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HomeMy WebLinkAboutPettygrove's 1st Block 6 Lots 2 & 4 - Geotechnical Evaluation of Slope Stability 1998.02.00't Cfy of Port Torneond RECEIVED FEB 2 6 t99S w-824241 hdfu & (qmudry Drmhpncnt Geotechnical Evaluation of SloPe Stability and Sife DeveloPment Solvik ProPertY (Lots 2 and 4, Block 6, Pettyg rove's F i rst Add iti o n) Port Townsend, Was hi ngton February 1998 B en t:,k-li t-' ;$ii - e.li""lJ Mr. Sven Solvik 2624 Northeast Skidmore Portand, Oregon 97211 ill - SI{ANNONAWLSON, ING GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS 400 N. 34th St. ' Suite 100 P.O. Box 300303 Seattle, Washington 98103 '632 .8020 I ) ) zrfu) UlV" '|@7rr#"b ,.F i,t'!r 4sta -t I I = I l t "s. "mNoN f?Wtt{FpN, I NF,, SEATTLE HICHLAND FAIRBANKS ANCHORAGE SAINT LOUIS BOSION II 1J l I ^.{ I February 20, L998 Sven Solvik 2624N.8. Skidmore Portland, Oregon 972tL RE: GEOTECHNICAL EVALUATION OF SLOPE STABILITY AI\D SITE DE\ZELOPMENT, SOLYTK PROPERTY (LOTS 2 AND 4, BLOC!( 6, PETTYGROVE' S FIRST ADDITION), PORT TOWNSEI\ID, WASHINGTON Dear Mr. Solvik: This letter summarizes our observations made during a site reconnaissance, and our conclusions and recommendations for the proposed residential development at the above site. The purpose of our work was to evaluate the stability of the site as required by the City of Port Townsend's Environmentally Sensitive Areas ordinance for steep slopes and provide geotechnical recommendations for the residential development of the site. Our work was conducted in accordance with our proposal dated January 29, 1998, as authorized by you on January 30, 1998. SITE AND PROJECT DESCRIPTION The site is situated on the west side and lower reaches of Morgan Hill in Port Townsend, Washington, at the southwest corner of Fir Street and H Street (cunently vacated). The property is approximately 100 feet wide (north-south) by 100 feet long (east-west) and slopes fromFir Street down to the west. The slopes along the north and south property lines were measured with an abney level and found to be approximately 11 degrees (19 percent). We understand that you plan to construct an approximately 27-foot wide (north-south) by 38-foot long (east-west), three story residence with a basement that daylights out to the west on this site. A small, single story cottage, approximately 20 feet wide (north-south) by 15 feet long (east- wes| is located on the northwest corner of the site. The remainder of the property is vegetated with small, deciduous trees (typically l6-inch diameter or less) and shrubs, including wild rose bushes which are indicative of relatively well drained soil conditions. Three fir trees, which are up to about 2 Yz feet in diameter, are located along the west property line. A bowed trunk on one of these firs is indicative of some slight soil creep. Soil creep, which is present on most hillsides, is the imperceptibly slow downslope movement of soils under the effects of gravity. r/4OO NORTH 34TH STREET.SUITE 1OO PO. BOX 300303 SEATTLE, WASHINGTON 981 03206'632'8020 FAX 206.633.6777 TDD: 1.800.833.6388 !J w-8242-01 Mr. Sven Solvik February 20,1998 Page2 SHANNON EWLSON,INC. No springs, seeps or other indications of near surface groundwater were observed at the site during our site visit. Geologic maps of the area indicate that the site is underlain by Vashon lodgment till. Till is a nonsorted mixture of clay, silt, sand, and gravel with scattered cobbles and boulders deposited by a glacier. Lodgment till is deposited at the base of a glacier and is subsequently overridden by the advancing glacier. The Vashon Ice sheet that deposited the lodgment till is estimated to have been up to 4,000 feet thick in the area and, as a result, the till and underlying soils have been overconsolidated to a very dense or hard state due to the great weight of the ice. While it is likely that this site is underlain by the till mapped in this area, no subsurface explorations were performed at the site confirm its presence or depth. With time, the surface of the till weathers and topsoil develops from the weathered till. The combined thickness of the topsoil and weathered till above the very dense unweathered till is typically about 3 to 6 feet (subsurface explorations would be required to further evaluate the depth to the unweathered till at this site). The United States Department of Agriculture (USDA)/Soil Conservation Service Soil Survey for Jefferson County indicates that the topsoil is a Clallam Series gravelly sandy loam and is described as well drai,ned with only a moderate erosion hazard on slopes between 15 and 30 percent. l CONCLUSIONS AND RECOMMENDATIONS Slope Stability Based on our observations, it