Presentation-John Gannon - Bored Pile Retaining Walls 2010[1]

7/27/2019 Presentation-John Gannon - Bored Pile Retaining Walls 2010[1]

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Bored Pile Retaining WallsBored Pile Retaining Walls

John GannonJohn GannonBylandByland Engineering LimitedEngineering Limitedwww.bylandengineering.comwww.b

ylandengineering.com



IntroductionIntroduction - - speaker speaker A a chartered civil engineer with >25 years experience with site A a chartered civil engineer with >25 years experience with siteinvestigation contractors and consulting engineers.investigation contractors and consulting engineers.

Director of independent geotechnical design practice since 2001.Director of independent geotechnical design practice since 2001.

Provides engineering services to Leeds City Council Geotechnical UnitProvides engineering services to Leeds City Council Geotechnical Unit

and therefore the other Yorkshire councils.and therefore the other Yorkshire councils.

Experience in most construction sectors including design andExperience in most construction sectors including design andconstruction of highway structures, earthworks and drainage.construction of highway structures, earthworks and drainage.

Familiar with ground conditions of West Yorkshire.Familiar with ground conditions of West Yorkshire.

Has designed dozens of bored pile walls across UK.Has designed dozens of bored pile walls across UK.

John Gannon Bored Pile Retaining WallsJohn Gannon Bored Pile Retaining Walls BylandByland EngineeringEngineering



John Gannon Bored Pile Retaining Walls Byland EngineeringJohn Gannon Bored Pile Retaining Walls Byland Engineering

Introduction : popularityIntroduction : popularity –– why?why?

•• Increased awareness / growing track recordIncreased awareness / growing track record•• Easier & quicker to design (software)Easier & quicker to design (software)•• Land scarce and values risingLand scarce and values rising

•• Faster hence reduced piling costFaster hence reduced piling cost•• Able to penetrate denser and harder ground Able to penetrate denser and harder ground•• Improved qualityImproved quality

•• Improved verticalityImproved verticality




Introduction – what are they ?•Replacement vertical piles formed of concrete

•Bored and cast-in-situ•Wholly or partially reinforced or not reinforced at all (usuallycages, might be sections e.g. UC, CHS)

•Concrete strength typically 35 MPa for structural piles•Piles may be spaced out up to 3d (king post), practically

touching (contiguous) or intersecting (secant)•Soil between piles “arches”•Temporary or permanent•Cantilever or propped•Usually drained, lined and / or clad

•Varying degrees of waterproofness•Can accept heavy imposed loads e.g. column reactions•Retained heights to say 30m•Cantilever retained heights to 8m or 10m



IntroductionIntroduction – – what are they ?what are they ?





Site InvestigationSite InvestigationPurposePurpose is to provide information to theis to provide information to the

designer designer andand contractor contractor for for •• Engineering feasibility studiesEngineering feasibility studies

•• Permanent works designPermanent works design•• Temporary works designTemporary works design•• Construction planningConstruction planning•• Risk assessmentRisk assessment•• PricingPricing



Site InvestigationSite InvestigationGood PracticeGood Practice is to undertake andis to undertake and reportreport thesethese

elements :elements :•• Thorough desk study including services searchThorough desk study including services search•• Topographic surveyTopographic survey•• Intrusive ground investigationIntrusive ground investigation•• Careful interpretationCareful interpretation

•• Design report with risk register Design report with risk register •• Performance feedbackPerformance feedback





Ground InvestigationGround Investigation Do’sDo’s1.1. Boreholes below pile toeBoreholes below pile toe2.2. Record groundwater ingressRecord groundwater ingress3.3. Measure groundwater levelsMeasure groundwater levels – –

several visitsseveral visits4.4. Record surface levelsRecord surface levels5.5. Test soil : SPT, index &Test soil : SPT, index &

classification, electroclassification, electro- -chemical,chemical,shear strength, stiffnessshear strength, stiffness

6.6. Retained ground slope andRetained ground slope andsurchargessurcharges

7.7. Long term groundwater level inLong term groundwater level inretained groundretained ground

8.8. Obstructions e.g. concrete,Obstructions e.g. concrete,masonry, bouldersmasonry, boulders

9.9. Variation in rockVariation in rock- -head level andhead level androck strengthrock strength



Ground InvestigationGround Investigation Do’sDo’s


Low water table : Za > 0.4h and > (t + 2m)High water table : Za > (H+2m) and > (t + 2m)



Site InvestigationSite Investigation


Soil Mechanics Reminders :

• Τ = c + σ’ n tan φ

• σ’n = σ n – u

• Τ can be measured directly in a total stress approach , pore fluid pressure notbeing considered. For example when the shear strength is measured in a vane test

or in a quick un-drained tri-axial compression test. Either S u (un-drained shear strength) or c u (un-drained cohesion which implies φ = 0) is provided.•Or Τ can be measured in an effective stress approach where the total stress andthe pore fluid pressure is considered. For example when shear strength ismeasured in a tri-axial compression tests in which drainage is permitted or porepressure measured so that c’ and φ’ are obtained.

