Site Investigation and It’s Importance

The process of exploring to characterize or define small scale properties of substrata at construction sites is unique to geotechnical engineering. In other engineering disciplines, material properties are specified during design, or before construction or manufacture, and then controlled to meet the specification. Unfortunately, subsurface properties cannot be specified; they must be deduced through exploration.

Charles H. Dowding (1979)

What is site investigation?

Site investigation is a process of collecting, assessing and reporting the information about subsurface condition. The information generally comprises of the pertinent physical properties of soil and rock such as its strength, compressibility, and permeability.

Sometimes, chemical nature of soil and groundwater may be desired to evaluate hazard of corrosion on the foundation structure.

Why is site investigation necessary?

Most engineers work with manufactured products that have very consistent and predictable engineering properties. For example, when a structural engineer designs a I-section 100×150 beam to be made of A36 structural steel, he or she can be confident the yield strength will be 250 N/mm². Moment of inertia can also be calculated accurately, and so on. There is no need to test A36 steel every time someone wants to design a beam; we simply specify what is to be used and the contractor is obliged to supply it.

But for geotechnical engineers, we need to work with whatever soil or rock is present at our site. So instead of specifying required properties, our task is to determine the existing properties at our site and hence site investigation becomes necessary.

Phases of site investigation:

A complete site investigation typically includes three phases:

First, the stratigraphical survey means subsurface exploration and sampling which will provide information regarding depth to ground water, sequence, thickness, extend and approximate identification of encountered strata. It also includes collection of samples of soil and rock for its identification and laboratory tests and making observations / field tests to determine condition of samples and to estimate physical properties of the subsoil.

The physical survey relates to field and laboratory tests to obtain the engineering properties of the subsurface materials.

In the evaluation and formulation stage, definite solution to the problem at hand has to be evolved utilizing the information obtained in the stratigraphical survey, physical survey and personal experience.

Benefits:

A proper site investigation is done to save the cost of the project. The notion may seem opposite since it involves costs on its own but on the long term it may help to prevent delays in the project or help for an economical design.

Let us consider a simple example, of a site where a housing construction project is scheduled. The house covers over a large portion of the area but the client suggests taking samples only from a single borehole and we end up specifying bearing capacity based on single borehole. The larger portion of the area remains uncertain. Underneath there may be a presence of some problematic soil that we are unaware of.

The design of the housing complex is finished, and the construction process is undertaken based on the bearing capacity of the soil and the foundation size that we specified.

Now during the construction process, we may come across the unforeseen problematic soil. Now, the whole design process would need revision as the problematic soil may not offer the same bearing capacity for the specified foundation size. This can result in delays and additional costs to the project. Such unforeseen cases can be avoided through proper site investigation.

Also through a thorough site investigation, we will gain more confident on subsurface properties. By that we won’t be needing to suggest lower strength values as no engineers would want to take the risk. By that, the foundation size or size of any other structures may be reduced. This results in an effective and economical design.

Pic: Washboring method adopted as a form for site investigation @Dilibazaar, Kathmandu by Soil Engineering Pvt. Ltd.