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Stockpile Stability Evahia[ion -3- Perniit N . M-1984-00/ <br />Surface Rock Pi[ tober 6, 1000 <br />relative density. Other testing may include Atterburg limits (unless it is obvious from fie d observation that <br />the material is non-plastic) and sheaz strength testing. Laboratory testing of shear strengt may not be <br />necessary if the stockpile material is classified under the Unified Soil Classification Syst m and <br />conservative values for shear strength are selected from the literature for input to the anal sis. Among the <br />most important field observations to be recorded are careful measurement of the stockpil slope angles and <br />heights and the foundation slope angles. <br />In geotechnical engineering, the shear strength of soils is the major concern because, for ost problems in <br />foundations and earthwork engineering, failure results from excessive applied shear styes es. Unfortunately, <br />soil strength is usually the parameter that is most difficult to evaluate, and it usually invo es considerable <br />uncertainty. Based on what is known about the stockpile, it is a fairly safe assumption th t it is a <br />cohesionless fill built on firm soil or rock. The following discussion of shear strength is redicated on the <br />field evaluations confirming this assumption. <br />The stability of fill slopes built of cohesionless gravel, sand, and silt depends on (a) the gle of internal <br />friction of the fill material, ~', (b) the slope angle, and (c) the pore pressures within the fi 1 and fill <br />foundation. Values for ~' for stability analyses can be determined by drained triaxial or irect shear tests, <br />or by correlations with grain size distribution, relative density, and particle shape combin d with <br />consultation of geotechnical literature. For a given soil the value of ~' increases as the r alive density <br />increases. For different soils at the same relative density, the value of ~' is affected by p icle size <br />distribution and particle shape. The value of ~' for swell-graded soil may be several de ees greater than <br />that for a uniform soil of the same average particle size and shape. The same is true whe a soil composed <br />of angular grains is compazed with one made up of rounded grains. Pore pressure due to eepage through <br />the fill reduces the stability of the slopes, and the potential for seepage to develop under s vere climatic <br />conditions must be considered even if there is no evidence of seepage at the present time. <br />Depending on the type of slope, and the amount of time and effort that can be appropriat y be devoted to <br />site investigation and analysis, a number of different procedures may be used for investig lion and design <br />Three frequently used procedures, which represent increasing levels of complexity and c t aze the <br />following. <br />1. Use of field observations and experience alone, with no test borings, laboratory tests, of slope stability <br />calculations. <br />2. Use of slope stability calculations in combination with field observations and a minimu~tt number of test <br />borings and laboratory tests. <br />3. Use of detailed slope stability calculations, in combination with a thorough program of site investigation <br />and laboratory tests. <br />Based on what is currently known about the stockpile, the Division envisions a procedure initially-that <br />would fall between items one and two listed above. Afield evaluation of stockpile and f dation slopes, <br />the potential for seepage and erosion, and the grain size, consistency, and density of the s pes and <br />foundation may be used to estimate shear strength for input to stability analyses. Howev ,when estimated <br />