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Geotech_Report_10-21-10.pdfGEOTECHNICAL ENGINEERING LETTER DANIALI DECK REPLACEMENT EDMONDS, WASHINGTON PREPARED FOR DR. SAEED DANIALI NGA ` Main Office 17311 — 135'h Avenue NE, A-500 Woodinville, WA 98072 (425) 486-1669 FAX (425) 481-2510 (425) 337-1669 Snohomish County October 21, 2010 Dr. Saecd Daniali 7812 175"' Street SW Edmonds, Washington 98026 NELSON GEOTECHNICAL ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS Geotechnical Engineering Letter Daniali Deck Replacement 7812 —175"' Street SW Edmonds, Washington NGA File No. 833710 Dear Dr, Daniali; Engineering -Geology Branch 437 East Penny Road Wenatchee, WA 98801 (509) 665-7696 FAX (509) 665-7692 This letter summarizes our opinions and recommendations for the planned deck replacement project at your residence located at 7812 --- 175`x' Street SW in Edmonds, Washington. INTRODUCTION The site is situated on a moderate to steep north- to northwest -facing slope below 175"' Street SW, as shown on the vicinity map in Figure 1. Improvement plans include the replacement of two existing decks on the northwestern and northeastern corners of the existing multi-level, single-family residence. The northwestern deck will extend over an existing rockery and will be as close as approximately 20 feet from the top of the steep slope. The northeastern deck will extend as close as five to seven feet from the top of the steep slope. It is our understanding that the City of Edmonds has requested a geotechnical study be completed for the project due to the close proximity of the deck foundations to the steep slope. You have requested that we explore the subsurface conditions in the area of the planned decks and provide recommendations for the new deck foundations. For our use in preparing this letter, we were provided with an undated site plan titled "Plot Plan," showing the existing conditions and the planned decks. We were also provided with a Ietter from Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833'10 October 21, 2010 Page 2 the City of Edmonds Development Services Department — Planning Division dated September 30, 2010. The letter requested a geotechnical report for the project since the site slopes have an inclination greater than 40 percent. SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions within the vicinity of the proposed deck, and provide geotechnical recommendations for deck support. Specifically, our scope of services includes the following: 1. Explore the subsurface soil and groundwater conditions within the site with hand tool excavations. 2. Map the conditions on the slope and evaluate current surircial slope stability conditions. 3. Provide recommendations for deck setback from the slope and minimum foundation embedment. 4. Provide recommendations for subgrade preparation. 5. Provide recommendations for site drainage and erosion control. 6. Document the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The site consists of an irregularly-shaped parcel and is occupied by a multi-level residence with a daylight basement, within the central portion of the site. The site is generally moderately sloping down from the southern portion of the site along the residence and then steeply down to the north and northeast into a ravine. The southern and central portions of the site are occupied by the residence and are vegetated with grass and landscaping plants. Several terraced retaining walls are located in the yard to the south of the residence. The northern portion of the property is steeply sloping down to the north/northeast and is vegetated with brush and scattered trees, with dense vegetation near a ravine at the toe of the slope. The current and proposed site layouts are shown on the Site Plan in Figure 2. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 3 The site is moderately sloping, with a steep north to northeast -facing slope on the northern portion of the site that descends to a ravine and creek. The steep slope has an approximate inclination of approximately 33 degrees (65 percent) as shown on cross-sections A -A' and B -B' in Figures 3 and 4, respectively. The upper portion of the steep slope is vegetated with brush and scattered trees, and the lower portion of the slope is heavily vegetated. We did not observe seepage on the slope during our site visit on October 12, 2010. We also did not observe signs of recent movement on the site slopes. Subsurface Conditions Geology: The Geologic Map of Edmonds East and Part of the Edmonds West Quadrangles, Washington, by James P. Minard (1983) was reviewed for this site. The site is mapped near as Vashon Till (Qvt). The till is described as a mixture of clay, silt, sand, pebbles, and cobbles with occasional large boulders. Our explorations typically encountered medium dense to dense, silty sand with gravel consistent with the description of Vashon Till. Explorations: We visited the site on October 12, 2010 to explore the subsurface conditions within the proposed development area with hand auger explorations. