Slope Stability

In remote areas the stability of the slopes in borrow pits is not necessarily a big issue. The volume of sand deposits that can be borrowed in near-shore areas, lakes, rivers and other in-land waterways are, however, is often limited by minimum allowable slope angles. These minimum slope angles are not only a function of the properties of the soil mass, but they may also be dictated by environmental issues and/or the safety of adjacent structures (revetments, buildings, infrastructure like roads, pipelines, etc.). Slope stability analyses may be divided into  

Analysis of the long term stability may require an assessment of loads that could cause failure in future. Such loads include both static loads like bearing loads of all types of structures and dynamic loads as a result of ship movements, earthquakes, etc. Special trimming of the slopes may be required to reduce the slope angles after dredging in order to ensure sufficient safety against failure in the long term. 

The analysis of flow slides is more complicated. Both the liquefaction and breaching phenomenon are discussed in section 8.6. 

Determination of the shear strength of cohesionless material may require in-situ tests like the Standard Penetration Test or the Cone Penetration Test in 

If the potential fill material has a cohesive nature, in-situ vane shear tests may be performed to determine the undrained shear strength (see section 3.4.2.1 and Appendix B.2.3.2). Alternatively, Cone Penetration Tests could be undertaken to indirectly assess the undrained shear strength using existing correlations (see Appendix C). Undisturbed samples of the cohesive strata could be recovered from boreholes which are subsequently tested in the laboratory. For more details on slope stability analysis reference is made to section 8.4.3.7 of this Manual. 

Submarine slides involve a series of idealized steps when they occur. These steps are as follows  

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Weakening of Foundations, Diminishing Slope Stability, Erosion Landslides from Increased Rainfall. 

An example of the effects of waste settlement can be illustrated by a recent incident at a hazardous waste landfill facility in California.5 At this facility, waste settlement led to sliding of the waste, causing the standpipes (used to monitor secondary leachate collection sumps) to move 60-90 ft downslope in 1 day. Because there was a very low coefficient of friction between the primary liner and the geonet, the waste (which was deposited in a canyon) slid down the canyon. There was also a failure zone between the secondary liner and the clay. A two-dimensional slope stability analysis at the site indicated a factor of safety (FS) greater than 1. A three-dimensional slope stability analysis, however, showed that the safety factor had dropped below one. Three-dimensional slope stability analyses should therefore be considered with canyon and trench landfills. 

Produces a synthetic soil (stabilized waste with soil-like qualities) that has greater erosion resistance than natural soil, enhanced slope stability, and has a greater affinity for soluble heavy metals. 

Sifting may occur while a pile is being formed. Fine particles are concentrated in the center under the fill point. However, as the pile is formed, the slope stability is such that layers of finite thickness intermittendy move from the central fill point, carrying some of the finer particles with them. 

Slope Stabilization. A rigid lining of sulfur composite was applied to the face of an unstable hillside (Figure 6). The hill had been cut to allow construction of several storage tanks. Exposed rock strata channeled rainwater to clay/shale sections below. These became slide-prone when saturated with water. Prior to installing the lining, several unsuccessful attempts were made to recontour the slope to improve the runoff patterns. However, just before the lining was installed, the first rains of the wet season caused another small slide on the face of the slope. 

Figure 6. Slope stabilization with a sulfur composite lining. (top left) Slope became unstable when saturated from heavy rains. (top right) Spray application of sulfur composite to the rim of the slope. (bottom left) The slope after the lining was installed.

Slope stabilization is just one potential application for composites in the broad area of erosion control. Several other related applications are bridge abutments, freeway cuts, storage tank embankments, mine tailings, sand dunes, and areas of turbulent water flow. 

Wieisum, K. F. 1984. Surface erosion under various tropical agroforestry systems. In Proceedings of a Symposium on Effects of Forest Land Use on Erosion and Slope Stability, eds. C. L. O Loughlin, and A. J. Pearce (lUFRO, Vienna, and East-West Centre, Honolulu, Hawaii), pp. 231-239. 

However, if sand becomes saturated with water (that is, its pores become completely water-filled as they are in quick sand), then the sand will flow in a process known as lateral spreading. Water-saturated sand flows because the weight of the sand is supported (at least temporarily) by the water, and so the grains are not continuously in contact. Apparently then, the slope of a pile of sand is dependent on water content, and either too little or too much water lowers the stable slope. This illustrates how slope stability is a function of water content. 

Plans for disposal of mine waste, including methods for controlling dust generation, slope stability, and seepage of contaminated water... 

Control slope stability and landslides. Of course, this is also necessary for successful mining operation, but inadequate control of slope stability may also lead to environmental damage. Slope failure may block or contaminate streams, damage wildlife habitat, and cause flooding that endangers wildlife and humans living near the mine. 

At regular, frequent intervals, monitor and control slope stability, water quality (pH, chemical content, and particulate content), and vegetation growth. 

Figure 8.2 Slope stabilization with plantings. (Courtesy of Bobbin B. Sotir Associates, Marietta, GA.)...

Plantings of various kinds have proven to be cost-effective alternatives to remedy slope stability and erosion problems. A minimum of simple and readily available equipment is needed for the installation of plantings. Trained personnel, however, are required in order to reap the maximum benefit. 

Benching. In light of previously discussed soil stability problems adjacent to the Lagoon, several precautionary measures were implemented in conjunction with closure activities to ensure slope stability. These measures included ... 

Rubber tires filler, paviors, mats (skid resistant), athletic tracks, dock systems, playground cover, slope stabilization, road fill, rubberized asphalt and concrete. 

Robinson, K. S. and T. B. Jones, Slope Stability of Electropacked Beds, Conference Record, IEEE Industry Applications Society Annual Meeting, Oct. 5-9, 1981. 1036-1042... 

Physical properties provide a lithological and geotechnical description of the sediment. Questions concerning the composition of a depositional regime, slope stability or nature of seismic reflectors are of particular interest within this context. Parameters like P- and S-wave velocity and attenuation, elastic moduli, wet bulk density and porosity contribute to their solution. 

Haq, B.U., 1998. Natural gas hydrates searching for the long-term climatic and slope-stability records, in Henriet, J.R, and Mienert, J., eds.. Gas Hydrates Relevance to World Margin Stability and Climate Change, Volume 137 Special Publications London, Geological Society, pp. 303-318. 

Hybrid model of Discrete Fracture Network (DFN) and Discrete Element Method (DEM) for coupled stress-flow analysis of rock slope stability (Wang, 2000) ... 

Wang H. 2000. Hybrid model of discrete fracture network and discrete element method for rock slope stability analysis. J. of Hydrogeology Engineering Geology, Vol. 2, pp. 30-34. 

Keywords, slope stability radar. Slope surface monitoring... 

Chen, Z. Recent developments in slope stability analysis. ISRM 8th Int. Cong. Rock Mech., 1995 Tokyo. 

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