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Casagrande’s Logarithm of Time Fitting Method
From the oedometer test (explained in Lesson 23) the dial reading (settlement) corresponding to a particular time is measured. From the measured data, dial reading vs time graph can be drawn (as shown in Figure 24.2). Select two points t_{1} and t_{2} in initial part of the curve such that t_{2 }= 4 t_{1}. The points corresponding to the chosen times are marked on the curve. The vertical distance (z) between the two points on the curve is measured. Select another point R_{0} such that the vertical distance between that point and point on the curve corresponding to the t_{1 }time is also z. R_{0 } is corrected zero reading i.e U = 0%. Determine the U=100% line by drawing two tangents form the straight portion of the curve as shown in Figure 24.2. Once U=0% and 100% lines are identified, U= 50% line is also determined by choosing the middle point between the U=0% and 100% lines. Time (t_{50}) corresponding to the 50% degree of consolidation is determined from the curve. The Coefficient of consolidation (c_{v}) is determined from Eq.(24.1).
Fig. 24.2. Casagrande’s Logarithm of Time Fitting Method.
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