土的压缩性及固结理论Word格式.docx

1、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 de

2、formation 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 ind

3、uced 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

4、，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；（3） 孔隙水和孔隙气体的排出。由于纯水的弹模约为2106kPa，固体颗粒的弹模为9l 07kPa，土粒本身和孔隙中水的压缩量，在工程压力（100600kPa）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

5、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 compre

6、ssion 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 terzaghis 1D consolidation theo

7、ryReasons of volumetric reduction of soil mass1 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 wate

8、r 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

9、; 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 compressibili

10、ty 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

11、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

12、 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 m

13、etal 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 vert

14、ically）: 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

15、 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

16、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

17、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 calcul

18、ating 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） So

19、il 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. sh

20、ows a saturated soil specimen after compression at effective stress p1. assume the height of the soil specimen is h, the volume of soil particle Vs, the corresponding void ratio e1,then the pore volume is vs and the total volume V1 is（1+e1） Vs. if the effective stress is increased to p2 equal to （p1

21、+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:而公式中分别为土粒比重，初始含水量，初始密度和水的密度。因此只要测的的稳定压缩量就可按上式算得相应的孔隙比，从而绘制土的压缩曲线。Wher

22、e, are the specific gravity of solids, initial water content, initial density of soil and density of water. So that we can calculate the void ratio in stable state after compression/consolidation from the dial gauge readings and draw the compression curve. 压缩曲线可按两种方式绘制。e-p 曲线 e-logp曲线e-p曲线可确定土的压缩系数，

23、压缩摸量等指标，e-logp曲线可确定土的压缩指数等压缩性指标。压缩系数（compression coefficient）的定义为“曲线上任意两点割线的斜率”。 From the curve e-p, the coefficient of consolidation and the compression modulus can be determined. It is defined as the change of void ratio over unit pressure increment, the slope of two points in the curve e-p.式中负号表示

24、随着压力p的增加，e逐渐减少。压缩性不同的土，其压缩曲线的形状是不一样的。曲线愈陡，说明随着压力的增加，土孔隙比的减小愈显著，因而土的压缩性愈高。a1-2 0.1MPa-1时，低压缩性土（low compressible soil）0.1a1-20.5MPa -1时，中压缩性土（intermediate compressible soil）a1-2 0.5MPa -1时，高压缩性土（high compressible soil） 自重应力p1增加到外荷作用土中应力p2 （自重与附加应力之和） In above equation, - means the void ratio decrease

25、with the increase of the pressure p. For soils having different characteristic of compressibility, the curves of compression are different too. If the slope is large, it means the void ratio change is remarkable, and the soil is high compressible.压缩指数（compression index）：e-lgp 坐标系统中的曲线上直线的斜率 （slope o

26、f e-lgp curve）Cc是无量纲系数，同压缩系数a一样，压缩指数Cc值越大，土的压缩性越高。虽然压缩系数a 和压缩指数Cc 都是反映土的压缩性指标，但两者有所不同。前者随所取的初始压力及压力增量的大小而异，而后者在较高的压力范围内却是常量，不随压力而变。 卸载段和再加载段的平均斜率称为土的回弹指数Ce,而CeCc。一般粘性土的Cc值在1.0左右，Ce值在（0.10.2） Cc之间。Same as the coefficient of compression a, the larger the value of Cc, the steeper the compression curve

27、and the higher the soil compressibility will be. Although both the coefficient of compression a and the compression index are indicators of soil compressibility, they are actually different in the following aspects. The former varies with the initial pressure and the pressure increment applied, wher

28、eas the later is essentially a constant within a majority range of the applied load.压缩模量（compression modulus）：土体在完全侧限的条件下，竖向应力增量与竖向应变增量的比值。（The ratio of vertical stress increment over vertical strain increment with no lateral strain） 由右图可知From the figure, 且， 故有， 由此可推得侧限条件下的应力应变模量为（the compression mo

29、dulus can be given as）： 土的压缩模量越小，土的压缩性越高。Es的倒数成为土的体积压缩系数mv（coefficient of volume compressibility），它表示单位压应力变化引起的单位体积变化（MPa-1） 与土的压缩系数，土的压缩指数一样，体积压缩系数植越大，土的压缩性越高。 The compression coefficient av, the compression index Cc, the coefficient of volume compressibility mv, the compression modulus Es, and the

30、 deformation modulus E are all indicators of the compression characteristics of soil. These parameters can be used for the settlement calculation, although they have different meanings.压缩曲线特征：土体变形机理非常复杂，不是理想的弹塑性体，而是具弹、粘、塑性的自然历史的产物（1）卸荷时，试样不是沿初始压缩曲线，而是沿曲线bc回弹，可见土体的变形是由可恢复的弹性变形和不可恢复的塑性变形两部份组成。（2）回弹曲线和

31、再压线曲线构成一迴滞环，土体不是完全弹性体；（3）回弹和再压缩曲线比压缩曲线平缓得多。（4）当再加荷时的压力超过b点，再压缩曲线就趋于初始压缩曲线的延长线.Typical plots of void ratio （e） after consolidation, against effective stress （p） for a saturated clay, are shown in the Fig., showing an initial compression followed by expansion and recompression, namely a loading-swelling-reloading sequence. During loading, the soil is compressed and becomes more dense, and therefore stiffer, so that the e-p relationship is curved and getting less steep. The shapes of the curves are related to the stress history of the clay. During compression, c