Concrete Basements, Design and Construction Guide - The Concrete Centre

Concrete Basements Guidance on the design and construction of in-situ concrete basement structures

29 May 2012

Charles Goodchild CEng., MCIOB, MIStructE Principal Structural Engineer The Concrete Centre

Concrete Basements 2007, issues for concrete basements: – – – –

Imminent introduction of the Eurocodes Withdrawal of BS 8110, BS 8007 etc Revision to BS 8102 ‘Recent’ information: • • • •

CIRIA C660 CIRIA C580 ICE Reducing the Risk Guide Research

– Previous references • CIRIA R139/R140 • IStructE Design and construction of deep basements

– Debate Recognised need for up-to-date guidance TCC proposal (with BSI B525/2 encouragement) Nary Narayanan approached and commissioned.

Concrete Basements Main Authors Nary Narayanan Clark Smith Partnership Charles Goodchild The Concrete Centre

1st full draft April 2008

Steering Group:

3 full meetings

Alan Gilbertson Consultant (Chairman) Stuart Alexander WSP; Edwin Bergbaum Waterman; John Caine Curtins; Donal Coughlan Halcrow ; Roger Davies Ramboll ; Graham Hardwick John Doyle ; Bill Hewlett Costain ; Ratnam Kugananthan Laing O’Rourke ; Andy Lyle Capita ; Stuart Marchand Wentworth House; Mahesh Parmar Team 4 Consulting Alan K Tovey The Basement Information Centre ; Robert Vollum Imperial College ; Bjorn Watson SKM Anthony Hunt ; Rod Webster Concrete Innovation and Design ; Derek S Winsor Mott Macdonald ; Corresponding members: Phil Bamforth The Solution Organisation; Tony Jones Arup; Deborah Lazurus Arup.

Contributions Robin Atkinson, Stephen Blundell, John Bungey, Sooren Chinnappa, John Clarke, Peter Goring, John Morrison, Zedi Nyirenda, Duncan Oughton, Ian Whyte & thanks to Andrew Bond.

15 versions/drafts

Concrete Basements Contents Symbols 1. Introduction 2. Outline of the design process 3. Planning of basements 4. Ground movements and construction methods 5. Selection of materials 6. Structural design – general 7. Calculation of lateral earth pressures 8. Design for ultimate limit states 9. Design for serviceability limit states 10. Worked example: basement slab and wall 11. Specification and construction details 12. Case studies References Appendix A: Design data Appendix B: Neutral axes and SLS stresses

page

iii 1 2 3 20 32 39 46 74 80 106 133 144 156 160 172

Concrete Basements Symbols

App. A: Design data

App. B: NA and SLS stresses

References 12. Case studies 11. Specification and details

1. Introduction 2. Outline of design 3. Planning of basements (17 pp) 4. Ground movements etc. 5. Selection of materials 6. Structural design – general

10. Worked example (26 pp) 7. Lateral earth pressures (28 pp) 9. Design for SLS (26 pp)

8. Design for ULS

Concrete Basements 1 Introduction • Basements are common– especially in urban areas • Successful design requires understanding of design, construction and resolution of many construction issues • Scope and drivers

Concrete Basements 1 Outline of the design process

1. Establish Clients requirements 2. Site surveys, etc 3. Outline designs, methodology and proposals

4. On approval do detailed design 5. Construction

Concrete Basements 3 Planning a basement: Grades (BS 8102) 1 basic utility 2 better utility 3 habitable (4) special Types (BS 8102) A Barrier (Membrane) protection B Structurally integral protection C Drained protection Forms RC box Contiguous/ secant piling Diaphragm

Concrete Basements 3 Planning a basement: Grades

Concrete Basements 3 Planning a basement: Types Type A Barrier protection

Type B Structurally integral protection

Type C Drained protection

Concrete Basements 3 Planning a basement: Forms

Table 3.4 Forms of rc basement construction related to site conditions and use of basement space Likely grade that can be achieved with different levels of vapour exclusion

Water table

Form

Method

Water exclLikely Additional measures uding property grade

Concrete Basements

Direction of increasing cost

Generally RC box

below 3 Forms floor level

Open excavation or – Table 3.4? in temporary works Contiguous Excavated piling with after piling: facing wall floors act as props

Good if designed 1 or 2 No additional measures as Type B to BS 3 (or (4)) Type A or (Type C) EN 1992-3 Otherwise insufficient Should be treated as Type A or as Type C. Insufficient. extnl. No additional measures Drained cavity 1 and 2 Designed concrete facing wallc necessary 1 and 2 Drained cavity or int. membraneb

