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Home My WebLink AboutGEOTech - BLD Engineering / Geo-tech Reports - 7/24/2019 SHANNON 6WILSON GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS July 24,2019 Mr. Ray Nelson Watermark Estate Management Services, LLC 10230 NE Points Drive,Suite 200 Kirkland,WA 98033 RE: GEOTECHNICAL EVALUATION OF GREAT LAWN DRAINAGE SOILS, HOODS CANAL PROPERTY, UNION WASHINGTON Dear Ray: This letter presents the results of our geotechnical field investigation of the sod in the Great Lawn area of the Hood Canal Estate. We understand that the Great Lawn is frequently saturated and soggy/unstable,especially during wet rainy weather. Our services consisted of exploring,sampling,and testing the near-surface soils below the sod layer to aid in the design of improved soil drainage systems within the Great Lawn area. The grounds maintenance staff at the property are proposing to construct a mitigation measure that will include use of a subsurface drainage composite material called Multi Flow Professional Drainage Systems to mitigate the saturated condition. Our findings will be used to help evaluate the causes of the saturation and to determine the appropriate depths for installation of the Multi-Flow System. Our scope of services included the following: ■ Perform hand-excavated shallow core sampling at six locations within the Great Lawn. ■ Collect soils samples and perform laboratory index tests. • Prepare a Geotechnical Letter documenting the core testing and laboratory testing results with recommendations for mitigation design. SOIL CORES We performed six test cores using a hand-operated,bucket-auger and shovel. The cores were located at selected locations within the Great Lawn,as shown in the enclosed Figure 1. The cores were excavated to depths ranging from 6 to 16 inches and in all cases were terminated on very dense soil or asphalt that prevented advancement of the hand auger. We collected soil samples from the ground surface down to 16 inches to allow us to determine the composition of the sand that underlies the sod and the fill soils that underlie the sand. 400 North 34th Street■Suite 100■ PO Box 300303 ■ Seattle, Washington 98103■ 206 632-8020 ■ Fax 206 695-6777 ■www.shannonwilson.com 0 1 Mr. Ray Nelson SHISHANNON VWILSON Watermark Estate Management Services, LLC July 24, 2019 Page 2 of 5 RESULTS OF GEOTECHNICAL CORES The Great Lawn ground surface is generally flat and level within the upper south side (former maintenance yard)and slopes gently down to the west. We understand that the Great Lawn was built over the septic drainfield for the estate, a former estate maintenance yard,the old abandoned alignment of SR-106 and various areas that were previously paved. The first core, designated C-1,was located at the north end of the Great Lawn, as shown in Figure 1 and Exhibit 1 below. Exhibit 1: Core C-1 located in west side of Great Lawn The cores were made using a 2-inch core barrel pushed into the sod,followed by excavating with a flinch-diameter hand auger and spade. The sod consists of a 2-inch-thick layer of grass and roots growing in a silt medium. This is underlain by clean sand to depths of about 6 to 7 inches. Exhibit 2 below is a photo of the typical sand bedding material that supports the sod. _-I ,,�- 21-1-22423-003 Mr. Ray Nelson =1IISHANNONWALSON Watermark Estate Management Services, LLC July 24, 2019 Page 3 of 5 �n f� u� v: sj V 00 Exhibit 2: Photo of Sod and Bedding Sand Core C-1 Subgrade soils below the sand layer consisted of very dense,silty,gravelly sand;very dense, sandy gravel;and, at core C-5,asphalt pavement. We collected samples of the clean sand bedding material that underlies the sod at all locations and we performed grain size distribution tests on three samples from cores C-2, C-5, and C-6. The results indicate that the bedding sand is relatively clean with 4 to 7 percent fines(silt). We also collected two representative samples of the underlying dense to very dense subgrade soils. These samples from cores C-1 and C-2 were tested for grain size distribution and found to contain 8 to 16 percent fines and 37 to 51 percent gravel. The results of the grain size distribution testing are presented in Figure 2. _�4__3 i0 �_,.d.,_„,ter,SKr 2 1-1-22423-003 Mr. Ray Nelson C Watermark Estate Management Services, LLC ��`�F�ANNON�WILSON July 24, 2019 Page 4 of 5 The relative density of the sod, sand bedding, and subgrade was evaluated using a '/2-inch- diameter steel rod pushed into the ground. The sod and underlying bedding sand was consistently loose. The subgrade soils ranged from dense to very dense. At all core locations, except C-5, we encountered very dense, compacted gravels at 6 to 16 inches deep that prevented advancing the auger. At core C-5, asphalt pavement was present at a depth of 6 inches, directly beneath the sand layer. The table below presents a summary of the soils encountered at each core location. Core Sod Thickness Bedding