The assessment of the risk of damage to buildings due to tunnelling and excavations - J Burland

8/7/2019 The assessment of the risk of damage to buildings due to tunnelling and excavations - J Burland

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The assessment of the risk of damage to

buildings due to tunnelling and excavations

AN HISTORICAL PERSPECTIVE

John Burland

mper a o ege on on

Routine guides on limiting distortion and

settlement

Classic work of Skempton and MacDonald (1956)

Examined records of nearly 100 buildings mainly

infilled steel or reinforced concrete framed, but a few

load bearing wall

Damage was correlated with angular distortion /L

Concluded that cracking occurs when /L > 1/300

and recommended designing to 1/500

Structural damage occurs when /L > 1/150


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Skempton and MacDonald (1956)

Definition of angular distortion with no rigid body tilt

Note: This measure of foundation distortion implicitly

assumes that the superstructure is deforming in shear

Skempton and

MacDonalds

analysis of field

evidence of damage

on traditional frame

buildings and load-

bearing brick walls


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Bjerrum (1963) supplemented the guidance with

the following recommendations for/L :

Difficulties with machines sensitive to settlement > 1/750

Danger for frames with diagonals > 1/600

Buildings must not crack 1/500

First cracks occur > 1/300

Tilting of high buildings noticeable > 1/250

ruc ura amage may occur

Limitations to the routine guidelines:

Based mainly on indirect evidence from the literature

Deals only with traditional buildings and structural

mem ers

Though presented by Skempton as tentative, often

stated as rules Damage not objectively defined or classified

Evidence for masonry walls very suspect

Angular distortion /L implicitly assumes that the

building is deforming in shear

It does not distinguish between hogging and sagging

It is sometimes difficult to define o.a. rotation


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Slowly we began to accumulate clear evidence

that buildings do not only deform in shear

For example the measurements that we made at the

Palace of Westminster during the construction of the

underground car park in New Palace Yard

Underground car park at the Palace of Westminster


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Palace of Westminster

Cracking in the south wall of the Annexe due tohogging - /L 1/300

Palace of Westminster

Cracking in the west wall of Westminster Hall due to

sagging and hogging


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Burland and Wroth (1974)

u w

needed in assessing limiting deformations and that

angular distortion was unsatisfactory in a number of

ways

was rs necessary o se ou e n ons ofoundation movement which do not make assumptions

about the mode of deformation of the superstructure

(a) Rotation or slope, ,and angular strain, .

Definitions of ground and foundation movementBurland and Wroth (1974)

(b) Relative deflection, ,

and deflection ratio, /L.

(c) Tilt, and relativerotation, (angulardistortion)


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Tensile strain as a parameter giving rise to

cracking

Burland and Wroth argued that there was a need to move away

from em irical deflection criteria and stud the fundamental

causes of damage

They noted that buildings usually become unserviceable before

there is a risk of structural collapse

Most damage to walls, cladding and finishes manifests as

cracking which results from extensional (tensile) strain

ey ere ore carr e ou a s u y o e wor carr e ou athe Building Research Establishment on the behaviour of

masonry and blockwork when subjected to a variety of loading

conditions

The concept of Critical Tensile Strain crit

They noted that the locally determined tensile strains at

which cracking became visible was reasonably well defined

and independent both of the tensile strength of the masonry

and blockwork and of the form of loading of the wall i.e.

whether it was subjected to racking in shear or in-plane

bending They concluded that the value ofcrit varied between about

0.05% and 0.1% and suggested using an average value of

0.075%

They stressed that crit is significantly larger than the strain

at which tensile failure occurs. It is also an average strain

measured over a gauge length of about a metre


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BRE large scale tests on composite action between

masonry walls and their supporting beams

Burhouse, 1969

BRE tests on the stiffness and strength of

masonry infilled frames

Mainstone 1971


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The cracking of simple beams in bending and

shear

Burland and Wroth then a lied the conce t of

critical tensile strain to evaluating the limiting

displacements of simple weightless elastic beams of

length L and height H.

Even though real buildings are much more complex

s s u y as e pe o us ra e a num er oimportant features that control limiting values of/L

Cracking of a simple beam in bending and in shear

actual building

equivalent deep beam

deflected shape of soffit

shear deformation

bending deformation


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Centrally loaded beam with both bending and

shear stiffness

Extreme fibre strain b max :

Maximum dia onal strain

max2

3

12b

G

E

yLH

I

t

L

L

max

2

181 d

H

G

I

HL

L

Burland and Wroth (1974)

Limiting relationships between /L and L/H for a uniform load and a central point load.

