Bored Pile Retaining Walls
John Gannon Byland Engineering Limited www.bylandengineering.com
Introduction - speaker A a chartered civil engineer with >25 years experience with site investigation contractors and consulting engineers. Director of independent geotechnical design practice since 2001. Provides engineering services to Leeds City Council Geotechnical Unit and therefore the other Yorkshire councils. Experience in most construction sectors including design and construction of highway structures, earthworks and drainage. Familiar with ground conditions of West Yorkshire. Has designed dozens of bored pile walls across UK. John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction - programme • • • • • • • • • • • •
Introduction 09.00 – 09.15 Site investigation 09.15 – 09.30 Design (general) 09.30 – 09.45 Exercise 09.45 – 10.00 Tea/coffee break 10.00 – 10.15 Design (geotechnical) 10.15 – 10.45 Eurocode 7 10.45 – 11.00 Design (structural) 11.00 – 11.15 Design standards etc 11.15 – 11.30 Exercise 11.30 – 11.45 Construction 11.45 – 12.15 Questions & Close 12.15 – 12.30
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction : popularity – why? • • • • • • •
Increased awareness / growing track record Easier & quicker to design (software) Land scarce and values rising Faster hence reduced piling cost Able to penetrate denser and harder ground Improved quality Improved verticality
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction – what are they ? •Replacement vertical piles formed of concrete •Bored and cast-in-situ •Wholly or partially reinforced or not reinforced at all (usually cages, 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 John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction – what are they ?
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction – what are they ?
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction – what are they ?
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Introduction – what are they ?
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Site Investigation Purpose is to provide information to the designer and contractor for • Engineering feasibility studies • Permanent works design • Temporary works design • Construction planning • Risk assessment • Pricing John Gannon
Bored Pile Retaining Walls
Byland Engineering
Site Investigation Good Practice is to undertake and report these elements : • Thorough desk study including services search • Topographic survey • Intrusive ground investigation • Careful interpretation • Design report with risk register • Performance feedback John Gannon
Bored Pile Retaining Walls
Byland Engineering
Ground Investigation Do’s 1. 2. 3. 4. 5.
6. 7. 8. 9.
John Gannon
Boreholes below pile toe Record groundwater ingress Measure groundwater levels – several visits Record surface levels Test soil : SPT, index & classification, electroelectro-chemical, shear strength, stiffness Retained ground slope and surcharges Long term groundwater level in retained ground Obstructions e.g. concrete, masonry, boulders Variation in rock rock--head level and rock strength
Bored Pile Retaining Walls
Byland Engineering
Ground Investigation Do’s
Low water table : Za > 0.4h and > (t + 2m) High water table : Za > (H+2m) and > (t + 2m) John Gannon
Bored Pile Retaining Walls
Byland Engineering
Site Investigation Soil Mechanics Reminders : • Τ = c + σ’n tan φ • σ’n = σn – u • Τ can be measured directly in a total stress approach, pore fluid pressure not being considered. For example when the shear strength is measured in a vane test or in a quick un-drained tri-axial compression test. Either Su (un-drained shear strength) or cu (un-drained cohesion which implies φ = 0) is provided. •Or Τ can be measured in an effective stress approach where the total stress and the pore fluid pressure is considered. For example when shear strength is measured in a tri-axial compression tests in which drainage is permitted or pore pressure measured so that c’ and φ’ are obtained. • Soil Mechanics meaning of drainage : condition of the soil which in a response to a change in total stress, the pore fluid pressure ( water and / or air) has reached equilibrium (drained) or not (un-drained). Rate of load change and soil permeability govern.
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Site 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) mudstone to very hard (gritstone)
•
shallow mine workings
John Gannon
Bored Pile Retaining Walls
Byland Engineering
General design The key considerations are : • Choice of wall type & construction method • Durability • Support arrangements and stiffness • Geotechnical analysis & design • Structural analysis & design • Current and emerging standards
John Gannon
Bored Pile Retaining Walls
Byland Engineering
General design – wall 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 years permanent highway structure. •Buildability – working width, height, allowable rig size, proximity to buildings, type of ground to be excavated, required verticality, difference between piling level and casting level. •Appearance – does wall need to be clad or lined or waterproofed. •Minimum cost always a driver. John Gannon
Bored Pile Retaining Walls
Byland Engineering
General design - durability •Concrete cast against the ground normally has a nominal minimum cover of 75mm which comprises minimum cover of 60mm and a fixing tolerance of 15mm. •Careful use of cover spacers to reinforcement cages essential. For Continuous Flight Auger (CFA) piles where cage pushed in last, rigidity of spacers can be important. 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 too stiff to plunge the cage. The load in the pile seldom justifies high strength concrete. •Good concrete placement technique, control of cover and reinforcement detailing to limit crack widths are best means of promoting good durability. •Extensive use of GGBS and PFA cement replacement and use of superplasticisers in mix designs common and aid durability at expense of slower strength gain. Slump class S3 (80-180mm) or class S4 (140 -240mm) usual. Might be pumped or tremied from a hopper. Can still obtain DC-3 and possibly also DC-4 classification without APM‘s ( see BRE SD1), depending on structural performance level, exposure class and section size.
