考虑坑壁位移和土压力方向的直撑力学特性

宋明健1, a 汤连生1, b 胡辉2 王大志1, b 徐通1, b
（ 1. 中山大学 a. 工学院 b. 地球科学系广州 510275）（2. 暨南大学土木工程学院广州 510632）

摘要：鉴于基坑支护中常见的直撑设计一般都没有考虑基坑位移和土压力的变化，文中首先总结了土压力和基坑位移之间非线性关系的研究现状，分析了随坑壁转动时土压力大小和方向的变化规律，在此基础上，推导了直撑轴力和坑壁位移之间、以及直撑纵向变形与坑壁位移之间的数学关系式。结合工程实例，进一步研究了直撑轴力和变形随基坑初始位移、支撑点、土压力作用点、土体与挡土结构间摩擦角等因素变化时的发展变化规律。研究表明，直撑轴力和变形随基坑位移、支撑点高度、土体与挡土结构之间摩擦角的增加而减小，随土压力作用点高度的增加而增大，与基坑初始位移值基本无关。
关键词：支撑；直撑；基坑；土压力；位移
中图分类号：TU 443 文献标识码：A

Mechanical Characteristics of Straight Brace Considering Displacement of Foundation pit and Direction of Earth Pressure

SONG Ming-jian1, a TANG Lian-sheng1, b HU Hui2 WANG Da-zhi1, b XU Tong1, b
(1. Sun Yat-sen University a. School of Engineering b. Department of Earth Sciences Guangzhou 510275)
(2. College of Civil Engineering, Jinan University Guangzhou 510632)

Abstract: It is generally realized that earth pressure will change with the deformation and displacement of foundation pit. But at present the design of straight brace does not almost consider earth pressure changing with displacement of foundation pit. Firstly, the researches and achievements on the nonlinear relation of earth pressure and the displacement of foundation pit are summarized. Secondly, the rules on earth pressure changing with the displacement and movement of the side of foundation pit are discussed. On the basis of the present researches and the change rules on earth pressure changing with displacement of foundation pit, new formulas of axis force and deformation of straight brace considering the rotation or movement of the side of foundation pit are presented. At last, the curves of axis force and deformation of straight brace changing with some factors are obtained. Those factors include different initial displacement of foundation pit, supporting location of straight brace, friction angle of soil and retaining structure, resultant force action point of earth pressure. Conclusions can be drawn as follows from these curves: axis force and deformation of straight brace will diminish with the increase of displacement of foundation pit, supporting location height of straight brace or friction angle of soil and retaining structure. The height enhancement of resultant force action point of earth pressure will improve axis force and deformation of straight brace. But the initial displacement of the side of foundation pit has no influence on axis force and deformation of straight brace.
Key words: brace; straight brace; foundation pit; earth pressure; displacement

1 引言
基坑现有支撑多为直线形式，简称直撑，如图1。直撑多按极限平衡条件和经典土压力理论进行设计计算[1-3,5]，经典土压力假定坑壁不发生位移、支撑材料绝对刚性、土压力大小和方向保持不变。根据以上假设，以单支撑为例（多支撑存在类似问题），土压力的作用方向始终沿直撑长度方向，直撑长度l在施工前后保持不变，如图2、图4所示。且由等值梁法可知，直撑两端的轴力恒等于，为坑后极限土压力，为作用点距弯矩零点的距离，为支撑点距弯矩零点的距离。
实际工程中，坑壁位置和支撑长度均并非一成不变[3,4]。在安设直撑之前，由于岩土的流变特性和施工扰动，支撑处坑壁已产生了一定的位移x0，如图3所示。直撑安装并发挥作用后，由于材料的弹塑特性，支撑必然会产生一定的压缩变形ε，导致支撑处坑壁位移增大至x1（）。压缩变形ε和坑壁位移x1将持续增大，直到坑壁对直撑的挤压作用和直撑对坑壁的抵抗作用相当，即“支撑－基坑”系统达到稳定平衡状态后，直撑将不再被压缩、坑壁不再发生位移。另外，在坑壁发生位变的过程中，土压力大小也在改变[3-20]，从而支撑能力，以及支撑两端的轴力，也不是如支护经典设计理论所描述的为常量，而是在不断地变化。
因此，结合土压力随位移变化而变化的研究成果，考虑坑壁位移和支撑材料本身的压缩特性，对直撑的受力和变形特征进行分析，对基坑及其支护系统的设计施工将具有很好的参考意义。

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考虑坑壁位移和土压力方向的直撑力学特性(一)