On-site ground testing providing informative in situ characteristics

Also known as field testing, In Situ Testing collects data and analyses soil on-site.

This step in the investigation process typically aims to determine the site’s ground conditions and geotechnical parameters to further supplement the Ground/Site Investigation.

Depending on the soil properties of interest, these in situ tests are often easier and quicker to perform than sampling for laboratory testing. They can also provide an accurate depth profile of the site.

There are four fairly common in situ tests. These include the two most common: Standard Penetration Test (SPT) and Cone Penetration Test (CPT), and the two less often used: Pressuremeter tests and Dilatometer tests.

Each of these tests has its own strengths and weaknesses and targets different soil properties.

Pressuremeter Testing

Pressuremeter testing assesses the strength and stiffness parameters of soils and rocks.

There are different types of pressuremeters. Pre-bored pressuremeters, sometimes referred to as the Ménard Pressuremeter, are lowered into a predrilled hole.

There are also full displacement pressuremeters that are pushed into suitable soils, and self-boring pressuremeters. These are different ways of installing the piezometer prior to testing, but the principle of the test remains the same.

The choice of pressuremeter is largely dependent on the soil type. In stiff soils and hard rock, where penetration or self-boring is not viable, pre-boring is the only option.

The self-boring pressuremeter is one of the best in situ testing instruments in the field. It causes minimal disturbance during installation (i.e., it tests the material at its in situ stress without altering the soil parameters of interest) and is extremely versatile in determining a range of soil properties.

Once installed, the operational principle involves a cylindrical flexible membrane that expands when pressure is applied, either by air or soil via an umbilical tube. This inflates the membrane like a balloon.

The applied pressure causes the borehole wall to deform. The deformation and the applied pressure are measured and used to derive many different properties, including soil strength, stiffness, and in situ horizontal stress.

Dilatometer Testing

The Flat Dilatometer Test is an in situ method that is quite similar to the pressuremeter, except that the instrument is not a cylinder but a blade-shaped probe.

It also uses the displacement of an inflatable membrane to derive stiffness and strength characteristics.

Dilatometer Testing is simple, quick, and cost-effective for obtaining parameters required for a wide variety of geotechnical design applications.