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Assessing the impact of excavation-inducedmovements on adjacent buildingsPaul Fok a , Bian Hong Neo a , Kok Hun Goh a & Dazhi Wen aa Land Transport Authority , SingaporePublished online: 18 Jul 2012.

To cite this article: Paul Fok , Bian Hong Neo , Kok Hun Goh & Dazhi Wen (2012) Assessing the impact of excavation-induced movements on adjacent buildings, The IES Journal Part A: Civil & Structural Engineering, 5:3, 195-203, DOI:10.1080/19373260.2012.696444

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TECHNICAL PAPER

Assessing the impact of excavation-induced movements on adjacent buildings

Paul Fok, Bian Hong Neo, Kok Hun Goh* and Dazhi Wen

Land Transport Authority, Singapore

(Received 9 March 2012; final version received 14 April 2012)

Activities associated with deep excavations can cause ground movements. It is thus important to assess the impactcaused by excavation-induced movements on existing buildings and facilities. This paper discusses the approach forassessing the risk of building damage due to deep excavations. This is largely similar to the methodology described inTR26: 2010 but slightly modified to include a desk-study on influence zone for excavations and to include the currentstructural condition from visual inspection surveys into the risk assessment framework. The paper further highlightsthe opportunities and limitations associated with including the influence of building stiffness into the damageassessment framework. Finally, the paper supplements the recommendations made in TR26: 2010 by reviewing theconsiderations that should be provided for reinforced concrete structures, buildings on piled foundations, buildingson mixed foundations, and buildings with poor structural condition.

Keywords: risk assessment; building damage; deep excavations

1. Introduction

One of the biggest challenges when constructing amajor underground infrastructure within a denselybuilt-up urban environment like Singapore is theimpact caused onto existing buildings and facilitiesthat are adjacent to the construction. Hence, buildingimpact assessment forms an essential part of the riskmanagement process to mitigate the risks of deepexcavation onto buildings in the vicinity (Fok et al.2012). In Singapore, the impact assessment forbuildings follows broadly the staged approach sug-gested by Mair et al. (1996), and this is describedin LTA Civil Design Criteria (2010) and TR26: 2010.This paper discusses the approach for risk assess-ment of building impact due to deep excavationsand proposes some adjustments on the currentmethodology.

2. Classification of building damage

In the absence of objective guidelines based onexperience, the classification of building damage canbecome a highly subjective and emotive exercise, whichwould be conditioned by the attitudes of variousinterest groups (insurers, owners, builders, etc.). As aresult, there could be unrealistic expectations andextreme attitudes toward damage classification. As amatter of fact, many buildings may already beexperiencing some degree of cracking, sometimesarising from shrinkage and thermal movements and

unrelated to foundation movements which can beeasily repaired during routine maintenance. An objec-tive system of classifying building damage has beendeveloped in the UK by Building Research Establish-ment (1995), and this has gained worldwide acceptanceas the basis for a logical and realistic framework toassess building damage.

The classification of damage associated withvarious degrees of severity and having particularreference to ease of repair of plaster and brickworkor masonry, is shown in Table 1. Building damage ismulti-faceted, and the tendency to classify buildingdamage solely based on crack width should be resisted.It should be highlighted that this classification systemis based on the ease of repair of the visible damage.There are six categories of damage, numbered 0 to 5 inincreasing severity. Generally, damage categories 0, 1and 2 relate to aesthetic damage which can be easilyrectified, and such damages usually arise from acombination of causes either from within the buildingitself (e.g. shrinkage, temperature effects) or due toground movements. Damage categories 3 and 4 relateto serviceability and functional damages which canaffect doors, windows, weather tightness, and requiresubstantial repair. Damage category 5 relates tostructural stability concerns and would require majorrepair jobs. The division between damage categories 2and 3 therefore provides the threshold on what isdeemed acceptable and what is deemed unacceptable inthe risk assessment process.

*Corresponding author. Email: kok_hun_goh@lta.gov.sg

The IES Journal Part A: Civil & Structural Engineering

Vol. 5, No. 3, August 2012, 195–203

ISSN 1937-3260 print/ISSN 1937-3279 online

� 2012 The Institution of Engineers, Singapore

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Furthermore, it was found that the onset ofcracking is closely related to the development of tensilestrains in the building. By verifying with full-scale testsand back-analysing actual case histories, Burland et al.(1977) and Boscardin and Cording (1989) showed thatthe category of building damage can be related to themagnitude of the tensile strains, and identified therange of tensile strains corresponding to variousdamage categories (as shown in Table 2). This makesit possible to relate the risk of causing visible damageby estimating the tensile strains that would be inducedin a building. Furthermore, although the classificationin Tables 1 and 2 was developed primarily forbrickwork or blockwork and stone masonry, thesecould also be adapted for other forms of cladding,including frame structures where the infills are usuallymasonry and blockwork.

