Three drained slope failures bordering I-37 in San Antonio, Texas, occurred during the Fall and early Winter of 1986. One failure resulted from adverse natural seepage conditions, where as the other two failures resulted from a broken water main and a disrupted surface drainage system. The latter two failures were stabilised by eliminating the water entering the slopes. Several procedures were considered for stabilisation of the first failure. Use of horizontal drains was judged to be the most effective in raising the long-term factor of safety for the slope. The design of horizontal drainage systems is reviewed and sample calculations are provided to illustrate that many combinations of drain length and spacing can result in approximately equal gains in factor of safety with time and that longer drains that can achieve higher long-term factors of safety can be only marginally more expensive than systems with shorter drains. A second finding is that there is a limit on the benefit of increasing drain length to increase long-term factors of safety.
Abstract