• Soil Mechanics meaning of drainage : condition of the soil which in a response toa change in total stress, the pore fluid pressure ( water and / or air) has reachedequilibrium (drained) or not (un-drained). Rate of load change and soil permeabilitygovern.



Site InvestigationSite Investigation


West Yorkshire ground issues for Walls :

• Ground sloping along and especially across

• Made ground and deep in-filled valleys

• High level rock-head, very irregular

• High water table

• Variable rock strength from very weak (clay like) mudstoneto very hard (gritstone)

• shallow mine workings




General designGeneral designThe keyThe key considerationsconsiderations are :are :

•• Choice of wall type & construction methodChoice of wall type & construction method•• DurabilityDurability••

Support arrangements and stiffnessSupport arrangements and stiffness•• Geotechnical analysis & designGeotechnical analysis & design•• Structural analysis & designStructural analysis & design•• Current and emerging standardsCurrent and emerging standards



General designGeneral design – – wall typewall type


Choice of wall type depends on :

•Function – e.g. Support to earth, water, imposed loads,allowable deflections.

•Durability& Service life – temporary works <2yrs to 120 yearspermanent highway structure.

•Buildability – working width, height, allowable rig size,proximity to buildings, type of ground to be excavated, requiredverticality, difference between piling level and casting level.

•Appearance – does wall need to be clad or lined or waterproofed.

•Minimum cost always a driver.



General designGeneral design - - durabilitydurability


•Concrete cast against the ground normally has a nominal minimum cover of 75mmwhich comprises minimum cover of 60mm and a fixing tolerance of 15mm.

•Careful use of cover spacers to reinforcement cages essential. For ContinuousFlight Auger (CFA) piles where cage pushed in last, rigidity of spacers can beimportant. Wheel spacers can come off.

•Normal pile mix is a C28/35 concrete with MAS of 20mm.

•Can use grade 40 or even grade 50 concrete but avoid in CFA because it is toostiff to plunge the cage. The load in the pile seldom justifies high strength concrete.

•Good concrete placement technique, control of cover and reinforcement detailingto limit crack widths are best means of promoting good durability.

•Extensive use of GGBS and PFA cement replacement and use of super-plasticisers in mix designs common and aid durability at expense of slower strengthgain. Slump class S3 (80-180mm) or class S4 (140 -240mm) usual. Might bepumped or tremied from a hopper. Can still obtain DC-3 and possibly also DC-4classification without APM‘s ( see BRE SD1), depending on structural performancelevel, exposure class and section size.



General designGeneral design – – support arrangementssupport arrangements




Geotechnical designGeotechnical design - - analysisanalysis


Analysis should consider :

• All stages of construction and service. Hence temporary andpermanent works design are integrated.

•Pile length required to ensure horizontal equilibrium.

•Pile length required to ensure vertical equilibrium. particularly wherethere are imposed loads, for example in embedded integral bridgeabutments.

•Pile diameter and length and support arrangements to control wall

deflections and associated ground movements.•Ability of the soil to arch where king post walls are required.



Geotechnical designGeotechnical design - - analysisanalysis


A few concepts :

Geotechnical category (as BS EN 1997 part 1) :

1 Small and simple, retained height less than about 3m2 Intermediate3 Large and unusual

Support Stiffness categories :

High : Top down construction, high level temporary props before high levelpermanent props (e.g. Integral bridge on embedded wall abutment)

Mod : Temporary props of high stiffness before permanent low level props

Low : cantilever walls, temporary props of low stiffness



Geotechnical designGeotechnical design – – limit stateslimit states


Limit state design philosophy : basis of Eurocodes and

some earlier standards e.g.BS8002, BS5400 etc.

ULS : loss of equilibrium, failure by rotation, structuralfailure of an element or of a structure supported by the wall

etc.

SLS : unacceptable wall deflections and associatedground movements, unacceptable leakage etc.



