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Home My WebLink AboutGEO2009-00049 - GEO Geological Review - 6/30/2009 SLOPE ASSESSMENT PROPOSED SINGLE FAMILY RESIDENCE 270 NE LAKE DRIVE TAH UYA, WA CLIENT: Mr. John O Baker 309 South Yantic Avenue Bremerton, WA 98312 BY: N.L. OLSON AND ASSOCIATES, INC. 2453 BETHEL AVE. SE PORT ORCHARD, WA 98366 (360) 876-2284 JOB N O. 7 O 235- 09 1Pyti bVflS,•��G��. i � �NAt June 2009 N.L. OLSON & ASSOCIATES INC. � 'Engineering, Planning and Surveying June 16, 2009 Project Number: 7235-09 Attn: Mr. John O Baker 309 South Yantic Avenue Bremerton, WA 98312 Subject: Slope Assessment Proposed Single Family Residence 270 NE Lake Drive Tahuya, WA Mason County Parcel #: 223305000188 Dear Mr. Baker; This report presents the results of our field investigation for the referenced property. In the following, we present our findings and conclusion for the slope assessment and proposed single-family development. We appreciate the opportunity to be of service to you on this project. If we can be of further assistance or if you have any questions regarding this project, please contact our office. Sincerely, Wesley R. Johnson, P.E. Project Engineer P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 CADocuments and SettingsMohnson\My DocumentsTroject Folder\7235 Baker-mason county\7235 Baker Final.doc APPENDIX A TABLE OF CONTENTS INTRODUCTION...................................................................................................................... 1 SITELOCATION....................................................................................................................... 1 SITECONDITIONS.................................................................................................................... 1 PROPOSEDDEVELOPMENT...................................................................................................... 1 FIELDINVESTIGATION.............................................................................................................. 2 SURFACE SEEPAGE AND GROUNDWATER ............................................................................... 2 AVAILABLE GEOLOGIC AND SOILS INFORMATION ....................................................................... 2 DEPARTMENT OF ECOLOGY WATER WELL REPORT................................................................... 2 SLOPE RECONNAISSANCE AND LANDSLIDE ACTIVITY................................................................. 2 HISTORICAL LANDSLIDE RESEARCH.......................................................................................... 2 UPSLOPEGEOMORPHOLOGY................................................................................................... 3 UPLANDWATER BODIES.......................................................................................................... 3 WETLANDS............................................................................................................................. 3 CONCLUSIONS AND RECOMMONDATIONS ........................................................................ 3 FOUNDATION RECOMMENDATIONS ........................................................................................... 4 STRUCTURALFILL ................................................................................................................... 4 CRUSHEDROCK ..................................................................................................................... 4 SETBACK................................................................................................................................ 5 REPORTLIMITATIONS........................................................................................................... 5 VICINITYMAP ...........................................................................................................FIGURE 1 SITESKETCH............................................................................................................FIGURE 2 SOILSURVEY MAP...................................................................................................FIGURE 3 SLOPESTABILITY MAP...........................................................................................FIGURE 4 CROSSSECTION A-A...............................................................................................FIGURE 5 APPENDIX A- MASON COUNTY DEPARTMENT OF COMMUNITY DEVELOPMENT SUBMITTAL CHECKLIST SLOPE ASSESSMENT PROPOSED SINGLE FAMILY RESIDENCE 270 NE LAKE DRIVE TAHUYA, WA INTRODUCTION This report presents the results of our site assessment for the referenced project. The assessment was done per Mason County Resource Ordinance, 17.01.100 landslide hazard areas, section E, special reports, page 6 of 15, dated December 27, 2006. NLO has included Mason County Department of Community Development Submittal Checklist for a geological Assessment