Response of Building Adjacent to Stiff Excavation Support System in Soft Clay

A three-story school supported by shallow foundations was affected by an adjacent 12.2-m-deep excavation in soft clay in which the excavation support system was a 0.9-m-wide secant pile wall braced by both cross-lot struts and tiebacks. The school is a reinforced concrete frame structure with exterior reinforced concrete foundation walls. This paper summarizes the conditions at the site and presents correlations among construction activities, measured deformations and distortions, and attendant damage in the school. The lateral ground movements associated with the excavation were monitored with four inclinometers placed around the school. The building movements were monitored with optical survey points established on interior columns, exterior walls and on the roof, and with tiltmeters installed on the exterior foundation walls. The damage to the school mainly consisted of 300 to 500-mm-long hairline cracks in nonload bearing walls. Only a few cracks had widths greater than 6 mm. The school deformed such that the portion closest to the excavation sagged and the remainder hogged. Damage was first observed in the area of sagging when angular distortions reached 1/940 and the excavation was approximately 5.5-m deep. Angular distortions as large as 1/300 were observed at the end of the project. The data suggest that angular distortions had to be less than 1/1000 to preclude any damage to the school.     

Ground Displacements from a Pipe-Bursting Experiment in Well-Graded Sand and Gravel

Pipe bursting is a construction technique that involves the replacement of an existing buried pipe with potentially much less surface disturbance than traditional cut and cover construction. However, excessive ground movements associated with pipe-bursting operations may lead to damage to surrounding infrastructure. A static pipe-bursting experiment was performed in sand and gravel within an 8-m-long, 8-m-wide, and 3-m-deep test pit to quantify the ground displacements from pipe bursting. An existing unreinforced concrete pipe buried 1.385 m below the ground surface was replaced with a high-density polyethylene pipe. Pulling force and the three-dimensional nature of surface displacements associated with pipe bursting are examined. The 4-m wide surface response had a peak vertical displacement of 6 mm. In addition, transverse displacements of 1.2 mm resulted in the formation of a tension crack in the ground above the concrete pipe. This experiment offers data that improves the understanding of the mechanisms of ground disturbance, and provides unique experimental data for calibration of numerical models.     

Interaction of Shallow Foundations with Reverse Faults

A series of centrifuge model tests is reported that investigated the effects of foundation position on the interaction of reverse, dip-slip faults with shallow foundations resting on sand. The model tests have allowed careful examination of both the soil and foundation deformation as a shear localization (fault) propagates through a 15?m thick sand layer for fault throws up to 5?m. By comparing results of the tests with foundations present with those from a “free-field” test, the effect of the foundation on the faulting pattern has been observed directly. The response of the foundation is very sensitive to the exact position of the fault and even when the fault emerged remotely from the foundation it sometimes caused significant foundation movements. Detailed results are presented for the tests and it is suggested that these results are used as: (1) indications of likely foundation–soil–fault interaction mechanisms; and (2) to allow future validation of numerical models for similar problems. Finally, foundation rotations measured during the fault–foundation interaction tests are compared to those predicted using a simple method based on free-field soil displacements. This simple method makes surprisingly good prediction of maximum fault rotations for different throws.     

Wall and Ground Movements due to Deep Excavations in Shanghai Soft Soils

An extensive database of 300 case histories of wall displacements and ground settlements due to deep excavations in Shanghai soft soils were collected and analyzed. The mean values of the maximum lateral displacements of walls constructed by the top-down method, walls constructed by the bottom-up method (including diaphragm walls, contiguous pile walls, and compound deep soil mixing walls), sheet pile walls, compound soil nail walls, and deep soil mixing walls are 0.27%H, 0.4%H, 1.5%H, 0.55%H, and 0.91%H, respectively, where H is the excavation depth. The mean value of the maximum ground surface settlement is 0.42%H. The settlement influence zone reaches to a distance of about 1.5H to 3.5H from the excavation. The ratio between the maximum ground surface settlement and the maximum lateral displacement of a wall generally ranges from 0.4 to 2.0, with an average value of 0.9. The factors affecting the deformation of the wall were analyzed. It shows that there is a slight evidence of a trend for decreasing wall displacement with increasing system stiffness and the factor of safety against basal heave. Wall and ground movements were also compared with that observed in worldwide case histories.     

