Unlike other in-situ tests, dilatometer test data are correlated with classical geotechnical parameters, Φ’, S,u, Pc, Υt, ko and M, that you are already familiar with. Marchetti originally developed three independent indices, material index (|D), horizontal stress index (KD), and dilatometer modulus index (ED) for the DMT test. Thrust (T) and pore pressure index (UD) have since been added. Geotechnical parameters are obtained using a "triangulation" method--correlations from two indices are used to obtain the third result or geotechnical  parameter
Dilatometer test data correlate well with the soil's shear strength. Because the test only takes about 1 minute to perform, clays are tested in undrained boundary conditions and sands are tested in drained conditions. The dilatometer defines clays as having an ID < 0.6, and sands as having an ID > 1.2. Silts and clay-sand mixtures have IDs between 0.6 and 1.2 and are not well correlated with shear strengths because they are tested in partially drained conditions.
The undrained shear strength for clays is correlated from KD and the effective vertical stress. The effective vertical stress is calculated from the soil's unit weight, which is determined from ID and ED. The DMT derived undrained shear strength values plot in the middle of other test methods from sites worldwide.
In 1982, Dr. Schmertmann developed a theoretical method for determining the plane strain drained angle of internal friction for sands. His method is based on elastic half-space bearing capacity theory developed by Drs. H. Durgunoglu and Mitchell for the Apollo space mission to the moon. Schmertmann's method is an iterative method that uses the thrust and KD index. The plane strain friction angle is higher than the triaxial friction angle for angles greater than 32 degrees. Schmertmann recommends the following equation to determine the triaxial value :
Φtx| = 32 + 2/3 (Φp.s. - 32)
Thrust measurements are relatively easy to do and should always be done. Campanella and Robertson (1990) showed that thrust measured at the surface was within 10% of the thrust at the blade when an appropriately sized friction reducer is used. Recently, I developed a push system that grabs the outside of a rod and measures thrust using 4 load cells at 90 degrees apart. As a test, we painted the outside of a rod with an anti-seize grease, and pushed it without any slipping.
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