软土地层管幕-箱涵顶进工具管网格自平衡设计理论研究

目前,国外在软土地区箱涵顶进施工中均对箱涵顶进工具管开挖面前的土体进行加固,以保证在箱涵高度范围内的土体能够自立稳定。为降低工程造价,上海市中环线虹许路—北虹路地下立交隧道工程是世界上首次在软土地区采用不加固软土方法进行管幕-箱涵顶进施工的工程,为维持开挖面土体平衡,工具管网格的合理设计是工程成败的关键。以此工程为背景,从力学原理出发,利用网格内土体在稳定的前提下摩擦力与土压力始终平衡——自平衡原理,建立平衡微分方程,推导出工具管内土压力、土-钢板接触面正应力、剪应力随工具管长度的分布及衰减规律——指数方程,由此方程可解出维持开挖面稳定所需的工具管网格临界长度,并建立网格的长、宽和高之间应满足的定量关系。影响网格长度的主要因素有网格前的土压力、土与网格壁的摩擦特性和土体性质等,分析表明,网格临界长度随土压力的增大而增大,随摩擦系数的增大而减小,为对数方程,所建立的公式可为工程设计提供理论依据。

STUDY ON SELF-BALANCE DESIGN THEORY OF CULVERT GRID ADVANCED IN PIPE-ROOF IN SATURATED SOFT SOIL LAYERS

At present,during the construction of advancing culvert box at home and abroad,the soil in front of box should be reinforced to retain stability. The tunnel of Shanghai Middle Ring Line located in the intersection of Hongxu Road and Beihong Road will be the first case to adopt pipe roof culvert box advancing scheme without reinforcing the saturated soft soil layer to decrease project cost. In order to sustain the stability of surface and control the ground settlement during the construction,the reasonable design of girding will be crucial. Based on the principle of mechanics and some hypothesis,this paper takes advantage of the balance between friction and earth pressure in grid,and presents a kind of self-balance design theory. A differential equation is set up and the distribution and attenuation rules of soil pressure,normal stress,shear stress and etc. are obtained. These rules of stress distribution and attenuation conform to a series of exponential equations. In addition,the critical length of grid and the relations among length,width and height have been obtained from the equations. Also,the main factors affecting the critical length of grid are analyzed from these equations and are obtained as follows:the soil pressure in front of grid;frictional property between soil and grid wall;soil friction angle and cohesion. Analysis indicates that the critical length is increased as the soil pressure increasing and decreased as the friction factor increasing. These equations and results can be regarded as academic foundation for grid design for soft soil layers.