Numberbg .d C-1 1 to 1.5 4.5 to 5 Dense to very dense,silty gravel(up to 2-inch-diameter)and sand. Refusal to excavation at 16 inches. C-2 2 4 Dense to very dense,silty,gravelly sand. Refusal at 16 inches. C-3 2 6 Dense to very dense,silty,sandy Gravel. Refusal at 16 inches. C-4 2 4 Very dense,compact(cemented)gravel with refusal at 6 inches C-5 2 4 Refusal on Asphalt Pavement at 6 inches. C-6 2 6 Very dense,silty,sandy Gravel to refusal at 14 inches. CONCLUSIONS In our opinion, the results of the site reconnaissance and subsurface explorations indicate that the Great Lawn was built over densely compacted gravel subgrades and old asphalt pavement. These subgrade materials are either very low permeability or, in the case of asphalt, impervious. We understand that the Great Lawn was built using rolled sod that was apparently grown in silt. This silt tends to hold moisture and probably contributes to the wet, soggy feel of the grass after rain and irrigation. The presence of relatively impervious subgrade causes the rain and irrigation water to accumulate above the subgrade, saturating the bedding sand layer. The saturated bedding sand layer would be unstable under foot and lawnmower traffic. In our opinion, the remedy for the wet soggy lawn conditions would include installation of subsurface drainage that can collect mounded water from above the subgrade layer and convey it downgradient to a suitable discharge location. Alternatively, if the entire Great Lawn will be removed and replaced, the underlying subgrade soils will be exposed and could be scarified and trenched with heavy equipment, such as a bulldozer with a ripping tool, to loosen the compacted subgrade soils and increase their permeability. The asphalt pavement in the vicinity of core C-5 could be completed 21-1-22423-003 Mr. Ray Nelson WIISHMNON8VAMN Watermark Estate Management Services, LLC July 24, 2019 Page 5 of 5 demolished and removed to expose underlying granular soils that would likely have adequate permeability to prevent saturation of the sod and bedding sand. CLOSURE The conclusions contained in this letter are based on site conditions as they existed at the time our site reconnaissance and explorations were performed, and further assume conditions interpreted from the explorations are representative of the subsurface conditions beneath the entire Great Lawn;that is, the subsurface conditions everywhere are not significantly different from those disclosed by the explorations. Within the limitations of the scope,schedule, and budget, the conclusions presented in this letter were prepared in accordance with generally accepted professional geotechnical engineering and geologic principles and practices in this area at the time this letter was prepared. We make no other warranty,either express or implied. The conclusions and recommendations are based on our understanding of the Project as described in this letter and the site conditions as interpreted from our site reconnaissance and field explorations. If you have any questions,please call me at(206)695-6875. Sincerely, SHANNON & WILSON N W. p co o Q 30308� O��CISIEI�'" Sy SSI�NAL ti�� Martin W.Page,PE,LEG Vice President Geotechnical Engineer MWP/mwp Enc. Figure 1 -Site Exploration Plan Figure 2-Grain Size Distribution 2 1-1-224 23-003-1I.doWw0kn 21-1-22423-003 f c-6 0 i _ o C-1 Great Lawn O Core Locations i — — _ — C-4 O I C-3 O OPERATIONS O - BUILDING i — i' � I _ Existing Septic Field n GUEST I LODGE BUNK HOUSE O TOY BOX -- ----- --_-- --" �o•o gyp. 1,5 1e 'moo.d Git o / O KITCHEN --------------- -- -- BUDDING JAMES DAVIDSON ARCHITECTS Hood Canal Estate-overall SRa Plan 0 SITE PLAN-1'=W-(r-014119 60 120 Figure 1- Site Exploration Plan =111 SHANNON 6WILSON,INC. GRAIN SIZE DISTRIBUTION PLOT Union Washington Residences Great Lawn Evaluation C-1� C-2, C-5, C-6 Union,Washington Gravel Sand Fines Coarse Fine Coarse Medium Fine Sift Clay-Size Mesh Opening in Inches Mesh Openings per Inch,U.S.Standard Grain Size in Millimeters �ry O o 00�0 0 00 00~ 00 100 95 5 90 10 85 15 80 20 75 25 70 30 65 35 -0 m Q) 60 40 a 55 45 o O C 50 50 N I.L N C45 55Q U � N 40 60 0- N 35 65 y 30 70 25 75 20 80 15 85 10 90 5 95 0 100 py p0 b0 00 ry0 ,O 0 0 ^b P h ^ O O O ♦' p. O' O' O' O' O'O' O' O' O' O' O' Grain Size(MM) Sample Depth USCSp USCS Gravel Sand Fines <20pm <2pm WC Tested Review ASTM Identification (ft) Symbol Group Name % % % % % % By By Std. •C-1,S-1* 1.0 GP-GM Poorly Graded Gravel with Silt and Sand 51 41 7.5 8.8 AKV C136 rn ■C-2,S-1, 0.3 SP Poorly Graded Sand 2 94 3.6 6.1 AKV C136 C-2,S-2 0.7 SM Silty Sand with Gravel 37 47 16 13.2 AKV C136 r- C-5,S-1* 0.3 SP-SM Poorly Graded Sand with SIR and Gravel 6.8 10.3 AKV D1140 J Z� O C-6,S-1 0.2 SP-SM Poorly Graded Sand with Sift 5.0 9.9 AKV D1140 < x a Test specimen did not meet minimum mass recommendations. vi N V N N N Z < Q to Q 4 n N Y N N N SHANNON&WILSON,INC. 400 NORTH 34TH STREET SUITE 100 SEATTLE,WASHINGTON 98103 MAIN(206)632-8020 FAX(206)695-6777