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Home My WebLink AboutGEO2016-00071 Slope Assessment - GEO Geological Review - 10/30/2016 Go SLOPE ASSESSMENT PROPOSED GARAGE STRUCTURE 1781 NE BLACKSMITH DR RECEIVED BELFAIR, WA OCT 17 2016 615 W. Alder Street CLIENT: Linda Whaley PL 7► D LL 4G P.O. Box 566 Belfair, WA 98628 BY: N.L. OLSON AND ASSOCIATES, INC. 2453 BETHEL AVE. SE PORT ORCHARD, WA 98366 (360) 876-2284 JOB NO. 9612-16 October 2016 N.L. OLSON & ASSOCIATES, INC. Engineering, Planning and Surveying October 14, 2016 Project Number: 9612-16 Attn: Linda Whaley P.O. Box 566 Belfair, WA 98528 Subject: Slope Assessment Proposed Garage Structure 1781 NE Blacksmith DR Belfair, WA Mason County Parcel #: 22310-78-00410 Linda, This report presents the results of our slope assessment for the referenced property. In the following, we present our findings and conclusion. 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. ohnson, P.E. Geotechn al Division Manager P.O. Box 637.2453 Bethel Avenue•Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360) 876-1487 USERVER\Users*ohnsonlproject tolderWroject Folder19612 Whaleylgeo assoasmont.dec TABLE OF CONTENTS INTRODUCTION ........................................................................................................................I SITELOCATION .........................................................................................................................I SITECONDITIONS......................................................................................................................1 PROPOSEDDEVELOPMENT ........................................................................................................1 SURFACE SEEPAGE AND GROUNDWATER ...........................................................................I.......1 AVAILABLE GEOLOGIC AND SOILS INFORMATION..........................................................................2 DEPARTMENT OF ECOLOGY WATER WELL REPORT.....................................................................2 SLOPE RECONNAISSANCE AND LANDSLIDE ACTIVITY....................................................................2 HISTORICAL LANDSLIDE RESEARCH............................................................................................2 UPSLOPEGEOMORPHOLOGY.....................................................................................................2 UPLANDWATER BODIES............................................................................................................3 WETLANDS...............................................................................................................................3 CONCLUSIONS AND RECOMMONDATIONS...........................................................................3 SETBACK..................................................................................................................................3 SLAB-ON-GRADE RECOMMENDATIONS.......................................................................................3 REPORTLIMITATIONS.............................................................................................................4 VICINITY MAP &SITE SKETCH................................................................................FIGURE 1 CROSSSECTION A-A...............................................................................................FIGURE 2 WETLAND..................................................................................................................FIGURE 3 APPENDIX A—MASON COUNTY DEPARTMENT OF COMMUNITY DEVELOPMENT SUBMITTAL CHECKLIST P.O. Box 637.2453 Bethel Avenue•Port Orchard, Washington 98366 Phone: 1(800)755-1282 Fax: (360) 876-1487 1\SERVER\UsersMjchnaon\pr0p t folderTroject Fokler\9612 Malefteo assessment.dec SLOPE ASSESSMENT PROPOSED GARAGE STRUCTURE 1781 NE BLACKSMITH DR BELFAIR, WA INTRODUCTION This report presents the results of our slope assessment for the referenced project. The assessment was done per Mason County Department of Community Development Submittal Checklist for a Geological Assessment, dated June 2008. NLO has included the checklist in Appendix A of this report. SITE LOCATION The property is located by the address of 1781 NE Blacksmith Drive, Belfair, WA and by, section 10, 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 north and west by NE Blacksmith DR, and to the south and east by single family residences. The property is parallelogram in shape and roughly 5.0 acres in size. Access to the property is provided from NE Blacksmith DR. For report purposes, the subject property has been split into an eastern and western portions by a stream channel. N.L. Olson has illustrated the subject property on the Site Sketch, Figure 1. N.L. Olson observed clearing and site improvements have been confined to the western portion on the subject property. The site improvements that have occurred included an access drive, shop, carport, septic tank and drainfield associated septic clean outs and water well. The mobile home and underlying concrete trailer pad was removed prior to our assessment. The western portion of the subject property was vegetated with cedar, douglas