Settlement Analysis for a Clay Layer Draining from Top and Bottom:

The settlement analysis for a clay layer draining from top and bottom shows a settlement of 2.5 cm in 4 years and an ultimate settlement of 10 cm. However, detailed subsurface investigation reveals that there is no drainage at the bottom. We need to determine the ultimate settlement in this condition.

Understanding Settlement:

Settlement refers to the downward movement of the ground or a structure. It occurs due to various factors such as consolidation of soil, shrinkage of clay, and the load applied to the ground or structure.

Settlement in Clay Layers:

Clay soils are highly compressible and undergo settlement when subjected to loads. Settlement in clay layers can be classified into primary and secondary settlements.

Primary settlement occurs due to the consolidation of the clay layer under the applied load. It can be calculated using the Terzaghi's one-dimensional consolidation theory. The settlement is directly proportional to the intensity of the applied load and the coefficient of consolidation of the clay.

Secondary settlement occurs due to the compression and reorientation of the clay particles. It is usually smaller in magnitude compared to primary settlement and takes place over a longer period.

Settlement Analysis:

In the given scenario, the settlement analysis shows a settlement of 2.5 cm in 4 years and an ultimate settlement of 10 cm. This settlement is likely a combination of primary and secondary settlements.

No Drainage at the Bottom:

When there is no drainage at the bottom of the clay layer, the excess pore water pressure cannot dissipate freely. As a result, the consolidation process is hindered, and the settlement takes place at a slower rate.

In this condition, the primary consolidation settlement will continue to occur, but the secondary settlement will be significantly reduced or even negligible. This is because the clay particles cannot reorient and compress as efficiently without drainage at the bottom.

Ultimate Settlement:

The ultimate settlement refers to the settlement that occurs after a long period when the consolidation process is complete. In this case, since there is no drainage at the bottom, the primary consolidation settlement will continue to occur but the secondary settlement will be minimal.

Therefore, the ultimate settlement in this condition will be equal to the settlement observed in the settlement analysis, which is 10 cm.

Conclusion:

In summary, when there is no drainage at the bottom of a clay layer, the ultimate settlement will be equal to the settlement observed in the settlement analysis. In this case, the settlement analysis showed a settlement of 2.5 cm in 4 years and an ultimate settlement of 10 cm, therefore the correct answer is option 'C' - 10 cm.

If we change the system of consolidation from 2way drainage to 1way drainage the time required for attain ultimate settlements in 1-way drainage system is increases 4times of time required in 2-way drainage system.but the magnitude of ultimate settlements is same in the both systems.ultimate settlements are depends upon only soil.