BUILDING RESPONSE TO EXCAVATION-INDUCED

SETTLEMENT

By Marco D. Boscardin1 and Edward J. Cording,2 Members, ASCE

ABSTRACT: Analytic models and field data are used to develop procedures to evaluate the tolerance of brick-bearing-wall and small frame structures to the ground displacements that develop during opencutting and tunneling. The role of hori­zontal and vertical ground displacements are discussed and the effects of grade beams, building orientation and building location relative to excavation are ex­amined. Case studies of structures adjacent to opencuts and tunnels are used to verify procedures for estimating potential for structures adjacent to excavations to sustain damage.

INTRODUCTION

A major concern during the planning and execution of underground con­struction is the impact of construction related ground movements on adjacent buildings and utilities. During excavation and support of tunnels and open-cuts, changes in the state of stress in the ground mass around the excavation and loss of ground occur. These changes in stress and ground losses are typically expressed in the form of vertical and horizontal ground movements. The ground movements, in turn, cause any structures supported by the af­fected ground to translate, rotate, deform, distort, and possibly sustain dam­age. As a consequence, important tasks facing both the engineer and the contractor are the estimation of the magnitude and distribution of the ground movements to be caused by the construction procedures and the tolerance of the structures and utilities to the deformations and distortions sustained as a result of the ground displacements.

This paper concentrates on the latter issue, the tolerance of structures to excavation-induced ground displacements, and describes techniques, based on field and analytical studies, for estimating the response of brick bearing-wall and small frame structures. The cost of excavation-related damage may then be evaluated, and a rational basis may be used for the evaluation and selection of remedial measures, protective measures, and/or alternative ex­cavation and support procedures.

Past studies of building response to ground displacement (Skempton and MacDonald 1957; Bjerrum 1963; Meyerhof 1953 and 1956; Polshin and To-kar 1957; Burland and Wroth 1974; and Wahls 1981) have primarily dealt with the tolerance of buildings to settlement under their own weight. In these studies, damage or building response was correlated with the angular dis­tortion or the deflection ratio of the structure (see Appendix I for defini­tions). These studies were typically based on cases where horizontal strain

'Asst. Prof., Dept. of Civ. Engrg., Marston Hall, Univ. of Massachusetts, Am­herst, MA 01003.

2Prof., Dept. of Civ. Engrg., Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.

Note. Discussion open until June 1, 1989. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on August 18, 1987. This paper is part of the Journal of Geotechnical Engineering, Vol. 115, No. 1, January, 1989. ©ASCE, ISSN 0733-9410/89/0001-0001/S1.00 + $.15 per page. Paper No. 23066.

1

was not a significant factor and distortions grew progressively worse with time.

More recent investigations of building response to ground displacement have included the response of structures to mining, tunneling, and open-cutting in both Europe and the United States (Brauner 1973; National Coal Board 1975; Littlejohn 1974; Geddes 1977; Breth and Chambosse 1974; Attewell 1977a, b; O'Rourke et al. 1976; Boscardin et al. 1978; Boscardin, 1980; and Mahar and Marino 1981). These studies have demonstrated the significance of both horizontal ground strain and the pattern of development of the ground displacement on the tolerance of structures to distortion.

The study presented herein examines case studies of building performance near tunnels and open cuts and compares the results to analytic studies. Brick bearing-wall and small frame structures were chosen as the subjects for this investigation because they comprise a large portion of the structures en­countered around and near such excavations. The study investigated the ef­fect of horizontal ground strain, differential settlement, building orientation relative to the excavation, and details of construction on building response. The results of the investigation are summarized in the form of figures for use in estimating potential damage to the structures once excavation-induced ground movements are estimated.

Two approaches to the study of the responses of structures to excavation-related ground movements are presented and discussed. The first approach is to model a bearing wall as a homogeneous isotropic, linear-elastic deep beam to simulate cases where the bearing walls are perpendicular to the excavation axis. The ground strains are then imposed on the structure and the concept of a critical tensile strain is employed to estimate the level of elastic deformation sufficient to create observable cracking. The second ap­proach uses a finite element model of an open cut adjacent to a plane frame structure to evaluate the effect of structure stiffness in modifying the dis­tortions imposed on it. This case is also used to simulate a bearing wall structure with bearing walls parallel to the excavation axis. Parametric stud­ies were performed to investigate the influence of number of bays, number of stories, soil stiffness, and grade-beam rigidity on the distortion of the structure.

DEEP BEAM MODEL

Most damage in masonry bearing walls is due to cracking after the tensile capacity of the material is exceeded. Polshin and Tokar (1957) used the concept that the onset of visible cracking (the start of observable damage) may be associated with a limiting or critical tensile strain. Burland and Wroth (1974) later applied the idea of a critical tensile strain to the initial visible cracking of a simple beam. The values of critical tensile strain for masonry used in the analyses were 0.0005 and 0.00075 (Burland and Wroth 1974; Polshin and Tokar 1957). It was assumed that the beam behaves in a linear-elastic manner at least until the tensile strain reaches the critical value and cracking becomes visible.