is our opinion that the existing slope on the site is relatively stable and that the risk of future slope instability affecting the proposed residence will be relatively low provided the following recommendations contained in this letter are implemented. Our opinion is based on the following: ' No signs of slope instability (e.g., slide scarps, jack-strawed trees) were observed at the site. The slopes are relatively gentle at about 11 degrees or 19 percent. w-8242-0r Mr. Sven Solvik February 20,1998 Page3 SFIqNNCN SWLSON,INC. The till mapped beneath the site is typically very dense with a relatively high shear strength and resistant to slope movement. Near surface groundwater, which often increases the risk of slope instability, does not appear to be present at the site, based on the vegetation at the site that is indicative of well drained conditions, and no observation of springs or seeps. The USDA soil map indicates the surficial soils are well drained with only a moderate erosion potential. i Please note that there is present on all slopes, such as those on this site, some risk of future instability that the owner must be prepared to accept. Such instability could occur because of future water breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events on a slope that may cause sliding. Foundations We recommend that the footings for the residence bear in the very dense till that presumably underlies the site and not in the topsoil or weathered material. By locating the footings in the very dense till, the risk to the residence from potential future soil creep or sliding of the topsoil and weathered till can be reduced. In addition, spread footings for lightly loaded structures (including residences) bearing in very dense till typically perform well and are not subject to significant differential settlements. As previously indicated in this letter, the depth to the very dense, unweathered till that presumably underlies this site is typically 3 to 6 feet; however subsurface explorations would be required to further evaluate the depth to the unweathered till at this site. While the very dense till will typically provide adequate bearing, it is commonly moisture- sensitive and may become muddy, unstable and erodable under wet conditions and subjected to construction traffic. We, therefore, recommend that excavations and footing construction be conducted during dry weather. In western Washington, May through mid-October are considered dry weather months. Excavation during wet weather requires extra care and usually results in greater expenditures. The judicious use of straw bales and silt fences on the down slope side of the excavations may also be used to further reduce erosion during construction. w-8242-01 Mr. Sven Solvik February 20,1998 Page 4 SHANNON SWLSON,INC. Drainage As previously noted, groundwater in soils on a slope often increases the risk of slope instability' Therefore, whatever measures can be taken to reduce the amount of water in the soils on the slope will have a positive impact on the stability of the slope. In additioq adequate drainage around the structure will reduce the potential for basement leaks and water beneath crawl spaces. We recommend that footing drains be installed along the outside perimeter of the structure. Footing subdrains should consist of slotted, 4-inch-diameter, plastic pipe bedded in washed 3/8- inch pea gravel. Weep holes should be provided through the down slope exterior footings (west side of the house) to provide a pathway for water that may occur beneath the house to drain to the exterior footing drains. Typical installation of these drains is shown on Figure 1" Roof drains should not be connected to the footing subdrains. The discharge from footing drains and roof drains should be collected and routed by means of a tightline to suitable discharge point (e.g., street ditch, storm sewer). Water should not be allowed to discharge onto the surface of the slope. All hard surfaces adjacent to the structures should be sloped to catch basins and the collected water disposed of as previously outlined. All outside grades should be sloped away from the structures. The need to reduce the water entering the topsoit on the site should be considered in developing landscape plans and planting. Landscaping and plants that do not need more water than typically occurs naturally should be used. Plants that require extensive irrigation should not be used. Additional Considerations We recommend that we be retained to review those portions of plans and specifications pertaining to earthwork and foundations to confirm that they are consistent with our recommendations. We also recommend that we be retained