fir, and large leaf maple, and alder in areas non disturbed areas. Along the eastern portion of the subject property east of the creek, no development or clearing was observed at this time of this report. Along the eastern portion of the subject property, vegetation consisted of secondary growth of cedar, douglas fir, and large leaf maple, and alder. PROPOSED DEVELOPMENT As presently conceived, a new slab-on-grade will be constructed within in the footprint of the former trailer pad area with a future garage/shop to be constructed at a later date. The new structure will be one story and of light steel frame construction. We have shown the proposed building area on the Site Plan - Figure 1. SURFACE SEEPAGE AND GROUNDWATER No surface seepage was observed along the slope area east of the proposed building area towards the steam or along the gradual sloping uplands areas located between NE Blacksmith DR and the proposed building area. P.O. Box 637.2453 Bethel Avenue• Port Orchard, Washington 98366 Phone: 1(800)755-1282 Fax: (360) 876-1467 ASERVERviwmwphnaord ea(oldw%Project FoldwW12 Whaleftw w5e3sment.doc Project No.9612-16 October 14,2016 Page No.2 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 Quantemary 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. NLO has provided SCS soil mapping as additional information for use by the client or contractors they may subcontract. The Web Soil Survey (WSS) indicates the site is underlain by Alderwood Gravelly sandy loam. SCS soil mapping corresponds with WDGER mapping that glacial till underlies the site. DEPARTMENT OF ECOLOGY WATER WELL REPORT A water well log drilled on the property indicated glacial till in the upper 70 feet of the soil horizon. The glacial till was underlain by cemented sand and gravel from 70 to 105 feet below ground surface (bgs). Sand, gravel and the ground water table were encountered 105 to 112 feet bgs. SLOPE RECONNAISSANCE AND LANDSLIDE ACTIVITY As part of our fieldwork performed on September 22, 2016, 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. The subject area, trailer pad area has been identified on the Site Sketch, Figure 1. The subject area was located on an area graded level for the previous structure that overlooked the underlying slope and stream channel to the east. The gradual slope located between the stream channel and building pad had a vertical slope height of almost 10 feet with gradients on the order of 15 percent to 30 percent. West of the building pad towards NE Blacksmith DR, gradients ascended at roughly 5 percent to 10 percent were observed. At the time of our site visit, the above mentioned slope areas appeared stable with no springs or surface seepage observed. N.L. Olson has provided a section view of the proposed building area and it's relation to the slope as shown on Cross Section A-A, Figure 3. HISTORICAL LANDSLIDE RESEARCH A review of "Mass Wasting Assessment for the lands of the Tahuya Block" Slaughter ET AL., August 2005 was inferred from the close proximity, near Kitsap County and Mason County boundary as Class 2. Class 2 is considered as a low hazard of slope instability and the upper surface profile described as ice scoured hills with a low relief till plain. 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 P.O. Box 637e 2453 Bethel Avenue • Port Orchard, Washington 98366 Phone: 1(800) 755-1282 Fax: (360)876-1487 \XSERVERWWMW jolinaonprcpd fokkvV\ Mlw foldert9612 Vfiefeygeo assessmentdoo i 4 , i Project No.9612-16 October 14,2016 Page No. 3 (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 i strength and low compressive characteristics. Locally, this soil unit is referred to as "hardpan". �I UPLAND WATER BODIES Black Smith Lake is an upland water body that comprises an area of roughly 20 to 25 acres. Other upland water body are also located near the property of interest. N.L. Olson has identified i those water bodies on Figure 3. In regards to slope instability, NLO does not anticipate negative impact from the proposed development to Black Smith Lake or other water bodies given their distance from the subject area and higher elevation. WETLANDS NLO has reviewed National Wetlands Inventory maps for this area and have identified their locality on Figure 3. We do not anticipate impacts from the proposed development to the wetlands areas given the topographic restrictions, highpoints, which reside between the proposed building area and wetlands. 