In this study, the model of the load-bearing wall is a weightless, linear-elastic, isotropic beam of length L, height H, and unit thickness. A convex deflected shape (hogging) is imposed (Fig. 1). The possible modes of de-

2

H

^ a) Simply Supported Beam ?m -%—

b) Deflected Shape

d) Shearing Mode

FIG. 1. Deep Beam Model

formation include bending, shearing, and combinations of both bending and shearing (Fig. 1).

Most structures do not typically deflect solely in bending or solely in shear, but rather in a combined mode of bending and shearing. The case of a simply supported beam with a central load was employed by Burland and Wroth (1974) to examine this condition. The expression, considering both shear and bending deformations, for the total midspan deflection, A, for this case is given by Timoshenko (1957) as

A = PV

48£7 1 +

18EI

L2HG Q)

In real structures, the foundation and soil would offer considerable re­straint. As a consequence, the condition where the neutral axis is at the lower edge of the beam model is of interest for the case of a hogging profile. Burland and Wroth (1974) examined this case by rewriting Eq. 1 in terms of deflection ratio, A/L, and the maximum extreme fiber strain, e6max, as-

3

1.0

0.5

0

7~\ 1 1

Diagonal Strain C r i t i ca l

Uniform Load ~ NA in Center

Point Load NA in Center

Bending Strain Cr i t ica l I I L

-<c"" Point Load ^ — N A a t Edg« Edge -

-L 4 L/H

5 6 7 8

[After Burland & Wroth,19741

FIG. 2. A/Z,ecri, versus L/H for Beams Deflecting Due to Combined Bending and Shear

suming a Poisson's ratio of 0.3. The resulting equation, for the neutral axis at the lower edge of the beam, is

A

L

L H 0 .083- + 1 .3 -

H L £fcmax • (2)

Similarly, Eq. 1 can be rewritten in terms of deflection ratio, A/L, and the maximum diagonal strain, erfmax, assuming Poisson's ratio equal to 0.3 and the neutral axis at the lower edge of the beam. The resulting equation is

A

L

L2

0.064 — + 1 H2 ^rfmax ' (3)

Similar relations can be derived for the case of a uniformly distributed load to show that the relationships are not sensitive to the type of loading (Burland and Wroth 1974). Plots of Eq. 2 and 3 are shown in Fig. 2, where the maximum tensile strains, either bending or diagonal tension, are related to the critical tensile strain. This plot shows that for structures with L/H ratios less than about one, the first damage will be in the form of diagonal tension cracking. Curves for the cases of a uniformly loaded beam with the neutral axis at the center and a central point loaded beam with the neutral axis at the center are also shown for comparison.

In contrast to cases where buildings settle under their own weight, ground movements related to mining, tunneling, and open excavation can include a substantial component of horizontal strain. Including the horizontal strain in the above analyses will result in the initiation of damage at smaller values of differential settlement and angular distortion. The critical tensile strain, ecrit, can now be considered to have two components for the case where the edge of the beam is in tension due to bending:

+ e* (4)

For the case of diagonal tension with horizontal extension, the tensile strain, ee, at any angel 0 from the horizontal is

69 = eh cos 8 + 2eAnax cos 9 sin 8

The maximum value of e9 (e6max) can be written as

ecrit = e8max = e / i ^OS 8 m a x + -^dmax COS 8 m a x Si l l 8 m a x . .

Substituting Eq. 4 into Eq.-2 yields

A

L

L H 0 .083- + 1 .3 -

H L [6crit e / i ]

Substituting Eq. 6 into Eq. 3 yields

A

L 0.064 •

H2 + 1 €h COS 9 „

(5)

(6)

(7)

(8)

For the case of a beam with a central point load and assuming the neutral axis at the lower edge, the angular distortion, p, the maximum change in slope along the beam (the slope at the support) can be determined from

3A 1 + 4 - —

1 + 6 1 -H'

(9)

This leads to p ranging from 3A/L to 2A/L for H/L ranging from 0 to oo, respectively. For typical values of H/L and E/G encountered in the field, p will vary between 2A/L and 2.3 A/L.

Evaluation of Eqs. 7-9 shows that the deflection ratio or angular distortion required to cause the critical tensile strain to be exceeded and damage to occur decreases when the structure undergoes lateral extension. Fig. 3 shows curves based on Eqs. 7-9 for the case of ecrit equal to 0.00075 and horizontal strain equal to 0.0005. Notice that for L/H less than about 1, first observable cracking will be controlled by shear related deformations. Furthermore, these analyses were performed assuming that the beam was composed of an iso­tropic material such that E = 2(1 + v)G and v is assumed to be as 0.3. In reality, masonry is an orthotropic material, and the relationship of Young's modulus, E, to shear modulus, G, depends on a number of factors. Typi­cally, the E/G ratio for a solid masonry beam would be expected to be somewhat greater than the E/G ratio for an isotropic material. In addition, the presence of openings in the wall would further increase the E/G ratio. Substituting various ratios of E/G into the equations for A/L versus L/H shows that the more flexible the structure is in shear (larger E/G ratio), the greater the range of L/H where shearing distortion and diagonal tension will control cracking. Refer to the case where E/G is equal to 12.5 versus cases where E/G equals 2.6 in Fig. 3.

