Memorandum - Western Federal Lands Highway Division - U.S. ...

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observed underlying the conglomerate. Seepage was observed in this area just above the contact of the two layers. See Figure 6 below. Figure 6. Photos of conglomerate (left) and sand (right) layers. LANDSLIDES A fairly significant landslide was observed on the opposite side of the drainage from the road. See location of landslide in Figure 1 (attached) and photo of landslide in Figure 7. The landslide encompassed an area of about one acre. It appears from the age of the trees on top of the slide mass that it occurred within the last 20 years. The slide is not impacting the roadway; however, if continued movement dams the drainage, it could cause significant flooding and erosion or a larger failure that could have significant impacts to the existing road. Page 6 of 10
Figure 7. <strong>Lands</strong>lide Observed Across Drainage from Refuge Access Road. Several smaller slumps were observed in the native slopes throughout the drainage. These slumps are shown as shallow bowl shaped depressions in the lidar imagery of Figure 1 (attached). The shallow slumping does not appear to be a significant problem other than having the potential to deposit debris on the road during heavy precipitation events. The shallow slumps and also the larger landslide previously mentioned are commonly observed throughout this geologic formation mapped as conglomerate by Evarts (2004). As previously stated, this formation is often included as part of the Troutdale Formation. The formation consists of very dense and slightly cemented gravel and cobbles so it will stand at vertical slopes as observed along this road. However, when the material becomes saturated it loses significant strength often resulting in landslides ranging from shallow slumps to massive ground movements; both observed with the drainage that the Refuge access road follows. Slopes in the conglomerate are sometimes cut vertical to minimize the amount of precipitation that directly impacts the slope. Once the slope deviates from vertical it must be laid back to an angle that will be stable considering the amount of precipitation that it will be exposed to. Generally these materials would be stable at slopes of 1V:1.5H or flatter. The vertical cuts in the conglomerate material were 30 to 40 feet high along the Refuge access road. The slope had about 10 feet (measured vertically) of accumulated sloughed material at the toe. The angle of repose of the sloughed material was between 35 degrees (1V:1.5H) to 40 degrees (1V:1.2H). DETENTION BASIN A detention basin was observed on private land near the intersection with Hillhurst Road. See aerial photo in Figure 8 showing its location. The detention basin is situated on the level Page 7 of 10
Page 1 and 2: Memorandum Western Federal Lands Hi
Page 3 and 4: thick. The conglomerate layer is of
Page 5: Site Observations Brian Collins, WF
Page 9 and 10: PILE Figure 9. Picture of East Abut

Memorandum - Western Federal Lands Highway Division - U.S. ...

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