Drilling and Insitu Tests

Dilatometer (DMT-SDMT)


Dilatometer is developed by Marchetti in Italy at 1980. With this experiment, dilotometer modulus, material index and horizontal stress index will be determined and also soil types, unit volume weight, vertical dranaiged captive modulus, in sands pore pressure and internal friction angle, in clays creep endurance out of drainage, intense consolidation ratio, consolidation coefficent, permeability coefficent, soil pressure coefficent will be assigned. Obtained parameters is used for determining fill control, slide surface, liquifaction potential, bearing capacity, settlement and pile capacity calculations.

The flat dilatometer is a stainless steel blade having a flat, circular steel membrane mounted flush on one side. The blade is connected to a control unit on the ground surface by a pneumatic-electrical tube (transmitting gas pressure and electrical continuity) running through the insertion rods. A gas tank, connected to the control unit by a pneumatic cable, supplies the gas pressure required to expand the membrane. The control unit is equipped with a pressure regulator, pressure gage(s), an audio-visual signal and vent valves. The blade is advanced into the ground using common field equipment, i.e. push rigs normally used for the cone penetration test (CPT) or drill rigs. Push rods are used to transfer the thrust from the insertion rig to the blade.

The test starts by inserting the dilatometer into the ground. Soon after penetration, by use of the control unit, the operator inflates the membrane and takes, in about 1 minute, two readings:

  1. the A-pressure, required to just begin to move the membrane against the soil ("lift-off")
  2. the B-pressure, required to move the center of the membrane 1.1 mm against the soil.

A third reading C ("closing pressure") can also optionally be taken by slowly deflating the membrane soon after B is reached.

The blade is then advanced into the ground of one depth increment (typically 20 cm) and the procedure for taking A, B readings repeated at each depth.

The pressure readings A, B are then corrected by the values ?A, ?B determined by calibration to take into account the membrane stiffness and converted into p0, p1.

The field of application of the DMT is very wide, ranging from extremely soft soils to hard soils/soft rocks. The DMT is suitable for sands, silts and clays, where the grains are small compared to the membrane diameter (60 mm). It is not suitable for gravels, however the blade is robust enough to cross gravel layers of about 0.5 m thickness. Due to the balance of zero pressure measurement method (null method), the DMT readings are highly accurate even in extremely soft - nearly liquid soils. On the other hand the blade is very robust (can safely withstand up to 250 kN of pushing force) and can penetrate even soft rocks. Clays can be tested from cu = 2-4 kPa up to 1000 kPa (marls). The range for moduli M is from 0.4 MPa up to 400 MPa.

CPT equipment is used for penetrating DMT probe to the soil. The measured value in DMT is more realistic according to CPT and SPT. The obtained data in Dmt is used directly with determining corelations while usage of foundation design, is anaother advantage of DMT.

+