Performance prediction and optimization of raise boring machines (RBMs)

Abstract

Raise Boring Machines (RBMs) continue to find extensive application in the mining and tunnelling industries because of the many advantages compared with drill and blast methods. These machines as the fast, safe, and efficient excavation machines are used for different purposes such as excavation of ventilation and ore transport shafts in the mines, switching lines between underground subway tunnels (horizontal), and ventilation shafts in the road and railway tunnels. In the conventional raise boring method, the machine is set up on the upper level of the two levels to be connected. Then, a small diameter hole (pilot hole) is drilled by a sealed bearing tricone bit to the lower level. Once the rod (drill string) breaks into the opening on the target level, the pilot bit is removed and a reamerhead with roller button cutters is connected to the rods and raised back up towards the upper level. The debris and cuttings from the reamerhead during drilling fall on to the lower level and from there a conventional method is used to take them out. Although a high price in foreign currency is paid for purchasing these machines, sometimes the performance (productivity) of the machines would be too low, rod jamming, and shaft deviation problems might be encountered. Consequently, the selection and design of these machines according to the project and excavation environment, predicting accurately their performance, and maximization (optimization) of the performance become very important in terms of the economics of the project in the feasibility and planning stages. Predicting the performance of an RBM is one of the important subjects in mechanized shaft / raise excavation to estimate costs and job completion time that directly affects the project economics. The parameters affecting the selection, design, and prediction of the performance of an RBM can be classified into three general categories: mechanical (machine-related) parameters, geological / geotechnical parameters, and operational parameters. A suitable RBM should be selected / designed for the geological / geotechnical conditions to be encountered during the shaft excavation. Utilizing the highest rotational speed and correct bit load for specific rock formation could lead to an optimum rate of penetration and bit service life. Cutters with larger row spacing are suitable for reaming in softer rocks. However, cutters with narrow spacing are suitable for reaming in very hard and tough rocks where the thrust forces cannot be fully utilized. Identification of fault zones and broken ground along the shaft alignment are the key requirements for successful shaft excavation with RBMs. A comprehensive rock-testing program is the most important requirement for the assessment of RBMs, and by use of the obtained data from the rock-testing program the best-suited raise bore machine for each project could be selected. Samples of all different rock types in the excavation area should be included in the laboratory studies. Moreover, site preparation plays an important role in the elimination of delay and increasing the efficiency of the raise boring operation. Load per cutter, reamerhead diameter, geological formation, and inclination of the shaft are some important factors affecting the torque requirements. Increasing shaft diameter causes additional torque effort on the drill string and reamer stem. In addition, with the increase in diameter, there is a greater potential for instability of the shaft walls and the advancing face of the shaft. The basic aim of this thesis research is to propose new empirical and deterministic models for proper selection, design, and prediction of operational and performance parameters including daily advance rate, rotational speed, weight on bit (pushing force of tricone bit), net reaming thrust (pulling) force, tricone bit / reamerhead torque and power, unit penetration rate, instantaneous penetration rate, field specific energy, and machine utilization rate of RBMs. In order to reach the goals of this thesis, firstly, the related mine and tunnel sites are visited for obtaining information on the geology of the field, the applied excavation method, and the specifications of the RBMs; collecting rock samples from the formation(s) excavated; and collecting field operational-performance data. The obtained rock core samples are used for the determination of the physical-mechanical properties and indentation tests, and block samples are used for the full-scale linear cutting tests. The experimental studies start by determining some important physical-mechanical properties of intact rock samples including uniaxial compressive strength, indirect (Brazilian) tensile strength, static and dynamic elasticity modulus and Poisson's ratio, acoustic wave velocities (P and S wave velocities), Schmidt hammer, Shore scleroscope, Cerchar abrasivity, and petrographic analysis. Rock Quality Designation (RQD) values are also determined based on geotechnical reports of the related sites. Then, indentation tests (by an insert tip with a diameter of 22.2 mm and width of 11 mm) are performed on the core samples from the fields and rectangular rock pieces obtained from the remnants of the block samples. Finally, a series of full-scale linear cutting tests in five different rock types (diabase, granodiorite, skarn, limestone-1, and limestone-2) with a 305 mm (12 inches) diameter button (kerf) cutter having an insert tip width of 11 mm are performed to generate a new RBM performance database. This research demonstrates the challenges of collecting long-term operational data in raise boring operations. Although this thesis includes raise boring operation data collected over six years, it has some limitations such as the number of data, rock mass parameters, different reamerhead diameters, and different raise inclinations. The models presented in this study are limited by the data content, the upper and lower limits of the data, and the assumptions made on a few issues mentioned in this thesis. The results of the experimental studies indicate that the mechanical properties of the rock samples vary in a wide range and the collected rock samples are in all types of geological origins (sedimentary, igneous, and metamorphic). In the inclined raise-bored shafts static elasticity modulus and Brazilian tensile strength could be good predictors for the field specific energy of reaming operations. In addition, the operational parameters of pilot hole drilling and the physical-mechanical properties of rocks may be used to predict the performance parameters of reaming in both vertical (90°) and inclined (70°) raise-bored shafts. Moreover, direct and indirect deterministic approaches are able reliably to predict the unit penetration and instantaneous penetration rates of RBMs. Besides, indentation tests indicate that the brittleness index of rocks may be predicted by using the velocity of the S-wave and the Cerchar abrasivity index parameters. Finally, this study shows the feasibility of using the power function on the relationships between the indentation force and the penetration values for predicting the performance of RBMs. The outcome of this study establishes a new database to assist on predicting the performance parameters of RBMs. The proposed empirical and deterministic models in this study can be used by the designer engineering companies, contractor companies, and project owner institutions in the feasibility and planning stages of the projects.