The effect of scale on the shear behaviour of joints is studied by performing direct shear tests on different sized replicas cast from various natural joint surfaces. The result show significant scale effects on both the shear strength and deformation characteristics. Scale effects are more pronounced in the case of rough, undulating joint types, whereas they are virtually absent for planar joints. The key factor is the involvement of different asperity sizes in controlling the peak behaviour of different lengths of joints. It is shown that as a results both the joint roughness coefficient (JRC) and the joint compression strength (JCS) reduce with increasing scale. The behaviour of multiple jointed masses with different joint spacing is also considered. It is found that despite unchanged roughness, jointed masses consisting of many small blocks have higher peak shear strength than jointed masses with larger joint spacing. These scale effects are related to the changing stiffness of a rock mass as the block size or joint spacing increases or decreases. Economic methods for obtaining scale-free estimates of shear strength are described.

Experimental studies of scale effects on the shear behaviour of rock joints

We investigate the source parameters of picoseismic and nanoseismic events ( M w >-4.1) recorded with a high-sensitivity seismic network at the Mponeng gold mine in South Africa to gain new insights into the scaling of small seismic events. The Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) network, composed of one three-component (3C) accelerometer (sensitivity 50 Hz to 25 kHz) and 8 acoustic emission (AE) sensors, (sensitivity 1 kHz to 180 kHz) is located at a depth of 3268 m and covers the limited volume of approximately 300×300×300 m. The AE sensors are calibrated with respect to the 3C accelerometer in the frequency band 400 Hz–17 kHz. The waveform data of two datasets are analyzed; (1) the aftershock sequence of an M w  1.9 event that occurred approximately 30 m from our network, and (2) the postblasting activity recorded during working days, located at a distance >90 m from the network at the exploitation level. For the analysis we applied spectral fitting and spectral ratio methods. The calculated values of M w range from -0.8 down to -4.1 with corner frequencies 0.8 kHz–13.6 kHz (source sizes from 8 cm to 1.3 m). We observe static stress drops ranging from 1 MPa to 10.0 MPa with apparent stresses of 0.01 MPa–1.00 MPa. Stress drops are independent of the moment, suggesting self-similar behavior in the analyzed magnitude range -4.1 M w

Source Parameters of Picoseismicity Recorded at Mponeng Deep Gold Mine, South Africa: Implications for Scaling Relations

[1] We determine stress drops from P-wave spectra of about 1000 earthquakes induced by hydraulic stimulation in crystalline rock for a deep heat mining project in Basel, Switzerland We observe an increase in stress drop by about a factor of five with radial distance from 10 m to 300 m, which suggests that stress drop correlates with pore pressure perturbations due to the injection We test this hypothesis by calculating the injection-related pore pressure perturbation based on a simple linear pore pressure diffusion model and find a good correlation of the expected pore pressure perturbation with the estimated stress drops

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011GL047498

Stress drop variations of induced earthquakes at the Basel geothermal site

Rockburst characteristics and microseismic monitoring of deep-buried tunnels for Jinping II Hydropower Station

https://era.library.ualberta.ca/items/5f4d6c3c-0f1a-417c-a5c1-4478e6e6b64b/view/e87fc3c3-1cd8-4811-8347-9490f0f202e4/Khazaei-20et-20al-20--20Submitted-20Jan-2020-20--20Clean-20version-20--20Post-20Print.pdf

Damage quantification of intact rocks using acoustic emission energies recorded during uniaxial compression test and discrete element modeling

A series of uniaxial compression tests were performed on 96-mm-diameter quartzite samples, 242 mm in length, to understand the scaling of rock fracture processes. Nine acoustic emission (AE) sensors glued to each sample monitored the AEs resulting from microcracking within the samples. In contrast to previous AE studies, the sensors were calibrated as velocity transducers so that the output could be compared to mining-induced seismicity and natural earthquakes. A new hybrid, relative moment tensor method was applied to obtain source mechanism solutions for eight clusters of events. Once the AE rate accelerated prior to failure, the event positions are associated with the observed failure planes. The moment tensors were found to have double-couple components, indicating that shearing was occurring. The stress drop appears to be constant over the range of moments suggesting the self-similar scaling of the fracture response from the laboratory sample to mine seismicity and natural earthquakes over a wide range of length scales. Similar conclusions can be drawn by considering the apparent stress and source radius. The scaling of the peak velocity and peak acceleration parameters is apparently consistent with mining-induced seismicity but is considerably affected by high-frequency attenuation and the limited bandwidth. This is confirmed by the frequency-magnitude plot, which has a slope of unity. The fracture processes in the laboratory are similar to those occurring underground near stope faces and pillars in deep-level South African gold mines, where there is a high vertical compression and low confinement.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JB000670

Source parameters of acoustic emission events and scaling with mining‐induced seismicity

