Cone Penetration Testing

Full-flow Penetrometers

Full flow penetrometers are a specialist form of penetrometer undertaken to provide estimates of peak shear strength and residual shear strength in soft and very soft soils, sediments or tailings. The larger surface area of the full-flow penetrometer and the behaviour of soft soils as it shears around the penetrometer can help provide a more uniform interpretation of these parameters compared with traditional cones, which have a smaller surface area and promote shearing rather than flow.

Dilatometer Testing

The Marchetti dilatometer was developed in 1980 by Silvano Marchetti. The apparatus was of a pioneering nature, conceptually focussed on establishing a reliable operational modulus for the particular problem of laterally loaded piles. Later developments of the apparatus have yielded a rich domain of interpretable parameters including stiffness, strength and strength history extending as far as material behaviour indexes and dissipation testing. The equipment is deceptively simple comprising a blade
containing a circular deformable membrane which can be pressurised to
a fixed deformation thus providing the basis of the dilatometer test.

Measurements are taken of pressures on inflation and deflation which are developed into the intermediate definitional (not correlational) parameters of Material Index, Horizontal Stress Index, Dilatometer Modulus and Pore Pressure Index. These indexes can then be correlated with geotechnical parameters. The DMT is particularly powerful when used in concert with direct seismic testing via geophones mounted on the apparatus.

Dilatometer Testing

Marchetti Dilatometer

Cone Penetration Testing

Full-flow Penetrometers

The Marchetti dilatometer was developed in 1980 by Silvano Marchetti. The apparatus was of a pioneering nature, conceptually focussed on establishing a reliable operational modulus for the particular problem of laterally loaded piles. Later developments of the apparatus have yielded a rich domain of interpretable parameters including stiffness, strength and strength history extending as far as material behaviour indexes and dissipation testing. The equipment is deceptively simple comprising a blade containing a circular deformable membrane which can be pressurised to a fixed deformation thus providing the basis of the dilatometer test. Measurements are taken of pressures on inflation and deflation which are developed into the intermediate definitional (not correlational) parameters of Material Index, Horizontal Stress Index, Dilatometer Modulus and Pore Pressure Index. These indexes can then be correlated with geotechnical parameters. The DMT is particularly powerful when used in concert with direct seismic testing via geophones mounted on the apparatus.

As the ground investigation industry moves (slowly) away from drilling and sampling for overburden soils, with geophysics and in situ testing becoming the state of the practice rather than the state of the art, we have introduced our customers to our latest offering in the direct-push in situ testing space. The Marchetti dilatometer has been a stalwart in specialist geotechnical testing circles and academia since the early 1980s hoever, we believe that it has now been perfected in the 4th generation “Medusa” – a fully automated hydraulically activated dilatometer. The instrument yields excellent repeatability, speed and precision augmented with downhole seismic S-wave geophones which produce a high-quality shear wave velocity profile allowing refined site characterisation to include fabric / microstructure delineation. 

The flat dilatometer is a steel blade with dimensions 95 x  200 x 15 mm, ending with a sharp lower edge. One side of the blade exhibits a circular steel membrane that is expanded during the test. The blade is advanced into the soil by pushing vertically on a series of rods. An electro-pneumatical cable connects the blade to the control unit at the ground surface. At fixed depth intervals (generally 0.20m) the penetration is stopped and the membrane is pressurized by gas traveling in the pneumatic tubing. At each depth the operator reads three values of the fluid pressure:

Po = pressure necessary to balance the total horizontal stress of the soil and to start the movement of the membrane (lift-off pressure)

P1 = pressure necessary to horizontally displace the centre of the membrane against the soil of 1.1 mm

P2 = pressure measured after membrane is deflated with the slow vent valve (after P1), when the membrane returns to the initial closed position

The instrument and the test procedure follow the recommendations contained in the documents ASTM, Eurocode 7, ISO and the TC16 documents listed below.

The three field readings P0, P1 e P2 are then used to obtain the four intermediate parameters:

ID            Material Index

KD          Horizontal Stress Index

ED           Dilatometer Modulus

UD          Pore Pressure Index

According to well established empirical correlations documented in the references below, the intermediate parameters are used to interpret the following geotechnical parameters:

M            Oedometer modulus (both in cohesive and cohesionless soils)

Su           Undrained shear strength (cohesive soils)

K0           In situ earth pressure coefficient (cohesive soils)

OCR       Overconsolidation ratio (cohesive soils)

                Friction angle (cohesionless soils)

ɣ              Unit weight

 

References

ASTM D6635-15 “Standard Test Method for Performing the Flat Plate Dilatometer”, ASTM International, West Conshohocken, PA, USA, 2015.

Eurocode 7. EN 1997-2:2007 2007: Geotechnical Design – Part 2: Ground Investigation and Testing. CEN European Committee for Standardization, Brussels, Belgium.

ISO 22476-11:2017(E) “Geotechnical Investigation and Testing – Field Testing – Part 11: Flat Dilatometer Test”, International Organization for Standardization, Geneva, Switzerland, 2017.

TC16 (2001) “The DMT in Soil Investigations”, A Report by the ISSMGE Committee TC16, 41 pp.

TC16 Updates (2015): Marchetti S. 2015, “Some 2015 Updates to the TC16 DMT Report (2001)”, Proceedings of the 3rd International Conference on the Flat Dilatometer (DMT’15), pp. 43–65, In Rome, Italy.