考虑蠕变三维效应和交通荷载的超固结软土沉降模型

Soft clay deposit is widely distributed in the coastal areas of South Eastern China. Long-term settlement of embankments constructed on deep and thick deposits of this soft clay can lead to unsafe incidents during service and heavy economic losses. The post construction settlement of embankment on the deep and thick soft clay foundation treated with drainage and surcharge preloading is prevalent in these coastal areas. The main reason of these phenomena is the creep deformation of overconsolidated clay formed by unloading of the surcharge loading, and also, the impact of cyclic traffic loading on low embankment constructed on the soft clay foundation. .In this research, a theoretical model of secondary consolidation coefficient of over-consolidated soft clay and a 1D creep model of it will be elaborated based on the theory of parallel time lines proposed by Bjerrum. According to the relationship between the lateral pressure coefficient and stress history of over-consolidated soft clay, an expanded theoretical 1D creep model will be derived. From this model and combing the 3D creep behavior of over-consolidated soft clay, a volumetric strain model will be created. The relationship between the volumetric strain and axial strain of clay will be deduced from the associated flow rule of plastic theory. The relationship between volumetric strain and axial strain of clay and the volumetric model will be combined to constitute a unified creep settlement model of over-consolidated soft clay considering 3D effect..The calculation method of creep settlement of over-consolidated soft clay considering 3D effect will be created from the unified creep settlement model combing the time of creep process of the field clay determined from the idea of "equivalent time". Using the creep model above, the settlement effect of low-frequency slightly cyclic loading on over-consolidated soft clay foundation will be simulated by superimposing many separate short period creep processes of constant static load. And the accumulated strain superposition model of cyclic loading will be obtained from this method. The settlement model for over-consolidated soft clay considering 3D effect of creep and traffic load will be proposed by combing the calculation method of creep settlement of over-consolidated soft clay considering 3D effect and the accumulated strain superposition model of cyclic loading. .The result of this proposal will be important for calculation theory of soft clay settlement.

我国东南沿海地区软土地层广泛分布。深厚软土路基工后长期沉降可导致安全事故和重大损失。超载预压法处理的深厚软基工程的工后沉降问题具有普遍性，主要原因是超载卸荷后形成的超固结软土的蠕变变形，软基低路堤还应考虑交通荷载的影响。本项研究基于Bjerrum时间平行线理论，建立超固结软土次固结系数的理论模型和一维蠕变沉降模型；根据超固结软土侧压力系数与应力历史的关系，建立一维蠕变理论拓展模型；结合超固结软土的三维蠕变性状，建立三维蠕变体变模型；根据流动法则建立体变和轴向应变的关系，构成三维蠕变沉降统一模型。基于"等效时间"的思想，建立现场软土蠕变过程时间状态的确定方法，形成考虑三维效应的蠕变沉降算法。依据蠕变模型，以分时段不同恒载蠕变过程迭加逼近低频小幅循环荷载过程，建立循环荷载累计应变迭加模型，最终形成考虑蠕变三维效应和交通荷载的超固结软土沉降模型。研究成果对土力学沉降计算理论也有重要价值。

超载预压深厚软土地基上的工后沉降与超固结软土的蠕变变形密切相关，若能较准确地估算出超载卸除后软基的蠕变沉降，即可以解决其工后沉降的控制问题。超载预压软基低路堤浅层软土需要附加考虑交通循环荷载的影响。由于受力状态不同，软土多维蠕变下的沉降全过程都存在三维效应，现有一、三维蠕变沉降模型存在参数多或功能局限的问题，不能直接反映应力历史的影响，也不能确切判定现场软土层蠕变过程的时间状态。本项研究对于土力学沉降计算理论和解决超载预压深厚软基的工后沉降控制问题有重要意义。.通过理论和实验研究，采用与应力历史相关的蠕变沉降参数，直接反映软土的蠕变过程受应力历史的影响，建立考虑三维效应的超固结软土蠕变沉降一维模型。将循环荷载长期的作用过程转化为近似的恒定荷载过程，用恒载方法解决循环荷载问题。.研究发现，超固结软黏土一维蠕变过程与空间应力状态参数（广义剪应力q和球应力p）具有良好的对应性，以此嫁接一维、三维蠕变过程成为建立蠕变统一模型的有效途径；以空间应力状态参数定义的超固结剪应力比能全面地反映超固结软黏土的应力历史，同时包含前期固结应力的量值和相对大小的影响；平面应变状态下的蠕变过程与超固结剪应力比有对应关系，同时受主应力比影响，据此建立的以主应力比和超固结剪应力比为变量的四参数二维蠕变经验模型，包含了正常固结状态，具有较好的适用性；循环荷载长期作用下超固结软土的一维、二维变形过程，孔隙水压力与循环荷载对应，量值渐次消减直至发生等值振荡，对土体的长期变形不产生明显影响，长期沉降变形可采用恒载蠕变模型近似计算；研制开发了低频多波形循环荷载重复加卸荷的动力砝码，建立了循环荷载下的软土动力特性实验仪器系统；研制开发了侧向变形量测装置和孔隙压力测量方法改进常规平面应变三轴仪，独立量测平面应变超固结软土的侧向变形和直接测量试样中心的孔隙压力，为动荷载下的二维蠕变实验创造了条件。