Laboratory & In-Situ Testing

Most site investigation programs involve in situ testing, sampling and then carrying out laboratory tests on these samples. With a tight budget, it is necessary to strike the right balance between the laboratory and in situ testing. The number and depths of boreholes, types of samples, and the types of laboratory and in situ tests determine the cost of the site investigation program, which can cost up to 1% of the project budget. At Purdue University, Late Professor GA Leonards used to tell us "You need to know what you are looking for." This is very true in planning a site investigation program.

Which is better – laboratory or in situ tests? You should never compare them. They have their pros and cons, and both have their roles to play. Laboratory tests have well defined and controllable boundary conditions, more rationale theory behind the interpretation, but test smaller volume of soil. In situ tests are quicker, test a larger volume of soil and hence more representative, but the interpretation is generally empirical and crude. Geophysical surveys (e.g., ground penetrating radars, electrical resistivity, and seismic refraction) can nicely complement a traditional site investigation program. And now we have drones!

About 80-90% of the in situ testing worldwide consist of standard penetration (SPT) and cone penetration tests (CPT). Vane shear has its special place for determining the undrained shear strength and sensitivity of soft clays. Dilatometer test (DMT) and pressure meter test are becoming popular. Seismic CPT or DMT are widely used for determining the shear wave velocities that can be useful in determining dynamic properties.

At ASSACO, we generally get the students to estimate the cost of a site investigation program (using the current rates used locally) for a specific job and they would be surprised to see how the cost escalates with addition of boreholes and tests. It is a pity that these are not covered in traditional textbooks (not in my books too), with the costs varying between countries. As fresh graduate engineers, you will be involved in preparing proposals, tenders and hence would get into these sooner than later.