11.4 Test procedure

11.4.1 Pre-test explorations of ground conditions

(1)P In case the existing ground conditions of the test location are not known, they shall be determined to a depth of more than 5 times (preferably 8 times) the diameter or width of the plate below the test level.

(2) For rock, a depth of less than 5 times the diameter, or the width of the plate may be sufficient for laboratory investigations.

11.4.2 Calibration and checks

(1)P The manometers, the load transducer system and the electric displacement transducers shall be calibrated at least every six months and be checked before each field operation.

(2)P The dial gauges and the displacement transducer system shall be checked before every test at the site by inserting a measuring block of known size under the tip of the gauge.

11.4.3 Preparation of test area

(1)P The ground shall be undisturbed if the test is conducted in natural soil or rock. Disturbed material shall be removed.

(2)P The contact surface of the soil or rock to the plate shall be smooth and horizontally levelled.

(3) If necessary this can be achieved by fill-in material with a higher strength than that of the ground. For tests on cohesive soils, as soon as possible after levelling the ground, the paste of a quick-setting plaster should be poured and spread to obtain a levelled surface not more than 20 mm thick. Immediately after the paste has been spread, the plate should be bedded.

(4) For tests on granular soils, any hollows should be filled with clean dry sand to produce a levelled surface on which to bed the plate.

(5) The final preparation of the test level, in both pits and boreholes should be made by hand, if possible.

(6)P In case several tests are to be conducted within a given area, the distance between the centres of neighbouring plates shall be at least 6 times the diameter or width of the plates.

(7) In the case of a plate of cast in situ concrete, no special preparation of the contact surface of the soil is necessary; in the case of rock, the surface should be cleaned by hand from debris.

11.4.4 Preparation and setting up of loading and measuring apparatus

(1)P When tension piles are used for the reaction, they shall be installed before the test area is exposed. Tension piles shall not influence the test ground.

(2)P The test plate shall not be preloaded during erection of the reaction and force measurement systems. The loading column shall be positioned centrally over the plate and perpendicularly so that the reaction load is applied direct to the plate without eccentricity.

11.4.5 Loading test

11.4.5.1 Incremental loading test

(1)P An incremental loading test shall be conducted if drained loading properties of the soil are to be derived.

(2)P The load shall be applied in equal increments (about ten steps) and shall be kept constant for a certain time within each increment.

(3)P In cohesive soils, primary consolidation shall have taken place at the end of each load increment; this shall be checked by evaluating the time-settlement curves including the creep at the end of each increment.

(4) In non-cohesive soils, each load step is normally applied for 8 min. and the settlements are measured after 1, 2, 4 and 8 min. or continuously 1 to 2 times per min. with an automatic measurement equipment. Sometimes, each load step is applied for 16 min. instead of 8 min.

(5) In the case of tests for direct design, the load increment up to the assumed working load may be extended over a longer time period (one to two months) in order to estimate longterm behaviour.

(6)P The test shall be conducted using loading and unloading cycles if an indication of the relative amounts of reversible and permanent settlements has to be obtained.

(7) While unloading, the decrements may be twice as large as in the case of loading.

(8) Unloading and reloading cycles should be applied before the critical load is reached; complete unloading should be avoided.

11.4.5.2 Constant rate of penetration test

(1) A constant rate of penetration test may be conducted if undrained loading properties of cohesive soil are to be derived. The rate of settlement should be chosen according to the permeability of the soil and to the size of the plate in order to ensure undrained conditions during the test.

(2) The test should be conducted until a settlement equal to fifteen percent of the plate diameter or width has been reached.