【作者】 肖成安；

【导师】 韩小雷；

【作者基本信息】 华南理工大学 ， 结构工程， 2013， 博士

【摘要】 广东粤北地区的韶关、清远、肇庆等地，作为岩溶相当发育的地区，存在较多的工程隐患，因此，迫切要求找到合适的基础形式和地基处理方式。在华南理工大学亚热带建筑科学国家重点实验室重点项目的资助下，就这一问题，华南理工大学土木与交通学院对岩溶地区地基处理关键技术进行了深入的研究，并在粤北地区进行了一系列的试验、测试和观测。采用单桩压板试验确定复合地基承载力，忽视了群桩对复合地基受力性能的影响，无法合理确定桩土承载能力的发挥情况。在进行考虑上下部相互作用的结构分析时，由于计算机的限制往往难以进行，提出符合工程实际的、便于计算分析的方法显得尤为重要。而且，在对筏基的实测中发现，筏板钢筋的应力测量值明显小于计算值，说明原有计算模型存在不足且偏于保守，所以要求设计方法应能更真实地反映筏板中钢筋的实际受力情况。同时，随着复合地基在抗震地区的广泛应用，对结构与复合地基之间动力相互作用的研究也十分迫切。本文采用理论分析与试验研究相结合的方法，以实际工程为背景，围绕刚性桩复合地基-筏基-上部结构体系进行了深入的研究，主要研究内容及创新性工作有以下几个方面：1）进行了24个复杂岩溶地质条件下的单桩复合地基载荷试验。结果表明，桩间土的承载能力发挥较早，在特征值荷载时桩土很难达到各自的设计值；无垫层，当土层承载力较高时，只要基础与垫层接触紧密，土体仍可发挥一定的承载力；砂石垫层厚100mm，特征值荷载下的桩间土平均应力为0.31MPa，桩-土应力比约19，土承担荷载比为46.5%，桩、土承载力发挥系数分别约为0.72、1.78。砂石垫层厚300mm，特征值荷载下桩间土平均应力约为0.38Mpa，桩-土应力比14.6，土承担荷载比为61%。分析表明该种处理方案可行。2）通过与单桩复合地基压板试验对比，验证了所采用的土体弹塑性模型、接触关系和所选参数的正确性，可以较为准确地分析刚性桩复合地基的受力、变形性能。在此基础上，研究了群桩复合地基载荷板的尺寸效应，得到了大尺寸压板的最终沉降估算公式，垫层厚度为100mm，基础边长为30m、40m、50m的最终沉降为16.7mm、17.5mm、18.1mm，垫层厚度为300mm，基础边长为30m、40m、50m的最终沉降为32.4mm、34.5mm、36.1mm，与实测值吻合。分析了群桩复合地基的宏观本构关系，结果表明，随着桩数的增多压板沉降增大，土体承载能力发挥度减小。垫层厚为100mm、200mm、300mm，特征值荷载作用下，49桩复合地基的平均沉降约是单桩复合地基平均沉降的1.6、2.18、2.2倍，此时，土应力分别为单桩复合地基土应力的46%、65%、78%；加载初期，单、多桩的刺入相差不大，随荷载增加群桩的刺入量比单桩大，垫层厚度为100mm、200mm、300mm，特征值荷载下单桩的刺入量分别为3.9mm、6.8mm、8.4mm，49桩的刺入量分别为6.8mm、18.7mm、19.2mm。群桩在达到49个，以及垫层达到300mm后，桩土应力、基础沉降、刚性桩的刺入等变化很少，褥垫层的调节作用趋于稳定。3）根据试验和群桩有限元分析，结合工程经验，提出了针对该类复合地基的二折线弹簧宏观本构模型，特征值荷载对应的沉降为12mm，极限荷载对应的沉降为40mm。借助SAP2000可以利用该模型进行考虑上部结构与复合地基相互作用的基础受力分析，特征值荷载对应的基础沉降从4mm到20mm进行变化，除个别地方的筏板弯矩变化幅值接近9%，其它地方变化都很小。结果表明，该方法既可以考虑上部结构刚度对基础的影响，又可以反应土体一定程度的非线性特性和基础的整体沉降。4）建立地基的实体单元模型，考虑筏板与垫层之间的摩擦，借助ABAQUS进行考虑上部结构与复合地基相互作用的弹塑性分析。结果表明，在弹塑性计算中荷载设计值下，截面应力分布较均匀，截面最大拉应力约为1.63Mpa，钢筋最大应力约80Mpa；荷载标准值下钢筋最大应力为48Mpa，计算结果与实测值吻合较好，验证了本文分析方法的正确性。通过计算分析，与常用方法的计算结果比较表明，地基反力模式、筏板与垫层之间的摩擦是影响筏板内力计算结果的主要因素。地基反力在筏板边缘剧增，最大值达到平均值的3.5倍，极易造成地基局部区域的提前破坏。最后，分析了地基土变形模量、褥垫层厚度、褥垫层模量、筏板厚度等参数对筏板内力及变形的影响。结果表明，地基反力和筏板应力与筏板同地基相对刚度有关。5）采用有限元与无限元耦合的方法，建立动力相互作用三维整体模型，进行了动力弹塑性时程分析。通过与普通桩筏基础的时程分析结果对比，研究了刚性桩复合地基-上部结构体系的抗震性能。结果表明，7度小震时，复合地基没有减震作用；7度大震时，桩基中桩体弯矩、剪力最大值是刚性桩复合地基中桩体弯矩、剪力最大值的1.8倍，褥垫层产生了较大的塑性变形和相对基底的滑移，具有一定的耗能减震作用，并且复合地基中筏基及上部结构的动力响应约是桩基体系中对应量的90%，减震系数在0.8-0.9之间。本论文的研究成果已应用于广东粤北地区的碧湖苑、时代美居、翠湖花园等工程项目建设中，取得良好效果。更多还原

【Abstract】 The karst geology situation of Guangdong Province is very complicated and of greatdisadvantage of engineering, especially in the northern area such as Shaoguan, Qingyuan,Zhaoqing. As a result, a more suitable foundation style and way of ground treatment isurgently needed. The School of Civil engineering and transportation of South ChinaUniversity of technology has conducted a series of experiments and engineering practices inthe northern area of Guangdong Province under the financial support provided by Key Projectof the State Key Laboratory of Subtropical Building Science South China University ofTechnology.Nowadays the bearing capacity of composite foundation is determined by the single-pilestatic loading test which ignores the pile-group effect and is not able to offer the forcedistribution suitably. And the interaction between the superstructure and the foundation isdifficult to be considered because of the restriction of computers. So further research shouldbe done to simplify the analyzing method to make it feasible. Moreover the field experimentalresults show that there is a remarkable difference between the experimental stress of raft steelrebar and the calculated value, and the measured values are smaller. It is indicated