5

1 r E/G = 2.6 E/G= 12.5

3 -

'O

ca c" * 2 o «-» w

0)

o>1

eh =0.5x10 3

c c r i t=0.75xld3

L/H

FIG. 3. Effect of £/G Ratio on Range of L/H Where Shearing is Critical

Another factor to consider is the manner in which the ground movements develop in response to excavation. The ground movements related to ex­cavation usually develop in the form of a traveling wave that gradually im­pinges on a structure. The ratio of deformed length to height of structure, L/H, will initially be very small and increase gradually as the ground move­ments propagate out from the immediate vicinity of the excavation. Thus, the initial building deformations will be primarily shear-related deformation.

The above discussions regarding E/G ratio and development of ground movements around an excavation suggest that shear deformation will be re­sponsible for most damage to masonry bearing-wall buildings when the im­posed distortions are a result of nearby excavation. This is, in fact, the case typically observed in the field where diagonal tension-related cracking nearly always occurs before bending-related cracking. As a consequence, angular distortion, a measure of shear strain, would be an appropriate parameter to correlate with building behavior if modified to account for the effects of horizontal strain.

6

ESTIMATING POTENTIAL FOR AND DEGREE OF DAMAGE

A set of curves relating horizontal extension, angular distortion, and de­gree of cracking damage for brick bearing-wall structures is presented in Fig. 4. The curves were developed using the deep beam analysis described above with the neutral axis at one edge and L/H equal to one. Each curve rep­resents a given value of critical tensile strain. The curves are based on Eqs. 8 and 9, and correspond to the maximum tension strains in the beam due to combined shearing and lateral extension. At very small angular distortions, the curves become horizontal, representing the condition where horizontal strains equal critical tensile strains for the various levels of damage. At very low levels of horizontal extension strain, the curves are inclined nearly 45° representing the condition where diagonal tension strains equal the critical tensile strains.

The critical tensile strains corresponding to the boundaries of the zone labeled very slight were taken as 0.0005 and 0.00075, the range suggested by Burland and Wroth (1974) and Polshin and Tokar (1957) for cracking to first become noticeable. (Refer to Appendix I for descriptions of the various damage categories). The upper bound of the zone in which damage is con­sidered slight was established at a critical tensile strain of 0.0015, which corresponds to an angular distortion of 1/300 for a horizontal strain of zero. This is the value of angular distortion that Skempton and MacDonald (1956), and Bjerrum (1963) denote as the threshold for first cracking in panel wall and load-bearing walls for structures settling under their own weight. The upper bound of the zone in which the damage is considered moderate to severe was established at a critical tensile strain of 0.0030, which corre­sponds to an angular distortion of 1/150 for a horizontal strain of zero. Skempton and MacDonald (1956), and Bjerrum (1963) found that an angular distortion of 1/150 corresponds to the threshold angular distortion for severe cracking and structural damage in structures settling under their own weight.

T 1 1 1 n r

/ SEVERE TO VERY SEVERE

Angular Distortion, (3 |x10 I

FIG. 4. Relationship of Damage to Angular Distortion and Horizontal Extension Strain

7

0.005

0.004

0.003

C/5

•z 0.002 c o N

o X 0.001

50 100 150 2 0 0

Length of St ructure | m e t e r s l

250

FIG. 5. Strain

lAfter The National Coal Board,19751

Relationship of Damage to Length of Structure and Horizontal Ground

Fig. 4 can also be compared with damage criteria developed for subsi­dence due to mining in which angular distortions are small and lateral strains alone are correlated with damage. The bounds of the damage zone indicated on the vertical axis (zero angular distortion) correspond to the recommen­dations of the National Coal Board, presented in Fig. 5, for structures ap­proximately 30 to 40 m long. For lateral strains alone, Fig. 4 will tend to overestimate damage to smaller buildings and underestimate damage to larger buildings in comparison to the National Coal Board criteria.

The horizontal strain criteria shown in Fig. 4 (vertical axis) are applicable in the following cases: (1) structures with linear dimensions in the range of 6 to 40 m parallel to the direction of straining; and (2) larger structures adjacent to tunnels and open cuts less than 35 m deep. In the latter case, the affected portion of a structure will typically be less than 30 m in length with the remainder of the structure's foundation located outside the zone of influence of the excavation. This should yield a somewhat conservative but not overly conservative estimate of the potential for damage.

The rays extending out from the origin of Fig. 4 indicate the region of the figure where data from various types of excavations are likely to plot. Deep mines, greater than 100 m deep, tend to cause very small changes in

8

Hogging

• w = H a l f Wid th of Trough

X= Location of Center of Structure

Smax= Maximum Settlement of Trough

FIG. 6. Angular Distortion Within a Tunnel Settlement Trough

the slope of the ground surface across the length of the structure but rather large horizontal ground strains; thus, most damage is in response to lateral extension. Shallow mines, tunnels and braced cuts less than 30 m deep, tend to include a moderate degree of change in the ground slope across a structure as well as horizontal straining. Structures settling due to their weight sustain little if any horizontal strain and data from these cases would tend to fall along the angular distortion axis.

