Building and excavating in the underground environment naturally results in changes in the stresses. Induced stresses can concentrate in sensitive areas leading to rock falls and high deformation. As construction proceeds to deeper levels, or as multiple excavations are created in the same area stresses accumulate and can overcome the strength of the geology hosting the excavations.

Specifically, problems can be encountered when excavating through geologies with highly varied properties. Stresses in these areas are concentrated not only under the effect of multiple different excavations, but by the natural capabilities of the different geologies to withstand and/or transmit that stress. It is a situation in which analytical and/or empirical methods are insufficient, and numerical methods can produce inaccurate or non-verifiable results. Improvements in modelling methodology are required to enable the numerical tool to be of greater help to design in these situations where it is potentially unsafe, and certainly inefficient, to design based solely on “what was done before”.

To better design openings and support systems in these areas, it is necessary to better understand how excavation-induced stresses and geologically driven stress concentrations interact to create rock failure. This project addresses this problem by using a combination of data collection in a controlled environment, and numerical modelling of complex, multi-opening excavations in order develop a method for improving design and support systems.

The research generated significant amount of data which can be used for further research and has already been assessed in the daily mine operations and planning.

The Norra Alliansen orebody of the Malmberget sublevel caving mine consists of iron ore interspersed with biotite schist and granitic inclusions. The schist is squeezed between the ore and the host rock and in direct contact with the ore along the majority of the length of the footwall. The schist exhibits high deformation when exposed to stress. SMART cable bolt roof deformation measurements are re-analyzed to draw conclusions regarding the patterns of deformation in the mine. Each bolt’s head is placed at the origin of a spherical coordinate system and the radius and inclination angle between the bolt and every production blast occurring during the bolt’s recorded lifetime are calculated. The deformation experienced by each instrument is investigated by comparing the long-term recorded movements with the developed geometric variables. Patterns of deformation magnitude and rate are found with respect to production-blast distance and inclination angle, instrument location, rock quality designation, and likely mining-induced stresses. Results show that deformation magnitude tends to be higher when driven by production blasting occurring on the production level above the instrumentation when accounting for the effects of distance, but average deformation magnitude for very-near production blasts tends to be higher than that for production blasts occurring directly above the instrument. Correlations also exist between the measured RQD, estimated rockmass parameters, and the measured deformation. Empirical evidence allows the identification of six scenarios which account for 91% of the recorded high-deformation-rate events. These scenarios help determine which production activities are most likely to cause high deformation rates.

Between 2007 and 2010, 20 SMART cable bolts collected crosscut deformation data on levels 932 and 962 of the Malmberget mine. Using 3D mapping techniques, the coordinates of each instrument were determined, along with the coordinates of every production blast occurring during their lifetimes. These coordinates were used to calculate the relative distance and inclination angle (vertical angle above the horizontal plane) from each instrument to each production blast. The instrument data was analyzed to determine average-weekly deformation magnitude and rate.Specific deformation trends were identified when the deformation data was compared to the average weekly distance and inclination angle of the blasts. When blasts detonated at the same distance were compared, those blasts occurring on the level above the instrumentation produced greater deformation rates on every recorded occasion. Also, decreased distances produced higher deformation rates. These rates were all highly influenced by the varying RQD present at each bolt location, with higher RQD giving less deformation. While these results are not unexpected, they resulting data produced further interesting connections when the actual position of each blast was considered.In total, 1026 data points were produced from the eight bolts that recorded significant movement. Of these data points, only 49 instances (5%) occurred where deformation rates higher than 150 μm/day were realized. Given that increased deformation rate is a known indicator of instability or approaching failure, the individual blasts that created these 49 instances were investigated to see if causal patterns could be determined.Of the 49 elevated-rate readings, specific production blasts could be identified for 45 of them. Six types of blasting events were identified as common triggers. Nearly two-thirds of these events were caused by types of stress redistribution including redistribution ontoartificial pillars created during mining, and redistribution caused by the opening blasts in a new crosscut.Using the causal events and deformation patterns as keys, it may be possible to develop a new method of empirical support design, wherein expected deformation could be anticipated and the necessary support installed in advance. This could prevent entry degradation, improve safety, and potentially lower support costs by tailoring support to suit the particular needs of a location, rather than using a blanket support policy.

In 2011, a test was installed in a crosscut on the 1022 level of the Malmberget Norra Alliansen orebody . The goal of this test was to investigate bolting support regimes and their impact on the long-term performance of the crosscut. They were installed in the most problematic geology found in the mine, weak biotite schist located primarily along the footwall contact. Three test zones included Kiruna bolts, D-bolts, and Swellex bolts, were instrumented with multi-point extensometer and strain-gaged bolts in an effort to capture the response of the bolts and the rock to both time and mine-induced stress changes. Full deformation and strain profiles in each test section indicate differences in bolt performance and stress-induced patterns occurring in the walls, shoulders and back of each bolting profile.

This paper describes a rib-control methodology that uses standing support to provide most of the lateral resistance to the rib. The method is analyzed, and its boundary conditions for rib control are described. In this method, an animal-fencing mesh product covers 80 percent of the rib surface and is sparsely bolted to the rib. Can supports or other similar stiff supports are installed primarily as roof support for the entry. As the rib spalls and dilates, the mesh catches the material and expands, pressing against the standing support. Mining proceeds, and vertical roof loads increase the normal force on the Can supports, causing friction at the top and bottom of the support to go up, thereby improving lateral resistance to rib swelling.At one mine in the western United States, this method has proved excellent at preventing the buildup of rib spall. It substantially reduces the number of bolts used for screen installation, improves installation efficiency, reduces manual labor and provides more rib coverage than many screening methods. Because standing support is typically part of the roof-control plan in longwall tailgates, no additional standing-support costs are incurred.