摘要
随着我国建设规模的扩大,桩基础领域发展迅速,长桩基础被越来越多的工程所采用。工程实测表明,桩侧土模量较高的非软土地区的长桩静力试桩的荷载传递和桩身压缩性状与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。目前的桩身压缩测试结果大都基于工程试桩所得,为了能够探讨研究考虑桩身压缩的群桩基础沉降性状,通过现场大型单、群桩模型试验,深入研究了群桩基础承载力、桩身压缩和土体压缩的沉降性状,本文主要完成了以下工作：
1.通过系统的模型试验研究了单、群桩基础的承载力性状。群桩中基桩桩身轴力、侧阻力的分布形式因位置的不同而不同。群桩端阻表现出明显的沉降硬化效应,即端阻力随着群桩基础沉降的增大端阻力都会得到不同程度的提高,端阻力在较小桩距条件下提高程度较大,在较大桩距条件下提高程度较小。
2.工程实测表明,桩身压缩产生的沉降始于加载初期,并伴随于加载全过程。桩侧土模量较高的非软土地区的长桩静力试桩的荷载传递和桩身压缩性状与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。
3.群桩试验表明,桩间土的竖向变形在桩顶处为最大,随着深度的增加桩间土的竖向变形而逐渐变小。桩间土除了产生剪切变形(桩、土相对位移)外,还出现压缩变形。
4.在同一荷载水平下,桩端以下土层的沉降值随深度的增加而减小,整体压缩主要产生在距桩端下一定厚度的土层范围内,即土层距桩端越近,单位厚度土层的整体压缩量越大；同一深度处,土体整体压缩沉降值随荷载的增大而增大。
5.当复合基桩分担荷载值一定时,较大桩距群桩桩端整体压缩沉降值较小桩距群桩为小；多桩数群桩桩端整体压缩沉降值较少桩数群桩为大,这是由于群桩效应增强所致,即桩距一定时,随着桩数的增多,桩与桩之间的增沉效应增强。
6.群桩桩端平面以下地基土整体压缩变形及压缩层深度因桩距大小差异很大,即在P=Pu/2荷载条件下,大桩距群桩基础地基土整体压缩变形及压缩层深度较小桩距为小。本次结论与粉土、软土中群桩试验的结果一致。
7.探讨研究了桩身压缩沉降与桩侧阻力分布模式相关的机理,给出了考虑不同侧阻分布模式确定桩身压缩系数的三个公式：ξe正三角=0.33α+0.67ξe矩形=0.50α+0.50ξe倒三角=0.67α+0.33
8.给出了小桩距群桩基础的沉降计算公式,该公式可考虑桩身的压缩、桩数、群桩的几何特征、侧阻力分布模式、端阻比例等因素对桩基沉降的影响：
9.基于目前规范的压缩层厚度确定方法,探讨研究了群桩基础的压缩层厚度确定方案,最终确定了压缩层厚度的计算公式。
10.在已有的考虑桩径影响Mindlin解竖向应力系数的研究成果上,给出了小桩距群桩基础的平均竖向应力系数的数值解,制作了相应的数据表格,该表格可供相关的工程设计人员手算沉降量时使用。
In recent years, the pile foundation field has a rapid development due to the scale enlargement of engineering construction in China. More and more types of long pile foundations, meanwhile, are adopted in many construction projects. Static testing results of long piles in different strata figure out the essential behaviors of long piles under vertical loading, e.g. load transfer mechanism and compression behavior. Generally, the loading behaviors of long piles in hard soil ground are similar to super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice. At the present stage, in-situ static testing is still one popular and authentic method to acquire compression behaviors of long piles. Consequently, in order to investigate the elastic compression behaviors of pile group foundation, the field tests are designed and carried out. Based on a series of large scale model tests of single pile and pile group, the settlement behaviors of pile group foundation are observed and the load transfer mechanisms are analyzed by the corresponding test data. Furthermore, the pile group effects of shaft resistance, point resistance, soil resistance beneath cap, and load-bearing capacity under vertical load are revealed. The principal conclusions may be summarized as follows.
