect intact

material. A range of values should be considered. Thesevalues should also allow for any softening due to potential

changes in moisture content and the effect of excavation

disturbance. 75

75



76/136

Many soil parameters are not true constants but depend uponfactors such as stress & strain levels, mode of deformation,

type of analysis, etc. Under working conditions while

deformations are comparatively small, some or all of the soil

will operate at below peak strength. Under ULS wheredeformations are comparatively large, the soil may operate

beyond peak strength conditions & may dilate to approach

critical state values (BS 8002, 1994).

The designer of an embedded wall in a stiff over-consolidated

soil should decide on the appropriate strength to use in a

particular circumstance. The residual strength might be

appropriate where sliding along a pre-existing polishedrupture surface represents a potential failure mechanism, but

it will in general be far too conservative in other situations.

76

76


The choice is therefore usually between the peak & the critical


77/136

The choice is therefore usually between the peak & the critical

state strength & the following points should be borne in mind:

for a given soil, the critical state angle of shearing

resistance, Ø’crit, is a constant over the range of stresses

normally encountered in geotechnical engineering.Conversely, the development of a peak angle of shearing

resistance, Ø’ peak , depends on soil-structure & potential for

dilation. The latter depends in turn on the soil density &

the average effective stress during shear

failure at the peak angle of shearing resistance is brittle.

With continued post-peak deformation the soil strains &

softens, leading to the possibility of progressive failure.The factor of safety adopted in design should therefore

ensure that displacements & strains will not be large

enough to take the material into the post-peak range 77

77



78/136

the onset of large deformations tends to occur when about80% of peak strength is mobilized. This applies to a wide

range of soils

in an over-consolidated soil that fails by rapture, the peak

strength is easier to identity than the critical state

at a given effective stress, denser soils (of a particular type)

have both a higher stiffness & a higher peak strength. This

is particularly relevant when retaining walls are designed by the application of a factor of safety to the soil strength.

If critical state strengths are used in the collapse

calculation, a higher factor of safety would be needed for a

retaining wall in a loose soil than for an identical retaining

wall in a dense soil, for the wall movements under working

conditions to be the same.78

78


79/136

CIRIA 580 – DETERMINATION OF SOIL PARAMETERS


80/136

Certain laboratory tests bear the full imprint of disturbance, The unconsolidated undrained triaxial

test is a good example since no attempt is made to

reimpose the in situ stresses, hence it is particularly

prone to sample disturbance.

The effects of sample disturbance & limitations of

many laboratory tests have contributed to poor field predictions. This has contributed to greater use of in

situ testing, or at least of integrated laboratory & in

situ testing. A balanced view should be taken of theadvantages & limitations of both types of tests so

that they are included appropriately in a ground

investigation. 80

80



81/136

There is no reason why a shear strength derived from anin sits vane, pressuremeter or cone test should coincide

with that measured in a laboratory triaxial compression

or simple shear test. For a soil of a given composition,

deposition & post-deposition history, peak shear strength

will be influenced by the initial effective stress state, by

drainage during shear, by the stress path and the rate &

direction of shear. These will vary between the differenttypes of in situ & laboratory test and also the measured

strength. The small-strain stiffness behavior will also be

affected by the recent stress or strain history, imposed bythe sampling process. In view of stress-strain non-

linearity, comparisons are only meaningful if they are

made at corresponding levels of strain. 81

81


Soil parameters should be determined from several


82/136

independent sources:

directly from the results of in situ & laboratory tests

from established empirical correlations between different

types of in situ & laboratory tests and with the soil’sgrading & index properties

from relevant published data, and local & general

experiencefrom back analysis of measurements taken from

comparable full-scale construction in similar ground

conditions.

The selected soil parameters should encapsulate the designer’s

expertise & understanding of the ground and be based on both

site-specific information & a wider body of geotechnical

knowledge and experience.

82

82

CIRIA 580 – Classification properties

Classification tests


83/136

The results of classification testing are essential in

understanding material characteristics & behavior and are

necessary in the interpretation of in situ & laboratory testing.

