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HomeMy WebLinkAboutMLA16-00027 Geotechnical Site Review651 Bachelor Road, Sequim, Washington Jefferson County Parcel # 002352019 Geotechnical Site Review Prepared For Terry Parks and Christine Clark July 2015 NORTHWESTERN TERRITORIES, INC. (io T1T SOUTH PEABODY STREET. PORT ANGELES, WA 98362 Engineers 0 Land Surveyors 0 Geologists Constnx,bon 0 Inspection 0 Matenals Testing (360) 452-8491 Emall: In(o(dnti4u.com NTI e' NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 96363 i Engineers Land Surveyors L GeOIOgIStS ConSM on Inspection ,. Matenals Tes6rg Port Angeles (360( 462-6491 E -Mail: inloMnti4u.com NT/ July 15, 2015 Mr. Terry Parks and Christine Clark 651 Bachelor Road Sequim, WA 98382 Subject: Geotechnical Site Review For Jefferson County Parcel iJ 002352019 Related To A Proposed Bedroom Addition To A Single Family Residence Dear Terry and Christine, and Others Concerned, 1.0 Introduction and Background In October of 2014 and in lune of 2015, Northwestern Territories, Inc. (NTI) reviewed the geotechnical conditions at Jefferson County Parcel if 002352019 belonging to Terry Parks. In October of 2014, NTI Engineering studied the general situation at the 651 Bachelor Road house after it was revealed that a small landslide had occurred there prior to the purchase of the property by Mr. Parks. The house is on the waterfront in Gardiner, Washington. A follow-up visit and site study was completed in June of 2015. By lune of 2015, the owners had a plan to build a new bedroom addition over an old carport that was hastily converted to a bedroom by the former owner, apparently without a building permit. Roughly five or six years ago, before the current owners occupied the house, a small landslide of the fill materials occurred in the area south of the house. Thereafter, a buttress revetment of large basalt rocks was built to arrest further sliding and the fill soil was replaced. The June 2015 review was aimed at assessing the slide buttress and the proposed new bedroom addition from the geotechnical and geologic hazard point of view. Our review, summarized herein, indicates that the remedial works constructed after the slide were built reasonably well and that they remain in good condition. In our opinion, the proposed bedroom addition is a prudent action with little or no residual geologic hazard if the related recommendations outlined below are followed during the construction. 2.0 Site Conditions and Relevant Site History At one time the site of the existing house was separated from Bachelor Road by a ravine whose average depth was perhaps 8 feet and width about 30 feet. Probably at the time of the original Geotechnical Site Review 12015 house construction, the ravine was filled with earth and a driveway was built over the ravine to allow an approach to the carport which was originally south of the house. Very little water now flows through the ravine in modern times and our observations suggest that the ravine is an "ancestral" drainage way that was carved out at the close of the Pleistocene (Ice Age) when there was an abundance of glacial meltwater in the area. The former carport south of the house was hastily re -constructed into a bedroom without deeper footings ora prescriptive stem -wall foundation. Instead, the bedroom was built over the carport's original concrete slab -on -grade. Site evidence and testimony of neighbors indicates that about six years ago there was a slope - failure within the filled ravine that affected the southerly portions of the house. It was reported by a neighbor -observer that the slide undermined the southeasterly corner of the carport conversion leaving two or three feet of the concrete floor slab cantilevered over the I slide scarp. Cracks in the concrete floor of the carport and a slight west to east fall of that floor probably arose from adjustments of the soil that occurred during or shortly after the landslide. A shallow concrete underpinning was constructed to reduce the motion of the carport floor sometime after the slide. The current owner, Mr. Parks, now intends to replace the inadequate bedroom with a fully -compliant Building Code structure within the same footprint. It is also our understanding that Mr. Parks is considering the possibility of expanding the footprint of the addition to the west. After the slide occurred, a heavy rock revetment/retaining structure was built below the slide area and a culvert was installed through an earth fill that was placed in the slide scarp. The photograph below shows the position of the carport on the south side of the house that was enclosed to form a hasty bedroom. The culvert beneath the driveway and approach to the house lies about eighteen (18) feet from the southerly limit of the proposed addition at a depth of about 8 feet below the surface. The approximate position of the culvert is shown in the attached site plan prepared by Creative Design Solutions. The bedroom lies about 21 feet from the rim of the retaining wall on the east. (To the right in the photograph below.) Geotechnical Site Review 12015 Photograph 1— View of Subject House and Carport/Bedroom Conversion Looking North 3.0 Geotechnical Observations At The Site Related To The Proposed Bedroom Addition As Photograph 1 above helps to illustrate, the principal geologic hazard at this site is the possible resumption of sliding of the soil placed in the slide zone. The southeastern