RC box

Good- if treated as Type B and design to BS EN 1992-3 Insufficient. Drained cavity nec. Piling accessible for repair Insufficient. Drained cavity necessary. Wall accessible for repair

In open excavation managing above ground water lowest Secant Excavated basement piling with after piling: facing wall floors act as floor props level – Diaphragm Excavated variable to walling after piling: floors act as high props

Permanently

Note : Based Key a b c

3 and (4) 1 or 2 3 (or (4))

Drained cavity/ membraneb / precautionsa No additional measures External or internal membrane or drained cavity and active precautionsa 1 and 2 Drained cavity and internal tanking 3 and 4 Drained cavity and/or internal membrane b and active precautionsa 1 and 2 A designed concrete facing wallc 1 and 2 Drained cavity and/or membrane 3 Drained cavity and internal (or (4)) membraneb and/or precautionsa

on CIRIA Report R140[20]. Active precautions relate to heating and ventilation requirements to achieve the required internal environment. Fully bonded waterproofing membrane applied on the inside face of the structural walls. Facing walls may be designed to BS EN 1992-3, so where integrated with a designed slab form an RC box with the properties

Concrete Basements 3 Planning a basement: Other subjects

•

Surveys and ground investigations

•

Precautions near underground tunnels, sewers & service mains

•

Working adjacent to existing structures: Party walls

•

Tolerance of buildings to damage

•

Space planning

•

Integrating basement with the superstructure

•

Fire safety considerations

•

Client approval

Concrete Basements 4 Ground movements and construction methods Vertical load relief

Horizontal load relief

Concrete Basements 4 Ground movements and construction methods Construction methods: • • • • 

Open excavation Bottom – up Top – down Semi-top down Groundwater

Options for basement walls:

• In open excavations: R C walls • Incorporating temporary embedded retaining walls o King post walls o Steel sheet piling o Contiguous piled wall o Secant piled wall o Diaphragm walls  Facing walls Temporary works

Concrete Basements 5 Selection of materials Concrete: • Benign soils: RC30/37? Cement IIB-V (CEM I + 21%-35% fly ash) or IIIA (CEM I + 36% - 65% ggbs). cf C35A?: requirements: C28/35 (equiv) -- WCR 0.55 CC 325 CEM I,, IIB-V,) RC30/3: requirements : C30/37 S3 WCR 0.55 CC 300 CEM I, IIA, IIB-S, IIB-V, IIIA, IVB-V B)

• Aggressive soils: Advise producer of DC Class. For DC-2: FND-2? (C25/30)? More aggressive soils: Cement IIIB (CEM I + 66% - 80% ggbs) or IIVB-V (CEM I + 36%-55% fly ash) • Car Parks: C32/40? + provisos

Concrete Basements 5 Selection of materials Waterproofing membranes and systems: • Category 1 – Bonded sheet membranes • Category 2 – Cavity drain membranes • Category 3 – Bentonite clay active membranes • Category 4 – Liquid applied membranes • Category 5 – Mastic asphalt membranes • Category 6 – Cementitious crystallisation active systems • Category 7 – Proprietary cementitious multi-coat renders, toppings and coatings Admixtures for watertightness Water stops • Preformed strips – rubber, PVC, black steel • Water-swellable water stops • Cementitious crystalline water stops • Miscellaneous post-construction techniques • (Re) injectable water bars • Rebate and sealant

Concrete Basements 6 Structural design – general Options for basement slabs • • •

Soil-structure interaction Beams on elastic foundations FEA

Options for basement walls • •

Temporary conditions: construction method and sequence Permanent condition

Loads to be considered: • •

Slabs: column & wall loads, basement slab load, upward water pressure, heave. Walls, lateral earth pressure, water pressure, compaction, loads from superstructure, imbalances.

Design ground water pressure • ‘Normal’ and ‘maximum’ levels Unplanned excavations •

Allowances for cantilever retaining systems

Concrete Basements Soil-structure interaction

≡ UDL 250kN/m2 @ SLS 20 mm settlement

Settlement for a soil with modulus, Es = 150 MPa – a very stiff clay)

Concrete Basements 7 Calculation of lateral earth pressures Angle of shearing resistance: • Granular soils: Estimated peak effective angle of shearing resistance ′max = 30 + A + B + C (A - Angularity, B - Grading, C - N blows)

• Clay soils In the long term, clays behave as granular soils exhibiting friction and dilation.

