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06/25/97 WED 10:12 FAX 503 684 0954 C,tRLSON TESTING ~001 2arlson Testing, Inc. Constniction Inspection & Related Tests C, eotechnical Consulting P,O. Box 23814 T~gard, Oregon 97281 Phone (50~3) 684-3460 FAX (503) 684-0954 June 20, 1997 CTI #97-G1106 Craig Robe~~ 24085 a~gb':te~flle-l~fiad Aurora, OR 97002---' GEOTECHNICAL REVIEW OF SLOPE FAILURE ROBERTS AND ALTON RESIDENCES BUTTEVILLE, OREGON JU/ 1 & t997 MARION COUNTY BUILDING INSPECTION At your request, our engineer and geologist have visited the site and performed two subsurface borings, one towards the rear of each residence. The purpose of our work is to asses the stability- of the riverbank and provide recommendations for slope stabilization. BACKGROUND ObseneatJons At the Alton residence, reportedly the outer 15 to 20 feet of riverbank west of the dock stairs has slid into the river and was mostly washed away. The scarp is very near the existing deck supports. The upper 10 feet of slope apoears to be uncompected fill. At mid-height in the freshly exposed river bank a layer of dense fine gravel was exposed. Seepage is evidence toward the bottom of this gravel layer. The Roberts Residence has experienced [ateral movement of varying degrees, increasing toward the slope. The bank did not completely fail, but moved several feet at the worst location. Evidence of lateral movement to a lesser degree was observed along the house, such as: tilted deck supports and downdropped deck, patio slab pulling away from house, and settlement cracks in the foundation. Nead3v Site We have previously worked on a similar failure at the nearby Ragan residence located at 24145 Butteville Road. The riverbank failure at that site was caused during the same flooding event in the winter of 1996. At that site. the home was constrHcted over the riverbank and experienced much distress. FIELD EXPLORATIONS The borings were advanced with a Simco 2800 drill rig on June 2, 1997. All borings were backfilled immediately after completion of logging. Standard Penetration Test samples were taken at 5 foot intervals. The N-values from these tests ranged from 2 to 9 in the upper 30 feet and 10 to 18 at 35 feet. Logs of the bodngs, showing changes in soil type, are presented on Figures A-1 and A-2. A geologist with our firm logged the borings with respect to soil type, relative strength, and ground water occurrence. Soil conditions were evaluated, described, and classified in accordance with the classification format described in the Oregon Department of Transportation Soil and Rock Classification Manual (ODOT Highway Division, 1987), which is based on the Unified Soil Classification System.