An unstable slope in an open-pit copper mine in Arizona was analyzed after a 250,000-ton slope failure had occurred. Data on all available time-dependent hydrogeologic factors were analyzed utilizing multiple regression techniques in order to build a mathematical model of the slide displacement. This allowed determination of those factors which were most influential in promoting instability. It was determined that a ground-water rise of approximately one foot, and rainfall in the week prior to the failure, were the most important factors contributing to this particular slide. This is believed to be due primarily to an increase in cleft-water, or hydrostatic, pressure, with seepage forces contributing a significant but relatively constant shear component. Because instability was observed to extend beyond the original slope failure, a dewatering analysis was undertaken. A steady-state finite element flow system model series was developed by progressively incorporating subsurface information, as well as water table location. The ground-water flow system within the granitic host rock was then simulated as a continuum, and a flow net derived. Analysis of this flow net, combined with known ground-water discharge into the open pit, comprised the basis to estimate the coefficient of permeability to be approximately 4 gallons/day/foot². This value was then utilized in a preliminary dewatering analysis incorporating standard aquifer equations to predict possible drawdowns. The computed drawdowns suggest that pumping rates on the order of 10 gallons/minute/well from a line of 12 wells spaced 50 feet apart would produce approximately 100 feet of drawdown after 1 year of pumping. Results of stability analyses for various ground-water levels indicated that when the water table is lowered by dewatering, a steeper slope could be maintained in the open pit. Such steepening could be as much as 1° when the water table is lowered one-half to one-quarter the height of the slope (135 feet), and approximately 4½° when the slope area is completely drained.

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Cover title: Draft technical appendix. / Phoenix Urban Study: Final Report / The work upon which this publication is based was supported by funds provided by the U.S. Army Corps of Engineers, Los Angeles District, as authorized by resolution of the Senate Public Works Committee on July 31, 1973. This report was prepared as a contribution to the Phoenix Urban Study under auspices of the U.S. Army Engineer District, Los Angeles.

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6 pp. / Well Owners' Guide to Ground Water Resources in Yavapai County. AZ1451 / Private well owners are responsible for the safety and quality of their water supply. The nearly 6,000 exempt wells in Gila County are not regulated by any state or federal agency, and lot splits and subdivisions may result in an unregulated water supply serving several homes. This publication is one in a series of county-based publications that identifies well maintenance and testing procedures for the well owner, with an emphasis on water quality concerns for Gila County.

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