Comparative evaluation of building responses toan adjacent braced excavation
S.J. Boone, J. Westland, and R. Nusink
Abstract: Construction data from a large braced excavation are evaluated in comparison to several methods ofpredicting the response of buildings to excavation-induced ground movements. The project included an excavation ofup to about 20 m depth, over 650 m long, and 20 m wide made through generally competent glacial overburden.Excavation support was achieved using a braced soldier-pile and lagging wall system. A detailed instrumentationprogram was undertaken by the owner to monitor contractor compliance with ground and structure movement criteria.Data from 46 structures, with damage ranging from negligible to moderate categories, are presented, with four casespresented in detail. A modified approach to estimating potential damage categorization is provided and compared tocase histories. Good agreement is demonstrated between actual and estimated damage categories.
Key words: building damage, excavations, cracking, angular distortion, settlement.
Resumé: Les données de construction d’une grande excavation étançonnée sont évaluées en comparant plusieursméthodes de prédiction de la réaction des bâtiments aux mouvements du terrain induits par une excavation. Le projetinclut une excavation dépassant environ 20 m de profondeur, 650 m de longueur, et 20 m de largeur réalisée dans unmort-terrain de matériau glaciaire généralement de bonne qualité. Le soutènement de l’excavation était assuré par unsystème de mur constitué de pieux verticaux étançonnés joints entre eux par des garnitures. Un programmed’instrumentation détaillé a été mis en place par le propriétaire pour s’assurer que le constructeur rencontrait lescritères de mouvement de la structure et du terrain. Les données provenant de 46 structures ayant subi des dommagesqui se situaient dans des catégories s’étendant de négligeables à modérés sont présentées de même que les détails dequatre cas. Une approche modifiée pour évaluer la catégorisation du dommage potentiel est fournie et comparée à deshistoires de cas. L’on démontre qu’il y a une bonne concordance entre les catégories de dommages réels et estimés.
Mots clés: dommage aux bâtiments, excavations, fissuration, distorsion angulaire, tassement.
[Traduit par la Rédaction] Boone et al. 223
Introduction
R.B. Peck (Peck 1984) stated that one of the main goalsduring the early design phase of urban construction projectsis to assess the influence of the work on adjacent facilities.Brierley (1988) noted that “...the tunnel owner must catalogall third-party impacts and make an honest effort to priori-tize them in terms of risk to the third party and the project.”In contrast to the “reasonably satisfactory” state of groundmovement prediction methods, Peck also stated that “...pro-cedures for determining the need for underpinning, or pro-tective measures in lieu of underpinning, are among the leastsatisfactory aspects of the state-of-the-art.”
In some large North American construction projects, ex-culpatory clauses are included in the contract documentswhich attempt to place all responsibility for damage to adja-cent properties on the contractor. In these instances the con-tract documents may only require that work be carried outsuch that “damage does not occur to neighbouring proper-ties,” but “damage” is often not defined. In other cases, con-
trol of the ground and building movements is limited tostatements requiring only that settlement be limited to amaximum, typically 25 mm (1 in.). Horizontal movementsare often ignored. Design and control of damage, however,should be an iterative process (e.g., Clough et al. 1989)whereby a number of retaining systems or underpinningtechniques can be evaluated on a cost and risk basis consid-ering various ground and structure movements and the de-tails of the structures in question.
A number of methods have been proposed for evaluatingthe potential effect construction will have on nearby struc-tures. Most methods of determining tolerable movements ofstructures are based on either angular distortion (e.g.,Skempton and MacDonald 1956) or displacement and slopeof the settlement trough (Rankin 1988). Boscardin andCording (1989) proposed an approach that recognized theimportance of horizontal strains and correlated measure-ments of angular distortion and horizontal strains to a lim-ited number of case histories. Others have correlated thedeflection ratio (∆yL, see Appendix 2) and horizontal strainfor other cases (e.g., Burland et al. 1977; Mair et al. 1996).Angular distortion and the deflection ratio, however, aresimple parameters that do not consider real differences instructure responses that depend on their height, length, andconstruction. Boone (1996) therefore proposed a stepwisemethod by which the geometric changes induced in a build-
S.J. Boone and J. Westland.Golder Associates, Ltd., 2180Meadowvale Blvd., Mississauga, ON L5N 5S3, Canada.R. Nusink. Morrison Hershfield Ltd., 4 Lansing Square,North York, ON M2J 1T1, Canada.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
ing by ground movement can be used to estimate cumulativecrack width. Examples of this approach are described furtherin Appendix 1 and Boone (1998). Construction data from alarge braced excavation are evaluated in this paper in com-parison to the methods proposed for estimating building re-sponse to excavation-induced ground movements by Rankin(1988), Boscardin and Cording (1989), and Boone (1996).
