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Home My WebLink AboutGEO BLD99-0607 Report Review, Soil Logs - BLD Engineering / Geo-tech Reports - 8/24/1999 MASON COUNTY PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton,Washington 98584 DATE: August 24, 1999 INTER-DEPARTMENTAL COMMUNICATIONS TO: Shandra O'Haleck,DCD- Planner FROM: Alan A. Tahja, P/W-Co. Hydr. Engr. DCD WO# 9969 SUBJ: Geo-Tech Report Review NAME: Joshua& Pamela Simpson BLD99-0607 The geotechnical report prepared by JEROME w. MORRISSETTE&ASSOCIATES INC., for the proposed Simpson residential construction has been received and reviewed by Public Works and determined to adequately investigate and overcome potential instability issues. The report contains a recommendation that the proposed structure be located no less than 50 feet from the top of the slope above Oakland Bay. This should be made a condition for permit approval. The report also recommends that ground cover be reestablished in disturbed areas,that surface drainage is handled appropriately, and wastewater drainfields are located well upslope of the top of the bluff. These recommendations should be incorporated into the project development and construction as much as is reasonably possible. In addition to the recommendations promoted in the geotechnical report, it is recommended that concentrations of surface runoff not be delivered to the top of the steep slope,but should be "sheeted" onto the slope by allowing runoff to deliver itself to the top of the slope by sheeting across the building setback area. It appears that only a geotechnical assessment was warranted by the engineer's report, but the report submitted contains all the required elements of a geotechnical report. The completeness of the investigation makes it easy to recommend approval of the project conditioned on the recommendations contained in the report. Please feel free to contact me at County extension 461 if you have any questions regarding this department's review,or these recommendations. Sincerely, go t:�Zl Alan A. Tahja File: H:\WP\GEO\REVIEWS\SIMPSON.RPT 'DECEIVED AUG 181999 Jerome W. Morrissette&Associates Inc., P.S. MASON 00.PLANNING DEPT. Civil•Municipal•Geotechnical Engineering and Planning 1700 Cooper Point Road SW, #13-2, Olympia, WA 98502-1110 (360) 352-9456/FAX (360) 352-9990 GEOTECHNICAL REPORT SIMPSON RESIDENCE: 670-131 EAST BAYVIEW DRIVE SHELTON, WASHINGTON August 17, 1999 INTRODUCTION The purpose of this report is to present a geotechnical review of the proposed building site for a single- family residence at 670-131 East Bayview Drive in Mason County, Washington (see Figure 1). This report has been prepared for Mr. and Mrs. Simpson, the owners of the property. The subject property lies above a waterfront property along the east shores of Oakland Bay. The property to be built upon is generally flat consisting of lawn and a few scattered trees. The property currently contains a mobile home, shop, and gazebo as shown on the site plan (see Figure 2). The property between the site and the shoreline is heavily vegetated and undeveloped. The waterfront property has a continuous moderate slope from the boundary of the subject property to the shoreline. The steepest portion of the waterfront property is approximately 35 percent with an average slope of 27 percent. The study area was inspected in July and August of 1999, after one of the most sever winter wet seasons on record for western Washington. GEOMORPHOLOGY The study area is located along the margin of a broad glacial drift plain that has been incised by seasonal and small stream valleys. The slope rises from the water contact along Oakland Bay to a maximum elevation of about 145 ft (see Figure 2), with the project site being