in the Appendix A of this report. SITE LOCATION The property is located at the address of 270 NE Lake Drive, Tahuya, WA and by, section 30, township 23 north, range 2 west, in Mason County, Washington. The Vicinity Map illustrating the location of the property is shown on Figure 1. SITE CONDITIONS The site is bordered to the west by NE Lake Drive, to the north and south by undeveloped land, and by Haven Lake to the east. The property is rectangle in shape and roughly 0.27 acres in size. Access to the property is provided from Lake Drive to the west. The property has been cleared to make way for the new residence. During our site visit, NLO observed a recently installed septic drain field and associated septic clean outs and a new water well. The proposed building area has been located on a slope that descends from NE Lake Drive to the west and trends towards Haven Lake to east. The gradients in the proposed building area ranged from 10 to 15 percent with a vertical relief of about 5 to 10 feet. The proposed building area overlooks a localized steep slope area with gradients on the order of 50 percent. The localized steep slope has a vertical slope height of roughly 10 to 15 feet and terminated along the shoreline of Haven Lake. The site has been logged roughly 20 year ago and since vegetated with a few cedar, Douglas fir, and large leaf maple in areas that have not been disturbed by previous grading activity. PROPOSED DEVELOPMENT As presently conceived, a portion of the site is proposed for development with a single-family residence. For report purpose, the building's finish floor has been approximated between elevation (el)=120 to el=130. Open cuts will be utilized to achieve the building's desired finish floor elevation. The structure will be one story and of light wood frame construction with a floor slab. The building's footing loads are anticipated to be light to moderate. We have shown the approximate building area on the Site Plan - Figure 2. P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C Tocuments and Settings\WJohnson\My Documents\Project Folder\7235 Baker-mason county\7235 Baker Final.doc Project No. 7235-09 June 16,2009 Page No. 2 FIELD INVESTIGATION The site's subsurface conditions were explored on May 20, 2009 by two test pits excavated with posthole diggers. Our limited site investigation revealed loose to medium dense silty sand with gravel underlain by dense to very dense glacial till. The glacial till was encountered 3 to 5 feet below current site grades. The test pit locations are shown on the Site Plan, Figure 2. SURFACE SEEPAGE AND GROUNDWATER In the area of test pit TP-2, light surface seepage was observed along a road cut that was graded to provide access to the new well location and in a drainage ditch that was graded along the west property line. On the adjacent property to the north light, groundwater seepage was observed along the steeper slope area upslope of the lake. The observed surface seepage has been illustrated on our Site Sketch - Figure 2. During our subsurface exploration, light ground water seepage was also encountered above the glacial till 3 to 5 feet below current site grades hand our hand excavated test pit locations. AVAILABLE GEOLOGIC AND SOILS INFORMATION The Washington Division of Geology and Earth Resource (WDGER), Geologic Map of Washington — Northwest Quadrant, dated 2002, has mapped the site as Quanternary sediments, dominantly glacial drift and includes alluvium. Glacial till consists of an unsorted, unstratified, highly compacted mixture of clay, silt, sand, gravel and boulders deposited by glacial ice. The Web Soil Survey (WSS) indicates the site is underlain by Alderwood Gravelly sandy loam. NLO has included the soil mapping for this area on SCS soil map, Figure 3. DEPARTMENT OF ECOLOGY WATER WELL REPORT A water well log for the property indicated glacial till in the upper 170 feet of the soil horizon. The glacial till was underlain by cemented sand and gravel from 170 to 292 feet below ground surface (bgs). Sand, gravel and the ground water table were encountered 292 to 296 feet bgs. SLOPE RECONNAISSANCE AND LANDSLIDE ACTIVITY As part of our fieldwork performed on May 20, 2009, the slope area was examined for indications of slope instability. Indications of slope instability include head scarps, hummocky terrain, inconsistent patterns of vegetation, tension cracks, seepage zones and course grain material overlaying silt and clay soils. At the time of our site visit, the slope comprising the property appeared stable. In our opinion, we do not anticipate the observed water seepage and encountered ground water perched above the glacial till will contribute to slope instability. HISTORICAL LANDSLIDE RESEARCH A review of "Mass Wasting Assessment for the lands of the Tahuya Block" Slaughter ET AL., August 2005 indicates the area in and around