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r~ <br />f~ <br />1991 <br />At the owners req~k the annul safety inspection was conducted an May 30°i. Early inspections provide <br />ample opport®ity to store additional water above the restriction should conditions be favorable. Results of Ote <br />past three yeas inspections and monitoring appeared to i~icate the IeveI of saturation and potential movement <br />of the huuislide comprising the left abutment was more athnbutabL: to mow mek water than leakage from the <br />reservoir. However, lack of subsmiace information and iostmmenis to monitor ground water levels and <br />progressive movements prevented confident aeeneemeltt of the long-teem safety of the dam. A temporary <br />revision to fire restriction was allowed, with the condition that a fomtai plan would be fonumlaled to investigate <br />the Lmdslide and install instiiurients to mgnitor sfide movement The reservoir ffied within 1 foot of spi'Il <br />without sign of structural weakness, ar>musaal perfo**~~ based on monitoring results. <br />1992 <br />The a~oal safety inspection, conducted on May 7m, revealed no visible evidence of movement and the two <br />embankment piezometes showed no annual signs of behavior. The reservofr waR allowed temporary stnsge <br />above the restriction. <br />A proposal for investigating the landslide was submitted for review and approved by the State Engineer- The <br />plan included cuing and logging of three holes for instaIlaton of inclinometer casing, installing pieaon>eters in <br />the landslide, and establishing two lines of survey monuments. The objectives of the plan were to identify <br />subsadace geologic features which could affect stability of the embankmen; monitor slide movement as it <br />related to operation of the reservos and drtem>iv if continued slide movement jeopardized the integrity of tbe <br />dam. The inslmmeNation plan is included on Figure 1. <br />Subsurface investigation and instrumentation of the landslide was completed in the Fall, a$er the iaigation <br />• season. Stiatigraphy was typical of sfide debris, consisting of sandstone cobbles and boulder in a fat clay <br />matrix. The sfide debris originated from the svaounding marine formations of sandstone and chtystone. The <br />investigation identified chazacteristics of at least two historic slide eveNs eprmxicing the IeR atmm~rnr <br />The boring of INGI, on a bench at the left downstream toe, identified a five foot thick L1yer, characteristic of <br />lake bed sediments. The lacashine deposit separated the two slide rcasses at a depth of 70 feet below the <br />elevation of are dam crest Topographic fcatnres 0.25 miles downstream of the dam site depict a landslide <br />feature sufficiently Loge enough to have blocked the stream at oce t®e, possibly forming a lake where <br />sediment would accumniatt. SIlckensided sadness were preser>f m the hicostrine deposit and in a zone <br />exteadmg five feet above t>ffi strata. A second shear zone, 30 foot thicl5 was identidied 45 feet below.the Lds <br />deposits, brrt revealed signs more indicative of mudflow type movement The lower slide mass would most <br />likely be canshrained by the opposing canyon wall based an the depth The upper sfide mass became the focus <br />for most probable movements which could affect &e integrity of the dam. Ahhough less conclusive, existence <br />of a third, shallower sfide sequence may have ocramed that forms the ridge along the fall line of the hiIl above <br />the dam and its terminus comprises the upper left portion of the abamient As typically observed in hnxlsfides, <br />boundaries exist that typically impede generalized ground water flow or disrupt groundwater flow regimes. <br />Pore pressm>; changes in the shear zone could be attributable to multiple pemzeable networks, hydraulically <br />comrected to independent sources. Therefore, piezometes were targeted io measure pore pressures within the <br />apps shear zone. FoIlowing installation, initial profiles of the throe inclinometer were obtained. <br />1993 <br />Ti®g abe importance of the imshumentation program was not fuIly realized rmnl Spring. With above normal <br />snowpack, runoff flows quickly exceeded ouflet capacity. The reservos ffied to OS feet above spffi during <br />pesknmoff flow. The spillway, located just beyond the left abutment of the dam, traveses over the slide mass. <br />With the reservoir spIlling over the sfide mass, all possible sources of water affecting slide movement were <br /> <br />