The Bushveld Complex is a large layered igneous intrusion which spans about 350 km from east to west. It is situated in the northern part of South Africa (Figure 1). The platinum group metals are concentrated in two planar orebodies known as the Merensky Reef, a mineralized pegmatoidal pyroxenite 0.7 m to 1.4 m thick, and, underlying this, the UG2 Reef comprising one or more chromitite seams of similar thickness. The strata generally dip at 8° to 15° toward the centre of the complex. The horizontal to vertical stress ratio (k ratio) varies from about 0.5 to over 2.5. The depth of mining ranges from outcrop to 2 300 m. In the mining depth range from surface to about 1 400 m, the vertical tensile zone often extends high into the hangingwall. If a sufficiently large mining span is achieved, or the stope abuts a geological feature, a large volume of hangingwall rock can become unstable, resulting in a stope collapse, or colloquially, a ‘backbreak’1. In order to prevent these backbreaks a high resistance support system is required. This is universally achieved by the use of small in-stope chain pillars orientated either on strike for breast mining (Figure 2) or on dip for up-or downdip mining. In the past, pillar design on the Merensky and UG2 reefs has been done using experience and strength formulae derived for other hardrock mines. The consequence of this uncertain methodology is to cut oversize pillars, which lowers the extraction ratio. In addition, pillars cut in the deeper levels are required to fail in a stable manner soon after being cut. These pillars are known as crush pillars and their residual strengths provide the required support resistance to prevent backbreaks and keep the stope hangingwall stable. A recent series of pillar bursts, with serious consequences, has raised questions about the design of these pillars. A maximum likelihood back-analysis study was conducted on failed and stable pillars, and strength formulae were developed for the Merensky Reef at Impala Platinum (just north of Rustenburg, Figure 1).

/pdf/merensky-pillar-strength-formulae-based-on-back-analysis-of-gs2dongdm4.pdf

Merensky pillar strength formulae based on back-analysis of pillar failures at Impala Platinum

To understand an earthquake preparation process or rockmass behaviour during the process, we should get closer to the hypocentre. We should put multiple sensors in the seismogenic zone to monitor the slightest stress within the seismogenic area in addition to external loading or bulk deformation of rockmass in a seismogenic area. However, it is not easy with natural earthquakes because hypocentres are usually a great distance from the earth’s surface. A lengthy term of seismograms tells us to some extent about sources. However, the surface layer with a low Q disguises information considerably on an earthquake source1. Some researchers have attempted to drill holes to seismogenic depths to measure the stress2. However, the costs are considerable and the holes are not always close enough to monitor seismogenic processes clearly. In South African deep gold mines, thin tabular gold reefs are excavated. Seismogenic areas move with the advance of mining3. Therefore, we can install instruments in situ in future seismogenic areas, and we can monitor earthquakes from the closest proximities. Earlier works, for example, McGarr and Green4, McGarr et al.5 attempted seismic monitoring and compared these with tiltmeters or the Sacks-Evertson dilatometer. Nicolaysen6 referred to such an attempt as a semicontrolled experiment on seismic events. In 1992 he showed the Japanese Seismological Society7 that South African deep gold mines are good experimental fields for the study of earthquakes. Accordingly, we looked for a good experimental field in some South African gold mines8 and established the first experimental field in 1996 at a mine in Carletonville, South Africa, in co-operation with ISS international, Welkom, South Africa9. In the first field, more than 20,000 events (-2 < M < 2) were monitored within 200 m from our network from February to October 1996. Most events occurred within 200 m of our monitoring network. In this report, we review some of our results9–11, particularly those Semi-controlled seismogenic experiments in South African deep gold mines

Semi-controlled seismogenic experiments in South African deep gold mines

This paper summarizes the most important developments in South African research in earthquake, exploration and mine seismology for the period January 2003 to December 2006. The South African National Seismograph Network (SANSN) has been considerably improved in this time, and the upgraded seismological stations now use the SeisComp recording system and GPRS cellular communi- cations for retrieving data in quasi-real time. The largest mining- related ever to occur in South Africa registered 5.3 on the local Richter magnitude scale (ML) on 9 March 2005. An unusually large earthquake of ML = 7.3 occurred in Mozambique on 23 February 2006, at the southernmost end of the East African Rift System. Improved seismic location and source parameter calculation methods have been developed for the data recorded by the SANSN, and significant research has been completed on seismic hazard and risk methods. The work in mining seismology has been wide-ranging in pursuit of improved understanding of the complex rock mass response to underground mining. Novel underground instrumentation has been developed. A new model of the Bushveld Complex uses seismically determined crustal thickness values to assess its isostatic responses and has important economic impli- cations.

/pdf/recent-research-in-seismology-in-south-africa-1ksdea2heq.pdf

M. O. Kataka

Papers

Source parameters of acoustic emission events and scaling with mining‐induced seismicity

Merensky pillar strength formulae based on back-analysis of pillar failures at Impala Platinum

Semi-controlled seismogenic experiments in South African deep gold mines

Recent research in seismology in South Africa

Experimental investigation of fundamental processes in mining induced fracturing and rock instability.