Performing the test
The procedures A, B, C and D as described in this Annex B shall be adhered to.
B.1 Load-unload-reload procedures B.1.1 Procedure A (see Figure B.1)
Both the loading and unloading parts shall be carried out in steps with pressure holds at each step. In addition, the minimum pressure in each reload loop shall be the same pressure p1.1. After having reached the maximum pressure, the load shall be decreased in steps with readings continued as before.
B.1.2 Procedure B (see Figure B.2)
The unloading and reloading phases of each loop shall be carried out either by steps or continuously.
NOTE The loop sizes are normally smaller than in the case of procedure A.
The size of the unloading phases shall be selected according to the design specifications of the test.
B.1.3 Common to procedures A and B
The maximum applied pressure pmax to be used during the test shall be decided by considering the maximum stress expected to be applied to the ground by the structure proposed. In both procedures, a minimum of three reload loops should be carried out. The programme for these unload/reload loops should be either given in the specifications for the test or decided according to the observed progress of the test.
Before commencing the descent phase of a reload loop, enough time shall be allowed for time-dependent effects to become insignificant.
B.1.4 Single loading procedure C (see Figure B.3)
At each step, the pressure shall be held until either the maximum specified pressure or the displacement capacity of the equipment is reached.
B.2 Details of procedure A
The flexible dilatometer membrane shall be expanded until it contacts the pocket wall indicated by an abrupt rise in pressure, i.e. the seating pressure ps. Further pressure shall then be applied until it equals approximately 2 % to 5 % of the expected maximum pressure.
Starting the test from this pressure ps, the soil or rock shall be loaded by stepped increments of pressure. The duration of the pressure holds should be between 1 min and 3 min. Simultaneous readings of pressure and pocket diameter shall be recorded.
Loading and unloading phases are shown in Figure B.1.
|Key for indices:
||Key for examples:
The following details shall be observed:
- a) First loop:
- the first loading shall be carried out at least in five equal steps. The position of the first load reversal point shall be decided by reference to the magnitude of the seating pressure;
- unloading to p1.1 shall be carried out at least in four steps. The pressure steps shall be the same as those used for the loading.
- reloading to p1 shall be carried out with the same steps as the unloading and shall take the same time;
- the duration p2 of the unloading phase of any reload loop shall not take less than half of the duration t1 of the loading phase.
- b) Following loops:
- loading to p2 such as 1,5p1 < p2 < 2p1 shall be carried out with equal steps;
- unloading to p1.1 shall be carried out in four equal steps;
- reloading to p2 shall be carried out using the same size steps in both pressure and time as the preceding unloading.
B.3 Details of procedure B
The whole descent phase from pmax shall be carried out continuously without steps except that one or more reload/unload loops are permitted (Figure B.2). These loops shall be carried out in a similar manner to unload/reload loops. For good resolution of reload loops, data should be recorded not less frequently than at 10 s intervals. A loop should contain about 20 data points. A pressure drop of about a third of the pressure at the start of the reload loop is recommended.
Loading and unloading phases are shown in Figure B.2.
|Key for indices:
||Key for examples:
In deciding the proper rate of pressure change, consideration should be given to rate effects and drainage in the material being tested.
B.4 Details of procedure C
Pressure shall be increased in steps until either the ground fails or the capacity of the equipment is reached (Figure B.3).
The first pressure increment Δp1 to be used shall be decided either by the operator or by instruction. Once the initial readings have been recorded, the operator shall observe the stability trend during a pressure hold and as a result may change the pressure increment so as to
— obtain enough points to evaluate the pseudo-elastic behaviour during the test, and
— record at least three pressure holds beyond this behaviour.
- L = loading phase
- U = unloading phase
B.5 Details of procedure D
Tests in which time-dependent effects are important shall be individually designed according to the precise data requirements.