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Marchetti DMT, SDMT & Medusa
DMT
The Marchetti Flat Dilatometer test consists in advancing a blade into the ground with any common field machine. At each test depth a circular steel membrane located on one side of the blade is expanded horizontally against the soil. The pressure readings from this action are recorded at specific moments during the membrane expansion. The blade is then advanced to the next depth, typically with a 200mm (0.2m) depth interval. Discover more ....
SDMT
The Marchetti Seismic Dilatometer is an add-on module that may be combined with the Dilatometer or with the CPT for measuring the shear wave velocity Vs (SDMT) and optionally the compression wave velocity Vp (SPDMT). The supplied software is also user friendly and provides shear wave velocity results in real time during the test execution. Discover more ....
Medusa
The Marchetti Medusa DMT is a probe able to autonomously perform dilatometer tests which generates and measures the pressure directly at depth. The obtained DMT readings are extremely accurate and repeatable because the the fluid contained within is a liquid and therefore incompressible and the pressure regulation is electronically controlled. New testing procedures are possible such as high resolution dissipation tests and horizontal pressure measurements during continuous penetration.
Sea Floor
The Marchetti Sea Floor DMT is a penetrometer for advancing the DMT, the SDMT and also the Medusa DMT with multiple short length strokes (0.100mm or 0.200mm). It is very cost effective compared to competitive devices because it does not require the probe to be advanced at a specific speed (200mm per second for CPT). The first prototype was built and tested however the system is still at a development stage (early 2021).
Software
Marchetti "In-house" developed software is required for data acquisition and for managing the results of the tests completed. Additional software is available for specific geotechnical applications using the output parameters of the test measurements.
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DMT Flat Dilatometer Composition
With reference to Fig. 1 below (DMT Testing Layout): The DMT flat blade is connected to a control unit on the ground surface by a pneumatic-electrical cable running through the insertion rods and transmitting gas pressure and electrical continuity.
The DMT blade has a cutting edge to penetrate the soil and it is advanced into the ground using common field equipment, such as "push rigs" normally used for the cone penetration testing (CPT) or drilling rigs. Push rods are used to transfer the thrust energy from the insertion rig to the blade. Dynamic insertion is not the preferred way, but in some countries, driving is the most common insertion method.
The pressure required to expand the Flat DMT membrane is provided by a gas cylinder (n.5), generally nitrogen or compressed air, connected to the control unit by a pneumatic cable. The gas cylinder is equipped with a pressure regulator (suitable to gas type) which must be able to supply a regulated output pressure of at least 7-8 MPa.
The Marchetti DMT control unit is equipped with pressure gauge(s), an audio-visual signal and vent valves and two pressure gauges, connected in parallel, that have different scale ranges: a low-range gauge (1 MPa), self-excluding when the end of the scale is reached, and a high-range gauge (8 MPa). The low-range gauge permits high resolution when the pression readings are small.
DMT Testing Procedure
The Marchetti DMT Testing Procedure consists of a sequence of operations. The procedure is relatively simple and in a condensed explanation the system is connected as per the adjacent diagram. The Flat DMT Blade is then pushed into the soil to the required test depth. The membrane is pressurised and deflated and the data is recorded. The DMT Blade is then pushed down to the next Test Depth and the process is repeated over and over until all of the tests have been completed. Please read our Full DMT Testing Procedure for a more in depth explanation which includes pushing techniques.
DMT Testing Procedures, Specifications, Forms & Tutorial Videos
Tutorial Videos
Main Applications
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Soil Parameters
The Marchetti DMT provides estimates of the oedometer modulus M, shear strength Su, OCR and Ko in clay, liquefaction resistance CRR.
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Settlement Prediction
Many of the world experts consider DMT one of the best tools for predicting settlements.
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Compaction Control
Marchetti DMT has been recognised to be twice more sensitive than CPT to compaction. Before and after DMT Tests are increasingly used to monitor not only the gain in modulus but also the gain in OCR due to compaction.
