2.0 Method of Construction in lift section

Dumps are usually constructed in a series of lifts following either descending or ascending sequence. Ascending construction is advantageous, as toe of each lift is supported on the preceding lift in this case. Figure 1 & 2 show the construction of mine waste dump in lifts. The method of construction selected is based on a combination of factors such as minimizing haulage distance, accessibility, available capacity and dump stability. Stability can be enhanced by judicious use of wrap-arounds, terracing, restricting lift heights to limit shear stresses on the foundations and the length of potential runout, and dumping in the direction of valley contours rather than downslope.

Construction in lifts depends on the geotechnical properties of dump material, cost of haulage, ease of final grading, etc. Surface slope of top of the dump depends on run-off of water, drainage and safety.  The control of run-off water and drain recharge is also very important in design of mine waste dump. As the dump is composed of blasted rock material, proper dust control and dust suppression methods should be used.

Figure 1 : Construction of mine waste dump in lifts

Figure 2: cross-section view of dump Lifts

 

 

 


 

 

Figure 3: Typical layout of Dump

 

 

 

 

 

 

 

 

General type of fills are valley fill, Sidehill, Cross-Valley fills, Heaped Fills, etc. Valley fills fille the valley partially or completely (figure 4). The surface of the dump is usually graded to prevent impoundment of water at head of the valley. Valley fills, which do not completely fill the valley, may require construction of culverts, flow-through rock drains or diversions depending on the size and characteristics of the upstream catchment. Sidehill fills are constructed on sloping terrain and do not block any major drainage course, as illustrated in Figure 4. Dump slopes are usually inclined in the same general direction as the foundation. Toes of Sidehill fills may be located on the slope or on flat terrain in the valley bottom. The Cross-Valley fill is a variation of the Valley fill. As illustrated in Figure 4, the embankment extends from one side of the valley across the drainage to the other side of the valley. The upstream portion of the valley is not completely filled and fill slopes are established in both the upstream and the downstream directions. Heaped fills are stacked or piled fills, consisting of mounds of waste with slopes formed on all sides. Foundation slopes are generally flat or gently inclined.

 

 

 

Figure 4: Dump Location & Type

 

Ascending construction requires development of lifts starting at base of the structure and progressing to the ultimate height. Material is placed in a controlled manner in relatively small thickness. Once a lift is completed, the next lift is placed on top, and the sequence is continued until the ultimate dump height is reached. This controlled construction technique is commonly used in situations where sensitive foundation soils exist and incremental loading allows these soils to drain and consolidate through strain hardening at rates suitable for foundation stability. Rapid loading of such soils may result in strain softening and static liquefaction due to generation pore pressure trending to the overlying weight of material. This type of construction is generally more expensive since it involves more material handling than other construction methodologies and requires monitoring.

Descending construction is placement of material from the operating height which allows the dump to develop based on natural material strength. Material is placed from the ultimate height, or crest of the dump. Typically, material is end-dumped from the haul truck at this elevation. Variations in this procedure include the dump-short-and-push method, where trucks dump on the safety of the platform without approaching a potentially unstable crest and the material is then pushed by crawler dozer over the crest.

A common development for descending dump is the construction of wraparound dumps. These dumps include the use of secondary benches and are another means of descending construction, where rock is placed on the slope of an existing dump at a lower elevation than the dump platform. This method is only possible by achieving access to the specified elevation, and is often carried out as mining progresses to lower elevation. The benefit of wraparound berms is a general increase in stability for the overall dump. Placement of material in a wraparound berm is analogous to placing a toe berm, effectively flattening the overall slope, increasing shear resistance of the mass, and lessening the shear stresses induced in the foundation materials.