The index properties below should be routinely determined for fine-grained & coarse-grained soils:

83

83

CIRIA 580 – Classification Permeability

The coefficient of permeability of soil, k, varies over a very


84/136

p y , , y

wide range of values from about 10-10m/s for practically

impervious clays to about 1 m/s for clean gravels. A range of

values for various soils is presented in BS 8004. This is

reproduced below which shows the mass permeability of fissured clays can vary over a wide range of values.

84

84

CIRIA 580 – Soil Stiffness


85/136

It is good practice to determine soil stiffness usingseveral different approaches. Current UK practice

includes specialist in situ self-boring preesuremeter

testing, geophysical testing & specialist sampling &laboratory small-strain stiffness measurement.

The self-boring pressuremeter is probably the most

robust means of determining soil stiffness at strainsrelevant for wall design across a broad range of over-

consolidated clays & very weak rocks in the UK.

The stress-strain behavior of soil is highly non-linear and soil stiffness decays with strain by orders of

magnitude.85

85

CIRIA 580 – Soil StiffnessAt very small strains of about 0.001%, the stiffness is large; at


86/136

strains close to failure, the stiffness is small. Atkinson andSällfors (1991) identity 3 regions of a typical stiffness strain

curve for soil:

86

86

CIRIA 580 – SELECTION OF DESIGN PARAMETERS

Th f ll i h ld b id d i l i i


87/136

The following should be considered in selecting appropriate parameters for use in design calculations:

geological & other background information, such as data

from previous projectsthe variability of the determined values, including

differences between the in situ conditions & the properties

measured by field and laboratory tests

the extent of the zone of ground governing the behavior of

the wall at the limit state being considered

the effect of construction activities on the properties of in

situ ground

changes that may occur in the field due to variation in the

environment or weather.87

87

CIRIA 580 – SELECTION OF DESIGN PARAMETERSUncertainty in the selection of soil strength, stiffness, loads and geometric

t f ti l i t i t i i ll d i Th i k


88/136

parameters are of particular importance in retaining wall design. The risksof soil strength and stiffness being less or greater than assumed, or

surcharge loads being greater, or of over-excavation or a rise in

groundwater pressures occurring, influence the factor of safety appropriate

for design. Three design approaches A, B & C are discussed in C580:

88

88



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Process for obtaining design values from test results.

89

89


( ) ( ) 1.64 ( )ck mCharacteristic Strength f MeanValue x Standard Deviation


90/136

)

( )Design Strength =

(ck

m

CharaceristicStrength f

Material Safety Factor

EXAMPLE:

10 concrete cubes were tested in compression at 28 days. The following

crushing strengths (N/mm²) were obtained:

44.5 47.3 42.1 39.6 47.3 46.7 43.8 49.7 45.2 42.7Mean strength xm = 448.9/10 = 44.9 N/mm²

Standard deviation σ = √[(x-xm)²/(n-1)] = √(80/9) = 2.98 N/mm²Characteristic strength f ck= 44.9 – (1.64×2.98) = 40.0 N/mm²

Design strength = 40/γm

= 40/1.5 = 26.7 N/mm²

stress

strain

40 N/mm²

26.7 N/mm²

2

1

1

n

m x x Standard Deviation

n

90

90


To find the 95% confidence level, for soil properties, as only a small portion of

th t t l l i l d i d i it ti i t t d it i t ibl t


91/136

the total volume involved in a design situation is tested, it is not possible to

rely on Normal Distribution.