Geotechnical designGeotechnical design – – groundground

movementsmovements


Source :• construction of the wall• excavation in front• loss of soil by flowing water • thermal affects (particularly integral bridges where deck acts as prop)



Geotechnical designGeotechnical design –– analysis stagesanalysis stages




Geotechnical designGeotechnical design – – earth pressuresearth pressures




Geotechnical designGeotechnical design ––earth pressuresearth pressures


Soil Mechanics Reminders :

• σ’h = K σ’ v

• starting value of K is earth pressure at rest, Ko

• Ko falls as soil is put into tension and expands as at points B

• Ko rises as soil is put into tension and compresses as at points A

• Limiting values (min and max) are, for cohesionless soils, known as Ka and Kp, theactive and passive earth pressure coefficients

• The value of K depends on soil stress history, the mobilised friction angle and

effective cohesion, the soil drainage state, the amount of movement that ispermitted and the friction force or adhesion which develops between soil andwall as movements occur

•Equations and charts are given in BS8002 and BS EN 1997 Part 1 (Annex C)





Walls movements required to enable active and passive pressures to be mobilised



Geotechnical designGeotechnical design – – earth pressuresearth pressures




Geotechnical designGeotechnical design – – stability analysisstability analysis•• Considers potential failure mechanisms.Considers potential failure mechanisms.•• Factor of safety calculation to establish toe elevation usesFactor of safety calculation to establish toe elevation uses limitinglimiting

values of earth pressure.values of earth pressure.•• Various definitions of FOS.Various definitions of FOS.•• Strength factor method most commonly usedStrength factor method most commonly used – – margin by which soilmargin by which soil

shear strength must be reduced to bring about equilibrium.shear strength must be reduced to bring about equilibrium.

•• Factored strength = characteristic strength / partial material factor.Factored strength = characteristic strength / partial material factor.c’des = c’ / γm ; cudes = cu/ γm ; tanφ’des = tan φ’ / γm (NB M inBS8002)

•• Partial factors depend on limit state being considered as well as howPartial factors depend on limit state being considered as well as how

soil strength is considered and the desired margin of safety.soil strength is considered and the desired margin of safety.




Geotechnical designGeotechnical design – – limit statelimit statecalculationscalculations

ULS (collapse) calculationsULS (collapse) calculations – – current practice (C580)current practice (C580)q min 10q min 10 kPakPa , unplanned over , unplanned over--dig 10% H or 0.5m, pile length that givesdig 10% H or 0.5m, pile length that gives

overturning equilibrium enhanced by 20%.overturning equilibrium enhanced by 20%.

Value of Value of δ δ,, cwcw etc variesetc varies – – see C580see C580Shear strength may be peak, critical state, residual, drained or unShear strength may be peak, critical state, residual, drained or un- -drained.drained.SLS calculationsSLS calculations –– current practicecurrent practiceDesign approach A or C,Design approach A or C, γ γm = 1.0m = 1.0


Approach γm Charac. shear strength

Comment

A 1.2 or 1.5 Mod cons.

B 1.0 Worst credible

C 1.2 or 1.5 Most probable With OM



Geotechnical designGeotechnical design – – stability analysisstability analysisSingle Pile Vertical Pile Capacity (Single Pile Vertical Pile Capacity (Q Q uu)) – – current approach (BS8004)current approach (BS8004)

QuQu == QusQus ++ QubQub

QusQus == ππ d ld l ΤΤsusu

QbuQbu == ππ/4 d^2/4 d^2 σσbubu

Conventionally (BS8004)Conventionally (BS8004) Qa Qa == QuQu / F ; F > 2.0/ F ; F > 2.0

IncreasinglyIncreasingly QaQa == QusQus / f1 +/ f1 + QusQus /f2/f2

QuQu also adjusted for pilealso adjusted for pile spacingsspacings, as in pile group design, as in pile group design




Geotechnical designGeotechnical design –– overall stabilityoverall stability




Geotechnical designGeotechnical design –– overall stabilityoverall stability


h l dh l d l l lblb



Geotechnical designGeotechnical design –– limitlimit equilbriumequilbrium

methodsmethods


For statically determinate structures e.g. cantilevers and single propped cantilevers

Fixed earth support

Free earth support



Geotechnical designGeotechnical design – – analysisanalysisSoftware :Software :

A Limit equilibrium. To calculate wall length e.g. A Limit equilibrium. To calculate wall length e.g. StawalStawal

BB SubgradeSubgrade reaction /reaction / PsuedoPsuedo finite element. To simulate wallfinite element. To simulate wall

behaviour. Will calculate all wall load effects using simple soilbehaviour. Will calculate all wall load effects using simple soilmodels, e.g.models, e.g. WallapWallap, , FrewFrew

C Continuum models (FE/FD). To simulate wall and ground behaviour.C Continuum models (FE/FD). To simulate wall and ground behaviour.

