There has been incidents where Kenyans have spent over 1 million shillings on building foundations of bungalows in black cotton soil zones. Others have spent an average of 500,000/= to 600,000/= on foundations alone, with a minimum of about 250,000/= being reported in some cases.
This raises the question, why are foundations so expensive, and what really can be done about it? The science of foundation design is too complex to be discussed in this article.
For specific information on foundation design, we recommend that you consult a structural engineer, a graduate engineer, an architect, a construction manager, an architectural or engineering technician or any other “qualified” person. The focus here, will be to discuss foundation design of bungalows and massionattes or 3 to 4 level buildings.
The site a building is the first determinant for the type and cost of foundation. The reasons being that some sites are on level ground, others on gently sloping terrain, others on steep slopes, others on different soils and moisture conditions, like gravel, sand, clay, black cotton soils type, swampy zones or wetlands and lower water table levels. In summary, the challenges that are brought by the site include the topography or relief, soil type and underground water moisture or water content.
Level grounds are easy to build on, but steeper slopes and marshy conditions and poor soils are difficult to build on. The details of this article is to focus, not on the type of foundation used, but the depth recommended for bungalows and massionatte building types.
The Kenya building code recommends that a foundation should at least be sunk one meter below ground. In temperate climatic zones, the recommended depth is often below the “frost line,” and a minimum of 24” depending on the nature of the soil beneath. The recommended foundation footing width is three times the width of the building wall, but a minimum of twice the width is acceptable on smaller structures.
A typical Kenyan bungalow foundation, therefore, looks like this: 3 to 4 feet deep and 6 feet wide trench excavation, 24” wide foundation footing that is 8” to 12” in height, and a masonry or stone building wall, normally 8” wide, rising to the ground level and extending 24” high to avoid flooding, moisture, termite and pest entry into the building.
On top of the 5 feet high structure from the bottom of the footing, is typically a 4” concrete slab, normally reinforced with welded wire mesh or lightly reinforced with #4 rebar (Y10). Below that is a 6 mill damp-proof membrane, a 2” layer of sand, and about 12” to 24” layer of compacted hardcore or 4” compacted gravel, resting on a compacted earth beneath.
The purpose here is to determine the depth of foundation required. This depth will give us the number of courses of masonry blocks or stones that will be needed, hence a great determinant of the cost component.
Recommended foundation depth for bungalows from the Finished Floor Level line to the Bottom of Footing is about 5 feet (or 6 courses of masonry blocks) and 7 feet ( or 9 courses of masonry blocks) for massionatte or 3 to 4 level building. For depths greater than 7 feet, it’s recommended that a basement or cellar be included in the building design.
This is because, the minimum floor to ceiling height is 8 feet and why do all the excavation and again cover it altogether? The efforts are well spent with a cellar or basement, because it makes sense in this application.
Assuming the prevailing cost of solid masonry or stone wall construction of 200/= per foot of one course, the cost of building with an 16” long x 8”wide x 8” thick masonry block could run in the neighborhood of 1,200/= to 1,600/= per foot of one course for bungalows and 1,800/= to 2,000/= per foot of one course for massionatte.
The implications of this is that a 50’ x 30’ (1,500 sft. or a typical 3 bedroom house) with similar amount of interior wall length as the perimeter length, will have a total foundation length of about 320 feet. That implies a cost of 512,000/= for bungalows and a cost of 640,000/= for massionatte foundations. Anything beyond the 30% margin of these values must raise a red flag and be investigated by the structural engineers and quantity surveyors. (N/B The prevailing cost of a solid masonry block or stone is about 50/= to 100/= in 2017)
Black cotton soils are inherently low in bearing capacity and generally, contractors tend to dig deeper to try to access the firm and better soils beneath. There was an outrageous incident where an individual reported to have spent close to 1.2 million on a bungalow foundation alone. It had 13 courses and the excavation was extensive. What really went wrong?
If no bearing soil is found within digging of 7 feet, an engineering solution need to be sought to remedy the solution. This can involve measures like: soil fill borrowing, soil mixing, soil strengthening with cement, thermal or other chemical means, measures done to harden the soil, using special types of foundation systems like slab on grade or raft foundation, using piers and piles etc. All these activities can be summarily called foundation engineering. Therefore, the remedy to foundation costs is foundation engineering and analysis. The role that these foundation professionals do cannot be overlooked or underestimated.
It is key that we get AEC professional and practitioners involved.
Have a blessed day.