For a small sample size the Student t value for a 95% confidence level may be

used to determine that Xck value, given by:

Some typical values of V (σ/xm) for different soil properties given by:

1ck m m

tV t x x x

n n

Soil PropertyRange of typical V

values

Recommended V Valueif limited Test results

available

tanφ’ 0.05 – 0.15 0.12

c’ 0.30 – 0.50 0.42

cu 0.20 – 0.40 0.32

mv 0.20 – 0.70 0.42

γ (unit weight) 0.01 – 0.10 0

91

91

df\p 0.40 0.25 0.10 0.05 0.025 0.01 0.005 0.0005

1 0.324920 1.000000 3.077684 6.313752 12.70620 31.82052 63.65674 636.6192

2 0.288675 0.816497 1.885618 2.919986 4.30265 6.96456 9.92484 31.5991

3 0.276671 0.764892 1.637744 2.353363 3.18245 4.54070 5.84091 12.9240

4 0 270722 0 740697 1 533206 2 131847 2 77645 3 74695 4 60409 8 6103

t table with right tail probabilities


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4 0.270722 0.740697 1.533206 2.131847 2.77645 3.74695 4.60409 8.6103

5 0.267181 0.726687 1.475884 2.015048 2.57058 3.36493 4.03214 6.8688

6 0.264835 0.717558 1.439756 1.943180 2.44691 3.14267 3.70743 5.9588

7 0.263167 0.711142 1.414924 1.894579 2.36462 2.99795 3.49948 5.4079

8 0.261921 0.706387 1.396815 1.859548 2.30600 2.89646 3.35539 5.0413

9 0.260955 0.702722 1.383029 1.833113 2.26216 2.82144 3.24984 4.7809

10 0.260185 0.699812 1.372184 1.812461 2.22814 2.76377 3.16927 4.5869

11 0.259556 0.697445 1.363430 1.795885 2.20099 2.71808 3.10581 4.4370

12 0.259033 0.695483 1.356217 1.782288 2.17881 2.68100 3.05454 4.3178

13 0.258591 0.693829 1.350171 1.770933 2.16037 2.65031 3.01228 4.2208

14 0.258213 0.692417 1.345030 1.761310 2.14479 2.62449 2.97684 4.1405

15 0.257885 0.691197 1.340606 1.753050 2.13145 2.60248 2.94671 4.0728

16 0.257599 0.690132 1.336757 1.745884 2.11991 2.58349 2.92078 4.0150

17 0.257347 0.689195 1.333379 1.739607 2.10982 2.56693 2.89823 3.9651

18 0.257123 0.688364 1.330391 1.734064 2.10092 2.55238 2.87844 3.9216

19 0.256923 0.687621 1.327728 1.729133 2.09302 2.53948 2.86093 3.8834

20 0.256743 0.686954 1.325341 1.724718 2.08596 2.52798 2.84534 3.8495

21 0.256580 0.686352 1.323188 1.720743 2.07961 2.51765 2.83136 3.8193

22 0.256432 0.685805 1.321237 1.717144 2.07387 2.50832 2.81876 3.7921

23 0.256297 0.685306 1.319460 1.713872 2.06866 2.49987 2.80734 3.7676

24 0.256173 0.684850 1.317836 1.710882 2.06390 2.49216 2.79694 3.7454

25 0.256060 0.684430 1.316345 1.708141 2.05954 2.48511 2.78744 3.7251

26 0.255955 0.684043 1.314972 1.705618 2.05553 2.47863 2.77871 3.7066

27 0.255858 0.683685 1.313703 1.703288 2.05183 2.47266 2.77068 3.6896

28 0.255768 0.683353 1.312527 1.701131 2.04841 2.46714 2.76326 3.6739

29 0.255684 0.683044 1.311434 1.699127 2.04523 2.46202 2.75639 3.6594

30 0.255605 0.682756 1.310415 1.697261 2.04227 2.45726 2.75000 3.6460

infinty 0.253347 0.674490 1.281552 1.644854 1.95996 2.32635 2.57583 3.2905

d

e g r e e s o f f r e e d o m d

f = n

−

1Probability density function

Cumulative distribution function

92

92


The Characteristic Value of the angle of shearing resistance ∅’ck

is required for

a 10m depth of ground consisting of sand for which the following ∅’ values


93/136

ck a 10m depth of ground consisting of sand for which the following ∅ values

were determined from 10 triaxial tests: 33°, 35°, 33.5°, 32.5°, 37.5°, 34.5°,

36.0°, 31.5°, 37°, 33.5°

Average angle of shearing resistance ∅’m = 34.4°

Standard Deviation σ = 1.97°

Coefficient of variation V = 1.97/34.4 = 0.057

Student t for a 95% confidence level with 10 test results = 2.26 (wrong!)

