Following the previous series on the Time-dependent behaviour of structures, this series change tack to talk about the time-dependent behaviour of the ground. What causes the foundation of a structure to settle? This blog explains the reasons from a soil engineering perspective.
Typically for an underground structure such as a foundation, some ground is removed and a structure is built on top. If the weight of the structure is greater than the weight of the removed soil, the bearing ground underneath effectively takes on more load from above and is further compressed. If this is the case, the structure will go through some settlement consisting of three parts:
1. Immediate settlement. As the name suggests, this happens immediately after the ground is loaded. A large part of this is elastic deformation. Therefore as soon as the load goes away, the deformed ground largely bounces back to where it was originally. Elastic methods can be applied here to calculate the settlement, by giving a soil mass an elastic modulus. However, it should be noted that the stiffness of soil changes significantly with pressure. There is usually a ‘stiffening effect’– the stiffness of soil goes up when it is compressed; the stiffer it gets the harder it is compressed.
2. Consolidation settlement. This happens over time, i.e. is time-dependent. When the ground is compressed, or compressed further from where it was, water is squeezed out of it like from a sponge. For types of ground where the ground particles are big, water gets squeezed out quickly; where the particles are fine, water flows out slowly. In soil engineering, the process of water being squeezed out is called ‘consolidation’. During the consolidation process, water takes on the full compression instead of the soil particles. My previous blog http://www.si-eng.org/post/shear-3-shear-in-soil explains why this happens. Water will keep being squeezed out as long as it still takes on compression. As more and more water is squeezed out, compression taken by water will gradually reduce until it reaches equilibrium in the end. Consolidation settlement for loose sand takes place almost immediately whereas that for clay takes place very slowly over years and years.
3. Creep settlement. After the consolidation settlement is done, i.e. enough water has been squeezed out, the soil particles will now take on the full compression. Similar to concrete, soil undergoes creep as well when subject to compression. In geotechnical engineering, the word ‘creep’ is always associated with the solid particles of the soil (note ‘consolidation’ in contrast is associated with ground water). Both the consolidation settlement and the creep settlement are largely irreversible, which means when the compression goes away, the ground does not bounce back to its original position.
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