Project description
A deep braced excavation, over 650 m long, from 9 to20 m deep, and up to 20 m wide, was made through glacialtill and highly overconsolidated glaciolacustrine sand, silt,and clay deposits. Standard penetration test (SPT)N valuesin the stiff to hard cohesive glacial till ranged from 25 togreater than 100, with an average per borehole ranging be-tween 40 and 70. Average SPTN values in the dense sandand silt deposits typically ranged between 60 and 100.Groundwater levels, observed in two distinct aquifers,ranged from near the ground surface to about 4 m above thebase of the main excavation.
Over 50 buildings were in the vicinity of the project, and46 of these were within the most critical “zone of influence”of the excavation, i.e., where the front of the structure waswithin a distance equal to or less than the depth of the adja-cent excavation. Some buildings were less than 2 m from theexcavation face. Most of the structures were between oneand three stories high with shallow basements and were con-structed of brick load-bearing exterior walls and wood fram-
ing within. A few structures were up to four stories high,one of which was a concrete frame structure with a wood-panelled exterior.
The excavation support system generally consisted of wide-flange steel beams (soldier-piles) placed in prebored holeswith wood lagging installed between the piles as excavationprogressed. Soldier-piles were typically installed on 3 m centre-to-centre spacings. Horizontal restraint was provided by deckbeams and preloaded pipe struts located at each pile (i.e.,there were no wales). The vertical spacing of struts generallyranged between 2.4 and 5.8 m, resulting in each pile pair be-ing restrained by the deck beam and two to three struts below.Because the excavation was made beneath a street, it wasfully decked during construction, except for small openingsfor removing spoil and equipment and lowering lagging, brac-ing, and other construction materials.
Fig. 1. Plot of level 1 assessment results from design stage ofproject assuming a maximum relative settlement of 0.2% and aparabolic settlement curve extending to a relative distance oftwice the excavation depth (after Boscardin and Cording 1989). Damage category Description of typical damage
Negligible (0) Hairline cracksVery slight (1) Very slight damage includes fine cracks
which can be easily treated duringnormal decoration, perhaps an isolatedslight fracture in building, and cracks inexternal brickwork visible on closeinspection
Slight (2) Slight damage includes cracks which can beeasily filled and redecoration would prob-ably be required, several slight fracturesmay appear showing the inside of thebuilding, cracks which are visible exter-nally and some repointing may berequired, and doors and windows maystick
Moderate (3) Moderate damage includes cracks thatrequire some opening up and can bepatched by a mason, recurrent cracks thatcan be masked by suitable linings,repointing of external brickwork and pos-sibly a small amount of brickworkreplacement may be required, doors andwindows stick, service pipes may frac-ture, and weathertightness is oftenimpaired
Severe (4) Severe damage includes large cracks requir-ing extensive repair work involvingbreaking-out and replacing sections ofwalls (especially over doors andwindows), distorted windows and doorframes, noticeably sloping floors, leaningor bulging walls, some loss of bearing inbeams, and disrupted service pipes
Very severe (5) Very severe damage often requires a majorrepair job involving partial or completerebuilding, beams lose bearing, walls leanand require shoring, windows are brokenwith distortion, and there is danger ofstructural instability
Table 1. Severity of cracking damage (modified after Burlandet al. 1977).
Fig. 2. Angular distortion as used in the application of themethod proposed by Boscardin and Cording (1989).