Will calculate system behaviour in 2D or 3D using complex soilWill calculate system behaviour in 2D or 3D using complex soilmodels if requiredmodels if required e.ge.g FlacFlac,, PlaxisPlaxis, Crisp, Crisp




Geotechnical designGeotechnical design – – analysisanalysis




EurocodeEurocode 7 7


Limit state design, verification by partial factor method of calculation :

LIMIT STATES

EQU – loss of equilibrium (ULS)STR – internal failure or excessive deformation of elements (ULS/SLS)GEO – failure or excessive deformation of ground (ULS/SLS)

DESIGN APPROACHESConsider action and material factor to check strength or resistance

In UK design approach 1 (DA1) , either combination (of partial factors) 1 or 2(C1, C2). C1 usually to determine pile toe depth, C2 usually to determine

section forcesDESIGN SITUATIONS

Persistent, transient, accidental, seismic





ACTIONS (LOADS) ( earth pressure, water pressure , imposed, thermal etc.) :

Fd = γ F F rep or Fd = γ E F rep and F rep = ψ F k (generally ψ = 1.0)

γF varies 0 to 1.7 depending on limit state, nature of action and combinationWater pressure may be dealt with by increase in level

MATERIALS (soil, concrete, steel)

Xd = Xk / γ m

γm varies 1.0 to 1.5 depending on material type and combination

GEOMETRICAL DATA

RESISTANCES

Rd = Rk / γ m

γm varies 1.0 to 1.6 depending on calculation set, pile type, nature of action (T,C)





GROUNDWATER

EC7 provision for water table very onerous :

dw = H/4 for H<4m

dw = 1m for H>4

“Where no reliable drainage present”





Philosophy reduced to its simplest :

Action x Partial Factor = Design Action (DA)

Action Effect x Partial Factor = Design Action Effect (DEA)

Representative Strength / Partial Factor = Design Strength (DS)

Resistance / Partial Factor = Design Resistance (DR)

DS ≥ DA or DEA

DR ≥ DA or DEA

Note practice of C580 not same as EC7 and several “conflicts” yet tobe resolved (design approaches and partial factors not the same)



Structural designStructural design –– failure modesfailure modes




Structural designStructural design - - checkschecks


Current routine calculation checks :

• Ac > N / 0.25 fcu ( N / 0.4 fcu when N is factored or design load)

• Asc > 0.5% Ac (or as Table 7.6.2.2 BS EN 1536) NB 0.3% in BD42

• Md / h^3 ; N / h^2 : use column design charts or calculation

to get Asc

• Asv : check Vd < 0.75 (fcu^0.5) or 4.75 N/mm2

Asv typically 0.4 bv Sv / 0.95 fy or similar

• N < buckling load of the column and check additional eccentricitymoments (e.g. section 5.5 of BS5400)





Detailing rules from BS EN 1536 :

• maximum clear spacing of bars 400mm (300mm BD42)

• minimum clear spacing of bars 100mm

• maximum aggregate size 20mm

• cover 75mm

• link diameter not less than 0.25 x main bar diameter, min

6mm dia

• link spacing not more than 0.75 d





Some design and detailing rules from BD 42/00 etc:

• Additional longitudinal shear steel Asv :

Asv > Vd / 2 (0.87 fy)

• Control of early thermal cracking :

As = fct Ac / fy where fct = 0.37 (fcu^0.5)

• Asc : minimum 6 bars, not less than 12mm diameter

Asc > 0.15N / fy

• Lap lengths and anchorage as per BS 5400 Part 2 Table 15

• Control of crack widths :

“ has little effect on the corrosion of reinforcement for buried elements innon-aggressive ground”




Cantilever wall he ight vs normalised moment - dry soil

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0

M/h^3 (N/mm2)

H ( m )

350/500 450/600 600/750 750/900 900/1050

Structural designStructural design -- preliminary pile sizingpreliminary pile sizing




Design standardsDesign standardsNowNowBS8002, BS004 & BS8081BS8002, BS004 & BS8081BS8106BS8106BS8110 & BS5400BS8110 & BS5400BS EN 206 & BS EN 8500BS EN 206 & BS EN 8500BS EN 1536BS EN 1536BRE SD1BRE SD1CIRIA C580CIRIA C580ICE SPERWICE SPERWHA advice and design notesHA advice and design notes

