Foundations are integral elements of many kinds of structures. These include buildings, roads, water tanks, silos, swimming pools, jetties and even breakwaters.
Because of their sub-surface nature, many novice builders and project owners often overlook the importance of foundations and why they should be designed critically. Engineers and architects, however, often have a good understanding of foundations and their requirements regarding various types of structures. Thus, it is important to involve these professionals in the geotechnical and critical design phases of structures, especially for medium-to-large scale projects.
So, what are the most important factors to consider for your foundation if you are a new builder or are just new to foundation design overall?
1. The Type of Sub-Surface Soil or On-Site Strata Profile
The type of soil or subgrade is the most important element to consider for foundation design. It is therefore important to determine the type of strata on-site before design or construction commence. In this regard, 4 main types of soils are prevalent, especially in the East African region. These are:
- Clays: These are extremely fine soils that have good bearing capacity when dry. However, clays exhibit plasticity and are highly expansive, affected by even the smallest changes in sub-soil moisture. They are also poorly drained. Thus, they are not the most ideal for typical building foundations unless minimum depth requirements to overcome moisture expansion are met (typically 750mm-1200mm for medium plasticity clays).
- Sands: Sands generally have a good bearing capacity and in most cases are usually well-graded in places where they naturally occur.
- Sandy loams: These are the most prevalent soils across East Africa, and they have a good bearing capacity for most typical buildings. They are also well-drained because of the combination of particle sizes.
- Rock: Rocks such as granite, limestone, sandstone, and hard chalk form the best kinds of foundations for any type of structure.
This data on soil strata can be obtained from extensive geotechnical surveys for large-sized structures and developments. Simple borehole studies will suffice for small-to-medium residential developments.
2. The Plinth Extents of the Structure
The area covered by the outer extents (plus a small margin) of the built-up area of a structure affects the type of foundation. Large developments and infrastructure works may require raft or mat foundations, while areas of weak soil such as clay or where the sub-surface strata aren’t uniform may require specialized piling works.
Where there is a good rock foundation below an expansive development, strip foundations or isolated pad footings with a ground beam may suffice.
3. Budgets
For smaller residential dwellings and single-family units, strip foundations or isolated footings will almost always be the most cost-effective. For larger developments such as malls and factories, it may be inevitable to use a raft foundation or piles where the soil strata are weak or aren’t uniform.
4. Topography
While this isn’t usually a major factor on small, single-unit developments, the topography of a site can grossly affect large areas of foundations. In particular, where substantial cut-&-fill operations are involved, soil properties may be changed or aggregated, thus warranting deeper foundations.
What Will Work for a Small Residential Development?
Never avoid a soil survey, even a limited one with two or three boreholes of 1-1.5 meters if this is possible. Rock foundations almost certainly suffice for almost all types of developments. Rock foundations are also the cheapest since they require the least depth to achieve structural stability.
For small residential developments in areas of unstable soils such as clay, stabilization methods such as lime or geotextiles can be used to improve soil drainage and minimize moisture infiltration. Areas with heavy clay or black cotton soils fit this scope. A small raft foundation may also be relatively inexpensive in such areas, although slightly more expensive than isolated footings or strip foundations.