Since most small to medium-sized masonry bearing-wall structures do lit­tle to alter the ground movements, the anticipated free-field lateral ground strains and change in slope of the ground settlement profile where the build­ing is located can be used as input into Fig. 4. This will tend to result in a somewhat conservative prediction of the potential for and the degree of dam­age. If the structure is large and extends beyond the zone of significant ground movements caused by the excavation, then the angular distortion can be as­sumed equal to the average slope of the settlement profile. (For a braced

9

cut this condition typically occurs where the structure is longer than 1 to 2 times the depth of the cut.) If the length of structure is small with respect to the width of the settlement zone, then the angular distortion should be estimated from the change in slope of the ground surface profile over the length of the structure. The magnitude of the angular distortion for such a structure will be smaller than the magnitude of the average slope of the settlement zone.

Fig. 6 can be used to select input angular distortions in the vicinity of a tunnel. The figure was constructed assuming that the settlement trough due to tunneling has the shape of a normal distribution curve. Once the centerline settlement (5max) and the width of the trough (w) are determined (Cording and Hansmire 1975), Fig. 6 permits an estimate of the angular distortion induced in a structure by tunneling based on the length of the structure rel­ative to the width of the settlement trough, and the location of the structure within the settlement trough.

In the absence of other information, the lateral strain adjacent to an open cut may be assumed to be in the range of 1/2 to 1 times the change in slope of the settlement profile for braced excavations, and 1 to 1.5 times the change in slope of the settlement profile for excavations with cantilevered walls (O'Rourke et al. 1976; Milligan 1974). When dealing with a structure ad­jacent to a tunnel, the magnitude of the lateral strain will depend on the location of the structure in the settlement trough and must be estimated ac­cordingly. The assumption that the lateral strains in the building are roughly the same as the lateral strains in the ground appear to be reasonable as long as there is no tensile reinforcement in the footing or in the walls. In cases where the structure has significant horizontal stiffness, such as a building with grade beams, the horizontal strain induced in a structure will be sig­nificantly less than the horizontal strain of the soil mass with no interference due to a structure. In these cases, Fig. 4 should still provide reasonable, though more conservative, estimates of threshold values for assessment of potential for damage. The difference between the extension of the ground and the extension of the building depends on the soil-structure interface, foundation embedment in the soil, the type and orientation of the structure under consideration, and the presence of grade beams or other elements ca­pable of resisting lateral extension. This will be examined further in a fol­lowing section.

COMPARISON TO FIELD CASES

To check the ability to predict the degree of damage from Fig. 4, field data have been plotted on Fig. 4 and the damage actually observed can be compared to the damage that would have been predicted. Damages observed in each case and brief descriptions are presented in Table 1. In some cases, a range of lateral strains has been assumed when lateral strain data was not available. The estimated lateral strain was assumed to be in the range of 1/ 2 to 1 times the change in the slope of the ground surface settlement profile over the section where the angular distortion was measured (O'Rourke et al. 1976). These structures have no grade beams, thus are relatively flexible. They do little to alter the ground movements caused by excavation therefore the angular distortion of the structure will be approximately equal to the change in slope of the ground settlement profile and the lateral building strains

10

TABLE 1. Case History Summaries

Case number

(1)

1

2

3 4 5 6

7 8

9 10 11 12

13 14 15 16 17

18

Structure8

(2)

5-sty BBW

6-sty SF

3-sty MBV 3-sty MBV 4-sty BBW 2 Brick walls

3-sty BBW 3-sty BBW

3-sty BBW 3-sty BBW 3-sty BBW 1 to 5-sty

BBW & SF

2-sty WF 1-sty Brick 1-sty WF w/ 1-sty WF Bi-level WF

1-sty WF

Excavation

(3)

Braced cut

Braced cut

Twin tunnels Twin tunnels Twin tunnels Braced cut

Twin tunnels Twin tunnels

Twin tunnels Twin tunnels Twin tunnels Braced cuts

and tunnels

Shallow mine Shallow mine Shallow mine Shallow mine Shallow mine

Shallow mine

Damage"

(4)

Sev

SI

Sev Si/Mod

Sev Negl/VSl

Slight VS1/S1

Negl/VSl Negl

VS1/S1 Slight

Si/Mod Negl/VSl

Mod Mod Mod

Sev/VSev

Pm„ (X 10~3)

(5)

8

1.8

7.2 1.1

5-12 =0

1.3 2.4

0.4 0.5 0.5 1

1 0.8 1.9 2.7 2.9

3

Stora*

(X 10~3)

(6)

•4 to 8

0.9 to 1.8

3.6 to 7.2 0.7 to 1.4

3 to 12 1.3 to 1.8

0.3 0.1

0.5 0.3 0.8

0.5 to 1

1.2 0.2 1.7 1.2 0.1

5.3

Comments (7)

Only underpinned wall nearest excavation

Underpinned footings settled also

Walls rotated as rigid bodies

In compression zone of trough

Onset of damage summary O'Rourke et al. (1976)

Brick veneer

Some masonry

Brick veneer and reinforced foundation

Brick veneer

aBBW ~ brick bearing wall; SF = steel frame; WF — wood frame; MBV = masonry barrel vault. bNegl = negligible; SI = slight; Mod = moderate; Sev = severe; VSev = very severe. Cases 13 to 18 from Marino (1985).

