Concrete Basements the New Design Guide

Charles Goodchild CEng., MCIOB, MIStructE

Principal Structural Engineer

The Concrete Centre

Concrete Basements Guidance on the design and construction of in-situ concrete basement structures 8 November 2012

Concrete Basements

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

Steering Group: 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 ORourke ; 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.

1st full draft April 2008

3 full meetings

15 versions/drafts

Concrete Basements

Symbols 1. Introduction 2. Outline of

design 3. Planning of

basements (17 pp)

4. Ground movements etc.

5. Selection of materials

6. Structural design general

7. Lateral earth pressures (28 pp)

8. Design for ULS

9. Design for SLS (26 pp)

10. Worked example (26 pp)

11. Specification and details

12. Case studies

References

App. A: Design data App. B: NA

and SLS stresses

Concrete Basements

1. Establish Clients

requirements

2. Site surveys, etc

3. Outline designs,

methodology

and proposals

4. On approval do

detailed design

5. Construction

2 Outline of the design process

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: Types

Type A

Barrier protection

Type B

Structurally integral

protection

Type C

Drained protection

Concrete Basements

3 Forms Table 3.4?

Table 3.4

Forms of rc basement construction related to site conditions and use of basement space

Water

table

Form Method Water excl-uding property

Likely grade that can be achieved with different levels of vapour exclusion

Likely

grade

Additional measures

Generally

below

lowest

floor level

RC box Open

excavation or

in temporary

works

Good if designed

as Type B to BS

EN 1992-3

1 or 2 No additional measures

3 (or (4)) Type A or (Type C)

Otherwise insufficient Should be treated as Type A or as Type C.

Contiguous

piling with

facing wall

Excavated

after piling:

floors act as

props

Insufficient.

Drained cavity

necessary

extnl. No additional measures

1 and 2 Designed concrete facing wallc

1 and 2 Drained cavity or int. membraneb

3 and (4) Drained cavity/ membraneb / precautionsa

Perman-

ently

above

lowest

basement

floor level

variable

to high

RC box In open

excavation -

managing

ground water

Good- if treated

as Type B and

design to BS EN

1992-3

1 or 2 No additional measures

3

(or (4))

External or internal membrane or

drained cavity and active

precautionsa

Secant

piling with

facing wall

Excavated

after piling:

floors act as

props

Insufficient.

Drained cavity

nec. Piling acc-

essible for repair

1 and 2 Drained cavity and internal tanking

3 and 4 Drained cavity and/or internal

membrane b and active precautionsa

Diaphragm

walling

Excavated

after piling:

floors act as

props

Insufficient.

Drained cavity

necessary. Wall

accessible for

repair

1 and 2 A designed concrete facing wallc

1 and 2 Drained cavity and/or membrane

3

(or (4))

Drained cavity and internal

membraneb and/or precautionsa

Note : Based on CIRIA Report R140[20].

Key a Active precautions relate to heating and ventilation requirements to achieve the required internal environment.

b Fully bonded waterproofing membrane applied on the inside face of the structural walls.

c Facing walls may be designed to BS EN 1992-3, so where integrated with a designed slab form an RC box with the properties

and likely grades indicated for RC boxes above.

Dir

ecti

on o

f in

cre

asi

ng c

ost

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

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).

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

cf C35A?: requirements: C28/35 (equiv) -- WCR 0.55 CC 325 CEM I, IIB-V,)

RC30/37: requirements : C30/37 S3 WCR 0.55 CC 300 CEM I, IIA, IIB-S, IIB-V, IIIA, IVB-V B)

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 at construction joints

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

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

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

gF for ground water

o Normal gF = 1.35

o Most unfavourable gF = 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


9.1 Causes of cracking and general principles of crack control:

9.1.1 Early thermal effects

9.1.2 Autogenous and drying shrinkage

9.1.3 Restraints

9.1.4 Cracking due to restraint (early thermal and shrinkage

effects)

9.1.5 Cracking due to flexure

9.1.6 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


9.3 Sequence for verification of cracking

8 Design for ULS

9.4 Check whether section is likely to crack

9.5 Check minimum reinforcement

9.6 Determine limiting crack width

9.7 Calculate crack width

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


9.4 Test for restraint cracking

A section will crack if:

r = Rax free = K[([cT1 +ca) R1 + ([cT2 R2) + cd R3] ctu

where K = allowance for creep = 0.65 when R is calculated using CIRIA C660

= 1.0 when R is calculated using BS EN 1992-3

c = coefficient of thermal expansion (See CIRIA C660 for values). See Table A6 for typical values

T1 = difference between the peak temperature of concrete during hydration and ambient

temperature C (See CIRIA C660). Typical values are noted in Table A7

ca = Autogenous shrinkage strain value for early age (3 days: see Table A9) R1, R2,

R3

= restraint factors. See Section A5.6

For edge restraint from Figure L1 of BS EN 1992-3 for short- and long-term thermal and long-

term 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.

