The Northeast Tripp Slide - A 11.7 Million Cubic Meter Wedge Failure at Kennecott's Nevada Mine Division

The Northeast Tripp Slide is one of the larger slope failures that can be attributed to open pit mining. It is a 11.7 million cubic meter (15.3 x l0 6 yd3) wedge failure created by two thick gouge-filled fault zones whose intersection plunges into the Tripp pit at 13°. One unique aspect of this failure is that the slide is now stable despite having moved an estimated 8.5m (28 ft.) towards the pit and having reached a maximum overall velocity of 2lmmlday (0.07 ft/day). The slide started to fail in 1968, accelerated until mining in the slide's toe area was halted in April 1970, then began to decelerate. With the cessation of mining in the toe area, rubblized rock sloughed into the pit bottom which added weight to the failures toe constraints and began the stabilization process. Measurements from a network of survey stations revealed a correlation between the slide's velocity and heavy precipitation. Runoff water accumulating in the tension cracks is believed responsible for this correlation. Moreover, the survey net measurements showed that during the failure's last stage the vertical component of the slide's movement greatly increased. It is proposed that this settling was caused by plastic-like flowage of the clay gouge underlying the failure The fault zone's effective shear strength was estimated by a back- analysis on the wedge's geometry and by performing direct shear tests. The tests, performed on cores taken from samples that were collected on the failure, yielded a residual shear strength of: cohesion 0.61 kg/cm2 (8.7 lb/in2), and a friction angle of 9.6°. This residual shear strength was near1y equal to the back-calculated strength indicating that the controlling fault's effective strength was essentially the same as the residual shear strength of the gouge filling