to monitor earthwork, including placement of drains and subgrade preparation for foundations. In this regard, it may be advantageous to have us complete explorations on the site to confirm the presence of till and its depth at the proposed location of structure. w-8242-01 Mr. Sven Solvik February 20,1998 Page 5 SHANNON &WLSON,INC. LIMITATTONS The conclusions and recommendations in this letter are based on site conditions observed during our site visit and assume that observed conditions are representative of the subsurface conditions throughout the site; i.e., the subsurface conditions are not significantly different from those observed during our site reconnaissance or indicated on geologic and soil maps. I[, during construction, subsurface conditions different from those inferred in this letter are observed or appear to be present, we should be advised at once so that we can review those conditions and reconsider our recommendations where necessary. If there is a substantial lapse of time between submission of our report and the start of work at the site, we recommend that this report be reviewed to determine the applicability of the conclusions and recommendations, considering the changed conditions and/or elapsed time. Within the limitations of scope, schedule, and budget, the analyses, conclusions, and recommendation presented in this report were prepared in accordance with generally accepted professional geotechnical engineering principles and practices in this area atthe time this report was prepared. we make no other warranty, either express or implied. This report was prepared for the use of Mr. Solvik in the evaluation of the stability of the site. With respect to construction, it should be made available for information on factual data only and not as a warranty of subsurface conditions, such as those interpreted from the site reconnaissance and discussion of subsurface conditions included in this letter. Unanticipated conditions are commonly encountered and cannot be fully determined merely by performing a reconnaissance of a site. Such unexpected conditions frequently require that additional expenditures be made to achieve a properly constructed project. Some contingency fund is recommended to accommodate such potential extra costs. please note that the scope of our services did not include any investigation for the presence or absence of wetlands or environmental assessment for the presence or absence of hazardous or toxic material in the soil, surface water, groundwater, or air on or below or around this site. We are able to provide these services and would be happy to discuss these with you if the need arises. w-8242-01 Mr. Sven Solvik February 20,1998 Page 6 SHANNON SWLSON,INC. Shannon & Wlson has prepared the attached, "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our report. If you have any questions regarding the observations, conclusions, or recommendations contained in this letter, please call us. We appreciate the opportunity to be of service. Sincerely, SHANNON & WILSON, TNC. Lof r? ,y'"/t* EXPIRES I a5 Gregory R. Fischer, Ph.D., P.E. Associate -) William f. Perkins, R.P.G. Principal Engineering Geologist WJP:GRF:HHD/wjp Enclosures: Figure 1 - Subdrainage and Backfilling Important Information About Your Geotechnical Report cc:Ms. Judy Surber, City ofPort Townsend OREGON W8242-0 l_ltr.d oc / w8242-Pec/ am w-8242-01 -l I I I I I i I IJ II lt I I 1 .J I f,J J 1l *i Solvik PropertY Port Townsend, Washington SUBDRAINAGE & BACKFILLING February 1998 W-8242-01 FIG. 1SHANNON & WILSON,INC. Geobchnical and Envhonm€ntal Consultants Pavement or 10" to 15" lmpervious Soil Backlill Meeting Gradation Requirements lor Structural Fill (See Note 2) Excavation Slope Contractor's Responsibility 6" Min. Cover of Pea Gravel (6" Min. on Sides of Pipe) Sloped to Drain Away From Structure Subdrain Pipe 2" lo 4" Wall Drainage Sand & Gravelor Washed Pea Gravel Damp Proofing Weep Holes (See Note 1) Vapor Barrier 4" Min.Washed Pea Gravel Not to Scale Drainage Sand & Gravelwith the Following Specifications: % Passing Sieve Size by Weight 1-1t2', 100 314" 90 to 100 114" 75 to 100 No. 8 65 to 92 No. 30 20 to 65 No. 50 5 to 20 No. 100 0 to 2 (by wet sieving) (non-plastic) SUBDRAIN PIPE 4" minimum diameter perforated or slotted pipe; tight joints; sloped to drain (6"/100' min. slope); provide clean-outs. Perforated pipe holes (3/16" to 1/4" dia.)to be in lower half of the pipe with lower quarter segment unperforated for water flow. Slotted pipe to have 1/8" maximum width slots NOTES Drainage gravel beneath floor slab should be hydraulically connected to subdrain pipe on the downslope side of the floor slabs only. Use of 2" diameter weep holes on S-foot centers as shown is one applicable method. 