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) was present during the explorations, examined the soils and geologic conditions encountered, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 5. The logs of our explorations are presented as Figure 6. The following paragraph contains a brief description of the subsurface conditions encountered in the explorations. For a detailed description of the subsurface conditions, the hand auger logs should be reviewed. At the surface of Hand Auger 1, we encountered approximately 0.5 feet of loose, dark brown to black, silty fine to medium sand with gravel and organics that we interpreted as undocumented topsoil. At the surface of Hand Auger 2, we encountered 1.0 feet of crushed rock. Below the topsoil and crushed rock, and from the surface of Hand Augers 3 and 4, we encountered loose, dark brown, sandy silt to silty fine to medium sand with gravel and organics. We interpreted this NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 4 material to be undocumented fill The thickness of the fill was about 1.5 to 2.0 feet in Hand Augers 1, 3, and 4, and up to 6.0 feet in Hand Auger 2. Underlying the fill in all of the Hand Augers, we encountered red -brown to brown gray, silty fine to medium sand with gravel that we interpreted native, weathered glacial soils. Hand Augers 1, 2, 3, and 4 were temlinated at depths of 4.0, 7.0, 4.0, and 2.5 feet below the existing ground surface, respectively. Hydrologic Conditions Groundwater seepage was not encountered in our explorations. However, perched water could occur on the site during periods of wet weather. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2006 International Building Code (IBC) for seismic site classification for this project. Since medium dense to dense soils were generally encountered underlying the site at depth, the site conditions best fit the IBC description for Site Class D. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion by soft deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense or better native glacial soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. The medium dense to dense glacial soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However, the loose surficial soil and undocumented fill on the slope have the potential for shallow sloughing failures during seismic events. Such events should not affect the planned deck provided the foundations are designed NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. $33710 October 21, 2010 Page 5 with the depths and setback distances from the slope as described in the Deck Setback subsection of this letter. Erosion Hazard The criteria used for determination of erosion hazard areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington, by the Soil Conservation Service (SCS) was reviewed to detennine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Everett gravelly sandy loam, 15 to 25 percent slopes. This unit is listed as having a moderate erosion hazard. The on-site soils should have a low to moderate hazard for erosion where the vegetation is not disturbed, and high where the vegetation cover is removed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The steep north to northwest -facing slopes descends from the yard area to the ravine at approximately 33 degrees (65 percent) with an overall height of up to 20 feet. We did not observe evidence of past erosion or sloughing on the site slopes during our site visit. We did not observe seepage on the slopes during our visit. However, we did observe standing water at the base of the slope that we interpreted to be perched water. Relatively shallow failures as well as surficial erosion are natural processes and could occur in the loose surficial soil on the steep slope. It is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial soils and undocumented fill on the site slopes, the potential for deep-seated slope failure under current site conditions is low. Safety factors against deep-seated sliding should be above 1.2 and 1.5 for dynamic and static conditions, respectively. Proper site grading and drainage as recommended in this letter should help maintain current stability conditions. CONCLUSIONS AND RECOMMENDATIONS NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA pile No. 833710 October 21, 2010 Page 6 General It is our opinion from a geotechnical standpoint that the site is compatible with the planned construction of two new decks along the west side of the residence. In our opinion the planned earthwork associated with the new decks should be minor and should not have adverse effects on the steep slope if conducted according to the recommendations presented in this letter. Our explorations generally encountered medium dense to dense glacial soils at depth in the planned development area. In our opinion, the native glacial soils should provide adequate support for the planned deck supports. The site slopes are considered stable with respect to deep- seated failures, but shallow failures could occur on the slopes in the loose surficial soil and undocumented fill, especially during adverse weather or due to improper human activity. We recommend that the new deck footings be extended through any loose soil or undocumented fall and be supported on the medium dense or better native soil. We understand that the proposed deck foundations will be located on and along the top of the moderate to steep slopes approximately in the same location of the existing deck foundations. We also recommend that the deck foundations be designed with an effective setback of at least 