Conclusion: The relationship between max and /L isinsensitive to the form of loading


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Burland and Wroth (1974) and Burland et al(1977)

Influence of mode of deformation and E/G on limitingvalues of/L for a simple beam


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Burland and Wroth(1974)

Frame buildings and

undergoing sagging and

hogging

Relationship between

/L and L/H for various

degrees of damage

Comparison with simple

elastic beams with

crit = 0.075%

Classification of damage, Burland et al(1977)

In 1975 and 1976 the UK, like much of Europe was subject to

severe droughts. As a consequence many buildings on clay

soils experienced damage.

n e c a ms or su s ence amage aga ns ouse o

insurers amounted to over 100m

An investigation by the BRE revealed that in the early 70s the

insurance companies introduced a subsidence clause (aimed at

protecting householders against landslip)

Nowhere in this clause was damage defined.

As a result claims escalated year by year and 1976 reached

disaster proportions

It became clear that an objective system of classifying damage

was needed


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Three broad categories of damage Aesthetic: affects only the appearance of the property

Serviceability: cracking and distortion which impairs

the weathertightness or other functions (eg sound

insulation, fracturing of service pipes, jamming of

doors and windows)

Stability: there is an unacceptable risk that some part

of the structure will collapse unless preventative

act on s ta en It is only a short step from these to the more detailed

classification proposed by Burland et al(1977) and

given in Table 1 of the paper

Classification based on ease of repair

The classification is based on ease of repair and is developed from a

large number of other studies

It applies only to masonry and blockwork

It relates to visible damage at a given time and not its cause or

possible progression these have to be considered separately

Classification is NOT based on crack width alone it is ease of repair

which is the key factor

corrosion, penetration of harmful liquids or gasses or structural failure

Categories 0, 1 and 2 represent aesthetic damage; categories 3 and 4

serviceability damage and 5 stability damage

JUDGEMENT AND EXPERINECE ARE ALL IMPORTANT


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Example of Category 1

damage

Fine cracks which are

normal decoration

The house was

un erp nne a a cos oabout 15,000 Euros in

1976!

The division between categories 2 and 3 damage

The dividing line between category 2 and 3 damage is

particularly important and is based on many case

records of building damage assembled by the BRE.

Damage up to category 2 can result from a variety of

causes (e.g. shrinkage, thermal effects, corrosion,

ground movement etc). Identification of the cause isusually very difficult.

If damage exceeds category 2 the cause is usually

much easier to identify and is frequently associated

with ground movement.

Thus the division between categories 2 and 3 is an

important threshold


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Example of Category 3damage - Moderate

Repointing of externalbrickwork. Somebrickwork requiredreplacing above and belowwindows

Example of Category 4 damage - Severe

Gable wall leaning outwards with some loss of bearing of beams


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Example of Category 5 damage Very severe

Danger of instability

Progression to the concept of limiting tensile

strain

Burland et al(1977) noted that the critical tensile strain causing theonset of visible cracking is not a fundamental material property. Theonset of visible cracking represents a level of damage of aboutCategory 1.

It would be better to think of the tensile strain as a serviceabilityparameter the magnitude of which can be chosen to take account ofdifferent materials and serviceability limit states.

ence ey rep ace crit y lim m ng ens e s ra n

It is also necessary to consider the likely progression of damage afterthe initiating of visible cracking


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The influence of building stiffness If realistic estimates are to be made of allowable relative

deflections of buildings it is necessary to take some account of

Burland et aldrew attention to the work of Fraser and Wardle

(1976) who published some very useful charts showing the

influence of the relative stiffness of rectangular rafts on their

relative deflections

It was shown that a quite small change in stiffness can change

. By representing the global stiffness of a building as a raft it is

possible to make some simple but valuable calculations on the

extent to which the stiffness of a building is likely to reduce

the calculated relative deflections

Fraser and Wardle (1976)

The influence of relative foundation stiffness on the differential

settlement of a rectangular raft


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Movements above tunnels and around excavations

The ground movements resulting from tunnelling and

components of displacement

These have to be taken account of in assessing

impacts on buildings and services

Surface settlement trough above an advancing

tunnel


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Horizontal displacements

point sink assumption

resultant vector of

sp acement po nts

towards tunnel axis

allows horizontal

displacements to be

determined

differentiate to obtain

horizontal strain, h

Observed and

predicted ground

surface

movements

around the New

Palace Yard car

park


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The work of Boscardin and Cording (1989)

Boscardin and Cording introduced two importantadvances:

1. The influence of horizontal ground strain h was addedto the beam model of Burland and Wroth by simplesuperposition. They then developed an interactiondiagram relating angular distortion and h fordifferent categories of damage. This interactiondiagram strictly relates only to L/H = 1 for a hogging

mode of deformation

2. From their work it is possible to assign a range ofvalues of limiting tensile strain lim to the differentcategories of damage defined by Burland et al(1977)

Boscardin and Cording (1989)

Interaction diagram between h and for L/H=1 and neutral

axis at one edge assuming that 2/L


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Relationship between category of damage and

limiting tensile strain (lim)(after Boscardin and Cording 1989)

y

0 0 0.05

1 0.05 0.075

2 0.075 0.15

3 0.15 0.3

4 to 5 > 0.3

A methodology for assessing the risk of damageBurland (1995)

The key objective of the assessment of potentialdamage is to make an assessment of the maximumtensile strain in the simplified building.

ur an , us ng e approac o oscar n anCording of superimposing the horizontal groundstrains, developed the equations for the resultant

bending or diagonal strains as given in the paper (10)and (11).

