1880热轧带钢工程加热炉区深基坑围护及土体位移有限元分析

软土地基深基坑开挖过程中,由于降水质量、基坑围护不同,或者土方开挖工况选择不当,常会引起土体位移,对管桩产生较大的土压力,出现基坑滑坡、桩身偏斜等质量事故,特别是大面积群桩整体位移,对工程本身将造成较大的经济损失和工期延误.采用桩土共同建模,桩土单元之间位移协调的有限元方法,将桩土单元集成总体刚度矩阵整体分析,对宝钢1880热轧带钢工程加热炉深基坑开挖过程进行研究,对比计算了基坑开挖对土体侧移和土体应力分布的影响,对施工工况的设计具有参考作用.     

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.     

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.     

Behavior of Braced and Anchored Walls in Soils Overlying Rock

This paper presents the results of analyses on the behavior of in situ walls, using the measured data collected from various deep excavation sites with multilayered ground conditions of soils overlying rock in Korea. A variety of in situ wall systems from >60 excavation sites were considered, covering a wide range of wall types, including H-pile, soil cement, cast-in-place pile, and diaphragm. The measured data were thoroughly analyzed to investigate the effects of wall and support types on lateral wall movements as well as apparent earth pressures. A series of 2D finite-element analyses were also performed to provide insights regarding the in situ wall behavior. Based on the results, lateral wall movements and apparent earth pressures are related to primary influencing factors affecting the wall behavior and information is presented in forms to provide tools that can be used for design and analysis.     

Pile Behavior due to Excavation-Induced Soil Movement in Clay. I: Stable Wall

A series of centrifuge model tests has been conducted to investigate the behavior of a single pile subjected to excavation-induced soil movements behind a stable retaining wall in clay. The results reveal that after the completion of soil excavation, the wall and the soil continue to move and such movement induces further bending moment and deflection on an adjacent pile. For a pile located within 3?m behind the wall where the soil experiences large shear strain (>2%) due to stress relief as a result of the excavation, the induced pile bending moment and deflection reach their maximum values sometime after soil excavation and thereafter decrease slightly with time. For a pile located 3?m beyond the wall, the induced pile bending moment and deflection continue to increase slightly with time after excavation until the end of the test. A numerical model developed at the National University of Singapore is used to back-analyze the centrifuge test data. The method gives a reasonably good prediction of the induced bending moment and deflection on a pile located at 3?m or beyond the wall. For a pile located at 1?m behind the wall where the soil experiences large shear strain (>2%) due to stress relief resulting from the excavation, the calculated pile response is in good agreement with the measured data if the correct soil shear strength obtained from postexcavation is used in the analysis. However, if the original soil shear strength prior to excavation is used in the analysis, this leads to an overestimation of the maximum bending moment of about 25%. The practical implications of the findings are also discussed in this paper.     

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.     

Ground Movements Associated with Wall Construction: Case Histories

The construction of diaphragm wall panels can cause movements to the adjacent ground. The magnitude of the movements depends on various factors such as the construction technique, soil type, and panel dimensions. These movements can be excessive if the wall dimensions and construction technique are not properly chosen or the process of wall construction is not properly controlled. This paper presents four case histories on the adjacent ground response to the construction of diaphragm wall panels. The aspects of performance monitored include lateral soil movements and soil settlements. The monitoring results indicated that the lateral soil movements caused by the construction of wall panels increased with increasing wall dimension. These results suggest that the magnitude of the lateral soil movements could be minimized by reducing the dimensions of the wall panels. The results also suggest that the use of high slurry levels during the construction of the wall panels would help to minimize the lateral soil movements.     

