Casagrande’s Logarithm of Time Fitting Method- Determination of Coefficient of Consolidation of Soil - Notes | Study Soil Mechanics Notes- Agricultural Engineering - Agricultural Engineering

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 t₁ and t₂ in initial part of the curve such that t₂ = 4 t₁. 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 R₀ such that the vertical distance between that point and point on the curve corresponding to the t₁ time is also z. R₀  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 (t₅₀) 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.