These equations can be used directly is assessing the

level of tensile strain and damage category.

The equations can also be used to develop simpleinteractive diagrams of/L versus h for a variety ofgeometries and deformation modes.


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Superposition of horizontal strain h

Resultant extreme fibre strain in bending:

hbbr max

Resultant maximum diagonal strain in shear:

2

max

2

2

2

1

2

1dhhdr

Relationship between (/L)/lim and L/H for rectangular

isotropic beams with the neutral axis at the bottom edge (using

elastic beam theory)


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Influence of horizontal strain on (/L)/lim for (a)

bending strain controlling, (b) diagonal strainscontrolling and (c) combinations of (a) and (b)

Relationship of damage category to deflection ratio

and horizontal strain for hogging (L/H = 1)


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Building deformation: partitioning between

sagging and hogging

The influence of building stiffness

Parametric FE analyses by Potts and Addenbrook (1997)


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Geometry for Potts and Addenbrooke parametric study

building length, L

depth to tunnel axis, z0

unne ame er, eccentricity, e

Influence of relative bending stiffness on settlement

profile (Potts and Addenbrooke, 1997)


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Bending stiffness modification factors

(Potts and Addenbrooke, 1997)

sagging

hogging

Comparison of observed and greenfield site settlements of

the Mansion House from driving a 3.05 m diameter tunnel at

15 m depth (Frischmann et al., 1994)


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Methodology for assessing the risk of damage

The concepts described previously may be combined

to provide a rational approach to the assessment of

.

The risk means the possible level of damage in

terms of the 6 Categories.

Most buildings are considered to be at low risk if

the predicted category of damage falls into categories

. A major objective of design and construction is to

maintain the level of risk below this threshold.

Special consideration may have to be given to certain

buildings of particular sensitivity for various reasons

A three stage approach (Burland, 1995)

Stage 1Preliminary assessment

Often in an urban situation a large number of buildings

are located within the settlement trough of a tunnel.

According to Rankine (1988) a building experiencing a

maximum slope of 1/500 and a settlement of less than10mm has negligible risk of damage.

B drawin round surface contours of settlement alon

the route it is possible to eliminate all buildings having

negligible risk.

Particularly sensitive buildings may be retained for the

next stage of assessment.


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Stage 2 second stage assessment

The faade of any building is represented by a simple beamwhose foundations follow the greenfield site displacementscause y t e tunne or excavat on.

The maximum tensile strains are calculated from the equationsand an appropriate category of damage is assigned to the

building.

Though much more detailed than the stage 1 assessment, thisapproach is still very conservative in that the stiffness of the

.

Sometimes the approach of Potts and Addenbrooke is includedat this stage.

Particularly sensitive buildings may be retained for the nextstage of assessment.


Stage 3 Detailed evaluation

Detailed evaluation is carried out on those buildings thatremain as being at risk of category 3 damage or greater.

.

Each building has to be considered in its own right andrequires a detailed expert inspection.

Particular attention has to be paid to: (1) tunnelling andexcavation sequence, (2) structural continuity, (3) thefoundations, (4) orientation of the building, (5) previousmovements and existing cracking.

Following detailed evaluation, which usually results in areduced category of damage from stage 2, considerationhas to be given as to whether protective measures need to

be adopted.


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Protective measures

Before considering surface measures, tunnellingprocedures should be examined. These tackle the rootcause of the problem and may prove much less costly and

srup ve an near sur ace measures.

The paper summarises a range of surface or near surfacemeasures including strengthening the ground, structural

jacking, underpinning and strengthening the building.

Recently compensation grouting has been used with greatsuccess on ver resti ious or sensitive structures.

However it is a very expensive measure and should not beused as a substitute for good quality tunnelling orexcavation procedures.

Conclusions

The methodology described here draws together a number of

related studies including:

The objective definitions of foundation movement

The concept of limiting tensile strain and its simple application

to identify key aspects of behaviour

The objective categorisation of damage which plays a key role

in bringing realism to emotive discussions on damage The importance of building stiffness in modifying

deformations

As ever there, is a need for carefully monitored case studies of

the progressive development of building response which are

then rigorously analysed. We took this opportunity during the

construction of the Jubilee Line Extension


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Cross-section through Westminster station box and

the Palace of Westminster

Protection of the Big

Ben Clock Tower by

means of

com ensation

grouting

Documents

The assessment of the risk of damage to buildings due to tunnelling and excavations - J Burland