Here & ComingHere & ComingBS EN ISO 14688BS EN ISO 14688BS EN ISO 22476BS EN ISO 22476BS EN 1990BS EN 1990BS EN 1991BS EN 1991BS EN 1992BS EN 1992BS EN 1997BS EN 1997

Design detailsDesign details




Design detailsDesign details

What the industry prefers :What the industry prefers :

CantileversCantileversSingle high level propsSingle high level propsCorner bracingCorner bracing

Avoid raking props Avoid raking propsPassive self drill tie backs (usually temporary)Passive self drill tie backs (usually temporary)

Anchor head blocks instead of Anchor head blocks instead of walingswalingsPermanent structure propsPermanent structure props



DesignDesign - - summarysummaryKey Points :Key Points :

•• Define geotechnical categoryDefine geotechnical category•• Define limit states and performance criteria to be metDefine limit states and performance criteria to be met•• Failures seldom due to structural failure of wallFailures seldom due to structural failure of wall

•• PermissablePermissable movements depend on sensitivity of surroundsmovements depend on sensitivity of surrounds•• Flexible walls with many props give similar displacements to stiff Flexible walls with many props give similar displacements to stiff

walls with fewer propswalls with fewer props•• Ground movements cannot be accurately predicted but they can andGround movements cannot be accurately predicted but they can and

should be estimated. All walls and supported ground moveshould be estimated. All walls and supported ground move•• Early high propping effective in controlling ground movementsEarly high propping effective in controlling ground movements


B d Pil C iB d Pil C i



Bored Pile ConstructionBored Pile Construction


Continuous Flight Auger (CFA)

Basic Technique

A hollow stemmed continuous flight auger is rotated into the ground to the required depth. As theauger is withdrawn, concrete is pumped down the hollow stem under balancing pressure forminga shaft of liquid concrete to ground level. A reinforcing cage is then inserted by hand or vibrator.

Other

Installations are monitored for a variety of parameters to improve pile quality.Where particular pile verticality is required, a cased CFA piling system may be used.

Strengths

Minimal disturbance - hence limited risk of damage to adjacent structures

No casing is requiredSuitable for all soil typesSpeed of installationNoise and vibration sensitive sitesWater bearing strataRapid installation





CFA instrumentation





Rotary Bored Piles

Basic Technique

An auger is used to excavate the earth, whilst a steel sleeve is inserted to maintain the borethrough the top layers of unstable ground. A steel cage of reinforcing rods is introduced into thebore before the concrete is poured. The steel sleeve is later withdrawn.

Oscillator Piles

Where minimum disturbance is critical (e.g. close to an existing building or railway line), anoscillator can be used to install the steel casing with great precision.

Drilling Fluids

Where unstable ground exists at greater depths, a vinyl polymer, or bentonite drilling fluid isintroduced to support the bore during excavation and before the concrete is poured.

Strengths Accommodates large loadsWide variety of sizes - up to 3m diameter Large depths possible - up to 70mSuitable for all soil types, including rock

C i Pili Ri S i iC i Pili Ri S i i




Construction : Piling Rig StatisticsConstruction : Piling Rig StatisticsMakeMake ModelModel Mass (T)Mass (T) Typ mastTyp mast

height (m)height (m)Max pile dia.Max pile dia.

(mm)(mm)TorqueTorque(kNm)(kNm)

CrowdCrowd(kN)(kN)

Bauer Bauer BG15BG15 4747 1818 15001500 151151 200200

BG36BG36 114114 2626 25002500 367367 400400

CasagrandeCasagrande CFA425CFA425 4242 2525 900900 112112 5050

B300B300 8080 2828 10001000 250250 137137

HutteHutte HBR205MPHBR205MP 1515 44 600600 5050 100100

KlemmKlemm KR709KR709 1010 55 600600 3.53.5KR701KR701 3.53.5 3.83.8 300300 1.31.3 5050

LlamadaLlamada P90TTP90TT 5050 2323 10001000 120120

P150TTP150TT 4646 2424 800800 6060

SoilmecSoilmec SR20SR20 2727 15.315.3 750750 100100 280280

SR90SR90 105105 28.528.5 12001200 334334 11601160

TwintechTwintech TD308TD308 1.51.5 33 300300

C iC i



ConstructionConstruction


C t tiC t ti





C t tiC t ti




Final ThoughtsFinal Thoughts




Thank you

Presentation-John Gannon - Bored Pile Retaining Walls 2010[1]

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