土工合成加筋砂土地基加载速率相关的时变特性研究

Compared to RC basements and structures, the geosynthetic-reinforced soil is undoubtedly a relatively soft basement or structure, thus the deformation control must be seriously considered in the design of th geosynthetic-reinforced soil, which includes the instantaneous deformation following construction and the long-term residual deformation after construction.The obvious loading rate-dependent behaviors of geosynthetic-reinforced soils are observed in the field measurement of actual engineering project, namely, on a step-change in the loading rate, the basement pressure-settlement relation is changed correspondingly. It is due to the viscous properties of the backfill and the geosynthetic-reinforcement and the interactions between them. The above behaviors seriously affect the short-term deformation following construction and the long-term residual deformation after construction. Although it is time-dependent, the traditional rheological theory and model with the state parameter of time(t), cannot interpret the deformation behaviors of geosynthetic-reinforced soil under the change of loading rate.. In the present study,therefore, a series of model tests of the geogrid-reinforced sands under the change loading rate will be performed in order to analyze the relationship between the loading rate and the time-dependent deformation. Based on the above experimental results, the nonlinear finite element method considering the effect of loading rate will be developed to more accurately simulate the time-dependent deformation of geosynthetic-reinforced soils. Therefore,the content of this study includes:(1)The experimental conditions and devices will be improved, which include the loading system and the measuring devices. (2)A series of model tests will be performed, which include: (a)constant strain rate on the stress-strain behaviour;(b)changes in the stress-strain behaviour when the strain rate suddenly or gradually increases or decrease from a certain value to another; (c)creep deformation and stress relaxation,and etc.;(3)The behaviors of the time-dependent deformation under the change of loading rate will be discussed, including: (a) load-deformation behaviour at different rates of loading; (b) creep deformation and stress relaxation behaviour following loading at different rates,and etc.;(4) By considering the effect of loading rate, an elasto-visco-plastic model will be presented and a highly non-linear FE method will be developed to reasonably simulate the time-dependent deformation behaviors of geosynthetic-reinforced sand.. The innovations of this study include:(1) To clarify the mechanism of the time-dependent deformation and the relationship between the loading rate and the time-dependent deformation of the geosynthetic-reinforced soils;(2)To put forward a highly non-linear elasto-visco-plastic FE aanalysis method with the effect of loading rate,instead of the traditional one with the state parameter of time(t).

与钢筋混凝土基础或挡土结构相比，土工合成加筋土复合地基无疑是一种相对柔软的土工构筑物。因此，施工时的瞬时变形及使用时的长期流变是土工合成加筋土设计上必须考虑的重要参数。实际工程监测表明加筋土的荷载沉降曲线与加载速率显著相关，这种特性严重影响加筋土地基的短期或长期与时间有关的变形行为，而以时间t为内在变量的传统粘塑性理论和流变模型不能完全合理解释这种现象。本项目以土工格栅加筋砂土为对象，基于加载速率变化开展一系列的模型试验和理论研究，分析加载速率变化与加筋砂土地基与时间相关变形的关系及内在规律。本项目的创新点主要包括：阐明加载速率变化下加筋砂土瞬时及长期变形发生机理；提出基于加载速率效应的加筋砂土时间有关变形的有限元预测方法。本项目的研究方法将突破直接以时间t为内变量的传统弹粘塑性理论框架，其成果对揭示加筋砂土与时间相关的变形破坏机理以及确立高精度的变形预测方法具有重要的理论意义及应用价值。

土工合成加筋土结构的速率相关的变形强度特性从本质上讲应归因于材料粘性特性的外在反映，即填土材料和土工合成加筋材料本身的粘性特性以及两者之间的相互作用。因此，本项目的研究思路是：从加载速率角度出发，先对填土材料（以砂土为例）和土工合成加筋材料的速率相关的粘性行为特性分别进行详细的探讨和研究，进而对土工合成加筋土结构的速率相关的变形强度特性进行分析。通过本构模型和有限元数值分析与计算探讨了砂土材料、土工合成加筋材料以及土工合成加筋土结构的加载速率效应、蠕变和应力松弛等速率相关行为特性，为改进现有设计理论提供了科学依据。本项目的主要研究内容及所取得的研究成果包括以下几个方面：.（1） 基于任意加载速率条件下室内模型试验，对砂土及土工合成加筋材料的速率相关粘性特性进行了分析，考察了砂土以及土工合成加筋材料的加载速率效应、蠕变以及应力松弛等特性。.(2) 将统一的非线性三要素弹粘塑性本构模型嵌入到基于动态松弛法的有限元程序中，从而可对砂土、土工合成加筋材料以及土工合成加筋土结构的速率相关的变形强度特性进行弹粘塑性有限元模拟，实现了包含应变速率变化加载、蠕变加载和应力松弛加载在内的全过程模拟以及砂土材料从硬化→峰值→软化→残余的变形破坏的全过程模拟。.(3) 对土工合成加筋砂土以及土工合成加筋挡土墙的速率相关变形强度特性进行弹粘塑性有限元分析，结果表明，综合考虑砂土材料和土工合成加筋材料粘性特性的有限元计算方法能很好地模拟土工合成加筋砂土结构的加载速率效应、蠕变变形以及应力松弛现象。.(4). 通过试验现象和相应有限元分析，发现蠕变并不是一种劣化现象，它对材料的最大强度几乎没有影响，为改进或取消现有蠕变设计理论中的“折减系数”法提供科学依据。