will be approximately equal to the lateral ground strains. In general, the data shown in Fig. 4 and Table 1 fall within the lines

denoting the range corresponding to shallow mines, braced cuts and tunnels and the damage observed corresponds well with the damage zones shown on the figure. Three obvious exceptions are cases 6, 8, and 17. The struc­tures of case 6 consist of two short (3.7-m-long) brick walls which accom­modated nearly all the change in ground slope via rigid body tilt with little if any angular distortion. As a consequence, damage was primarily caused by horizontal extension. In contrast, case 8 examines a structure located in the compression zone of a tunnel settlement trough where lateral extension strains are negligible and any damage is related to angular distortion. Case 17 is a home with a reinforced concrete foundation which provided sufficient tensile restraint so that the lateral extension trains sustained by the structure were negligible and damage was in response to angular distortion.

It is also worth noting that cases 1 and 2 consider underpinned structures. In case 1, only the wall nearest the excavation was underpinned. As the

11

-Zone of Lateral Extension

(1 = Angular Distortion (3S= Building Slope a=Rig id Body Tilt P=PS+ a

FIG. 7. Example of Building Response to Tunnel Excavation

ground settlements propagated out away from the excavation, the walls and columns not underpinned began settling while the underpinned wall did not. This caused rather severe distortion and damage in the structure. The damage and distortion were greater than would have occurred without underpinning. In case 2, it was found that the underpinning sustained sufficient down drag to cause settlement as though the underpinning was not present. It should also be noted that most underpinning systems provide vertical support with little if any horizontal support. However, any underpinning for buildings adjacent to excavations must include provisions to control the horizontal as well as vertical movements of the structures.

These trends are shown in the example presented in Fig. 7. In this case, settlements, strains, distortions, and damage were monitored in two, three-story brick-bearing-wall structures during and after adjacent tunneling op­erations. The figures shows settlements, diagonal strains, horizontal strains, rigid body rotations or tilts, and angular distortions. From examination of the ground surface settlement profile and the building settlement profiles, it is apparent that the buildings settled with the ground with little if any re­straint or bridging type action. Ground surface settlement data were from reference points sufficiently far from the buildings so that they were not affected by the buildings.

12

Building I, located nearer the center of the final ground surface settlement trough was in the zone of compression and showed very little final lateral extension. However, this is a bit misleading because Building I sustained larger lateral extensions (at least 1/3,300 at ground level) during interme­diate stages of trough development as tunnel excavation proceeded past the building. Building II was located near the edge of the ground surface set­tlement trough in the zone of lateral extension and sustained a larger final lateral extension. The lateral extension strain at ground level was approxi­mately 65 percent of the change in slope of the ground surface settlement profile across building II.

Visible cracking in buildings I and II was minor. The lateral extensions in building II were concentrated in a single vertical crack between the south bearing wall and the facade walls. The pre-existing hairline crack opened to 3 mm in the lower story and 6 mm in the second story. Tilt of building I caused a separation of 10 mm between the tops of the bearing walls of the two buildings. The shearing distortions in building I were evident in the distortion of the door frame on the east facade wall of the building, and in opening of existing cracks by 0.4 to 0.8 mm in the mortar joints between bricks above the doorway and windows on the facade wall. The plaster in building I was cracked prior to tunneling so that the distortions due to tun­neling caused only slight widening of some of the pre-existing cracks.

The horizontal strain and shear distortion data from these two structures falls in the very slight damage to slight damage ranges indicated on Fig. 4. Cases 7, 8, and 9 correspond to various portions of building I at various stages of development of the tunnel settlement trough while cases 10 and 11 correspond to building II.

FRAME STUDY

Finite element simulations were used to study the behavior of frames and cases where the bearing walls are parallel to the edge of an excavation or parallel to the axis of a tunnel. For this study, a building is modeled as a plane frame structure with bents oriented perpendicular to the edge of an opencut. In cases where bearing wall structures were simulated, the columns of the frame represent the bearing walls while the cross beams were assigned a sufficiently low modulus in order to simulate the floor system tying the bearing walls together by friction at the joist-wall juncture. These models were used to perform parametric studies to investigate the relative signifi­cance of soil stiffness, number of stories, number of bays and grade beam stiffness on building behavior.

The excavation modeled was 18 m wide, 12 m deep, infinitely long, and symmetrical about its centerline. Initially, ground movements were deter­mined for the excavation without interference from adjacent structures. This free-field case was checked against actual field data to ensure it was rea­sonable. After a reasonable free field case was obtained, frame structures were superposed on the soil mesh and the resulting responses and distortions were noted. Based upon these parametric studies, some general statements may be made regarding the angular distortion, diagonal tension strain, and horizontal extension strain induced in a structure.

