lateral support - saice - geotechnical division support.pdf · geotechnical parameters design for...

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Back to Basics

Lateral Support

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Back to Basics

Lateral Support

Back to Basics

The ease or the fascination of carrying outcalculations leads many to believe that realistic results will emerge even if vital

subsurface characteristics are undetected, ignored or over simplified.

Ralph B Peck

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Back to Basics

The advent of powerful numerical analyses hasmade it possible to solve a wide range of boundaryvalue problems.The errors associated with simple classicalmethods of analysis are small with those that canoccur with sampling and testingThe emphasis should be on the accuratedetermination of simple parameters coupled withsimple calculations.

John Burland 1977

Back to Basics

MALEM EST CONSILIUM QUOD MUTARI NON POTEST

BAD IS THE PLAN THAT CANNOT BE CHANGED

Julius Caesar

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Historical Events Formalizing Lateral Support• Formalization required mainly due to deep basements

associated with the development of the city centre of Johannesburg (SAAN, Trust Bank, Standard Bank, Carlton Centre).

• 1963 Johannesburg City Engineers Dept. Design procedures for cable anchors.

• 1966 meeting with SAICE, SAACE and the City Engineer of Johannesburg.

• 1967 SAICE symposium on deep basements. • Code of Practice 1972.• Period of major technological advances in design and

construction• Code of Practice 1989.

Historical Events: Lateral Support Development• 1958 Stressed cable anchors. SA Mutual Port Elizabeth. Anchored

into rock. 400 kN anchors.• 1963 Carlton hotel site. Anchoring into residual soil. • Mid 1960,s: Carlton Centre, Trust Bank, Standard Bank.• Mid to late 1960’ s. Stressed cable anchors used for many

basements in Johannesburg CBD (SAAN, The Star, Holmer House).• +/- 1970: Reinjection of the fixed anchorage length.• Late 1970’s: The advent of design and construct.• +/- 1980: The concept of using the lateral support system as

temporary and permanent support. Temporary anchors with perimeter concrete piles and gunite arches.

• 1987: The soil nail revolution.• +/- 1990: The use of a permanent gunite wall together with

temporary soil nails.• 2005 : Self drilling anchors/soil nails.

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SAICE 1989 Code of Practice

SAICE 1989 Code of Practice

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Geotechnical Investigation

A client will pay for a good geotechnical investigation whether he has one done or not.......

Geotechnical InvestigationCLASS A: SIMPLELimited height and low surcharge loadsMovements not criticalWell known geological/geotechnical conditions

CLASS B: MODERATEModerate height and/or low to moderate surcharge loadsReasonable estimate of movement is requiredSome experience with similar geological/geotechnical conditions

CLASS C: COMPLEXDeep excavation and/or high and complex surcharge loadsRestriction s on lateral support system due to adjacent developmentsMovements are criticalComplex and variable geological/geotechnical conditions

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Geotechnical Investigation

Class ASimple

Class BModerate

Class CComplex

Good description of the soil profile for full depthTrial holes/test pits to the full depth of excavation.Basic laboratory tests

Good description of the soil profile for full depthTrial holes and boreholes.Strength tests.

Trial holes and boreholes with in situ tests.Good description of the geological conditions and the soil and rock profile.Assessment of geological/geotechnical conditions outside the site boundaries.Strength and/or compressibility tests.Special in situ tests.

Geotechnical Parameters

Design for Stability

• Effective strength parameters • Peak or Residual. Structure of residual

soil. Relic joints slickensides.• The cohesion conundrum

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Laboratory Testing for Shear Strength Parameters

• Triaxial • Shear Box• The effects of testing procedures

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No Strength Testing Available(Usually the case!!!!)

• A good description of the soil profile is essential.• Index property tests are a help.• Previous experience of similar site conditions.• Back analysis of existing slopes.

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Externally Stabilized Lateral Support Systems

Summary of Input Design Parameters• Surcharge loads• Water table• Effective Cohesion• Effective angle of friction • Bulk density and submerged density• Active earth pressure coefficient: Ka = 1-sinϕ/1+sin ϕ

pa = (ƔZ+Q)Ka – 2C’ √Ka• Passive earth pressure coefficient: Kp = 1 + sin ϕ/1-sin ϕ

pp = (ƔZ+Q)Kp + 2C’ √Kp• Coefficient of earth pressure at rest

Ko = Normally Consolidated =1-sin ϕ. Over consolidated = 1-sin ϕ √OCR

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Wall Friction

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Cantilever Wall

Simple analysis methods using active andpassive pressure distributions accuratelypredict the embedment lengths for stabilityand produce conservative estimates ofstructural forces.

