9 FLAT DILATOMETER TEST (DMT)

9.1 General

(1)P The flat dilatometer test DMT covers the determination of the in situ strength and deformation properties of fine grained soils using a blade shaped probe having a thin circular steel membrane mounted flush on one face.

(2) Results of DMT tests are mostly used to obtain information on soil stratigraphy, in situ state of stress, deformation properties and shear strength.

(3)P The basis of the test consists of inserting vertically into the soil a blade-shaped steel probe with a thin expandable circular steel membrane mounted flush on one face and determining, at selected depths or in a semi-continuous manner, the contact pressure exerted by the soil against the membrane when the membrane is flush with the blade and subsequently the pressure exerted when the central displacement of the membrane reaches 1,10 mm.

(4) The DMT test is most appropriate in clays, silts and sands where particles are small compared to the size of the membrane.

(5)P The tests shall be carried out in accordance with a method that complies with the requirements given in this section.

(6)P The test method used shall be reported in detail with the test results.

(7) The test method may be reported by reference to a published standard.

9.2 Definitions

9.2.1 Equipment and testing procedure

(1) The main parts of a dilatometer equipment are defined in figure 9.1.

(2)P dilatometer blade or dilatometer probe: blade-shaped steel probe that is inserted into the soil to run a DMT test.

(3)P membrane: circular steel membrane that is mounted flush on one face of the blade and is expanded when applying a gas pressure at its back.

(4)P switch mechanism: apparatus housed inside the blade, behind the membrane, capable to activate and disconnect an electric contact which in turn shall respectively set off and on an audio and/or visual signal when the membrane expands and reaches two preset deflections equal to 0,05 mm and 1,10 mm respectively.

(5)P pneumatic-electric cable: cable that connects the control unit to the blade, delivers gas pressure at the back of the membrane, and provides electric continuity between the control unit and the switch mechanism.

(6)P control and calibration unit: set of suitable devices capable of supplying gas pressure to the back of the membrane and measuring the pressure when the switch mechanism activates and disconnect the electric contact behind the membrane.

(7)P earth wire: wire connecting the control unit to the earth.

(8)P pressure source: pressurized gas tank filled with any dry nonflammable and noncorrosive gas.

Dilatometer equipment and definition of calculated in situ soil pressure
Figure 9.1: Dilatometer equipment and definition of calculated in situ soil pressure

(9)P membrane calibration: the procedure to determine the membrane calibration pressures equal to the suction and the pressure that must be applied in air to the back of the membrane to retract its centre to 0,05 mm expansion or to expand it 1,10 mm respectively.

(10)P dilatometer profiling: the execution of a sequence of dilatometer tests from the same station at ground level along a vertical direction at closely spaced intervals with depth increments ranging between 150 mm and 300 mm.

9.2.2 DMT parameters

(1)P The following parameters are defined:

  • A-pressure: the pressure (PA)that must be applied to the back of the membrane to expand its centre 0,05 mm in soil;
  • B-pressure: the pressure (PB)that must be applied to the back of the membrane to expand its centre 1,10 mm in soil;
  • A-membrane-calibration-pressure: the suction, (ΔPA)recorded as a positive value, that must be applied to the back of the membrane to retract its centre to the 0,05 mm deflection in air;
  • B-membrane-calibration-pressure: the pressure (ΔPB) that must be applied to the back of the membrane to expand its centre to the 1,10 mm deflection in air;
  • ΔPA;avg and ΔPB;avg:the averaged values of the membrane calibration pressures obtained from the respective values of ΔPA and ΔPB measured before and after each dilatometer profiling or single dilatometer test;
  • Zm-pressure: gauge pressure deviation from zero when venting the blade to atmospheric pressure;
  • soil pressure p0: soil pressure against the membrane when it is flush with the blade (e.g. at zero expansion), also termed contact pressure, as shown in figure 9.1;
  • soil pressure p1: soil pressure against the membrane when its centre is expanded 1,10 mm as shown in figure 9.1;
  • in situ pore water pressure prior to blade insertion u0:in situ pore water pressure prior to blade insertation at the elevation of the centre of the membrane;
  • the in situ effective vertical stress Pv0:vertical stress prior to blade insertion at the elevation of the centre of the membrane;
  • dilatometer material index IDMT:an index related to the type of soil;
  • dilatometer horizontal stress index KDMT:an index related to the in situ horizontal stress;
  • dilatometer modulus EDMT: a parameter related from theory to the modulus of elasticity of the soil.

Eurocode 7 Geotechnical design — Part 3: Design assisted by fieldtesting