Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
Fig, 2.—Box Model of Bolted Aggregate.
The amount of movement depends on the magnitude of the stress, the redistribution of stress consequent upon the change in shape caused by excavation, and the elastic modulus of the rock.
The first and last of these can be determined by the flat jack test of stress and the intermediate item by photoelastic methods.
Method:
The test method has been adapted from the Freysinnet jack test as used by Tincelin and his co-workers(10). A slot cut in a wall or roof frees the rock on either side of the cut of part of the stresses which existed prior to the cutting. This relief brings about expansion of the rock into the cut.
Measurements may be made either of the contraction across the slot or of the extension on the rock faces on either side of the slot. A flat hydraulic jack is embedded in the slot, pressure is raised by a hand pump until the displacements which took place on slot-cutting are reduced and finally cancelled. The cancellation pressure is then essentially equal to the pressure in the rock, normal to the plane of the slot, which existed before cutting the slot.
Fig. 3.—Plate Bearing Test of Bolted Rock Aggregate.
Field Technique:
The slots used are 13 in. long, 1⅞ in. wide, and l3 in. deep, used with 12 by 12 by 3/16 in. flat jacks. A Huggenberger Deformeter is used for measuring the movement across the slot between pairs of pins 10 and 6 in. apart (normal accuracy 0.0002 in.).
The slot method measures a stress in one direction only, which is averaged over an area of rock of dimensions comparable with those of the jack. It is therefore subject less to scatter than methods which measure stress at a point.
