At the site of a large scale land reclamation project 2 m of sand overlie a silty clay layer 5 m thick, which overlies highly fractured permeable rock. The project involved rapidly placing 5 m of fill over a large area that applied a stress of 100 kPa to the underlying soil. It was initially expected that after 2 years consolidation would be complete. However, after 2 years the excess pore pressures in the silty clay layer were still significant and had the values shown in the table below.
Depth below
top of clay layer (m) Excess pore pressure(kPa)
0 0
1 25
2 40
3 50
4 40
5 25
6 0
To speed up consolidation it is proposed to place additional fill, starting at t = 2 years, so that the stress on the clay increases at a rate of 10kPa/month for 6 months, at which time all the additional load will be removed.
a.Assuming that consolidation is occurring under 1-Dimensional conditions show that the pore pressures measured at 2 years are consistent with a cv of approximately 1.7 m2/yr.
b.Use a numerical method to estimate the excess pore pressure distribution in the clay layer between 2 years and 2.5 years from the start of the land reclamation, and hence determine the settlement at 2.5 years. Use ?t = 2 months, ?z = 1 m, and take mv = 0.0001 m2/kN.