基坑开挖引起土体侧移对桩的影响 毕业论文外文翻译.doc

pile response due to excavation-induced lateral soil movement by h. g. poulos, fellow, asce, and l. t. chen abstract: in this paper, a two-stage analysis involving the finite-element method and the boundary-element method is used to study pile response due to excavation-induced lateral soil movements, with specific attention being focused on braced excavations in clay layers. influences of various parameters on pile response are investigated and design charts for estimating pile bending moments and deflections are presented. these may be used by practicing engineers to assess the behavior of existing piles due to the excavation. the application of the charts is demonstrated via a study of two published historical cases. comparisons are presented between measured pile behavior and that predicted both from the chart solutions and the computer analyses. it is found that the chart solutions may be extended approximately to cover other soil types, but are not applicable to the case of unsupported excavations. introduction there are several examples where pile foundations have been affected or damaged by excavation-induced lateral soil movements, for example, finno et al. (1991), amirsoleymani (1991), and chu (1994). it is thus important for practicing engineers to be able to estimate the construction impact on adjacent piles before and/or during excavation. in principle, a finite-element analysis may be used to make such an estimation and indeed this has been shown to be a powerful method, for example, as demonstrated by finno et al. (1991) and hara et al. (1991). however, in many cases, there is a lack of detailed site or geotechnical information, and a finite-element analysis is neither warranted nor feasible. in such cases, the use of soundly based, but simplified, design charts may be more appropriate, and the development of such charts forms the primary purpose of the present study. although an excavation will cause both vertical and lateral soil movements, the latter component is considered to be more critical for adjacent piles, especially concrete piles, as piles are often not designed to sustain significant lateral loadings. therefore, in the present study piles are considered to be affected only by excavation-induced lateral soil movements, and their response is analyzed by the combination of a finite-element method and a boundary-element method. the finite-element method is used first to simulate the excavation procedure and to generate free-field soil movements, that is, the soil movements that would occur without the presence of the pile. these generated lateral soil movements are then used as input into a boundary-element program to analyze pile response. solutions for pile response (bending moment and deflection) are presented in chart form that may readily be used in practice. two published case histories are then analyzed to demonstrate the applicability of the present method. problem analyzed the problem analyzed is shown in fig. 1, where an existing single pile is situated near an excavation. as excavation proceeds, the surrounding soil sr.prin., coffey partners int. pty. ltd., 12 waterloo rd., north ryde, australia 2113; and prof. of civ. engrg., univ. of sydney, sydney, australia 2006. 2geotech. engr., coffey partners int. pty.ltd., 12 waterloo rd., north ryde, australia; and sr. res. assoc., school of civ. and min. engrg., univ. of sydney, sydney, australia. note. discussion open until july i, 1997. 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 may 22, 1995. this paper is part of the journal of geotechnical and geo environmental engineering, vol. 123, no.2, february, 1997. asce, issn 1090-0241/97/0002-0094-0099/$4.00 + $.50 per page. paper no. 10772. s will move toward the excavation and their movement will induce bending moments and deflections in the pile. key factors influencing the response of a single pile may include excavation dimensions, excavation support conditions, construction procedures, soil properties, and pile properties. to avoid undue complication, the soil is assumed to be a uniform clay layer and to be under undrained conditions during excavation. the parameters selected are shown in fig. 1 and are considered to be typical of a clay soil. the excavation is assumed to be sufficiently long that a two dimensional plane strain analysis is applicable. the basic problem analyzed and the parameters selected are shown in fig. 1, where b = half width of excavation; h = total thickness of soil layer; x = distance from excavation face; c; = undrained shear strength of soil; e, = soil youngs modulus; y = unit weight of soil and wall; lp = pile length; d = pile diameter; ep = pile youngs modulus; e1w = stiffness of wall; s = strut stiffness; l = length of wall; and hmax = maximum depth of excavation. the excavation was carried out from top to bottom in 10 steps, with each step involving removal of a 1-m-thick layer. four levels of struts were simulated, the first being placed after the first excavation step and the remaining three at steps 4, 7, and 10. the struts were not preloaded. for convenience, the depth of excavation may be expressed by the well-known stability factor nc; of the following form: finite-element and boundary-element analyses a two-dimensional finite-element program was used to simulate the plane-strain excavation without the presence of the pile. the finite-element program used is named avpull (for analysis of vertical piles under lateral loading) and has been described elsewhere (see chen and poulos 1993; chen and poulos 1994; chen 1994). the program was originally developed to analyze pile groups subjected to either lateral loadings or lateral soil movements, and was modified to also accommodate excavation analyses. in the program, eight-node isoparametric elements are used to model the