| - HOME - | - profile - | - services - | - projects - |
Foundation design is possibly the most important and also the least predictable part of structural design. Soil is a natural material and so it can be very variable.
There are a number of fairly common conditions in Botswana and this article looks specifically at foundations for low rise buildings on collapsing sands. This sounds a bit specific but it is a remarkably common occurrence. Of course you should be warned that it is the uncommon conditions that normally cause the problems.
This is not intended to be a treatise on soil mechanics but a general introduction to the subject of collapsing sands.
Strata, or the layers in the ground can be of three types; sand, clay or rock. They can also be any combination of these and you may see the term, residual granite in a clayey sand matrix covering all three types.
Botswana has a lot of sand and this is not just restricted to the surface, it can be up to 100m deep in places. In a way this is fortunate as sand tends to behave a lot better than clay. Sand behaves much the same whether wet or dry which clay does not.
This is the problem with some of Botswana's sand. It contains a small amount of clay. Anyone who has worked in the construction industry in Botswana for any length of time has heard of collapsing sands. This sounds fairly dramatic but generally leads to cracked buildings rather than buildings collapsing into holes. The sand grains are held apart by tiny clay particles. This is generally a fairly stable arrangement until it gets wet. When the clay bridges get wet they collapse and the sand particles, which were previously fairly loosely packed can now pack down tighter, especially when encouraged to do so by a hefty building on top of them.
There are laboratory tests for collapsing sand which will give you fairly reliable information on the degree of risk they pose to a particular loading. However, there are indicators which should show you if you need to be concerned or not. First of all if the soil is dense it is unlikely to be collapsing but beware because often collapsing soils look very good when dry. The origin of the soil can give you a clue too. Aeolian or wind blown sands, hillwash and residual granites are often collapsing. Take a clod of the soil (Collapsing soils should be cemented together when they are not saturated) and put it in a small amount of water. If it is collapsing you should see the water soak up through the tiny pores and then if you push it gently when it is wet it will collapse as if it had been dissolved by the water. You may also be able to see tiny pores in the soil with a magnifying glass. Beware that a collapsing soil can look very much like good gravel soil because it has some larger particles in it. The other thing to look for in collapsing soils is the slightly rounded particles indicating that they have been abraded.
So what can be done? There are three options, avoidance, allowance and precollapse. Avoidance means placing the foundations below the collapsing stratum in a better stratum. This may not be practical if the collapsing stratum is deep. Allowance means allowing for the settlements you expect the building to undergo. This can be done by introducing sufficient movement joints and reinforcing the footings between joints. Precollapse is a fairly popular method in which the sand is collapsed before you put a structure on it. This involves digging out the soil and then putting it back in shallow enough layers to be able to wet them and compact them. These are usually about 150mm thick. In theory all the soil under the building and even the soil next to the building is affected by the loading. That would be a lot of soil to dig out and replace. Fortunately the load effects generally become fairly negligible under strip footings at 1.5 times the width of the footing below the bottom of the footing and outside a width of 1.5 times the width of the footing.