that theCalculation Model exists insufficiently and the design method is conservative. So the designmethod should give a more accurate steel rebar stress in the raft. Furthermore, with the wideuse of composite foundation technology in earthquake region, it is urgent to study thedynamic interaction of composite foundation and superstructure. This thesis adopts themethod of theoretical analysis and experiments to study the bearing and settlementperformance of the rigid-pile composite ground-raft foundation-superstructure system, with apractical engineering background. The main contents and innovative works are listed asfollows:1) The results of twenty-four load tests of single-pile composite foundation incomplicated karst geology show that the bearing capacity of soil between piles exerts earlyand it’s difficult for both pile and soil to reach their design value under the characteristic load.With0mm thick sand-gravel cushion and high soil’s bearing capacity, part of the soil’sbearing capacity still works as long as the foundation is in close contact with the soil. With100mm thick sand-gravel cushion, under the characteristic load, the mean stress of soil is0.31MPa, the pile-soil stress ratio is about19, the load bearing ratio of soil is46.5%, and thebearing capacity factors of pile and soil is about0.72and1.78respectively. With300mmthick sand-gravel cushion. Under the characteristic load, the mean stress of soil is0.38MPa, the pile-soil stress ratio is about14.6and the load bearing ratio of soil is61%. The analysisshows that this kind of treatment is feasible.2) Compared with the single pile composite foundation loading test, the correctness ofeffectiveness of the adopted soil elastic-plastic model is veirified. The contact relationship andthe selected parameters can be used to analyze the stress of the rigid pile compositefoundation, and the deformation performance more accurately. On this basis, the size effect ofgroup piles composite foundation loading plate is researched and the formulas for calculatingthe final settlement of large size loading plate is worked out. The thickness of the Cushionlayer is100mm, the final settlements of the foundations with the side length of30m,40m and50m,is respectively16.7mm,17.5mm and18.1mm; while the thickness of the cushion is300mm, the final settlements of the foundations with the side length of30m,40m and50m,isrespectively32.4mm,34.5mm and36.1mm. The results match well with the measured values.The results of analyzing the macroscopic constitutive relation of the group piles compositefoundation show that with the increase of the number of the piles, the settlement of theloading plate increases, the bearing capacity of the soil decreases. In this geologicalconditions, under the effect of characteristic value of loads. As the thickness of cushions isrespectively100mm,200mm and300mm, the average value of49piles compositefoundation settlement is respectively about1.6,2.18and2.2times the foundation settlementof a single pile composite on average and the soil stress is respectively46%,65%,78%ofsingle pile composite foundation soil stress. In early loading period, the single and multiplepiles were of the similar penetration into the cushion, while the penetration of pile groupproved to be larger as the load increases. Thickness of the cushion layer is respectively100mm,200mm and300mm, on the characteristic loading value, the penetration of the singlepile into