The assessment of the risk of damage to
buildings due to tunnelling and excavations –
AN HISTORICAL PERSPECTIVE
John Burland
I
Imperial
i l College
C ll
London
L d
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 – δ/L
• Concluded that cracking occurs when δ/L > 1/300
and recommended designing to ≤ 1/500
• Structural damage occurs when δ/L > 1/150
1
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
MacDonald’s
analysis of field
evidence of damage
on traditional frame
buildings and loadbearing brick walls
2
Bjerrum (1963) supplemented the guidance with
the following recommendations for δ/L :
•
•
•
•
•
•
Difficulties with machines sensitive to settlement
Danger for frames with diagonals
Buildings must not crack
First cracks occur
Tilting of high buildings noticeable
St t l damage
Structural
d
may occur
> 1/750
> 1/600
≤ 1/500
> 1/300
> 1/250
> 1/150
Limitations to the routine guidelines:
• Based mainly on indirect evidence from the literature
• Deals only with traditional buildings and structural
members
b
• Though presented by Skempton as tentative, often
stated as ‘rules’
• ‘Damage’ not objectively defined or classified
• Evidence for masonry walls very suspect
• Angular distortion δ/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
3
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
4
Palace of Westminster
Cracking in the south wall of the Annexe due to
hogging - δ/L « 1/300
Palace of Westminster
Cracking in the west wall of Westminster Hall due to
sagging and hogging
5
Burland and Wroth (1974)
It became clear that a more fundamental
f ndamental approach was
as
needed in assessing limiting deformations and that
angular distortion was unsatisfactory in a number of
ways
It was fi
firstt necessary tto sett outt definitions
d fi iti
off
foundation movement which do not make assumptions
about the mode of deformation of the superstructure
Definitions of ground and foundation movement
Burland and Wroth (1974)
• (a) Rotation or slope, θ,
and angular strain, α.
• (b) Relative deflection, Δ,
and deflection ratio, Δ/L.
• (c) Tilt, ω and relative
rotation, β (angular
distortion)
6
Tensile strain as a parameter giving rise to
cracking
• Burland and Wroth argued that there was a need to move away
from empirical deflection criteria and study 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
• They
Th therefore
th f
carried
i d outt a study
t d off the
th workk carried
i d outt att
the Building Research Establishment on the behaviour of
masonry and blockwork when subjected to a variety of loading
conditions
The concept of Critical Tensile Strain εcrit
• They noted that the locally determined tensile strains at
which cracking became visible was reasonably well defined
and independent
p
both of the tensile strength
g of the masonryy
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 of εcrit varied between about
0.05% and 0.1% and suggested using an average value of
0.075%
• They stressed that εcrit 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
7
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
8
The cracking of simple beams in bending and
shear
• Burland and Wroth then applied the concept 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
thi study
this
t d hhas hhelped
l d tto illustrate
ill t t a number
b off
important features that control limiting values of Δ/L
Cracking of a simple beam in bending and in shear
• actual building
• equivalent deep beam
• deflected shape of soffit
• bending deformation
• shear deformation
9
Centrally loaded beam with both bending and
shear stiffness
Extreme fibre strain εb max :
3I E 
  L
  b max
 

L  12 t 2 yLH G 
Maximum diagonal
g
strain εd max
  HL2 G 
  d max
 1 
L  18 I H 
Burland and Wroth (1974)
Limiting relationships between Δ/L and L/H for a uniform load and a central point load.
Conclusion: The relationship between εmax and Δ/L is
insensitive to the form of loading
10
Burland and Wroth (1974) and Burland et al (1977)
Influence of mode of deformation and E/G on limiting
values of Δ/L for a simple beam
11
Burland and Wroth
(1974)
Frame buildings and
Loadbearing walls
undergoing sagging and
hogging
Relationship between
Δ/L and L/H for various
degrees of damage
Comparison with simple
elastic beams with
εcrit = 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.
• In 1976 the claims for subsidence damage against household
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
12
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
action
i is
i taken
k
• 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
• More stringent criteria may be necessary where cracking could lead to
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
13
Example of Category 1
damage
Fine cracks which are
easily treated during
normal decoration
The house was
underpinned
d i d att a costt off
about 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 is
usually 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
14
Example of Category 3
damage - Moderate
Repointing of external
brickwork. Some
brickwork required
replacing above and below
windows
Example of Category 4 damage - Severe
Gable wall leaning outwards with some loss of bearing of beams
15
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 the
onset of visible crackingg is not a fundamental material pproperty.
p y The
onset of visible cracking represents a level of damage of about
Category 1.
• It would be better to think of the tensile strain as a serviceability
parameter the magnitude of which can be chosen to take account of
different materials and serviceability limit states.
• Hence
H
th
they replaced
l d εcrit by
b εlim – limiting
li iti tensile
t il strain
t i
• It is also necessary to consider the likely progression of damage after
the initiating of visible cracking
16
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
their stiffness
• Burland et al drew 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
a raft from being relatively very flexible to very stiff.
stiff
• 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
17
Movements above tunnels and around excavations
• The ground movements resulting from tunnelling and
from excavations often include significant horizontal
components of displacement
• These have to be taken account of in assessing
impacts on buildings and services
Surface settlement trough above an advancing
tunnel
18
Horizontal displacements
• point sink assumption
• resultant vector of
di l
displacement
points
i
towards tunnel axis
• allows horizontal
displacements to be
determined
• differentiate to obtain
horizontal strain, εh
Observed and
predicted ground
surface
movements
around the New
Palace Yard car
park
19
The work of Boscardin and Cording (1989)
Boscardin and Cording introduced two important
advances:
1. The influence of horizontal ground strain εh was added
to the beam model of Burland and Wroth by simple
superposition. They then developed an interaction
diagram relating angular distortion β and εh for
different categories of damage. This interaction
diagram strictly relates only to L/H = 1 for a hogging
mode of deformation
2. From their work it is possible to assign a range of
values of limiting tensile strain εlim to the different
categories of damage defined by Burland et al (1977)
Boscardin and Cording (1989)
Interaction diagram between εh and β for L/H=1 and neutral
axis at one edge assuming that β ≈ 2Δ/L
20
Relationship between category of damage and
limiting tensile strain (εlim)
(after Boscardin and Cording 1989)
Categor of damage
Category
0
1
Limiting tensile strain (%)
0 – 0.05
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 damage
Burland (1995)
• The key objective of the assessment of potential
damage is to make an assessment of the maximum
tensile strain in the simplified building.
• Burland
B l d (1995),
(1995) using
i the
th approachh off Boscardin
B
di andd
Cording of superimposing the horizontal ground
strains, 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
potential for damage using the relationship between
level of tensile strain and damage category.
• The equations can also be used to develop simple
interactive diagrams of Δ/L versus εh for a variety of
geometries and deformation modes.
21
Superposition of horizontal strain εh
• Resultant extreme fibre strain in bending:
 br   b max   h
• Resultant
maximum diagonal strain in shear:
 1 
 dr   h 
 2
2

