Strength Anisotropy and Time Effects of Two Sensitive Clays: Discussion

Abstract

strength, Cu, uscd for both sets of curves is gcnstern's charts is normally less than 50% of the pcak strength (or undisturbcd field vane thc short term (undrained) factor of safety strength). In the practical design range, taking calculated from Taylor's chart. Short term accoui2t of the strain softening behavior reduces stability calculations are often used if an excathe calculated factor of safety by 25-30%. vation is open for periods up to 3 months. The During an excavation on Slater Street in Ottawa reduction in factor of safety over this time (sce McRostie et a[. 1972) about 30 ft (9 m) period is normally associated with the dissipaof Champlain Sca clay was excavated to rock in tion of pore water suctions in the soil and is onc lift with an average angle of about 70. The related to it? .\itu permeability, a factor not undraincd strcngth of the clay is 0.63 kg/cmonsidered in calculations prcsented on the and thc calculated factors of safety are: from author's Fig. 18. Taylor's chart, F.S. = 2.0; from the modified curves, F.S. = 1.5. Sloughing was noted to occur 011 this slope when the excavation was EDEN, W. J. , and MITCHELL, R. J. 1970. The mechanics discontinued for 3 days indicating that the of landslides in Leda clay. Can. Geotech. J. 7(3), pp. lowcr factor of safety inight be more appropri285-296. JARRETT, P. M.. and EDEN, W. J. 1970. Groundwater atc. Whilc there have been occasional failures flow in eastern Ottawa, Can, Geotech, J. 7(3), pp. of tcmporary slopes in Champlain Sea clays, 326-332. thc writers have 'no documenktion for corieLAWRENCE, V. M. 1971. Slope stability in naturally lative pLIrposes. -l-his discussion of short term cemented soils. Ph.D. thesis. ~ u e e n ' s ~niversit;, Kingston, Ont. stability, however, rclates to the author's Fig. MCROSTIE, G. c., BURN, K. N., and MITCHELL, R. J. 18. On this figure the time scale begins at day 1972. The performance of tied back sheet piling in onc and a small decreasc i n factor of safetv is clay. Can. Geotech. J . 9(2), pp. 206-218. after 3 months, F~~ paranle;ers SKEMPTON, A. W. 1970. ~ i r s t i i h e slides in overconsolidated clays. Geotechnique, 20(3), pp. 320-324. a ~ ~ l i c a b l c to in the Ottawa area WEBB, G. S. 1971. Geotechnical properties of the weaththe long term (fully drained) safcty factor of a ered crust of a Leda clay. M.Sc. thesis. Queen's givcn slope calculated from Bishop and MorUniversity, Kingston, Ont.