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Detecting Slip Surfaces in Clay Slopes
Values of the Marchetti DMT parameter KD = 2 found in a slope signal the presence of slip surfaces, active or quiescent.
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Liquefaction
A recently published Asce paper (2016) provides an updated KD-CRR correlation for estimating the liquefaction resistance CRR from KD. The paper also includes a chart for estimating CRR based at the same time on CPT and DMT Tests.
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Stress History Information
The Marchetti DMT parameter KD is considerably more sensitive to Stress History than other in situ tools. Stress history is important, as it significantly increases moduli and liquefaction resistance. If Stress History is not felt, its benefits are wasted, leading to an uneconomical design.
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G-Gamma Decay Curves
SDMT provides the small strain modulus Go and the working strain modulus MDMT, i.e. two points of the G-γ curve. The availability of two points is helpful while selecting the design G-γ curve.
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Laterally Loaded Piles
The Marchetti DMT was originally conceived to provide estimates of the horizontal soil modulus for laterally loaded piles.
SDMT Software
The Marchetti SDMT Pro Software is the official software for handling Dilatometer and Seismic Dilatometer Test Data.
It is fully back compatible with the older SDMT Elab and may import and handle any DMT, SDMT, SPDMT, Medusa DMT and Medusa SDMT testing data.
Automatic data acquisition is also available for DMT, S-wave and P-wave Testing Equipment.
The Marchetti SDMT Pro contains options for the processing of DMT and Seismic Test results, which are displayed with graphs and tables.
Report documents are generated in MS Word and MS Excel, if pre-installed with a valid license (not included). The Word report contains graphs with all test results and optional introductory pages for test explanation. The report pages may be customized using editable templates, to include company information on each page (including logo) and to allow also for standard Letter USA paper format.
The Excel document contains excel sheets with numerical data of the tests.
If an internet connection is available when SDMT Pro is launched, any recent update may be downloaded at no cost.
The licence is valid for 5 years.
About SDMT
The Marchetti seismic dilatometer is an add-on module that may be combined with the dilatometer or with the CPT for measuring the shear wave velocity Vs (SDMT) and compression wave velocity Vp (SPDMT)
Thanks to the true-interval test configuration (two receivers), the repeatability of the Vs measurements is very high, approximately 1 %, i.e. a few m/s.
The Vs values are calculated automatically and displayed in real time, at a cost and time considerably lower than Downhole or Cross hole.
SDMT provides the small strain modulus Go and the working strain modulus MDMT, i.e. two points of the G-γ The availability of two points is helpful while selecting the design G-γ curve. (Amoroso et al. 2014).
Based on SDMTs executed in many different soils and geographical regions, a chart (Fig. 5, Marchetti 2008) has been constructed permitting estimates of Vs from just mechanical DMT results.
A Vs profile can be obtained in impenetrable soils by executing the test in a borehole backfilled with gravel (Fig. 7, Totani et al. 2009).
Seismic Dilatometer Composition
The Marchetti SDMT is the combination of the flat dilatometer with an add-on seismic module for the measurement of the shear wave velocity. The seismic module (Fig. 1a) is a tubular element placed above the DMT blade, equipped with two receivers located at 0.5m distance apart.
When a shear wave is generated at the surface, it reaches the upper receiver first, then, after a delay, it reaches the lower receiver.
The seismograms acquired by the two receivers, amplified and digitized at depth, are transmitted to a PC at the surface, that automatically calculates the delay using the Cross Correlation algorithm.
VS is obtained (Fig. 1b) as the ratio between the difference in distance between the source and the two receivers (S2 – S1) and the delay ΔT from the first to the second receiver.
The true-interval test configuration with two receivers avoids possible inaccuracy of the “zero time” at the hammer impact, sometimes observed in the pseudo-interval one-receiver configuration.
Fig. 1a DMT blade & seismic module
Fig. 1b
Moreover, the two seismograms recorded by the two receivers at a given test depth corresponds to the same hammer blow.
The repeatability of the VS measurements (see example in Table 1) is remarkable (observed VS repeatability » 1 %, i.e. a few m/s). The Vs values are calculated automatically and displayed in real time, at a cost and time considerably lower than Downhole or Crosshole.
