DMTC (P2) Dissipation

Since it has already been established that the recontact pressure, P₂, may be used to give an indication of the excess pore water pressure, a procedure in which sequential pressures are obtained over time should give an indication of the change or dissipation of pore water pressure following penetration of the blade. This procedure is informally referred to as a DMTC test or a С-Reading dissipation test. The test is conducted as follows:

• 1. Following the penetration and release of the pushing thrust, a stopwatch is started to log the elapsed time after penetration. This then becomes time zero for the test.
• 2. The normal sequence of obtaining an A-, B-, and С-Reading is first performed as a routine part of the test. The elapsed time to obtain the С-Reading is noted.
• 3. At various time intervals, the sequence of obtaining the A-, B-, and С-Reading is repeated, but only the С-Reading and the elapsed time are recorded.
• 4. A sufficient number of data points are obtained over time until a minimum of 50% of the excess pore water pressure has dissipated. (This may require a test to run for a few minutes to several hours, depending on the drainage characteristics.)

Typical results obtained from a DMTC dissipation test (corrected to P₂) are shown in Figure 6.32. As can be seen, these results show the characteristic shape of time-rate of consolidation tests. How then is the coefficient of consolidation obtained?

In a manner similar to that described in Chapter 4 for the CPTU, the dissipation test can be used to obtain c_h according to

where

c_h = coefficient of consolidation in the horizontal direction T = theoretical time factor for a given % dissipation H = length of drainage path

t = elapsed time to achieve a given % of dissipation.

Figure 6.32 Typical DMTC dissipation test.

In the theory developed for the CPTU, H is taken as the radius of the cone (for cylindrical cavity expansion) and T is a function of the rigidity index of the soil (or E/s_u), the pore pressure parameter at failure, A„ the cone tip geometry and the location of the porous element with respect to the cone tip. For example, Gupta & Davidson (1986) suggested that for A_f = 0.9, E/s_u = 200 that T = 0.97 for a 60° cone with the pore water pressure measured at the cone base.

Robertson et al. (1988) proposed that as an initial means of estimating c_h, an “equivalent radius” procedure could be used, i.e., back calculating the equivalent radius of a cylindrical cone given by the projected end area of the DMT blade and then using theory for a cylindrical probe. This gives a radius about 22mm. Schmertmann (1988) suggested that an equivalent radius of 24 mm would produce approximately the same results for c_h for both CPTU dissipation and DMTC dissipation tests in the same cohesive soil. While the “equivalent radius” technique may be used to provide an initial estimate of c_h, Kabir & Lutenegger

(1990) found that when compared to results from horizontal flow oedometer tests, the back calculated value of the equivalent radius ranged from 5 to 40 mm for different clays. This suggests that an alternative technique might be more appropriate.

There are several potential sources of error that may occur using this technique:

• 1. The apex angle of the DMT blade is only about 10°-20° in comparison to the 60° apex of most cones. This means that the initial excess pore pressure distribution will be different.
• 2. The point of measurement of the pore water pressure in the DMT is about 7.6 blade thicknesses behind the base of the leading wedge and about 12.7 blade thicknesses behind the wedge tip. This is considerably different than the base of the wedge where most CPTUs measure pore pressure. This may affect the rate of pore pressure dissipation observed.
• 3. The geometry between the two instruments is different. In some soils, e.g., soft clays, this has little or no effect and the DMT blade approximates a cylindrical cavity expansion, while in other soils, the aspect ratio of the blade may have a significant influence on the results.

An alternative procedure is to treat the DMT blade as a cone with a radius equal to the half-thickness (7.5 mm) and use the theoretical time factor for an 18° cone with pore water pressure measured 10 radii behind the tip. For 50% dissipation, Gupta & Davidson (1986) gave T_so = 25. Schmertmann (1988) suggested using the time for 50% dissipation of pore water pressure in which case Equation 6.52 may be simplified to

The excess pore water pressure at any time can be obtained from

The initial excess pore water pressure would be

Since the value of P_2j is actually not measured in the test, because it takes about 1 min

after penetration to obtain the first P, reading, P_2i must be obtained by back extrapolation.

Schmertmann (1988) suggested to simply plot the values of P₂ on a square-root-of-time plot and extrapolate the initial few data points as a straight line back to t = 0.

Once P_2i is obtained, the normalized excess pore water pressure may be determined for each P, reading as

The value of t₅₀ can be taken as the value corresponding to U = 0.5. The value of u₍₎ must either be measured from piezometers, estimated from ground water conditions, or measured from the test if sufficient time is given in the test to allow P₂ to reach a constant value, indicating complete dissipation and u₍₎ conditions.

DMTA Dissipation

A procedure using only the А-Reading dissipation to estimate the horizontal coefficient of consolidation, c_h, was presented by Marchetti 8c Totani (1989) and is referred to as a DMTA dissipation test. In this method, only the А-Reading is obtained from the test at selected time intervals following the penetration to the test depth. Like P₂ dissipation tests, a plot of A vs. log time shows a characteristic “S” shape as shown in Figure 6.33. Unlike the DMTC dissipation, no В-Reading or С-Reading are obtained in this procedure. Using the А-Reading vs. log time curve, the point of contraflexure is defined as T_flcx. The coefficient of consolidation is obtained from

The value of c_h estimated by Equation 6.61 is the overconsolidated value and would need to be appropriately adjusted (for example, using local experience) to give an estimate of the normally consolidated value. Table 6.10 gives a qualitative rating of the rate of consolidation based on T_tlex suggested by Marchetti & Totani (1989).

Figure 6.33 Typical DMTA dissipation test.

Table 6.10 Rating of consolidation speed based on T_flex