Prediction of Ground Movements due to Pile Driving in Clay

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements δL∕R2 are functions of their dimensionless position x∕L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x∕L depend only on the wall thickness w; i.e., δ∕w = f[x∕L]. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of (1) building movements caused by installation of large pile groups; (2) uplift of a pile caused by driving of an adjacent pile within a group; and (3) spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the shallow strain path method analyses.     

Simplified Approach for the Seismic Response of a Pile Foundation

Pseudostatic approaches for the seismic analysis of pile foundations are attractive for practicing engineers because they are simple when compared to difficult and more complex dynamic analyses. To evaluate the internal response of piles subjected to earthquake loading, a simplified approach based on the “p-y” subgrade reaction method has been developed. The method involves two main steps: first, a site response analysis is carried out to obtain the free-field ground displacements along the pile. Next, a static load analysis is carried out for the pile, subjected to the computed free-field ground displacements and the static loading at the pile head. A pseudostatic push over analysis is adopted to simulate the behavior of piles subjected to both lateral soil movements and static loadings at the pile head. The single pile or the pile group interact with the surrounding soil by means of hyperbolic p-y curves. The solution derived first for the single pile, was extended to the case of a pile group by empirical multipliers, which account for reduced resistance and stiffness due to pile-soil-pile interaction. Numerical results obtained by the proposed simplified approach were compared with experimental and numerical results reported in literature. It has been shown that this procedure can be used successfully for determining the response of a pile foundation to “inertial” loading caused by the lateral forces imposed on the superstructure and “kinematic” loading caused by the ground movements developed during an earthquake.     

Simplified Model for Evaluating Damage Potential of Buildings Adjacent to a Braced Excavation

This paper presents a simplified model for evaluating damage potential of a building adjacent to a braced excavation. New developments include several component models for estimating the lateral ground movement profile, angular distortion and lateral strain in a building, and damage potential of a building adjacent to an excavation. These new developments along with an existing model for the vertical ground movement profile are integrated into the proposed simplified model. Case histories are used to verify the developed simplified model. Furthermore, the uncertainty in the developed model is quantified, which enables a probabilistic assessment of the excavation-induced building damage potential. An example probabilistic assessment of building damage potential is presented.     

Liquefaction-Induced Softening of Load Transfer between Pile Groups and Laterally Spreading Crusts

Laterally spreading nonliquefied crusts can exert large loads on pile foundations causing major damage to structures. While monotonic load tests of pile caps indicate that full passive resistance may be mobilized by displacements on the order of 1–7% of the pile cap height, dynamic centrifuge model tests show that much larger relative displacements may be required to mobilize the full passive load from a laterally spreading crust onto a pile group. The centrifuge models contained six-pile groups embedded in a gently sloping soil profile with a nonliquefied crust over liquefiable loose sand over dense sand. The nonliquefied crust layer spread downslope on top of the liquefied sand layer, and failed in the passive mode against the pile foundations. The dynamic trace of lateral load versus relative displacement between the “free-field” crust and pile cap is nonlinear and hysteretic, and depends on the cyclic mobility of the underlying liquefiable sand, ground motion characteristics, and cyclic degradation and cracking of the nonliquefied crust. Analytical models are derived to explain a mechanism by which liquefaction of the underlying sand layer causes the soil-to-pile-cap interaction stresses to be distributed through a larger zone of influence in the crust, thereby contributing to the softer load transfer behavior. The analytical models distinguish between structural loading and lateral spreading conditions. Load transfer relations obtained from the two analytical models reasonably envelope the responses observed in the centrifuge tests.     

