隐伏倾向断层作用下群桩失效机理与减灾措施研究

Field investigations after earthquake show that buildings around surface fault rupture tend to be severely damaged. The corresponding mechanism behind the failure of pile group is not yet understood. Appropriate setback distance and disaster mitigation strategies are needed. In this study laboratory testing, centrifuge modeling, numerical simulation and theoretical analysis are conducted to investigate the failure mechanism of pile group in sand induced by a buried dip-slip faulting. The conditions inducing different failure modes of pile group are also studied. The differences in the fault rupture propagation, the location of scarp at the ground surface, the width of surface fault rupture and the failure mode of pile group under normal faulting and reverse faulting are compared. Considering the stress-dependent characteristic and dilatancy of sand material, the effects of soil resistance under pile cap on interaction between pile and fault rupture propagation are studied. Based on a general error function, an analytical solution is developed for prediction of setback distance between the pile group and the bedrock fault. The influences of soil layer thickness, seismic intensity and dip-slip angle of bedrock fault are considered in this analytical solution. For existing pile group, a mitigation strategy of arching-shape retaining wall by deep mixing method is proposed and verified. By parametric study of key design parameters in numerical simulation, a design chart for the arching-shape retaining wall is developed. The outcomes from this study not only reveal the mechanism behind the faulting-soil-pile interaction but also are of benefit to the development of mitigation strategies to disaster induced by bedrock fault movement.

地震活动断层引起地表破裂带附近建筑物集中破坏，相应群桩基础近断层失效机理尚不明确，缺乏针对性的安全避让与减灾措施。本项目通过室内土工试验、离心机试验、数值模拟和理论分析相结合的研究方法，探索隐伏倾向断层引起上覆砂土中群桩基础失效机理和不同破坏模式的诱发条件。对比正、逆断层作用下断层破裂扩展路径、地表陡坎出露位置、地表破裂带宽度和群桩破坏模式的差异。基于砂土材料的应力相关和剪胀特性，考察承台底土反力水平对桩基－破裂扩展相互作用的影响规律。采用一般形式误差方程，考虑土层厚度、抗震设防烈度、断层倾角等主要影响因素，建立群桩基础与断层间安全避让距离的理论预测模型。针对既有桩基，建立水泥搅拌桩隔离拱墙变形控制方法并验证其有效性，通过关键设计参数的数值分析，建立隔离拱墙设计流程。该课题的研究成果不仅揭示断层－土体－桩基相互作用机理，对近断层灾害控制理论的发展和完善也起到积极推动作用。

地震活动断层引起地表破裂带附近建筑物集中破坏，相应群桩基础近断层失效机理尚不明确，缺乏针对性的安全避让与减灾措施。本项目通过室内土工试验、离心机试验、数值模拟和理论分析相结合的研究方法，探索隐伏倾向断层引起上覆砂土中群桩基础失效机理和不同破坏模式的诱发条件。项目研究成果表明，逆断层错动引起的断层破裂扩展更为平缓，使得断层破裂向下盘一侧倾斜，并使得地表破裂带远离基岩断层的地表投影线。对于正断层，随着断层错动的发展，桩基承台与土体可能发生脱空，使得桩基承担的荷载由承台转移至复合基桩，导致基桩受压和受弯破坏。而对于逆断层，桩基承担的荷载则主要由复合基桩转移至承台，使得基桩以受拉破坏和受弯破坏为主。对于上覆砂层，断层破裂扩展的方向主要受砂土剪胀角的影响；砂土剪胀角使得断层破裂向上盘一侧偏移。承台底土反力倾向于抑制断层破裂带的形成和扩展，使得断层破裂带绕过群桩承台，并于承台两侧出露地表。采用一般形式误差方程，可以综合考虑土层厚度、基岩错动量、断层倾角等主要因素的影响，建立了上覆土体的位移表征方程，进而将土体的位移简化为等效荷载，基于p-y曲线法建立基桩的内力计算模型，从而进一步建立群桩基础与断层间安全避让距离的理论预测模型。针对既有桩基，建立了膨润土隔离墙桩基保护措施，验证了其对变形控制的有效性。通过关键设计参数的数值分析，分析了隔离墙关键设计参数的影响规律。该课题的研究成果不仅揭示了断层－土体－桩基相互作用机理，对近断层灾害控制理论的发展和完善也起到积极推动作用。