Q. Settlement due to Reduction in Volume of Soil?
The weight of the structure causes reduction in volume of soil due to expulsion of pore water from the voids. This reduces the void ratio, maintaining same volume of solid particles. The effect of this decrease in volume will result in vertical downward movement of the superstructure.
The compressible soil layer under the foundation of the structure is stressed upto a large depth due to the weight of the structure. The stress will be maximum at the foundation level which will gradually decrease with depth. Hence, any compressible structure lying within the depth of the stressed zone will undergo settlement.
Settlement on sandy soil is usually less and whatever settlement has to take place occurs at fast rate. In case of cohesive soil, the settlement is usually more and the rate of settlement is also very much prolonged.