Cantilever Wall

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Cantilever Wall

Cantilever Wall

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Cantilever Wall

Wall Stability. Moments about the Toe

1. Factor of safety on passive resistance 2. Factor of safety on net passive resistance3. Methods 1 and 2 with a factor of safety

on strength4. The use of a multiplying factor

Cantilever Wall. Stability

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Cantilever Wall. Bending Moment

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Walls with one prop

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Wall with one prop. Free earth supportWall can deflect and rotate at toe

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Wall with One Prop. Free Earth Support

STABILITYSimple limit equilibrium methods of analysisusing Ka and Kp give an acceptable depth of embedment.

Moments of earth pressure about the anchor/prop

Active and passive pressure Net active and passive pressure

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Wall with one prop

Wall Stability. Moments about the Anchor

1. Factor of safety on passive resistance 2. Factor of safety on net passive resistance3. Methods 1 and 2 with a factor of safety

on strength4. The use of a multiplying factor

Walls with one prop

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Anchor force

ANCHOR FREE LENGTH

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Bending MomentsEarth pressure at working conditions difficult to predictCalculated for earth pressure at limiting equilibriumWall stiffness an important parameter. Likely to be an over prediction for walls of low stiffness.

Hadfield Wall.Day and Potts

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Hadfield Wall.Day and Potts

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Wall StiffnessPotts and Bond (1994)

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Multi propped walls

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Multi propped wallsMain Calculations• Anchor/prop loads required for stability• Magnitude and distribution of bending moments

Complex soil structure interaction problem. Dependant mainly on:• Type of soil (strength and stiffness)• Type and sequence of construction• Stiffness of the structure

Simple/limit equilibrium methods of analysis are based on certain theoretical considerations together with empirical procedures. In many cases the accuracy of these methods needs to be treated with caution.

Analyses must be carried out for all stages of construction to determine the maximum anchor forces/prop loads and bending moments

Force for stability• Pecks diagrams.• Assume an earth pressure distribution.

Ka to Ko depending on an assessment of wall stiffness and wall movement. Kp if applicable.

• Slope stability analyses (single wedge, multi wedge, log spiral, compound failure surfaces).

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Force for stability. Simple wedge analysis

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Bending moments simple limit equilibrium

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Bending moments.

Bending moments

Fictitious prop where net earth pressure is zero

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Anchor free length

JHB CED Design ProcedureThe anchors should extend deeply enough so as to ensure that no surface which passes behind the points of anchorage has a factor of safety of less than 1,5 calculated on the strength of the retained material

Anchor free length

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Other design considerations

Stability of the lateral support system• Overall stability• Base heave• Vertical movement of wall elements due to steep

anchor inclinationsAnchors• Stressing and testing• Corrosion protection

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Anchors

AnchorsWorking capacity controlled by COP requirements for stressing.Temporary: 80% of yield at 125% of working loadPermanent: 80% of yield at 150% of working load

Strand: 265 kN. Temporary: 170 kN/strandPermanent: 140 kN/strand

Threaded bars: 550 MPa to 1050 MPaSelf drilling systems: Yield 180 kN to 1600 kN

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Soil nails

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Summary of Input Design Parameters• Surcharge loads• Water table• Effective Cohesion• Effective angle of friction • Bulk density and submerged density• Active earth pressure coefficient: Ka = 1-sinϕ/1+sin ϕ

pa = (ƔZ+Q)Ka – 2C’ √Ka• Passive earth pressure coefficient: Kp = 1 + sin ϕ/1-sin ϕ

pp = (ƔZ+Q)Kp + 2C’ √Kp• Undrained shear strength of cohesive soils• Tensile strength of soil reinforcement

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Internal Stability

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Important design concept. Internal Stability

With an adequate overall FOS the maximumshear force that is developed in the nails is alwaysa very small proportion of the axial tensile capacityof the nails.It is therefore acceptable to base the design ofinternal stability only on the mobilization of tensileforces in the nails.

