Physical Properties: Soil Strength
The soil strength gives a measure of the capacity of a soil mass to withstand stresses without giving way to those stresses by rupturing (failing suddenly and quickly) or becoming deformed (failing gradually by compression). The failure of a soil to withstand gravitational forces can be seen when a structure settles as its weight exceeds the soil's bearing strength, when unstable roads and hillslopes slide downhill, and when earthen dams give way under the pressure of impounded water. In the context of mine reclamation, soil strength information is important for construction of: (i) access roads, (ii) reclaimed hillslopes, (iii) containment structures for contaminated material, and (iv) earthen dams for diversion of water from one watercourse to another, to capture contaminated water, or to provide water for the reclamation site. Soil strength tests are conducted to determine a soil's ability to withstand sudden soil failure and gradual soil compression.
physical Properties: Soil Strength: Sudden Failure
Sudden failure of soil involves the rupture of the soil matrix that gives way to gravitational forces when the soil strength no longer exceeds gravity. This phenomenon can be seen on hillslopes that simply give way unexpectedly. A classic example of this is during a mudslide when the soil moisture decreases the soil strength and the slope fails and moves downhill.
Cohesive Soils
Cohesive soils (soils with a clay content of more than 15%) have two components of strength that act against gravitational forces: (i) the inherent electrostatic attractive forces between clay particles and the water in very fine pore spaces, and (ii) the frictional resistance to movement between soil particles of all sizes. The simplest and most common laboratory test used to estimate soil strength is the direct unconfined compression test. A cylindrical specimen of cohesive soil is placed vertically between two flat porous stones (which allow water to escape from the compressed soil pores) and a slowly increasing downward force is applied. The soil column will first bulge out a bit and then fail suddenly and collapse when the force exceeds the soil strength. The greater the downward force the soil can withstand, the greater ability the soil has to withstand sudden failure. Cohesive soils become less stable when they are wet because the particles are forced apart so neither the cohesive nor the frictional components are very strong, such as in a mudslide situation (Brady and Weil, 1999).
Noncohesive Soils
The strength of dry, noncohesive soil materials such as sand depends entirely on frictional forces, including interlocking of rough particle surfaces. One reflection of the strength of a noncohesive material is its angle of repose, the steepest angle to which it can be piled without slumping. Smooth rounded sand grains cannot be piled as steeply as can rough, interlocking sands. If a small amount of water bridges the gaps between particles, electrostatic attraction of the water for the mineral surfaces will increase the soil strength. Interparticle water bridges explain why cars can drive along the edge of the beach where the sand is moist, but their tires sink on loose dry sand.
Soil Strength: Settlement (Gradual Compression)
While embankments and hillslopes commonly fail suddenly due to stresses that exceed the soil's strength, most buildings and roads are unlikely to provide loads that cause the soil to rupture. Instead, most foundation problems result from slow, often uneven, vertical settlement of the soil. The compactability and compressibility of the soil indicate how easily a soil will tend to settle.
Proper compaction ensures the stability of foundations for structures as well as adequate impermeability for the compacted layers of a containment structure. Foundations and structures built on them will tend to settle when gravitational forces acting on the soil mass exceed material strength and frictional resistance forces. The Proctor test is the most common method used to obtain data that can guide efforts at compacting soils. A specimen of soil is mixed to a given water content and placed in a holder where it is compacted by a drop hammer (weighing 2.5 kg for the standard Proctor test). The bulk density (usually referred to as dry density by engineers) is then measured. The process is repeated several times with increasing water contents until enough data is collected to produce a Proctor curve. The curve indicates the maximum bulk density to which the soil may be compacted by a given force. The test also indicates the water content of the soil that is optimum for maximum compaction. When the soil is wetter or drier, compaction is more difficult. On the construction site, water trucks will be used, if needed, to bring the water content of the soil to the determined optimum level (Brady and Weil, 1999).
