The foundation is undoubtedly the most important part of any building or structure. However, foundations typically require stable sub-soil strata with adequate bearing capacity to sustain the loadings from the super-structure and in-service loads such as machinery, vehicular movement, and human weight.
All builders prefer to build on bedrock since this is the most stable foundation and also the cheapest to build upon. Such ideal conditions are not always possible on every construction site. In some cases, you might find clay, silty, sand, loose, or water-logged soil on site all of which presents a new set of design and construction challenges. In such cases, the usual strip foundations may not suffice since they become quite uneconomical after about 6 feet.
If you do encounter such unstable foundations as a builder or a contractor, is there a way around this to improve the sub-strata and optimize your construction budget? Let’s dive into a few different types of unstable foundations and possible cost-effective remedies for these.
1. Clayey soils
Clay soils are relatively stable and even have a reasonable bearing capacity when dry and compacted. However, their performance significantly reduces with the uptake of water, for example, rising groundwater or even when it rains. Besides this, clayey soils are highly expansive and also shrink when dry.
This makes foundations built wholly on clay soils unstable and also prone to structural failure since parts of or the entire foundation can easily be displaced during expansion or shrinkage. A great example of a 2-foot strip foundation is shown in the infographic below.
If this simple structure was to be built on clayey soil without remediation measures being applied, the uneven movement due to clay expansion would probably damage parts of the strip foundation. In the case of a small residential dwelling, the damage can be localized to one section of the building. However, for a structure with a large plan area such as a commercial building, a simple strip footing will not suffice. In areas with good subgrade, isolated footings for a large commercial building might do the trick.
A conventional raft foundation is one way to keep clay expansion and contraction in check for large plan areas. The cross-section of the ring beam supporting the raft is shown above. A raft foundation ensures that the foundation loading is spread out under soil pressures, or that the deformation will only be slight.
The raft foundation is also the basis of an alternative foundation like the waffle pod foundation, illustrated above. The waffle pod is an ideal raft for smaller buildings and is very cost-effective since beam reinforcement isn’t used in concrete.
If the waffle foundation isn’t an option, soil removal and replacement with compaction provide another cost-effective remedy for clay foundations.
2.  Sand foundation
Sand isn’t necessarily a bad foundation, and has good bearing capacity when compacted and tested. The problem with sand is that it can be easily washed away and thus the foundation will degrade before the expected design life is accomplished.
Sand depth usually varies between 1.0 -1.5m inland although it can be deeper in river basins and near estuaries. In most cases, it will stretch to a depth of 0.5 meters with good strata beneath it. Therefore, the remedy would be to cut the sand until good bedrock is achieved. This should always be preceded by a borehole test to establish the depth of the sand. Sand can also be compacted although this should be done in three passes of 100 -150mm each.
3.  Peat
Peat is a type of soil usually found in marshes and wetlands that is made up of decaying organic matter such as vegetation mixed with topsoil. The remedy for such a foundation type is to excavate all of the unsuitable material and replace it with a more suitable base course such as compacted murram or hardcore. However, further geotechnical tests should be done if the bedrock around the project area is unknown.
Conclusion
Your soil foundation is an important part of your structure achieving its design lifespan. Borehole testing, even rudimentary types, should be carried out before any building project starts. Soil should always be compacted where possible (for example, by vehicular traffic) unless it is known that good bedrock or strata underly the foundation. Seek the advice of an expert such as a structural or geotechnical engineer if results from borehole tests are unclear or inconclusive.
