Significance of Geotechnical Engineering – Part II – Total Settlement
Posted by Kshitija Nadgouda on May 5, 2006
This is a continuation, rather the next part, of the topic I posted earlier on Differential Settlement.
Total settlement is the uniform “sinking” of the structure due to various factors such as the self-weight of the structure, the loads imposed on it, the nature of the soil on which the structure’s foundation rests, etc.
The settlement of any structure can occur immediately (during or post-construction) or it may take years to show up (or sink in!), depending on nature of the soil.
The immediate settlement occurs due to re-organization of the soil particles in response to the weight imposed on it. This is typically observed in sandy soils. Sandy soil or coarse-grained soil is permeable to water, that is, it allows water to move through it easily. When the foundation of any structure rests on coarse-grained soil, the air gaps (voids) are either compressed to a small extent or removed by the re-arrangement of the soil particles, causing the soil to become more dense. The water within the voids, since it cannot be compressed, will move away. This high permeability attributed to these soils results in immediate (quick) settlement.
Long term Settlement
The long term settlement (also called consolidation settlement) is a phenomenon exhibited by fine-grained, saturated, clayey soils, in simpler words, sticky, muddy soil saturated with groundwater. Fine-grained clay soil shows low permeability, that is, it takes very long for the water to move through it from one point to another. This causes a time lag in the settlement to occur! Silty soils are the gray area between the sandy and clayey soils. Silty soil may appear to be similar to very fine sand, but exhibits many properties like clay.
However, in nature, soil is not often “purely sandy” or “purely clayey”. In most cases, soil will be a mixture of sandy, silty and clayey particles. So estimation of the total settlement is important, and limit it within safety standards.
For safety and aesthetic reasons, the total settlement is typically limited to 25 mm (1 inch).
It is the responsibility of the Geotechnical Engineer to determine the soil properties, assess the predicted load imposed on the soil due to the foundation and super-structure and estimate the total settlement.
A classic and drastic example of total settlement is the Palacio De Bellas Artes (Palace Of Fine Arts), cultural centre in Mexico City, which has sunk more than 15 feet (4.6m) since its construction in the early 20th century.
Copyright Kshitija Nadgouda.