Performance of a Stiff Support System in Soft Clay

The performance of an excavation support system for a subway station renovation project in Chicago and its effects on an adjacent, shallow-foundation supported building are presented. The 13-m-deep excavation was made through soft to medium stiff clays and was supported by a 900-mm-thick secant pile wall, one level of cross-lot bracing, and two levels of tiebacks. Design considerations are discussed and construction procedures are summarized. Field performance data were collected, including lateral soil movements at five locations, building settlements along the exterior wall and interior columns, support system loads, and observations of building damage. As planned in the design, minor damage occurred to nonload bearing portions of the building. Of the 38 mm of maximum lateral movement adjacent to the building, 9 mm occurred during wall installation, 16 mm developed as the soil was excavated, and 13 mm occurred during tunnel demolition and station renovation as a result of soil creep and reduction of wall stiffness. Settlements extended beyond the secant pile wall a distance approximately equal to the depth of the secant pile wall. The effect of excavation was to cause larger settlements within the affected zone, but not to expand the width of the settlement trough. When distortions exceeded approximately 1/960, damage began to manifest itself in the nonload bearing portions of the building.     

Three-Dimensional Responses Observed in an Internally Braced Excavation in Soft Clay

Several three-dimensional effects were observed in the performance monitoring data collected during excavation for the Ford Engineering Design Center in Evanston, Illinois. The elevations of the soil around the excavation varied and the excavator removed the soil in a nonuniform excavation process, both of which contributed to the observed three-dimensional (3D) effects. This paper describes the excavation support system and subsurface conditions at the site, summarizes the construction procedures, and presents the lateral soil movements measured by inclinometers, ground-surface movements measured by an automated total station, tilt of components of an adjacent structure, and forces in internal braces. These responses are compared with expected responses from current design methods. The 3D nature of the excavation resulted in smaller movements on the side of the excavation, where the retained soil was lowest, an unexpected pattern of axial strut loads and very slight damage to an exterior wall that paralleled one of the excavation walls.     

Database for Retaining Wall and Ground Movements due to Deep Excavations

A database of some 300 case histories of wall and ground movements due to deep excavations worldwide is presented. Although recognizing the weakness in the approach, a large database is used to examine general trends and patterns. For still soil sites, movements are generally less than those suggested in the well-known relationships proposed by Clough and his coworkers. However, for walls that retain a significant thickness of soft material but have a high factor of safety against basal heave, movements are similar to those calculated using the Clough charts. In these cases, when soft ground is actually present at dredge level, the Clough charts will underpredict movement and need to be used with care. For the above cases there is no discernible difference in the performance of propped or anchored systems but there is some evidence to suggest top-down systems perform better. In cases where there is a low factor of safety against excavation base heave, large movements can occur, but the Clough charts will give reasonable preliminary estimates of the likely movement in such cases. Cantilever walls have shown displacements that are often independent of the system stiffness. There is evidence to suggest that, in the case of cantilever walls and for all walls in stiff soils worldwide, design practice is conservative. Finally, the inclusion of a cantilever stage at the beginning of a construction sequence seems to be the main cause of unusually large movements.     

Fully Probabilistic Framework for Evaluating Excavation-Induced Damage Potential of Adjacent Buildings

This paper presents a framework for a fully probabilistic analysis of the potential for damage to buildings adjacent to an excavation. Herein, the damage potential index (DPI), which is a function of angular distortion and lateral strain, is used to assess building damage potential. A serviceability limit state is established in which the resistance is expressed in terms of the “limiting” DPI, and the load is represented by the “applied” DPI. In this context, damage to the building adjacent to an excavation is said to occur deterministically if the applied DPI is greater than the limiting DPI. For the fully probabilistic analysis, both parameter and model uncertainties of the limiting and applied DPIs are first characterized. The analysis framework is then presented and demonstrated with a case history. Finally, sensitivity analysis is performed to identify the factors to which the probability of damage is most sensitive and to analyze the effect of various assumptions of the input parameters on the computed probability of building damage.     

