Study On The Effect Of Unsaturated Soils On Excavation And Slope

With the increasing demand for living space, the modern buildings become taller and deeper. At the same time, the bigger excavations come into being. As early as forties of last century, many scholars did research on the excavation engineering. From then on, a lot of research results sprang up. Nowadays, with the rapid development of underground spaces, excavation engineering becomes one of hot issues in the civil engineering. But most of those research results still have a common problem that the theoretical research lags behind the practice application. Many studies are just the empirical summaries of practice projects, which lack rigorous theoretical foundation. Therefore, some engineering problems are very difficult to eliminate. Lacking rigorous theoretical foundation in the design methods is one of the important reasons, except the poor construction quality. In the light of this situation, the effects of the common properties differences between saturated soil and unsaturated soil on excavation are analyzed and compared from the theory in this thesis, in order to explore new directions for the excavation study.This thesis summarizes and analyzes the status quo of excavation and finds that changes of hydraulic environment often lead to engineering accidents. The special hydraulic properties of unsaturated soil in the practical engineering are ignored in many studies and even in the design codes. For this reason, the preliminary study of excavation is made in the thesis, combining with the theory of unsaturated soil.Firstly, classical constitutive models of unsaturated soil are sorted and some problems in these models are found out. This study is focused on the stress variables of unsaturated soil from micro-structure, energy conservation and mechanical equilibrium. A selection principle of stress variables for unsaturated soil is proposed based on these discussions and analyses. Existing stress variables are also analyzed to propose a reasonable guideline for the research of constitutive theory of unsaturated soil.Subsequently, the effective stress of unsaturated soil based on Lu's suction stress is selected as a stress variable for the analyses. Combining with the classic Mohr-Coulomb model, a simple constitutive relation of unsaturated soil was established. The constitutive model is used in ABAQUS program to analyze the influence of rainfall infiltration on stress and strain of a slope. The comparison results of measured values and simulation values show that changes of slope deformation in rainfall infiltration can be well modeled by using this model. The trend of the stress change in the slope can also be well reflected. Overall, this constitutive model can reflect the actual impact of rainfall on unsaturated slopes well. Finally, the constitutive model is applied to the excavation engineering. In order to observe the effect of unsaturated soil on excavation deformation and slope stability in rainfall infiltration, an ideal model of an unsupported excavation of silt soil is modeled in ABAQUS. The results show that the deformation difference of the pit between saturated soil and unsaturated soil is very obvious. The unsaturated soil pit is more stable than the saturated soil pit with no other conditions changed. States of slope under different rainfall intensities are also analyzed. The results demonstrate that rainfall would lead the slope surface moving inside and the safety factor descends simultaneously. The safety factor of slope becomes smaller with the higher intensity of the rainfall. The numerical results show that the characteristics of unsaturated soil can not be ignored during excavation process. More attention should be paid to the slope stability, especially in the rainfall case.