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
Design estimation of damage
Early in the project it was recognized that damage to adja-cent facilities could not be entirely avoided. Although a vari-ety of protection or underpinning techniques could be usedto minimize damage, for the dense and hard soils at the sitemany such methods have the potential to induce movementsthat could be larger than those from a conventionally sup-ported excavation. It was also recognized that costs for dam-age mitigation could greatly exceed costs for repairs,replacement, or purchase of the affected facilities. Therefore,
a two-step process, loosely following Rankin (1988), was setout for the evaluation of potential building and utility dam-age, the level 1 and level 2 assessments. For typical build-ings adjacent to the excavation, a goal of limiting damage to“slight” or less was also established. If the level 1 assess-ments indicated that slight building damage could be ex-ceeded, the subject facility was to be analyzed in greaterdetail. Level 2 assessments were generally separated intotwo substeps: (1) rigorous analysis of potential groundmovements, and (2) close examination and analysis of thebuildings–utilities to ascertain specific effects the total and
Fig. 3. Relative lateral movements of ground behind the excavation support system expressed as a percentage of excavation depth.
Fig. 4. Relative vertical movements near the excavation support system expressed as a percentage of excavation depth. Closest groundand building monitoring points are located 1.5–2 m from the excavation edge (0.08H to 0.22H).
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
differential movements could elicit. Depending on the re-sults of the level 2 analyses, alternative excavation support,underpinning, or ground improvement measures were con-sidered and their consequent effects on building damage po-tential were evaluated to choose the most cost effectivesolution that would also manage risk to acceptable levels.This two-step process is described in more detail by Booneet al. (1998).
A geotechnical design report prepared for this project pro-vided generalized forms of ground movement to be expectedfrom excavations for level 1 assessments. These predictionswere made on the basis of local experience coupled with avariety of published data (e.g., Peck 1969; Clough andO’Rourke 1990) and considered that maximum horizontalmovements would be equal to maximum vertical move-ments. Using an empirical zone of influence, the designersapplied the generalized ground movement predictions to allfacilities within this zone. The potential for building damagewas assessed by the designer using the method suggested byBoscardin and Cording (1989), summarized in Fig. 1, andpotential structure damage was classified according to Ta-ble 1. For this project, the designer elected to use the slopeof the settlement curve at the front of the structure as thevalue for angular distortion without subtracting rigid-bodytilt (as this was a more conservative interpretation) (seeFig. 2). Based on the level 1 and level 2 analyses completedfor this site, it was thought that a stiff and closely controlledsoldier-pile and lagging system would limit the movementsto the acceptable levels determined for the project. Since theproject was close to many buildings, ground and buildingmovement limitations, based on the damage potential assess-
ments, were included in the contract. In general, groundmovements, both horizontal and vertical, were limited to be-tween 0.1 and 0.2% of the excavation depth.
Assessment of ground movements
To monitor the contractor’s compliance with the specifiedground and structure movement limitations, the owner incor-porated a detailed instrumentation program into the project.Instrumentation relevant to this paper included (1) inclinom-eters installed in the ground 1–2 m behind the shoring sys-tem, (2) ground surface monitoring points consisting of steelrods grouted into the ground about 1.8 m below the groundsurface (to isolate them from frost and pavement move-ments), and (3) structure monitoring points consisting ofthreaded bolts and inserts installed on the buildings. Sur-veying of building and ground movement was performedwith electronic levels achieving a typical accuracy of ±2 mmor less. Near the west end of the project, an area wasoverexcavated below a planned strut level resulting inground movements that were nearly double those in mostother areas. Near the middle and east end of the project, anumber of struts were cut prematurely during backfilling.The premature removal of struts caused additional groundmovements that were readily quantified from the regular in-strumentation readings. The last part of the excavation wasmade near the middle of the project and load transfer duringpreloading was optimum and ground movements were mini-mized in this area. Ground settlements of up to 31 mm weremeasured. Figures 3 and 4 summarize all the relative lateraland vertical ground movements along the excavation asmeasured by the instrumentation program. A line has beendrawn in these diagrams indicating the estimated envelopeof maximum ground movement in each area.