Pile Responses Caused by Tunneling

In this paper, a two-stage approach is used to analyze the lateral and axial responses of piles caused by tunneling. First, free-field soil movements are estimated based on an analytical method, and, second, these estimated soil movements are imposed on the pile in simplified boundary element analyses to compute the pile responses. Through a parametric study, it is shown that the influence of tunneling on pile response depends on a number of factors, including tunnel geometry, ground loss ratio, soil strength, pile diameter, and ratio of pile length to tunnel cover depth. Simple design charts are presented for estimating maximum pile responses and may be used in practice to assess the behavior of existing piles adjacent to tunneling operations. A published case history has been studied in which the measured lateral pile deflections are compared with those computed using the present method and fair agreement is found.     

Effects of Jet Grouting on Adjacent Ground and Structures

The basement excavation of the Singapore Post Center involved extensive jet grouting to improve the soft marine clay present within the excavation. The treated soil mass, with much improved strength and deformation characteristics, was intended to act as an internal strut below the bottom of the excavation level, reducing movements caused by the basement excavation. This paper presents the performance of production grouting carried out during the construction of the building's basement. Results of monitoring suggest that the jet grouting caused the retaining diaphragm walls to move between 9.7 and 36.4 mm. The soils behind the walls also moved away from the excavation. Movements ranged from 35.3 to 53.6 mm within 1–2 m from the wall to 13.5 to 32.8 mm at 4.5–20.5 m away from the wall. The recorded soil heave ranged from 2 to 24 mm, with the majority of the measurements being less than 10 mm. The backward movements induced by the production grouting are similar to those induced by preloading a strut of braced excavation system. Provided the backward movements do not exceed the allowable limits, they would help in minimizing the ultimate positive movements induced by the subsequent basement excavation. The jet grouting also induced some bending moments on the diaphragm walls and caused the adjacent structures to tilt and move away from the jet grout area.     

Use of Jet Grouting to Limit Diaphragm Wall Displacement of a Deep Excavation

Excessive lateral diaphragm wall displacement and the associated ground settlement are often the primary cause of damage of nearby buildings. It is therefore imperative to minimize diaphragm wall displacement during basement excavation if the integrity of adjacent buildings is of concern. This paper describes the application of a jet grouting scheme to reduce the diaphragm wall displacement of a six-level basement excavation. Based upon field experience of similar projects, buildings adjacent to the construction site may settle well beyond an acceptable limit if excavation is carried out without any protection measures being taken. In this excavation project, the soil mass within the excavation zone was partially jet grouted in an attempt to increase its passive resistance as an effective measure to limit wall displacement. Numerical analyses were carried out to assess the effects of jet grouting. Field measurements on wall displacement and ground settlement confirm the effectiveness of the improvement scheme.     

Measurement of Particle Movement in Granular Soils Using Image Analysis

The flowing behavior of granular soils in the form of localized deformations and shear bands is a difficult phenomenon to define explicitly in the modeling of soil-structure interface problems. However, for development of accurate numerical solutions for these problems, an estimation of particle movements is important in determining the nature of displacement fields within the granular media. Video images from direct shear tests were used to capture the movement of individual grains relative to the movement of a ribbed structural surface. Two different algorithms have been developed to determine particle displacements in an image sequence of the interface during a direct shear test. BMAD employs a block-matching algorithm using unit patterns of images to determine two-dimensional particle displacement vectors. MATCH, on the other hand, computes displacements based on centroid locations of segmented particles that are matched using a four-level filter algorithm in successive image frames. Measured hardware dependent noise was reduced during image acquisition by means of a frame averaging technique implemented in BMAD. BMAD was successfully employed to determine particle displacements in Ottawa sand images obtained during shearing on a ribbed surface. The results were verified with MATCH using the same image frames.     

Combined Finite- and Boundary-Element Analysis of the Effects of Tunneling on Single Piles

An efficient and practical method of analysis to predict the effects of tunneling on existing single pile foundations is described. The method involves a combination of the finite- and boundary-element (FAB) methods, with free-field ground movements predicted by the finite-element method and the response of an embedded pile to these ground movements predicted by the boundary-element method. The method allows prediction of the full three-dimensional (3D) response of the pile as tunnel excavation proceeds towards the pile and away from it. Very good agreement is obtained between predictions of the pile response obtained by the FAB method and a 3D finite-element analysis which specifically includes the pile in the finite-element mesh. The vastly superior computational efficiency of the FAB method over the full 3D finite element approach is also illustrated.     

