Experimental Testing Of Geomechanical Behavior Of Fiber Reinforced Cemented Paste Backfill Fr Cpb Under Warmer Curing Temperature
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Experimental Testing of Geomechanical Behavior of Fiber-reinforced Cemented Paste Backfill (FR-CPB) Under Warmer Curing Temperature
Backfilling techniques enable improved ore recovery and structural stability to underground mines employing a material to fill the voids after the excavation. Fiber-reinforced cemented paste backfill (FR-CPB) is this material and it consists of mine tailings, cement, mixing, and fibers. After placed into the underground space (called stope), FR-CPB provides sufficient ground support, enables the exploration of larger amounts of ore since no orebody pillars are required to sustain the excavations, and thus enhances mining production. The reinforcement technique has been considered as a promising approach for the backfilling design. However, regarding that mining activities may take place at a depth of more than 1000 meters, the geothermal gradient can not only change the temperature of FR-CPB but also affect its geomechanical behaviors due to its temperature-dependent characteristics. Therefore, the objective of this research is to experimentally investigate compression, tension, shear, triaxial, and fracture behaviors of FR-CPB subjected to different warmer curing temperatures (20°C, 35°C, and 45°C). Moreover, to identify the mechanisms responsible for the evolution of geomechanical behavior, a series of mold-based monitoring programs have been designed and performed to measure changes related to matric suction, electrical conductivity, and temperature in FR-CPB. Additionally, to determine the progress of binder hydration and associated microstructure change, extensive X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) observation have been conducted at the microscale. The obtained results evidenced that warmer curing temperature can significantly affect the fiber-CPB matrix interfacial interaction. Correspondingly, the geomechanical (including tensile, compressive, shear, and fracture) behavior show strong temperature sensitivity from early to advanced ages. Therefore, the obtained results from the present study can not only improve the understanding of the geomechanical behavior of FR-CPB but also contribute to the safe design of backfill structures in underground mines.
Testing and Multiphysics Modelling of the Shear Behaviour of Rock-Cemented Paste Backfill Interface
Cemented paste backfill (CPB) is an innovative technology developed in the mining industry during the last few decades. It has been adopted worldwide by many underground mines for its tremendous advantages: (1) mining space is stabilized by pumping cemented paste backfill into the underground cavities created by mining activity, which is critical to the safety of mine workers; (2) the consumption of tailings (which is stored at the ground surface and is a major source of acid mine drainage (AMD)) is beneficial for environmental protection and community safety; (3) due to the supporting effect of the CPB structure on underground cavities, the recovery ratio is significantly increased; and (4) CPB structures can also carry heavy equipment when mining the adjacent orebody, facilitating mining operations. How to design a safe and cost-effective CPB structure is a key task or challenge for mining engineers and researchers. Mechanical stability is one of the most important design criteria. This stability is mainly a function of the uniaxial compressive strength (UCS) of CPB body and the shear strength/behaviour of the CPB-rock interface. Given the lower friction angle and adhesion of the CPB-rock interface (in comparison with the friction angle and cohesion of CPB body), a thorough understanding of the shear strength/behaviour of the interface is critical for a cost-effective geotechnical design of underground CPB structures. However, only limited studies have been conducted to date on the shear performance of the CPB-rock interface, and no studies have taken into consideration the effects of different factors (e.g., temperature, sulphate ions, self-weight or surface morphology) on the shear behaviour of the CPB-rock interface. Moreover, no multiphysics interface model is currently available that incorporates the aforementioned factors to describe and predict the CPB-rock interface shear behaviour. This research gap was therefore addressed in this PhD study. In this PhD research, a series of laboratory tests were conducted assessing the effects of sulphate content, temperature, curing stress, drainage condition and interface roughness on the shear strength/behaviour of the interface between CPB and rock. The results obtained so far indicated that sulphate and temperature can either positively or negatively affect the shear strength of the CPB-rock interface, depending on the initial sulphate contents and curing time. In terms of the effect of temperature, the shear strength and shear strength properties generally increased with temperature. However, high temperature (≥ 35°C) resulted in an adverse effect on the shear strength because of the crossover effect. In addition, higher curing stress benefitted to the shear strength acquisition of the interface and, due to the increased effective stress and matrix suction, the drained condition increased shear strength as well. As for the effect of surface morphology, the shear strength of the CPB-rock interface rose with surface roughness. Furthermore, chemo-elastic as well as coupled thermo-chemo-mechanical cohesive zone models (CZMs), which take the sulphate attack and temperature-induced acceleration in the cement hydration into consideration, are also developed to simulate the shear strength and behaviour of the CPB-rock interface. The proposed models can well capture the shear behaviour of the interface under different loading conditions. Besides, they also numerically attest to the importance of the shear resistance of the CPB-rock interface in controlling stress distribution in CPB structures. The results obtained from experimental tests, numerical modelling and simulations concerning the shear behaviour of the CPB-rock interface under different multiphysics conditions provided useful information for understanding and more effectively assessing the shear strength and behaviour of the interface between a CPB structure and rock mass, which is critical for the design of safer and more cost-effective CPB structures.
An Experimental Study of Cementing Paste Backfill
[Truncated abstract] This thesis focuses on experimental element testing of cementing paste backfill (CPB) to examine and improve on existing laboratory testing techniques. Specifically, the work focuses on developing a framework to account for differences in curing conditions between in situ and laboratory environments, given the recognised improvement in mechanical properties of in situ cured CPB. This has been explored within the effective stress framework by making use of a hydration cell testing apparatus. The existing hydration cell set-up was modified to allow control over the rate of temperature increase and final temperature of specimens during curing, to replicate in situ curing conditions. The combination of elevated curing temperature and effective stress generation was found to significantly increase the mechanical properties of CPB compared with curing at elevated effective stress and ambient temperatures. The current procedure for curing under effective stress can be expensive and time consuming. As such, the standard test method for chemical shrinkage of hydraulic cement paste (ASTM C1608-07) was investigated as a potential index/screening test for use in the design of CPB mixes through comparison of the chemical shrinkage-induced strain generated in both the ASTM and hydration cell tests. The appropriate use of this ASTM standard with CPB was validated despite the high w/c ratio of typical CPB mixes.