REV96671 - Laserfiche WebLink

Home Browse Search

the analysis. The addition of the load of the new fill likely caused settlement of the entire embankment. This is evidenced by the increase in pipe movement during the period substantial fill was placed. Addition of the new fill, particularly fine-grained fill with substantial clay likely caused ground water level rise in both the new and old embankment, compared to original ground water levels. This is evidenced by the increase in the marshy area upstream and flows observed from downstream springs. The old embankment was apparently stable at the observed slope of about 1.3H:1 V for many years. No problems with embankment stability or the penstock were reported by PSCO for years prior to Pathfinder's initiation of grading activities. The new fill is less stable than the old, gravel fill. This is evidenced by recent mudflow and slump features observed. Truck traffic has reportedly increased substantially in the last few months compared with historic uses of the road. These considerations allowed us to make a pertormance-based evaluation of relative stability for judging strength of the various embankment materials. The strength of the original fill was judged based on the uphill slope and the granular nature of the soils in the fill. The soils in the downhill (new embankment) appear to contain considerably more clay and silt sizes and the strength was modeled accordingly. We analyzed the existing embankment for two ground water conditions, i.e., the "dry" condition, which places the ground water at the estimated filllnatural soil interface, and a "wet" condition, with the ground water level up into the fill. The dry condition is meant to model conditions over the portion of the year when the ground is either frozen or receives relatively little rainfall with high average temperatures. The wet condition models conditions of increased ground water as might be expected during spring snowmelt and late summer-fall rainy season. Our stability analyzes were performed using the computer program SLOPENV, by Geo-Slope International, Ltd., version 4.20. The attached cross sections are taken directly from SLOPENV output (Figs. 11-16). The configuration of the existing embankment (old and new) is shown on Fig. 11, where we postulate a higher ground water level, as would be expected if the relatively finer soils in the new embankment resulted in decreased drainage capacity. Field observations support modeling this condition as marginally stable. We obtained a relative FS of 1.1 for this condition. An FS of 1 means that the forces resisting failure just PUBLIC SERVICE COMPANY OF COLORADO AMES HYDRO PENSTOCK CTL? 31,&L8 5