∅’ck = 34.4 - 1.97×2.26 / √10 = 33.0°

Design Value XD = Xck /γm & Applying γm = 1.25 for Case C

∅’D = arctan[(tan ∅’ck ) / 1.25] = 27.8°

Average angle of shearing resistance ∅’m = 34.44°

Standard Deviation σ = 2.91°Coefficient of variation V = 0.0509/0.6858 = 0.0742

Student t for a 95% confidence level with 10 test results = 1.833

tan∅’ck = 0.6858 – 0.0509×1.833 / √10 = 0.6563 (∅’ck = 33.27°)

Design Value XD = Xck/γm & Applying γm = 1.25 for Case C∅’D = arc tan [(tan ∅’ck ) / 1.25] = 27.70°

93

93


The Characteristic Value of the angle of shearing resistance ∅’ck

is required for

a 10m depth of ground consisting of sand for which the following ∅’ values


94/136

a 10m depth of ground consisting of sand for which the following ∅ values

were determined from 10 triaxial tests: 33°, 35°, 33.5°, 32.5°, 37.5°, 34.5°,

36.0°, 31.5°, 37°, 33.5° (Exel has built-in function to calculate these values)

wrong method σ 1.969207 in φ'

(X-Xm)² 1.96 0.36 0.81 3.61 9.61 0.01 2.56 8.41 6.76 0.81 34.9 in φ'

Average

φ' 33 35 33.5 32.5 37.5 34.5 36 31.5 37 33.5 34.4 in φ'

X = Tanφ' 0.649408 0.700208 0.661886 0.63707 0.767327 0.687281 0.726543 0.612801 0.753554 0.661886 0.685796

φ'm 34.44218

Xm = tan

φ'm0.685796

(X-Xm)² 0.001324 0.000208 0.000572 0.002374 0.006647 2.2E-06 0.00166 0.005328 0.004591 0.000572 0.023279

σ 0.050858 in Tanφ'

Correct method σ 2.91143 in φ'

94

94

CIRIA 580 – Design Approach A

M d t l ti il t


95/136

Moderately conservative soil parameters,

groundwater pressures, loads & geometry are

selected and safety factors are applied. Moderately

conservative in a cautious estimate of the value

relevant to the occurrence of the limit state. It is

considered to be equivalent to representative values

as defined in BS 8002 & to characteristic values asdefined in EC7 (1995). This should not be confused

with characteristic values (5% fractile) adopted in

structural engineering for materiel properties.

95

95

CIRIA 580 – Design Approach B

W t dibl il t d t


96/136

Worst credible soil parameters, groundwater

pressures, loads & geometry are selected and safety

factors lower than those in Design Approach A are

applied. This value is the worst that the designer

reasonably believes might occur - a value that is

very unlikely. As a guide, it may be regarded as the

0.1% fractile. Design Approach B is notappropriate for SLS calculations.

96

96

EUROCODE 7: Singapore Technical Reference for

Deep excavationThe term “moderately conservative” is taken to mean the “cautious


97/136

The term moderately conservative is taken to mean the cautiousestimate” of the value relevant to the occurrence of the limit state as in

CIRIA C580. It is also considered to be equivalent to the “representative

value” as in BS 8002 and to the “characteristic value” as in EC7.

“Worst credible” value is the worst value which is reasonably believedmight occur – a value that is very unlikely. It is considered to be equivalent

to the “conservative” value as in BS8002.

The ULS design shall be based on the most onerous of:

(a) Approach 1: Earth pressures derived from design values as defined inthis Section in which the reduction factors m in Tables 3.1a or 3.1b areappropriately applied to the moderately conservative parameters.

(b) Approach 2: Earth pressures derived from the worst credible parameters.