Compressibility
A consolidation test may be conducted on a soil specimen to determine its compressibility - how much volume will be reduced by a given applied force. The reduction in volume occurs due to the soil particles rearranging themselves into a more compact form. Because of the relatively low porosity and equidimensional shape of the individual mineral grains, very sandy soils resist compression once the particles have settled into a tight packing arrangement. They make excellent soils to bear foundations. Clay soils with high porosity have a high compressibility and soils with organic matter have the highest compressibility. Clays and organic soils are not recommended for foundations for this reason. Compression of wet clayey soils may occur very slowly after a load (i.e. building, traffic) is applied because compression can occur only as fast as water can escape the soil pores - which for fine pores in clayey materials is not very fast (Brady and Weil, 1999).
Project Construction
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Topographic Reconstruction: Hillslope Reconstruction
The reconstruction of hillsides requires the formation of a stable structure, which will not displace downslope or fail quickly and unexpectedly. Stability exists when soil strength and frictional forces exceed gravitational forces acting upon the structure. It is important to maintain stability since movement can disrupt vegetative cover, result in exposure of toxic materials, and/or result in accelerated erosion processes. Hillsides are built by laying down lifts (layers) of soil and then compacting the soil to a desired gradient. Downward movement of hillsides often occur slowly due to two processes: (i) settlement, the movement caused by a reduction in void space between particles due to compression of fill and foundation material; (ii) subsidence, the movement caused by fine-grained material migrating into void spaces between coarse-grained material. Slow downward movement and rapid failure may occur due to several possible stresses: (i) soil-mass weight, (ii) pore-water pressure, (iii) weight-loading by construction on the surface, tree growth, or traffic, (iv) accelerations due to earthquakes, and (v) stresses applied by engineering structures. Sloping or vertical bedrock structures located underneath newly graded soils may also create a failure surface since soil strength and frictional forces are less at the soil/bedrock interface. They also may yield water from bedding planes, joints and cracks that can increase pore-water pressure and consequently, contribute to hillslope stability. Near surface groundwater and precipitation also cause stability problems since increased moisture content decreases soil strength and frictional forces. Slope aspects that face south or west decrease soil moisture content due to the drying effect by solar radiation and, consequently, increase stability.
When grading a hillside, three form properties should be considered: (i) length, (ii) gradient, and (iii) shape. As length of a hillside increases, runoff will increase in volume and velocity which can cause erosion problems towards the bottom of the slope. To shorten the length of slopes, benching should be considered which breaks up a long slope into a series of shorter ones by adding in horizontal benches along the contour. Water collected on the benches can be conveyed of the slope by ditches or pipes where it is directed to collection ponds. For more information, see the surface water control section. As slope gradient increases, shear stresses (gravitational forces) within the mass increase and at some critical gradient, the shear stresses will exceed the soil strength and frictional forces. Steep slope gradients decrease the effectiveness of compaction and attainable soil strength. In these situations, design should include hillside support by structures such as retaining walls, reinforced earth, or rock anchors. The form (shape) of the hillside influences water flow which, in turn, influences erosion potential. For example, convex slopes tend to be more erosive than concave slopes since the steepest section of the slope is near the bottom where runoff volumes are greatest. If hillsides are not designed correctly, hydrologic and erosive problems will result, with rill erosion being the most common. For more information on hillside erosion control, see the hillside erosion control section.
Consolidation & Containment
Consolidation involves grouping similar waste types in a common area for subsequent management and treatment. Once materials are consolidated, containment technologies can be employed to minimize the influx of atmospheric oxygen to the mine waste, limit moisture infiltration and divert surface water from the contaminated media. These measures minimize oxidation of sulfide minerals and subsequent formation of acid mine drainage. Containment also reduces the potential health risk that may be associated with exposure (direct contact or airborne releases of particulate) to the contaminated media. Containment technologies can be divided into two categories: dry covers and water covers. The various types of dry and wet covers will be discussed in the following sections.
Water Controls: Surface Water Controls
The key objectives of surface-water control structures are to: (i) protect the site from runon that may come into contact with tailings and waste rock by diverting water around the site, (ii) protect engineered covers from erosion and exposure of contaminates, and (iii) protect water treatment systems from surface water runon and possible overtopping.