Concrete Basements 7 Calculation of lateral earth pressures

Design angle of shearing resistance: tan ′d = tan ′k/

(NB  according to Combinations 1 and 2)

Pressure coefficients • Active pressure at depth z below ground surface ′ah = Kad ′v + u

• Passive pressure at depth z below ground surface ′ph = Kpd ′v + u • At rest pressure at depth z below ground surface ′ph = K0d ′v + u Surcharge loadings: • Imposed loads: general, highways • UDLs, point loads, strip loads, rectangular loads : Boussinesq • Compaction pressures

Concrete Basements 7 Calculation of lateral earth pressures

Boussinesq: Method for ‘adding’ superimposed vertical pressures (and therefore lateral pressures) due to : • UDLs, • Point loads, • Strip loads and/or • Rectangular loads.

It is recommended that horizontal earth pressures against ‘rigid’ walls determined using Boussinesq’s theory of stresses in an elastic half space should be doubled for design purposes.

Concrete Basements 7 Calculation of lateral earth pressures

Examples: 1. Active pressures 2. At-rest pressures 3. Surcharge from imposed loads 4. Surcharge from pad foundation 5. Compaction pressures

Concrete Basements 8 Design for Ultimate Limit State EQU – Equilibrium Limit State STR & GEO – Structural and geotechnical Limit States • Combinations 1 and 2

• F for ground water o Normal F = 1.35 o Most unfavourable F = 1.20

• Structural design o As ‘normal’ elements o 3D nature of design

Concrete Basements 9 Design for Serviceability Limit State ≡

Control of cracking

9.1

Causes of cracking and general principles of crack control

9.2

General principles of crack control and minimum reinforcement

9.3

Sequence for verification of cracking

9.4

Test for restraint cracking

9.5

Minimum reinforcement

9.6

Crack widths and watertightness

9.7

Crack width calculations

9.8

Crack control without direct calculation

9.9

Deflection control

9.10

Minimising the risk of cracking

Concrete Basements 9 Design for Serviceability Limit State 9.1

Causes of cracking and general principles of crack control: 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5 9.1.6

Early thermal effects Autogenous and drying shrinkage Restraints Cracking due to restraint (early thermal and shrinkage effects) Cracking due to flexure Cracking due to combinations of restraint and loading

Assumed that target limiting crack widths will give satisfactory performance 9.2

General principles of crack control and minimum reinforcement Provision of minimum reinforcement does not guarantee any specific crack width. It is simply a necessary amount presumed by models to control cracking; but not necessarily a sufficient amount to limit actual crack widths.

Concrete Basements 9 Design for Serviceability Limit State 9.3

Sequence for verification of cracking Design for ULS (Section 8) Check whether section is likely to crack (Section 9.4) Check minimum reinforcement (Section 9.5) Determine limiting crack width (Section 9.6) Calculate crack width (Section 9.7) 9.7.1 Crack width and crack spacing, wk = sr,max cr Crack inducing strain: 9.7.2 cr due to edge restraint and early thermal effects. 9.7.3 cr due to edge restraint and long term effects 9.7.4 cr due to end restraint

9.7.5 cr due to flexure (and applied tension) 9.7.6 cr due to a combination of restraint and loading

Concrete Basements 9 Design for Serviceability Limit State 9.4

Test for restraint cracking A section will crack if:

r = Rax free = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] where

K c T1 ca R1, R3

= = = = = = R2, =

T2

=

cd

=

ctu

=

 ctu

allowance for creep 0.65 when R is calculated using CIRIA C660 1.0 when R is calculated using BS EN 1992-3 coefficient of thermal expansion (See CIRIA C660 for values). See Table A6 for typical values difference between the peak temperature of concrete during hydration and ambient temperature °C (See CIRIA C660). Typical values are noted in Table A7 Autogenous shrinkage strain – value for early age (3 days: see Table A9) restraint factors. See Section A5.6 For edge restraint from Figure L1 of BS EN 1992-3 for short- and long-term thermal and longterm drying situations. For base-wall restraint they may be calculated in accordance with CIRIA C660. Figure L1 may be used with CIRIA C660 methods providing an adjustment for creep is made (See Figure A2 and note). For end restraint, where the restraint is truly rigid 1.0 is most often used, for instance in infill bays. This figure might be overly pessimistic for piled slabs. long-term drop in temperature after concreting, °C. T2 depends on the ambient temperature during concreting. The recommended values from CIRIA C660 for T2 are 20°C for concrete cast in the summer and 10°C for concrete cast in winter. These figures are based on HA BD 28/87[60] based on monthly air temperatures for exposed bridges. Basements are likely to follow soil temperatures so T2 = 12°C may be considered appropriate at depth. drying shrinkage strain, dependent on ambient RH, cement content and member size (see BS EN 1992-1-1 Exp. (3.9) or CIRIA C660 or Table A10). CIRIA C660 alludes to 45% RH for internal conditions and 85% for external conditions. tensile strain capacity may be obtained from Eurocode 2 or CIRIA C660 for both short term and long term values

