4.3 Dump Material Properties
Properties of dump materials include gradation, shear strength and durability, etc. The most favourable dump materials are composed of hard and durable coarse rock with little or no fines and are commonly associated with metal mines. The least favourable materials are formed of soft and degradable rocks with significant fines, such as mudstones or shales, which are commonly associated with coal measures or heavily weathered or altered rock masses. However, of primary importance is the percentage of tine particles which may absorb water (especially clay minerals), settle and blind natural drainage paths within the base of dump, and may impart some degree of apparent cohesion causing oversteepening in localized areas. In general, the higher the percentage of fines, the poorer is the overall stability of the dump.
The shear strength of dump material depends on rock types and its compositions, the unconfined compressive strength, weathering and slaking potential, and the respective particle sizes. The strength of the resulting combination of material is generally based on a comparison of similar rock types and their relative compressive strengths and gradation.
Materials with low durability weather rapidly to finer particles in the natural dump environment due to exposure to atmospheric moisture, wetting and drying, freezing and thawing, and high particle to particle loads. Movement within the dump also degrades the material and results in a rounded rather than angular shaped rock, with a coincident reduction in friction angle for the material.
The mine geology may change dramatically from bench to bench. Under such condition, strong and competent rocks may be placed under weak and altered rocks on the platform and down the face. In this scenario, further loading even of competent material, may result in instability due to the presence of weak planes.
Most mining operations encounter a range of material types in their rock and soil materials. Soil should be excluded from mine dumps, as it hinders drainage and introduces zones of lower shear strength. Such soil should be placed in specifically designated and designed dumps or stockpile sites.