Mechanical properties of soils determined in situ

A wide variety of parameters are used to describe the physical and mechanical properties of soils:

• identification and status parameters ;

• deformability parameters ;

• resistance parameters ;

• permeability parameters.

The most common in situ soil mechanics tests are penetrometer tests (static or dynamic, including the SPT core penetration test), Ménard pressuremeter tests and site scissometer tests.

This article presents these classic tests (equipment, implementation methods, interpretation of measurements). The use of the results for soil investigation and structural design will only be briefly discussed, as the subject is covered in other articles.

There are other interesting in-place tests, but they are less frequently used due to their more complex nature or more limited field of application. Examples include the self-drilling pressure meter, the pressure-penetrometer and the phicometer. The section of this article devoted to them also outlines the additions that can be made to conventional tests (in particular piezocones and friction sleeves for the static penetrometer) and presents the most common plate loading and water tests.

In-situ tests can be arbitrarily subdivided into two main categories: tests that give a soil characteristic at the limit (or, if you like, at "failure") and tests that also give a stress-strain relationship –.

Static and dynamic penetrometers belong to the first family. They are driven into the ground either at a set speed or by repeated shocks. The soil beneath the point is constantly in a state of limit as the tip of the instrument punches through it.

Pressure meters and scissometers belong to the second family, as they stress the soil from its resting state to a limit value.

By definition, therefore, penetrometers cannot be used to determine soil deformation characteristics, except by correlation. However, pressure and scissometers can be used to measure deformability parameters, useful for determining settlement or deformation of structures. Depending on how the equipment is used and the test conditions, we will examine the advantages and limitations of this particular feature.

It is very rare for all these parameters to be measured at a sufficient number of points on a given site to be able to assess the entire soil mass. Usually, geotechnical reconnaissance is limited to the strict minimum, with the values of some parameters available at some points and other parameters at others. The geotechnical engineer has to make the most...