97

97

CIRIA 580 – Design Approach CMost probable soil parameters, groundwater pressures,

loads & geometry are selected and the safety factors of


98/136

loads & geometry are selected and the safety factors of Design Approach A are adopted. Most probable values have

a 50% probability of exceedance. Design Approach C should

only be used within an Observational Method process. It

should be used in conjunction with Design Approach B, toenable contingency measures to be developed for rapid

implementation in the event that conditions actually

encountered are less than the most probable. Thus, it is

unacceptable to proceed solely on the basis of Design

Approach C. The construction cost saving of this approach

should be offset against the costs relating to the additional

calculations to Design Approach B & those associated withthe development of contingency measures, the additional

monitoring & measurement systems necessary for the

implementation of the Observational Method. 98

98

CIRIA 580 – DESIGN PHILOSOPHY

Limit state design philosophy


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Limit state design philosophy

Design calculations should satisfy the ultimate limit states

(ULS) of wall stability & structural strength and the

required serviceability limit states (SLS) by verifyingsatisfactory performance in respect of wall deflections,

associated ground movement, wall watertightness criteria

etc. Neither ultimate or serviceability limit states should be

exceeded in the envisaged design.The factor Fs, should be applied on soil strength. The soil

design parameters derived therefrom should be used in

conjunction with the groundwater pressures, loads and

design geometries for collapse (ULS) calculations, SLS

calculations and the accidental design situation respectively.

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99


Fs factors appropriate for use in design calculations


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1. Effective stress: tan ø’d = tan ø’ / Fsø’ & C’d = c’ / Fsc’

Total stress: S ud = S u / Fssu

2. The design strength parameters in note 1 above are used to deriveearth pressure coefficients.

3. Not appropriate for SLS calculations.

F factors appropriate for use in design calculations

100

100


Deep excavation

Partial factors are to be used in the Ultimate Limit State


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Partial factors are to be used in the Ultimate Limit State

(ULS) design of the excavations system. The design

values of the geotechnical parameters Xd shall be derived

using:

Xd = Xk / m

in which Xk is the moderately conservative estimate of thesoil parameter and m is the reduction factor for the

parameter. For designs based on EC7, the reduction

factors (which are termed as partial factors in EC7) are

shown in Table 3.1a. For designs based on BS8002, thereduction factors (which are termed as mobilization

factors in BS8002) are shown in Table 3.1b.101

101


Deep excavation

Table 3.1a – EC7 Partial factors for soil parameters (m)


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p ( )(no case classification)

Soil parameter Symbol Value

Angle of shear resistance* ’ 1.25

Effective cohesion (1.6) c’ 1.25Undrained shear strength cu 1.4

Unconfined strength qu 1.4

Weight density g 1.0

* This factor is applied to tan’102

102

BS8002: Singapore Technical Reference for

Deep excavation

Table 3.1b – BS8002 Minimum factors for soil parameters (m)


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Soil parameter Symbol Value

Angle of shear resistance* ’ 1.2Effective cohesion c’ 1.2

Undrained shear strength cu 1.5

Unconfined strength qu 1.5

Weight density g 1.0

* This factor is applied to tan’

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103


Deep excavation


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This example (from CIRIA report 104, 1984) illustrates the ULS design to

determine the minimum penetration depth for a wall restrained with a strut (prop)

at a depth of 2 m below the original ground surface. The groundwater table is

assumed to be well below the tip of the wall. This worked example uses Approach

1 with moderately conservative values.

Soil and interface properties Values

Soil unit weight (kN/m³) 20

Friction angle ’ 25o

Cohesion (kPa) c’ 0

Interface friction (active side) = (2/3) ’

Interface friction (passive side) = (1/2) ’

104

104


Deep excavation


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105

105


Deep excavation

A l i d ti f t f ’ f 1 25


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Applying a reduction factor for

’ of 1.25,

the design d’ = tan

-1{tan(25o)/1.25} = 20.5o

Based on Caquot and Kerisel (1948) with design d’ = 20.5o, the active

coefficient K a = 0.44, and the passive coefficient K p = 2.70.