the property has been classified as Class 2. Class 2 is considered to have a low hazard of slope instability and described ice scoured hills with a low relief till plain. The slope stability mapping for the site has been illustrated on Figure 4. P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C:\Documents and Settings\WJohnson\My Documents\Project Folder\7235 Baker-mason county\7235 Baker Final.doc Project No. 7235-09 June 16, 2009 Page No. 3 UPSLOPE GEOMORPHOLOGY According to Eric J. Schuster's, "Geologic Map of Washington State", the site and its surrounding geologic makeup was classified as Pleistocene Continental Glacial Drift (Qgd). This includes the sediments as mentioned by the WDGER above. The geomorphology of this area shows signs of glacial scouring in generally a northeast-southwest direction. These trending ridges and valleys were carved by the retreat of glacial ice in the Pleistocene (approximately 10,000 to 12,000 years ago to the end of the Epoch). During the retreat and melting of the glacial ice, the scoured areas and closed depressions were filled with outwash material (alluvium). Alluvium consists of glacial fluvial sediments, such as sand and gravel that are deposited near the terminus of the glacier in a size-sorted manner (fines up sequence). The property and surrounding areas consists of glacial till. Glacial till is comprised of glacially consolidated sand, gravel, and cobbles with variable amounts of silt and clay that exhibits high strength and low compressive characteristics. Locally, this soil unit is referred to as "hardpan". UPLAND WATER BODIES Haven Lake is an upland water body that comprises an area of roughly 69 acres and the only upland water body located near the property of interest. In regards to slope instability, NLO does not anticipate negative impact from the proposed development to Haven Lake, which borders the property to the east. Our opinion assumes that our report recommendations will be implemented in the proposed development as well as Mason County guidelines. WETLANDS NLO has reviewed maps in the area of the proposed development and no other documented wetland areas other than Lake Haven are noted in the general vicinity of the site. CONCLUSIONS AND RECOMMONDATIONS Following our review of Mason County Resource Ordinance, Ordinance Number 77-93, we conclude that large scale sliding or deep-seated rotational failures resulting from the proposed construction appears negligible. The overall global stability of the property under the present site conditions appears good. In our opinion, the property is not located within or near a landslide hazard area and an additional geotechnical engineering report will not be necessary in regards to the proposed development. Further discussion of the building setback is provided later in this report. Our site work revealed portions of the proposed building's foundation system will be placed on dense to very dense glacial till that transitions to loose uncontrolled fill. The loose uncontrolled deeper fill soils were encountered along the east side of the proposed building area. In our opinion, differential settlement may occur in areas the foundation transitions from loose to dense soil and contribute to cracking of the buildings foundation or floor slab. In order to prevent differential settlement, NLO has provided foundation recommendations as additional information for the client's use in the Foundation Recommendation section of this report. P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C Tocuments and Settings\WJohnson\My DocumentsTroject Folder\7235 Baker-mason county\7235 Baker Final.doc I Project No. 7235-09 June 16, 2009 Page No. 4 FOUNDATION RECOMMENDATIONS In the building's foundation area that daylights or shallows up near the east side of the structure, an over excavation of an additional 1 to 5 feet may be required below the bottom level of the foundation to achieve hardpan. The subsequent excavation can be backfilled to the desired construction elevation with a clean crushed rock or structural fill. We recommend that the over excavation should extend horizontally the depth of the excavation. For example, if the over excavation extends 3 feet below the bottom level of foundation, the excavation should extend horizontally 3 feet each side of the foundation. This means a foundation width of 2 feet will require an 8 feet wide excavation. An alternative to over excavation and backfill would be to extend the foundation down to hardpan. NLO recommends field verification of the foundation subgrade areas prior to placement of concrete formwork or rebar. The field verification of the footing's foundation subgrade area is necessary to make certain that the foundation have been established on dense to very dense soil condition or on properly compacted crushed rock or structural fill. During our field