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Nonreproducible relative density test results for an hydraulic fill of clean river sands, suggested errors either in assumed control densities or methods or techniques of field testing. Control densities had been assumed to vary only slightly because sample gradations and sizes appeared quite similar. To verify this assumption, separate maximum and minimum density (ASTM Test for Relative Density of Cohesionless Soils, D 2049-69) and gradational parameters were determined for each field density test. No dependable correlation of maximum or minimum densities or range between them with gradational characteristics was apparent for results reported.
Doctoral Thesis / Dissertation from the year 2010 in the subject Engineering - Geotechnology, , course: Civil Engineering, language: English, abstract: With growing infrastructure developments in hilly areas and due to economical constraints of using locally available rockfill materials for construction of embankments, practicing engineers must be acquainted with geotechnical response of non-conventional granular soils. These materials are most likely to disintegrate with time due to physical and chemical weathering. In general, the laboratory investigations on durability characteristics of such materials are only made through simple slaking tests. However, studies examining the effects of slaking-induced disintegration of soil grains on the geotechnical engineering analysis and design parameters are rather limited. This is essentially due to the reason that the grains of standard laboratory sands are mostly durable and hence, the stress-strain response is considered to be unaffected by the presence of water. In order to explore the possible effects of deterioration of soil grains on static and dynamic properties of granular soils, a series of consolidated drained torsional shear tests on various crushed soft rocks were performed under saturated and dry conditions and compared with a well reported Toyoura silica sand consisting of durable grains. Due to the sensitivity of crushed rockfill to deteriorate upon water-submergence, test under dry conditions represented the response of a soil with intact grains, whereas a similar test under saturated condition simulated the potential reduction in strength and stiffness of the soil with time. From the grain size distributions determined after each test, a degradation index was defined to quantify the degree of disintegration of grains. Strength and deformation properties determined from monotonic as well as cyclic shear tests were then compared with this index. Possible correlations of water-induced deterioration of soil grains with consolidation behaviour, peak shear strengths, friction angles, dynamic shear stiffness, and volume-change characteristics during shearing were explored. In addition, the effects of confining stress and shear strain level on particle breakage were also investigated. It was concluded that time-dependent characterization of rockfill materials by monitoring the degree of deterioration can be helpful to avoid catastrophic geotechnical failures. Nonetheless, this study is a caution to conventional soil mechanics in which decay of grains and loss of soil strength with time are often uncared.
The use of relative density correlations based on an "average" sand to predict soil behavior without considering the particle shape can result in poor or misleading predictions. Experimental data show that the particle shape has a pronounced effect on all engineering properties studied. Angularity of the particles increases the maximum void ratio, strength, and deformability of cohesionless soils. Variations in engineering properties due to particle shape can be as large as variations associated with large differences in relative density. Penetration tests in small containers with small rods suggest that the Standard Penetration Test is influenced by both the angularity and density of cohesionless soils.
Several physical methods are described for the practical measurement and rating of angularity (shape) of cohesionless soil particles. Angularity is determined by utilizing the fundamental property of a sphere: a sphere has the smallest contact surface area of any shape for a given volume. Therefore, any other shape will exhibit a greater contact surface area and consequently will have a greater frictional resistance which is a function of its degree of angularity. The effects of angularity on the physical behavior (e.g. strength) of cohesionless soils was investigated at various relative compaction densities. For this purpose a combined compaction and direct shear test device constructed from a modified standard Proctor compaction mold was devised. The samples used to determine the effect of particle shape on the physical behavior of cohesionless materials were produced in the lab from pure quartz. This was done in order to avoid the problem of variations due to mineral composition and grain size distributions. It was hoped that this would insure a greater uniformity of test results. In addition, the shear test results derived from lab-produced quartz samples were compared to those of natural field samples in order to determine whether the behavior observed during lab tests was representative of natural field soils. These experiments demonstrated that the strength of a cohesionless material increases with degree of angularity and relative density to an optimum point. Surpassing the optimum value implies substantial particle crushing which reduces the particle interlocking effect and can result in a reduction of soil strength. Crushing is greatest when cohesionless particles are poorly graded, highly angular, and large in size. Generally, the degree of particle crushing influences strength, and particle shape determines the degree of crushing. Shape (angularity), therefore, significantly controls the overall strength of a cohesionless soil.