Take moments about the strut.

For equilibrium (Factor of safety = MP/MA = 1.0), the minimum depth of

penetration of the wall, d = 4.10 m.

Force (kN/m) Lever arm (m) Moment (kNm/m)

PA = 0.5K a(h+d)²= 3.6(8+d)²

LA = (2/3)(h+d) - 2

= (2/3)(5+d)

MA = 2.4(8+d)²(5+d)

PP = 0.5K pd²= 34.7d²

LP = (2/3)d + 8 - 2

= (2/3)d + 6

MP = 34.7d²(6 +2d/3)

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Design Approach A, subscript is mcFor effective stress analysis the limiting value of wall friction


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For effective stress analysis, the limiting value of wall friction, max,

should be taken to be:

max ≤ k ’crit,mc

where:

’crit,mc = moderately conservative critical state angle of shearing

resistance

k = 1.0 for rough concrete (e.g. concrete cast directly against

soil) and for a rupture surface within the soil;

k = 0.67 for smooth concrete (e.g. precast concrete or

concrete cast against formwork) and other smooth

surfaces (e.g. steel) and for driven or jacked in walls.

The value of the design effective wall adhesion, S’wd, should be taken aszero.

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107


For total stress analysis, design s u = s ud = s umc

where:


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s umc = modemtely conservative value of undrained shear

strength, s u.

The limiting value of wall adhesion, Swmax, should be takenas: Swmax = S ud

where:

= 0.5 in stiff clay. Smaller values of may apply in particular circumstances, e.g. steel sheet piles

driven through overlying soft clay.

For design approach B & C, subscript is wc & mp

respectively

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108

CIRIA 580 – STRUCTURAL DESIGN OF WALL

The structural design of the wall should conform to therelevant code of practice for the particular material namely


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relevant code of practice for the particular material, namely

ES8110 Part 1 (1997), BS 5400 Part 4 (1990) or EC2 Part 1

(DD ENV 1992-1-1: 1992) for reinforced concrete and ES

5950 Part 1 (2000), BS 449 Part 2 (1969) or EC3 Part 5 (ENV

1993-5, 1998) for structural steelwork.

The design of the structural members should allow for the

loads generated by the temporary & permanent constructionstages and the installation method.

Installation stresses are generated in pushed, driven or

vibrated sections. For concrete cast in situ, into a pre-

formed hole, the reinforcement detailing should allow for

the method of placing the reinforcement & concrete.

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CIRIA 580 – STRUCTURAL DESIGN OF WALL

ULS wall bending moments and shear forces for use in thestructural design of the wall should be obtained as the


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structural design of the wall should be obtained as the

greater of:

the values obtained from limit equilibrium calculation or soil-structure interaction analysis.

1.35 times the SLS values, where SLS calculations are

undertaken

the values calculated for accidental design

situation/progressive failure check.

values arising from the use of the Distributed Prop Load

method for the design of temporary propping to thewall.

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110

CIRIA 580 – Steel sheet pile walls

For driven sheet piling, the forces induced during thedriving process should out exceed the capacity of the


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driving process should out exceed the capacity of the

section.

Durability: Steel corrosion rates are generally low and steel

piling may be used for permanent works in an unpainted or

unprotected condition. The degree of corrosion and the

need for protection depends upon the working environment,

which can vary along the length & depth of the pile andwith time. Underground corrosion of steel piles driven into

undisturbed natural soils that do not comprise peat and are

not chemically contaminated is negligible. This is attributed

to the low oxygen levels present in undisturbed soils.Corrosion rates are higher where steel piling is exposed to

atmospheric conditions, fresh water and marine

environments. 111

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CIRIA 580 – Steel sheet pile wallsCorrosion rates for steel piling in natural environments (after BS 8002 1994)


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Note:

1. Fresh waters are variable. Corrosion losses in fresh water immersion zones are

generally lower than for seawater. 112

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CIRIA 580 – Steel sheet pile walls

The analysis o

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