British Columbia Mine Reclamation Symposia

Some observations relative to the performance of flow-through rock drains 2009

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Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 119  April, 1989 SOME OBSERVATIONS RELATIVE TO THE PERFORMANCE OF FLOW-THROUGH ROCK DRAINS by D.B. Campbell, Principal, COLDER ASSOCIATES LTD. Consulting Engineers INTRODUCTION In Sept 1986, an International Symposium on Flow- Through Rock Drains was held in Cranbrook, British Columbia. At the Cranbrook Symposium, Campbell (1986) addressed some of the concerns that have been expressed, regarding the long-term performance of rock drains. The 1986 paper presented arguments and data which showed that some of the concerns that have been expressed, are not supported by field and laboratory data that have been collected. The author has been requested to provide an update of data and information that have been obtained, subsequent to the Cranbrook Symposium. This paper presents a brief summary of some field observations that have been made at rock drains, together with an assessment of what these observations indicate with respect to rock drain performance.  Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 120 April. 1989 ____________________________________________________________ PARTICLE BREAKAGE DUE TO HIGH STRESSES John Wilkins, an Engineer who worked with the Tasmanian Hydroelectric Authority, was one of the pioneer investigators of the parameters governing turbulent flow through coarse rock (Wilkins, 1956). The metric version of the results of Wilkins' investigations, is as follows: VV = 5.28(eD/7.5)0.5 S0.5 (Equation 1) Where: VV is the average velocity of flow-through the voids within the mass of the coarse rock fragments. D is the mean stone size (metres) e is the void ratio S is the hydraulic gradient through the rock drain The term inside the brackets in equation 1 represents the hydraulic radius, that is, the mean area through which flow takes place, divided by the area of the wetted surface on the flow boundaries. Equation 1 shows that the rate of flow-through a rock drain, per unit area of the wetted cross section, is governed by the mean size of the rock fragments, and by the void ratio of the mass of the rock fragments comprising the rock drain. When the zone of coarse segregated rock comprising a rock drain becomes covered, the weight of the waste rock above the drain must be carried by transfer of load, at the points of contact between the adjacent rock fragments. Broken rock is angular. Consequently, the contact areas between the adjacent blocks of coarse rock are small. These small contact areas result in high stresses at the points of contact, which in turn result in rock crushing and rock fracture.   Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 121 April, 1989_____________________________________________________________ Both crushing and fracture of the rock result in a reduction in the mean size of the rock fragments and a reduction in the void ratio, relative to what would have been observed before the rock drain materials became covered. Reduction in particle size, and in void ratio result in a reduction in the hydraulic radius. Equation (1) shows that a reduction in the hydraulic radius results in a reduction in the through-flow capacity of the rock drain. There is seldom an opportunity to observe the effect of rock fracture and point-to-point crushing within a rock drain after it has become covered. However, one such opportunity became available at the Fording Coal Ltd., Fording River Operations during the winter of 1987, as No. 1 Spoil, was being excavated. No. 1 Spoil was the first waste rock dump developed at the Fording Coal property. Development of this spoil was required to permit commencement of the open pit mining operations on the property. It was known that the spoil was located above coal reserves, and that its removal would be required at a later date. In January, 1987, a shovel breakdown occurred as excavation was in progress in an area where the base of No. 1 Spoil is in contact with the natural foundation on which it rests. The short interruption in the mining activity provided an opportunity to examine the condition of the zone of coarse segregated rock at the base of the No. 1 Spoil. Temperatures at the time were below freezing, and the small amount of cohesion provided by minor ice in the waste rock permitted the excavated spoil face to stand nearly vertical. Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 122 April, 1989 ____________________________________________________________ The condition of the rocks comprising the coarse segregated zone at the base of the No. 1 Spoil, after it had been covered for about 12 years by 50 metres of waste rock fill, is illustrated in photos 1, and 2. Fracturing of individual rocks, as a result of high stresses at the points of contact between adjacent blocks, is evident in the photographs. However, the zone of coarse rock at the base of the dump remained porous. POTENTIAL PARTICLE MIGRATION WITHIN A DUMP In reference 1, gradation curves for a laboratory model waste dump are presented. The distribution of particle sizes in the vertical direction, as obtained from the model dump, constitutes a well graded filter, and indicates that downward migration of particles within the body of a waste rock dump cannot occur. A similar conclusion is indicated by the gradation curves from field trials conducted by Nichols (1986), and by the gradation of the rock sizes on the face of the Crows Nest Resources ' West Line Creek Dump, as presented in Reference 1. Examination of the waste rock at the base of the No. 1 Spoil did not reveal any evidence of the presence of fines that could have originated from segments of the dump above the segregated zone at the base. The January '87 inspection of No. 1 Spoil provided field data that supports the conclusion that downward particle migration within the body of a waste rock dump is precluded. BEDLOAD AND SUSPENDED SEDIMENT During the winter of 1986-87, a rock fill was advanced across the North Fork of Rose Creek near Faro, Yukon. This rock fill was constructed   to provide access to a  Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 123 April, 1989_____________________________________________________________ proposed new mining area. At the location where it crosses the bottom of the North Fork valley, the roadway fill is approximately 60 metres high. The fill for the roadway, at the location of the creek crossing, consists of calcium silicate, a hard metamorphic rock. The rock fill was placed by end dumping from roadway level. Segregation of the large fragments of the waste rock, in the course of their transit