The integrity of a building structure depends ultimately on the stability and strength under loading of the soil or rock underlying the foundation.
The type of foundation to be adopted for a building depends upon the load to be transmitted and the bearing capacity of the soil. When the site with a good bearing capacity of reasonable depth is available it is not difficult to design on the type of foundation. On the other hand on certain soil, a close investigation is necessary soils of low bearing capacity of bearing depends. Coastal areas where the upper stratum is sand and water level is high are cases where the investigation is necessary.
For a large building project such a multi-story structure the site exploration may be both extensive and costly and may include on of the following
- Trial pits
- Bore holes
- Load test
A pit dug to a depth on type of foundation expected usually going up to about 3m and rarely to 6m. The pit should be large enough accommodate. A trial pit does expose a section through the ground revealing for examinations the character and depth of stratum. A sample of soil for caboratory testing can be readily obtained. A pit also gives the contractor valuable data regarding the cost of excavation of foundations such as foundation. Whether timbering is necessary and whether pumping of underground water is necessary for a building covering a large area four trail pits may be required one near each corners. These pits should not be dug too close to be the proposed building as they would be a source of weakness to the foundation it should be filled with concrete.
As much work is involve in digging trail pits a simpler method is the use of boreholes. These are used for raising samples of soil for examination. A simple tool for mating bore hole is the Augur which may be operated manually or mechanically.
In this method, a weight of known cross-sectional area dropped from known height. The depth of impression on the soil by the drop of the weight is noted for performing this experiment a pit of the required depth is excavated.
The depth of pit will be equal to depth of the foundation. After excavating the pit is cleaned and leveled but should not be computed.
The concrete cube block is them drop very gently without impact from the specified height and the impression is noted down. The experiment is repeated at last 5 times in different pits or along the foundation trench at different poles and the average depth of impression noted. The bearing capacity is worked out as follows. Let R is equal to
Let R is equal to resistance of soil that is resistance to the penetration or resistance to settlement.
H – height through which the weight is dropped
W – weight
D – depth of impression
A – Area of cross section of the weight.