Engineering and Construction

The second basic type of settlement of a building is caused by secondary influence, which may develop at a time long after the completion of the structure. This type of settlement is not directly caused by the weight of the structure. For example, the foundation may settle as water infiltrates the ground and causes unstable soils to collapse. The foundation may also settle due to yielding of adjacent excavations or the collapse of limestone cavities or underground mines and tunnels. Other causes of settlement that would be included in this category are natural disasters, such as settlement caused by earthquakes or undermining of the foundation from floods.

Subsidence is usually defined as a sinking down of a large area of the ground surface. Subsidence could be caused by the extraction of oil or groundwater that leads to a compression of the underlying porous soil or rock structure. Since subsidence is due to a secondary influence (extraction of oil or groundwater), its effect on the structure would be included in the second basic type of settlement described earlier.

A special case is the downward displacement of the foundation due to the drying of underlying wet clays, which will be discussed later. Often this downward displacement of the foundation caused by the desiccation of clays is referred to as settlement. But upon the introduction of moisture, such as during the rainy season, the desiccated clay will swell and the downward displacement will be reversed. The foundation could even heave more than it initially settled. When dealing with expansive clays, it is best to consider the downward displacement of the foundation as part of the cyclic heave and shrinkage of expansive soil and not permanent settlement.

Determining the settlement behavior of the proposed structure is one of the primary obligations of the geotechnical engineer. The following parameters are often required:

1. Total settlement ρmax. Also known as the maximum settlement, it is the largest amount of settlement experienced by any part of the foundation, such as shown in Fig. 7.1.

2. Maximum differential settlement ∆. The maximum differential settlement is the largest difference in settlement between two different foundation locations, such as shown in Fig. 7.1. The maximum differential settlement does not necessarily occur at the same location as the total settlement.

3. Rate of settlement. It is often desirable to know if the settlement will occur during construction as the dead load is applied to the soil, or if the settlement will occur over the life of the project, in which case there may be on-going cracking to the structure.

4. Maximum angular distortion δ/L. The angular distortion is defined as the differential settlement between two points divided by the distance between them less the tilt, where tilt equals rotation of the entire building (Skempton and MacDonald, 1956). As shown in Fig. 7.1, the high- est value of δ/L would be the maximum angular distortion. The location of the maximum angular distortion does not necessarily occur at the location of the total settlement or maximum differential settlement.

One approach for the design of the foundation is to first calculate the total and maximum differential settlement of the proposed structure for the site soil conditions. Then these values can be compared with allowable values. The foundation type may have to be changed or mitigation measures adopted if the calculated total settlement of the structure exceeds the allowable settlement.