T2 = 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 20C for concrete cast

in the summer and 10C 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 = 12C may be considered appropriate at depth.

cd

ctu

=

=

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


9.5 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. Act 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

[1] 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 6070% of its instantaneous value.

The area of reinforcement obtained using this value may well

need increasing during the remaining design process

Table 9.2 Tightness Classes

Concrete Basements



Tightness Classes- notes:

Concrete Basements



Concrete Basements


9.6 Crack widths and watertightness -recommendations

Table 9.4

Summary of crack width recommendations

Construction

typea and water

table

Expected

performance of

structure

Crack width requirement Tight

-ness

Class

wk mm

Flex-ural wk,max[9]

Restraint/ axial wk,1

[10]

A 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 0 0.30 0.30e

B high permanently high

water table

Structure is almost

watertight


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

1 0.30b 0.05 to

0.20 (wrt hd/h)

B variable fluctuating water

table

Structure is almost

watertight


Table 9.2. Generally 0.3 mm for flexural cracks

but 0.2 mm for cracks that pass through the

section

1 c 0.30 b 0.20

B lowd water table

permanently below

underside of slab

Structure is watertight

under normal conditions. Some risk under exceptional conditions.


Table 9.2. Generally 0.3 mm for RC structures 0 c 0.30 0.30

C Structure itself is not considered watertight


Table 9.2. Generally 0.3 mm for RC structure.

Design to Tightness Class 1 may be helpful for

construction type C

0

(1)c

0.30

(0.3)

0.30e

(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 thickness}) and variations in strain should be less than 150 106.

Concrete Basements


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)

cr = Crack-inducing strain = (Restrained strain effect of crack formation)over 2 debonding lengths = (Mean strain in steel mean strain in concrete)over 2 debonding lengths = (cs - cm ). . . . . . . . . . . . . . .

where

c = nominal cover, cnom

k1 = 0.8

(CIRIA C660 suggests 1.14)

k2 =

=

=

1.0 for tension (e.g. from restraint)

0.5 for bending

(1 + 2)/21 for combinations of bending and tension = diameter of the bar in mm.

p,eff = 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.

sm

c

s

cm = 0

sm

c

s

cm = 0

Sr,max

S0S0S0S0

Consider a crack in a section:

sm - cm

sm

c

s

cm = 0

sm

c

s

cm = 0

Sr,max

S0S0S0S0

ctu

Strain

Plan (or section)

Strain in reinforcement

Strain in concrete

sm

c

s

cm = 0

sm

c

s

cm = 0

Sr,max

S0S0S0S0

Concrete Basements

sm

c

s

cm = 0

sm

c

s

cm = 0

Sr,max

S0S0S0S0

sm

c

s

cm = 0

sm

c

s

cm = 0

Sr,max

S0S0S0S0

sr,max

wk = sr,max cr

Concrete Basements


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 = 0.5e 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


9.8 Crack control without direct calculation

dont do it!

9.9 Deflection control

As normal design

9.10 Minimising the risk of cracking

9.10.1 Materials use cement replacements, aggregates with low ac, avoid high strength concretes

9.10.2 Construction construct at low temperatures, use GRP or steel formwork, sequential pours

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

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 Miscellaneous

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

11.4 Inspection, remedials & maintenance

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

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

A5.3 T1, difference between the peak temperature of concrete

during hydration and ambient temperature C

A5.4 ca, autogenous shrinkage strain

A5.5 cd, drying shrinkage strain

A5.6 R, restraint factors

A5.7 ctu, tensile strain capacity of concrete

A5.8 Moduli of elasticity of concrete Ecm and modular ratio, ae

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, c and reinforcement, s (cracked section and no axial stress)

B2.1 Singly reinforced section

B2.2 Doubly reinforced section

B3 SLS stresses in concrete, c, and in reinforcement, s due to flexure and axial load (cracked

section)

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 water-

resisting structures.

The guide has been written for generalist

structural engineers who have a basic understanding of soil mechanics.

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

Thank you

Documents

Concrete Basements the New Design Guide