1.1 - Introduction

This chunk introduces the concept of consolidation in civil engineering. As civil engineers design and construct buildings, they must understand how the weight of these structures affects the soil beneath them. When a structure is built, the load it imposes causes stress in the soil, leading to strain or deformation. This process results in settlement, which is the gradual sinking or shifting of the soil mass underneath the structure. It's crucial to recognize that this settlement occurs because the volume of the soil decreases, primarily as water is expelled from the soil's voids.

In a fully saturated soil system, the water filling the voids between soil particles is considered incompressible, which means it cannot be squeezed into a smaller volume. Instead, when stress is applied (like when a building is constructed), the soil’s volume decreases mainly because water is forced out of these voids. This expulsion of water results in a rearrangement of the soil particles, affecting the overall structure of the soil and leading to settlement.

Total settlement of a structure can be categorized into three components: immediate settlement occurs right after the load is applied, due to a change in shape of the soil without any volume change; primary consolidation occurs over time when pore water is expelled from soil voids; and secondary consolidation (or creep) refers to the slow rearrangement of soil particles at constant stress, leading to continued settlement. Understanding each type of settlement helps engineers predict how much a structure will sink over time.

Elastic settlement happens almost instantaneously once a load is applied, occurring due to the flexing or deformation of soil particles without a significant change in volume. This type of settlement can be observed in less permeable soils where water doesn't flow out quickly. The behavior of soil under load can be calculated using elastic theory, which models how materials deform.

Primary consolidation occurs over time due to the gradual expulsion of pore water from the soil. As water moves out, the pressure within the soil decreases, leading to a rearrangement of soil particles and an increase in effective stress. This process takes time since the rate of water flow depends on the soil's permeability. The settlement during this stage is critical for predicting how much settlement will occur as a structure settles.

Secondary consolidation occurs after primary consolidation is complete. This settlement is a result of the slow rearrangement of soil particles under constant effective stress, occurring over a longer time frame. It’s slower than primary consolidation and results from the adjustment of the soil structure rather than the drainage of pore water. Understanding secondary consolidation is pivotal for long-term settlement predictions.

Several factors influence the total settlement: the amount of pressure applied to the soil and the thickness of the soil layer that is saturated with water. Greater loads lead to more substantial settlements, while thicker layers of saturated soil can also impact how the soil responds to loading, potentially leading to increased deformation.