Soil is a living material and the condition, quality, and behavior play a key role in construction projects. All structures, buildings, gas, and chemical plants, harbor, airports, roads, bridges, dams and even life itself are based on the soil. The soil is the natural foundation that supports all structures and investments.
As an important material, Geotechnical Engineers and Engineering Geologists have to perform geotechnical investigations to obtain information on the physical and chemical properties of soil and rock underlying a site to design earthworks and foundations for proposed structures, and for the repair of distress to earthworks and structures caused by soil conditions. Also, Geotechnical investigations enable the assessment of the suitability of the soil materials in the site by providing vital data for informed site development planning and decision making.
However, the paradox is that soil is the most important yet most neglected in construction projects in Nigeria. Despite the importance of Geotechnical investigation, it is one of the most overlooked parts of the construction industry. It is not easy to understand why, but one can speculate that the reason is that asset owners and their consultants want to save money, while the Architects do not want to take liability for the soil conditions of the site.
Again, most clients see soil investigation as a waste of funds despite being the cheapest activity in the construction process, hardly is the cost of soil test up to one percent of the cost of construction. Some contractors too ignore the importance of proper soil investigation and analysis and base their design on assumed bearing capacity and rate of settlement.
A geotechnical investigation generally includes surface and subsurface appraisal of a site. Occasionally, geophysical methods are used to obtain preliminary qualitative data about sites. Soil Investigation usually involves in-situ testing such as the Standard Penetration Test and Cone Penetration Test. Additionally, site investigation will often include soil sampling and laboratory testing of the retrieved soil samples. Test pits, trenches, and large diameter borings may also be used to learn about soil conditions at depth for direct visual and manual examination of the soil and rock stratigraphy. The extent of investigation depends on whether the area is built up or not and the size of the structure.
Geotechnical investigation helps Geotechnical Engineers to correlate varying physical and Engineering properties of soil, which can vary from place to place and from layer to layer even within the limits of the proposed structure. Soil characteristics can change considerably within a small area. Weather, climatic changes, and site management can in the future affect the bearing qualities of the soil if the foundation is not designed properly to the bearing capacity of the soil, then they will fail and so will the structure too.
An early geotechnical investigation help to determine the bearing capacity of the soil which determine the soil’s load sustenance capability, helps understand the rate of settlement of the soil which affect the rate at which any structure placed on it settles and stabilise, to select a type and depth of foundation, to select suitable construction technique, to predict and resolve probable foundation problems, to determine if the land can be subjected to subsidence and cause sinking of the building, to determine water table which affects humidity within the foundation and greatly affects the character of a soil which varies considerably with water content, mineral or chemical composition of the soil that might affect the choice of construction materials.
Whatever type of soil one encounters, peat, gravel, clay, silt, sand, or loam soil, understanding the soil properties of a site helps make good construction decisions leading to the success of the project. Therefore, the Structural Engineers can efficiently and accurately design the structural elements for the long term viability and soundness of the project. The geotechnical investigation also helps to determine whether there is a need for soil stabilization and the foundation depth required to attain the desired soil bearing capacity.
In summarising the benefits of geotechnical investigation before any project starts are as follows:
- Asset owners and Developers can be informed of potentially problematic soils in the sites. This implies that damage that could be caused by these soils can be minimized at the inception of the project. Consequently, the associated costs are known from the outset, thus enabling owners and developers to make informed decisions regarding the most appropriate foundation system for the project.
- In urban development projects there could be the possibility of a group of neighbors and owners agreeing to treat a large area of soil on a once-off basis, and sharing the cost of the required treatment process (e.g. the impact rolling of a site located on collapsible soil).
- In general, early geotechnical investigation reduces maintenance costs, resulting from problematic soils during a project life cycle.
In light of the above, it is clear that asset owners and property developers are taking a huge risk by failing to perform an early rigorous geotechnical investigation of their proposed site.
- By Professor Goodwill Ofunne