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The following data were obtained from a consolidation test on an undisturbed clay sample: 01 = 140 kPa, el = 0.912 o2 = 280 kPa, e2 = 0.749 The average value of the coefficient of permeability of the clay in this pressure increment range is 9.2 x 10-8cm/s. Compute and plot the decrease in thickness with time for a 12 m layer of this clay which is drained (a) on the upper surface only and (b) on the upper surface, and at a depth of 2.5 m by a thin horizontal sand layer that provides free drainage. (After A. Casagrande.)