Module 5. CONSOLIDATION (Lectures 27 to 34)

Transcription

1 Module 5 CONSOLIDATION (Lectures 27 to 34) Topics 1.1 FUNDAMENTS OF CONSOLIDATION General Concepts of One-dimensional Consolidation Theory of One-Dimensional Consolidation Relations of and for Other Forms of Initial Excess Pore Water Pressure Distribution Numerical Solution for One-Dimensional Consolidation Consolidation in a layered soil Degree of Consolidation under Time-Dependent Loading Standard One-Dimensional Consolidation Test and Interpretation Preconsolidation pressure. Compression index Effect of sample disturbance on the e vs. log cirve Calculation of one-dimensional consolidation settlement Calculation of coefficient of consolidation from laboratory test results Logarithm-of-time method Square-root-of-time method Su s maximum slope method Sivaram and Swamee s computational method Secondary Consolidation Constant Rate-of-Strain consolidation Tests Coefficient of consolidation Interpretation of experimental results Constant-Gradient Consolidating Test Interpretation of experimental results One-Dimensional Consolidation with Visoelastic Models 1.2 CONSOLIDATON BY SAND DRAINS Dept. of Civil Engg. Indian Institute of Technology, Kanpur 1

2 1.2.1 Sand Drains Free-Strain Consolidation with no Smear Equal-Strain Consolidation with no Smear Effect of Smear Zone on Radial Consolidation Calculation of the Degree of Consolidation with Vertical and Radial Drainage Numerical Solution for Radial Drainage PROBLEMS Dept. of Civil Engg. Indian Institute of Technology, Kanpur 2

3 Module 5 Lecture 27 Consolidation-1 Topics 1.1 FUNDAMENTS OF CONSOLIDATION General Concepts of One-dimensional Consolidation Theory of One-Dimensional Consolidation According to Terzaghi (1943), a decrease of water content of a saturated soil without replacement of the water by air is called a process of consolidation. When saturated clayey soils-which have a low coefficient of permeability-are subjected to a compressive stress due to a foundation loading, the ore water pressure will immediately increase; however, due to the low permeability of the soil, there will be a time lag between the application of load and the extrusion of the pore water and, thus, the settlement. This phenomenon is the subject of discussion of this chapter. 1.1 FUNDAMENTS OF CONSOLIDATION General Concepts of One-dimensional Consolidation To understand the basic concepts of consolidation, consider a clay layer of thickness located below the groundwater level and between two highly permeable sand layers as shown in Figure 5.1. If a surcharge of intensity is applied at the ground surface over a very large area, the pore water pressure in the clay layer will increase. For a surcharge of infinite extent, the immediate increase of the pore water pressure,, at all depths of the clay layer will be equal to the increase of the total stress,. Thus, immediately after the application of the surcharge. Figure 5.1 Dept. of Civil Engg. Indian Institute of Technology, Kanpur 3

4 Since the total stress is equal to the sum of the effective stress and the pore water pressure at all depth soft the clay layer the increase of effective stress due to the surcharge (immediately after application) will be equal to zero (i.e., where is the increase of the effective stress). In other words, at time t = 0, the entire stress increase at all depths of the clay is taken by the pore water pressure and none b y the soil skeleton. This is shown in Figure 5.2a. (It must be pointed out that, for loads applied over a limited area, it may to be true that the increase of the pore water pressure is equal to the increase of vertical stress at any depth at time t = 0. Figure 5.2 Change of pore water pressure and effective stress in the clay layer shown in Figure 5. 1 due to the surcharge After application of the surcharge (i.e., at time ), the water in the void spaces of the clay layer will be squeezed out and will flow toward both the highly permeable sand layers, thereby reducing the excess pore water pressure. This, in turn, will increase the effective stress by an amount since. Thus, at time, And This fact is shown in Figure 5.2b. Theoretically, at time the excess pore water pressure at all depths of the clay layer will be dissipated by gradual drainage. Thus, at time, Dept. of Civil Engg. Indian Institute of Technology, Kanpur 4

5 And This shown in Figure 5.2c. This gradual process of increase of effective stress in the clay layer due to the surcharge will result in a settlement which is time-dependent and is referred to as the process of consolidation Theory of One-Dimensional Consolidation The theory for the time rate of one-dimensional consolidation was first proposed by Terzaghi (1925). The underlying assumption in the derivation of the mathematical equations are as follows: Figure 5.3 Clay layer undergoing consolidation 1. The clay layer is homogeneous. 2. The clay layer is saturated. 3. The compression of the soil layer is due to the change in volume only, which, in turn, is due to the squeezing out of water from the void spaces. 4. Darcy s law valid. 5. Deformation of soil occurs only in the direction of the load application. 6. The coefficient of consolidation [equation (15)] is constant during the consolidation. With the above assumptions, let us consider a clay layer of thickness as shown in Figure 5.3. The layer is located between two highly permeable sand layers. In this case of one-dimensional consolidation, the flow of water into and out of the soil element is in one direction only, i.e., in the z direction. This means that are equal to zero, and thus the rate of low into and out of the soil element can be given by: (1) Where (2) we obtain Dept. of Civil Engg. Indian Institute of Technology, Kanpur 5