The effect of number of stories on normalized angular distortion (P/P«)> normalized diagonal strain (€d/$g) and normalized lateral strain (e,,/€,,g) for

13

* w

Ed

in Ehg

0.5

No Grade — Beams

Light Grade — Beams

X 2 5

Number of Stories %

10

9

_ e h / E h g

FIG. 8. Effect of Number of Stories on Angular Distortion, Diagonal Strain, and Lateral Strain

JL 1-0 Pg'

chg

0.5

No Grade Beams •

Moderate Grade Beams

I X 1 2

Number of Bays P/i pg WPg e n / E h g

FIG. 9. Effect of Number of Bays on Angular Distortion, Diagonal Strain, and Lateral Strain

14

the cases of no grade beams and light, reinforced grade beams is shown in Fig. 8. The curves are for three bay structures. Increasing the number of stories decreases the angular distortion and diagonal tension strain induced in the structure. The lateral strain induced is rather less sensitive the the number of stories and tends to increase with the number of stories for the case of no grade beams. It is apparent from Fig. 8 that even rather light grade beams can dramatically reduce the distortions sustained by the struc­ture.

Adding bays, thus increasing the length of the structure relative to the excavation-induced settlement profile, increases the angular distortions and diagonal extensions in the structure. Horizontal extension strains were in­sensitive to the number of bays composing the structure. These trends are illustrated for two- to five-story structures in Fig. 9 and are primarily a result of forcing the structure to distort more to conform to the ground surface profile. The shorter one-bay cases can accommodate more of the change in ground surface slope via rigid body rotation and thus sustain less distortion. Fig. 9 also indicates dramatic reductions in distortion when grade beams are present.

Using grade beams to tie the walls/columns together results in substantial reductions in the maximum angular distortion, diagonal extension, and hor­izontal extension induced in the structure. The curves are for two-story struc­tures with three bays. However, they may be used for five- to ten-story structures with only slight errors which are on the conservative side. The greatest benefit is derived when going from no grade beams to a light grade

f 1.5

JL 3g'

1.0

ii. P g '

0.5 eh ehg

0 2 4 6 8

Eg A

EgHS

FIG. 10. Effect of Grade Beams on Angular Distortion, Diagonal Strain, and Lat­eral Strain

15

beam where the grade beam is about the same stiffness as a typical column (EgA/EsHS ~ 1 to 2 for the case examined). Increasing the grade beam stiffness much beyond this nets little additional reduction in building dis­tortion as shown in Fig. 10. The horizontal axis of Fig. 10 is the normalized grade beam stiffness, EgA/EsHS, where Es is the soil stiffness, H is the depth of the cut, S is the spacing of the grade beams perpendicular to the edge of the cut, Eg is the Young's modulus of the grade beam, and A is the area of the grade beam.

When the beam stiffnesses were reduced to simulate a bearing wall struc­ture rather than a concrete frame structure, the maximum angular distortions induced in the structure decreased slightly. However, the angular distortion sustained by the upper floors increased substantially due to the decreased restraint provided by the less stiff beams. Maximum horizontal extension strains at ground level were about the same in both cases, but horizontal extension strains in the stiffer beam cases were much less in the upper sto­ries.

SUMMARY AND CONCLUSIONS

In this study, the tolerance of buildings to nearby excavation was inves­tigated. Model studies were used to develop a method of estimating potential building response, and these were compared with field data. The following conclusions are drawn from this study:

1. Buildings sited adjacent to excavations are generally less tolerant to ex­cavation-induced differential settlements than similar structures settling under their own weight. This is due to the lateral strains that develop in response to most excavations.

2. As a structure is subjected to increasing lateral strains, its tolerance to dif­ferential settlement decreases. As a consequence, measures to mitigate excava­tion-related building damage should include provisions to reduce the lateral strains sustained by the structure.

3. Since the ground movements develop in the form of a traveling wave grad­ually impinging on a structure, the ratio of the length of the portion of the struc­ture affected by the excavation-induced ground movements to height of the struc­ture will initially be very small and grow gradually. Thus, the tolerance of a building to initial deformation will be governed by its tolerance to shearing de­formation and horizontal extension.

4. Increasing the ratio of longitudinal stiffness to shear stiffness, E/G, as openings in a wall are assumed to cause, increases the range of deformed length to height ratio, L/H, in which the diagonal strain capacity or shearing resistance is the limiting factor.

5. Points 3 and 4 suggest that angular distortion, a measure of shearing strain, would be an appropriate parameter to correlate with observed response.

6. If reasonable estimates of ecril and L/H can be made for a structure in the vicinity of proposed underground construction, the limiting deflection ratio and angular distortion for that structure can be estimated and compared to the esti­mated ground movement to permit engineers to assess the potential for damage and suitability of possible remedial measures.

7. Frame-type structures, depending on size and other details, can often resist some of the ground movements. Increasing the number of stories creates a struc-

16

ture stiffer in shear that would tend to tilt rather than distort. Conversely, in­creasing the number of bays typically increases the length of a structure and causes it to distort more to accommodate the ground movements caused by the excavation.

8. Horizontal ties in the form of reinforced grade beams or similar items are effective means of controlling the strains and distortions in both bearing-wall and frame structures adjacent to excavation.

9. This study only examines cases where the lateral strains are tensile in na­ture. However, compressive strains may create a critical condition for structures sited over the center line of a tunnel or mine. The effects of compressive lateral strain are examined by the. National Coal Board (1975).

The use of Fig. 4 to predict degree of cracking in bearing-wall and small frame structures adjacent to excavation appears to have promise, based upon the limited field check. Further field observations with measurements of lat­eral displacement as well as settlement will help verify the relations. In this study, only cracking and distortion-related damage are considered. However, the effects of tilt on the use should be included in any evaluation of potential response to adjacent excavation.

