In Nigeria, as prescribed in the National Building Code of 2006, when a proposed building is more than one storey or 6m high or where the proposed foundation type is raft or a deep foundation, subsurface soil investigation is to be carried out. Such soil investigation will be by boring, test pitting, cone penetration testing amongst others to provide information enough for determining character, nature and load bearing capacity of the soil.
The Building Code further specified that the investigation/exploratory borehole is to be drilled to bedrock or adequate depth into load bearing strata. As a minimum, one borehole is required for every 200m2 of the proposed building footprint. However, when non-uniformity in the bearing characteristics of the soil strata is observed, additional boring is required to determine strata levels of equal bearing capacity. Sometimes, soil boring for subsurface investigation is performed to determine the engineering characteristics of existing structures as well as contamination status.
However, soil boring for site investigation is required to characterise the basic geologic materials of the chosen site, by:
- defining geologic stratigraphy and structure;
- collecting samples for index testing as well as determination of engineering properties;
- obtaining groundwater data;
- performing in-situ tests;
- installing instrumentation; and
- establishing foundation elevations for structures.
With respect to National Building Code specification of one (1) borehole per 200m2, in many circumstances, additional guidance would be required for decision making. Thus, the following are applicable in determining number and distribution of soil investigation points.
- Large areas covering industrial and residential colonies, the whole area may be divided into grid pattern and Cone Penetration Tests performed at every 100m grid points. While the number of boreholes or trial pits be decided by examining the variation in penetration curves. At least 65% – 70% of the required number of borings or trial pits to be located within the area under the building.
- In compact building sites covering an area of 0.4 hectare (4,000m2), one borehole or trial pit in each corner and one in centre.
- For widely spaced buildings covering an area of less than 90m2 and a height less than four (4) storeys, at least one borehole or trial pit in the centre shall be done.
It is often difficult to determine the scope / extent of boring, where multiple points are required to assure quality and safe building, but there is veritable guide to resolving this challenge.
The number and locations/distribution of investigation points (test pits, boreholes, CPTs, etc.) should be selected, by taking into consideration the geological conditions of the site, the dimensions of the structure and the engineering problems involved. Hence, the number and distribution of test points should be such that soil stratification information across the site is availed; and test points are placed at critical points relative to the shape, structural behavior and expected load distribution (corners of the foundation area). In addition, investigation points should be arranged so that they do not present a hazard to the proposed structure, the construction work, or the surroundings including groundwater conditions.
The locations and spacing of CPTs, pits and boreholes should be such that the soil profiles obtained will permit a reasonably accurate estimate of the extent and character of the intervening soil or rock masses and will disclose important irregularities in subsurface conditions.
Note however, that where ground conditions are relatively uniform, or the ground is known to have enough strength and stiffness properties, wider spacing or fewer investigation points may be applied. The choice shall be based on local experience.
Subsoil investigation techniques are relatively expensive and therefore should be carefully planned and controlled to obtain accurate and relevant data/information. It should be noted that the quality of the information produced can vary significantly and radically different conclusions reached if procedures are not carefully followed and data not properly interpreted.