Fig. 1c shows an example of seismograms obtained by SDMT at various test depths at the site of Fucino.
Fig. 1c Example of seismograms as recorded and rephased
Main Applications
Shear wave source
Fig. 2 shows a pendulum hammer generating the shear wave at ground surface level.
Fig. 2 Pendulum hammer for generating a shear wave
Example of SDMT results
Shown in Fig. 3 below are the profiles obtained from two adjacent soundings. The fifth profile is the VS profile obtained by the seismic module. It can be seen that the repeatability of VS in the two soundings is similar to the repeatability of the other four DMT parameters.
Fig. 3 Example of SDMT results obtained in two adjacent SDMT's
Repeatability
Table 1 shows the repeatability of Vs obtained from multiple hammer blows at a given depth, typically a couple of m/sec.
Table 1. Repeating of the Vs values from multiple hammer blows at a given depth
G-gamma curves
Fig. 4 shows the two points of the G-γ curve, namely the small strain modulus and the working strain modulus. They can be of help while selecting the design G-γ curve (Amoroso et al. 2014).
Fig. 4. Small strain stiffness (Go) and working strain stiffness (MDMT). Two points of the G-y curve
Estimating Vs from mechanical DMT results
The chart in Fig. 5 has been constructed based on the results of 34 SDMT sites in numerous different soils and geographical regions. The chart can be used to obtain estimates of Vs from just mechanical DMT results, i.e. Vs = f(ID, KD, MDMT). It can be noted that the ratio Go/MDMT is highly variable. Hence it is not possible to estimate the operative modulus M by dividing Go by a constant, as it has been suggested sometimes.
Vs measured vs Vs estimated
Fig. 6 shows a comparison between the Vs profiles measured by SDMTs and the profiles of Vs estimated from mechanical DMT data using the diagram in the previous Figure. Amoroso (2013) compares the DMT and CPT correlations for estimating Vs and concludes that Vs estimates based on DMT are closer to Vs measured.
Fig. 5 Chart for estimating Vs (Go) from just mechanical-DMT data i.e. Go = f(ID,KD,MDMT). Marchetti et al. (2008)
Fig. 6 Comparisons Vs measured by SDMT with Vs estimated by mechanical DMT data using the previous Figure in the area of L'Aquila (Amoroso et al. 2013)
Executing SDMT in impenetrable soils
In impenetrable soils VS profiles can be obtained by SDMTs carried out inside boreholes backfilled with gravel (Totani et al. 2009).
The possibility of such measurement descends from the fact that the path of the shear wave from the surface to the upper and lower receiver includes a short path in the backfill of very similar length for both receivers. The backfilling material has to be clean coarse sand – fine gravel (grain size 2 to 12 mm, no fines), i.e. material sinking in the borehole without leaving cavities in the filling, that might later reduce the contact between the seismic probe and the soil. DMT measurements – meaningless in the backfill soil – are not taken in this case. Comparative tests at various sites indicate that the values of VS obtained in a backfilled borehole are nearly coincident with the VS obtained by penetrating the “virgin” soil (Fig. 7). See also Test Specifications SDMT (en/ita).
About Medusa
The Marchetti Medusa DMT is a probe able to autonomously perform dilatometer tests, which generates and measures the pressure directly at depth. The obtained DMT readings are extremely repeatable, because the fluid is liquid (incompressible) and the pressure regulation is electronically controlled. New test procedures are possible, such as high resolution dissipation tests and horizontal pressure measurements during penetration.
Medusa DMT Cableless
When the Medusa DMT is operated "cableless", a programmable period (TMCP) determines when to start each measurement cycle. In the first part of the period, the A, B, C readings are taken and stored in the EPROM memory.
The system will then stay in an idle state, waiting for the penetration to the next test depth.
A typical period for TMCP is of 1 minute, where the measurements are taken in the first 30 seconds and the device is idle in the remaining seconds for completing the period.
