46.2 Plate tests

46.2.1 General

The plate load test is probably the most common vertical bearing test in use. Plates tests should be used to determine the stiffness and strength of the ground; they are also used for quality control of fills, etc. Test procedures should be carried out in accordance with BS 1377-9:1990, 4.1. They can be performed as surface tests either at ground surface or in pits and also at the base of boreholes.

The test comprises applying a load to a square or circular plate while monitoring the movement or penetration of the plate into the ground. Loading can be either using increments of load which are maintained for specified periods of time or until movement becomes minimal or by applying load so that the plate penetrates into the ground at a known speed/rate of penetration. The rate of application of the load determines if the test is taking place under undrained or drained conditions. The size of the plate used should be appropriate for the scale of features controlling the ground behaviour. Undertaking plate load tests in boreholes has specific problems.

Where, for reasons of economy, the test is conducted in a small diameter borehole, the cleaning of the bottom and the bedding of the plate should be done from the surface, and it is, therefore, very difficult to be certain that the plate is not resting on disturbed material. This limits the value of the results. Care should be taken to ensure that the plate movements are measured directly and that the effects of compression of the load column are eliminated. The strain distribution beneath the plate can be measured using an extensometer array (see Marsland and Eason, 1973 [104]).

The techniques for tests in large and small diameter boreholes differ in some respects mainly related to ability to clean and prepare the base of the borehole and is discussed further below. The diameter of the plate used should, so far as practicable, be equal of that of the borehole, provided that care is taken to eliminate cohesion or friction on the side of the plate.

NOTE Where the diameter of the plate is significantly less than that of the borehole, the results of the test become difficult to interpret. At a hole-diameter to plate-diameter ratio greater than about 3:2, the parameters being measured are those pertaining to a load at a free surface and not at depth under confined conditions, which are usually the conditions of interest.

46.2.2 Limitations

COMMENTARY ON 46.2.2

For the general limitations of the vertical load test, see 46.1.2. They apply similarly to the borehole test. Additionally, in the bottom of a borehole that is too small to allow hand preparation of the test surface, it is more difficult to achieve a satisfactory bedding of the loading plate on the test surface and, therefore, values obtained for the deformation parameters might be of limited significance.

Casing should be used to support the sides of the borehole and to seal off water seepage from strata that are above the test elevation as necessary. When the test is to be carried out below the prevailing water table, dewatering by pumping or baling from within the borehole might cause seepage, which disturbs the ground and adversely affects its deformation characteristics, leading to a need for external dewatering.

If the test is undertaken only for measuring the strength parameters, disturbance due to groundwater seepage might be a less significant factor and the borehole could be emptied, if this is possible, while the plate is being installed. The water should be allowed to return to its normal rest level before the test is commenced. Alternatively, the plate can be installed under water, although it might not then be possible to set the plate sufficiently accurately for the deformation characteristics to be measured.

In small diameter boreholes the cleaning should be carried out by means of a suitable auger or hinged bucket operated at the end of a drill rod assembly. A layer of neat cement mortar should then be placed at the bottom of the borehole by means of a tremie or bottom opening bucket, and the plate lowered down the hole and lightly pressed on to the surface of the mortar. Plaster and resins may also be used for bedding.

Surface plate loads tests should be used to assess the performance of fills and compacted materials.

The test should be used to determine the strength and deformation characteristics of the mass ground. It may also sometimes be used to establish the working load of piles.

In small diameter boreholes the deformation characteristics obtained can often be of limited value owing to doubts about the elimination of ground disturbance and errors resulting from unsatisfactory bedding of the plate, although satisfactory unload/reload parameters can be obtained. The main use of the test is for measuring the strength characteristics of those cohesive soils in which undisturbed samples cannot be obtained, e.g. some gravelly clays and weaker rocks. The plate diameter should be large in relation to the structure of the ground.

NOTE Plate load tests are also be used as an indicator of CBR values in fills.

46.2.3 Long-term load tests

COMMENTARY ON 46.2.3

The long-term settlement of fills is usually of much greater significance to the satisfactory functioning of structures built on the site than the movements that occur during construction. A variation of the bearing test can be used and is known as the long term maintained test. This test usually consists on casting a concrete slab on the surface of the fill and loaded by a deadweight which can be applied either by a skip or skips filled with sand or other suitable material (often referred to as the BRE skip test) or simply kentledge. The settlement of the pad is observed with time and referenced to a stable datum remote form the test.

A long-term maintained load test should be carried out in accordance with BS 1377-9:1990, 4.2.

NOTE The actual period over which the test is carried out is inevitably a compromise between the theoretically desirable requirement of a period comparable with the life of the structure and the practical requirement of early development of the site. A month would seem to be a minimum for the test and it would be highly desirable for tests to be carried out over periods of 3 to 6 months, whenever possible.

46.3 Lateral and inclined loading tests

Lateral and inclined loading tests, which are essentially the same test procedures as vertical loading tests, should be carried out and analysed in a comparable way. Particular characteristics of the ground should be investigated by loading tests at a preferred orientation. These should be carried out in rock for investigations concerning tunnels and underground excavations.

NOTE A simple lateral loading test, using an hydraulic jack between the opposite sides of a trial pit, forms a very convenient means of measuring in situ the shear strength of soils. It is often used in soils that are not suitable for undisturbed sampling, e.g. clays containing gravel and cobbles (and boulders).