The assessment of the risk of damage to buildings due to tunnelling and excavations AN HISTORICAL PERSPECTIVE

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1 The assessment of the risk of damage to buildings due to tunnelling and excavations AN HISTORICAL PERSPECTIVE John Burland Imperial College London 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

2 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

3 Bjerrum (1963) supplemented the guidance with the following recommendations for δ/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 Structural t ld damage may occur > 1/150 Limitations to the routine guidelines: Based mainly on indirect evidence from the literature Deals only with traditional buildings and structural members 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

4 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

5 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

6 Burland and Wroth (1974) It became clear that a more fundamental approach was needed in assessing limiting deformations and that angular distortion was unsatisfactory in a number of ways It was first necessary to set out definitions iti of 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

7 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 therefore carried out a study of the work carried out at 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 ε 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 of ε crit varied between about 0.05% and 0.1% and suggested using an average value of 0.075% They stressed that ε 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 7

8 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

9 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 this study has helped to illustrate t a number of 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

10 Centrally loaded beam with both bending and shear stiffness Extreme fibre strain ε b max : L L 12t 3I 2yLH E G b max Maximum diagonal strain ε dmax L 2 HL 18 I G H 1 d max 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 ε max and Δ/L is insensitive to the form of loading 10

11 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

12 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 ε 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. 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

13 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 is taken 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

14 Example of Category 1 damage Fine cracks which are easily treated during normal decoration The house was underpinned dat a cost of 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

15 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

16 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 cracking is not a fundamental material property. p 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 they replaced ε crit by ε lim limiting iti tensile strain It is also necessary to consider the likely progression of damage after the initiating of visible cracking 16

17 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. 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

18 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

19 Horizontal displacements point sink assumption resultant vector of displacement points towards tunnel axis allows horizontal displacements to be determined differentiate to obtain horizontal strain, ε h Observed and predicted ground surface movements around the New Palace Yard car park 19

20 The work of Boscardin and Cording (1989) Boscardin and Cording introduced two important advances: 1. The influence of horizontal ground strain ε h was added to the beam model of Burland and Wroth by simple superposition. They then developed an interaction diagram relating angular distortion β and ε h 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 ε lim to the different categories of damage defined by Burland et al (1977) Boscardin and Cording (1989) Interaction diagram between ε h and β for L/H=1 and neutral axis at one edge assuming that β 2Δ/L 20

21 Relationship between category of damage and limiting tensile strain (ε lim ) (after Boscardin and Cording 1989) Category of damage Limiting tensile strain (%) 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 (1995), using the approach of Boscardin and 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 ε h for a variety of geometries and deformation modes. 21

22 Superposition of horizontal strain ε h Resultant extreme fibre strain in bending: br b max h Resultant maximum diagonal strain in shear: dr 1 h h d max Relationship between (Δ/L)/ε lim and L/H for rectangular isotropic beams with the neutral axis at the bottom edge (using elastic beam theory) 22

23 Influence of horizontal strain on (Δ/L)/ε lim 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

24 Building deformation: partitioning between sagging and hogging The influence of building stiffness Parametric FE analyses by Potts and Addenbrook (1997) 24

25 Geometry for Potts and Addenbrooke parametric study building length, L depth to tunnel axis, z 0 tunnel ldiameter, D eccentricity, e Influence of relative bending stiffness on settlement profile (Potts and Addenbrooke, 1997) 25

26 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

27 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. 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 0to2 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 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 θ of 1/500 and a settlement of less than 10mm has negligible risk of damage. By drawing ground surface contours of settlement along 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

28 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 caused dby the tunnel or excavation. 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. 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. 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, (5) previous 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

29 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 disruptive than near surface 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 very prestigious or sensitive s structures. 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 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

30 Cross-section through Westminster station box and the Palace of Westminster Protection of the Big Ben Clock Tower by means of compensation grouting 30