General information on the C-reading can be found in Section 5.2 of the TC16 DMT Report.
The C-reading is particularly useful in granular soils, where it permits to know the equilibrium pore pressure. In practice it replaces, in sand, a piezometer.
Reason why the corrected C-reading (C-Zm+DA) equals the equilibrium pore pressure in granular soils: In clay, during the membrane deflation, the displaced soil returns back and remains in contact with the retracting membrane. In sand, during the deflation, the grains do not return, i.e. remain away, with the formation of a gap between soil and membrane. The gap is filled just with water. Since there is no effective pressure, total pressure = water pressure.
The deflation must be slow (say 0.5 min), using the “slow vent valve”, to prevent sand grains from caving in the gap. If this happens, the equilibrium pore pressure will be overestimated. Therefore it may be wise to repeat the deflation several times and use the lowest C-value.
The C-reading can also be taken in clay, but in clay the total interface pressure is the sum : equilibrium pore water pressure + excess pore water pressure + effective (grains) pressure, difficult to separate. Therefore the C-reading in clay is difficult to use quantitatively. However if the C-reading exceeds the equilibrium pore water pressure, this is an indication of a not freely draining material.
The C-reading needs not to be taken at every depth interval. It can be taken e.g. every 1 m. The C-reading gives valuable information in particular in granular soils.
NOTE. A frequent mistake in C-reading.
As remarked in DMT Digest Winter 1996 (edited by GPE Inc., Gainesville, Florida), several users have reported poor C‑readings, mostly due to improper technique. The frequent mistake is the following. After B, i.e. when the slow deflation starts, the signal is on. After some time the signal stops (from on to off). The mistake is to take the pressure at this first signal inversion as C, which is incorrect (at this time the membrane is the B-position). The correct instant for taking C is some time later, when, completed the deflation, after say 30 sec, there is a second signal inversion, when the membrane returns to the “closed” A-position, thereby contacting the supporting pedestal and reactivating the signal.
It can be of help (to avoid the mistake) to remind that the C-reading is typically a low number, say less than 2-3 bar in sand, much smaller than B.