Jewell and Pedley (1990)

Internal Stability

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Soil nails

Grout/soil bond

Classic conceptT = c’ + δ’r tanφ’δ'r = K δ’v

Implies that there is a linear increase in bond with depth

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Grout/soil bond

Grout/soil bond

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Grout/soil bondResidual granite (silty sand). Ultimate bond = 200 tanφ’ or 4 φ’

Grout/soil bondResidual diabase cohesive clayey soil Ultimate bond: 2N

Cu = 5N. Ultimate bond = 0,4Cu

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Internal Stability

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Soil Nail Applications• Lateral support for excavations of limited height and

where moderate movements can be tolerated.• Stabilization of road and railway cuts.• Stabilization of cuts for road widening.• Stabilization of abutment fills for road widening.• Repair and reconstruction of existing retaining walls.• Hybrid walls for road widening together with other soil

reinforcement systems.

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Soil nail problem areas• Sites where deformations need to be limited• High surcharge loads. Usually occur together with the

requirement to limit deformations.• Requirement for stable excavated benches. Problem with

non cohesive soils or closely jointed residual soils below the water table.

• Soft clays. High support forces required are incompatible with the low grout/soil bond available.

• Drilling in boulder and gravel formations. Can be overcome with self drilling systems.

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Additional Essential Design Processes

ENGINEERING JUDGEMENT

The application of sound “Judgement” is the Heart of good geotechnical engineering.

Gut feel based on the analysis of data is a valid part of good geotechnical engineering practice.

Engineering judgement involvesassembling all the data you can, doing allthe calculations you can and then on thebasis of a good bottle of brandy and agood nights sleep , making your decision.J. E. Jennings

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Engineering Judgement

Sub- surface engineering has developed largely on the basis of case histories tempered with scientific knowledge. If we ever reach the situation in which we believe that engineering science can replace experience and judgement based on such experience, disaster is not far away.

John Burland 1975

Additional Essential Design ProcessesThe Observational Method of Design

Continuous managed and integrated process of construction control, monitoring and review to enable modifications to be incorporated during construction if necessary.

The objective is to apply the most appropriate design and construction procedures without compromising safety.

The construction phase essentially becomes an extension of the design phase

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Failures Near Failures and Movements

No excavation can be made without causing some movement of the surrounding ground1989 Lateral Support COP

Stress is an abstract concept.Strain is the physical realityJohn Burland


Correctly applied limit equilibrium analyses are acceptable in terms of the analysis of overall stability of a retaining system.

Failures defined as the overall collapse of the lateral support system are rare.

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Failures Near Failures and MovementsWith limit equilibrium analysis procedures it is not possible to predict movements.

Predictions are based on experience of excavations in similar conditions and general rules of thumb.


Rules of thumbActive support: 0,1% to 0,2% of the heightPassive support: 0,15% to 0,3% of the height

Unfortunately mother nature does not always play by the rules

The role of advanced numerical analyses

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Case history: Bank City

GrabenSite

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Case history: Bank CityM

ovem

ent

(mm

)

Maximum

Average


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Case history: Bank CityM

ovem

ent

(mm

)

Str

ess

B’

Str

ess

C’


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Mov

emen

t (m

m)

Str

ess

B’

Str

ess

C’

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Failures Near Failures and MovementsMain Contributing Factors (Never a single Factor):• Inadequate geotechnical investigation leading to unforeseen

conditions.• Inadequate definition of surrounding site conditions.• Incorrect/optimistic design concept or design procedures.• Poor lateral support construction practice .• Localized small failures or fall outs during excavation.• Unauthorized over excavation by the earthworks contractor.• Vibrations due to uncontrolled blasting. • Not adopting or the incorrect application of the observational

method of design.• Unreasonable pressure from the client and/or the project

manager.

If you can keep your head when all about youare losing theirs and blaming it on you,If you can trust yourself when all men doubt you,but make allowance for their doubting too;

If.......

If you can meet with Triumph and DisasterAnd treat those two imposters just the same

If....

And which is more you will be a Man, my Son

Rudyard Kipling

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Lateral Support Back to BasicsWhere to now???Experience has shown that our engineers have the capability to design lateral support for deep and complex excavations.

Explore the benefits of numerical analyses.

Improve the information provided for design.• Geotechnical investigation• Structural and layout details

Full scale monitoring of excavations.

lateral support - saice - geotechnical division support.pdf · geotechnical parameters design for...

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