Simplified Lateral Load Analyses of Fixed-Head Piles and Pile Groups

The characteristic load method (CLM) can be used to estimate lateral deflections and maximum bending moments in single fixed-head piles under lateral load. However, this approach is limited to cases where the lateral load on the pile top is applied at the ground surface. When the pile top is embedded, as in most piles that are capped, the additional embedment results in an increased lateral resistance. A simple approach to account for embedment effects in the CLM is presented for single fixed-head piles. In practice, fixed-head piles are more typically used in groups where the response of an individual pile can be influenced through the adjacent soil by the response of other nearby piles. This pile–soil–pile interaction results in larger deflections and moments in pile groups for the same load per pile compared to single piles. A simplified procedure to estimate group deflections and moments was also developed based on the p-multiplier approach. Group amplification factors are introduced to amplify the single pile deflection and bending moment to reflect pile–soil–pile interaction. The resulting approach lends itself well to simple spreadsheet computations and provides good agreement with other generally accepted analytical tools and with values measured in published lateral load tests on groups of fixed-head piles.     

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.     

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.     

Behavior of Pile Subject to Excavation-Induced Soil Movement

This paper presents the results of centrifuge model tests on unstrutted deep excavation in dense sand and its influence on an adjacent single pile foundation behind the retaining wall. It is found that, in the case of a stable wall, the induced pile bending moment and deflection decrease exponentially with increasing distance between the pile and the wall. Pile head boundary condition plays an important role in affecting the pile responses due to an adjacent excavation. In the case of retaining wall collapse, the failure pattern of the soil behind the wall features a slip plane projecting from near the wall toe to the ground surface. Soil within the failure zone demonstrates large lateral movement and induces significant bending moment and deflection on pile located within the zone. Soil movement and pile responses outside this zone are noted to be significantly less. A comparison between the experimental results and the theoretical predictions by an existing numerical method shows good agreement, provided that appropriate assumptions are made on the soil parameters and conditions, especially in the case of retaining wall collapse.     

Predicting RC Frame Response to Excavation-Induced Settlement

In many tunneling and excavation projects, free-field vertical ground movements have been used to predict subsidence, and empirical limits have been employed to evaluate risk. Validity of such approaches is largely unknown given that ground movements are in fact not one-dimensional and that adjacent applied loads are known to have an impact. This paper employed analytical and large-scale experimental efforts to quantify these issues, in the case of excavation adjacent to a reinforced concrete frame with tieback anchors and a sheetpile wall in dry sand. With this flexible system, a disproportionate amount of the soil and building movements occurred prior to installation of the first tieback, even when conservative construction practices were applied. Furthermore, free-field data generated a trough as little as one-half the size of that recorded near the building frames. Empirically based relative gradient limits generally matched the extent and distribution of the damage, while the application of various structural limits did not fully identify local damage distribution but did generally reflect global response. The use of fully free-field data or a failure to include lateral soil displacements both underpredicted building displacements by as much as 50% for low-rise concrete frames without grade beams on sand.     

Performance of Sheet Pile Wall in Peat

To study the performance of sheet pile wall in peat during roadway construction, a long-term instrumentation program was conducted over a period of two years, measuring total lateral earth pressures, sheet pile deflections, soil movements, and water table level variances during construction. The analysis of field data indicated: (1) The earth pressure distribution in peat matched well with the classic Rankine earth pressure; (2) the expected long-term postconstruction sheet pile movement due to the creep behavior of peat was not observed; (3) fully passive earth pressure in peat was mobilized once the maximum measured sheet pile deflection exceeded 0.8% of sheet pile length; and (4) arching effect due to the protruding cross section of sheet pile caused pressure differences of 3–10?kPa between the inside web and outside web of the sheeting. Then, all the construction stages were continuously modeled by finite-element method and the calculated results were compared with the field measurements. The comparisons showed that the calculated results were consistent with the field data and provided reasonable explanations and helpful insights to understand soil–structure interaction mechanism. Finally, some conclusions and suggestions for sheet pile design and construction in peat were reached.     

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.     