2. lmported structural fill should consist of well-graded granular soil with not more than 5% fines (by weight based on minus 3/4" portion) passing No.200 sieve (by wet sieving) with no plastic fines. 3. Backfill within 18" of wall should be compacted with hand-operated equipment. Heavy equipment should not be used for backfill, as such equipment operated near the wall could increase lateral earth pressures and possibly damage the wall. 4. All backfill should be placed in layers not exceeding 4" loose thickness and densely compacted. Beneath paved or sidewalk areas, compact to at least 95% modified Proctor maximum density (ASTM: D1557'70, Method C). Otherwise compact 1o92"/o minimum. MATERIALS Floor Slab Min. 19" o o0oo o0to D6 0a'o o 0 t' o'a Department of Public Works 5210 KUHN STREET PORT TOWNSEND, WASHINGTON 98368 360/385-7212 360/385'7675 rnx CrrvENcrNsnn MEMORANI)T]M To: cc: From: Subject: Date: S{Iddy'Surber David Sven Solvik - Geotech Evaluation March 13, 1998 I reviewed the Shannon and Wilson Report for the Sven Slovik property. I concur with their conclusion in their fax dated 3109 that an ESA permit should not be necessary for this site. One minor not just for the record. The report on page one states that H Street is currently " vacated." In fact H Street is an unopened right-of-way, but not vacated. { ,t\$" frtA Nt'''' MEMO To: f,Yom: Subject: Date: IanTimme\ Judy Surber Sven SoMk March 13, I Spears development Mr. SoMk orrrms Lots 2 and4, Block 6, Pettygrove's First Addition. At the request of Colette Kostelec, I rwiewed the site for ESAs. As tle topography appeared ovet L5%o slope, additional information was requested. Mr. Bill Perkins of Shannon & Wilson submitted a Geotechnical Evaluation ofthe site datedd February 20,1998) and a fax dated March 9, 1998. Mr. Perkins concluded tlat con'ditions on site do not meet the defnition ofESAper oity ordinance. Based on the additional information received, Dave Peterson and I concur with IvIr. Perkins. No ESA pemit is required for proposed single-family development on the above referenced property. I have verbally recoilrmended that the applioants incorporate Mr. Perkins' recommendations into their design. TTIANK YOU MnMo To: From: Subject: Date: JanZimmer, Judy Surber Sven Solvik March 13, 1 development Spears Mr. Solvik owns Lots 2 and4, Blook 6, Pettygrove's First Addition. At the request of Colette Kostelec, I reviewed the site for ESAs. As the topography appeared over l5o/o slope, additional information was requested. Mr..Bill Perkins of Shannon & Wilson submitted a Geotechnical Evaluation ofthe site datedd February 20,1998) and a fax dated March 9,1998. Mr. Perkins concluded that conditions on site do not meet the definition of ESA per crty ordinance. Based on the additional information received, Dave Peterson and I ooncur with Mr. Perkins. No ESA pemit is required for proposed single-family development on the above referenced property. I have verbally recomme,nded that the applicants incorporate Mr. Perkins' recommen dations into their design. TI{ANK YOU t]310{f./98 1(J;.lfi FAI l0(J 61,1'87:i7 SH.{}{NUN & IIILS{JN @ oor ffiffiffiffi SHANNON fu WI LSON.WqWW GEOTECHNICAL AND ENVIRoNMENTAL coNs INC. U LTANTS SAIITIE RICHLAN0 FAIRBANKS ANCHOHAGE SAINT LDUIS BO.STON FAX Sven $olvik Property (lots 2 & 4, block 6, Pettygrove's First Addition), ESA Determinstion 1 (including cover) CC: Fax Subject: Proiect No.r W-8242-01 To: Company: Fax: Date: Timel Total Pages; From: Wiltiam J. Perkins, H,P,G. Direct Phone; 206-633-6879E.mail: wjp@shanwil.com Judy $urber City of Port Townsend (360) 385-42s0 March 9, 1998 Sven $olvik (503) 287-ee73 Message: As discussed in our phone conversation this morning, it is our opinion that the above referenced propefi is gq!located in an Environmentally Sensitive Area (ESA) as described in the Pofi Townsend Municipal Sode for geologically hazardous areas. We base this opinion on the site conditions observed during our visit and review of geologic and soils maps of the area. A report of these observations and reviews is provided in our letter to Mr. $otvik dated February 20, 1998, a copy of which was provided to you. Please feel lree to call if you have any gue$tions regarding our determination or if we can be of fufther assistance. R grdr,E)\ll Original of this fax may be mailed if requested. NOTICE: The attached information is proprietary in its entiretil any teproduction 0r u$e of said inlormation by anyone other than its intended E IJ have recgived this facsimile in enor,Shannon & Wilson 4OO NOR]H 34THSTHEFT. SUITE lOO P.O. BOX 300303 SEATTLE, WASHINGTON 98109 ?06€32-8020 FAX20s633.67f7 TDOr 1-80G833{388