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<br />0033.28 <br /> <br />( <br />'. <br /> <br />3. As much as 40 ft of earthflow material consisting of unconsolidated <br />"soil" derived from bedrock units is involved in surficial slope <br />failures that probably present no serious threat other than to the t' <br />improved roads and irrigation ditches that cross the area, These <br />earthflows account for most of the recent and obvious landslide <br />morphology in the area, <br /> <br />4. Although monitoring of the most critical high runoff season has not <br />been done as yet, water levels in drill holes probably will vary <br />considerably seasonally and may correlate with increased rate(s) of <br />landslide movement, Drill hole 4 had artesian flow to the surface <br />to mid summer suggesting that in this part of the landslide near the <br />headscarp, water pressures at or near the basal landslide surface <br />are relatively high. It should also be noted that no irrigation <br />ditches were being actively used in this area at this time. <br /> <br />S, As indicated earlier there was concern from the outset of the <br />project about the concentrations of surface water on the old slide <br />mass, There were observable concentrations of water in natural <br />drainages, ponds and bogs, and in irrigation ditches., It was <br />suspected that this water could contribute to both localized and <br />major lands1iding on Meadow Mountain, Water flows were high in <br />several irrigation ditches, across the upper part of the area, as <br />well as in the natural drainages during snowmelt season. With the <br />exception of the ephemeral pond immediately upslope of the <br />"intennediate" landslide it has been difficult as yet to establish a <br />direct causal relationship between the surface water and specific <br />areas of instability. This is probably because the percolation of <br />massive amounts of water into the old slide mass is a very general <br />phenomenon and the actual instability it produces may occur a <br />considerable distance downslope where other conditions also promote <br />instability. A more definitive finding on this would require <br />additional exploration of slide plane geometry and continuing <br />monitoring of movement over 2 or 3 more full seasons. However, the <br />presence of artesian pressure in groundwater at borehole 4 even into <br />mid summer demonstrates that water pressure is high at least locally <br />in the slide mass, and based on geotechnical principles the observed <br />rapid percolation must be considered a serious contributory factor <br />to instability, <br /> <br />i <br />'. <br /> <br />- 6 - <br />