1. The load transfer mechanism of single pile and group piles under vertical loads are analyzed by large scale model tests. The distribution modes of axial force and lateral resistance along pile body differ with the location within the pile group. Analysis results show that the point resistance of pile group has an obvious hardening effect of settlement, which means that the point resistance increases distinctly with pile group settlement. On the other hand, the increase magnitudes of point resistance are different in the cases of small and large intervals between two adjacent piles. Popularly, the point resistance has a greater magnitude in a smaller interval condition than that one in a larger condition.
2. The engineering static load testing results show that pile elastic compression settlement appear at the initial stage of loading process and then keep existing and developing in the whole process. Generally, the settlement behaviors of long piles in hard soil ground are similar to the super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice.
3. By the analysis of soil settlement behavior within group piles, the largest settlement appears at the top position of pile closed to the cap, and the settlement decreases with the burial depth. Meanwhile, the settlement covers two kinds of deformations, i.e. shear and compression.
4. Under the same loading conditions, it is disclosed that the soil settlement below pile tip decreases with the burial depth by the analysis results of soil settlement at different depths within pile groups. Furthermore, one conclusion may be obtained that the compression settlement of soil mass mostly occurs in a certain range of soil closed to the pile tip. Actually, the compression settlement of per unit thickness of soil mass closed to the pile tip is greater than that one with a larger distance away the tip; the settlement value of soil mass increases with the load magnitude at the same burial depth.
5. For a certain average loading magnitude on each pile within pile groups, the total compression settlement at the tip is greater in a smaller interval condition between two adjacent piles than that one with a larger interval. On the other hand, the settlement is smaller in the case of less numbers of piles than the case of more piles. This observation result may be connected with the pile group effect, which means that at a certain interval between two adjacent piles within pile groups, the settlement-increase effect of pile group becomes sharper with the increase of pile number.
6. The thickness of compressed layer and total settlement behavior of soil ground below the tip of pile group both differ with the intervals between two adjacent piles within pile group. The field test results show that at the loading condition of P=Pu/2, the total settlement and thickness of compression soil layer both are smaller in a case of lager intervals between two adjacent piles within pile group than that one with an opposite case. These results agree with the previous field tests in silt and soft soil.
7. Based on the analysis results of the pile elastic compression settlement and the lateral resistance distribution pattern, three linear equation of compression coefficient, which considers the difference of lateral resistance distribution pattern, are proposed as follows.ξe正三角=0.33α+0.67ξe矩形=0.50α+0.50ξe倒三角=0.67α+0.33
8. By the analysis of settlement mechanism of pile group foundation, one equation to calculate the foundation settlement of pile groups in the less interval cases is obtained. In the equation, the critical factors influencing pile foundation settlement, such as compression behavior of pile body, pile number, geometrical characteristics of pile group, lateral resistance distribution pattern, and resistance ratio of tip to lateral, etc., are considered completely and the detailed format, which is more reasonable than current methods, is showed as the follow.
9. Based on the assessment method of compression soil thickness in current code and the observation and analysis results in this dissertation, the author take more efforts to optimize assessment scheme for compression soil thickness in the cases of pile group and one more reasonable calculation formula is acquired finally.
10. According to the research achievement of vertical stress coefficient solved by Mindlin method considering the influence of pile diameter, numerical solutions for the mean vertical stress coefficient of pile group with small pile interval are proposed and listed in one table. These results in the table are convenient for engineer to calculate the compression settlement of pile group foundation by hand.
引文
[1]刘金砺.桩基础设计与计算[M].北京：中国建筑工业出版社,1990.
[2]刘金砺,高文生,邱明兵.建筑桩基技术规范应用手册[M].北京：中国建筑工业出版社,2010.
[3]中国建筑科学研究院.建筑桩基技术规范(JGJ 94-94)[S].北京：中国建筑工业出版社,1994.
[4]中国建筑科学研究院.建筑桩基技术规范(JGJ 94-2008)[S].北京：中国建筑工业出版社,2008.
[5]刘金砺,迟铃泉.桩土变形计算模型和变刚度调平设计[J].岩土工程学报,2000,22(2)：151-157.
[6]赵锡宏等.上海高层建筑桩筏与桩箱基础设计理论[M].上海：同济大学出版社,1989.
[7]朱向荣,方鹏飞,黄洪勉.深厚软基超长桩工程性状试验研究[J].岩土工程学报,2003,25(1)：76-79.
[8]阳吉宝,钟正雄.超长桩的荷载传递机理[J].岩土工程学报,1998,20(6)：108-112.
