What is soil suction?

The water in soil voids below the water table is normally continuous. The soil may be saturated, with voids full of water or there may be occluded air bubbles present in the water. Pore pressures at depths below the water table are derived from a combination of the weight of the water lying above the given elevation and the drainage conditions below. The pore pressure normally has a positive value and can be measured using a saturated piezometer with a porous filter that is making intimate contact with the water in the soil.

If the water contained in the voids of a soil were subjected to no other force than that due to gravity, the soil lying above the water table would be completely dry. However, powerful molecular and physico-chemical forces acting at the boundary between the soil particles and the water cause the water to be either (a) drawn up into the otherwise empty void spaces or (b) held there without drainage following infiltration from the surface. The attraction that the soil exerts on the water is termed soil suction and manifests itself as a tensile hydraulic stress in a saturated piezometer with a porous filter placed in intimate contact with the water in the soil.

The magnitude of the attractive force that soil above the water table exerts on water is governed by the size of the voids in manner similar to the way that the diameter of a small bore glass tube governs the height to which water will rise inside the tube when it is immersed in water. The smaller the void, the harder it is to remove the water from the void.

The meniscus formed between adjacent particles of soil by the soil suction creates a normal force between the particles, which bonds them in a temporary way. Thus soil suction, if it can be relied upon, can enhance the stability of earth structures. However soil suction also provides an attractive force for free water, which can result in a loss of stability in loosely compacted soils or swelling in densely compacted soils.

Soil suctions can be found in all ground that lies above the water table. This may be natural level ground or slopes, fill materials and other earth structures that are constructed above the water table. Soil suctions will also be present in samples that have been recovered from a ground investigation. Laboratory measurements of suction can be very useful for assessing the quality of the samples, estimating the in situ effective stress and detecting the presence of desiccation.

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