Raked Piles—Virtues and Drawbacks

This technical note examines some of the characteristics of behavior of pile groups containing raked piles, via a simplified and hypothetical example. Three cases are examined: (1) a group subjected to vertical and lateral loadings, with no ground movements; (2) a group subjected to vertical and lateral loadings, but with vertical ground movements also acting on the group; and (3) a group subjected to vertical and lateral loadings, but with horizontal ground movements acting on the group. In each case, the effect of pile rake on typical behavioral characteristics (group settlement, lateral deflection and rotation, and pile loads and moments) are examined. It is found that, while the presence of raked piles can provide some advantages when the group is subjected to applied vertical and lateral loadings, especially in relation to a reduction in lateral deflection, some aspects of group behavior may be adversely affected when either vertical or horizontal ground movements act on the group. Thus, caution must be exercised in employing raked piles when such ground movements are expected to occur.     

Three-Dimensional Analysis of Performance of Laterally Loaded Sleeved Piles in Sloping Ground

Development of urban cities in hilly terrain often involves the construction of high-rise buildings supported by large diameter piles on steep cut slopes. Under lateral loads, the piles may induce slope failure, particularly at shallow depths. To minimize the transfer of lateral load from the buildings to the shallow depths of the slope, an annulus of compressible material, referred to as sleeving, is usually constructed between the piles and the adjacent soil. However, the influence of the sleeving on the pile performance in a sloping ground is not fully studied and understood. To investigate the influence, a 3D numerical analysis of sleeved and unsleeved piles on a cut slope is described in this paper. The influences of relative soil stiffness on the response of sleeved piles are also examined. The load transfer from the laterally loaded sleeved pile to the sloping ground is primarily through a shear load transfer mechanism in the vertical plane. Under small lateral loads, the sleeving can lead to a significant reduction in subgrade reaction on the sleeved pile segment and may considerably increase the pile deflection and bending moments. Under large lateral loads, the influence of the sleeving on pile performance appears to diminish because of the widespread plastic zones developed around the pile.     

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.     

Behavior of Slender Piles Subject to Free-Field Lateral Soil Movement

Soil movements associated with slope instability induce shear forces and bending moments in stabilizing piles that vary with the buildup of passive pile resistance. For such free-field lateral soil movements, stress development along the pile element is a function of the relative displacement between the soil and the pile. To investigate the effects of relative soil-pile displacement on pile response, large-scale load tests were performed on relatively slender, drilled, composite pile elements (cementitious grout with centered steel reinforcing bar). The piles were installed through a shear box into stable soil and then loaded by lateral translation of the shear box. The load tests included two pile diameters (nominal 115 and 178?mm) and three cohesive soil types (loess, glacial till, and weathered shale). Instrumentation indicated the relative soil-pile displacements and the pile response to the loads that developed along the piles. Using the experimental results, an analysis approach was evaluated using soil p-y curves derived from laboratory undrained shear strength tests. The test piles and analyses helped characterize behavioral stages of the composite pile elements at loads up to pile section failure and also provided a unique dataset to evaluate the lateral response analysis method for its applicability to slender piles.     

Shaking Table Model Tests on Pile Groups behind Quay Walls Subjected to Lateral Spreading

This paper presents experimental results of 1-g shaking table model tests on a 3×3 pile group behind a sheet-pile quay wall. The main purpose was to understand the mechanisms of liquefaction-induced large ground deformation and the behavior of the pile group subjected to the lateral soil displacement. The sheet-pile quay wall was employed to trigger the liquefaction-induced large deformation in the backfill, and a study was made of the effect of several parameters such as soil density, amplitude and frequency of input motion, pile head fixity, and superstructure on the magnitude of soil lateral displacement and the maximum lateral force of liquefied soil. Furthermore, distribution of the maximum lateral force within the group pile was thoroughly studied. It was found that the force varies depending on the position of individual piles in the group. To evaluate the contribution of each pile in the total lateral force, a new two-dimensional parameter that is called contribution index was introduced and recommended values for each pile were suggested. Finally, it is concluded that displacement and velocity of soil are the most important parameters that affect the distribution of the lateral forces in the group pile, and these two parameters are highly dependent on the configuration of the ground (geometry).