Question

The soil profile at a site consists of a clay layer 10 m thick, sandwich between two sand layers. The top sand layer has a density of 1.9 Mg/m3 and is 5 m thick. Below this layer is a 10 m thick clay layer with a saturated density of 1.8 Mg/m3 and a saturated water content 42%. The deeper sand layer has a saturated density of 1.9 Mg/m3 and extend to the depth investigated at the site. The water table was detected at the top of the clay layer.

The results of a consolidation test performed on a sample recovered from the mid point of the clay layer indicated the following properties for the clay:

Effective pre-consolidation pressure of 130 kPa;

Compression index Cc = 0.40;

Re-compression index Cr = 0.03; and

Consolidation coefficient Cv = 0.544 * 10-2 cm2/sec.

The site is being proposed to be graded by compacting an additional layer.

The new compacted layer will impose a total stress increment of 75 kPa on the present ground surface. It is desired to obtain the following information:

Estimate the ultimate consolidation settlement due to grading;

Estimate the time in weeks that it will take to reach the average degree of consolidation Uave = 90%; and

At the time calculated in 2.), estimate the excess pore pressure still present at the mid-point of the clay layer.

Answer 1