11 PLATE LOADING TEST (PLT)
11.1 General
(1)P The plate loading test covers the determination of the vertical settlement and strength properties of soil and rock masses in situ by recording the load and the corresponding settlement when a rigid plate is loading the ground.
(2) This section covers plate loading tests carried out on a thoroughly levelled ground surface or on the bottom of an excavation at a certain depth or the bottom of a prebored, large diameter borehole.
(3) The test is applicable in all soils, fills and rocks but is normally not suitable for very soft cohesive soils.
(4)P The test shall be carried out in accordance with a method that complies with the requirements given in this section.
(5)P The test method used shall be described in detail with the test results.
(6) The test method may be described by reference to a published standard.
(7)P Any deviation from the requirements given below shall be justified and in particular its influence on the results shall be commented upon.
(8) Experience with deviations exists with respect to
- plate size;
- test procedure (incremental loading, constant rate of deformation).
11.2 Definitions
(1)P applied contact pressure p: contact pressure equal to the applied load including the weight of the apparatus acting directly on the plate and the weight of the plate divided by the area of the base of the plate.
(2)P ultimate contact pressure pu: largest possible contact pressure or contact pressure where the settlement reaches a specified level or increase.
11.3 Equipment
11.3.1 Apparatus
11.3.1.1 Plate
(1)P The plate shall be rigid to avoid bending, and nominally flat on the bottom. The top shall contain a guide to locate the loading column, particularly where the test is to be made in a bore hole. The longitudinal axis of the loading column and the centre of the plate shall coincide and the contact shall be provided with a bearing.
(2)P When performing test loading of a plate in very heterogeneous ground, the plate shall be of such a size that it is not influenced by any random firm or weak spot. The width of the plate (b) shall be at least five, preferably ten times larger than the largest spot.
(3) In this section circular and square shaped plates are treated. For deriving settlement and strength properties, circular plates are preferably used. For direct design, square plates are normally used. If failure is to occur in a certain direction, this can be achieved by using a rectangular test plate. The ratio of the smaller dimension of the rectangular plate and the larger dimension should be more than 0,8; the smaller dimension should normally be more than 1 m.
(4) For circular plates, diameters or more than 0,6 m are normally used.
11.3.1.2 Reaction loading system
(1)P The reaction load shall be designed such that the required contact pressures below the plate can be produced and the required settlements can be achieved.
(2) The reaction load may be created by jacking against a counterweight, against tension piles or anchors or against an existing abutment (see figure 11.1).
(3)P The reaction load or its supports shall be placed sufficiently far from the proposed test position so as to reduce the influence on the results to an acceptable level.
(4) Between the centre of the plate and the reaction system, a distance of 3,5 times the width or diameter of the plate is normally sufficient.
(5)P The loading column shall have sufficient strength to prevent undue buckling under the maximum load and in the case of borehole tests shall be clear of the borehole walls.
(6) The loading column should have ball-joint connections to the reaction beam and to the plate, provided the horizontal stability is secured.
11.3.2 Measuring equipment
11.3.2.1 Load measurement system
(1)P A load measurement device shall be placed between the reaction loading system and the test plate which shall be calibrated to the expected maximum test load and the required accuracy of the test.
(2)P The accuracy of the load measuring system shall allow to measure any applied load with an accuracy of 5 %.
(3) The load measuring system should be free of hysteresis effects.
11.3.2.2 Settlement measuring system
(1)P The settlement measurements of the centre of the plate shall be made with reference to fixed points which shall not be influenced by the loading of the plate or the reaction loading system.
(2)P The accuracy of the complete settlement measurement system shall be within 2 % or at least 0,1 mm.
(3) If dial gauges or electrical displacement transducers to measure the settlement are used, they should be fixed to a measuring frame which is sufficiently stiff in order to avoid creep and vibration in the frames.
(4) The measuring bridge should be protected against wind, sun and frost.
(5) If large plates of more than 1 m width are used, it is recommended to measure the settlements with a telescope level possessing an accuracy of 0,1 mm. Two fixed reference points are to be chosen which are close enough but outside the area influenced by the loading.
(6) The settlement measurement system should be able to measure the average settlement as well as the tilting of the plate. At least a three-point measuring system should be centrically located on the plate; the three measuring points should be equally spaced around the plate.
