Seismic Site Response Analysis Considering Partially Saturated Soil Conditions

部分饱和土的地震反应不同于干燥或水饱和土的地震反应。然而，现有的场地响应分析方法忽略了土壤局部饱和的影响。本研究将研究局部饱和对地震场地反应分析的影响，以及不同的吸力（饱和度）剖面对土层地震反应的影响。由于部分饱和引起的吸力增加了作用在土壤上的有效应力，改变了剪切模量、剪切波速、非线性变形响应和阻尼。因此，预计吸力随深度的分布将影响地震波的传播和由此产生的地面加速度。这反过来又影响了对土壤和地表结构施加的地震要求。目前场地响应分析方法中采用的剪切波速虽然考虑了土体的局部饱和，但仅反映了测量剪切波速时土体的饱和程度。因此，地下水位的季节波动及其对饱和程度的影响可能会改变场地的响应。目前的实践状态通常依赖于包括水饱和或干燥土壤动态材料特性的程序，这是最保守的情况。对于处理土的强度和变形的问题，这可能是一个适当的假设。然而，由于颗粒间吸力的作用，较硬的部分饱和土会导致较高的场地固有频率和较低的阻尼，这可能会对场地响应产生不利影响。在本研究中，将通过土工离心机内部非饱和土层的稳态渗透来控制土壤层？S的饱和度。在不同吸力剖面的土层上施加不同幅值、频率的循环荷载和不同烈度的地震运动。这些剖面受渗透速率和离心加速度的控制。实验数据将与数值估计的现场响应进行比较，其中包括基于有效应力的、依赖于吸力的动态材料特性。本研究将通过纳入饱和程度的季节性波动对场地响应的影响，推进非饱和土动力学和岩土工程系统抗震性能的基础知识。这项工作将有助于更好地理解波浪在非饱和土层中传播的基本机制。这个项目将是朝着更可持续和更安全的建筑和基础设施抗震设计迈出的重要一步。该项目的潜在发现将有助于评估当前评估地震现场反应方法的性能，并为考虑当前地震现场反应分析方法中的局部饱和提供实用建议。研究生发展将通过在岩土地震工程、水渗透和物理建模方面的高级技术培训，以及指导机会，为学生领导该行业做好准备。地震工程和地震危害的重要性将通过本科研究、课程和推广项目介绍给大学土木工程和K-12学生。

The seismic response of partially saturated soils differs from that of dry or water saturated soil deposits. Yet, the available site response analysis methods ignore the influence of partial saturation in the soil. This research will study the effects of partial saturation on the seismic site response analysis and how different suction (degree of saturation) profiles affect the seismic response of a soil layer. Suction due to partial saturation increases the effective stresses acting on the soil, altering the shear modulus, shear wave velocity, nonlinear deformation response, and damping. As a result, the distribution of suction with depth is expected to affect the propagation of seismic waves and the resulting ground accelerations. This, in turn, influences the seismic demand imposed on the soil and surface structures. Although the shear wave velocity employed in current site response analysis methods accounts for partial saturation of the soil, it only reflects the degree of saturation at the time of shear wave velocity measurement. Hence, the seasonal fluctuation of the water table and its impact on degree of saturation may alter the site response. The current state of practice commonly relies on procedures that include dynamic material properties of either water saturated or dry soils as the most conservative scenarios. This might be an appropriate assumption for problems dealing with soils' strength and deformation. However, stiffer partially saturated soils due to inter-particle suction forces result in a higher site natural frequency and lower damping, which may adversely affect the site response. In this study, the steady state-infiltration through an unsaturated soil layer inside a geotechnical centrifuge will be implemented to control the soil layer?s degree of saturation. Cyclic loads with different amplitudes and frequencies, and earthquake motions with different intensities will be applied to the soil layers with various suction profiles. These profiles are controlled by the infiltration rate and the centrifugal acceleration. The experimental data will be compared with the numerically estimated site responses incorporating effective stress-based, suction-dependent dynamic material properties. This research will advance the fundamental knowledge of unsaturated soil dynamics and seismic performance of geotechnical systems by incorporating the effects of seasonal fluctuation of degree of saturation on site response. The work will assist in better understanding of fundamental mechanism of wave propagation through an unsaturated soil layer. This project will be a substantial step towards more sustainable and safer seismic designs of buildings and infrastructure. The potential findings of the project will help in assessing the performance of current approaches in evaluating the seismic site response and provide practical recommendations to consider the partial saturation in current seismic site response analysis methods. The graduate student development will prepare students to lead the profession through advanced technical training in geotechnical earthquake engineering, water infiltration, and physical modeling, as well as mentoring opportunities. The importance of earthquake engineering and seismic hazards will be introduced to college Civil Engineering and K-12 students through undergraduate research, courses, and outreach programs.