Site investigation is an integral part of any construction project, and should be attributed as much significance as would be ascribed to any other part of the project. The risk associated with undertaking a project without adequate site investigation is monumental. It's wiser to be scared and prepared than to be daring and unprepared.
It has been proven over and over again by overconfident contractors that undertaking a project without site investigation is not only risky, it is foolish and dangerous. Risk is something that can be managed or its consequences minimized, but it cannot be ignored.
Reliable information obtained from a site investigation report is what enables designers to design strong and lasting civil projects. (Proper planning is essential for any construction project. Learn more in The Planning Process: How to Prepare for a Trenchless Project.)
Unforeseen risks are still a large part of a project, and the finances related to the risk is allocated to the contractor or owner, or shared between both. The cost of a project will vary based on what option is chosen. Site investigation is unique to every project and should be planned based on the project requirements. The reports are not transferable, even if another project is to be undertaken at the same site, but it can be used as a reference.
There are numerous published documents outlining the guidelines for conducting site investigations. The ASTM's (American Society for Testing and Materials) geotechnical engineering standards are developed to aid in specifying, testing and investigating the properties of surface and subsurface materials relevant to a specific project.
Boring, drilling, sampling, testing and nondestructive testing methods, as required, are employed to conduct site investigation. Sampling and testing are done remotely from the surface at some distance below the ground. The testing sample should remain undisturbed until testing is complete, in order to get a clear picture of subsoil conditions.
Some soil characteristics that can be obtained are its material and chemical constituents, strength, thickness and orientation.
Stages of Site Investigation
Site investigation can be broadly classified into four stages: reconnaissance, data and map study, in-depth investigation and laboratory testing.
Simply scouting around the site can sometimes provide a lot of information such as topography, vegetation, geological features and utility lines. Observations made during site reconnaissance include:
- Topography of the land including water bodies, estuaries, reserved land and quarries
- Slopes' angles and orientation
- Presence of structures, heritage structures, trees and utility lines
- Presence of hazardous industries or waste disposal sites that could be potential public health hazards
- Areas with loose soil, patches of soil discoloration, excess growth of vegetation or foul smell
- Comparison of available plan with current situation, i.e., addition of new structures or utility lines
Data and Map Study
Details obtained from reconnaissance now need to be thoroughly understood in order to determine the necessary tests and samples to be collected for the next stages of site investigation, i.e., in-depth investigation and lab testing. Data can be obtained from local municipalities, libraries, county record and survey offices, utility and service providers, and from commercial databases through the internet.
Geological maps, old plans and photographs can be obtained from the survey or records department. The study includes:
- Presence of licensed water abstraction and discharge consents, landfills, and waste disposal sites
- History, if any, of hazardous incidents
- Quality of surface water and groundwater vulnerability
- Past local borehole records
- Presence of coal and other mines
- Historical mapping
- Buried and overhead supply and utility lines
Based on the above study, locations for trial pits and boreholes can be selected. The selection should be such that a complete geologic sub-surface perspective can be obtained. At least three points should be selected with spacing of 10 to 30 meters depending on site conditions and type of project. Commonly boreholes, probes and trial pits are used to undertake in-depth investigation. (To learn more about investigations, see Getting Technical: Information Required in a Geotechnical Investigation Report.)
Boreholes are used extensively for site investigation because they are less disruptive to the surface than trial pits, and can be taken to a greater depth. Also, boreholes can be drilled through any type of subsurface strata using percussion drilling, rotary drilling, wash boring and power augering.
Boreholes drilled using the percussion method are suitable for installation of groundwater monitoring systems, which provide valuable information. In rotary drilling, a drill bit is rotated at the bottom of the borehole using open-hole drilling or core drilling.
Core drilling is preferred because the core is retained within the core barrel and brought to the surface for investigation. Sometimes smaller, portable drilling equipment may be required where accessing ideal sampling locations is difficult.
This method consists of dynamic and static cone penetration methods. Dynamic probing is similar to a standard penetration test (SPT) and the number of blows required to drive the cone 100 mm is recorded. In static probing, the head of the cone has a sensor that records the resistance to driving force.
These pits are excavated in soils that can support themselves for the required time needed to conduct the investigation. In loose soils, shoring may be required. There are depth restrictions in trial pits, and the consequent area of ground that is disturbed is also quite large.
In environmentally sensitive locations, trial pits may not be permitted. The advantages are that it indicates vertical and lateral variations in the subsoil strata.
The soil recovered during the in-depth soil investigation is tested in the lab at this stage. The material obtained is classified and characterized, and based on the project, geotechnical parameters are provided for the design phase. The basic tests are:
For cohesive soils, the tests are done for moisture content, plasticity index, particle size distribution and bulk density. For granular soils, tests are done for particle size distribution and bulk density.
Shear Strength Test
For cohesive soils, tests are done for short-term stability, long-term stability and residual shear strength properties using the shear box test. For granular soils, the shear box test is done for both short-term and long-term stability analysis.
What We've Learned
Site investigation varies from project to project and needs to be as comprehensive as possible. Visual inspection and desk studies using maps and data should not be ignored, because they form the basis for in-depth investigation. Based on the project type, the most suitable intrusive investigation should be selected.
Monitoring groundwater by using standpipes and piezometers is also very important, as they provide crucial data required for design purposes. The tests mentioned above are not exhaustive, nor are they the only ones available.
Many stipulated testing methods are available for perusal in the standards prescribed by ASTM and the Occupational Safety and Health Administration (OSHA).