3. Assessment of potential damage to buildings

3.1. Zone of influence

Figure 1 suggests a framework for assessing the risk ofinducing damage to adjacent buildings due to excava-tion and tunnelling. The first step in the methodologyis to ascertain the zone of influence of the constructionso that all buildings and structures within the zone ofinfluence can be assessed for potential damage. Thezone of influence should be defined based on previousexperience in excavation activities through a desk-study of previously completed case histories, and

verified using numerical modelling when details ofconstruction method are available. As an example ofthe desk study, the upper bound envelopes by Cloughand O’Rourke (1990) suggest that the zone of influenceextends up to three times the excavation depth behindthe wall. Where there is a thick layer of soft soil belowthe final excavation level, the zone of influence wouldrelate to the depth of the soft material rather than theexcavation depth, so that the zone of influence wouldbecome three times the depth of soft soils.

However, when the effects of far-field consolidationare persistent, it is observed from local practice andexperience that the excavation zone of influence couldextend to a few hundred metres away from theexcavation. This can be attributed to various reasons,

Table 1. BRE classification of visible damage to walls with particular reference to ease of repair of plaster and brickwork ormasonry (after Mair et al. 1996).

Categoryof damage

Normal degreeof severity Description of typical damage (Ease of repair is italicised)

0 Negligible Hairline cracks less than about 0.1 mm.1 Very slight Fine cracks which are easily treated during normal decoration. Damage generally restricted to

internal wall finishes. Close inspection may reveal some cracks in external brickwork ormasonry. Typical crack widths up to 1 mm.

2 Slight Cracks easily filled. Re-decoration probably required. Recurrent cracks can be masked bysuitable linings. Cracks may be visible externally and some repointing may be required toensure weathertightness. Doors and windows may stick slightly. Typical crack widthsup to 5 mm.

3 Moderate The cracks require some opening up and can be patched by a mason. Repointing of externalbrickwork and possibly a small amount of brickwork to be replaced. Doors and windowssticking. Service pipes may fracture. Weathertightness often impaired. Typical crack widthsare 5–15 mm or several greater than 3 mm.

4 Severe Extensive repair work involving breaking-out and replacing sections of walls, especially overdoors and windows. Windows and door frames distorted, floor sloping noticeably1. Walls orbuilding leaning1 noticeably, some loss of bearing in beams. Service pipes disrupted.Typical crack widths are 15–25 mm but also depends on the number of cracks.

5 Very severe This requires a major repair job involving partial or complete rebuilding. Beams lose bearing,walls lean badly and require shoring. Windows broken with distortion. Danger ofinstability. Typical crack widths are greater than 25 mm but depends on the number of cracks.

1Note: Local deviation of slope, from the horizontal or vertical, of more than 1/100 will normally be clearly visible. Overall deviations in excess of1/150 are undesirable.2Note: Crack width is only one factor in assessing category of damage and should not be used on its own as a direct measure of it.

Table 2. Relationship between damage category and limit-ing tensile strain (after Boscardin and Cording 1989).

Damage categoryNormal degreeof severity

Limiting tensilestrain 2lim (%)

0 Negligible 0–0.051 Very slight 0.05–0.0752 Slight 0.075–0.153 Moderate* 0.15–0.34–5 Severe to very severe 40.3

*Note: Boscardin and Cording describe the damage corresponding to2lim in the range 0.15–0.3% as ‘moderate to severe’. However, noneof the cases quoted by them exhibits severe damage for this range ofstrains. There is therefore no evidence to suggest that tensile strainsup to 0.3% will result in severe damage.

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including under-drainage of the marine clay (Wen andLin 2002), water leakage in the retaining wall (Halimand Wong 2006) and the amount and duration ofexcavation dewatering. However, if the extent ofdewatering can be controlled during the construction,such as by enhancing hydraulic cut-off below theexcavation with ground improvement or longer wallsand/or by implementing recharge wells, far-fieldconsolidation effects can be mitigated and it was foundthat the classical envelope by Clough and O’Rourkedoes provide a reasonable bound on observed data.This was observed by Goh and Cham (2011) from the

ground settlement monitoring data at some excavationsites in Circle Line Stage 5. Figure 2 shows a schematicfor the zone of influence of excavations where far-fieldconsolidation effects are absent.

