Analysis Of Kaolin Clay

:

Consolidation

1 Problem Statement

A kaolin clay which is used in laboratory tests has the following properties:

 Plastic limit: 30%

 Liquid limit: 70%

 Compression index: 0.53

 Void ratio of normally consolidated kaolin under 50kPa effective overburden pressure: 1.75

 Specific gravity of soil grains: 2.70

 Coefficient of consolidation: 2m2/yr

(a) As part of a laboratory experiment, it is desired to prepare a clay layer with a target thickness of 1m by re-consolidation from clay slurry. To do this, clay slurry is first made by mixing clay powder with water to a water content of twice its liquid limit. The slurry is then poured into a cylindrical consolidation tank, which is lined at the top and bottom by thin layers of sand for drainage

(b) In the second stage of the preparation process, it is desired to create a vertical effective stress profile in the kaolin clay layer which varies from 10kPa at the top to 80kPa at the bottom, see Figure 2. This is to be achieved by applying appropriate level of vacuum suction (i.e. negative pore pressure) to the bottom sand layer, while the top sand layer is kept saturated and submerged under a thin layer of water, as shown in Figure 2. During this stage, the 10kPa surcharge pressure at the top of the model is to be maintained. The self-weight of the sand layers can be neglected.

(i) Sketch, showing significant values, the effective stress profile after the first stage; that is after the surcharge pressure is applied and completion of consolidation, but before vacuum is applied to the bottom sand layer.

(ii) Hence, or otherwise, determine the vacuum suction (i.e. negative pressure) which needs to be applied to the bottom sand layer.

(iii) Assuming that the vacuum is applied instantaneously, sketch the pore pressure profile in the clay layer just after the application of the vacuum and a long time after vacuum application (i.e. after excess pore pressure has