verification, NLO's representative can also provide recommendations for over excavation and backfill. STRUCTURAL FILL Structural fill is defined as compacted fill placed under buildings or pavements that consist of free draining gravelly sand having a maximum size of 1-1/2 inches and no more than 5.0% fines passing the No. 200 sieve. Soils with a fine content greater than 5 percent passing the 200 sieve will degrade if exposed to excessive moisture. All imported fill material should conform to the above recommendations regardless of the site's weather conditions. All structural fill should be placed on a firm, properly prepared subgrade in loose layers approximately 8 inches in thickness, conditioned to a moisture content suitable for compaction, and compacted to 95% of the maximum dry density as determined by ASTM D-1557 — Modified Proctor. All Structural fill material should be submitted for approval to the Geotechnical Engineer at least 48 hours prior to delivery to the site. Native soils with a fine content greater than 5 percent passing the 200 sieve will degrade if exposed to excessive moisture, and compaction and grading of this material will be difficult or impossible if soil moisture significantly increases. If used, the on-site silt and clay soils utilized for structural fill should be moisture conditioned to within plus/minus 2 percent of the optimum moisture content, and compacted to 95 percent of the maximum dry density based on the Modified Proctor ASTM 1557. Additional fill layers shall not be placed until the previous lift meets the compaction requirements presented in this report. CRUSHED ROCK The crushed rock used for back fill of the building's foundation system should have a particle size of 2 inches and washed of fines. The crushed rock will not require in place density tests to determine relative compaction of the material. The crushed rock will still require compaction with a jumping jack or similar compaction device to properly seat the material. We recommend compacting the crushed rock in one-foot lifts. P.O. Box 637. 2453 Bethel Avenue• Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C\Documents and Settings\WJohnson\My Documents\Project Folder\7235 Baker-mason county\7235 Baker Final.doc i Project No. 7235-09 June 16, 2009 Page No. 5 SETBACK With the provided foundation guidelines, in our opinion, the building can be sited 25 feet from the top edge of the steep slope. The 25 feet setback can be measured from the water well located along the grade brake overlooking the lake. NLO has illustrated the building setback as well as the water wells approximate location on the Site Sketch Figure 2 and the Cross Section Figure 5. REPORT LIMITATIONS This report has been prepared for the client regarding the subject project. Information presented in this report has been collected and interpreted in a manner consistent with the level of care and skill ordinarily exercised by members of the profession currently practicing under similar conditions, and in accordance with sound and generally accepted principles consistent with normal consulting practice. No other warranty, expressed or implied, including (but not limited to) any warranty or merchantability or fitness for a particular use has been made. In the event that change in the nature, design, or location of the proposed construction is made, or any physical changes to the site occur, recommendations are not be considered valid unless the changes are reviewed by NLO and conclusions of this report are modified or verified in writing. Any site involving sloping terrain has inherent risk of earth movement. As a result, the CLIENT agrees to accept full responsibility for all risks associated with steep slopes. The CLIENT acknowledges that this risk cannot be completely eliminated and that engineering and geologic analysis is intended to reduce the inherent risk associated with slopes. No amount of geotechnical engineering and geologic analysis can provide a guarantee of stable slopes. Geotechnical engineering and geologic analyses are based heavily on subjective interpretation, professional judgment, and opinion regarding the physical conditions at a specific site. Subsurface conditions are only documented at those points where samples were taken and interpolation and extrapolation is necessary between and around sample locations. Conditions can vary between samples and can change over time due to natural processes and/or human activity. Analyses and recommendations provided in this report are based in part upon the data obtained from the subsurface explorations. P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C\Documents and Settings\WJohnson\My Documents\Project Folder\7235 Baker-mason county\7235 Baker Final.doc R o�N�P\Nv`E�No NEM 0 NE V/gTA Pt NE HAVEN WAY SITE HAVEN LAKE R. Jpy A 3r FIGURE 1 FOR: SCALE: NONE L_`JN.L.Olson&Associctes,Inc. VICINITY MAP MR. JOHN BAKER DATE: YAY2009 Engineering, Planning and Surveying �0.