Ground movements adjacent to the excavation were alsoevaluated to determine a most-likely mathematical model ofthe settlement profile perpendicular to the shoring line. Thegenerally parabolic shape of the settlement profile suggestedby Peck (1969) was chosen with the mathematical represen-tation suggested by Bowles (1996) as a basis for determin-ing a graphical “best fit” to the observed movements (seeFig. 5). From six arrays of multiple ground monitoringpoints aligned perpendicular to the excavation, the modelwas shown to be reasonably representative as illustrated inFigs. 5 and 6. Data from a seventh array, array 2, did not fol-low the patterns suggested by the other arrays because themagnitude of movements (less than 0.04%H, whereH is theheight (depth) of the excavation and retaining system) andsurvey error (± 0.022%H) in this area obscured any trends asillustrated in Fig. 5. This ground movement model in the vi-cinity of four example buildings is shown in Fig. 6 alongwith inclinometer data nearest these buildings. As shown inFig. 6, the lateral movement was primarily cantilever inshape but also included some “bulging” between the sup-ports. It is noted that, although the majority of the groundmovement for maximum deformations between 0.1%H andabout 0.15%H took place within a distance from the excava-tion equal toH, where maximum lateral and vertical defor-mations exceeded about 0.2%H, a wider zone of influenceup to about 2H was observed, approaching the groundmovement profile used during design (see Fig. 5). Based on
Fig. 5. Ground movements at the project site showing surveyerror bars, envelope of movements used during design, andtypical pattern of displacement at array locations.
Fig. 6. Profiles of buildings and excavation support systems, building and ground settlement measurements, and lateral movementmeasurements from nearest inclinometer for four example buildings.
the available monitoring results (see Figs. 3 and 4) and thework of O’Rourke et al. (1976) and Milligan (1974), it wasassumed that lateral displacement was equal to vertical dis-placement; little work has focused on this aspect of defor-mation, and this project did not include research-appropriatemeasurements to determine such ratios.
Building responses
Prior to construction, condition surveys were carried outfor each of the neighbouring buildings. These surveys werecarried out in general accordance with the guidelines pre-pared by the Building Research Establishment (BRE 1989a,1989b), with reference also to Table 1. Sketches and photo-graphs of visible cracks on interior and exterior walls werecompiled along with summaries of the building constructionand condition. Crack widths were generally noted as “hair-line” (being visible on close inspection but a fraction of1 mm wide), or estimated to 1 mm to 2 mm intervals ofwidth. Following construction, each building was visitedonce again, and changes to existing conditions or new dam-
ages were noted on the preconstruction survey reports.Crack widths in the postconstruction survey reports were es-timated to the nearest 1–2 mm where they were greater thanhairline widths. For the “negligible” and “very slight” dam-age categories, where new hairline cracks of unmeasuredwidth were noted in walls, the hairline cracks were arbi-trarily assigned an average thickness of 0.5 mm for compar-ative purposes in this paper. Where damages were observedto be slight or greater in the postconstruction survey reports,subsequent detailed crack width measurements were made
Table 5. Table of crack widths for the building illustrated inFig. 6d.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
later with an automotive thickness gauge (gap gauge) andsketches were prepared. Ground movements, building move-ments, measured cracks, and other relevant details for fourbuildings are summarized in Fig. 6 and Tables 2–5, in whichthe notation 1/2Hb, for example, indicates the presence ofthe crack prior to construction but to one half of the buildingheight. The buildings in Fig. 6 are also shown with their po-sition aligned with the horizontal distance scale of the settle-ment plot. All postconstruction condition surveys werereviewed and the damages were categorized according toTable 1.
Comparative evaluation of buildingresponse estimation methods
The three building response estimation models proposedby Rankin (1988), Boscardin and Cording (1989), andBoone (1996) were compared based on the assessed groundmovement patterns. The purpose of the comparison was toevaluate and illustrate the validity of each method and theadvantages or disadvantages of each, assuming that theground conditions were appropriately modeled. A computerspreadsheet was developed incorporating building data suchas height above ground surface, depth to bottom of basementwall, length, and distance from front wall to excavation face.Additional data relevant to each building location were also
Fig. 7. Height and length ratios and lengths of building walls.
Fig. 8. Comparison of estimated frequency of damage categories to actual damage categories. The solid lines represent actualfrequency of damage categories and bars indicate damage categories as estimated by the methods indicated. N, negligible; VS, veryslight; S, slight; M, moderate; SV, severe; VSV, very severe.
Note: na, not applicable.aCombination of deformation modes producing values less than those provided above for each category of damage can result in damage of equal
classification. See Figs. 1, 9, and 12.
