For investigating the geological activity of faults, constructing underground electric power stations or deep tunnels and evaluating their stability, it is very important to estimate the geo-stress components of the surrounding rock.
Civil Engineering Laboratory of CRIEPI has developed two kinds of methods for determining geo-stress as follows;
Over Coring Method based on the principle of stress release method.
Acoustic Emission Method based on the principle of Kaiser-effect.
Kaiser-effect is explained that when a material had stressed to a certain value, acoustic emission does not clearly occur within stress below the value. Rocks which had once stressed are transported to laboratory and after cutting out as specimens acoustic emissions are measured by compressing those specimens. Geo-stress can be determined, when Kaiser-effect is clearly detected.
Acoustic Emission Method can, therefore, determine maximum geo-stress of the site where specimens are sampled. By comparing actual geo-stress obtained by O.C. method with maximum ones obtained by A.E. method, It may be expected that the activity of fault is estimated or the occurrence of earthquake is forecasted. These are problems to be solved in future.
In this report, not only in case of soft rock but also in case of hard rock, methods and several results of tests such as A.E. method or O.C. method are described.
The embedded gauge using the over coring method is shown in Fig. 1. This gauge is embedded with cement milk in borehole. It can be measured for diameter deformation in 4 directions and hole-axis deformation.
Figure 2 shows the measurement procedures for the O.C. method that is our traditional met
The determination of the measurement place is carried out through by boring survey around measurement place. This procedure is necessary because the place must be well conditioned in rock-masses.
After determining the place of measurement, for the convenience of boring work, a small widening of the test adit should be carried out. Then, for fixing the boring machine, the base of the concrete footing, using several bolts for anchoring, is secured. Boring of a rather large diameter of 218 mm should be carried out to a depth of 4–5 meters from the surface of the test adit. This is to avoid relaxed or damaged zones when excavation had been executed. The actual measurement of the geo-stresses should be carried out in the far deepened zone from the bottom of the above mentioned large diameter boring.
After inserting the apparatus for maintaining the center of the next pre-boring the boring hole of the diameter of 56 mm should be drilled to length of about 70 cm. After checking whether it is suitable for inserting gauges by sampling boring cores from these parts (depth of 70 cm), it is necessary to determine if the large diameter's drilling should be repeated to find a more suitable place for embedding gauges.
(Figure in full paper)