After establishing the zone of influence, analysis ofexcavations in the ‘greenfield’ condition would beconducted to estimate the displacements correspondingto each building location. The prediction of grounddisplacements would typically be done using empiricaland numerical methods, and would include effects suchas wall installation, support installation and removal,movements during excavation and soil removal, and

Figure 1. Framework for assessing the risk of building damage.

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consolidation. In Singapore, ground displacements areusually predicted using finite element models. Potts(2003) highlighted two key challenges on the applica-tion of numerical models in geotechnical analysis,firstly that the results are user dependent and variesaccording to the assumptions made during numericalmodelling, and secondly that there are limitations withcurrent constitutive models and their ability to modelactual soil behaviour such as nonlinearity, small-strainstiffness, anisotropy, etc. Hence, it is useful to checkthe results against empirical formulae that has beendeveloped and verified with actual excavation data.Furthermore, the desk-study to establish the zone ofinfluence based on actual case histories could also beused as a verification check on the predictions fromnumerical analysis. Concurrently, the as-built structur-al drawings and records would be obtained for allbuildings within the zone of influence and a visualinspection of structure would be conducted for eachbuilding.

The objective of the visual inspection is to ascertainthe existing structural condition of the building.Generally, this requires all structural elements to bevisually inspected and logged for defects includinglocating and measuring existing cracks. The visualinspection should attempt to identify signs of structur-al distress, including (1) the presence of cracks, (2)structural defects such as concrete spalling, (3) unusualdeformations of structure, (4) material deteriorationand water seepage, (5) overloading of structure due tochange in use of building and (6) addition andalteration works that are causing adverse effects onthe structure. Incorporating the existing condition ofthe building into the risk assessment gives a sense ofwhether the building can take additional movementsarising from excavation activities or if protective andstrengthening measures should be undertaken so thatthe building can tolerate the additional ground move-ments. If the existing condition of the building cannotbe ascertained due to lack of as-built information,diagnostic inspections could be conducted and thiswould include measurements of structural dimensions,conduct of trial pits, and non-destructive testing ofstructural elements.

3.2. Staged approach to risk assessment

As seen in Figure 1, a staged approach to assessing thepotential damage of buildings is adopted. From the as-built structural records and the visual inspectionsurvey, the buildings are classified into the followingcategories: (1) building on shallow foundation, (2)building on deep foundation, (3) building on mixedfoundation, (4) building in poor structural conditionand (5) sensitive structures. For building on shallowfoundation, the approach follows the outline given inMair et al. (1996) closely. This consists of three stages:-

. Stage 1 preliminary assessment. A building withina settlement zone of less than 10 mm andexperiencing a slope of less than 1:500 can beconsidered to have a negligible risk of damage,and can be eliminated in this first stage.

. Stage 2 assessment using tensile strain method.This is based on estimating the maximum tensilestrain induced in the building by idealising thebuilding as a beam, and calculating the max-imum tensile strain using the deflection ratiofrom the settlement profile and the horizontalstrain from the horizontal displacement profile.The induced maximum tensile strain is thenchecked against the limiting tensile strain corre-sponding to various building damage categoriesas shown in Table 2. This approach assumes thatthe building has no stiffness and will deflectaccording to the deflection ratios and horizontalstrains corresponding to the greenfield. This is aconservative assumption as the inherent stiffnessof buildings will reduce both the deflection ratioand horizontal strains. Buildings assessed to have‘negligible’, ‘very slight’, and ‘slight’ damagecategories are considered to be at low risk ofdamage, and can be eliminated from the assess-ment at this stage.

. Stage 3 detailed evaluation. For buildings as-sessed to have a moderate risk of damage andhigher, detailed evaluation is to be undertaken.This would require, although not necessarilylimited to, evaluating the structural details of thebuilding, giving full consideration of the se-quence and method of excavation, and consider-ing soil-structure interaction effects. Theinfluence of building stiffness is considered atthis stage. Where required, the detailed evalua-tion may also include a review on the additionalstresses caused onto structural elements underthe influence of external-induced distortions andmovements, and to check that the serviceabilitylimit states of the building are not exceeded. Thiswould require a structural analysis of the keyelements in the building.

Figure 2. Zone of influence for excavations without far-field consolidation effects.

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If the risk of damage is still unacceptable even afterthe detailed evaluation (i.e. moderate risk of damage orhigher), mitigation and/or protective measures wouldbe provided for the buildings. This would include (1)internal structural measures which refer to actionstaken within the excavations during its construction toreduce the ground movements generated at source,such as increasing wall thickness, stiffening struttingsupports and construction sequencing, (2) externalstructural measures which reduce the impact of groundmovements by increasing the capacity of the adjacentstructure to resist, modify or accommodate thosemovements, such as underpinning and structuralstrengthening and (3) ground treatment measureswhich include all methods of reducing or modifyingthe induced ground movements through improving orchanging the engineering response of the ground.