2  1  
2







h
d max
 2 

Relationship between (Δ/L)/εlim and L/H for rectangular
isotropic beams with the neutral axis at the bottom edge (using
elastic beam theory)
22
Influence of horizontal strain on (Δ/L)/εlim for (a)
bending strain controlling, (b) diagonal strains
controlling and (c) combinations of (a) and (b)
Relationship of damage category to deflection ratio
and horizontal strain for hogging (L/H = 1)
23
Building deformation: partitioning between
sagging and hogging
The influence of building stiffness
Parametric FE analyses by Potts and Addenbrook (1997)
24
Geometry for Potts and Addenbrooke parametric study
•
•
•
•
building length, L
depth to tunnel axis, z0
t
tunnel
l diameter,
di
t D
eccentricity, e
Influence of relative bending stiffness on settlement
profile (Potts and Addenbrooke, 1997)
25
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)
26
Methodology for assessing the risk of damage
• The concepts described previously may be combined
to provide a rational approach to the assessment of
risk of damage due to tunnelling and excavation
excavation.
• ‘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
0 to 2.
2
• 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 1 – Preliminary assessment
g number of buildings
g
• Often in an urban situation a large
are located within the settlement trough of a tunnel.
• According to Rankine (1988) a building experiencing a
maximum slope θ of 1/500 and a settlement of less than
10mm has negligible risk of damage.
y drawingg ground
g
surface contours of settlement alongg
• By
the route it is possible to eliminate all buildings having
negligible risk.
• Particularly sensitive buildings may be retained for the
next stage of assessment.
27
A three stage approach (Burland, 1995)
Stage 2 – second stage assessment
• The façade of any building is represented by a simple beam
whose foundations follow the ‘greenfield site’ displacements
causedd by
b the
h tunnell or excavation.
i
• The maximum tensile strains are calculated from the equations
and an appropriate category of damage is assigned to the
building.
• Though much more detailed than the stage 1 assessment, this
approach is still very conservative in that the stiffness of the
building is neglected.
neglected
• Sometimes the approach of Potts and Addenbrooke is included
at this stage.
• Particularly sensitive buildings may be retained for the next
stage of assessment.
A three stage approach (Burland, 1995)
Stage 3 – Detailed evaluation
• Detailed evaluation is carried out on those buildings that
remain as being at risk of category 3 damage or greater.
Also any buildings of particular sensitivity.
sensitivity
• Each building has to be considered in its own right and
requires a detailed expert inspection.
• Particular attention has to be paid to: (1) tunnelling and
excavation sequence, (2) structural continuity, (3) the
foundations, ((4)) orientation of the building,
g ((5)) pprevious
movements and existing cracking.
• Following detailed evaluation, which usually results in a
reduced category of damage from stage 2, consideration
has to be given as to whether protective measures need to
be adopted.
28
Protective measures
• Before considering surface measures, tunnelling
procedures should be examined. These tackle the root
cause of the problem and may prove much less costly and
di
disruptive
ti than
th near surface
f
measures.
• The paper summarises a range of surface or near surface
measures including strengthening the ground, structural
jacking, underpinning and strengthening the building.
• Recently compensation grouting has been used with great
success on
o very
ve y prestigious
p est g ous or
o sensitive
se s t ve structures.
st uctu es.
However it is a very expensive measure and should not be
used as a substitute for good quality tunnelling or
excavation 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
• The need for a staged approach to assessing the risk of damage
• 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
29
Cross-section through Westminster station box and
the Palace of Westminster
Protection of the Big
Ben Clock Tower by
means of
compensation
p
grouting
30