Early sDMT set-up
(Martin and Mayne 1998)
The flat dilatometer (DMT) was first developed in Italy by Prof. Marchetti in the mid 1970’s as a tool to investigate soil modulus values for laterally loaded driven piles. Further experimental work was undertaken to determine other practical applications for the test to obtain correlations with geotechnical parameters (Marchetti 1980) and the equipment was further refined. Since 1980, however, the mechanical DMT equipment has remained largely unchanged.
The seismic DMT (sDMT) was first developed in 1988 with a single triaxial geophone located just above the standard mechanical DMT blade. Through the 1990’s there were subsequent improvements to the system, but only one geophone was still used. The current sDMT with two geophones was developed in Italy in 2007.
The test is now used all over the world and is gaining increasing popularity. Much research has been undertaken on the SDMT to improve correlations with soil parameters and to develop new applications for the test.
The seismic dilatometer is a combination of a mechanical flat dilatometer (DMT) and a seismic module located above the DMT blade. The combined DMT and seismic module is referred to as the seismic dilatometer (sDMT), but the DMT and seismic tests are two independent tests that are carried out concurrently in the same sounding.
The DMT blade is a stainless steel blade approximately 15 mm thick and 96 mm wide with a 60mm diameter circular membrane on one side. The blade is connected to a pneumatic-electric tube that transmits both gas pressure through the flexible nylon tube and an electric current through a single wire that runs through the tube. The other end of the tube is connected to a control box. Nitrogen gas is connected to the control box, which controls and records the pressure delivered to the blade.
With the circular membrane pushed flat against the blade, the membrane closes an electrical circuit that runs along the single wire to the control box. This closed circuit causes a buzzer to activate on the control box. When the membrane is inflated it ‘lifts off’ its seating breaking the circuit and causing the buzzer to deactivate. When the membrane is inflated by a set distance of 1.1 mm from the blade, the internal mechanism reconnects the circuit and the buzzer reactivates.
The DMT blade is pushed into the ground using our Pagani TG63-150 CPT rig. At depth intervals of 250 mm, penetration is stopped and the membrane inflated. When the membrane ‘lifts off’, the buzzer is de-activated and the pressure required to do so is recorded. This is the ‘A’ reading, which is corrected by membrane calibration to give p0 (lift-off pressure). Inflation of the membrane continues until the buzzer reactivates, which is when the membrane has inflated by a distance of 1.1 mm. This is the ‘B’ reading, which corrects by calibration to p1 (expansion pressure). The pressure is then released after which the buzzer will deactivate. At this stage a third reading (‘C’ reading or p2) can be obtained by slowly releasing the pressure until the membrane sits flat on its seating against the blade and the buzzer reactivates again.
The seismic part of the equipment is a separate add-on test carried out in combination with the DMT test. The seismic module is attached immediately above the DMT blade and comprises a stainless steel tube with two geophones, which are 500 mm apart with the centre point between the two geophones being 500 mm above the centre of the membrane on the DMT blade.
The seismic test is carried out at 500 mm depth intervals. A beam at the ground surface is struck with a hammer to generate a shear wave that propagates through the ground. We normally use an electrically operated Autoseis Hammer, which is designed to optimise shear wave generation and provide consistent energy for each hammer blow. Alternatively we use a 4.5 kg hammer on a timber beam. The beam is held down to the ground under the levelling feet of the rig.
After the beam has been struck, the shear waves that are generated in the ground are recorded by the geophones in the seismic module. The geophone signals are sent back up to a computer on the ground surface as seismographs. As one geophone is 500 mm deeper that the other, the seismographs are shown as similar waves but with a time lag. The computer program automatically re-phases the two seismographs to obtain the time difference in the arrival of the shear waves at the two geophones. The difference in the distances between the shear wave source and the two geophones is divided by the time difference to give the shear wave velocity applicable to the soil layer between the two geophones. This provides a very rapid and accurate assessment of shear wave velocity.
A pressure transducer seismic box connected to a laptop computer is used with the DMT control box for automatic recording of the DMT and seismic tests using the Marchetti software, Sdmt Elab.
The results of the test are presented in a report format, which includes the basic raw data from the test and correlations to soil parameters. The Marchetti acquisition and data reduction program, Sdmt Elab is used to generate the results report. The results are presented as a pdf report and in tabular form in an excel file. The report can also be provided in MS Word format.
We are not consulting engineers and any interpretations that we provide are not our professional opinion, but are provided as a guide for the client to assess for himself.
The results can be provided immediately after completion of the test. Usually, the results are emailed to the client at the end of each day’s work. The results reports and excel files are provided at no extra cost.
Due to its relative accuracy of estimating modulus values and its sensitivity to insitu horizontal stresses, the sDMT test is particularly well suited to problems involving settlement prediction and any situations where lateral loading or stress history are factors. The most common applications for the test are:
Compaction control (e.g. Evaluating effect of ground improvements)
Laterally loaded piles
Finite Element Modelling
Liquefaction assessment of some New Zealand soils (particularly the pumaceous soils of the central North Island) is not well represented by the CPT test. The seismic dilatometer (sDMT) may provide a better method for liquefaction in these soils. Research is currently underway on this topic as well as others.
For more information on sDMT testing, interpretation and applications, please see our Papers and links page.