Soil Movement: Interaction between Soils and Buildings

When the water present between soil particles is removed, the latter will tend to move closer together: conversely, when water is absorbed, they will tend to move apart. Large movement can occur with clays, for these are capable of absorbing and relinquishing large quantities of moisture: drying leads to shrinkage and a gain in strength, and absorption to swelling and a loss in strength. Movement in sands is for the most part negligible, for they have little capacity to hold water. Silts have movement which lies between that of clays and sands. Peat can exhibit very large movement and has little bearing capacity. 

Changes in water content of soils may be caused in several ways. The most obvious is that caused when the soil is loaded by the weight of the foundations and the superimposed building. Water is then squeezed out of the soil and the soil particles move closer together. As the ground is compressed or consolidated in this way, the foundations settle, until equilibrium is achieved between the load imposed on the soil and the forces acting between its particles. 

The more clay there is contained in the soil, the longer does it take for this equilibrium to be achieved. With soils wholly of clay, such settlement may go on for years while, with sands, it is rapid and is substantially finished by the time building is completed. It may be of interest to note that a reduction in loading, such as will be caused by demolition or excavation, can lead to water migrating towards the unloaded soil, causing it to swell—again, appreciable with clays and negligible with sands.