Concrete Basements 9 Design for Serviceability Limit State 9.5

Minimum reinforcement As,min = kc k Act (fct,eff /fyk)

where

kc

= =

A coefficient to account for stress distribution. 1.0 for pure tension. When cracking first occurs the cause is usually early thermal effects and the whole section is likely to be in tension.

k

= =

A coefficient to account for self-equilibrating stresses 1.0 for thickness h < 300 mm and 0.65 for h > 800 mm (interpolation allowed for thicknesses between 300 mm and 800 mm).

Act

=

area of concrete in the tension zone just prior to onset of cracking. A ct is determined from section properties but generally for basement slabs and walls is most often based on full thickness of the section.

fct,eff

==

fctm mean tensile strength when cracking may be first expected to occur:  for early thermal effects 3 days  for long-term effects, 28 days (which considered to be a reasonable approximation) See Table A5 for typical values.

fyk

= =

characteristic yield strength of the reinforcement. 500 MPa

CIRIA C660 Recent research[61] would suggest that a factor of 0.8 should be applied to fct,eff in the formula for crack inducing strain due to end restraint. This factor accounts for long-term loading, in-situ strengths compared with laboratory strengths and the fact that the concrete will crack at its weakest point. TR 59 [62] concludes that the tensile strength of concrete subjected to sustained tensile stress reduces with time to 60–70% of its instantaneous value. [1]

The area of reinforcement obtained using this value may well need increasing during the remaining design process

Concrete Basements 9 Design for Serviceability Limit State 9.6

Crack widths and watertightness

Table 9.2 Tightness Classes

Concrete Basements 9 Design for Serviceability Limit State 9.6

Crack widths and watertightness

Tightness Classes- notes:

Concrete Basements 9 Design for Serviceability Limit State 9.6

Crack widths and watertightness -recommendations

Concrete Basements 9 Design for Serviceability Limit State 9.6

Crack widths and watertightness -recommendations

Table 9.4 Summary of crack width requirements for different types of in-situ concrete basement construction Construction Expected Crack width requirement Tight wk mm a type and water performance of -ness FlexRestraint/ table structure Class ural axial

wk,max[9] wk,1[10]

A B – high permanently high water table

B – variable fluctuating water table

B – lowd water table permanently below underside of slab

C

Structure itself is not considered watertight Structure is almost watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure Design to Tightness class 1 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm to 0.05 mm for cracks that pass through the section Structure is almost Design to Tightness class 1 of BS EN 1992-3. See watertight Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm for cracks that pass through the section Structure is watertight Design to Tightness class 0 of BS EN 1992-3. See under normal conditions. Table 9.2. Generally 0.3 mm for RC structures Some risk under exceptional conditions.

0

0.30

0.30e

1

0.30b

0.05 to 0.20

Structure itself is not considered watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure. Design to Tightness Class 1 may be helpful for construction type C

(wrt hd/h)

1c

0.30 b

0.20

0c

0.30

0.30

0

0.30

0.30e

(1)c

(0.3)

(0.05 to 0.20 or 0.20)

Key b Where the section is not fully cracked) the neutral axis depth at SLS should be at least xmin (where xmin > max {50 mm or 0.2 × section –6

Concrete Basements 9 Design for Serviceability Limit State 9.7

Crack width calculations

9.7.1 Crack width, wk = sr,max cr where

sr,max = Maximum crack spacing = 3.4c + 0.425 (k1k2 /p,eff) where c k1 k2  p,eff

= = = = = = =

nominal cover, cnom 0.8 (CIRIA C660 suggests 1.14) 1.0 for tension (e.g. from restraint) 0.5 for bending (1 + 2)/21 for combinations of bending and tension diameter of the bar in mm. As/Ac,eff Ac,eff for each face of a wall is based on 0.5h; 2.5(c + 0.5); (h – x)/3 where h = thickness of section x = depth to neutral axis.

cr = Crack-inducing strain

= Strain between cracks = Mean strain in steel – mean strain in concrete, (cs - cm ). . . .