Table 6. Summary of principle damage category criteria.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
included, such as the depth of excavation and maximum set-tlement (as a percentage of excavation depth, from Fig. 4).From this data, the following information was calculated:(1) length of building within the settlement profile (l) (val-
ues of this length and ratios of total building length andl tobuilding height,Hb, are shown in Fig. 7); (2) settlement atthe front of the building and at the far end of the building orlimit of building length, whichever governed (see Fig. 6);
Correct 15 19 18 21Overestimate by 1 0 7 16 14Overestimate by 2 0 1 2 1Underestimate by 1 15 17 9 10Underestimate by 2 or more 16 2 0 0
Table 7. Summary of damage category estimation.
Fig. 9. Summary plots of damage estimation methods. (a) Results using the method of Boscardin and Cording (1989) and assessedground movement behaviour. (b) Cumulative crack width estimation using the method of Boone (1996). Symbols indicate reporteddamage categories.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
(3) lateral movement at the front of the building and at thefar end of the building or limit of building length; (4) slopeof the settlement profile at the front of the building for usein all methods; (5) angular distortion and horizontal strain atthe front of the building for the Boscardin and Cording(1989) method; and (6) crack widths according to Boone(1996) (see Appendix 1).
From these analyses, each building was evaluated and cat-egorized for potential damage according to the three meth-ods and the principal criteria listed in Table 6. Fortunately,all methods referred to the crack width values outlined inTable 1 as one basis for judging damage severity, thus mak-ing comparison useful. In Fig. 8, the frequency of categoriespredicted by each method is compared with the actual fre-quency determined by the postconstruction surveys. Table 7(see also discussion below) compares the results on a build-ing by building basis. Summary plots derived from the as-sessed ground movements and actual damage categories areshown in Fig. 9 for the methods suggested by Boscardin andCording (1989) and Boone (1996).
The method proposed by Boscardin and Cording (1989),used during the design stage of this project, underestimateddamage in a number of cases. Where moderate damage actu-ally occurred, the design criteria suggested that slight dam-age would be expected and that two of the cases would beexpected to exhibit damage near the threshold between thevery slight and slight damage categories. Although the moreconservative interpretation of angular distortion was utilizedduring design, horizontal deformation remained the key esti-mator of damage severity. Boscardin and Cording refer toTable 1 as an indicator of damage, but the criteria illustratedin Figs. 1 and 9a and Table 7 are based on curves preparedby the National Coal Board (1975), a height to length ratioof 1, a total building length of 30–40 m, and absolute valuesof strain. For example, the division between slight and mod-erate damage (cracks of 5 mm or greater) is defined at a hor-izontal strain of 0.15%, or about 15 mm of horizontalextension for every 10 m length of building. If this distortion
occurred as one single crack 15 mm wide, the damagewould be categorized as severe, whereas if it occurred asfour equally wide cracks, it would be considered slight dam-age. This method therefore, like all others, depends highlyon the number and size distribution of cracks if damage iscategorized according to Table 1. As one of the few designtools available at the time, however, this approach allowed aconsistent and logical process for completing the stepwisedamage potential assessment process for this project. The re-sulting specification requirements and construction controladequately managed risks to adjacent properties.
The method proposed by Boone (1996) typically overesti-mates damage categories but reasonably estimates cumula-tive crack widths (see Fig. 10). Damage categoryoverestimation arises from the use of cumulative crackwidth in comparison to Table 1. All cases where the damagecategory was underestimated fell within the negligible toslight categories. Measured cumulative crack widths fromalong the top of each wall are compared in Fig. 10 with theestimated cumulative crack width because the main defor-mation mode was likely to be horizontal with a componentof bending (“hogging”). The cumulative crack widths inFig. 10 that are greater than the estimated cumulative crackwidth are likely the result of including some preexistingdamage in the summation of measured crack widths as wellas inevitable differences in the simplified mathematicalmodel and actual localized ground and building responses(see Fig. 6). The analysis results indicated that bending-related tensile strains accounted for 20–40% of the cumula-tive tensile strain in about 70% of the buildings. Diagonalcracking from principal tensile stresses was the critical modeof damage for about 22% of the buildings, but most of thesecases fell within the negligible to very slight damage catego-ries. Figure 11 summarizes crack width frequency for thefour buildings shown in Fig. 6. These four buildings wereexamined in detail because of their similar construction andmoderate damage. It can be seen that for cumulative mea-sured crack widths (along the top of the walls) of betweenapproximately 12 mm and 20 mm, the maximum crackwidth is generally less than two thirds of the cumulative andoften closer to one half of the cumulative crack width.
