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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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EUROCODE 7: GEOTECHNICAL DESIGN
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
1.258
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EUROCODE 7: GEOTECHNICAL DESIGN
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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EUROCODE 7: GEOTECHNICAL DESIGN
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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EUROCODE 7: GEOTECHNICAL DESIGN
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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EUROCODE 7: GEOTECHNICAL DESIGN
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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EUROCODE 7: GEOTECHNICAL DESIGN
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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EUROCODE 7: GEOTECHNICAL DESIGN
Design water pressures are affected by Tide and Rainfall
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EUROCODE 7: GEOTECHNICAL DESIGN
& 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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EUROCODE 7: GEOTECHNICAL DESIGN
& 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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Apparent Pressure Diagram for braced excavations in
soft clay with diaphragm wall
Soil profile andgeometry adopted for
parametric study using
computer program
EXCAV95
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Apparent Pressure Diagram for braced excavations in
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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Apparent Pressure Diagram for braced excavations in
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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Apparent Pressure Diagram for braced excavations in
soft clay with diaphragm wall
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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Apparent Pressure Diagram for braced excavations in
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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Apparent Pressure Diagram for braced excavations in
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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Apparent Pressure Diagram for braced excavations in
soft clay with diaphragm wall
Correlation among strut force exceedance ratio, Ei/cu value and cu*/H value.
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Apparent Pressure Diagram for braced excavations in
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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Condition For the Use of DPL Method
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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Condition For the Use of DPL Method
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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Condition For the Use of DPL Method
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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Condition For the Use of DPL Method
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
St t l id ti
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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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Structural considerations
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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Structural considerations
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Structural considerations
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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Structural considerations
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Structural considerations
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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Structural considerations
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Structural considerations
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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Structural considerations
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Structural considerations
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
44
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)
45
45
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
46
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
47
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.
48
48
CIRIA 580 – ON ANALYSIS
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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
49
CIRIA 580 – ON ANALYSIS
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CIRIA 580 ON ANALYSIS
Forces acting on a stabilizing base retaining wall
Relief Platform
50
50
CIRIA 580 – ON ANALYSIS
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CIRIA 580 ON ANALYSIS
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
51
CIRIA 580 – ON ANALYSIS
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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
CIRIA 580 – ON ANALYSIS
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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
CIRIA 580 – ON ANALYSIS
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Changing wall EI to allow for cracking, creep of concrete54
54
CIRIA 580 – ON ANALYSIS
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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
CIRIA 580 – ON ANALYSIS
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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
CIRIA 580 – ON ANALYSIS
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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
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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
CIRIA 580 – on steel sheet pile walls
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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
CIRIA 580 – on steel sheet pile walls
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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
CIRIA 580 – on steel sheet pile walls
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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
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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
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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
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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.
64
64
CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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Cantilever Wall - effective stress analysis
65
65
CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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Propped Wall - effective stress analysis
66
66
CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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Cantilever Wall - total stress analysis 67
67
CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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Propped Wall - total stress analysis
68
68
CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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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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CIRIA 580 – EFFECT OF METHOD OF ANALYSIS
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• 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
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CIRIA 580 – DESIGN PARAMETERS
Soil parameters required for various design approach
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Soil parameters required for various design approach
Note: 1. Special input parameters required depending upon analytical medal adopted
73
73
CIRIA 580 – DESIGN PARAMETERS
Knowledge of the soil density (unit weight) & shear strength is
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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
CIRIA 580 – DESIGN PARAMETERS
st
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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
CIRIA 580 – DESIGN PARAMETERS
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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
CIRIA 580 – DESIGN PARAMETERS
The choice is therefore usually between the peak & the critical
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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
CIRIA 580 – DESIGN PARAMETERS
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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
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CIRIA 580 – DETERMINATION OF SOIL PARAMETERS
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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
CIRIA 580 – DETERMINATION OF SOIL PARAMETERS
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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
CIRIA 580 – DETERMINATION OF SOIL PARAMETERS
Soil parameters should be determined from several
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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
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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
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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
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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
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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
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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
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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
CIRIA 580 – SELECTION OF DESIGN PARAMETERS
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Process for obtaining design values from test results.
89
89
CIRIA 580 – SELECTION OF DESIGN PARAMETERS
( ) ( ) 1.64 ( )ck mCharacteristic Strength f MeanValue x Standard Deviation
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)
( )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
CIRIA 580 – SELECTION OF DESIGN PARAMETERS
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
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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
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CIRIA 580 – SELECTION OF DESIGN PARAMETERS
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
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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
CIRIA 580 – SELECTION OF DESIGN PARAMETERS
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
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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'
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94
CIRIA 580 – Design Approach A
M d t l ti il t
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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
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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
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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
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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.
99
99
CIRIA 580 – DESIGN PHILOSOPHY
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
EUROCODE 7: Singapore Technical Reference for
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
EUROCODE 7: Singapore Technical Reference for
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’
103
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EUROCODE 7: Singapore Technical Reference for
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
EUROCODE 7: Singapore Technical Reference for
Deep excavation
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105
105
EUROCODE 7: Singapore Technical Reference for
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)
106
106
CIRIA 580 – DESIGN PHILOSOPHY
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.
107
107
CIRIA 580 – DESIGN PHILOSOPHY
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
108
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.
109
109
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.
110
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
111
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
112
CIRIA 580 – Steel sheet pile walls
The analysis o