John Gannon
Bored Pile Retaining Walls
Byland Engineering
General design – support arrangements
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design - analysis Analysis should consider : • All stages of construction and service. Hence temporary and permanent works design are integrated. •Pile length required to ensure horizontal equilibrium. •Pile length required to ensure vertical equilibrium. particularly where there are imposed loads, for example in embedded integral bridge abutments. •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.
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design - analysis A few concepts : Geotechnical category (as BS EN 1997 part 1) : 1 2 3
Small and simple, retained height less than about 3m Intermediate Large and unusual
Support Stiffness categories : High : Top down construction, high level temporary props before high level permanent 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
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – limit states Limit state design philosophy : basis of Eurocodes and some earlier standards e.g.BS8002, BS5400 etc. ULS : loss of equilibrium, failure by rotation, structural failure of an element or of a structure supported by the wall etc. SLS : unacceptable wall deflections and associated ground movements, unacceptable leakage etc.
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – ground movements Source : • construction of the wall • excavation in front • loss of soil by flowing water • thermal affects (particularly integral bridges where deck acts as prop)
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis stages
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis stages
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis stages
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis stages
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – earth pressures
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design –earth 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, the active 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 is permitted and the friction force or adhesion which develops between soil and wall as movements occur •Equations and charts are given in BS8002 and BS EN 1997 Part 1 (Annex C)
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design –earth pressures
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design –earth pressures Walls movements required to enable active and passive pressures to be mobilised
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – earth pressures
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – stability analysis • • • • •
•
Considers potential failure mechanisms. Factor of safety calculation to establish toe elevation uses limiting values of earth pressure. Various definitions of FOS. Strength factor method most commonly used – margin by which soil shear strength must be reduced to bring about equilibrium. Factored strength = characteristic strength / partial material factor. c’des = c’ / γm ; cudes = cu/ γm ; tanφ’des = tan φ’ / γm (NB M in BS8002) Partial factors depend on limit state being considered as well as how soil strength is considered and the desired margin of safety.
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – limit state calculations ULS (collapse) calculations – current practice (C580) q min 10 kPa , unplanned overover-dig 10% H or 0.5m, pile length that gives overturning equilibrium enhanced by 20%. Approach
γm
Charac. shear strength
A
1.2 or 1.5
Mod cons.
B
1.0
Worst credible
C
1.2 or 1.5
Most probable
Comment
With OM
Value of δ, cw etc varies – see C580 Shear strength may be peak, critical state, residual, drained or unun-drained. SLS calculations – current practice Design approach A or C, γm = 1.0 John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – stability analysis Single Pile Vertical Pile Capacity (Q (Qu) – current approach (BS8004) Qu = Qus + Qub Qus = π d l Τsu Qbu = π/4 d^2 σbu Conventionally (BS8004) Qa = Qu / F ; F > 2.0 Increasingly Qa = Qus / f1 + Qus /f2 Qu also adjusted for pile spacings, spacings, as in pile group design John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – overall stability
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – overall stability
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – limit equilbrium methods For statically determinate structures e.g. cantilevers and single propped cantilevers
Fixed earth support
Free earth support
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis Software : A Limit equilibrium. To calculate wall length e.g. Stawal B Subgrade reaction / Psuedo finite element. To simulate wall behaviour. Will calculate all wall load effects using simple soil models, e.g. Wallap Wallap,, Frew C Continuum models (FE/FD). To simulate wall and ground behaviour. Will calculate system behaviour in 2D or 3D using complex soil models if required e.g Flac Flac,, Plaxis Plaxis,, Crisp
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Geotechnical design – analysis
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Eurocode 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 APPROACHES Consider 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 forces DESIGN SITUATIONS Persistent, transient, accidental, seismic John Gannon