approximately 550 feet from the toe of the slope. Figure 3 shows an estimated surface profile from Oakland Bay to the upper reach of the residential property to be developed. The elevations used for this profile and the contours shown on Figure 2 were based off of the USGS mapping for this area, with contour estimations developed from aerial photographs and site inspections. The average slope between Oakland Bay and the subject property is estimated to be 27% with the steepest portion at about 35%. The property between the subject site and Oakland Bay is undeveloped containing a dense upper story consisting mainly of Douglas Fir and Western Red Cedar. The under story is moderate to dense, comprised of willow salal, fern, and other low rowing bushy vegetation. Ground cover within the subject site generally consists of lawns, landscaping and a sparse upper story of alder, Douglas Fir, and Western Red Cedar. There are no clearly defined surface water courses within the project site. There is, however, a small seasonal drainage course that starts approximately 100 feet from the top of the slope that appears to convey groundwater seepage down the slope face to Oakland Bay. This seasonal drainage course is approximately one foot in width with no significant cross sectional area. All stormwater runoff in this area either infiltrates, is absorbed by vegetation, or flows down the slope into Oakland Bay. There were no noticeable seeps or springs along the slope outside of the seasonal drainage course. One seepage area was observed along the upper slope, immediately west of the proposed building site. This seep has precipitated a shallow incision into the bluff face. This seep is located approximately ten feet below the top of the slope. This seep appears to be the result of a shallow block failure in the upper bluff glacial till, which has partially blocked the above said seam and appears to be causing the build up of shallow groundwater immediately upslope. The seep is identified by wet surface soils and"facultative" vegetation. This area presently appears to be stable with dense vegetation cover. One soil boring was performed west of the proposed building site as part of this investigation. The boring confirmed the upper glacial till overlying a wet sand strata followed by blue clay. The property between the site and Oakland Bay shows no signs of recent massive slides, failures, or movements in the recent past. Bluff toe bank erosion/undermining along the shoreline from tidal activity is evident (approximately 550 feet from the project site). The only signs of land movement are bends in the trunks of a couple of the largest cedar trees near the toe of the slope at Oakland Bay. Due to the apparent age of these trees and other trees that do not show this characteristic in this vicinity, it appears that this movement happened more than 50 years ago. A moderate sized slope failure was observed on the adjacent property to the north, consisting of the vertical displacement of about 800 feet of bluff crest. This failure appears to be quite old. Outside of this area, there were no outward signs of distress along the tope of the slope of the immediately adjacent areas. There are no indicates of shallow block slippage or deep rotational slope failure in the area. SOILS The soils in the study area are classified by the Coastal Zone Atlas as the Vashon Till, generally consist of sands and gravel above a clay layer. Sediments located below the surficial soils consist mainly of very competent highly overconsolidated glacial till. The shallow till forms a prominent surface for shallow groundwater flow and perching. The clay layer, acting as an aquatard, is located approximately 30 feet below the surface, allowing the glacial