3.3. Effect of building stiffness

In reality, the stiffness of a building will have an influenceon its response to excavation-inducedmovements.Whilea building with low stiffness would follow the grounddeformations in the greenfield condition, a building withhigh stiffness tends to rotate and move as a rigid body,and experiences lower tensile strains. This sectiondiscusses the impact of building stiffness on horizontaldisplacement and settlement behaviour of the building.

In term of horizontal displacement, a building oncontinuous foundation can reduce the horizontal strainsfrom thegreenfield condition greatly. This is observedbyDimmock and Mair (2008) from the actual monitoringofMurdochHouse andCleggHouse inLondon, that thehorizontal strains induced due to bored tunnelling belowthe buildings are negligible compared to the horizontalstrains induced in the greenfield. Other than horizontalslip occurring between the soil and the foundations(Elshafie 2008) which reduced the horizontal strainsimparted to the building, it was also reported thatbuildingswould have sufficient axial stiffness in reality sothat the horizontal strains induced would be negligible(Goh andMair 2011a). Figure 3 illustrates some findingsfrom the above studies on the horizontal strains inbuildings on continuous footings. However, for build-ings with walls/columns on isolated or individualfootings, there would be significant horizontal strainsinduced to the structural elements at the ground floor ofthe building. This was reported by Goh and Mair(2011b) from back-analysing tape extensometer databetween columns during the bored tunnelling worksdirectly below the rows of shop-houses at Pasir PanjangRoad (see Figure 4). It is important to give dueconsiderations on the arrangement and condition ofthe foundations when assessing the horizontal strainstransferred into the building.

In term of vertical displacement, several studies havebeen published from case histories and numerical studiesto show that a building with higher bending stiffness willgenerally reduce the deflection induced into the building.Using data monitored during the Singapore Circle Lineconstruction, Goh and Mair (2010, 2011c) reported thatthe Singapore Art Museum and the Pasir Panjangshophouses exhibited stiffer settlement responses com-pared to the ground and the greenfield conditionsrespectively (Figure 5). Design charts relating thereduction in deflection ratio of buildings with theirbending stiffness have been developed by Potts andAddenbrooke (1997) for tunnelling and Goh and Mair(2011a) for braced excavations. However, there is noguideline on estimating the representative bendingstiffness. Back-analysis using field studies showed thatthe representative bending stiffness depends on a varietyof factors, including the effect of wall openings, frameaction and also the difference between sagging andhogging deformation modes. This makes it difficult toestimate the bending stiffness of actual buildings so thatits influence can be properly accounted for in the damageassessment. More research and verification using back-analysis of actual field cases would be required.

To summarise, horizontal strains in buildings can bereduced considerably if the building is on continuousfootings, but horizontal strains can be quite significantfor buildings on isolated or individual footings. It isimportant to assess the foundation arrangement prop-erly before deciding if horizontal strains could be omi-tted from the tensile strain estimates. On the other hand,for settlement behaviour of buildings, although designcharts have been developed to relate building deflectionto bending stiffness, the difficulty in estimating therepresentative bending stiffness of actual buildings couldrestrict the application of such design charts in incor-porating the influence of bending stiffness. Nevertheless,design guidance such as the one shown in Figure 6 doesgive broad indication of whether the building is rigidenough for deflection ratios to be neglected.

3.4. Reinforced concrete framed structures

The actual damage on reinforced concrete structuresdue to induced ground movements can only beascertained using structural analysis and checkedagainst the structural capacities of the key load-carrying elements. Nevertheless, the potential damageinduced onto reinforced concrete structures may beapproximated by the damage induced onto theircladding and infill walls which is often brickworkand blockwork. Hence, reinforced concrete structurescan be assessed using a similar methodology outlinedfor masonry buildings using tensile strains, except witha slight modification on the Young’s modulus to shear

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modulus (E/G) ratio to account for the greater shearflexibility observed in reinforced concrete frame build-ings (after Mair et al. 1996).

3.5. Building on pile foundation

Where a building or structure is founded on piles,lateral and vertical ground movements caused by anadjacent excavation could induce lateral and verticalforces onto the piles. The induced effects can beestimated by using numerical analysis tools that takeinto account the three-dimensional nature of pilebehaviour – but this can be rather complicated to

model. A more commonly used approach is to firstdetermine the greenfield ground deformations causedby the excavation and corresponding to the pilelocation using a 2D finite element method, and thenusing a pile-soil interaction programme to estimatethe forces and moments induced onto the piles. Othermethods to estimate the induced effects are providedin TR26: 2010.