soils and the supporting wall, while goodman-type interface elements are used to model the interaction between the soil and the wall. in the present study, the interface between the soil and the wall was assumed to be rough, i.e. no slip occurred. struts were modeled as springs whose stiffness was assigned to an element node corresponding to the strut position. the soil and the interface elements were modeled as elasto-plastic materials, obeying the tresca failure criterion and a nonassociated flow rule. the wall was modeled as a linear elastic material. it should be noted that, as analyzed by hashash and whittle (1992), the computed soil deformations by the finite-element method may be dependent on constitutive soil models. the relatively simple elasto-plastic soil model adopted in the present study may not be able to capture localized strains associated with failure within the soil mass, especially for cases with high stability numbers (nc approaching 6). the finite-element mesh used for analyzing the basic problem is shown in fig. 2. only half of the excavation was simulated because of symmetry. in the finite-element simulation, the wall was assumed to be installed prior to excavation and to have no effect on surrounding soils. the computed lateral soil movements from the finite element analysis were then used as input into a boundary element program for pile response analyses. the boundary element program used is named pallas (for piles and lateral loading analysis) and has been described elsewhere (poulos et al. 1995). pallas uses a simplified form of boundary-element analysis in which the pile is idealized as an elastic beam and the soil as an elastic continuum, but with limiting pressures at the pile-soil interface to allow considerations of local failure of the soil adjacent to the pile. the program can consider both a single pile and a group of nonidentical free-head piles, but cannot handle capped pile groups. the input parameters for the piles consist of the bending stiffness, and the diameter and length of each individual pile within the group. the soil model requires specification of the poissons ratio (although this generally has little influence) and the distributions with depth of youngs modulus. although in principle the ultimate lateral pile-soil contact pressure, pu, could have been obtained by performing finiteelement analyses as described by chen and poulos (1994), in the present study p was assumed to be 9cu (c, is undrained shear strength of soil) for simplicity. this assumption may be conservative for the case of x less than about four pile diameters, especially when excavation support conditions are very flexible, because an excavation or cut has been found to tend to reduce p., as shown by poulos (1976) and chen and poulos (1994). the effect of pu on pile response has been examined by chen (1994). the substructuring approach adopted here has been successfully used for many problems involving piles subjected to externally imposed ground movements (e.g. poulos and davis 1980; poulos 1989; poulos et al. 1995). compared to single analysis method that incorporate the soil, the excavation support and the piles the substructuring approach has a major advantage in that the response of a wide variety of piles to excavation may be analyzed readily, using the same computed soil movements from an excavated analysis. such an ability facilitates greatly the development of parametric studies and design charts. results for basic problem fig. 3 shows the computed wall movement profiles and soil movement profiles at different distances from the excavation face and for four stages of excavation. it can be seen that both the wall and soil movements increase with increasing stability number, nc, and that the rate of increase is much more rapid as the soil approaches failure. the soil movement profiles are seen to be smoother away from the excavation face than at or near the wall. it is worth noting that the wall movement profiles shown in fig. 3(a) are similar in nature to those predicted by other researchers, e.g. hashash and whittle (1992). the maximum lateral soil movement, ymax, corresponding to different stability numbers is plotted against distance, x, in fig. 4. as would be expected, ymax decreases with increasing x, especially for larger stability numbers. solutions for pile response due to the free-field soil movements at x = 1 m as shown in fig. 3(b) are presented in fig. 5 for the pile shown in fig. 1. the pile deflections are very close to the free-field soil movements, reflecting the fact that the pile is relatively flexible. pile bending moment profiles are shown in fig. 5(b) and are found to exhibit a double curvature, with the maximum values increasing with increasing stability number. the rate of increase of bending moment with stability number increases rapidly at larger stability numbers when the soils approach failure. it was also found that pile deflections follow soil movements closely at all distances from the excavation face and therefore the maximum pile deflections can be taken conservatively to be equal to the soil movements shown in fig. 4. the pile bending moment profiles at various distances, x, are quite similar in shape, but the maximum value decreases with increasing distance x, as shown in fig. 6. parametric studies and design charts to investigate the influence of key parameters on pile response, a number of different cases were studied in which the following parameters were varied: undrained shear strength c.; wall stiffness elw, strut stiffness k, strut spacing s and pile diameter d. the soil youngs modulus was assumed to be 400cu the parameter studies revealed that 1. pile response (bending moment and deflection) increases with increasing c; (and e,) due to an increased ultimate lateral soil pressure. 