G. Ward Wilson

Mechanical response of highly gap-graded mixtures of waste rock and tailings. Part II: Undrained cyclic and post-cyclic shear response

Mixing of waste rock and tailings as a homogeneous mixture (referred to as “paste rock”) has been suggested as a favourable approach to overcome deficiencies associated with traditional methods of mine waste disposal. In consideration of the current limited understanding of the fundamental mechanical response of paste rock, a laboratory research program was undertaken to investigate the monotonic and cyclic shear response of paste rock. This paper presents the findings from undrained cyclic triaxial shear testing conducted on saturated paste rock specimens reconstituted such that the tailings would “just fill” the void spaces between the coarse particles of the skeleton. During undrained cyclic loading, paste rock typically exhibited a cumulative decrease in effective stress along with a progressive degradation of shear stiffness. The material generally displayed a higher tendency for strain development under cyclic loading when compared with rock-only and tailings-only specimens subjected to similar cons...

The characterization of hydrologic properties and moisture migration pathways of a waste rock pile

A multiphase research program has been initiated on a large waste rock pile which has undergone partial excavation. The focus of the research is to define the distribution of moisture migration pathways in the waste rock and to develop a conceptual model of the heat and mass transfer processes within the pile. The first phase of the research program was conducted simultaneously with the excavation process. This phase included a field logging and sampling program of exposed waste rock from numerous test pits. The test pits were excavated along predetermined transect lines established on the benches of the waste rock pile which were exposed during the excavation process. Phase I included visually logging the exposed rock and insitu measurements of matric suction, temperature, relative humidity and water content. Samples of each waste rock unit described in phase I were obtained for further analysis. The second phase of research includes a laboratory testing program to determine saturated and unsaturated hydraulic and thermal properties of the rock. The properties being measured include grainsize, porosity, moisture retention characteristics, saturated hydraulic and thermal conductivity and specific heat capacity. In summary this paper outlines the research program to date and discusses the initial findings of the field program. INTRODUCTION The mining industry is working towards improved environmental technologies. Research for modelling and predicting seepage quality, especially in the long term, is an important part of these research efforts. Prediction of effluent releases from waste rock piles requires an understanding of both geochemical and hydrogeological characteristics of the sulphide bearing waste rock material found within waste rock piles. This paper describes the first phase of a research program to determine the hydrogeologic properties and moisture migration pathways of a large waste rock pile. The first phase of the research program was conducted for a waste rock pile located at a large open pit gold mine in southwestern Montana. The

Volumetric Changes and Point of Saturation around a Pressuremeter Probe Used in Unsaturated Soils

AbstractThe interpretation of pressuremeter test results in cohesive soils assumes that the soil is saturated and the rate of loading is such that undrained conditions apply. Pressuremeter tests in compacted soils or heavily overconsolidated clays may have a degree of saturation that is less than 100%. Consequently, the soil may undergo volume changes during the initial stages of loading. Loading of the soil will result in the compression of occluded air and consequently the initial circumferential strains may be a function of volumetric changes and not a result of borehole expansion alone. A method to determine the applied radial total stress that will result in saturation for the pressuremeter test is developed based on Hilf’s method of pore water pressure prediction during compressive loading of an unsaturated soil. A method to quantify the volume change that will occur within the plastic region around the pressuremeter probe prior to saturation is also developed.

Soilcover, a new computer model to aid in the design of soil cover systems for acid generating waste rock and tailings

Evaluation of the flow of water across the soil atmosphere boundary is an essential component in the design of soil cover systems. The design of soil cover systems as oxygen barriers for the long term closure of sulphitic tailings and waste rock requires the accurate prediction of moisture fluxes between the cover surface and the atmosphere. SoilCover is a new software package which uses a theoretical method for predicting the exchange of water between the atmosphere and a soil surface. The theory is based on the well known principles of Darcys Law and Picks Law to describe the flow of liquid water and water vapour in the soil profile below the soil atmosphere boundary. A modified Penman formulation is used to compute the rate of evaporation to the atmosphere above the soil atmosphere boundary. SoilCover predicts the actual rate of evaporation from both saturated and unsaturated soil surfaces. The model accounts for atmospheric conditions, soil properties, and the effects of vegetation. In addition, SoilCover performs a water balance on the basis of infiltration, evapotranspiration, surface runoff, surface ponding, and the soil profile. The change in water content, suction, vapour pressure, temperature, and hydraulic conductivity with respect to time and depth within the soil profile are also calculated. SoilCover provides a one dimensional transient analysis which can be used interactively with other commercially available software packages for modelling groundwater flow in mine tailings and waste rock. The two dimensional flow system is modelled using the transient flux boundary conditions at the soil cover atmosphere boundary predicted by SoilCover.

ENHANCED GEOMORPHIC DESIGN FOR RECLAMATION OF RURAL WASTE-SCAPES

Many inventive concepts for the adaptive re-use of waste landscapes, or waste-scapes, have been proposed and constructed in the last decade. These are often located near or within large, urban populations, which provide much of the incentive for adaptive re-use. A different challenge presents itself when a waste-scape is rurally located, near a small - though equally important - population. How do we address complex socio-cultural, economic, and environmental objectives without the economic incentive provided by a large nearby population? This project looks at the mineable oil sands region of northern Alberta, Canada: a rural waste-scape covering 895 km2 in Canada’s boreal forest. Specifically, this project discusses the geomorphology and native substrate of northern Alberta, juxtaposed with the traditional design of waste storage landforms, in order to show that there are no natural analogues in the region. A geomorphic approach to the design of waste-scapes in this region has been developed using a Landscape Evolution Model (LEM) for long-term projections, and is being tested in the region. This project sheds new light on the rarely acknowledged issue of waste design in rural areas and the wide range of benefits achieved through use of an enhanced geomorphic design approach.

The modelling of moisture movement in engineered soil covers for mine waste rock dumps

A two-phased cover instrumentation and modelling research program has been initiated at a waste rock dump site. The first phase of the research program involves field instrumentation and monitoring of the inplace cover. The second phase involves modelling infiltration and evaporation in the cover system using a soil/atmosphere flux model that has been developed at the University of Saskatchewan. The soil/atmosphere model used is a one dimensional, transient, finite element, water and heat transport model that uses a physically based method to predict the exchange of water between the atmosphere and a saturated or an unsaturated soil surface. This paper describes the application of the soil/atmosphere model in predicting the field responses of the in-place cover system at the waste rock dump site.