Considering Fig. 11, data from Boone (1996) and fromthis project have been reevaluated using damage thresholds
Fig. 10. Measured and estimated cumulative tension crack width.Fig. 11. Crack width frequency for four brick and masonry blockstructures.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
for cumulative crack width equal to 1.5 times those valueslisted in Table 1 (see Fig. 12). The frequencies of each dam-age category estimated using this approach are depicted inFig. 13. Damage categorization appears difficult in the neg-ligible to very slight categories. This difficulty likely arisesbecause in these categories subjectivity may be more signifi-cant in simple visual surveys, critical strains for crackingcan differ, and building details and age may have a morepronounced effect. However, it is more critical to be able toappropriately estimate damage that exceeds the easily re-paired aesthetic damage of these categories. As illustrated inFigs. 12 and 13, the modified approach is more able to cor-rectly discern the boundaries between slight and moderatedamage. Most importantly, those structures for which amoderate damage category was estimated were those thatexhibited moderate damage in the field. Damage categoryunderestimation was relatively evenly distributed betweenthe negligible to very slight and very slight to slight catego-ries in all but one case. In the one case where moderatedamage was estimated to be slight, estimated crack widthsfell close to the threshold between slight and moderate. Forthis project, Table 7 and Figs. 12 and 13 demonstrate rea-sonably good agreement with the proposed modifiedmethod.
Conclusions
From the results of this study, a number of conclusionscan be drawn. In general, the two methods that consideredthe effects of lateral movement (Boscardin and Cording
1989; Boone 1996) provided far better results than the onemethod that did not (Rankin 1988). It is suggested thatmethods that do not consider horizontal movement are inap-propriate for estimating damage severity resulting from anadjacent excavation. Correctly predicting the ground re-sponse was a critical factor in using any of the evaluatedmethods. The method proposed by Boscardin and Cording(1989) carries a number of assumptions that may or may notbe appropriate for particular projects or structures. Boone(1996) provided a reasonable means of evaluating cumula-tive crack width which accommodated site-specific detail.As used in this study, the geometry-based mathematical rep-resentation of building deformation proposed by Boone andas modified in this paper readily allowed multiple iterationsof damage potential assessment for many buildings. Withbuilt-in logical functions of spreadsheet software, damagecategories were also easily estimated. Although cases wherehorizontal strains are not critical are needed to further evalu-ate the general applicability of this damage categorizationapproach, the category thresholds in Tables 1 and 7 andFig. 12 are both consistent and appropriate for evaluation oflow-rise, brick and masonry buildings of typical dimensionsadjacent to excavations.
Acknowledgements
The writers would like to thank S. Pang, S. McGaghran,and S. Poot of Golder Associates Ltd. for their efforts indata collection during construction, and the owner for per-mission to publish this paper.
Fig. 12. Modified damage thresholds based on cumulative crack width. Numbers on some data points refer to case numbers providedin Boone (1996).
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
References
Boone, S.J. 1996. Ground movement related building damage.Journal of Geotechnical Engineering, ASCE,122(11): 886–896.
Boone, S.J. 1998. Ground-movement-related building damage: clo-sure. Journal of Geotechnical and Geoenvironmental Engi-neering, ASCE,124(5): 463–465.
Boone, S.J., Garrod, B., and Branco, P. 1998. Building and utilitydamage assessments, risk and construction settlement control.InProceedings of the 24th World Tunnelling Congress, Sao Paulo,Brazil. Edited byA. Negro and A. Ferriera. A.A. Balkema, Rot-terdam, The Netherlands, pp. 243–248.
Boscardin, M.D., and Cording, E.J. 1989. Building response toexcavation-induced settlement. Journal of Geotechnical Engi-neering, ASCE,115(1): 1–21.
Bowles, J.E. 1996. Foundation analysis and design. 5th ed.McGraw-Hill, New York.
BRE. 1989a. Simple measuring and monitoring of movement inlow-rise buildings. Part 1: cracks. BRE Digest 343, BuildingResearch Establishment, Watford, U.K.
