The test starts by inserting the dilatometer into the ground. Immediately after the penetration the operator starts inflating the membrane and takes, in » 15 seconds from stopping the penetration, the A-pressure. Then he continues the inflation and takes B in additional » 15 seconds.
As a consequence of the indicated times, the pressurizations rate is very slow in weak soils and faster in stiff soils.
There are two reasons why pressures A and B must be reached slowly : (1) Give time to the gas to flow downwards to achieve pressure equalization (2) To have a reasonably slow movement of the needle of the pressure gage, so that accurate readings can be taken by the operator. With the advent of automatic acquisition the readings are recorded instantaneously even if the inflation rate is fast, so that the reason (2) is no longer a limitation. But reason (1) remains. Therefore, even with automatic acquisition, the above recommended times for reading must be respected.
The above time intervals typically apply for cables lengths up to approximately 30 m. For longer cables the flow rate may have to be reduced to allow pressure equalization along the cable.
During the test, the operator may occasionally check the adequacy of the selected flow rate by closing the micrometer flow valve and observing how the pressure gage reacts. If the gage pressure drops in excess of 2 % when closing the valve (ASTM 1986), the rate is too fast and must be reduced.
Seasoned operators often reduce the testing time by inflating “rapidly” until say 70% of the expected reading (A or B), then slowing to accurately read the pressure. This inflation method minimizes the pressure drop along the cable when it is time to take the reading. In this way the recommended 15 sec may be reduced to say 7 sec.
NOTE: Time required for the test
The time delay between stopping the penetrations and start of inflation is generally 1-2 seconds. The complete test sequence (A, B readings) requires »30 sec. If the C-reading has to be taken, additional 30 sec will have to be added to the sequence (thereby total duration »1 minute at each depth). The total time needed for obtaining a “typical” 30 m profile (if no obstructions are found) is about 3 hours.