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Title: Strength and deformation properties of earth-rock mixtures
Authors: Donaghe, Robert T.
Torrey, Victor H.
Keywords: Earth-Rock Mixtures
Q Tests (soils)
R Tests (soils)
Triaxial Shear Tests
Shear strength
Soil mechanics
Rock mechanics
DeGray Dam, Arkansas
Publisher: Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-85-9.
Description: Technical Report
Abstract: The results and analyses of a series of strain-controlled, consolidated-undrained (R) and unconsolidated-undrained (Q) triaxial compression tests performed on various gradations of artificially blended earth-rock mixtures and on a natural earth-rock mixture obtained from borrow area F associated with the construction of DeGray Dam, Caddo River , Ark., are presented. The tests were conducted to determine the validity of two widely used modeling techniques in cases where available triaxial testing equipment sizes are smaller than those dictated by the maximum particle size of the full-scale gradation. The objective was achieved by comparing results of Q and R triaxial tests performed on 15-in.-diam by 38.5-in.-tall specimens of a full-scale gradation with those obtained from 6-in.-diam by 13.6-in.-tall specimens of scalped/replaced and scalped gradations derived from the full-scale gradation. Three artificial full-scale earth-rock gradations (3-in. maximum particle size) were created by blending three percentages (20, 40 , and 60) by weight of washed gravel (GP), three percentages (55, 35, 15) of mortar sand (SP), and a constant percentage (25) of clay (CL). The three full-scale gradations so created represented a range of earth-rock materials used in constructing many earthen dams. Scalping/replacement (to 3/4-in. maximum particle size) and scalping (minus No. 4 fractions) procedures were then applied to the full-scale gradations to assess effects on strength-deformation characteristics. Tests were performed at confining pressures of 4.22 and 14.06 kg/cm^2. Effects due to equipment and specimen size were assessed by also testing scalped/replaced specimens 15 in. in diameter. A separate suite of Q triaxial tests was performed on scalped mater ial compacted to the initial conditions of the minus No. 4 fractions of corresponding scalped/replaced specimens to determine the contribution of gravel fractions to strength. Finally, a separate series of R triaxial tests was performed on the natural earth-rock gradation (3-in. maximum particle size, 48 percent gravel, 26 percent clay) from DeGray Dam as a check on trends observed for the blended materials. The full-scale and scalped/replaced specimens were compacted to 95 percent of their respective standard effort maximum dry densities at water contents obtained by combining minus No. 4 fractions prepared at their respective standard effort optimum water contents plus 1 percentage point with saturated surface-dry gravel. Scalped specimens were tested at 95 percent of their respective maximum dry densities and at water contents corresponding to their optimums plus 1 percentage point. Data presented include stress-strain and pore pressure-strain curves , effective stress paths, strength envelopes based on total and effective stresses, and comparative trend plots. For Q and R tests, it was concluded that neither scalping/replacing nor scalping procedures yield satisfactory estimates of total stress strength parameters for the parent full-scale gradations. R tests on scalped/replaced or scalped gradations may significantly underestimate the strength of the fullscale material, whereas Q tests on the altered gradations may overestimate the true strength. The DeGray Dam material exhibited higher plasticity of the minus No. 40 fraction than that of the blended material (identical PL but PI 10 percentage points higher) and somewhat less difference between full-scale and scalped/replaced strength data. Effect of plasticity of the fines was not a program objective but results suggest additional research to determine if there exists a level of plasticity of fines above which scalped/replaced gradations can be reliable for estimating full-scale gradation strengths. The scalping/replacing procedure is satisfactory for estimating effective stress strength parameters of full-scale materials. Earth-rock mixtures compacted to 95 percent standard effort maximum dry density and near optimum water content can be expected to develop considerable pore pressure during undrained shear which results in total stress angles of internal friction as low as 11 deg. Effects due to differences in sizes of specimens or equipment were not significant. Additional research to establish effects of plasticity of fines on both triaxial shear parameters and compaction test data between full-scale and scalped/replaced gradations is recommended. Research is also needed to eliminate equipment size effects on earth-rock compaction test results which have been documented by Donaghe and Townsend.
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