46 Bearing tests

46.1 Vertical loading tests

46.1.1 General

In-situ vertical loading tests can be made by measuring the applied load and penetration of a rigid flat object of known dimensions being pushed into a soil or rock mass and should be carried out in accordance with BS 1377-9:1990, 4.1, for soils (see also the ISRM document Suggested methods [55]). The test should be carried out in shallow pits or trenches or at depth in the bottom of a borehole, pit or adit (see Figure 14 and Figure 15). In soils, the test should be carried out to determine the shear strength and deformation characteristics of the material beneath the loaded plate. The ultimate load is often unattainable in rocks, where the test is more frequently used to determine the deformation characteristics.

NOTE 1 The test can be carried out either under a series of maintained loads or at a constant rate of penetration (see Powell, et al., 1989 [101] and Marsland and Powell, 1985 [102]). In the former, the ground is allowed to consolidate under such a load before a further increment is applied; this yields the drained deformation characteristics and, if the test is continued to failure, also the strength characteristics. In the latter, the rate of penetration is often such that little or no drainage occurs, and the test gives the corresponding undrained deformation and strength characteristics.

NOTE 2 The results of a single loading test apply only to the ground that is significantly stressed by the plate; this is typically a depth of about one and a half times the diameter or width of the plate. The depth of ground stressed by a structural foundation is usually far greater than that stressed by the loading test and the results of loading tests carried out at a single elevation do not normally give a direct indication of the allowable bearing capacity and settlement characteristics of full-scale structural foundation.

To determine the variation of ground properties with depth, a series of plate tests should be carried out at different depths. These should be carried out such that each test subjects the ground to the same effective stress level it would receive at working load.

Where tests are carried out in rock, blasting for rock excavation can seriously affect the rock to be tested. This effect can be minimized by using small charges, and by finishing the excavation by hand methods.

Key

1 Jack	7 Tight liner
2 Dial gauge	8 Loading column
3 To reference beam	9 Cone to locate loading column
4 Loading column	10 Skirt
5 Measuring column support	11 Loading plate
6 Measuring column	12 Bedding material

Key

1 Cored samples taken from anchorage hole for testing RQD, fracture, frequency observations and laboratory tests	6 Steel beams
	7 Settlement gauge
	8 Reference rod on axis of assembly
2 Mortar	9 Grouted anchor at depth
3 Circular loaded area
4 Hydraulic load capsule
5 Timber packing

46.1.2 Limitations of the test

COMMENTARY ON 46.1.2

The main limitations of the test are the possibility of ground disturbance in the course of the excavation to gain access to the test position, plus the significant expense of the excavation.

Unavoidable changes in the ground stresses are caused by an excavation, which can produce irreversible effects on the properties the test is intended to study. For example, in stiff, fissured, clay, some swelling and expansion of the clay inevitably occurs during the setting-up process, due to the opening of fissures and other discontinuities; this can considerably reduce the values of the deformation moduli (see Marsland, 1972 [103]). The moduli determined from plate tests are still more reliable, however, and often many times higher than those obtained from standard laboratory tests. In a project that involves a large excavation, e.g. a building with a deep and extensive basement, the excavation could cause disturbance to the ground beneath, with a consequent effect on the deformation characteristics. In such a case, it is necessary to allow for this unavoidable disturbance when interpreting the results of loading tests.

When carrying out the test below the prevailing ground water table, the seepage forces associated with dewatering might affect the properties to be measured. This effect is most severe for tests carried out at significant depths below the water table in soils and weaker rocks. It might, therefore, be necessary to lower the water table by a system of wells set outside and below the test position.

The test is sometimes used to measure the ultimate bearing capacity. In cases where settlements and elastic deformation characteristics of the ground need to be determined, as in rock foundations, care should be taken to work at stresses that are relevant. The observation of deformation, particularly at low stress levels, requires the utmost care in surface preparation and setting-up to achieve meaningful results. The errors that can be introduced by sample disturbance and inaccuracies of measurement can often be similar in size to the data sought or indeed larger.

The effect of sample disturbance can be reduced, to some extent, by carrying out preliminary cycles of loading and unloading. The maximum load in these cycles should not exceed the intended load. The rate of loading should be sufficiently rapid to prevent any significant consolidation or creep.

NOTE After two or three cycles, the stress/settlement graph tends to become repeatable, and the test can then be extended to the main testing programme. The data from the preliminary load-cycles gives an indication of the effect of the sampling disturbance. The undrained deformation moduli, as measured after preliminary load-cycling, are normally a more reliable indication of the true properties of the undisturbed ground. Alternatively, displacement measuring systems can be installed in the ground beneath the plate, thus eliminating the effects of shallow disturbance (see Marsland and Eason, 1973 [104] and Powell et al., 1989 [101]).

46.1.3 Interpretation of results

The correct interpretation of the behaviour of the mass of ground should be made with a careful examination of the results, not only of the loading tests, but also of other data pertaining to the ground.

NOTE Depending on the objects under investigation, such data might include the geological structure, the nature and distribution of discontinuities, lithology and the variability of the ground.

Several deformation moduli can be obtained from these tests, depending on the method used and the application. The results reflect the effects of the width and frequency of the discontinuities, and give an indication of the mass material behaviour under loading. The stress level at which these parameters should be examined depends on the working stress levels. In the case of tests on rock in adits, it might be necessary to consider the in-situ stresses in the test sample.

The moduli to be used for design purposes should be those relating to the ground, both at the time of construction and after it has been affected by the construction procedures, e.g. a deep excavation might affect the deformation moduli of a soil, and blasting might affect the properties of a rock. Sometimes, the effect of a construction procedure might be sufficiently severe to justify the examination of alternative methods of construction.

On completion of the testing, full identification of the material (see Section 6) beneath the loaded area should be carried out by sampling and testing in the laboratory (see Section 3 and Section 5). In many cases, results obtained from these tests assist in extrapolating the test results to other areas on the site.