2. pile response increases with increasing stability number due to larger lateral soil movements. 3. pile response decreases with stiffer excavation support conditions (i.e. larger wall stiffness e1w and strut stiffness k, and smaller strut spacing s) because such support conditions result in smaller soil movements. 4. pile bending moment increases with increasing pile diameter, due to its larger stiffness (for a solid pile); pile deflection tends to decrease slightly with pile diameter but generally follows the soil movement unless the pile is very stiff (e.g., d 1.0 m). based on the previous parametric studies, it has been found that the maximum pile bending moment and deflection may be approximated as follows: where mm = maximum bending moment, len m; pm = maximum deflection, mm; mh, ph = basic bending moment and deflection, respectively; kcu, k;u = correction factors for undrained shear strength; kd k = correction factors for pile diameter; knc kjvc = correction factors for depth of excavation; ke/ ,kk = correction factors for wall stiffness; kb k = correctioit fators for strut stiffness; and k k; = correction factors for strut spacing. both m, and p, values and all the correction factors are shown in figs. 7 -10. the variations of mb and pb with x, as shown in figs. 7 and 9, respectively, are extracted from the results presented in figs. 6 and 4, respectively, corresponding to n, = 3, and all the correction factors are based on the basic problem and also correspond to n; = 3. to use (2) and minimize the number of charts, there is inevitably some discrepancy between values calculated from (2) and those computed directly from the boundary element program. however, the discrepancy is within 15%, and in most cases it is below 10% unless the clay is very soft (e.g. c. is less than about 20 kpa). it should be noted that (2) computes only the additional response of the pile, assuming that the pile has zero initial bending moment and deflection prior to the excavation. if this is not the case, then the existing bending moment and deflection may be increased or decreased due to excavation construction, depending on the bending moment and deflection in the pile under the working load. it should also be pointed out that the lateral behavior of a pile will generally be affected by the presence of closely spaced adjacent piles, and the extent of the group effect depends on factors such as number of piles, space between piles,pile head condition, soil properties, and so forth. eq. (2) can also be used to assess approximately a pile group. to avoid undue complication, (2) does not cover the case of unsupported excavations, although such a case may be analyzed in a similar way. in fact, a limited study has shown that the pile response is quite different for the case of unsupported excavations, and therefore it is considered to be more appropriate to present separate chart solutions for this case, due to space limitations, such chart solutions are not presented in the present paper. application to case studies to examine the ability of the present method to predict pile response, two published cases were studied and are described briefly as follows. in each case, site-specific analyses were carried out using the finite-element program avpull to estimate soil movements and the program pallas to compute pile response, and (2) was also used to compute the maximum pile bending moment and deflection for the first case. case 1 finno et al. (1991) have described a case where a large excavation was carried out inside a framed structure. the structure was supported by groups of capped step-tapered piles that were very close to the excavation face. although the excavation was provided with temporary tie-back supports during the excavation, the piles had moved significantly towards the excavation by the time the extraction of the sheet-pile wall was about to begin. the excavation procedure was divided into a number of stages and deflections of the sheet-pile walls and piles were measured for all stages except the first one. fig. 11 shows the finite-element mesh used for the present study to generate free-field soil movements, in the present simulation of the excavation, the tie-backs were simulated with four levels of supports whose stiffnesses were chosen as to match the computed wall movements with those measured. the excavation was simulated by avpull in four steps (without stimulating the extraction of the sheet pile wall), each level of struts being placed immediately after the excavation step. the generated lateral soil movement was input into pallas to analyze the response of a single pile representative of those closest to the excavation face, the pile being 1.5 m from the excavation face and the head being restrained from rotation. the step-tapered pile was simulated as a uniform pile, having an average diameter of 327 mm and a length of 25 m. soil parameters were chosen on the basis of the available information and are shown in fig. 11. because the program pallas could not analyze a capped pile group, the analysis was carried out for an isolated pile, ignoring the effects of pile-soil-pile interaction, this interaction can have two effects: (1) it usually reduces the induced shears and bending moments in the piles, as compared with a single pile, when conditions are essentially elastic (i.e. for small soil movements); and (2) it may modify the ultimate lateral pile-soil pressure pu for a number of practical pile con-figurations, the effect of grouping on p is not large, and the considerations of piles within a group as isolated piles may in fact be conservative (chen 1994). fig. 12 shows the predicted pile deflection profiles for two excavation steps, together with those measured. it can be seen that the predicted and the measured profiles agree quite well. the predicted bending moment profiles from the pres