Drainage and settlement characteristics of hydraulic fills

Mining is one of the major export industries in Australia. When the ore is removed from the ground voids are backfilled. Hydraulic backfill and paste fill are most common backfills. These backfills provide safe working conditions and increase the strength of the ground. Hydraulic backfills consist of classified mine tailings with not more than 10% by weight of size less than 10 μm and approximately 70% by solid weight. These are prepared on the surface by dewatering the mine tailings and transported to the stope transported by gravity through boreholes and pipelines. The solid particles of hydraulic fills settle rapidly after entering the stopes. Excess water initially tends to pond on the surface building up from the lowest corners of a stope and then commences a vertical path of drainage due to gravity. Several miners and equipment had been trapped in mines due to barricade failures and the resulting in rush of mine fills into the drives. An extensive study of drainage and settlement characteristics of hydraulic fills is required to improve design of drainage and barricades. Most permeability measurements reported in the literature are from undisturbed or reconstituted samples in the laboratory under no surcharge. However, in the mine stopes the hydraulic fill is subjected to significant surcharge due to the overburden. In this research, extensive laboratory tests have been performed to measure permeability of hydraulic fill samples subjected to vertical stress. An analytical solution has been developed to estimate vertical stress development in a vertical and inclined mine stope using continuous compression arch of principal stresses. It has been verified with different analytical model discussed in literature. Degree of saturation of hydraulic fills within mine stopes decreases with continuous drainage of water through barricades. Laboratory tests have been performed to study the moisture content variation in a mine stope with depth and time. Soil-water characteristic curves have been developed using Filter Paper Method for hydraulic fills. Permeability of hydraulic fill has been computed using vertical stress and moisture content variation in a stope. It has been shown graphically. The anecdotal evidence suggests that the consolidation of the deslimed hydraulic fill is instantaneous. In this dissertation, laboratory tests have been conducted using conventional oedometer and modified triaxial cell to measure the settlement of hydraulic fills. It has been found that the immediate settlement and time dependent settlement contributes equally to the total settlement. Pore water pressure dissipation has observed using modified triaxial cell. Time of consolidation and coefficient of consolidation have been estimated and reported. Settlement- effective stress- time plot has been used to investigate the variation of coefficient of secondary compression (Cα) and it has been observed that Cα of hydraulic fill increases with effective stresses and is independent of time. Concept of equivalent permeability has been used to estimate drainage and maximum pore water pressure in a stope using method of fragments. Empirical equation has been proposed to estimate water level in stope at any time after filling. Reliability analysis has been carried out using First Order-Second Moment Analysis Method to measure the effect of parameter and modeling errors in drainage and settlement analysis. It has been verified using Monte Carlo Simulation.