Various instrumented “research” dilatometers have been constructed in the last decades.
The instrumentation in most cases has consisted in displacement sensors, monitoring the movement of the center of the membrane, in order to define the pressure-displacement curve. Other researchers have instrumented the face of the blade with a pressure transducer to monitor the contact pore water pressure.
The blades instrumented with displacement sensor have shown that the pressure-displacement curve during the 1.1 mm travel is essentially rectlinear. This rectilinear behaviour has in fact made less interesting instrumented dilatometers, being sufficient to know the initial and the final point of the “straight-line”.
Fig. 1. Two diagrams obtained by instrumented blades. The pressure deflection curve is nearly linear.
The blades instrumented with pressure transducers to monitor the contact pore water pressure have indicated that, during a test, soil conditions in clay are essentially undrained, in sand essentially drained, in silt partially drained. If a DMTA dissipation is executed, then in sands the A-reading (and pore pressure) is constant because there is no excess pore pressure caused by the penetration. In clay the A-reading (and pore pressure) is constant because the excess pore pressure caused by the penetration has no time to dissipate appreciably during the test. In silts the A-reading (and pore pressure) decreases during the test due to partial drainage. More information on DMT testing in partially draining silts: Schnaid (2015) and Marchetti (2016).