Abstract
Lagged settlement is a typical accident induced by shield construction in sandy cobble strata. This paper analyzed the process and mechanism of lagged settlement, results show that all phases are in accord with the characteristics of ellipsoid theory of particle flows. Based on this theory, a method for calculating coefficient of lateral earth pressure and loosened earth pressure is proposed in this research. For the coefficient of lateral earth pressure, the boundary of loosened ellipsoid is divided into two parts, the arch zone and the excavation zone, and the lateral pressure coefficients are derived respectively according to the stress state. For loosened earth pressure on tunnel, the Terzaghi earth pressure theory and Protodyakonov earth pressure theory are adapted in different conditions according to the state of loosened cobble soil. Theories developed in this study can be applied on determination of shield excavation parameters, as well as calculation of loosened earth pressure and control of tunnel support.
Highlights
Sandy cobble soils are mainly composed of cobbles, gravels, sand and some clayey soil, the structure of sandy cobble soil is discrete and easy to be disturbed
For the coefficient of lateral earth pressure, the boundary of loosened ellipsoid is divided into two parts, the arch zone and the excavation zone, and the lateral pressure coefficients are derived respectively according to the stress state
Lagged settlement or collapse is a typical accident induced by shield excavation in sandy cobble soil, there are several phases throughout the evolution including disturbance in front of excavation face, over excavation, formation of cavity, cavity moves upward and ground collapse
Summary
Sandy cobble soils are mainly composed of cobbles, gravels, sand and some clayey soil, the structure of sandy cobble soil is discrete and easy to be disturbed. There are many investigations on the process and mechanism of lagged settlement in sandy cobble soils [4,5,6,7], but the research of stress distribution in disturbed zone and evolution of earth pressure on shield tunnel is still insufficient. In different phases of lagged settlement, the stress distribution in disturbed zone and earth pressure on tunnel are different and they need to be analyzed respectively. This paper aims to investigate the stress distribution characteristics in loosened sandy cobble stratum, which is related to the stability of soil stratum and mechanical control of shield excavation. For the earth pressure on tunnel, the evolution of lagged settlement in different phases including soil disturbance, formation of cavity, cavity moving upward and land subsidence are analyzed, and theoretical formulas of loosened earth pressure are derived respectively. Results are helpful for understanding the stress distribution in disturbed cobble soil, providing theoretical guidance for control of shield thrust and calculation of tunnel support
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