Question

Variation of U, with T, and z/Hdr Proposed fill y 19.2 kN/m3 3,0 m 0 Compressibility of Soil teh 2.0 Y -18.5 kN/m3-; 1.5m-U Fine to medium sand (SP) 2.0 m 2 1.5 0.7 0.8 3.0 m 0.9 4.0m Soft clay (CL) γ 16.0 kN/m3 0.5 10.0m (4 3 batto oLthe soil specin 0 0.1 023 04 03 06 7 08 09 10 Degree of eomsolidasion,U Example: Degree of consolidation Dense sand and gravel The proposed sand fill is 3 m deep, with a unit weight equal to Y-19.2 kN/m3. Consider the soft clay strata. According to a laboratory test Cy 0.0021 m2/day, compute the hydrostatic, excess and total pore water pressure at point B, 2000 days after placement of the fill.

Answer 1

Initial excess hydrostatic pressure is equal to Increase in effective stress due to fill i.e

$u_{i}=\Delta \sigma =19.2\times 3$

= 57.6 kN/m²

We know that

$T_{v}=\frac{c_{v}t}{d^{2}}$

where T_v = time factor

c_v = 0.0021 m²/day

t = 2000 days

d = drainage path = 10/2 = 5 m

Putting values

T_v = 0.0021x2000/5²

= 0.168

Degree of Consolidation (U) for point B is calculated from graph

For point B

z = 3 , H = 5

z/H = 0.6 and T_v = 0.168

We get

U = 0.31

Also

U = 1 - u_z/u_i

where u_z = excess pore pressure remaining

u_i = initial excess pore pressure = 57.6 kN/m²

0.31 = 1 - u_z/57.6

u_z = 39.744 kN/m²

Also pore pressure at point B = u = 9.81x5 = 49.05 kN/m²

There total pore pressure at point B after 2000 days = u + u_z

= 49.05 + 39.744 = 88.794 kN/m²