Abstract
M.Sc.
Weltevreden Park, just like many other suburbs around Johannesburg, has been an area of rapid development over the years. The development of low-rise buildings such as houses, townhouse complexes, office-block buildings, and shopping centers is set to continue.
One of the potential problems facing construction in the area is expansive soils. Expansive soil is a term generally applied to any soil or rock material that has a potential for shrinking or swelling under changing moisture conditions and it is caused by the presence of swell clays, which absorb water and expand, in the soil.
Expansive soil can cause damage to engineering structures, and this can result in significant financial losses.
Due to its geological and geomorphic setting, Weltevreden Park is susceptible to expansive soil. Clay minerals, and possibly swell clays, tend to accumulate in the valley where Weltevreden Park is situated, due to the catena concept. The ridges above the area are composed of quartzite and shale, with the shale being a possible source of the clay minerals.
Soil samples and a shale sample were collected to perform a mechanical and a mineralogical analysis. The mechanical analysis was performed by sieving each soil sample, then weighing each fraction and determining the silt/clay percentage of each sample.
The mineralogical analysis involved analyzing each sample using various X-Ray diffraction techniques to determine the mineralogy of the soil and the shale. The X-Ray diffraction analysis also assists in determining if the clays are expansive or not. This was done by running an X-Ray diffractogram of an air-dried and a glycollated orientated sample of each soil and shale sample. A scanning electron microscope was also used to assist in both the mechanical and the mineralogical analysis.
The mechanical analysis determined that there is an increase in the silt/clay percentage in the soil downslope from the ridges, with accumulation of clays in the valley. The mineralogical analysis determined that the most abundant clay minerals in the soil are kaolinite and illite, with some swell clays such as montmorillonite, nontronite and vermiculite occurring in the soil in the valley. These swell clays only showed partial expansion when comparing the X-Ray diffractograms of the air-dried and glycollated orientated samples. The study also shows that the shale layer is a major source of kaolinite and illite, but it is not the source of the swell clays.
The study shows that there is a potential for expansion to occur in the soil in Weltevreden Park. The study also suggests a number of engineering techniques that can be used to mitigate the problem and prevent any damage to present and future construction in the area.