基于触变与时间效应的湛江组结构性黏土桩基安全性评价

The time effect of pile foundation is very obvious in Beibu Gulf coastal areas, due to the thixotropy of Zhanjiang group structural clay in those areas.The theoretical calculation value of file foundation bearing capacity in long term loading is very different with the measuring value in field, since pile foundation time effect influence is not considered in current theory and methods of pile foundation design and calculations, which means that traditional experiences are not suitable in those areas.Since Zhanjiang group structural clay is widely distributed and pile foundation is common in Beibu Gulf coastal areas, this project is supposed to conduct theoretical analysis, macro-meso and field model tests and numerical simulations, to reveal changing regulations of the structural, physical and mechanical properties before and after disturbance of the clay, to analyzed the thixotropy pattern, to recognize the thixotropy mechanism deeply, and to establish the indexes system to judge the thixotropy intensity of the structural clay.Based on the above work, this project fatherly attempts to reveal the soil thixotropy influence to the pile foundation time effect, to analyze the bearing property and mechanism of the pile foundation in different interregna when the pile foundation sinks into the Zhanjiang group structural clay in Beibu Gulf coastal areas, to raise up the theory and methods for design and calculations considering pile foundation time effect in structural clay, to analyze the bearing capacity influence factors for the pile foundations in long term loading in Zhanjiang group structural clay in Beibu Gulf coastal areas, and to establish the corresponding safety evaluation methods, which will supply theoretical basis and technology support for relative engineering design and constructions.

北部湾沿岸地区湛江组结构性黏土具有触变性，导致该土层中桩基时效性明显。现有的桩基设计计算理论和方法因没有考虑桩基时效性的影响，导致在长期荷载作用下桩基承载力理论计算值与实测值存在较大的误差，传统的经验在该地区存在适用性问题。本项目结合北部湾沿岸地区湛江组结构性黏土分布广泛且桩基普遍的实际，通过理论分析、宏微观和现场模型试验、数值模拟等方法，全面揭示该黏土在扰动前后结构、物理力学特性的变化规律，分析其触变模式，深化认识其触变机理，建立判定结构性黏土触变性强弱的指标体系；在此基础上，进一步揭示土体触变性对桩基时效性的影响规律，分析北部湾沿岸地区湛江组结构性黏土桩基在沉入土体后不同休止时间段的承载性状和作用机理，提出考虑结构性黏土桩基时效性的设计计算理论和方法，分析北部湾沿岸地区湛江组结构性黏土桩基在长期荷载作用下的承载力影响因素，并建立其安全评价方法，为相关工程设计施工提供理论依据和技术支持。

北部湾沿岸地区广泛分布着湛江组结构性黏土，因其触变性使桩基工程承载时效性明显。北部湾沿岸地区湛江组结构性黏土中桩基面临着长期荷载作用下承载性能安全评价的问题。本项目通过现场地质调查，查明了湛江组结构性黏土地层分布区域；通过湛江组结构性黏土触变性宏观试验，获得了含水率、孔隙比、灵敏度等对湛江组结构性黏土触变性强弱的影响规律；通过湛江组结构性黏土微观特性试验，诠释了湛江组结构性黏土触变机理；进行了湛江组结构性黏土触变过程中微观水转换对黏土触变性的影响试验，揭示了湛江组结构性黏土在触变过程中微观水的存在形式及转换对其触变性的影响机理；基于现场原位十字板剪切试验，设计模型装置，采用室内振动试验模拟土样受扰过程，在不同扰动程度、不同静置龄期下对湛江组结构性黏土进行微型十字板剪切试验及无侧限抗压强度试验，获得了扰动程度对土体触变恢复强度的影响规律；在不同上覆压力作用下对扰动后的湛江组结构性黏土在不同静置时间进行直接剪切试验和扫描电镜试验，从宏微观角度阐明了上覆压力对湛江组结构性黏土触变性影响机制；根据触变灵敏度随养护时间的关系，建立衡量湛江组结构性黏土触变性的强弱的指标体系；基于模糊数学理论的分级方法，对雷北河流相、河口相、浅海相及雷南河流相湛江组强结构性黏土触变性强弱进行分级；通过无侧限抗压强度试验，研究了土的灵敏度、含水率、密度变化对土重塑后强度增长的影响程度，建立了考虑灵敏度、含水率、密度影响的湛江组黏土触变过程中的强度恢复模型；通过现场原位测试与模型试验，研究了北部湾沿岸地区湛江组结构性黏土桩基在沉入前及沉入后不同静置龄期段内桩周土体结构、物理力学性质的变化以及桩基承载时效性的变化规律，建立了湛江组结构性黏土触变性与桩基时效性的量化关系，提出了考虑北部湾沿岸地区湛江组结构性黏土触变性与桩基时效性的设计计算方法，为北部湾沿岸地区的桩基工程设计与施工提供理论依据和技术支持。