[9]阳吉宝.超长桩荷载传递机理与桩箱(筏)基础优化设计研[D].上海：同济大学,1996.
[10]池跃君.大直径超长灌注桩承载性状的试验研究[J].工业建筑,2000,30(8)：26-29.
[11]费鸿庆,王燕.黄土地基中超长钻孔灌注桩工程性状研究[J].岩土工程学报,2000,22(5)：576-580.
[12]刘金砺.桩和桩基础若干机理与理论问题[C]//基工程技术进展.北京：中国建筑工业出版社,2009：104-115.
[13]刘金砺.桩基研究与应用若干进展浅析[J].岩士工程界,2000(2)：10-14.
[14]刘金砺.我国建筑基础工程技术的现状和发展述评[J].建筑技术,1997(7)：466-468.
[15]刘金砺.建设行业岩土工程五十年[J].建筑科学,2000(2)：1-4.
[16]刘金砺.竖向荷载下的群桩效应和群桩基础概念设计若干问题[J].土木工程学报,2004,37(1)：78-83.
[17]张雁,李华.软土地基超长灌注桩承载力试验研究[A].第七届全国土力学及基础工程学术会议论文集[C].北京：中国建筑工业出版社,1994.
[18]吴鹏.超大群桩基础竖向承载性能及设计理论研究[D].南京：东南大学,2006.
[19]马晔.超长钻孔灌注桩桩基承载性能研究[D].武汉：武汉理工大学,2008.
[20]方鹏飞.超长桩承载性状研究[D].杭州：浙江大学,2003.
[21]辛公锋.大直径超长桩侧阻软化试验与理论研究[D].杭州：浙江大学,2006.
[22]张忠苗.基于桩顶与桩端沉降的钻孔的钻孔桩受力性状研究[J].岩土工程学报,1997,19(4)：88-93.
[23]张忠苗.软土地基超长嵌岩桩的受力性状[J].岩士工程学报,2001,23(5)：552-556.
[24张忠苗,辛公锋,夏唐代.深厚软土非嵌岩超长桩的受力性状试验研究[J].土木工程学报,2004,37(4)：64-69.
[25]郑刚,顾晓鲁.软土地区超长灌注桩承载性状的讨论[A].高层建筑桩基工程技术[C].北京：中国建筑工业出版社,1998.
[26]中国建筑科学研究院.建筑地基基础设计规范(GB50007-2002)[S].北京：中国建筑工业出版社,2002.
[27]桩基工程手册编写委员会.桩基工程手册[M].北京：中国建筑工业出版社,1995.
[28]史佩栋主编.桩基工程手册[M].北京：人民交通出版社,2008.
[29]邹东峰,钟尔波,徐寒CCTV新址主楼Φ1200mm钻孔灌注桩承载特性研究[C]//桩基工程技术进展.北京：知识产权出版社,2005：90-96.
[30]秋仁东,孙轶斌,石玉成.不同场地条件下长桩的承载性状研究[J].岩土工程技术,2010,24(3)：114-118.
[31]Terzaghi,K. Theoretical Soil Mechanics[M].New York:Wiley,1943.
[32]Meyerhof,G.G. Compaction of Sands and Bearing Capacity of Piles[J]. J.S.M.F.D.,ASCE,1959,82:1-19.
[33]Focht, J. A. Discussion to Paper by Coyle and Reese[J]. J.S.M. F.D.,ASCE,1967,93:133-1138.
[34]Kezdi,A. The Bearing Capacity of Piles and Pile Groups.Proc.4th Int.Conf.S.M & F.E.,1957,2:46-51.
[35]佐藤·悟.基桩承载力机理[J].土木技术,1965,20：1-5.
[36]Seed,H,B.&Reese,L.C.The Action of Soft Clay Along Friction Piles[J]. Trans.ASCE,1957,122:731-754.
[37]Coyle,H.M.& Reese.L.C. Load Transfer for Axially Loaded Piles in Clay[J]. J.S.M. F.D.,ASCE,1966,92: 1-26.
[38]D'Appolonia & Romualdi. Load Transfer in End-Bearing Steel H-Piles[J]. J.S.M. F.D.,ASCE,1963,89: 1-25.
