Expansive soils and collapsible soils are among the commonest problem soils that pose a great challenge to construction professionals. Expansive soils tend to expand excessively when in the presence of water and shrink excessively when dry. Most expansive soils are noted to possess the mineral montmorillonite. The collapsible behaviour of soils depends on:
(1) The percentage of fines present
(2) Initial water content
(3) Initial dry density
(4) The energy and process used in compaction
Foundations on these soils are usually difficult and costly. I would dwell only on expansive soils because they seem to be more widespread in the world. It may interest you to know that expansive soils were listed alongside earthquakes, landslides, hurricanes, tornadoes, and floods as the six major natural hazards because enormous losses are incurred due to their effects on structures, especially buildings.
The characteristics of expansive soils are:
(1) They increase in volume with the addition of water.
(2) On drying, they shrink and develop cracks on the surface.
(3) They possess a high plasticity index.
(4) Their colour varies from dark grey to black and sometimes reddish-brown.
Find below an attached video of expansive soil effects on structure (Source: RMG Engineers)
Due to the fact that signs of expansive behaviour of soils are not usually noticeable during the rainy season, one may proceed to build a house with confidence on such soils without taking their effect into consideration. When the dry season comes and there is shrinkage due to dryness, it affects the building causing cracks that make buildings appear unserviceable. It is very important that signs of expansiveness of soils are sought before commencing any construction work, especially during the dry season. It is also recommended that building works should be done on expansive soils during the dry season. We will see why.
If it has been found that a proposed foundation for structure is on expansive soil, the following types of foundations should be adopted.
(A) Structures can be kept isolated from the swelling effect of soils. Generally, all parts of buildings are not affected by the swelling potential of the soil. Beneath the centre of the building, the soil is protected from sun and rain, thus moisture changes are minimal and soil movement is mild. But beneath the outside walls, the movement is severe. Damage to buildings is greatest on the outside walls due to soil movement and starts from there to affect other parts of the buildings. Figure 1 shows a measure that can be used to control this situation on a typical foundation of an external wall.
This involves the provision of a granular bed and cover below and around the foundation of external walls through the following steps.
- Excavate the foundation of an external wall by an extra 200 – 300 mm of the normal foundation width.
- Place and compact freely draining granular material such as a mixture of sand and gravel up to the base level of the foundation.
- Cast reinforced concrete footing at this level.
- Fill up the remaining fill space using some coarse free draining material.
- Cast a concrete apron of about 2 m wide around the building to prevent moisture from directly entering the foundation.
- This should be done for all foundations of external walls only.
- Proceed with the other aspects of the foundation works.
Note:
(i) The granular material below absorbs the effect of swelling and this reduces its effect on the foundation.
(ii) This type of foundation should be constructed only during the dry season when the soil has shrunk to its lowest level.
(iii) Arrangements should be made to drain away the water from the granular base during the rainy seasons.
(B) Designing of Foundations that would remain undamaged in spite of swelling. In this case, one can design a rigid foundation that can resist the swelling pressure as one unit. One example here is the use of a network of interconnected foundation beams that are tied to footings in one singular unit. Such a system can be effective though it may be more costly than option (A).
Read Also......
Foundation Types and Uses
C. Elimination of Swelling Potential of the Soil: Swelling potential is defined as the percentage of swell of a laterally confined sample in an Oedometer test which is soaked under surcharge load of 7kPa after being compacted to maximum dry density (MDD) at optimum moisture content (OMC) based on AASHTO compaction test. Expansive soils are known to have high swelling potential. The swelling potential of soils can be evaluated using either of:
(i) Atterberg limit tests
(ii) Linear shrinkage tests
(iii) Free swell tests and
(iv) Colloid content tests
Assuming Atterberg limit tests were done on the soil, the relation in Table 1 below can be used to assess the swelling potential of the soil
Table 1: Relationship between plasticity index and swelling potential
The swelling potential of soil can be eliminated through chemical stabilization of soil using lime. The soil should be mixed thoroughly with about 3 – 8% lime and compacted at about the optimum moisture content (OMC) of the soil. The lime reduces the plasticity of the soil, reduces its swelling potential, and the soil’s tendency to expand under the presence of water.
Always make the best choice.
Also, find attached a video on sophisticated methods to manage expansive soils
References
Dekker, M. Foundations on collapsible and expansive soils. New York
