Principle and application
The seismic dilatometer (SDMT) combines the Marchetti dilatometer with a seismic module. Besides the Marchetti indices taken every 20 cm, two geophones in the blade body measure the shear wave velocity Vs by depth. In the same hole, it delivers the soil deformability and the small strain dynamic modulus (G0), without relying on correlations.
The test in steps
Blade penetration
The dilatometer blade is pushed into the ground using the same reaction system as the CPTu.
Membrane expansion
The metal membrane is expanded with gas and the A and B pressures are recorded.
Reading by depth
The sequence is repeated every 20 cm, building the profile of the Marchetti indices.
S wave measurement
Two geophones in the blade body pick up the shear wave generated at the surface and measure Vs by depth.
Per Marchetti
| Stiffness | Dilatometer modulus and constrained modulus |
|---|---|
| Horizontal stress | KD index and K0 estimate |
| History | Overconsolidation ratio (OCR) |
| S wave | Vs velocity and small strain shear modulus G0 |
Quality and field operation
Data quality
- Membrane calibration before and after each vertical.
- Procedure following established Marchetti practice.
- Verification of geophone spacing and S wave triggering.
- Interpretation of the indices and Vs profile by a geotechnical engineer.
Safety and operation
- Ready for high demand areas: mining, industry and ports.
- Organized, signposted work front with a health and safety procedure.
- Own crews and fleet, including on night shifts.
The deliverable
| DMT profile | ID, KD, ED indices by depth |
|---|---|
| Parameters | Derived modulus, OCR, K0 and Su |
| Vs profile | Shear wave velocity and G0 by depth |
| Typical turnaround | Depending on depth and number of verticals |
Sectors and project types
Frequently asked questions about the SDMT test
What is the SDMT test for?
The seismic dilatometer (SDMT) combines the Marchetti dilatometer with a seismic module. Besides the Marchetti indices taken every 20 cm, two geophones in the blade body measure the shear wave velocity Vs by depth. In the same hole, it delivers the soil deformability and the small strain dynamic modulus (G0), without relying on correlations.
What does the SDMT add compared with the DMT?
In addition to the Marchetti indices, the SDMT measures shear wave velocity Vs by depth, giving direct access to the small strain shear modulus G0 and to the seismic response of the ground.
How does the SDMT measure the S wave?
Two geophones installed in the blade body pick up the shear wave generated at the surface. From the difference in arrival time between the two geophones, Vs is computed for each depth interval.
When is the SDMT preferable to a borehole geophysical test?
When the project needs stiffness, stress history and Vs in the same profile and on the same field day. The SDMT delivers the deformability parameters of the DMT and the S wave without requiring a separate hole for crosshole or downhole testing.
Bulletin 07: Vs waves in seismic tests
How shear wave velocity is measured in the field and what it means for the dynamic modulus and the seismic response of the ground.
Do you need an SDMT test on your project?
Tell us the project context and we will put together the investigation plan and the quote.