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the canyon walls as well as the fact that the width of the new embankment at its toe <br />is less than one-half the width of the embankment at its crest ensure that no major earth <br />movement could take place. <br />It is possible that heavy rains falling on the outer slopes of the embankment could <br />cause minor gulleying or small surficial movements of material, particularly during the <br />period before vegetation has been established on the exposed slopes. To minimize and <br />control the extent of this type of damage, 30-foot wide terraces will be provided at <br />100-foot elevation intervals on the outer slope of the embankment. Small contour fur- <br />rows will be constructed across the face and the area mulched to decrease runoff. The <br />terraces will have a slight slope inward toward the fill in order to contain surface runoff <br />from the slope above, and be canted to the north to convey any impounded runoff to the <br />new drainage channel along the north abutment. Moreover, they will provide access to <br />repair any erosional or vegetation damage, should it occur. <br />Over a period of time, minor differential settlements in localized areas may <br />occur as a result of gradual consolidation of the embankment ond~or subsidence of the <br />underlying foundations, but such occurrences should not have any significant effect on <br />the overall mass stability of the embankment. It can be expected that for several years <br />following completion of the embankment, it may be necessary to carry out occasional <br />maintenance work to eliminate ponding of surface runoff and to regrode the terrace areas <br />to ensure efficient flow of water to the drainage channel. The amount of such work <br />should diminish with the passage of time; and when vegetation has been completely es- <br />tablished on the new slopes, the need for further maintenance should be eliminated. <br />Commentary on OSM Interim Regulations <br />The spoil placement operations contemplated under the interim Regulations of <br />the Office of Surface Mining would, in our opinion, be appropriate under particular <br />conditions but not in others. The requirement for placement of spoil materials in lifts <br />not greater than four feet with adequate compaction would appropriately apply in cases <br />where: <br />8 <br />