BRE. 1989b. Simple measuring and monitoring of movement inlow-rise buildings. Part 2: settlement, heave and out-of-plumb.BRE Digest 344, Building Reasearch Establishment, Watford,U.K.
Brierley, G. 1988. Discussion on “The risks associated with tunnel-ling projects.” Tunnelling Technology Newsletter, Number 64,December 1–5, U.S. National Committee on Tunnelling Tech-nology, Washington, D.C.
Burland, J.B., Broms, B.B., and DeMello, V.F.B. 1977. Behaviourof foundations and structures: state-of-the-art report.In Proceed-ings of the 9th International Conference on Soil Mechanics andFoundation Engineering, Japanese Geotechnical Society, Tokyo,Japan, pp. 495–546.
Clough, W.G., and O’Rourke, T.D. 1990. Construction inducedmovements of in situ walls.In Design and performance of earthretaining structures.Edited by P.C. Lambe and L.A. Hansen.American Society of Civil Engineers, Geotechnical Special Pub-lication 25, pp. 439–470.
Clough, W.G., Smith, E.M., and Sweeney, B.P. 1989. Movementcontrol of excavation support systems by iterative design.InFoundation engineering, current practices and principles.Edited
by F.H Kulhawy. American Society of Civil Engineers,Geotechnical Special Publication 22, pp. 869–884.
Mair, R.J., Taylor, R.N., and Burland, J.B. 1996. Prediction ofground movements and assessment of risk of building damagedue to bored tunnelling.In Proceedings of the InternationalSymposium on Geotechnical Aspects of Underground Construc-tion in Soft Ground, London.Edited by K. Fujita and O.Kusakabe. A.A. Balkema, Rotterdam, The Netherlands,pp. 713–718.
Milligan, G.W.E. 1974. The behaviour of rigid and flexible retain-ing walls in sand. Ph.D. thesis, Cambridge University, Cam-bridge, U.K.
National Coal Board. 1975. Subsidence engineers handbook. Na-tional Coal Board Production Department, London, U.K.
O’Rourke, T.D., Cording, E.D., and Boscardin, M. 1976. Theground movements related to braced excavations and their influ-ence on adjacent structures. U.S. Department of TransportationReport No. DOT-TST 76T-23.
Peck, R.B. 1969. Deep excavations and tunnelling in soft ground:state of the art report.In Proceedings of the 7th InternationalConference on Soil Mechanics and Foundation Engineering,Mexico City, pp. 225–290.
Peck, R.B. 1984. State of the art: soft-ground tunnelling.In Tun-nelling in soil and rock.Edited byK.Y. Lo. American Society ofCivil Engineers, New York, pp. 1–11.
Rankin, W.J. 1988. Ground movements resulting from urban tun-nelling: predictions and effects.In Engineering geology of un-derground movements.Edited byF.G. Bell, M.G. Colshaw, J.C.Cripps, and M.A. Lovell. Geological Society, London, pp. 79–92.
Skempton, A.W., and MacDonald, D.H. 1956. The allowable set-tlements of buildings. Proceedings of the Institution of Civil En-gineers, Part 3,6: 727–768.
Appendix 1
The method proposed by Boone (1996) was used to esti-mate the cumulative crack width caused by ground deforma-tion in response to the excavation. In essence, the methodutilizes the geometric changes induced by ground deforma-tion in combination with an assumption that the buildingwall will deform as a simply supported, uniformly loaded,deep beam. Definitions of geometry parameters are providedin Fig. A1. Equations used for calculation of estimated cu-mulative crack width, derived from Boone (1996), are sum-marized in Table A1 and were utilized in a computerspreadsheet. Critical tensile strains,εc, were taken from Ta-ble 1 in Boone (1996). For negligible damage, an upperthreshold ofεc = 0.03% was chosen where cracks less than1 mm first appear, and strains below this value were consid-ered to be accommodated by the building materials withoutreadily visible cracking (i.e., microcracking). For the divisionbetween very slight and slight damage, the average of theupper bounds of theεc values (from 0.3 to 0.6 for bending-tensile strains) provided in Table 1 (Boone 1996), where finecracks are evident during cursory inspection, were used, re-sulting in anεc of about 0.04%. Beyond this limit of strain,it is considered that the existing fine cracks open and newones form at positions often governed by discontinuities(windows, doors, and previous repairs).