Concrete Basements Consider a crack in a section:

Plan (or section)

Strain

S00 S

S00 S

S0

εss

εs

εcc

εc Sr,maxSr,max

S0

Strain in reinforcement εsm sm εsm

ctu

sm - cm

cm εcm εcm ε = 0ε = 0 Strain in concrete

Concrete Basements 9 Design for Serviceability Limit State cr = Crack-inducing strain = . . . . . . . . . . . . . . . 9.7.2 Early age crack-inducing strain

cr = K[cT1 +ca R1 – 0.5 ctu 9.7.3 Long term crack-inducing strain

cr = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] – 0.5 ctu 9.7.4 End restraint crack-inducing strain

cr = 0.5e kckfct,eff [1 + (1/e ) /Es 9.7.5 Flexural (and applied tension) crack-inducing strain

cr = (sm – cm) = [s – kt (fct,eff /p,eff) (1 + e p,eff /Es cr  0.6 (s)/Es

Concrete Basements 9 Design for Serviceability Limit State 9.8

Crack control without direct calculation don’t do it!

9.9

Deflection control As ‘normal’ design

9.10

Minimising the risk of cracking 9.10.1

Materials

9.10.2

Construction

9.10.3

Detailing

use cement replacements, aggregates with low ac, avoid high strength concretes construct at low temperatures, use GRP or steel formwork, sequential pours

use small bars at close centres, avoid movement joints, prestress?

Concrete Basements 10 Worked Example

Concrete Basements 10 Worked Example

Concrete Basements 10 Worked Example

Concrete Basements 10 Worked Example

Concrete Basements 10 Worked Example

Commentary: In slab 53T : end restraint critical In walls 10T: edge restraint critical Iterations required. Refinements: fct,eff, e, ct =0.8, end restraint, concrete, Construction methodology Use CIRIA C660 rather than BS EN 1992-3

Concrete Basements 11 Specification and construction details 11.1

Specification: • BS EN 13670 • NSCS / NBS • ICE specification for piling and embedded retaining walls

11.2

Joints • Construction joints • Water stops

11.3

• Preformed strips –PVC, black steel Miscellaneous • Water-swellable water stops • •Kickers (Re) injectable epoxy water bars

• • • • • • 11.4

Formwork ties Membranes & coatings Admixtures & additives Service penetrations Drainage Underpinning

Inspection, remedials & maintenance

Concrete Basements 12 Case studies

Concrete Basements 12 Case studies

Concrete Basements References

Concrete Basements Appendix A: Design data A1

Combination factors

A2

Design angle of shearing resistance

A3

Pressure coefficients Kad and Kpd

A4

Bending moment coefficients for rectangular plates

A5

Design data for crack width formulae A5.1

fctm (≡ fct,eff), mean tensile strengths of concretes

A5.2

c, coefficient of thermal expansion T1, difference between the peak temperature of concrete

A5.3

during hydration and ambient temperature °C

A5.6

ca, autogenous shrinkage strain cd, drying shrinkage strain R, restraint factors

A5.7

ctu, tensile strain capacity of concrete

A5.8

Moduli of elasticity of concrete Ecm and modular ratio, ae

A5.4 A5.5

Concrete Basements Appendix A: Design data

Concrete Basements Appendix B: Neutral Axes and SLS stresses B1

Neutral axis at SLS (cracked section and no axial stress)

B2

SLS stresses in concrete, σc and reinforcement, σs (cracked section and no axial stress)

B3

B2.1

Singly reinforced section

B2.2

Doubly reinforced section

SLS stresses in concrete, σc, and in reinforcement, σs due to flexure and axial load (cracked section)

Concrete Basements Symbols

App. A: Design data

App. B: NA and SLS stresses

References 12. Case studies 11. Specification and details

1. Introduction 2. Outline of design 3. Planning of basements (17 pp) 4. Ground movements etc. 5. Selection of materials 6. Structural design – general

10. Worked example (26 pp) 7. Lateral earth pressures (28 pp) 9. Design for SLS (26 pp)

8. Design for ULS

Concrete Basements This guide covers the design and construction of reinforced concrete basements and is in accordance with the Eurocodes. The aim of the guide is to assist designers of concrete basements of modest depth, i.e. not exceeding 10 metres. It will also prove relevant to designers of other underground structures. It brings together in one publication the salient features for the design and construction of such waterresisting structures. The guide has been written for generalist structural engineers who have a basic understanding of soil mechanics.

Concrete Basements

Thank you

Concrete Basements Guidance on the design and construction of in-situ concrete basement structures

29 May 2012

Charles Goodchild CEng., MCIOB, MIStructE Principal Structural Engineer The Concrete Centre