非饱和路基冻土的土水特征及温湿动态响应

The evolution of the mechanical responses of frozen pavement subgrade soils under dynamic traffic loadings (i.e. dynamic responses, including resilient modulus and accumulative plastic strain) during freezing-thawing is quite sophisticated. Currently, empirical relationships are generally used in practice to describe the dynamic responses-temperature relationships. Pavement subgrade soils are typical unsaturated soils whose mechanical behaviors are similar during freezing-thawing and wetting-drying. This project proposes comprehensive experimental studies on the soil-water characteristics and volumetric change, dynamic responses, soil structure evolutions during wetting-drying and freezing-thawing of same pavement subgrade soils. Based on the comparison, this project intents to (i) clarify the similarities and divergences between the soil-water characteristics and dynamic responses during wetting-drying and freezing-thawing; (ii) combine the microscopic structural change with macroscopic volumetric behavior to explain the influence of soil structure on the soil-water characteristics and dynamic responses; (iii) use soil-water characteristics to define frozen soils’ suction-unfrozen water content-ice content relationships and to facilitate the comparison between the variation of the dynamic responses with suction and water content under positive temperatures, and with suction, unfrozen water content, ice content under negative temperatures. Such comparison helps to reveal the respective contributions of suction, water content and ice content (i.e. phase change) to the dynamic responses; (iv) propose constitutive model for the dynamic responses of frozen soils that uses suction and water content to represent the temperature change and considers the influence of the phase and structural changes. Studies proposed in this project would provide the pavement design practice in cold regions with theoretical references.

路基土在交通动荷载下的力学响应（即动态响应，包括回弹模量和累积塑性应变）在冻融过程中的变化非常复杂，目前一般采用经验方法描述其与温度的关系。路基土是典型的非饱和土，其力学行为在负温冻融过程与常温干湿过程中具有相似性。据此，本项目拟通过测定相同路基土在负温冻融和常温干湿过程中的土水特征以及动态响应、体变、微观结构的演变，明确土水特征及动态响应在两种过程中的异同；将微观结构与宏观体变相联系，揭示冻土结构对其土水特征和动态响应的影响规律；基于土水特征所确定的冻土中吸力-未冻水含量-冰含量的关系，比较动态响应在常温下与吸力、含水量的关系和其在负温下与吸力、未冻水含量、冰含量的关系，藉此分别探明吸力和未冻水含量，以及冰含量（相变）对冻土动态响应的影响机制；以吸力和未冻水含量表征温度变化，考虑相变和土体结构的影响，基于非饱和土刚度与强度理论，建立路基冻土的动态响应模型，为寒区路基的设计提供理论依据。

本项目以山东粉土和黑龙江黏土两种路基土为研究对象，开展了大量的试验和理论研究，揭示了路基土在室温和负温下的微观结构、土水特性、体变、静态刚度强度和动态响应（包括回弹模量和累积塑性应变）在冻融和干湿作用影响下的演化规律，明确了微观结构、吸力、含水率、干密度、应力状态、正负温度等因素对于路基土动态响应特性的影响机制，建立考虑冻融和干湿作用影响的路基土动态响应预估模型，为寒区路基的设计提供理论依据。本项目取得的主要研究成果：（1）路基土在冻融作用下产生不可恢复的裂隙和大孔隙，土体结构变得离散。土体中团粒内部的小孔隙基本不受冻融循环作用的影响。路基土孔隙密度分布曲线由单峰形式转变为双峰形式。在1000 kPa范围内，冻融循环作用降低了两种黏性土的持水能力，特别是产生裂隙和大孔隙较多的黏土。不同冻融循环作用下，压实黏性土的孔隙比-质量含水率-吸力关系可用同一本构曲面模型描述。（2）路基土的动静力学特性的衰减主要发生在第一次冻融循环阶段，且冻融作用降低了力学特性对湿度变化的敏感性。冻融循环作用后，回弹模量对围压变化的敏感性有所增加；回弹模量与偏应力的关系取决于土体结构的完整程度。粉土的累积塑性应变为塑性稳定型或塑性过渡型，而黏土由于压实含水率较高，在高偏应力低围压水平下累积塑性应变表现为递增破坏型。粉土的累积塑性应变对含水率的变化较为敏感，黏土对冻融循环次数的变化较为敏感。（3）脱湿路径下土水特征曲线与冻结路径下的冻结特征曲线形状相似但不一致。路基土的持水能力越强，干密度越高，其冻结特征曲线的滞回现象更明显。经过3次冻融循环以后，路基土的冻结特征曲线达到稳定状态。（4）提出了考虑冻融和干湿作用影响的回弹模量理论模型以及累积塑性应变半经验模型，取得了较好的预估效果。本项目相关研究所取得成果共发表SCI期刊论文11篇，中文期刊论文4篇；项目执行期间培养或协助培养研究生8人，申请发明专利2项。