Fig. 13. Damage category frequencies estimated using modifiedcumulative crack width thresholds. The solid line representsactual frequency of damage categories, and bars indicateestimated damage categories.
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
OF
SOU
TH
AM
PTO
N H
IGH
FIE
LD
on
03/1
1/12
For
pers
onal
use
onl
y.
Appendix 2: List of symbols
Cp: cumulative diagonal (principal) crack width, see Boone(1996)
Ct: cumulative tension crack widthD: distance from excavation edgeDmax: distance to point where settlement – lateral movement
is zeroDfront: distance from excavation edge to front of buildingg1, g2: slope, or grade, of line tangent to vertical parabolic
curve related to horizontal, at points 1 and 2H: height (depth) of excavation and retaining systemHb: height of building wallh: horizontal movementhmax: maximum horizontal movement at excavation edgel: length of straight line between curve endpoints, i.e., length
of part of building within settlement profileL: original horizontal length of building in settlement profileM: momentR: radius of curvatureRM: radius of curvature defined by moment portion of total
deflectionRNA: radius of the neutral axis curvaturer: linear regression coefficientS1, S2: settlement at points I and Q in profile, respectivelySmax: maximum settlement at excavation edge∆S: differential settlement between endpoints oflt: rigid-body tiltV: shearα: slopeβ: angular distortion defined as the maximum change in
slope along the beam, or the slope at the support usedby Boscardin and Cording (1989)
∆yL: deflection ratio: maximum deflection between thecurved beam deflection line and the straight line lengthbetween the two end points (chord), divided by thechord length as defined by Burland et al. (1977)
θ1, θ2: angle of rotation at support of simple beam at points1 and 2, respectively
θM : angle of rotation at support due to momentεc: critical tensile strainεg: strain due to elongation of the ground movement profile
along the curved profileεle: direct lateral extension strain, equivalent to horizontal
strain εh as used by Boscardin and Cording (1989)εM : tensile strain due to moment-induced bending, either +
for tension or – for compressionεp: principle tensile strain, see Boone (1996)εt : cumulative tensile strain =ε ε εM + +g leγ: shear strain defined as a measure of distortion in units of
radians (tanγ υ= ′( )V )υ: deflection of beam in relation to chord between beam
endpoints; although deflection is often notated asδ, υ isretained as the general notation for deflection consistentwith the work of Timoshenko, whereυmax = δ, and toavoid confusion with prior uses ofδ in this particularsubject
υmax: maximum deflection of beam in relation to chord be-tween beam endpoints
Fig. A1. Definitions of geometry and terms used to estimatecumulative crack width. Note thatυ′ at beam ends is equivalentto β as defined by Boscardin and Cording (1989).
Parameter Equation
Settlement,S, and horizontal movement,h S = Smax[(Dmax – D)/Dmax]2; h = hmax[(Dmax – D)/Dmax]
2
Slope,g g = 2Smax(Dmax – D)/(Dmax)2
Rigid-body tilt, t t = (S1 – S2)/L
Rotation slope,ν′ ν′ ≈ g1 – t
Maximum curve deflection,νmax ν νmax = ′ Ly4
Proportion of deformation due to moment,ν′( )M , νmax( )M ν ν νmax( ) maxM MH L v H L= + ′ = ′ +y y y y( . ); ( . )( )1 2 88 1 2 882 2 2 2
Proportion of deformation due to shear,ν′( )V , νmax(V) ν ν νmax( ) max max( )V M V Mv v v= ′ = ′ ′– ; –( ) ( )
Cumulative diagonal crack width,Cp C L Hp p= +ε [( . ) ]0 5 2 2 12
Note: Settlement curve derived from Bowles (1996), and cracking damage equations derived from Boone (1996). Basic assumptions are as follows: (1)horizontal deformation profile at ground surface is equal to vertical deformation profile; (2) building moves with the ground; (3) equations provided forprimarily cantilever-type wall–ground lateral movement pattern; (4) equations are provided only for load-bearing walls; and (5) lengthening of groundsurface simply by virtue of changing straight line to a curve is ignored (see Boone 1996).
Table A1. Table of equations for estimation of building damage.