Buried Pipeline Response to Braced Excavation Movements

This paper presents a detailed study to ascertain the response of a buried minor sewer pipeline consisting of three new manholes of 3.5- to 4-m deep and 89.7 m of 225-mm-diameter vitreous clay pipes to braced excavation ground movements. The pipeline and the manholes were supported on 6-m long, generally, of 65- to 75-mm-diameter Bakau (timber) piles embedded in 8- to 15-m-thick soft marine clay and peaty clay with an overconsolidation ratio of 1.2. The 34-m pipeline connecting two of the manholes ran parallel to a row of sheetpiles 9 m away. The sheetpiles formed one side of a strutted basement excavation about 9- to 10.5-m-deep. The excavation is part of an adjacent construction project. A recent oversite fill of 1.1 m has been placed to raise the site platform to avoid possible flooding throughout the area encompassing the pipeline and the adjacent construction project. Useful lessons which have been learnt from the study will be presented also.     

Novel Approach to Integration of Numerical Modeling and Field Observations for Deep Excavations

Precedent and observation of performance are an essential part of the design and construction process in geotechnical engineering. For deep urban excavations designers rely on empirical data to estimate potential deformations and impact on surrounding structures. Numerical simulations are also employed to estimate induced ground deformations. Significant resources are dedicated to monitor construction activities and control induced ground deformations. While engineers are able to learn from observations, numerical simulations have been unable to fully benefit from information gained at a given site or prior excavation case histories in the same area. A novel analysis method, self-learning in engineering simulations (SelfSim), is introduced to integrate precedent into numerical simulations. SelfSim is an inverse analysis technique that combines finite element method, biologically inspired material models, and field measurements. SelfSim extracts relevant constitutive soil information from field measurements of excavation response such as lateral wall deformations and surface settlement. The resulting soil model, used in a numerical analysis, provides correct ground deformations and can be used in estimating deformations of similar excavations. The soil model can continuously evolve using additional field information. SelfSim is demonstrated using two excavation case histories in Boston and Chicago.     

Sequential Analysis of Ground Movements at Three Deep Excavation Sites with Mixed Ground Profiles

Field measurements of settlement and lateral deformation obtained from three deep excavation sites constructed in mixed ground profiles are presented and analyzed. Settlement measurements were obtained throughout the construction process, categorized in three stages as: (1) preexcavation (i.e., preliminary site work and support wall installation); (2) main excavation and bracing/anchor installation; and (3) postexcavation (i.e., removal of bracing as basement construction proceeds). Maximum preexcavation stage settlements of 0.03%Hw to 0.06%Hw (where Hw = wall or trench depth) were measured at two sites, with the maximum settlements occurring adjacent to the wall during its installation. Maximum ground surface settlements during the main excavation stage ranged from about 0.15%He to 0.30%He (where He = final excavation depth) and the distribution of ground settlement extended to a distance of 1.5He to 2.0He from the wall. Maximum settlements occurred at distances of about 0.3He to 0.5He from the wall at two sites where the wall consisted of concrete cast in situ (concrete diaphragm and concrete secant pile walls), creating a significant reverse curvature in the settlement distribution. The maximum postexcavation stage settlements ranged from 0.07%He to 0.10%He for the three sites, representing roughly 10 to 60% increases in settlement over the main excavation settlements, depending greatly on the specific support removal methods as well as the basement floor construction details employed at an individual site. Lateral deflections during the main excavation stage were consistent with trends reported in the literature, ranging from 0.12%He to 0.23%He, while lateral movement during postexcavation stage ranged from 0.03%He to 0.09%He. Finally, the settlements measured during the main and postexcavation stages are related to the support system stiffness.