till to provide pressures on the water bearing sand layer to cause seepage along the slope face. A soil boring that was conducted on August 18, 1999 supports the soils classification of the Coastal Zone Atlas. The top two feet of soil consists of generally loose surficial sandy gravel. From 2 to approximately 15 feet is the glacial till (sand and gravel) that is common to this area. Water was also encountered at a depth of 16 feet. From 16 feet to 30 feet was sand with small amounts of gravel, and at blue clay was encountered at 30 feet (end of boring). A soil log of this boring can be found in Appendix A. COASTAL ZONE MAPPING Figure 4 shows a copy of the slope stability designations provided in the Coastal Zone Atlas, Volume 9 of Mason County,Washington. The project site area has been designated as stable with a small portion of the slope property just above Oakland Bay being designated as unstable. SUMMARY OF FINDINGS Based on the foregoing it has been observed that: • The average bluff slope between the subject property and Oakland Bay is 27%, less than the 33% typically identified as areas of potential Landslide Hazard. • The study area shows signs of one small surficial slope movement along the bank. The remainder of the area appears to be stable presently with no signs of past shallow or deep ground movements. • There are no surficial watercourses along the study area slope with the exception of the seasonal streambed. Hence, no stream incisions, stream bank erosion or undercutting is evident. • Groundwater seepage within the study area was observed to be minor(but is expected to be moderate during the wet months), occurring along the slope between the subject property and Oakland Bay. CONCLUSIONS AND RECOMMENDATIONS Based on the foregoing, it is concluded that: • Site soil conditions indicate a thin mantle of colluvium over highly overconsolidated glacial till, which provides competent support of the slope and residential construction in this area. • The study area slope, although the lower portion above the shores of Oakland Bay is designated as unstable in the Coastal Zone Atlas,Volume 9, appears to be reasonably stable. • The glacial till and clay substructures may be expected to provide moderate to good resistance to deep rotational slope failures. • Shallow surficial soil movements may occur in this area in locations where the slope is artificially over-steepened by cutting or filling,or where localized seepage undermines the upper till. • Construction of the residence should not adversely impact slope stability,provided measures are taken to reestablish any disturbed ground cover, surface drainage is handled appropriately, and wastewater drainfields are located well upslope of the top of the bluff. It is therefore recommended that: • The proposed residential structure will be located no less than 50 feet from the top of the slope above Oakland Bay(edge of lawn). • All disturbed surfaces should be restabilized with a suitable system of fertilizing and seeding or other permanent vegetation for soil stabilization. LIMITS OF LIABILITY Jerome W. Morrissette &Associates Inc., P.S. is responsible for the opinions, conclusions and recommendations presented in this report, which have been developed from the site data acquired at the specific locations described herein. It is noted that this data has been extended to cover the immediate vicinity of the related construction effort,using our best engineering judgment. However,comprehensive continuous data is not available and localized variations may exist. Such variations are not reflected in this report and their nature and extent may not become evident until construction has been initiated. This report has been prepared within the standard and