Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
Fig. 1. —Plate Bearing Test Equipment.
The test influences only a small area and depth of rock, and the deformation modulus obtained can vary over a wide range, depending on the degree of fissuring, e.g., in one group of tests, carried out in chert, the values from six different sites varied over a range of 30 to 1, from 7.5 × 106 down to 0.26 × 106 p.s.i. With this range of values, engineering judgment is required to arrive at an effective value for a length of tunnel.
In order to average out local variations, and to influence the rock to as great a depth as possible, some workers prefer a plate size as large as possible. Kujundzic(8) has achieved a size 4 ft. square, by using a large flat jack, embedded in a narrow trench, under plate bearing test conditions.
It appears desirable, however, that with the plate size used, the load should be sufficient to give a rock deflection of the same order as the deflection of the tunnel under design pressure. The load thus required, for Authority works, is 50-80 tons on a 1-ft. dia. plate.
Special features of the loading plate used in the S.M.A. permit the use of a rubber pad for equalization of pressure under the plate(3,5,6), and at the same time permit observations of rock deflection to be made at the centre of the area and near the circumference.
Rock deflections measured on the surface at various distances from the loaded area have been found to be less than those of the Boussinesq distribution, e.g., at a distance of 18 in. from the centre of a 12 in. diameter loaded area, the deflection (the average of six tests) was one-quarter of that of the Boussinesq theory, and only 5% of that at the centre(9).
Nevertheless, the plate bearing test appears to be the only one (excluding full-scale tunnel pressure tests) applicable to loose-jointed rock.
A supplementary test, to increase the knowledge of the amount and depth of fissuring, and of the modulus normal to the rock surface, is proposed in Sec. 8.
4.—Laboratory Plate Bearing Tests on a Rock Aggregate
In addition to the in-situ tests, plate bearing tests have also been carried out to determine whether a rock-bolted aggregate possesses quasi-elastic properties.
The tests were done on a box model (Fig. 2) with varying aggregate sizes. Loads up to 13,000 lb. were applied to a rock bolt plate by a hollow cylinder which slipped over the head of the bolt, and deflections of this plate were measured. A typical deformation curve is shown (Fig. 3). The curve type is similar to that for rocks in situ.
5.—Rock Stress Test
Rock stress lies at the basis of rock activity such as rock movements during excavation.