the cushion is3.9mm,6.8mm and8.4mm respectively. The penetration of themultiple piles is6.8mm、18.7mm、19.2mm respectively. When the pile number reaches theamount of49, and cushion layer of foundation up to300mm, the soil stress of the piles,foundation settlement, penetration of rigid pile, change little. Regulation of the cushion layertends to be stable.3) According to the test results and FEM analysis, combining engineering experiences,the bilinear spring macro constitutive relation of composite foundation is put forward. Thecorresponding settlement of characteristic load is12mm, and the corresponding settlement ofultimate load is40mm. The interaction between superstructure and composite foundation canbe simulated by SAP2000, and the corresponding foundation settlement of characteristic loadchanged from4mm to20mm. In addition to changes of raft moment in some individual area are close to9%, others are negligible. The results show that the method can not only considerthe effect of the stiffness of superstructure, but also can reflect the nonlinear property of theground as well as the overall settlement.4) Solid element model of the foundation has been established, considering the frictionbetween the raft and cushion. Elastoplastic analysis was done with ABAQUS to consider theinteraction of superstructure with composite foundation. The results showed that under thedesign load of elastoplastic calculation, section stress distribution is appropriately uniform,cross-sectional maximum tensile stress is about1.63Mpa, the maximum stress ofreinforcement is about80Mpa; the maximum stress of reinforcement under standard load is48Mpa，the calculated results agree well with the measured values to verify the correctness ofthis analysis in this article. By calculation and analysis, compared to calculation results withcommon methods, it shows that ground reaction force model, and the friction between raftand cushion are major factors that affect the calculation results of the forces in the raft.Ground reaction force at the edge of the raft surges, the maximum value reaches3.5times theaverage value, which can easily cause damage to the foundation of local area in advance.Finally, the foundation soil deformation modulus, thickness cushion, cushion modulus,thickness and other parameters on the raft force and deformation have been analyzed. Theresults show that the ground reaction force and stress in the raft are related to the relativestiffness of the raft and ground.5)The modeling method of FEM-Infinite element coupling is adopted to set up theintegral three-dimension analyzing model for the dynamic elastoplastic time-history analysis.By comparing with the results of ordinary pile raft foundation dynamic time-history analysis,the results show that the composite foundation has no seismic reduction effect at the stage offrequent earthquake of intensity of7; but at the same degree of rare earthquake intensity, themaximum pile moment and shear force of ordinary pile raft foundation is1.8times of thosevalues in the rigid pile composite foundation, and there is a large plastic deformation and sliprelative to the substrate in the cushion, which shows a certain role in energy dissipation andseismic reduction. The dynamic response of raft foundation in the composite foundation andsuperstructure is90％of the corresponding amount in ordinary pile raft foundation, theseismic decrease coefficient lies between0.8and0.9.更多还原