DMT dissipations (Totani et al. 1998) are generally executed in low permeability silts and clays in order to obtain estimates of the consolidation and permeability coefficients. Dissipations are not executed in permeable soils where instead C-readings may be taken, to get an estimate of the equilibrium pore pressure (uo = C-Zm+DA).
Information and procedure can be found in Section 8 of the TC16 DMT Report.
The DMTA dissipations consist in taking a timed sequence of the A reading (e.g. at 0.5, 1, 2, 4, 8, 15, 30 etc. minutes after stopping the advancement), deflating immediately (using the toggle valve) after reaching the A pressure (i.e. no expansion of the membrane). The A-values, usually plotted in a log t scale, provide an S-shaped curve, which is the same obtainable e.g. by measuring the interface total pressure by means of a pressure cell. The dissipation is stopped when the A–log t curve has flattened sufficiently to clearly identify the time at the contraflexure point Tflex.
Relatively permeable soils dissipate fast and have a low Tflex. Relatively impermeable soils dissipate slowly and have a high Tflex. The slower the dissipation, the lower the consolidation and permeability coefficients. The consolidation and permeability coefficients are estimated based on Tflex, the inflection point of the S-shaped curve.
In low permeability clays the DMTA dissipations can be quite slow, they can take days. Therefore DMTA dissipations are generally executed in projects where consolidation and permeability coefficients are critical.
Interestingly Mesri et al. (ASCE Jnl GGE, Aug 1999, p. 716) advocate the use of the “inflection point method” even for estimating Cv by oedometer, in place of the usual Casagrande or Taylor methods.
Short dissipations (same procedure, but limited in time to two minutes) are also executed in soils suspected to be “Niche silts”, namely silts where there is appreciable excess pore water pressure dissipation during the duration of the DMT test (» 0.5 min). If in the two minutes there is appreciable A reduction, then the silt is a “niche silt” and a special interpretation is needed. A detailed treatment can be found in “Niche silts“