Most articles about rock stresses start with a statement similar to the one above. Such a statement has even more relevance 30 years later, with the high reliance on powerful numerical models commonly used to carry out the design of underground and near-surface excavations. Good knowledge of the respective stress magnitudes, their directions and trend with depth is fundamental to the proper use of such modelling tools.
This article illustrates some important reasons why a single location stress measurement program may not suit all sites, whether the geological setting is complex or relatively simple.
This article illustrates some important reasons why a single location stress measurement program may not suit all sites, whether the geological setting is complex or relatively simple.
The in-situ rock stress condition was recently measured at the Toowoomba Bypass pilot tunnel. The two boreholes in which measurements were carried out were located at the base of a high strength 20m thick basalt layer, which was overlain by a low strength claystone and a layer of highly to slightly weathered jointed basalt. The two test sites, about 70m below the surface, were separated by approximately 7.5m, horizontally.
The measured maximum principal stress was sub-horizontal, trending approximately north-south and parallel to a nearby irregular, but generally north-south trending, escarpment approximately 500m to the east of the tunnel. This result was not unexpected.
However, the maximum and intermediate principal stresses measured at one location were about twice that measured at the other location; the magnitude of the measured principal stresses from the four tests ranged from 14 MPa to 32 MPa. The vertical stress determined from the four tests was 2 to 3.5 times that expected from the depth of overburden.
It was concluded that the multiple interlaying of strong and weak sub-horizontal layers significantly affected the stresses at this location.
Had stress measurements not been undertaken in this instance, the high vertical stress would not have been discovered until well after tunnel construction had commenced.
Case Study 2: Ballarat East gold mine
In early 2007, the rock stress condition was measured at two levels at the Ballarat East Gold Mine. The vertical stress component at both sites was near equal to, or higher than the major horizontal stress component. These results were consistent with the situation of the significantly folded and altered sandstone and siltstone units that contain the quartz hosted gold deposits at Ballarat. The measured vertical stress was largely in agreement with the stress calculated from the mass of the super incumbent rock.
In mid-2008, a second set of stress measurements was undertaken at lower levels in the mine, again at two sites. The measured orientation of the principal stresses was consistent between sites and to the orientation of the principal stresses measured in 2007.
The measured vertical stress component at one of the site was reasonably close to the theoretical value (within ±15 per cent, from three tests), while at the other site (from two tests) the vertical stress component was 55 per cent and 95 per cent higher than that expected from the overburden rock.
Another important result from the 2008 test program was that the minor principal stress magnitude was close to zero for four of the five tests. The magnitude of minor stress from the fifth test was also low at less than 4 MPa. The direction of all of these low stresses was approximately north-south along the strike of the gross fold hinge. It was concluded that the north-south stress at some locations could have been inhibited by slip on puggy fault zones striking at low angle to this stress direction.
Stress measurement, in this case, showed clearly that there was potential for a lack of confinement in the North South direction, which could lead to instability problems in the backs.
Summary The results from all three sites presented in the case studies clearly demonstrate that the rock stress condition at any given location should be well measured and the results carefully considered in the overall geological context.
The extensive stress measurement experience gained by Coffey Mining staff over the past 35 years also endorses the requirement for regular stress measurement programs. Stress measurement history gained from various sites has shown that the use of a reliable, consistent and repeatable stress measurement technique such as HI cell overcoring is highly beneficial.
The data obtained is fundamental to good modelling and understanding of rock mass behaviour. This leads to an efficient design of all mining systems for modern Australian mines.
In summary, a well planned stress measurement program, undertaken at regular intervals during a mine’s lifespan, will deliver significant economic and safety benefits.