10 feet from the face of the steep west -facing slope, To maintain this recommended effective setback, the northern deck foundations will need to be embedded a minimum of five feet into the competent native soils. The southern deck supports need only reach the competent native soils. The recommended effective setback and foundation embedment should allow for normal slope recession during a reasonable life span of the decks provided that the steep slope is not disturbed during or after construction. This is further discussed in the Deck Setback and Deck Foundations subsection of this letter. The planned earthwork associated with the deck addition should be minor. However, the soils encountered on this site are considered moisture -sensitive and may disturb easily when wet. To lessen the potential impacts of construction on the slopes and to reduce cost overruns and delays, we recommend that foundation preparation take place during dry weather if possible. If carthwork takes place during wet weather, additional expenses and delays should be expected. These extra expenses could include additional erosion control and temporary drainage measures to protect the slope. In any case, the excavation spoils should be hauled off site and not allowed NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 7 to reach the slope. Also, the vegetation on the slope should not be disturbed. Any disturbed areas should be promptly repaired. Erosion Control and Slope Protection Surface water appears to be adequately controlled on the site under current conditions. We do not anticipate that the planned deck addition would significantly alter drainage patterns at the site. Adequate spaced decking on joist framing should not concentrate runoff from the deck. If the decking will not be spaced and will concentrate runoff from the deck, the water should be collected in gutters and downspouts and be directed to the roof drain system. The on-site soils can have a moderate to high potential for erosion, depending on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the excavations and the steep slopes. If any vegetation is removed from the area above the steep slope, silt fences or straw bales should be erected to prevent muddy water from flowing over the site slopes. Disturbed areas should be replanted with vegetation at the end of construction and the vegetation should be maintained until it is established. Runoff generated within the site, including roof downspouts, yard areas, and hard surfaces should be collected into permanent catch basins and yard drains and tightlined into an approved stornwater management system. Under no circumstances should runoff be allowed to concentrate on the steep slope either during construction or after construction has been completed. Vegetation on the slopes should be maintained and not be disturbed. Yard waste, grass clippings, or any other material should not be cast over the slope. Stockpiled materials should not be placed on or near the steep slope. Deck Setback Uncertainties related to building along the top of steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a "buffer zone" between the NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 8 structure and the top of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback, the lower the risk of slope failures to impact the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, slope angle, material composition, and hydrology. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. Based upon the conditions described above, it is our opinion that the potential for shallow sloughing -type failures during severe weather exists on the steep slope. We also recommend that the deck foundations be designed with an effective setback of at least 10 feet from the face of the steep west -facing slope. To maintain this recommended effective setback, the northern deck foundations will need to be embedded a minimum of five feet into the competent native soils. The southern deck supports need only reach the competent native soils. We should be retained to evaluate the deck footing subgrade soil and setback distances prior to placing the footings. Protection of the setback and steep slope areas should be performed as required by the City of Edmonds. Specifically, we recommend that the setback area and top of slope not be disturbed or modified through placement of any fill or removal of the existing vegetation. No material of any kind should be placed within the setback area or be allowed to reach the slope, such as excavation spoils, lawn clippings, debris, and soil stockpiles, Any sloping areas disturbed during construction should be planted as soon as practical to reduce the potential for erosion. The new vegetation should be maintained until it is established. Replacement of vegetation should be performed in accordance with the City of Edmonds code. Under no circumstances should water be allowed to concentrate on the slope. Deck Foundations We recommend that the new deck supports be supported on 12 -inch minimum diameter concrete piers (Sonotubes) placed within undisturbed medium dense or better native soils. We also recommend that the concrete piers maintain the recommended embedments discussed in the previous section of this letter. If undocumented fill or less dense soils are encountered at the planned footing elevation, the subgrade should be over -excavated to expose suitable bearing soil. