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Soil mechanics - Chapter 18

Nếu lý thuyết hợp nhất, trình bày trong các chương trước, là một mô tả hoàn hảo của các hành vi vật lý của đất, nó nên được khá đơn giản để xác định giá trị của hệ số cv hợp nhất các dữ liệu thu được trong một thử nghiệm hợp nhất. Ví dụ, người ta có thể đo thời gian đó 50% của sự biến dạng cuối cùng đã diễn ra. Từ lý thuyết sau này đạt được khi cv t/h2 = 0,197, bởi vì sau đó giá trị của U = 0,5, xem công thức (16,19). Khi. | Chapter 18 CONSOLIDATION COEFFICIENT If the theory of consolidation presented in the previous chapters were a perfect description of the physical behavior of soils it should be rather simple to determine the value of the coefficient of consolidation cv from the data obtained in a consolidation test. For instance one could measure the time at which 50 of the final deformation has taken place. From the theory it follows that this is reached when cvt h2 0.197 because then the value of U 0.5 see formula 16.19 . As the values of time t and the sample thickness h are known it is then possible to determine the value of cv . Unfortunately there are some practical and some theoretical difficulties. The procedure would require an accurate determination of the initial deformation and of the ultimate deformation and that is not so simple as it may seem. The initial deformation of the sample Aho is the deformation at the moment of application of the load and at the moment of loading the indicator of the deformation will suddenly start to move with a sudden jump followed by a continuous increase. It is difficult to decide what the value at the exact moment of loading is as the moment is gone when the indicator starts to move. Also it usually appears that no final constant value of the deformation AhTO is reached as the deformation seems to continue even when the pore water pressures have been dissipated completely. For these reasons somewhat modified procedures have been developed to define the initial deformation and the final deformation. In this chapter the two most common procedures are presented. 18.1 Log t -method A first method to overcome the difficulties of determining the initial value and the final value of the deformation has been proposed by Casagrande. In this method the deformation of the sample as measured as a function of time in a consolidation test is plotted against the logarithm of time see Figure 18.1. It usually appears that there is no horizontal .