In Situ Soil Tests: Methods, Parameters and Limitations

In Situ Tests: Parameters, Methods, Limitations

In situ tests: Values of different parameters are obtained from theoretical interpretation of the test result. Compared with laboratory tests, in situ tests reduce some limitations such as:

• Disturbance of soil samples (matters less; no transport of the sample).
• Change in the stress state of the sample.
• Small size of laboratory samples may be unrepresentative (a larger portion of the soil is involved).

Other limitations appear:

• Some disturbance is always present.
• Less control over test conditions (in the laboratory we can control water conditions).
• If the parameters of the theoretical model are two or more, only a relationship between them is obtained (for example, E and ν, or c and φ).

Common In Situ Tests

Pressuremeter (presumeter)

Pressuremeter: It gives information on deformability and strength. We drill the borehole, remove the drill rod and introduce the pressuremeter tool. The pressuremeter has a rod with a flexible membrane. This membrane is inflated until it makes contact with the soil. We keep applying pressure and measure the volume of the membrane to determine the soil strength. With this test we can express the relationship between pressure and volume. There is a type of pressuremeter called the self-boring pressuremeter, which can carry out the test by advancing itself and testing without removing the tool.

Vane test

Vane test: The vane test gives the undrained shear strength. We introduce the instrument into the hole, push it into the soil and apply a torque movement until failure. We obtain the strength in soft soils. In hard soils we cannot obtain the maximum torque.

Lefranc test

Lefranc test (permeability): A borehole is used to obtain the permeability of soft soils. Two types:

• Constant head: We pump water into the borehole until the flow is constant, then determine the permeability of the media.
• Falling head: We introduce a cylinder into the borehole, fill it with water to a height h1 and measure the time for the water level to drop to h2.

Lugeon test

Lugeon test (permeability): It is used to obtain permeability of rock masses, mainly to evaluate permeability for foundation design. Water at constant pressure is injected into the rock mass through a slotted pipe by means of pneumatic packers. Secondary permeability is through discontinuities (usually much higher than through the intact rock mass).

Plate loading test

Plate loading test: Determines the bearing capacity of the soil by loading a steel plate located at foundation level and recording the settlements produced by the load. The test is carried out in increments until the plate settles at a rapid rate. We obtain the deformability of the soil. We assume a value for ν (Poisson's ratio) and estimate E (Young's modulus).

Direct shear test

Direct shear test: Measures the shear strength properties of a soil. From this test we obtain shear strength parameters (such as cohesion and friction angle).