6 (3) Where is the coefficient of permeability [k= ]. However, (4) where is the unit weight of water. Substitution of equation (4) and (3) and rearranging gives During consolidation the rate of change of volume is equal to the rate of change of the void volume. So, (5) (6) Where is the volume of voids in the soil element. But (7) Where is the volume of soil solids in the element, which is constant, and is the void ratio. So, (8) Substituting the above relation into equation (5), we get (9) The change in void ratio, related, then, is due to the increase of effective stress; assuming that these are linearly (10) Combining equations (9) and (11), (12) Where (13) Or (14) Where (15) Equation (14) is the basic differential equation of Terzaghi s consolidation theory and can be solved with proper boundary conditions. To solve the equation, assume u to be the product of two functions, i.e., the product of a function of z and a function of t, or So, (17) (16) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 6

7 And (18) From equations (14), (17), and (18), or The right-hand side of equation (19) is a function of z only and is independent of t; the left-hand side of the equation is a function of t only and is independent of z. therefore, they must be equal to a constant, say-. So, A solution to equation (20) can be given by Where and are constants. Again, the right-hand side of equation (19) may be written as The solution to equation (22) is given by Where is a constant. Combining equations (16), (21), and (23), (19) (20) (21) (22) (23) Where. The constants in equation (24) can be evaluated from the boundary conditions, which are as follows: 1. At time (initial excess pore water pressure at any depth) Note that H is the length of the longest drainage path. In this case, which is two-way drainage condition (top and bottom of the clay layer), H is equal to half the total thickness of the clay layer,. (24) The second boundary condition dictates that, and from the third boundary condition we get Where n is an integer. From the above, a general solution of equation (24) can be in given the form (25) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 7

8 Where is the nondimensional time factor and is equal to To satisfy the first boundary condition, we must have the coefficients of such that (26) Equation (26) is a Fourier sine series, and can be given by (27) Combining equations (25) and (27), (28) So far we have not made any assumptions regarding the variation of Several possible types of variation for are considered below. with the depth of the clay layer. Constant with depth. if is constant with depth i.e., if (Figure 5.4) referring to equation (28), So, (29) Figure 5.4 Initial excess pore water pressure-constant with depth (double drainage) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 8

9 Note that the term in the above equation is zero for cases when n is even; therefore, u is also zero. For the nonzero terms, it is convenient to substitute where m is an integer. So equation (29) will no read (30) Where. At a given time, the degree of consolidation at any depth z is defined as Where is the increase of effective stress at a depth z due to consolidation. From equations (30) and (31), Figure 5.5 shows the variation of with depth for various values of the non-dimensional time factor, ; these curves are called isocrones. (31) (32) Figure 5.5 Variation of with and In most cases, however, we need to obtain the average degree of consolidation for the entire layer. This is given by (33) The average degree of consolidation is also the ratio of consolidation settlement at any time to maximum consolidation settlement. Note, in this case, that. Combining equations (30) and (33), Dept. of Civil Engg. Indian Institute of Technology, Kanpur 9

10 (34) Figure 5.6 gives the variation of (also see table 1) Figure 5.6 Variation of average degree of consolidation (for conditions given in figs. 4, 7, 8, and 9) Terzaghi suggested the following equations for to approximate the values obtained from equation (34): For (35) For (36) Sivaram and Swamee (1977) gave the following equation for varying from 0 to 100%: (37) Or (38) Equations (37) and (38) give an error in of less than 1% for 0% and less than 3% for 90%. Table 1 Variation of [equation (34) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 10

11 It must be pointed out that, if we have a situation of one-way drainage as shown in Figure 5.7a and b, equation (34) would still be valid. Note, however, that the length of the drainage path is equal to the total thickness of the clay layer. Figure 5.7 Initial excess pore pressure distribution-one way drainage, constant with depth Linear variation of. The linear variation of the initial excess pore water pressure, as shown in Figure 5. 8, may be written as (39) Substitution of the above relation for into equation (28) yields Figure 5.8 linearly varying initial excess pore water pressure distribution-two-way drainage Dept. of Civil Engg. Indian Institute of Technology, Kanpur 11

12 Figure 5.9 Sinusoidal initial excess pore water pressure distribution-two-way drainage The average degree of consolidation can be obtained by solving equations (40) and 33): (40) This is identical to equation (34), which was for the case where the excess pore water pressure is constant with depth, and so the same curves as given in Figure 5.6 can be used. Sinusoidal variation of. Sinusoidal variation (Figure 5.9) can be represented by the equation The solution for the average degree of consolidation for this type of excess pore water pressure distribution is of the form The variation of for various values of is given in Figure 5.6. (41) (42) Dept. of Civil Engg. Indian Institute of Technology, Kanpur 12