CAVE PROPAGATION ANALYSIS AT DEEP MINE BLOCK CAVING METHOD USING TIME-LAPSE TOMOGRAPHY

• Main Author: Hidayat, Wahyu
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/78977
• Institution: Institut Teknologi Bandung

As the demand for mineral resources grows, mining operations have begun to extend beyond open pit mining and venture into deeper regions of the earth. In certain conditions, there may be a need to transition from open pit mining to underground mining due to technical and economic considerations. However, deep mining poses significant challenges such as insitu stress, high stress perturbation, and the presence of strong rock masses. These problems must be addressed when conducting underground mining operations. Events such as rockbursts, deformations, seismic hazards, and collapse of roofs or walls are consequences of stress increase in underground mines that occur within strong and brittle rock masses. Ensuring the stability of underground mines is crucial for maintaining continuous production and ensuring the safety of mine workers. To mitigate the risks associated with stability issues in block caving mining operations, special attention must be given to strong rock masses distant from the surface that are subject to insitu stresses and induced stresses caused by large cave dimensions. The implementation of monitoring measures becomes crucial in order to prevent extensive and uncontrolled rock damage, particularly in the abutment zone and undercutting levels where seismic induction is heightened during block caving activities. The study was carried out at the Deep Mill Level Zone, an underground mine located in Indonesia. This mine is known as the deepest underground mine in the country. The research focused on mining activities using the block caving method, which occurs in a geological setting with incline intrusions. In this type of mining, there is a significant concentration of stress in particular areas such as the seismogenic zone and abutments, especially near the front end of sloping areas. In mines with incline intrusion geological settings, seismometer networks are typically located within the facility area in the footwall region. This can lead to incomplete coverage of the seismic network and suboptimal recording of microseismic activity. In this study, we explore an alternative approach used in the petroleum and geothermal industries, where borehole seismometers are placed outside mining facilities to improve data collection. In order to enhance resolution and reduce uncertainty in underground mines, efforts have been made to maximize theaccuracy of monitoring. To evaluate the minimum resolution, a scheme was implemented involving the placement of seismometers: (i) deploying a network of seismometers along the incline intrusion mining level within the mining facility area, and (ii) including additional seismometers beyond the facility area, with a maximum distance of 300 meters from the production zone. To evaluate the raypath response and sensitivity of both schemes, the Checkerboard Resolution Test was conducted. The results indicated that incorporating seismometers in the areas beyond the mining facility can enhance resolution by approximately 30% in the seismogenic and abutment zones. In this study, the analysis of rock mass response under specific stress conditions relies on the mechanical properties of the rock mass. To gain further insights into the dynamic properties of underground mines, various tests were conducted to determine the physical, mechanical, and ultrasonic characteristics of rocks. The objective of these laboratory tests is to establish a connection between microseismic monitoring findings and geotechnical monitoring, in order to maintain ongoing mine production. Successful laboratory experiments were carried out to measure ultrasonic velocities in drill core samples with artificial joints. Empirical equations were derived from the results, which demonstrate the correlation between bridging density and P-wave and S-wave velocities, as well as the rock velocity model specific to our study area. Laboratory testing has demonstrated that the addition of approximately one jointt in Skarn, Diorite, and Limestone can result in a reduction of Vp by about 1.8%, 1%, and 1.2% respectively, as well as a decrease in Vs by approximately 5.9%, 3.7%, and 2.3%. These laboratory test results have been effectively utilized alongside drilling data to accurately assess the quality of rock masses within the designated study area. The Rock Quality Designation values recorded in this area vary from fair (50-75%) to excellent (91%-100%). Seismic tomography was used to monitor cave propagation and stress redistribution in the DMLZ underground mine. The data used is a 57-day microseismic catalog (August 29 to October 24, 2022) with a total of 14,821 events, recorded by 84 stations consisting of 176,265 P phases and 133,472 S phases. To see the velocity evolution based on cave propagation, the data is divided into 4 (four) data subsets. Checkerboard Resolution Test (CRT) and Derivative Weight Sum (DWS) were used to test the maximum resolution that can be applied to the study area. Based on the distribution of cave propagation velocity evolution ± 60 meters to the southwest about 100 m above the undercut level. The tomogram results were validated with Time Domain Reflectometry (TDR) data which showed growth in the same direction. Cave propagation is indicated by low Vp and Vs values caused by the fracturing process along with cave propagation. Stress redistribution is observed along with cave propagation, which is characterized by the appearance of high velocity anomalies (Vp and Vs) to compensate for low velocities in the cave front area, which begins to collapse. In this study, the potential seismic hazard due to the fault structure was successfully observed in the microseismic catalog from February 14 to March 13, 2022. The data is divided into 2 (two) subsets of data, namely subset 1 data dated February 14 to 27, 2022 andsubset 2 data dated February 28 to March 13, 2022. Subset 1 consists of 8492 microseismic events and subset 2 consists of 9644 events. Based on the analysis of seismicity and microseismic event clusters can be divided into 3 clusters. The event cluster of moment magnitude- 1.6 to 0 Mw, caused by mining activities and the initial process of rock fracture, the event cluster 0 to 0.7 Mw is associated with the process of cave propagation/fault structure, the event cluster > 0.7 Mw with a decrease in velocity value ? 82% is associated with cave propagation. The fault structure/weak zone is indicated by the microseismic event cluster that occurs between the low and high anomaly contrast. This fault structure/weak zone causes damage to the rock and decreases the seismic velocity value.