APPENDIX I. DEFINITIONS OF DISTORTION AND DAMAGE

Definitions of Distortion Angular distortion or relative rotation, (3, is a measure of the shearing

distortion of a structure. Angular distortion is often approximated as the ro­tation, due to settlement, of the straight line joining two reference points on the structure minus any rigid body tilt that the structure may have incurred. In general, the angular distortion calculated is an average value for the por­tion of the building bounded by the reference points, as shown in Fig. 11.

Deflection ratio, A/L, is the relative deflection, A, divided by the distance between two reference points, L, where relative deflection is the maximum displacement of the settlement profile of a structure relative to the straight line connecting two settlement reference points, as shown in Fig. 11. De­flection ratio is often correlated with bending related distortions.

Horizontal strain or lateral distortion, e,„ is the average strain due to the relative horizontal movement of two reference points, as shown in Fig. 11.

Definitions of Building Damage Classification of building damage has been traditionally divided into three

general categories after those employed by Skempton and MacDonald (1956). These classifications, architectural damage, functional damage, and struc­tural damage are described as follows:

Architectural damage affects the appearance of structures, and is usually related to cracks or separations in panel walls, floors, and finishes. Cracks in plaster walls greater than 0.5 mm wide and cracks in masonry or rough concrete walls greater than 1 mm wide are considered to be representative of a threshold where damage is noticed and reported by building occupants (O'Rourke et al. 1976; Burland et al. 1977).

Functional damage affects the use of the structure, and is exemplified by jammed doors and windows, extensively cracked and falling plaster, tilting

17

o o-I , TTiaxj

, - O T T

634

p Iheavel "max

a] Settlement and Differential Settlement

L14 1-12 1 L24

6 2o 3o i> L45l 5 L 5 6

jys_

bl Relat ive De f lec t ion and Def lect ion Rat io

c| T i l t and Angular D is tor t ion (Re la t ive Rota t ion !

S

D X • A 1 ZL.

PhF, L 23 \ s c -Ph3-Ph2 L23

d] Horizontal Displacement and Hor i zon ta l St ra in

FIG. 11. Building and Ground Movement Parameters

of walls and floors, and other damage that would require nonstructural repair to return the building to its full service capacity.

Structural damage affects the stability of the structure, usually related to cracks or distortions in primary support elements such as beams, columns, and load bearing walls.

These classifications are obviously quite general with no clearly defined limits. As a result, there may be considerable overlap of the categories de­pending upon the type and use of the particular structure considered. Burland et al. (1977) present a classification of visible damage based upon the work of Jennings and Kerrich (1962), The National Coal Board (1975), and MacLeod and Littlejohn (1974). This classification system, shown in Table 2, is based

18

TABLE 2. Classification of Visible Damage

Class of damage

(D Negligible

Very Slight

Slight

Moderate

Severe

Very Severe

Description of damage0

(2)

Hairline cracks Fine cracks easily treated during normal

redecoration. Perhaps isolated slight fracture in building. Cracks in exterior brickwork visible upon close inspection.

Cracks easily filled. Re-decoration probably required. Several slight fractures inside building. Exterior cracks visible, some re-pointing may be required for weathertightness. Doors and windows may stick slightly.

Cracks may require cutting out and patching. Recurrent cracks can be masked by suitable linings. Tuck-pointing and possibly replacement of a small amount of exterior brickwork may be required. Doors and windows sticking. Utility service may be interrupted. Weathertightness often impaired.

Extensive repair involving removal and replacement of sections of walls, especially over doors and windows required. Windows and door frames distorted, floor slopes noticeably. Walls lean or bulge noticeably, some loss of bearing in beams. Utility service disrupted.

Major repair required involving partial or complete re-construction. Beams lose bearing, walls lean badly and require shoring. Windows broken by distortion. Danger of instability.

Approximate widthb

of cracks, mm (3)

<0.1 < 1

<5

5 to 15 or several cracks > 3 mm

15 to 25 also depends on number of cracks

usually >25 depends on number of cracks

"Location of damage in the building or structure must be considered when classifying degree of damage.

bCrack width is only one aspect of damage and should not be used on alone as a direct measure of it.

Note: Modified from Burland et al. (1977)

in part on the ease of repair of the damage, and thus provides a defined framework for the evaluation of damage.

APPENDIX I. REFERENCES

Attewell, P. B. (1977a). "Ground movements caused by tunnelling in soil." Proc, Conf. on Large Ground Movements and Structures, Halstead Press, New York, N.Y., 812-948.

Attewell, P. B. (1977b). "Large ground movements and structural damage caused by tunnelling below the water table in a silty alluvial clay." Proc, Conf. on Large Ground Movements and Structures, Halstead Press, New York, N.Y., 307-356.

19

Bjerrum, L. (1963). "Discussion session IV." Proc, European Conf. on Soil Mech. and Found. Engr., Wiesbaden, Germany, II, 135-137.