down the face of the fill, and the accumulation of these coarse fragments at the toe of the advancing fill, constitutes a rock drain. This rock drain conducts the flows in the North Fork of Rose Creek through the base of the roadway fill. The North Fork rock drain first operated in the spring of 1987, when it passed a maximum flow of 7 cubic metres per second, as measured on the downstream side of the rock drain. Concerns have been expressed about the potential clogging of rock drains, as a result of deposition of suspended sediments within the drain, or as a result of deposition of bedload at the inlet end of the drain. Inspection of the North Fork rock drain, in May, 1987, provided some information germane to both of these concerns. The presence of even the most pervious rock drain within a drainage course represents an impedance to flow, relative to the conditions in the open channel prior to development of the rock drain. Thus for quasi-equilibrium to be maintained, the area of the gross wetted cross section within the drain must be greater than the area of the wetted cross section in the open channel leading to the drain. If the rate of flow in the open channel above the rock drain were to remain constant, the water level in the rock drain would rise to a level such that the combination of gross wetted cross section and the Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 124 April, 1989 hydraulic gradient, would result in a rate of discharge though the drain equal to the rate of flow in the open channel. The fact that the free water surface in the rock drain must rise to accommodate the flow, implies that a pool must form at the inlet end of the rock drain. This is true for all rock drains. As the water in the open channel enters the pool upstream of the inlet to a rock drain, the flow velocities decrease abruptly. As a result of this reduction in velocity, the bedload sediments that may be in transit along the bottom of the stream are deposited at the inlet to the pool. It is this mechanism that is responsible for the formation of deltas at the locations where rivers and streams enter lakes and oceans. Since the bedload sediments are deposited at the upstream end of the pool, they are not transported to the inlet of the rock drain, and do not result in blockage of the inlet to the drain. When the North Fork rock drain was inspected in May 1988, it was evident that the pool on the upstream end of the drain had been at a higher level than it was at the time of the inspection. The level to which the pool had previously risen is indicated by the wood debris on the upstream face of the rock fill, as shown in Photo No. 3. Within the range of fluctuation in the pool surface level, fine-grained sediment had been deposited on the upper surface of boulders at the inlet end of the drain. This sediment is visible on the surface of the boulders shown in Photo No. 3. The grain size of the sediments that had settled on the surface of the boulders, is an indication of the size of the suspended solids that had been transported to the inlet of the rock drain. A size    Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 125 April, 1989 analyses of a sample of the sediment, collected from the surface of the boulders, showed that all of the particles were smaller than 0.074 mm; the No. 200 US Standard Sieve size. By use of equation (1), the velocity of flow through the void spaces within the rock drain is estimated to have been approximately 0.1 metres per second. The settling velocity of a 0.074 mm size particle would be approximately 3.4 mm metres per second, or about l/30th of the average velocity of flow through the drain. In a turbulent flow field, particles having a settling velocity of about 3 per cent of the flow velocity, would not settle within the drain; these particles would be swept through the rock drain with the flow. The field observations that the author has made subsequent to the September '86 Cranbrook Symposium on through-flow rock drains provide confirmation of some of the conclusions stated in reference 1. These conclusion are: 1.     End dumping results in a distribution of particle sizes grading from fine to progressively coarser proceeding from the level of the dump platform toward the base of the dump. This gradual reduction in particle sizes constitutes a well-graded filter that precludes downward migration of particles within the body of a dump. 2     During periods of significant stream discharge, a pool will always be present at the inlet end of a rock drain. This pool serves to trap bedload at the location where the stream inters the pool. Consequently deposition of bedload does not result in blockage at the inlet to the rock drain. 3.    The pool at the inlet end of a rock drain also results in deposition of suspended sediment. The data collected at the North Fork rock drain, as well as data for the Swift Creek rock drain (reported in reference 1) indicate that potential deposition of suspended sediment is not a factor that would result in reduction in the through-flow capacity of a rock drain.     Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 126  April, 1989 REFERENCES 1. Campbell, D.B., "Discussions of Concerns Regarding the Long-Term Performance of Rock Drains," Proceedings of the International Symposium on Flow-Through Rock Drains, Cranbrook, British Columbia, September 1986. 2. Nichols, R.S., "Rock Segregation in Waste Dumps", Proceedings of the International Symposium on Flow-Through Rock Drains, Cranbrook British Columbia, September 1986. 3. Wilkins, J.K., "Flow of Water Through Rockfill and its Application to the Design of Dams". Proceedings, Second Australia-New Zealand Conference on Soil Mechanics and Foundation Engineering, 1956. DBC/gg 2/dc-pape           Golder Associates Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 127  Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation 128 PHOTOGRAPH No 3 Illustrating the size of the rocks at the upstream side of the North Fork rock drain. The wood debris at the level of the man' hardhat. indicates a previous level of the pool on the upstream side of the rock drain. Silt had settled onto the surface of the boulders at the time of previous high water. The size of the silt particles is an indication of the size of the suspended solids that had been transported to the upstream end of (he rock drain. A grain size analyses showed that all of the silt was finer than 0.074 mm. All particles of this size would have been transported throught the rock drain.              PHOTOGRAPH No 4 Showing the flows exiting the downstream toe of the North Fork rock drain. The rate of discharge is between 3 and 4 cubic metres per second. 

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