ENVIRONMENTAL SITE SURVEY AT MLABANI SEC. SCHOOL. (Posted by Benedict Mgubike)
MSABI sanitation team is making preparations to install a septic tank, 10 latrine rooms and mound-wastewater treatment system at Mlabani Secondary school.We carred out a site survey at the construction site to understand the soil structure/profiles present. This was prompted by huge cracks on the surface which is an indication of clayey soils.
Figure 1. Benedict holding auger bit extracting soil samples from the ground.
We used a soil auger to collect samples upto a depth of 4m from the surface. The results shows there is a 3.5m clay soil layer. The samples at 4m showed some silica content indicating the presence of a sandy layer. This is consistent with a drilling log form from a MSABI borehole located 100m from this site which indicates a sandy layer from 5m to 13.5m.
Figure 2. An auger bit with sample soil from the ground.
Figure 3. Collected soil sample at different depths as shown in this picture.
The survey results shows the site poses a high risk to the structural integrity of the septic tank if extra measures are not put in place. The cracks on the surface (clay) indicates the clay soils is an expansive clay which swells during the rainy season and contracts during the dry season. This intermittent expanding and contracting will induce stress on the septic tank walls.
The technical strategy to be used to mitigate against this is;
·When escavating, all the clay will be removed in the pit and backfilled with sand till the desired level for the septic tank foundation is attained.
·The pit to be dug will be wider than the tank. This is so as to leave a gap between the tank walls and pit walls which will later be filled with sand. This will reduce lateral pressure of the clayey soil.
·Extra reinforcement will go into the septic tank walls to withstand lateral pressures from the earth surrounding the walls.
Figure 4. A 5 meter long auger bit that was uesd to get soil samples.