Diversion Structures
Surface water diversion technologies help protect reclaimed lands from the consequences of water erosion and large precipitation events. Water erosion from runon and large storm events can damage or destroy the workings completed during mine reclamation. Surface water diversion technologies help keep reclaimed hillslopes, containment structures, and water treatment structures from structural failure or erosion damage and flooding which may cause the release of significant concentrations of acid and metals into the environment. These diversion technologies are considered permanent since they protect the reclaimed areas over the long term. They differ from the diversion technologies discussed in the site preparation sectionwhich are constructed on a temporary basis to divert clean water around contaminated sites before and during reclamation. Different permanent diversion structures include dikes and berms, terraces and benches, ditches and drainways, and chutes and downpipes. The permanent diversion structures are often similar in design to the temporary structures except the permanent structures must be able to carry a lower frequency, higher duration storm than the temporary structures. This entails larger, better-designed structures with a larger factor of safety.
Dikes and berms reshape the surface of the ground to form earthen walls that prevent runoff and off-site runon from reaching sensitive areas. Dikes and berms are often constructed around the base of containment structures and water treatment systems. Behind the berm there may be a diversion ditch to direct the flow to a nearby surface water body, sedimentation basin, or water treatment system. These measures prevent degradation of engineered covers and subsequent release and transport of contaminants to off-site locations. They also prevent flooding and overtopping of water treatment systems during large storm events.
Benches and terraces help protect hillslopes from excessive water erosion by creating barriers along the contour which captures flow and prevents excessive erosion. Surface roughening techniques such as dozer basins, pittes, and gouges also protect the hillslopes from excessive water erosion and subsequent instability, settlement, or sudden failure problems. For more information on these topics, see the hillslope erosion control section.
Ditches and drainage ways are constructed to collect and direct overland flow and runoff/runon captured by dikes, berms, benches, and terraces away from reclaimed hillslopes, containment structures, and water treatment systems. Water is diverted in ditches to nearby surface water bodies, sedimentation basins, and/or water treatment facilities. Ideally, a ditch has minimal and consistent gradient. Ditches are intended to convey the peak flows from the design event with adequate "freeboard", the distance between the calculated surface of the peak flow and the top of the constructed channel. For more information on ditch and drainage way design, see the site preparation section.
Chutes and downpipes are used to collect precipitation and runon and convey the flow down steep gradients, beneath roads or other structures and across areas that would obstruct the flow. The pipe material must be compatible with water quality. For example, corrugated metal pipes should not be used to convey acidic drainage. Pipes generally are designed with inlet controls to prevent complete filling and excessive joint pressure on the pipe. They are also designed with seepage collars to prevent undermining of pipes and with trash racks to prevent clogging. For more information on chutes and downpipes, see the site preparation section.
Water collected from ditches, drainage ways, chutes, and downpipes are commonly directed to sedimentation and detention basins where the water is detained for a given residence time in order to capture and drop out eroded sediment from the surface runoff. Depending on the water quality of the drainage, the water may then be diverted to a water treatment system or it may be released to a nearby surface water body.
Dams have various applications in mine reclamation. A dam may be built to: (i) store water for use on the reclamation site, (ii) impound contaminated water for treatment, (iii) divert waterways away from contaminated sites to prevent the migration of contaminants off-site, or (iv) dampen the effects of peak flows to reduce the effects of erosion.
Most reclamation dams are earthen dams made from indigenous materials. This material may not always be suitable for the structural stability of the dam therefore, in certain cases, it may be necessary to import some or all of the dam material. Maintaining structural stability of a dam is the most important design component since the pressure of the water against the embankment wall is always substantial. Embankments made of earthen material may become saturated due to the porous nature of the material. Once the embankment becomes saturated, the structural integrity of the dam is low and the dam could fail. To prevent this, most earth-filled dams have an impermeable core that allow a substantial amount of the downstream portion of the dam to be unsaturated. Often the toe of the dam on the downstream side is made from large rocks (rip rap) that ensure large, well-drained pores which aid in keeping the downstream portion of the dam dry.
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