During these additional 30 seconds the instrumentation is advanced to the next test depth. The time origin for the synchronization (T = 0) is set with the ON/OFF switch.
The USB connection enables to program test parameters, such as TMCP, and to download the data at the end of the test, when the probe is retrieved.
Medusa DMT with Cable
The Medusa DMT may also operate with an electric cable running from a computer laptop at ground surface down to the probe at depth.
In this configuration, the operator may activate the measurement cycle from the computer as soon as the test depth is reached. During the cycle all automation parameters, such as the battery status, the voltage and current provided to the engine, the position of the piston of the motorized syringe, the probe inclination and other additional information, are available in real time. The DMT parameters, in particular the current pressure and membrane contact status, are displayed in real time during the measurement, as for the traditional DMT pneumatic technology.
For the Medusa DMT the firmware embedded in the electronic board implements the procedure for inflating and deflating the DMT membrane. The hydraulic pressurization of the motorized syringe actuates a volume controlled expansion of the membrane, which enables to impose a programmable timing for achieving the readings. Therefore the Medusa DMT is capable to perform dilatometer tests with the recommended timing suggested in the international standards. At the same time, the highly accurate and repeatable time-for-reading facility provided by the instrument prompts for its potential use for performing dilatometer tests adopting variable pressurization rates in intermediate soils.
The motorized syringe, controlled by the electronic board, is also able to maintain the membrane in equilibrium with negligible displacements of the membrane. This capability enables to obtain continuous measurements of the total horizontal pressure of the soil against the membrane. The Medusa DMT may then be used to obtain continuous measurements of the total horizontal pressure during penetration (equivalent A-reading at T = 0 seconds instead of T = 15 seconds), thus providing useful indications for the assessment of the in-situ stress state and the at-rest lateral earth pressure coefficient (K0).
As in the standard pneumatic DMT, the A, B, C readings must be corrected with the calibration offsets ΔA and ΔB to obtain p0, p1, p2, respectively (Marchetti et al. 2001). All subsequent steps of data processing and interpretation of soil parameters, based on the corrected pressures p0, p1, p2, are the same as for the traditional pneumatic DMT equipment.
Medusa DMT advantages
The Medusa DMT has several advantages over the traditional pneumatic equipment, both in terms of simplification of the probe and test procedure, and in terms of increased accuracy of the measurements (Marchetti et al. 2019). The major advantages are listed below.
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The overall equipment occupancy is reduced to the size of the standard blade with a rod connected on its top, for a total height of about 1 m. The gas tank, the control unit and the pneumatic cables are no longer required.
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The probe may operate in cableless mode, which is a significant practical advantage, especially in the offshore industry. An optional electric cable may be used for obtaining real-time results during test execution.
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The pressure is generated and measured locally at depth, not at ground surface. This eliminates any possible problem of pressure equalization along the pneumatic cable of the traditional equipment.
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The pressurisation rate of the membrane is independent of the operator. The automatic (volume controlled) procedure of membrane pressurisation operated by the motorised syringe is highly repeatable and capable to impose the correct timing to obtain the A and B pressure readings, strictly according to the specifications of the international standards or any other required timing.
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The capability of the Medusa DMT of measuring (virtually continuously) the total horizontal pressure against the membrane with time enables new research possibilities.
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Short A-dissipations, consisting in repeated A-readings (without expansion of the membrane from A to B) for a couple of minutes, may be executed to detect intermediate or partially draining soil layers (Marchetti 2015, Marchetti and Monaco 2018). The duration of such short A-dissipations (much shorter than conventional DMT-A dissipation tests that provide the entire A-decay curve) is sufficient to discover whether an appreciable reduction of the total contact pressure A, reflecting pore pressure dissipation, occurs during the test. The Medusa DMT permits to execute routinely short A-dissipations before recording each standard A-reading, which is taken 15 s after reaching the test depth. In clays no appreciable pore pressure dissipation occurs in 15 s and the A-readings remain nearly constant, indicating fully undrained conditions, while a substantial reduction of A in 15 s prompts for partial drainage.