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- N.L. Olson has observed that a stream does reside on the subject property and flows east of the area proposed for development. In our opinion, we do not anticipate erosion or slope instability will be a concern for the gradual slope located between proposed building area and stream channel. SETBACK In our opinion, the new slab-on-grade can be placed within the old footprint of the previous trailer pad. The proposed slab-on-grade can be placed as shown on the Site Sketch, Figure 1. SLAB-ON-GRADE RECOMMENDATIONS To reduce floor slab cracking potential, NLO has provided slab on grade recommendations as a suggestion and additional information for the client's use. P.O. Box 637.2453 Bethel Avenue• Port Orchard,Washington 98366 Phone: 1(800)755-1282 Fax: (360)876-1487 USERVER1llssrs%*Jorrson prged fokk%V1romd Fo10e W12 V*W18yrga0 aaswsmer t.doc Project No. 9612-16 October 14,2016 Page No.4 Our assessment revealed portions of the future floor slab may have a subgrade comprised of dense to very dense glacial till that transitions to loose fill. The previously placed fill would be susceptible to settlement by both the weight of the slab and self-weight of the fill if constructed on present grades can contribute to cracking. To provide a stable subgrade for the slab-on-grade area, we recommend that area should be stripped of all sod, organic soil, existing fill and debris. In most undisturbed areas of the site, a stripping depth of about 6 to 12 inches should be anticipated. However, deeper excavations along the east side of the pad may be necessary because of previous fill placement. Compaction of the stripped subgrade should be continued until field density tests indicate a minimum compaction of 95% of the maximum dry density, as determined by ASTM method D- 1557, has been achieved along the slab-on-grade subgrade area. Soft or weaving areas disclosed during proof rolling with a fully loaded dump truck should be excavated and replaced with compacted 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. 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 P.O. Box 637e 2453 Bethel Avenue•Port Orchard, Washington 98366 Phone: 1(800)755-1282 Fax: (360) 876-1487 VSERVERIUsars\wpin9onl3rojed fWarWrojed Fok1srW12 Whalefteo assessment.doc Project No. 9612-16 October 14,2016 Pa�e No_5 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 637a 2453 Bethel Avenue• Port Orchard, Washington 98366 Phone: 1(800)755-1282 Fax: (360)876-1487 14SERVERXUxMk*g0h M1projed MWerV"iect FokWM12 VAwWfteo assossmwtt.dw a n U Q 4^r » u Y BElORE Y'Ql Oct O� SCALE I b1ch.wI 'YIC1NIlY -�,} Q LL I I r ®QQ H AD AS L TED MAPF1 REEK td A !,,PUMP H BE I AS LOCATED 8Y - is 2 7dnm � SRO �d$333 PORT / J 7 - )))) J i R ILER PAOI � < � RAIN 51ELO // / w HOP FIGURE 1 SRE SKETCH DEVELOPED FROM DRAWING PROVIDED 8V CLIENT. SlTE SKETCH d NClNlTYMAP iO6 LlDAWHALU .vem �itjN.LOlsondcAssociates,inc. 1781NEBlACKSMlTHDRNE POBC0f1li awe"A" BELFAIR WA eaPAw.YMoe®e "'� " PARCEL It71310 78-W410 s_ 0 T 1 —— — —— — I I FB1 AREA I I 1 I - ---- ACCE39*--PROPOSED --T— — —I— ROAD I BLpd I ATEMp — L 1 I I 430T-- --'^ j -- - -- --F- - — � -- -I— RANG T+ -- —I---- t--- � I 420+---- - -- --- -I--- -'1' --- —� I I I I I 410J-——— ——1'———J——— IQTQL1 0+00 RANGED DOWN 4 FEET BELOW CIMRENT SRE GRADES 1 W0 CROSS SECTION A-A LOOKING NORTH SCALE 1"=20' =-Firm' "�'� "+ WAWOMCM Oil FIGURE 2 SITE PLAN DEVELOPED FROM DRAVNNO PROVIDED BY CLEINT. MAN2-WE=70I �Lf70AYMiKEY n.orrw A0A N.Lgsm&As"dote*Jm 17871E BLACK SMRH DRIVE PDSDXM a rww uMrM Fk-b. sw.eq BELFAIR.WA MFAR WAMM PARCEL A 223104"W 1D i NE$Ifl/f�yE Ln Wetlan Wetlam pond v lake ■ Df ❑ E5 Fr' SCALE 3> ■ 1 inch=500 feet w m U z 4- m Fr. ko ■ La E:] 01 ■ Rt FRESHWATER EMERGENT WETLAND Nf: Hlackamrth tar j FRESHWATER FOREST r site SHRUB WETLAND FRESHWATER EMERGENT WETLAND pond FIGURE 3 SITE FUN DEVELOPED FROM DRAVY W PROVIDED BY CLEW er o.s /� MILANDS roa Le"WHKEY .o+c osw A#APLI.OlsonUnwietes.Inc. M NEBIACK SMITH DRIVE P00OXIR! a'� ""� BELFAN.WA es>:Aa WAOFmII ems•• PARCEL t.22310-75-OD410 ' APPENDIX A MASON COUNTY DEPARTMENT OF COMMUNITY DEVELOPMENT SUBMITTAL CHECKLIST The assessment was done per Submittal Checklist for a Geological Assessment, dated June 2008. NLO has included Mason County Department of Community Development Submittal Checklist for a geological Assessment in this appendix. Located on page(s) _ �, c L�_s(a. (8) If the presence of a hazard is determined within 3D0 feet of the proposed development, then the following are delineated on a geologic map/site map: (a) the area of the proposed development, / h Located on Map(s) J fP (b) the boundaries of the landslide hazrddaarea(top, both sides, and toe), Located on Map(s) (c) the associated buffers(top, both j)s, nd toe) Located on Maps) (d) building or other setbacks(top, both sides and toei ' � r_ 3 Located on Map(s) I\ 1 l� � (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) 1, hereby certify under penalty of perju tat I am a civil engineer licensed in the State of Washington with specialized knowledge of geotechnical/geological engineering or a geologist or engineering geologist licensed in the State of Washington with special knowledge of the local conditions. I also certify that the Geological Assessment, dated /a ��le and entitled 5COPC- P057ko t MyY,,(- SC��t-Ttt2 17.-IN6 e&fg silt eregfuire entsFof�thA��` eesa Mason County Resource Ordinance, Landslide Hazard Section, is complete and true,that the assessment demonstrates conclusively that the risks posed by the landslide hazard can be mitigated through the included geotechnical design recommendations, and that all hazards are mitigated in such a manner as to prevent harm to property and public health and safety. (Signature and Stamp) Disclaimer: Mason County does not certify the quality of the work done in this Geological Assessment. Page 2 of 2 Form Effective June 2008