corner of the proposed bedroom addition is just over 20 feet from the top of the retaining wall, as seen in the photograph above. Probes of the fill soil were strongly resisted by all of the soil in the zone south of the house suggesting that the soil is in a moderately dense state probably due to having been compacted in place. The three -tiered retaining wall was examined closely to assess its stability after heavy rains or during seismic events. Rocks were found to be thoughtfully placed and fairly -well interlocked. Photographs 2 and 3, below, shows Trent Adams, Geotechnical Engineer in Training, examining the rock revetment. The rock revetment has an arcuate shape when viewed from above so that the convex side of the rock revetment faces the retained earth. (See the arc -shaped top of wall to right of the southerly addition in Photograph 1.) This circular -arc shape enables the wall to operate like an arch in resisting the lateral forces created by the fill soil and water. The three -tiers of the rock revetment have an overall downward slope of about 55 degrees from the horizontal. A lesser Geotechnical Site Review 12015 slope would have been desirable, but the wall, as -constructed, is not excessively steep for this type of rock revetment. Photograph 2—The Retaining Wall/Revetment Has Favorable Rock -To -Rock Interlock Drainage from the access road reportedly flows over the retained soil at times so that there is an ongoing possibility of saturation of the soil behind the retaining structure. The rock structure is, by its nature, well drained. In general, it appeared from field surface observations that the retaining structure was thoughtfully built under the direction of someone who understood the principles involved and the mode of construction. Geotechnical Site Review 12015 Photograph 3 —Tiers Of The Rock Revetment Are Well -Constructed and Not Overly Steep 4.0 Conclusions Concerning Geologic Hazard In our opinion, the retaining wall/revetment is sufficiently strong and stable to permanently protect the house provided that the recommendations below are followed during the construction. If the site drainage work and foundation construction recommendations are carried out as described, the on-going geologic hazard related to the proposed bedroom addition action will be minimal in our opinion. 5.0 Recommendations To Be Followed 1. A total structure setback of fifteen (15) feet from the rim of the retaining wall to the building line of permanent structures is recommended for this site. This is equivalent to the current setback to the bedroom addition and in our opinion, such a setback will provide a prudent margin of safety against geologic hazard. Geotechnical Site Review 12015 2. A geologic hazard buffer often (10) feet in width is recommended for this site. Plant native trees in the zone between the retaining wall and the bedroom addition and encourage the growth of a native plant understory to enhance the stability of the slope and the rock revetment. 3. Embed the southerly footing and stem wall of the addition to a total depth of 4 feet from the surface of the ground to the bottom of the footing. This will ensure that the footing is wholly founded on undisturbed earth. If in doubt at the time of excavation and construction, notify the undersigned Engineers for field assistance. A design bearing of 2500 pounds per square foot may be used for the footings placed at that depth. 4. Lower the inlet at the catch basin approximately 8 inches and construct a gentle Swale so that water coming down the driveway readily flows to the drainage inlet south of the former slide area. We recommend that stormwater runoff from the proposed bedroom addition be tightlined to the existing catch basin for stormwater management. Call on the undersigned Engineers for field assistance at the time of construction for driveway and Swale grading instructions. 6.0 The Limitations of This Study and Report The observations and conclusions of this report apply only to the subject property and they are not transferable to nearby or adjoining property. This report is the property of Mr. Terry Parks and may be used by others only with his permission. The observations of this report were made during a limited reconnaissance study in which inferences were made from surface conditions. No borings, detailed measurements, soil testing or observations over time were made. Unknown subsurface conditions that were unseen could affect the outcomes of the study. Users who need a high level of reliance on the observations and conclusions of the study may wish to obtain further investigations. NTI Engineering and Land Surveying warrant that this study and the related report were conscientiously completed in accordance with the practice of professional engineering and according to the principles of geotechnical and geologic sciences. No other warranty, neither express, nor implied, is provided herewith. Please call on the undersigned Engineers if you have questions about the contents or the meaning of this report. Geotechnical Site Review 12015 Please call on the undersigned Engineers if you have questions or need clarifications during construction or permitting. We appreciated the opportunity to perform this work for you. NTI Engineering and Land Surveying Sincerely yours, Steve S. Luxton MSc, PE Senior Geotechnical Engineer Trent T. Adams BSCE, EIT Geotechnical Project Manager APPENDIX ' APN.002352019 i HER= a" ao-mare � a