[39]Thurman & D'Appolonia. Computed Movement of Friction and End-Bearing Piles Embedded in Uniform and Stratified Soils[C]. Proc.6th Int.Conf. S.M & F.E.1965,2:323-327.
[40]Salas & Belzunce.Discussion on Div.4[C]. Proc.6th Int.Conf. S.M & F.E.1965,2:309-313.
[41]Nair, K. Load-Settlement and Load Transfer Characteristics of a Friction Pile Subject to a Vertical Load[C]. Pro.3rd Pan-Amer. Conf. S.M & F.E.1967,1:565-590.
[43]Poulos, H. G.& Davis,E.H. The Settlement Behavious of Single Axially-Loaded Incomprssible Piles and Piers[J]. Geot.1968,18:351-371.
[43]Mattes, N. S.& Poulos, H. G. Settlement of Single Compressible Pile[J]. J.S.M. F.D.,ASCE,1969, 95:189-207.
[44]Poulos, H. G & Mattes N. S.The Behaviour of Axially-Loaded End-Bearing Piles[J].Geot.,1969,19: 285-300.
[45]Butterfield,R.& Banerjee,P.K.The Elastic Analysis of Compressible Piles and Pile Groups[J]. Geot.,1971,21(1):43-60.
[46]Banerjee,P.K.& Davies,T. G. Analysis of Pile Groups Embedded in Gibson Soil[C]. Proc.9th Int.Conf. S.M & F.E., Tokyo,1977,1:381-386.
[47]Randolph,M.F.& Wroth,C.P. Analysis of Deformation of Vertically Loaded Piles[J]. ASCE,1978,104(12):1465-1488.
[48]Poulos, H. G.& Davis,E.H. Pile Foundation Analysis and Design[M]. New York:1980,Wiley.
[49]Poulos, H. G & Mattes N. S. Displacements in a Soil Mass Due to Pile Groups. Aust. Geomechs,1971a, G1(1):29-35.
[50]Poulos, H. G & Mattes N. S. Settlement and Load Distribution Analysis of Pile Groups. Aust. Geomechs, 1971b, G1(1):18-28.
[51]Aschenbrener,T.B.& Olson,R.E. Prediction of Settlement of Single Piles in Clay[C]. Geotechnical and Geoenvironmental Engineering Conf.California,1984,41-58.
[52]Cook, R.W. The Settlement of Friction Pile Foundation[C]. Proc. Conf. Tall Buildings, Kuala Lumper, 1974.
[53]Cook, R.W. Jacked Piles in London Clay:A Study of Load Transfer and Settlement under Working Condition[J]. Geotechnique,1979,39 (2).
[54]宰金珉,杨荣吕.桩周土非线性变形分析的广义剪切位移法[J].南京建筑工程学院学报,1993(1)：1-16.
[55]《桩基工程手册》编写委员会.桩基工程手册[M].北京：中国建筑工业出版社,1995.
[56]中国建筑科学研究院.建筑地基基础设计规范GB 50007-2002[S].北京：中国建筑工业出版社,2002.
[57]Mindlin R. Force at a point in the interior of a semi-infinite soil. Physics,1936(5).
[58]Geddes J D. Stress in Foundation Soil due to Vertical Subsurface Load[J]. Geotechnique.1966,16(3), 231-255.
[59]上海现代建筑设计有限公司.地基基础设计规范[S].上海：上海市工程建设标准化办公室,1999.
[60]Vesic, A.S. Disscussion. Proc.7th Int. Conf. S.M.& F.E.1969,3:242-244.
[61]Ellison, R.D., D'Appolonia, E.,& Thiers, G. R. Load-Deformation Mechanism for Bored Piles[J]. J.S.S.F.D.,ASCE,1971,97:661-678.
[62]Desai, C.S. Numerical Design-Analysis of Piles in Sands[J]. ASCE,1974,100:661-678.
[63]陈雨孙,周红.纯摩擦桩荷载-沉降曲线拟合方法及其工作机理[J].岩土工程学报,1987,19(2).
[64]倪新华.无限元在桩土共同作用分析中的应用.[C]//首届全国岩士工程博士学术讨论会[C].上海：同济大学出版社,1990.
[65]Trochanis, A. M., Bielak.J.& Christiano, P.. Simplified model for analysis of one or two pile[J]. ASCE, 1991,117(3):448-465.