Bored Pile Retaining Walls
Byland Engineering
Eurocode 7 ACTIONS (LOADS) ( earth pressure, water pressure , imposed, thermal etc.) : Fd = γF Frep or Fd = γE Frep and Frep = ψ Fk (generally ψ = 1.0) γF varies 0 to 1.7 depending on limit state, nature of action and combination Water pressure may be dealt with by increase in level MATERIALS (soil, concrete, steel) Xd = Xk / γm γm varies 1.0 to 1.5 depending on material type and combination GEOMETRICAL DATA RESISTANCES Rd = Rk / γm γm varies 1.0 to 1.6 depending on calculation set, pile type, nature of action (T,C)
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Eurocode 7 GROUNDWATER EC7 provision for water table very onerous : dw = H/4 for H<4m dw = 1m for H>4 “Where no reliable drainage present”
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Eurocode 7 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 to be resolved (design approaches and partial factors not the same) John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design – failure modes
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design - checks 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 eccentricity moments (e.g. section 5.5 of BS5400)
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design - checks
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design - checks 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
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design - checks 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 in non-aggressive ground”
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Structural design - preliminary pile sizing Cantilever wall height vs normalised moment - dry soil 10
H (m)
8
6
4
2 0.0
1.0
2.0
3.0
4.0
M/h^3 (N/mm2) 350/500
John Gannon
450/600
600/750
750/900
Bored Pile Retaining Walls
900/1050
Byland Engineering
Design standards Now BS8002, BS004 & BS8081 BS8106 BS8110 & BS5400 BS EN 206 & BS EN 8500 BS EN 1536 BRE SD1 CIRIA C580 ICE SPERW HA advice and design notes John Gannon
Here & Coming BS EN ISO 14688 BS EN ISO 22476 BS EN 1990 BS EN 1991 BS EN 1992 BS EN 1997
Bored Pile Retaining Walls
Byland Engineering
Design details What the industry prefers : Cantilevers Single high level props Corner bracing Avoid raking props Passive self drill tie backs (usually temporary) Anchor head blocks instead of walings Permanent structure props John Gannon
Bored Pile Retaining Walls
Byland Engineering
Design - summary Key Points : • • • • • • •
Define geotechnical category Define limit states and performance criteria to be met Failures seldom due to structural failure of wall Permissable movements depend on sensitivity of surrounds Flexible walls with many props give similar displacements to stiff walls with fewer props Ground movements cannot be accurately predicted but they can and should be estimated. All walls and supported ground move Early high propping effective in controlling ground movements
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction Continuous Flight Auger (CFA) Basic Technique A hollow stemmed continuous flight auger is rotated into the ground to the required depth. As the auger is withdrawn, concrete is pumped down the hollow stem under balancing pressure forming a 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 required Suitable for all soil types Speed of installation Noise and vibration sensitive sites Water bearing strata Rapid installation
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction CFA instrumentation
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Bored Pile Construction Rotary Bored Piles Basic Technique An auger is used to excavate the earth, whilst a steel sleeve is inserted to maintain the bore through the top layers of unstable ground. A steel cage of reinforcing rods is introduced into the bore 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), an oscillator 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 is introduced to support the bore during excavation and before the concrete is poured. Strengths Accommodates large loads Wide variety of sizes - up to 3m diameter Large depths possible - up to 70m Suitable for all soil types, including rock
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction : Piling Rig Statistics Make
Model
Mass (T)
Typ mast height (m)
Max pile dia. (mm)
Torque (kNm)
Crowd (kN)
Bauer
BG15
47
18
1500
151
200
BG36
114
26
2500
367
400
CFA425
42
25
900
112
50
B300
80
28
1000
250
137
Hutte
HBR205MP
15
4
600
50
100
Klemm
KR709
10
5
600
3.5
KR701
3.5
3.8
300
1.3
P90TT
50
23
1000
120
P150TT
46
24
800
60
SR20
27
15.3
750
100
280
SR90
105
28.5
1200
334
1160
TD308
1.5
3
300
Casagrande
Llamada
Soilmec
Twintech
John Gannon
Bored Pile Retaining Walls
50
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Construction
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Final Thoughts
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Final Thoughts
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Final Thoughts
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Final Thoughts
John Gannon
Bored Pile Retaining Walls
Byland Engineering
Final Thoughts
Thank you
John Gannon
Bored Pile Retaining Walls
Byland Engineering