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Home My WebLink AboutBLD2003-00472 - BLD Engineering / Geo-tech Reports - 5/27/2003 e MASON COUNTY PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton,Wmhington 98584 DATE: MU 27 h, 2003 INTER-DEPARTMENTAL COMMUNICATIONS TO: Scott Longanecker, DCD-Planner FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WO# PLG-03 SUBJ: Geo-Tech Assessment Review NAME: Cook SFR(a, 150 W Pyramid Court BLD2003-00472 Scott, The geotechnical assessment prepared for the proposed Keith Cook Single Family Residence (SFR)to be constructed at 150 West Pyramid Court,has been received and reviewed by Public Works. The assessment appears to dispel the need for further geotechnical investigation, conforms to standards of engineering principles and practices for this time and place,and appears to conform to the minimum requirements of DCD for geotechnical assessment reporting. The evaluation raises no issues or concerns, and appears acceptable for the level of reporting required. Recommendations contained in the report should be incorporated into the site's development. The applicant and his contractor to should be alerted to their responsibility to protect neighboring properties during and after construction, from adverse stormwater runoff impacts. Please feel free to contact me at County extension 461 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. Si cerely, �an . Tahja File:H:IWPIGEOL&sessmentslCook.doc WOrkOrder PERMIT M. =?3 —oD L(—J 2 Number ,-, Requested by: 1.141I"-f —5-- L_ / y Authorized by: Date: Type of Work: CHARGE TO: NAME AGENCY/COMPANY BILUNG ADDRESS PHONE (: Pub. Works Person In Charge: tc) Project Time Une. (from to dat Project Start date: Estkrsated Finlsh Date: TO APProxhade haws: ESTIMATED TOTAL i'S: ','� COST ESTIMATE (D) P3t! _ Emp1oY« Lliit! U29M BOW Etitms� __TOTAL= 5 27/03 c l -�_�� `3,( ' �r _. Sty l�fI TOTAL t5 EQUIPMENT USED: BAD FAATERL 6L USED: i _ �.-....- . (� Actual Cost = BARS: PROJ#. DATE WORKED Name Hours "4U1 ErInoe% TOTAL$ i i EQUIPMENT USED: atu4 gal£ TOTAL SS MATERIAL USED: TOTAL ALL (G) BILLED DATE INV i PAID DATE REC.s CFC>r Lcook] ��; Y C'1 � Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 20 Ma 2003 y R TPA 47 MAY 212003 Mr. Keith Cook 700 East Hiawatha Boulevard Shelton, Washington 98584 Subject: Geotechnical assessment of slope stability at your new residence site, 150 West Pyramid Court, Mason County Tax Parcel Number 420187500020 , BLD 2003-00472 . Dear Mr. Cook: As requested, we have visited the above site in order to observe site conditions and to prepare a report addressing slope stability and its probable long-term effect on your proposed construction. This letter will present the results of our reconnaissance level survey and our research, and our opinion on the stability of the slope. At the time of our site visit, the site had been cleared and the building corners staked. Test pits for drain field design wE�re open, and we were able to observe the near-surface soils . The project site is located on a low, north-facing ridge above Egypt Valley. This slope has been modified in this development as the slope has been cleared of stumps, and only grasses are growing on the slope. Measuring slope angles using a Brunton Pocket Transit, we found the slope angles to average less than 22 degrees or 40% slope. Examination of the g P slope face at this lot and in front of adjacent lots did not indicate that erosion or geomorphic features associated with mass wasting events are present. The slope as it exists at this time appears to be stable. To verify our understanding of the area geology, we review Plate 1 of Water-Supply Bulletin 29 , Geology of Southeastern Mason County Washington by John B. Noble 03050601 Page 1 of 3 _� � 03050601 Page 2 of 3 and Dee Molenaar. This area is mapped as Qvt, Vashon subglacial till . We also reviewed the Geologic Map of the South Half of the Shelton and South Half of the Copalis Beach Quadrangles, Washington compiled by Robert L. Logan, Open File report 87-9 , 1987 (Revised Dec . 1987 ) and published by the Washington Division of Geology and Earth Resources . This area is mapped as Qdvt, Vashon subglacial till . No landslides or areas of mapped mass wasting events are indicated on either of these maps for this general area. In the drainfield test pits, we found in the exposed trench walls sands and gravels associated with Vashon recessional outwash Qvr. We expect that the outwash deposits mantle the upland and slope area at this site and that subglacial till underlies these soils . The upper soils of Vashon recessional outwash are granular and have densities in the firm to dense range . Using Table 3-2 in Bowles Foundation Analysis and Design, 3rd edition estimated angles of internal friction are presented. For these soils in a medium dense range, angles of internal friction from 30 to 35 degrees is indicated. With good internal drainage characteristics and slope angles less than the expected angle of internal friction, slope stability is inferred. The underlying Vashon subglacial till is a highly overconsolidated soil unit. Subglacial till was deposited at the base of the advancing ice sheet and is a mixture of silt, sand, and small gravels . The grain size distribution and loading by the ice sheet, thought to have been about one-half to one mile thick in this area has created a soil unit with many engineering properties similar to those of lean concrete. This soil unit can stand steeply with apparent stability and only slow erosion occurring on its surface . The soil units at this site are not prone to deep rotational type of failure. We expect that the only active erosional process working on the slope is soil creep, the slow downslope movement of the near surface active soil zone due to saturation and freeze thaw cycles . 03050601 Page 3 of 3 At the indicated setback of the residence from the top of slope, the setback will meet the minimum requirement of the Uniform Building Code for setback from descending slopes . We consider the setback to be reasonable and prudent. Soil creep will not have an impact on foundation soil support at this setback. Ground water will not have an adverse effect on the proposed development. The outwash sands and gravels are expected to infiltrate water rapidly. Flows of ground water may develop on the surface of the subglacial till as it is for all practical purposes impermeable. This flow will not affect the proposed development unless foundations extend into the subglacial till. In that event, good perimeter drainage will be required to intercept this flow to aid in ensuring dry crawl spaces . Water from roofs should not be directed towards the slope. Concentrated flows of water can create areas of localized saturation and induce slope erosion. We recommend that the roof rain leaders be collected into a tightline system with the water conveyed towards the road area on the south side of the residence for infiltration. Based on our site visit and research, it is our opinion that this site is not a landslide or erosion hazard area. Construction of this residence at the indicated setback is at no greater risk than the existing residences to the west of this lot in this development. If you have any questions, or if we may be of additional service to you on this project, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES ° Wash/n � �e 90 ?David C. S rong, L.E.G. 1 'c 1129 e'7Sed Ge°\off DAVID C. STRONG