Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
(c) Sites giving low to zero stress are rarely encountered. There is thus significance in the tests below the abutments of Tumut 2 machine hall, where very low vertical stresses have been found, as predicted by photoelastic tests.
(d) The effect of blasting on nearby sites, during tunnelling, has been found to be very small. The changes obtained (Tooma-Tumut Tunnel tests, Sec. 7) can rather be explained as due to the change of profile of the tunnel across section. It appears that the shattered rock may be very superficial.
(e) The flat jack tests generally indicate a nearly elastic in situ rock behaviour in the sound, stressed rock. The modulus depends on jointing however, and more information is needed on the magnitude of the elastic modulus for rock “en masse”.
7.—Comparison of Methods of Measuring Young’s Modulus
As the in-situ rock modulus is considerably affected by jointing, either tight or open, it is desirable to compare firstly the values of modulus of rock cores as determined by two different laboratory methods.
The sonic resonance test is the most rapid method of laboratory test, and several comparisons with results by the compression test have been made. The values for sonic cores taken from Tumut 2 Power Station are compared in Fig. 10. The difference between the two methods is generally less than 15%. The figure also shows the large differences in elasticity that sometimes occur between different cores from the same test site.
The effect of tight jointing on the elasticity of rock in situ is seen by comparing the results from flat jack tests with the results from laboratory tests on the cores.
Fig. 11. compares six different test sites. For sites with few or no joints, the points in the figure lie close to the 45° line, indicating approximate equality between the site modulus and the average for the cores from the site.
Fig. 10.— Young’s Modulus Comparison—Sonic and Compression Tests.
Sites with close jointing are represented by points lying above the 45° line, and have a low site modulus compared with the average from the cores. Points lying below the 45° line represent sites with anomalous behaviour. In two cases shown (sites l and 6), concavity of the rock surface caused the very high effective site moduli.
Fig. 11.—Young’s Modulus—Flat Jack and Compression Tests.