�'+an.+0 .%` 270 NE LAKE DRIVE 309 S. YANTIC AVE DRAWING NUMBER (360) 876-2284 TAHUYA, WA — MASON COUNTY BREMERTON, WA 98312 7235-09 2453 SetW Avenue,P.O.Bev 637.PM Ordwd,WA 9M #223305000188 T F p N A.G.al g O► ml DJloa'Zs oe0 -- ap t o_ 8 g I , c c - P y i Nui N � NO Lnp o Lu d1 M1 tit 0 m� - - j C �A A N 0 `� Rl z W $ L) W n. Z o � 1 � W - � J - J N ri IL 0 _v zQ z 0 .64 8'a Rl ate.,._ :�o,xn�� oK,-- L Z J Lu Q W J R.J FROM: SCS ONLINE SOIL CLASSIFICATION USDA WEB SOIL SURVEY (WSS) A 3 ` � z FIGURE 3 FOR: SCAE: NOW LOJ N.L.Olson&Associates,lnc. SCS SOILS MAP MR. JOHN BAKER DATE: Engineering, Planning and Surveying ` DRAWNG NUMBER �moxtsMd' 270 A LAKE DRIVE 309 S.BREMERTONTIWAAVE 8312 (360) 876-2284 TAHUYA, WA — MASON COUNTY 7235-09 24M Or"AmL%P.O.Do.W,Pon O{a{o4 0 9m � #223305000188 SHEET =1 - i h; 1 III ave , SITE Lake 1� ff 31 . �I � r ® 1:Alluvium--Low Hazard 2:Low Relief,Till-Plain with Ice-Scoured Hllls--Low Hazard 3:Lower Angle Terrace Edges—Moderate Hazard ■ 4:Active Meander Bends--High Hazard ■ 5:Steep Gorge Systems and Other Steep Convergent Topography--Very High Hazard ® 6:Terrace face—Very High Hazard ® 999:Groundwater Recharge Areas of Selected Deep Seated Landslides(does not include all recharge areas) MAPPING FROM " MASS WASTING ASSESSMENT FOR THE STATE LANDS TAHUYA BLOCK" SLAUGHTER ET AL, AUGUST 2005 e; R. dq�.y Q n 3 r 'Ca 2 FIGURE 4 FOR: SCALE: NOW L:,N.L.Olson&Associates,lnc. SLOPE STABILITY MAP MR. JOHN BAKER DATE: MAY2009ER Engineering, Planning and Surveying �� 270 NE LAKE DRIVE 309 S. YANTIC AVE 7235—DRAWING NUMBMB (360) 876-2284 OVAL TAHUYA, WA — MASON COUNTY BREMERTON, WA 98312 24M Befit Avenue.P.O.Box 637.Pat Odwd,WA 96366 E:::� #223305000188 SHEET APPENDIX A Mason County Department of Community Development Submittal Checklist for a Geological Assessment Instructions: This checklist must be submitted with a Geological Assessment and completed, signed, and stamped by the licensed professional(s) who prepared the Geological Assessment for review by Mason County pursuant to the Mason County Resource Ordinance. If an item found to be not applicable, the report should explain the basis for the conclusion. Applicant/Owner Mr. John O Baker Parcel 223305000188 Site Address 270 NE Lake Drive, Tahuya, WA (1) A discussion of geologic conditions in the general vicinity of the proposed development, with geologic unit designation based on referenced maps. Located on page(s) 2&3 (2) (a) A discussion of the ground water conditions at the site. Located on page(s) 2 (b) A discussion of the estimate depth to water Located on page(s) 2 (c) A discussion of the quantity of surface seepage Located on page(s) 2 (d) A discussion of the upslope geomorphology Located on page(s) 3 (e) A discussion of location of upland waterbodies and wetlands. Located on page(s)3 (3) The approximate depth to hard or dense competent soil, e.g. glacial till or outwash sand. Located on page(s) ? (4) A discussion of any geomorphic expression of past slope instability (presence of hummocky ground or ground cracks, terraced topography indicative of landslide block movement, bowed or arched trees indicating downslope movement, etc.). Located on page(s) 2 (5) A discussion of the history of landslide activity in the vicinity, as available in the referenced maps and records. Located on page(s) 2 (6) An opinion on whether the proposed development is within the landslide hazard area or its associated buffer or setback and the potential for landslide activity at the site in light of the proposed development. Located on page(s) 3 (7) A recommendation by the preparer whether a Geotechnical Report should be required to further evaluate site conditions and the proposed development of the subject property. Located on page(s) 3 (8) If the presence of a hazard is determined within 300 feet of the proposed development, then the following are delineated on a geologic map/site map: (a) The area of the proposed development, Located on Map(s) N/A (b) The boundaries of the landslide hazard area (top, both sides, and toe), Located on Map(s) N/A (c.) The associated buffers(top, both sides, and toe) Located on Map(s) N/A (c) Building or other setbacks(top, both sides, and toe). Located on Map(s) N/A Note: NLO has provided discussion for a building setback from a steep slope area. Our slope assessment has indicated no landslide hazard exists with in 300 feet of the area proposed for development. (9) A site map drawn to scale showing the property boundaries, scale, north arrow, and the location and nature of existing and proposed development on the site. Located on Map(s) -Site Rkatrh E4ire 9 P.O. Box 637. 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 C1Documents and Settings\WJohnson\My DocumentsTroject Folder\7235 Baker-mason county\7235 Baker Final.doc