Choosing Foundation based on Soil Types

It is highly important to determine the type of soil before you begin a construction project as it helps choose the appropriate foundation installation. Without further ado, let us explore how a foundation is selected based on different types of soil.

Different Types of Soil and Selection of Foundation


Clay is an expansive soil made up of tiny particles. When wet, clay greatly expands, but when it is dry, it will shrink significantly. When clay is moist, it is very pliable and can easily be moved, manipulated, and shifted. These extreme changes can put a great deal of pressure on foundations, usually causing them to shift up and down or crack, which is why clay generally is not the best soil on which to construct a residential or commercial building.

The first 900-1,200mm layer of clay is subject to movement due to expansion and shrinkage depending on moisture content, so it is generally necessary to excavate foundations to a depth where the moisture content of the clay remains stable. British Standard 8004 recommends a minimum depth of 1m for foundations But if there are, or were, trees nearby, depths of up to 3m may be necessary.

In clay, prior to concreting the foundations, the trench is often protected from heaven by lining it with a compressible layer (e.g. Clayboard).

Firm clay over soft clay

A traditional strip foundation is sometimes acceptable but it is important not to over-dig as this may increase the stress on the softer clay beneath. A common solution is to dig wide strip foundations with steel reinforcement — however, an engineered foundation may be necessary.


Peaty soil is typically dark brown or black in color and is easily compressible because of how much water it can hold. This soil type is formed by decomposed organic material, is usually found near wetlands, and is extremely porous. Like clay, peat expands when wet, and in extremely dry conditions, it not only shrinks, but it also is a potential fire hazard. It is a very poor subsoil when it comes to supporting, as foundations are most stable on soil that does not shift or change structure depending on weather conditions and that doesn’t have a low bearing capacity.

Peat and lose waterlogged sand are very poor subsoils. If the peat can be stripped back to find a suitable load-bearing ground of at least 1.5m depth, strip foundations may be suitable. A reinforced raft foundation will likely be required.

Recommended foundation

  • Concrete piles extended to the firm soil layer below
  • For small projects, pad and beam foundation took to firm strata blow.
  • Raft foundation for the case where firm strata are not available at reasonable depth but there is hard surface crust with 3-4m thick of suitable bearing capacity.

Factor to be considered

  • Pile types include bored cast in place with the temporary casing, driven cast in place, and driven precast concrete.
  • Allow for peat consolidation drag on piles
  • If the raft foundation is used, entries to the building shall be flexible.
  • Special high grade and protection are likely to be required in aggressive peats.
  • If the peat layer is shallow over a firm layer of soil, dig it out and replace it with compacted fill. For this, use a raft or reinforced widespread foundation dependent on the anticipated settlement.
  • Frequently, Sub-soil can be improved using Vibro treatment, and it would an economical solution if employed in conjunction with a strip or raft foundation.


Silty soil is made of smaller particles, which is why it is able to retain water longer. However, because of its tendency to retain moisture, the soil is cold and drains poorly. This causes the silty soil to expand, putting pressure against the foundation and weakening it, making it not ideal for supporting a foundation.

Sand and Gravel

When compacted with gravel and other materials, sand does not retain water. Therefore, it will not cause any structures above it to shift. Sand and gravel have the largest particles of the various soil types, which is why it doesn’t retain moisture but drains easily. When soil and sand are compacted and moist, it holds together fairly well. Additionally, if the two are compacted, they make for good soil to support a foundation due to their non-water-retaining properties. However, when moist, the particles will lose their friction and can be washed away, which can leave gaps beneath the foundation and cause settlement issues down the road. Luckily, quality helical piers are an effective fix for foundations that are built on and supported by sand.

Dry compact gravel, or gravel and sand subsoils are usually adequate for strip foundations. Generally, a depth of 700mm is acceptable, as long as the ground has adequate bearing capacity.

If the water table is high (i.e. the gravel is submerged), the bearing capacity is halved, so it’s important to keep the foundations as high as possible. A shallow, reinforced, wide strip foundation may be suitable.

Sand holds together reasonably well when damp, compacted and uniform, but trenches may collapse and so sheet piling is often used to retain the ground in trenches until the concrete is poured


There are varieties of rock, such as limestone, bedrock, and sandstone — all of which have exceptionally high bearing capacities, making them a suitable soil type for supporting residential or commercial buildings. It’s crucial that a rock surface is a level before building a foundation, otherwise, the foundation must be held into place with anchors.


When it comes to the ideal soil type for foundations, loam may be the best option. Generally, loam is a combination of clay, silt, and sand. Loam is dark in color and soft, dry, and crumbly to the touch. Loam is great for supporting foundations due to its evenly balanced properties, especially how it handles moisture in an evened way and will generally not expand or shrink enough to cause damage. Loam is good soil for supporting a foundation and building, as long as there are no miscellaneous soils that find their way onto the surface.

The type of foundation selected for a structure is controlled by numerous factors including earlier site usage, adjacent construction, soil type, size of the development process, etc. While considering these factors, the type of soil and its properties play a very crucial role.

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