Abstract:The effect of the embedded depth of composite ground on its bearing capacity has not been investigated deeply. The bearing capacity of composite ground is underestimated due to the conservative value of embedded depth considered in engineering practice. A finite difference model is established to study the failure mechanism of granular column under rigid foundation and the ultimate bearing capacity factors of granular column-reinforced composite ground without weight. The results indicate that the potential failure modes of granular column-reinforced composite ground can be categorized into shallow foundation failure, composite failure and block foundation failure, depending on the replacement ratio, length of piles and embedded depth. The depth correction factor increases with the length of columns and decreases with the embedded depth. When the length of column and the embedded depth of foundation keep constant, there is an optimum value of replacement ratio to obtain the maximum value of depth correction factor. According to the calculated results, the effect of embedded depth is underestimated by depth correction factor of 1.0 in the existing code. Based on the Mohr-Coulomb yield criterion, a formula for the equivalent strength method of composite ground is deduced and the bearing capacity factor of the shallow failure mode is evaluated by the limit analysis method, which agrees well with the results of finite difference method.
[1] 郑 刚, 龚晓南, 谢永利, 等. 地基处理技术发展综述[J].土木工程学报, 2012, 45(2): 127-146. (ZHENG Gang, GONG Xiao-nan, XIE Yong-li, et al. State-of-the-art techniques for ground improvement in China[J]. China Civil Engineering Journal, 2012, 45(2): 127-146. (in Chinese)) [2] GB50007—2011 建筑地基基础设计规范[S]. 北京: 中国建筑工业出版社, 2011. (GB50007–2011 Code for design of building foundation[S]. Beijing: China Architecture and Building Press, 2011. (in Chinese)) [3] 张在明. 北京地区高层和大型公用建筑的地基基础问题[J].岩土工程学报, 2005, 27(1): 11-23. (ZHANG Zai-ming. Problems on foundations of high-rise and large-scale public building in Beijing area [J]. Chinese Journal of Geotechnical Engineering, 2005, 27(1): 11-23. (in Chinese)) [4] JGJ79—2012 建筑地基处理技术规范[S]. 北京: 中国建筑工业出版社, 2012. (JGJ79—2012 Technical code for ground treatment of buildings[S]. Beijing: China Architecture and Building Press, 2012. (in Chinese)) [5] GB/T50783—2012 复合地基技术规范[S]. 北京: 中国技术出版社, 2012. (GB/T50783—2012 Technical code for composite foundation[S]. Beijing: China Technology Press, 2012. (in Chinese)) [6] 韩云山, 白晓红, 梁仁旺. 浅论地基承载力的深度修正[J]. 岩土力学, 2003, 24(增刊): 541-544. (HAN Yun-shan, BAI Xiao-hong, LIANG Ren-wang. Analysis of depth modification of subgrade bearing capacity[J]. Rock and Soil Mechanics, 2003, 24(S0): 541-544. (in Chinese)) [7] LADD C C. Stress-strain modulus of clay in undrained shear[J]. Journal of the Soil Mechanics and Foundations Division, ASCE, 1964, 90(5): 103-132. [8] AMBILY A P, SHAILESH R. Behavior of stone columns based on experimental and FEM analysis[J]. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 2007, 133(4): 405-415. [9] 魏 芸, 闫澍旺. 碎石墩复合地基的承载机理及破坏过程[J]. 土木工程学报, 2011, 44(7): 111-118. (WEI Yun, YAN Shu-wang. Bearing mechanism and destruction process of stone column composite foundations[J]. China Civil Engineering Journal, 2011, 44(7): 111-118. (in Chinese)) [10] HUGHES J M O, WITHERS N J. Reinforcing of soft cohesive soils with stone columns[J]. Ground Engineering, 1974, 18(4): 267-290. [11] HUGHES J M O, WITHERS N J, GREENWOOD D A. A field trial of reinforcing effect of stone column in soil[J]. Géotechnique, 1975, 25(1): 31-44. [12] BRAUNS J. Die anfangstraglast von schottersaulen im bindigen untergrund[J]. Die Bautechnik, 1978, 8(2): 263-271. (BRAUNS J. The initial load of graval pile in the clay foundation[J]. The Construction Technology, 1978, 8(2): 263-271. (in Germany)) [13] MADHAV M R, VITKAR P P. Strip footing on weak clay stabilized with a granular trench or pile[J]. Canadian Geotechnical Journal, 1978, 15(4): 605-609. [14] ABOSHI H, ICHIMOTO E, HARADA K, et al. The composer—A method to improve the characteristics of soft clays by inclusion of large diameter sand columns[C]// Proceedings of International Conference on Soil Reinforcement. Paris, ENPC-LCPC 1979: 211-216. [15] BOUASSIDA M, HADHRI T. Extreme load of soils reinforced by columns: the case of an isolated column[J]. Soils and Foundations, 1995, 35(1): 21-35. [16] MUIR WOOD D, HU W, NASH D F T. Group effects in stone column foundations: model tests[J]. Géotechnique, 2000, 50(6): 689-698. [17] BARKSDALE R D, BACHUS R C. Design and construction of stone columns[C]// Rep No FHWA/RD, Federal Highway Administration. Washington D C, 1983. [18] 赵明华, 刘建华, 刘代全, 等. 碎石桩复合地基承载力分析[J]. 公路, 2003, 1(1): 21-24. (ZHAO Ming-hua, LIU Jian-hua, LIU Dai-quan, et al. Analysis of bearing capacity of stone columns compound ground[J]. Highway, 2003, 1(1): 21-24. (in Chinese)) [19] 张爱军, 谢定义. 复合地基三维数值分析[M]. 北京: 中科学出版社, 2004. (ZHANG Ai-jun, XIE Ding-yi. Three- dimensional numerical analysis of composite foundation[M]. Beijing: Science Press, 2004. (in Chinese)) [20] 刘 杰, 张可能. 散体材料桩复合地基极限承载力计算[J]. 岩土力学, 2002, 23(2): 204-207. (LIU Jie, ZHANG Ke-neng. Calculation of ultimate bearing capacity of composite foundation on discrete material pile[J]. Rock and Soil Mechanics, 2002, 23(2): 204-207. (in Chinese)) [21] BOUASSIDA M, BUHAN P DE, DORMIEUX L. Bearing capacity of a foundation resting on a soil reinforced by a group of columns[J]. Géotechnique, 1995, 45(1): 25-34. [22] ABUSHARAR S W, HAN J. Two-dimensional deep-seated slope stability analysis of embankments over stone column-improved soft clay[J]. Engineering Geology, 2011, 120(1): 103-110. [23] BUHAN P D, MANGIAVACCHI P, NOVA R, et al. Yield design of reinforced earth walls by a homogenization method[J]. Géotechnique, 1989, 39(2): 189-201. [24] HASSEN G, BUHAN P D, ABDELKRIM M. Finite element implementation of a homogenized constitutive law for stone column-reinforced foundation soils, with application to the design of structures[J]. Computers and Geotechnics, 2010: 40-49. [25] GREENWOOD D A. Mechanical improvement of soils below ground surface[C]// Proceedings Conference on Ground Engineering. London: Institution of Civil Engineers, 1970: 11-22.