Boscardin, M. D. (1980). "Building response to excavation-induced ground move­ments." Thesis presented to the University of Illinois, at Urbana-Champaign, 111., in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

Boscardin, M., and O'Rourke, T. D. (1977). "Building response to ground move­ments caused by an 18-meter-deep excavation." Proc, Conf. on Large Ground Movements and Structures, Halstead Press, New York, N.Y., 162-183.

Boscardin, M. D., Cording, E. J., and O'Rourke, T. D. (1978). Case studies of building behavior in response to adjacent excavation. Final Report prepared by the University of Illinois at Urbana-Champaign for the U.S. Dept. of Transpor­tation, Report No. UMTA-IL-06-0043-78-2.

Brauner, G. (1974). Subsidence due to mining: 1. Theory and practices in predicting surface deformation; 2. Ground movements and mining damage. U.S. Dept. of the Interior, Bureau of Mines.

Breth, H., and Chambosse, G. (1974). "Settlement behavior of buildings above sub­way tunnels in Frankfurt clay." Proc, Conf. on Settlement of Structures, Pentech Press, London, England, 329-336.

Burland, J. B., and Wroth, C. P. (1974). "Settlement of buildings and associated damage." Proc, Conf. on Settlement of Structures, Pentech Press, London, En­gland, 611-654.

Burland, J. B., and Broms, B. B., and de Mello, V. F. B. (1977). "Behavior of foundations and structures." State-of-the-Art Report. Proc, 9thlnt'l. Conf. on Soil Mech. and Found. Engr., II, Tokyo, Japan, 495-546.

Geddes, J. D. (1977). "The effect of horizontal ground movements on structures." Proc, Conf. on Large Ground Movements and Structures, Halstead Press, New York, N.Y., 623-646.

Geddes, J. D. (1977). "Construction in areas of large ground movements." State-of-the-Art Report, Proc, Conf. on Large Ground Movements and Structures, Hal­stead Press, New York, N.Y., 949-974.

Jennings, J. E., and Kerrich, J. E. (1962). "The heaving of buildings and the as­sociated economic consequences, with particular reference to the Orange Free State goldfields," The Civ. Engr. in South Africa, 5(5), 122.

Littlejohn, G. S. (1974). "Observations of brick walls subjected to mining subsi­dence." Proc, Conf. on Settlement of Structures, Pentech Press, London, En­gland, 384-393.

MacLeod, I. A., and Littlejohn, G. S. (1974). "Discussion of session 5." Proc, Conf. on Settlement of Structures, Pentech Press, London, England, 792-795.

Mahr, J. W., and Marino, G. G. (1981). "Building response and mitigation measure for building damages in Illinois." Proc. Workshop on Surface Subsidence Due to Underground Mining, West Virginia Univ., 238-252.

Marino, G. G. (1985). "Subsidence damaged homes over room and pillar mines in Illinois." Thesis presented to the University of Illinois, at Urbana-Champaign, 111., in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

Meyerhof, G. G. (1953). "Some recent foundation research and its application to design." Struct. Engr., 31, 151-167.

Meyerhof, G. G. (1956). Discussion of "The allowable settlements of buildings," by A. W. Skempton and D. H. MacDonald," Proc, Inst. Civ. Engrs., Part II, 5, 774.

Milligan, G. W. E. (1974). "The behavior of rigid and flexible retaining walls in sand." Thesis presented to Cambridge University, Cambridge, England, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

National Coal Board (1975). Subsidence engineers handbook. National Coal Board Production Dept., London, England.

O'Rourke, T. D., Cording, E. J., and Boscardin, M. (1976). The ground movements related to braced excavation and their influence on adjacent structures. University of Illinois Report for U.S. Dept. of Transportation, Report No. DOT-TST-76T-22.

20

Polshin, D. E., and Tokar, R. A. (1957). "Maximum allowable non-uniform settle­ment of structures." Proc, 4th In?I. Conf. on Soil Mech. and Found. Engr., 1, London, England, 402-405.

Skempton, A. W., and MacDonald, D. H. (1956). "The allowable settlement of buildings." Proc, Inst, of Civ. Engrs., Part III, 5, 727-784.

Timoshenko, S. (1957). Strength of Materials—Part 1, D. Van Nostrand Co. Inc., London, England.

Wahls, H. E. (1981). "Tolerable settlement of buildings." J. Geotech. Engrg., ASCE, 107(11), 1489-1504.

APPENDIX II. NOTATION

The following symbols are used in this paper:

A E G H I L n P S

c ^max

W

X

a P

Ps P, A

A/L 8 e

e V

p

= =

= =

=

=

—

=

= =

= = = = = =

cross-sectional area of a beam; Young's modulus; shear modulus; height of beam, wall, or cut; moment of inertia about neutral axis; span length of beam, wall or reference point to reference point; ratio of length of structure to w; point load; spacing of grade beams; settlement of center of tunnel settlement trough; half width of tunnel settlement trough; location of structure relative to center of tunnel; rigid body tilt; angular distortion; change in slope of ground at location of structure; building slope; relative deflection; deflection ratio; differential displacement; strain; angle from horizontal; Poisson's ratio; and displacement.

Subscripts b

crit d g h

max * a

e

= = = = = = = = =

bending; critical; diagonal; ground or grade beam; horizontal; maximum; soil; rigid body rotation or tilt; and angle from horizontal.

21