. Mass soil movements in the Andrews Experimental Forest. Earth movements Oregon; Soil mechanics Oregon. 3. Road backslope slump causing fill failure. Six events in the Andrews fit within this type (fig. 4). In effect, this is a "chain reaction" with the general sequence as follows: A portion of the backslope becomes saturated and slumps down onto the road surface, thus blocking the in- side drainage ditch. The drainage water is then diverted across the road surface and down onto the fill slope from the outside portion of the road. The fill slope becomes saturated with water and
. Mass soil movements in the Andrews Experimental Forest. Earth movements Oregon; Soil mechanics Oregon. 3. Road backslope slump causing fill failure. Six events in the Andrews fit within this type (fig. 4). In effect, this is a "chain reaction" with the general sequence as follows: A portion of the backslope becomes saturated and slumps down onto the road surface, thus blocking the in- side drainage ditch. The drainage water is then diverted across the road surface and down onto the fill slope from the outside portion of the road. The fill slope becomes saturated with water and an earth- flow type of movement follows, en- croaching to a varying extent onto the road surface. In some cases, the "chain" is further lengthened by the fact that the earthflow on the fill slope may trigger channel scour- ing in a drainage below. 4. Mass movement caused by concen- tration of road drainage water. There are two classes of events under this heading: (a) those due to the concentration of road drainage water, even though the road drain- age system was apparently function- ing properly (fig. 5), and (b) those caused by failure of some portion- of the drainage system--for exam- ple, a clogged culvert or inside ditch. Events caused by concentration of road drainage water. - -Four events in the Andrews fall within this category. In all cases, the event was associated with stream channel scouring. The typical se- quence was as follows: Extremely large quantities of water flowing from a culvert completely saturated the soil mantle below the road. In most cases, this saturated area was. Figure 4. —View of typical backslope slump which has also caused roadfill failure. near the head of a tributary drain- age. When the soil became satu- rated, the entire mantle either slumped or flowed downslope and temporarily blocked the drainage channel. When a sufficient head of water was built up, this temporary dam breached, and a wall of water and debris rushed down the
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