Consequently, an assessment on the adequacy ofthe pile capacity should be carried out by estimatingthe additional bending moments and down draginduced from excavation activities. Furthermore, thepotential for displacements at the top of the piles

Figure 3. Horizontal strains for buildings on continuous foundations.

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(particularly for short piles) and the effect on thebuilding or structure due to differential pile movementsshould also be assessed. For high rise buildings,particular attention has to be given to assess theallowable tilt arising from pile head movements androtations.

3.6. Building on mixed foundation and sensitivebuildings

Buildings are extremely sensitive to differential settle-ment when one part of the building settles with theground and another part does not. This can be due tothe building being partly on shallow foundations andpartly on pile foundations, or even a building beingessentially on shallow foundations but one side beingeffectively restrained from settling freely by anabutting building which is piled. It is not uncommonto find buildings or structures in Singapore to beconstructed or renovated such that they have mixedfoundations. For such buildings, a Stage 3 detailedevaluation should be carried out taking into accountthe nature of the building or structure and thefoundations.

There are also some buildings that are classified asmonuments and conservation buildings. Due to theirage, portions of the building may have deteriorated,such as the plaster holding the tiles in place andthe mortar between masonry materials becoming morebrittle with time. Together with the associated heritagevalues, there could be a lower tolerance for suchbuildings to withstand external-induced movements.These are classified as sensitive structures, and a Stage3 detailed evaluation should be carried out taking intoaccount the tolerable movements allowed by thefinishes, claddings and masonry.

3.7. Buildings in poor structural condition

The tensile strain approach assumes that buildingbehaviour is elastic and the building is in a goodcondition. This assumption would not be true forbuildings with significant cracks and/or are showingunusual deformations. The existing structural condi-tion of the building can be ascertained through adetailed visual inspection, and then incorporated intothe building damage assessment works. In particularly,for buildings that are found to be in poor structuralcondition, a Stage 3 detailed evaluation should always

Figure 4. Horizontal strains for shop-houses on individual footings at Pasir Panjang (after Goh and Mair 2011b).

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be carried out in the assessment of excavation impacton the building including an evaluation on theadditional stresses induced into the structure.

4. Conclusion

The approach for assessing the risk of building damagearising from excavation-induced movements on adja-cent buildings is discussed in this paper. The stagedassessment approach is largely similar to the current

methodology described in TR26: 2010, but with somerefinements to include a desk-study on the influencezone for excavations and to include the currentstructural condition from a visual inspection surveyinto the risk assessment framework. The paperelaborates on the influence of building stiffness onhorizontal and vertical displacement response of thebuilding. Horizontal strains in buildings can bereduced considerably if the building is on continuousfootings, but not for buildings on isolated or individual

Figure 5. Building settlement behaviour (a) during excavation in front of Singapore Art Museum, and (b) tunnelling directlybelow Pasir Panjang shophouses.

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footings. Although the settlement response of buildingmay be influenced by its stiffness, the difficulty inestimating bending stiffness properly limits itsapplication.

The paper also reviews the considerations thatshould be provided for reinforced concrete structuresand buildings in poor structural condition and/or onmixed foundations, and discusses the approach forbuildings on piled foundations. With the suggestionsmade in this paper to supplement the guidance alreadyprovided in TR26: 2010, it is hoped that the process ofbuilding impact assessment can be further strength-ened to mitigate the risks of excavation onto adjacentstructures in order to achieve the dual objectives ofsafety and cost-efficiency.

References

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Building Research Establishment, 1995. BRE Digest 251:assessment of damage in low rise buildings withparticular reference to progressive foundation move-ments, revised 1995. Can be obtained from BREAdvisory Service, Garston, Watford, WD2 7JR.

Clough, G.W. and O’Rourke, T.D., 1990. Constructioninduced movements of in-situ walls. In: ASCE Geotech-nical special publication No.25 – Design and Performanceof Earth Retaining structures. New York: ASCE, 978-0-87262-761-1, 439–470.

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Goh, K.H. and Mair, R.J., 2011b. The horizontal response offramed buildings on individual footings to excavation-induced movements. In: Proceedings of 7th internationalsymposium TC28 geotechnical aspects of undergroundconstruction in soft ground, 16–18 May, Rome.

Goh, K.H. and Mair, R.J., 2011c. Building damageassessment for deep excavations in Singapore and theinfluence of building stiffness. Geotechnical EngineeringJournal of the SEAGS & AGSSEA, 42 (3), 1–12.

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Figure 6. Proposed design guidance for deflection ratio(after Goh and Mair 2011c).

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