土的压缩性及固结理论

第4章土的压缩性及固结理论

基本内容

这是本课程的重点。在学习土的压缩性指标确定方法的基础上，掌握地基最终沉降量计算原理和地基固结问题的分析计算方法。

学习要求：

1. 掌握土的压缩性与压缩性指标确定方法；

2．掌握有效应力原理；

3．掌握太沙基一维固结理论；

4.1 概述(outline)

土在自重应力或附加应力作用下，地基土要产生附加变形，包括体积变形和形状变形。对于土来说，体积变形通常表现为体积缩小。我们把这种在外力作用下土体积缩小得特性称为土的压缩性(compressibility)。

It is well recognized that the deformations will be induced in ground soil under self-weight or net contact pressure. The load-induced soil deformations can be divided into volumetric deformation and deviatoric deformation (namely, angular distortion or deformation in shape). The volumetric deformation is mainly caused by the normal stress, which compact the soil, resulting in soil contraction instead of soil failure. The deviatoric deformation is caused by the shear stress. When the shear stress is large enough, shear failure of the soil will be induced and soil deformation will develop continuously. Usually shear failure over a large area is not allowed to happen in the ground.

土的压缩性主要有两个特点：

（1）土的压缩性主要是由于孔隙体积减少而引起的；

（2）由于孔隙水的排出而引起的压缩对于饱和粘土来说需要时间，将土的压缩随时间增长的过程称为土的固结。

在建筑物荷载作用下，地基土主要由于压缩而引起的竖直方向的位移称为沉降。

研究建筑物沉降包含两方面的内容：

一是绝对沉降量的大小，亦即最终沉降；

二是沉降与时间的关系，主要介绍太沙基的一维固结理论

土体产生体积缩小的原因：

(1)固体颗粒的压缩；

(2)孔隙水和孔隙气体的压缩，孔隙气体的溶解；孔隙水和孔隙气体的排出。由于纯水的弹模约为2×106kPa，固体颗粒的弹模为9×l 07kPa，土粒本身和孔隙中水的压缩量，在工程压力(100～600kPa)范围内，不到土体总压缩量的1/400，因此常可略不计。所以，土体压缩主要来自孔隙水和土中孔隙气体的排出。孔隙中水和气体向外排出要有一个时间过程。因此土的压缩亦要一段时间才能完成。把这一与时间有关的压缩过程称为固结。

土体的变形计算，需要取得土的压缩性指标，可以通过室内侧限压缩试验和现场原位试验得到。

室内压缩试验亦称固结试验，是研究土压缩性最基本的方法。

现场载荷试验是在工程现场通过千斤顶逐级对置于地基土上的载荷板施加荷载，观测记录沉降随时间的发展以及稳定时的沉降量s，并绘制成p-s曲线，即获得地基土载荷试验的结果。

反映土的压缩性的指标主要有压缩系数、压缩模量、压缩指数和变形模量。土的压缩性的高低，常用压缩性指标定量表示，压缩性指标，通常由工程地质勘察取天然结构的原状土样进行. Characteristic of soil compression

(1)Compression of soil is mainly due to the decrease of void volume.

(2)The compression for a clay increases with the times (consolidation)

Ground soil will deform vertically due to structure load. The contents on studying structure settlement include

1 The absolute settlement (final settlement)

2 Relationship between settlement and time. Introducing terzaghi’s 1D consolidation theory Reasons of volumetric reduction of soil mass

1 The compressive deformation of the soil particles.

2 The compressive deformation of the pore water and air. The partial discharge of the pore water and air.

The consolidation process of saturated soils is in effect a process of discharge of the pore water and corresponding reduction of the pore volume. For saturated sands, pore water is apt to discharge under pressure due to high permeability; hence the consolidation process completes in a short length of time. For saturated clays, pore water discharges slowly under pressure due to low permeability; hence the consolidation process completes in a long length of time.

To calculate the deformation of the soil mass, it is necessary to know the compression indexes. These indexes can be obtained from laboratory compression test (consolidation test) and field load tests.

4.2 土的压缩性（soil compressibility charateristic）

4.2.1 固结试验及压缩性指标(Oedometer test, Consolidation test and Compression indexes) 侧限压缩试验亦称固结试验。所谓侧限就是使土样在竖向压力作用下只能发生竖向变形，而无侧向变形。

室内压缩试验采用的试验装置为压缩仪或固结仪(参照图4-1)。试验时将切有土样的环刀置于刚性护环中，由于金属环刀及刚性护环的限制，使得土样在竖向压力作用下只能发生竖向变形，而无侧向变形。在土样上下放置的透水石是土样受压后排出孔隙水的两个界面。压缩过程中竖向压力通过刚性板施加给土样，土样产生的压缩量可通过百分表量测。常规压缩试验通过逐级加荷进行试验，常用的分级加荷量p为：50kPa,100kPa,200kPa,300kPa,400kPa。

Compression test with zero lateral strain is also called Oedometer test. In test, there is vertical deformation but no lateral deformation under vertical load.

The characteristic of a soil during one-dimensional compression can be determined by means of the oedometer test (see Fig.4-1). The test specimen (2 cm high and a diameter to height ratio of 2.5) is in the form of a disc, held inside a metal ring and lying between two porous stones. The upper porous stone, which can move inside the ring with a small clearance, is fixed below a metal loading cap through which pressure can be applied to the specimen. The whole assembly sits in an open cell of water to which the pore water in the specimen has free access. The ring confining the specimen may be either fixed (clamped to the body of the cell) or floating (being free to move vertically): the inside of the ring should have a smooth polished surface to reduce side friction. The confining ring imposes a condition of zero lateral strain on the specimen, the ratio of lateral to vertical effective stress being K0, the coefficient of lateral earth pressure at rest. The compression of the specimen under pressure is measured by means of a dial gauge operating on the loading cap. Usually the specimen is gradually loaded, and the load grades are often set as p=50kPa,100kPa,200kPa,300kPa, 400kPa。It should be noted that the relationship between the void ratio and the effective pressure shown in fig. is not fixed for the same soil. It depends on the magnitude of the applied load and the length of the loading period in the standard oedometer test, each load is normally maintained for a period of 24 hours for a 2 cm thick clay to complete the compression.

如下图，为求得土样稳定后的孔隙比，利用土粒子体积不变和土截面不变的两个条件，可得出：The soil compression characteristic has been discussed in the last section. This section discusses further the calculation method of the magnitude of the soil compression under an effective stress increment. In the current engineering practice, the widely used method for calculating the foundation settlement is the one-dimensional consolidation method, which is established based on the calculation formulae of soil compression under zero lateral strain condition, namely unidirectional compression. The basic assumptions made for obtaining the calculation formulae are:

(1)Soil compression is fully the result of the deformation of soil skeleton due to reduction in pore volume.

The compression of soil particle is omitted;

(2)Deformation is only in the vertical direction, without

lateral strain;

(3)Stress is uniformly distributed along the height of the soil

layer.

Fig. shows a saturated soil specimen after compression at

effective stress p1. assume the height of the soil specimen is h,

the volume of soil particle V s, the corresponding void ratio

e1,then the pore volume is v s and the total volume V1 is(1+e1) V s.

if the effective stress is increased to p2 equal to (p1+△p),the

height of the soil specimen after compression is H2,

As shown in the below figures, because the volume of soil

particle and the soil cross section do not change, the void ratio

after compression can be calculated as follows: