Fig. 10.6 Typical consolidation test results.

The square root of time 'fitting' method

It will be appreciated that the curve described above is an actual consolidation curve and would not be obtainable from one of the theoretical curves of Fig. 10.4, which can only be used to plot the primary compression range. To evaluate the coefficient of consolidation it is necessary to establish the point C, representing 100 per cent primary consolidation, but it is difficult from a study of the test curve to fix C with accuracy and a procedure in which the test curve is 'fitted' to the theoretical curve becomes necessary.

A method was described by Taylor (1948). If the theoretical curve U against \f T is plotted for the case of a uniform initial excess pore pressure distribution, the curve will be like that shown in Fig. 10.7a. Up to values of U equal to about 60 per cent, the curve is a straight line of equation U= 1.13^/T, but if this straight line is extended to cut the ordinate U = 90 per cent the abscissa of the curve is seen to be 1.15 times the abscissa of the straight line. This fact is used to fit the test and theoretical curves.

With the test curve a corrected zero must first be established by projecting the straight line part of the primary compression back to cut the vertical axis at E (Fig. 10.6). A second line, starting through E, is now drawn such that all abscissas on it are 1.15 times the corresponding values on the laboratory curve, and the point at which this second line cuts the laboratory curve is taken to be the point representing 90 per cent primary consolidation (Fig. 10.7b).

To establish cv, T90 is first found from the theoretical curve that fits the drainage conditions (the curve m= 1); t90 is determined from the test curve:

rj. Cyt90

It is seen that the point of 90 per cent consolidation rather than the point for 100 per cent consolidation is used to establish cv. This is simply a matter

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