Dilatometer Testing

DMT

The Flat Plate Dilatometer Testing (DMT) is an in situ test for the determination of various key soil parameters used for geotechnical design.

The test is rapid, accurate, simple and cost effective. The results are highly repeatable and independent from the operator.

The blade may be advanced with any field machine, including CPT rigs and drill rigs.

DMT measurements are performed in situ, directly on the soil in its original position and state. This eliminates the disturbance caused by drilling, sampling, transportation to the laboratory and sample preparation at the laboratory.

At each test depth, a circular steel membrane located on one side of the blade is expanded horizontally against the soil. The pressure readings are recorded at specific moments during the membrane expansion. The blade is advanced to the next test depth, typically with 0.20 m depth interval.

DMT result profiles are available in real time during the test execution.

The direct measurement of soil deformation enables accurate estimations of elastic modulus.

The KD parameter provides stress history information of the soil, a very difficult property to assess with other testing methods.

Main Applications:

Soil stratigraphy
Stress history (OCR, Ko)
Settlement prediction
Ground Improvement qualification
Slip surface detection in clayey slopes
P-y curves for laterally loaded piles
Liquefaction potential
Permeability in clay
FEM input parameters (e.g. Plaxis)
Subgrade modulus for pavements
Subgrade modulus for retaining walls

The Medusa DMT

The Medusa DMT is an automated Dilatometer testing probe able to autonomously perform dilatometer tests, which generates and measures the pressure directly at depth. The obtained DMT readings are extremely repeatable, because the fluid is liquid (incompressible) and the pressure regulation is electronically controlled. New test procedures are possible, such as high resolution dissipation tests and horizontal pressure measurements during penetration.

A rechargeable battery pack powers an electronic board, connected to a pressure transducer and to a custom designed motorized syringe. The operator controlled software in the electronics activates the motorised syringe for generating the pressure required to obtain the DMT readings. The maximum operating pressure is 100 MPa. A high accuracy pressure transducer is used to measure the pressure generated by the syringe and operating on the membrane. An electric wire provides the contact status of the membrane to the electronic board. The A, B, C pressure readings are recorded by the Medusa software.

IGS operate the Medusa DMT with an electric cable running from a computer laptop at ground surface down to the probe at depth. In this configuration, the operator may activate the measurement cycle from the computer as soon as the test depth is reached. During the cycle all automation parameters, such as the battery status, the voltage and current provided to the engine, the position of the piston of the motorized syringe, the probe inclination and other additional information, are available in real time. The DMT parameters, in particular the current pressure and membrane contact status, are displayed in real time during the measurement, as for the traditional DMT pneumatic technology.

For the Medusa DMT the firmware embedded in the electronic board implements the procedure for inflating and deflating the DMT membrane. The hydraulic pressurization of the motorised syringe actuates a volume controlled expansion of the membrane, which enables to impose a programmable timing for achieving the readings. Therefore the Medusa DMT is capable to perform dilatometer tests with the recommended timing suggested in the international standards. At the same time, the highly accurate and repeatable time-for-reading facility provided by the instrument prompts for its potential use for performing dilatometer tests adopting variable pressurisation rates in intermediate soils.

Advantages of The Medusa DMT

The Medusa DMT has several advantages over the traditional pneumatic equipment, both in terms of simplification of the probe and test procedure, and in terms of increased accuracy of the measurements (Marchetti et al. 2019).

The major advantages are listed here below:-

The overall equipment occupancy is reduced to the size of the standard blade with a rod connected on its top, for a total height of about 1 m. The gas tank, the control unit and the pneumatic cables are no longer required.
The pressure is generated and measured locally at depth, not at ground surface. This eliminates any possible problem of pressure equalization along the pneumatic cable of the traditional equipment.
The pressurization rate of the membrane is independent of the operator. The automatic (volume controlled) procedure of membrane pressurization operated by the motorized syringe is highly repeatable and capable to impose the correct timing to obtain the A and B pressure readings, strictly according to the specifications of the international standards or any other required timing.
The capability of the Medusa DMT of measuring (virtually continuously) the total horizontal pressure against the membrane with time enables new research possibilities.
Short A-dissipations, consisting in repeated A-readings (without expansion of the membrane from A to B) for a couple of minutes, may be executed to detect intermediate or partially draining soil layers (Marchetti 2015, Marchetti and Monaco 2018). The duration of such short A-dissipations (much shorter than conventional DMT-A dissipation tests that provide the entire A-decay curve) is sufficient to discover whether an appreciable reduction of the total contact pressure A, reflecting pore pressure dissipation, occurs during the test. The Medusa DMT permits to execute routinely short A-dissipations before recording each standard A-reading, which is taken 15 s after reaching the test depth. In clays no appreciable pore pressure dissipation occurs in 15 s and the A-readings remain nearly constant, indicating fully undrained conditions, while a substantial reduction of A in 15s prompts for partial drainage.

Our DMT and Medusa DMT equipment has been developed and supplied by our good friends at Marchetti - Please follow the link to visit the Marchetti Official Website

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DMT

The Medusa DMT

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