accepted practice of our industry. Should the data or conclusions presented herein be used by others to formulate conclusions and recommendations for this or other projects in the immediate vicinity of this work, Jerome W. Morrissette&Associates accepts no responsibility unless we are given the opportunity to review them and concur with them in writing. Jerome W. Morrissette & Associates Inc., P.S. W*Wu .OF WASH F� Trent J. Lougheed, P.E. Project Geotechnical Engineer/Associate 4 - 3sivi a ,4 CIST I AL ,►itrM'W* FXPiRFS:7i12iD1 FIGURES i 9 1 >i �•• t MIM st" —j ] LPS�P�E ��2 LAKE ] z i A I d � I I I I I I JOHN'S CREEK ---�1511liAIGUEBr_--BAiSf9pR t ---- ---------- 1------ -------- ---wt"^- t-----F r+P7 IBAVSNOFf I I _ I 5 I GotfI- t I 1 PRAIRIE RD. I CLUB 1 I i Irr 1 1 .y I I �` •r • I .r I 1 I z 1 1 I --------t------ _———_——-t---------------- I_____-____---- __-L 9 I 1 it i !mm ro, 11 12 ^f 1 I � C 1 I I c• I I 14.J I I I 1 M ff I I I � I I 1O 1 I otEt I I I 1 �>• cr I I I I I 1 i I I � J 1 l 1 I 1 tJ\ 1 rcnriEzo, °0 I I I 1 10 : 11 �„� ---------------- ---- ------==:=---- - �. I ______________ 1 4 13____________—__ 1 s 14 I I I CHAPIVAN COVE ' I I - i 'hEr b• I I I I I i I 1 I I I ¢ 1 0. AGATE Zcmnm L. Is : 1S sr�r[• is : 14 I d 14 13 AGATE I JWM&A JEROME W. MORRISSETTE &ASSOCIATES INC., P.S VICINITY MAP CIVIL AND MUNICIPAL ENGINEERING AND PLANNING 670-131 E. BAYVIEW DRIVE 1700-132 COOPER POINT ROAD SW MASON COUNTY, WASHINGTON OLYMPIA,WASHINGTON 98502 TEL(360)352-9456 FAX(360)352-9989 JWMA#99141 AUGUST 17, 1999 FIGURE 1 SCALE: 1"=40 FEET s s O NfLL o TRANSFORMER 0 20 40 80 PO vauLT 30" FIR % \ �o 30 �ED bt ` Ff n /oi�°j♦°'tio7CN f LA� 3 C 24" CEO cn pi 0 HOSE Bt c— 4 ❑ \ \ HORSE SHOEO / \ I ! POWER c \ \ PIT l BOX 'no 130" CEO �4v 30" CED 1 1 28 30" CED 36' CEO , 24' CEO 4- 11 { PROPOSED TUB HORSE SHOE 1 1 \ �010 t l - HOME N PIT 1 t `\\ \` \ \�♦ - LOCATION SEPTIC----9 II /00"___�� t \ LID I t \♦ \\ '. --- �`� II DL 36" CEO JI --_ ------------ - �\\ DRt� Ii / 3O" CED 30" CED Ex15nt4G 1 Or0 CED J (36" CEO -=� ` I EXISTNG MOBILE kOME ♦ `, \ EX/SANG 30" CED \\ ♦\ \ `1 \ GAZEBO 30' CED \ 1 1 \ `---------- 1 � �110 co cc 9,>, \ r \ O NO DATE BY APPS REMSIONS CM M01c d.G.ot.oMkd &Vhow*,o and P1ambV 41?V.� GEOTECHNICAL REPORT CtiD.D.Br DATE SfTE PLAN SIMPSON )ot�ors,tta W. Mor*issdttw at Associwtss Inc., P.S. QCco=er Dn rE 1700 Cooper FoYK Road SW,*6-2,04wVt16 WA M M-1110-0"352-94S6-FAX(36M 3S2-9M t"T. 99yiH FCUE 2 50' MIN. �c BUILDING Z SETBACK m I 0 CL 0 140 130 120 110 100 �-AVERAGE SLOPE = 279 90 ESTIMATED MAXIMUM SLOPE = 359 80 I 70 i 60 50 2 40 1 30 20 10 0 550, I 1"=50' HORIZ 1"=25' VERT OAICLAND BAY SHORLINE ,.ro....aye/. Morrise+tte s.AssociA&WM IR7C., P.S. PROPILE GEOTECHNICAL REPORT 1700 Coopr IoMtt Rod AM,K-2,Cttvoik WA%502-1110•LIAO 332 9 6%•FAX 0"352A170 99141 ROLFE a � ^ a �• = � l Sit lip ,� '' ��s:t 15 Ag f f l {;�1 ' �Yy��f' 1<. •< 1 1. 4 ' AP r IL Frt�• t• ,!+ ��. ,,.t �"{ .�� f � .,f��r`��';' �'' lam• 'r°�; ♦ +'' • '•a ,< � it � .. ��++ �,}`y •�� ,?i Ar ir o !JWM v • 1.�� 4 a �L '•tr+lry�.1� V.�E,�,�.t� �,sl ']��Nx�s� �'' •'�i • JEROME • - PHOTOGRAPH CIVIL • MUNICIPAL ENGINEERING AND PLANNING 670-131 DRIVE 1700-132 COOPER POINT ROAD SW MASON COUNTY, WASHINGTON OLYMPIA,WASHINGTON1 TEL 1 (360)352-9989 APPENDIX A SOIL BORING LOG FROM GEOTESTLAB PHONE NO. 3607544848 Aug. 17 1999 03:27PM P2 r G EOTECHNICAL TESTING LABORATORY LOG OF TEST BORING Project No : M4rrlsvette &_•Assoc. Test- Boring No . 1 Date:�B-16-99 tz Location fi70 ga view Dr- Hason collcty w BLOWS Per 6" LOG DESCRIPTION AND REMARKS I i • i 5 30 43 Ref isal Firm Sandy silt with gravel 10 60 semi cemented silty sand with gravel 15 27 47 semi cemented sand with gravel Water 20 17 26 35 Sand 25 17 40 40 Sand 30 67 ' Ref sal Blue Clay