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HomeMy WebLinkAboutBLD2009-00159 Geotechnical Report Oc G E()TECH 13256 Northeast 20th Street,Suite 16 Bellevue,Washington 98005 CONSULTANTS, INC. (425)747-5618 FAX(425)747-8561 July 17, 2009 JN 09158 Dean Rienstra and Kathy Gager 40 Libby Court Port Ludlow, Washington 98365 Subject: Geotechnical Engineering Considerations Existing Single-Family Residence 40 Libby Court Port Ludlow, Washington Dear Mr. Rienstra and Ms. Gager: via email to: kathygager©yahoo.com On July 9, 2009, the undersigned Principal of Geotech Consultants met with you at your residence at Port Ludlow. The purpose of this visit was to observe the existing conditions, and to develop opinions regarding the stability of the residence. The recommendations and conclusions presented in this report are professional opinions based on the visual observations made during our site visit and on previous experience with other projects in the Puget Sound region. No deep subsurface explorations were conducted for our work. SITE CONDITIONS The Rienstra/Gager residence, which we understand was built in the 1970's, is located just north of Port Ludlow and just upslope of the Puget Sound. It sits on the mostly gentle slope that descends east to the Sound. Several other residences are located on this slope. A few large, scattered trees are also located on the slope. The residence has a driveway that enters along the property's western side. The main level of the residence has a grade that is near the driveway elevation. The main level is situated over a crawl space, with the exception of the garage that is on the southwestern side of the residence. The crawl space is somewhat shallow at the upslope, western side of the residence, but is_about 5 feet deep at the downslope eastern side._ One of the most common soil in the Puget Sound region is a glacially compacted mixture of sand, silt, and gravel. This soil is referred to as "hardpan" by many non-geotechnical engineer personnel in the construction industry because of its dense to very dense condition. It is common that the hardpan is located about 3 to 4 feet below the ground surface, covered by a looser, weather version of the soil mixture. We used a 1/2-inch steel rod to probe the soils outside the residence and within the crawl space. We found that indeed the hardpan was about 3 to 4 feet below what we believe was native ground. It is apparent that the residence is founded on a conventional footing system. It appears that most of the perimeter footings are buried rather shallow, in the range of 12 to 18 inches. There is an approximate 3-foot-tall bank in the middle of the crawl space that extends in the north-south direction. Several individual footings are located along the top of the slope, and several just east of the base of this bank. However, two individual footings that are in line with the footings at the top of the slope have been placed down below the bank equal to the grade of the lower level; it appears that these two were altered after the original foundation was installed. The soil at the top of the bank, where most of the footings existed, was loose for a depth of about 3 feet. However, the apparently altered footings were about 3 feet lower, and were found to bear on dense hardpan. fFnTFr..H ffNSI II TANTS INr. Rienstra/Gager • • JN 09158 July 17, 2009 Page 2 We understand from our discussions with you that settlement of the main level is evident in a "bump-out" area near the northeastern portion of the residence and in a hallway area near the center of the residence. A large fireplace is located at the eastern edge of the bump-out. A level survey done by a contractor indicates that the bump-out area is indeed about 2 inches below the western entry, while the center is down slightly less than that. The remainder of the residence is down in the range of 0.5 inches. We did not observe any cracks in the perimeter foundation other than some very minor ones. We determined that the area of central settlement was very near where the two altered footings in the crawl space are located. CONCLUSIONS AND RECOMMENDATIONS It is apparent to us that dense to very dense hardpan underlies the site and the area. However, this soil is located about 3 to 4 feet below the ground surface. The soil above is much looser. It appears that most of the footings of the house are founded in the looser soil, with some founded in the dense soil. The loose soil does have a tendency to settle over time, especially where significant loads and/or significant events occur. It is apparent that some settlement in the range of 2 inches has occurred in the bump-out area, and a similar amount has occurred in the middle of the residence where the altered foundations exist. A significantly heavy fireplace is located at the eastern edge of the bump-out, and that is likely the reason for the settlement there. We understand that a broken water line at/in the residence may have occurred fairly recently (before you bought the residence), and such an event could definitely have settled/damaged previous footings in the middle of the residence before the new footings noted earlier were installed. Portions of the remainder of the residence have settled in the range of 0.5 inches; the relatively light buildings loads founded on the looser soil is likely the explanation of this small settlement. We understand that you intend to underpin the bump-out foundation with pipe piles. The pipe piles may also be used along the remainder of the eastern perimeter foundation and the eastern half of the northern and southern perimeter foundations. In addition, you intend to repair the line of interior footings that are along the section where the central settlement occurred. We believe that the bump-out underpinning is quite necessary. However, we believe that underpinning the northern and southern foundations is of little benefit. The underpinning of the remaining eastern foundation has some benefit, but it is difficult to quantify it; the decision to underpin this portion should be made by you. As for repairing the interior foundations, we recommend that all of the existing foundations that are on the top of the soil bank be replaced with new footings that bear on the hardpan soil. An excavation of about 3 feet should be suitable to reach this soil. The two repaired footings that are already down about 3 feet are adequately founded on hardpan soil in our opinion. LIMITATIONS The analyses, conclusions, and recommendations contained in this report are based on site conditions as they existed at the time of our site visit. If the subsurface conditions encountered during construction are significantly different from those anticipated, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. GEOTECH CONSULTANTS,INC. • • Rienstra/Gager JN 09158 July 17, 2009 Page 3 This report has been prepared for the exclusive use of Mr. Rienstra and Ms. Gager and their representatives for specific application to this project and site. Our recommendations and conclusions are based on the site materials observed and on previous experience with sites that have similar observed conditions. The conclusions and recommendations are professional opinions derived in accordance with current standards of practice within the limited scope of our services. No warranty is expressed or implied. We trust that this report meets your immediate needs for the proposed development. Please contact us if we can be of further service. Respectfully submitted, GEOTEC CONSUL ANTS, INC. f( A. :,; , 7ik t/1,, .w�Il1 ,,,, i ......„,/, . 4./0 ♦L/ * 7 7 /7 0) D. Robert Ward, P.E. Principal cc: CG Engineering— Dennis Titus via email to: dennist@cgengineering.corn DRW:jyb GEOTECH CONSULTANTS,INC. • DAILIELD REPORT CONSUL G E OTAN T T E S, INC.C H TRAVEL/PREP TIME JOB NUMBER • IL TIME ON SITE DATE t,' t ., 13256 NE 20th Street,Suite 16 TIME OFF SITE DAY OF WEEK "! Bellevue,WA 98005 `/'.` 425-747-5618 FAX 425-747-8561 HOURS CHARGED WEATHER • {)ilt JOB LOCATION CLIENT/OWNER _, GENERAL CONTRACTOR GENERAL CONTRACTORS SUPERINTENDENT MILES PERMIT NUMBER GRADING CONTRACTOR GRADING SUPERINTENDENT • VISITORS PAGE OF r • i ��f 1i r j ( L. _. �,r-- rl ill t . ; r' �.faj� �t'%��2 ri 1 i, f. 1_ 4 I ` �' i4-4C+ f; '. 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