The flat dilatometer test (DMT)

Dilatometric investigation is one of the most popular test of the subsoil exploration (both: strength and deformation properties of the soil can be determined). The test can be applied to the objects

II and III geotechnical category, particularly the bridge foundations and retaining walls. It is recommended to use this penetrometer together with the static soundings.

DMT test results may be used to obtain information about the stratigraphy of the subsoil, stress condition, deformation properties and shear strength.

What is the DMT sounding:

The Marchetti dilatometer (DMT) consists of a flat steel blade having a thin, expandable, circular steel membrane mounted on its surface. The blade is connected to the control-measurement unit by pneumatic wire used to transfer gas pressure exerted on the membrane. The test involves pressing of the blade which membrane deforms under the influence of the acting gas pressure in the ground. The following measurements at the given depth are performed: the gas pressure needed in the initial phase of the test to obtain contact of the membrane with the surrounding soil, the gas pressure required to the deformation of the membrane center in the direction of the soil and the gas pressure after the controlled return of the membrane to the position in the first measurement.

Practical application of the DMT sounding:

Dilatometer can be used to identify the type and state of the soil, determine the geological profile and history of the stress condition, estimate values of the geotechnical parameters (undrained shear strength, coefficient of the at-rest earth pressure of the non-cohesive and cohesive soils, overconsolidation stress, deformation modules).

Based on the in situ measurements of the values of pressure and the estimated values of the effective vertical stress and pore pressure, the following dilatometric parameters of the subsoil are determined (at the depth of the measurement):

• dilatometer modulus - on the basis of which the compressibility modulus and the deformation modulus could be calculated
• horizontal stress index – used to determine the earth pressure coefficient, overconsolidation ratio and undrained shear stregnth
• coefficient characterizing the water flow conditions in the soil

Listed parameters defined in the vertical profile from the empirical dependencies should be selectively verified by laboratory tests.