9.3 Equipment

9.3.1 Dilatometer equipment

(1)P The equipment for dilatometer testing shall comprise:

  • dilatometer blade with suitable threaded adaptor to connect to push rods;
  • pneumatic-electrical cable;
  • earth wire;
  • control and calibration unit;
  • pressure source.

(2)P The dimensions of the blade, of the apex angle of the penetrating edge and of the membrane shall be comprised within the limits indicated in fig. 9.1.

(3)P The pneumatic-electrical cable, which provides pneumatic and electrical continuity between the control unit and the dilatometer blade, shall have stainless steel connectors with wire insulators to prevent short circuit and washers to prevent gas leakage.

(4)P The control and calibration unit shall accomplish the following:

  • it shall be earthed;
  • it shall control the rate of gas flow while monitoring and measuring the pressure of gas transmitted from the control unit to the blade and the membrane;
  • it shall signal the instants when the electric switch changes from on to off and vice versa.

(5)P The pressure measurement devices of the control and calibration unit shall allow to determine the pressure applied to the membrane with intervals of 10 kPa and a reproducibility of 2,5 kPa at least for pressures lower than 500 kPa.

(6)P The pressure source shall be provided with a suitable regulator, valves and pressure tubing to connect to the control unit.

9.3.2 Insertion apparatus

(1)P The equipment for inserting the dilatometer blade shall comprise:

  • thrust machine to insert and advance the dilatometer blade into the soil;
  • push rods with suitable adaptor to connect to the blade;
  • hollow slotted adaptors for lateral exit of the pneumatic-electrical cable.

(2)P The thrust machine shall be capable of advancing the blade vertically with no significant horizontal or torsional forces.

(3) Penetration rates in the range of 10 rnm/s to 30 mm/s should be applied. Driving should be avoided except when advancing the blade through stiff or strongly cemented layers which cannot be penetrated by static push.

(4)P Push rods shall be straight and resistand against buckling.

9.4 Test procedure

9.4.1 Calibration and checks

(1)P All the control, connecting and measuring devices shall be periodically checked and calibrated against a suitable reference instrument to assure that they provide reliable and accurate measurements.

(2)P The dilatometer blade and membrane shall be checked before penetrating in the soil. The blade shall be mounted axially with the rods. It shall be planar and coaxial and have a sharp penetration edge. The membrane shall be clean of soil particles, free of any deep scratches, wrinkles or dimples and expand smoothly in air upon pressurization.

(3)P The maximum out of plane deviation of the blade, defined as the maximum clearance under a 150 mm long straight edge placed along the blade parallel to its axis, shall not exceed 0,5 mm; the maximum coaxiality error of the blade, defined as the deviation of the penetration edge from the axis of the rods to which the blade is attached, shall not exceed 1,5 mm.

(4)P The control unit and the tubing shall be checked for leaks before starting a sequence of dilatometer profilings by plugging the blade end of the pneumatic-electrical cable and checking for any pressure drop in the system. Leakage in excess of 100 kPa/min shall be considered unacceptable and shall be repaired before testing begins.

(5) With the dilatometer equipment assembled and ready for testing the switch mechanisms should be checked by hand pushing the membrane flush with the blade to check that the audio and/or visual signals on the control unit are activated.

9.4.2 Membrane calibration procedure

(1)P The membrane shall be calibrated to measure the values of the ΔPA-suction and ΔPB-pressure with the dilatometer equipment assembled and ready for testing immediately before inserting the blade into the soil and upon retrieval to the ground surface, both when running a dilatometer profiling or even a single test.

(2)P If the values of the membrane calibration pressures ΔPA and ΔPB, obtained before penetrating the blade into the soil, fall outside the limits ΔPA = 5 kPa to 30 kPa and ΔPB = 5 kPa to 80 kPa respectively, the membrane shall be replaced before testing commences.

(3) After a membrane has been replaced, the new one should be exercised to improve the stability of the ΔPA and ΔPB values. Such exercising may consist in pressurizing the membrane in air to 500 kPa for a few seconds. Care should be taken to avoid overexpansion and permanent deformations of the membrane.

(4)P After any membrane calibration the values of ΔPA and ΔPB shall be promptly recorded. All the obtained values of ΔPA and ΔPB shall be available on site.

(5)P During calibration the audio and/or visual signal activated by the electric switch shall stop and return sharply and unambiguously while sensing the 0,05 mm and 1,10 mm expansions respectively.

(6) When testing soft soils the membrane calibration procedure should be performed more than once to assure that stable values of ΔPA and ΔPb, falling within the prescribed limits, are constantly determined.

9.4.3 Performing the test

(1)P After the blade has been inserted into the soil and advanced to the selected test depth, the load applied to the push rods shall be released and the blade pressurized without delay to expand the membrane.

(2)P The rate of gas flow to pressurize the membrane shall be such that the PA- reading is obtained within 20 s from reaching the test depth and the PB- reading is obtained within 20 s after the PA- reading.

(3)P Once Pb- has been determined the membrane shall be depressurized immediately, in order to prevent further expansion and permanent deformations, and the blade advanced to the next test depth or retrieved to the ground surface.

(4) Depending on the system used to advance the blade, the pneumatic-electric cable connected to the blade should be pre-threaded through the push rods for protection or left outside, using a slotted adaptor to egress it, and taped to the rod every 1 m.

(5) If a friction reducer is used to limit the thrust needed to advance the blade, it should be located at least 200 mm above the centre of the membrane.

(6)P After the blade has been retrieved to the ground surface and the membrane calibration procedure performed the values of ΔPA and ΔPbshall be recorded and compared with those measured previously. If the values of ΔPA and ΔPB measured before inserting the blade into the soil and after retrieval to the ground surface differ by more than 25 kPa then the test performed between the two successive calibration procedures shall be discarded.

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