A world first – Heavy Penetrometer automatically adapts to the soil hardness

The GRIZZLY is the first Dynamic Probing Super Heavy (DPSH) in the world to automatically adapt its impact energy to the hardness of the soils. This enables it to measure the dynamic resistance of the soils even more accurately.

Heavy dynamic penetrometers are the most suited for work on hard or very hard geological layers, where the soil resistance is over 50 MPa. However, for soft or very soft soils, the advantages of this type of equipment become its disadvantages: a single impact can sink the cone tip in by 10, 20, 30 centimetres or more.

In this case, the measurement made loses a lot of its finesse: for a depth of 1 metre, 30 measurement points are better than 3 to make an evaluation. However these soft zones are of great interest to geotechnical engineers.

To get around this problem, if it is possible with the equipment, the operator can lower the impact force (M) or the height from which M is dropped (H). The problem is that this requires several interventions during the tests to adapt it to the varying resistance of the ground. In other words, heavy and long operations (removal and refitting of protective cage, lifting the masses etc). In addition the soil resistance needs to be recalculated each time using the formula with M and H.

The technical solution developed by the R&D team of Sol Solution (Variable Energy GRIZZLY) still uses a fixed impact mass (M), but automatically varies H between four different values. This is all automatically commanded by the GRIZZLY integrated data acquisition system. Depending on the depth of penetration on the previous impact, using previously defined parameters, the onboard computer instantly decides on the value of H for the following test, so as to adapt in real time to the resistance of the soil encountered. During an evaluation, the more variations in resistance that are found in the soil, the more often the value of H changes. This requires no intervention by the operator or any interruption in the testing.

Furthermore, using the Dutch formula, the resistance of the soil is calculated automatically each time taking into account the value of H and thus the impact energy supplied. Again, no intervention is required by the operator. They simply need to select the impact mode at the start of the test: either the classic constant energy impact (DPSH-B), or the servo- controlled variable energy option. In this option, the 4 values of H available correspond to European standardised impact energies (EN ISO 22476-2). In extreme cases where the set of rods sink in themselves (no impact, just under their own weight), the penetration is still measured constantly in spite of this, and the following impact will update the total depth so that no information is lost.

This innovative technology is already installed on Sol Solution light dynamic cone penetrometers (PANDA®), and is now being adapted to the GRIZZLY® where its advantages can be seen when combined with the dynamic measurements at each impact to obtain other important rheological properties of the soil – load penetration graph, deformation modulus, wave velocity, viscosity… – and to measure the energy transmitted in the impacts.