[66]马晔,张学峰,张小江.超长钻孔灌注桩承载性能研究与试验[M].北京：人民交通出版社,2009.
[67]池跃君,敖立新,顾晓鲁.超长钻孔灌注桩荷载传递机理的计算分析[J].特种结构,2000(4)：12-15.
[68]Butterfield,R.& Banerjee,P.K.The Problem of Pile Groups and Pile Cap Interaction[J]. Geot.,1971,21(2):135-142.
[69]张问清,赵锡宏,宰金珉.任意力系作用下的层状弹性半空间无限体的有限层法[J].岩土工程学报,1982(3)：27-42.
[70]张崇文.有限层有限元混合法研究桩土相互作用[J].天津大学学报,1995(6)：165-772.
[71]曹志远.解析与数值结合法及其应用[J].计算结构力学及其应用,1992(2)：16-21.
[72]王玉杰,严宗达,刘作春等.有限棱柱法在群桩与土相互作用分析中的应用[J].天津大学学报,1997,30(2)：170-174.
[73]O'Neill,M.W.,Ghazzaly,O.I.& Ha, H. B. Analysis of Threedimensional Pile Groups with Nonlinear Soil Response and Pile-Soil-Pile Iteraction, Proc.9th Annual Offshore Technology Conference, Houston, 1977.
[74]Chow Y. K. Axial and Lateral Response of Pile Groups Embedded in Nonhomogeneous Soils[J]. International Journal for Numerical and Analytical Methods in Geomechanics,1986,11(6):621-638.
[75]Lee, C.Y.& Small, J.C. Finite Layer Analysis of Laterally Loaded Piles in Cross-Anisotropic Soils[J]. Int. J. for Numerical and Analytical Methods in Geomechanics,1991,15:785-808.
[76]潘时声.桩基础分层位移迭代法计算理论及其应用[D].上海：同济大学,1993.
[77]邓文龙,赵锡宏.横观各向同性土介质与结构物共同作用的有限元与边界元耦合分析[J].工程力学,1996,13(4)：74-81.
[78]阳吉宝.超长桩荷载传递机理与桩箱(筏)基础优化设计研究[D].上海：同济大学,1996.
[79]Hain, S. J. and I. K. Lee. The Analysis of Flexible Raft Pile System. Geotechnique,1978,28(1):65-83.
[80]Franke, E., Lutz, B.& El-Mossallamy, Y. Measurements and Numerical Modelling of High rise Building Foundations on Frankfurt Clay[C]. Proceedings of a Conference on Vertical and Horizontal Deformations of Foundations and Embankments. ASCE Geotechnical Special Publication,1994,40(2): 1325-1336.
[81]Sinha, J. Piled Raft Foundations Subjected to Swelling and Shrinking Soils[D]. Australia:University of Sydney,1996.
[82]袁建新,钟晓维.桩荷载与变位的数值模拟分析[J].岩土力学,1991,12(1)：1-8.
[83]周国林.单桩极限承载力的灰色预测[J].岩土力学,1991,12(1)：37-43.
[84]阳吉宝.超长桩荷载-沉降曲线的拟合方法[J].工业建筑,1995,25(8)：29-32.
[85]蒋镇华.有限里兹单元法及其在桩基和复合地基中的应用[D].杭州：浙江大学,1996.
[86]王建华.神经网络法预估水泥搅拌单桩沉降[J].土木工程学报,1996,29(1)：55-61.
[87]金波,李志飙.层状地基中的单桩沉降分析[J].岩土工程学报,1997,19(5)：35-42.
[88]刘金砺,邱明兵.单桩、单排桩、疏桩基础及其复合桩基的沉降计算[J].土木工程学报,2007, 40(s1)：146-151.
[89]刘金砺,邱明兵.软士中群桩承载变形特性与减沉复合疏桩基础设计计算[J].岩土工程学报,2008,30(1),51-55.
[90]刘金砺,袁振隆,张国平.钻孔群桩工作机理与承载能力的研究[R].中国建筑科学研究院建筑科学研究报告,No.2-1,1984.
[91]刘金砺,黄强,李华,胡文龙等.软弱地基高重建筑群桩基础工作性状与变形的研究[R].1989(内部资料).
[92]刘金砺,黄强,李华,高文生.竖向荷载在群桩变形性状及沉降计算[J].岩士工程学报,1995,17(6)：1-12.