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 9 The deck foundations should be designed in accordance with the 2006 International Building Code (IBC). Water should not be allowed to accumulate in footing excavations. All loose or disturbed soil should be removed from the footing excavation prior to placing concrete. For foundations constructed as outlined above, we recommend an allowable design axial compression capacity of 2,500 pounds for each pier to be used for determining the number and spacing of the pier supports. We should be retained to evaluate the foundation subgrade soils and embedment depths prior to placing foundation forms. USE OF THIS LETTER This letter has been prepared for Mr. Saeed Daniali and his agents for use in the planning and design of the new deck planned on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors' methods, techniques, sequences, or procedures, except as specifically described in our letter. There are possible variations in subsurface conditions between the explorations and also with time. Our letter, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the project budget and schedule. We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not deck support installation complies with our recommendations. We should be contacted a minimum of one week prior to construction activities. All people who own or occupy homes on or near hillsides should realize that landslide movements are always a possibility. The landowner should periodically inspect the slope, especially after a winter storm. If distress is evident, a geotechnical engineer should be contacted for advice on remedial/preventative measures. The probability that landsliding will occur is substantially reduced by the proper maintenance of drainage control measures at the site (the NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 10 runoff from the roofs should be led to an approved discharge point). Therefore, the homeowner should take responsibility for performing such maintenance. Within the limitations of scope, schedule and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this letter was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a paeans to identify and reduce the inherent risks to the owner. NELSON GEOTECHNICAL ASSOCIATES, INC. Geoteclu-tical Engineering Letter Daniali Deck Replacement Edmonds, Washington NGA File No. 833710 October 21, 2010 Page 11 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further inforinatioti, please call. Sincerely, Bala Dodoye-Alali Project Geologist Khaled M. Shawish, PE Principal BD:KMS:kmn Three Copies submitted Six Figures Attached 6130 rn 6130 f t I Mm t 7t -mapquest P(j,f3iET al "A 1Vr;1 J 'j Uot to ScRde 1."Mi"I P1 'I WI: I I 11 ;,ores rti 1 74 1 h f' I 1 201 Maj) toe Ploitiophs @2010 NAV E, 0 , lurteulp%� I Project Number NELSON GEOTECHNICAL No. Date I Revision By CIK ,1 . ASSOCIATES, INC. 833710 Daniali Deck Replacement NG 1 10115110 orlqMal DRN 8D Vicinity Map 'GEOTECHNECAL ENGINEV.RS & GEOLOGoSTS Figure 1 t7311-13 thA—NE.A'kQ V*,d'-1W,WA08Q72 W-=,T)Ch.i-15MW576DS E 'M :j L) z -2 roject Number 833710 Daniali Deck Replacement Site Plan NELSON GEOTECHNICAL .00 NGA '—�— ire SSOCIATeS, INC. GEOTECHNICAL ENGINEERS 8c GEOLOGISTS Figure 2 11311, 11111, — NC A,1- S-h—,h C—ky �426}3V-1669 W-d—flbWA MV2 0� �5L65-74'95) 4a,,. 1 Fax 4W�251 0 "A'-1—gsM—h — 1 110115/10 1 Cngjnaq' I DPN I BD UNIFIED SOIL CLASSIFICATION GROUP MAJORIVI I GROUPNAME SYMBOL CLEAN GW WELL-GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL 1- POORLY-GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO. 4SIEVE WITH FINES C CLAYEY GRAVEL SAND CLEAN Sw WELL-GRADED SAND, FINE TO COARSE SAND SAN SID POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE, THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND Sm SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY H SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FLAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIIGIHLY ORGANIC SOILS RT PEAT NOTES:. 1) Field classification is based on visual SOIL MOISTURE MODIIFIIERS: examination of soil In General accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using Laboratory tests is based on ASTM D 24813-93. Moist - Dump, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soilis obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. ry Project N9U�If I3G1 NELSG1N EOTEC�INICAd No. Este Revision fiy CK Y� I f7 X33710 LJaniali Deck Replacement � °i"' A " .ASSOCIATES, INC. 1 10115110 Orug�nal DPN sl] r.7dII CIaSSI is tan ChaGEOTECHNICAL ENGINEERS & GEOLOGISTS 1I 1]31413 J Ar NE grypp $11h,,6h C1 1Y (926)9'37-1669 FigureVNo Cle. WA h 98072 W Y heslGh (09) BE 16 Q420) 486-1 P1'69 f Ear: 481,25510 tnnn.ue,s<enggeoM_G,e:uroi LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION HAND AUGER ONE 0.0-0.5 DARK BROWN TO BLACK, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANICS (LOOSE, MOIST) (TOPSOIL) 0.5-2.0 DARK BROWN, SANDY SILT WITH GRAVEL AND TRACE ORGANICS (LOOSE, MOIST) (FILL) 2.0-4.0 BROWN -GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) SAMPLE WAS COLLECTED AT 3.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER MET REFUSAL AT 4.0 FEET ON 10/12/10 HAND AUGER TWO 0.0-1.0 CRUSHED ROCK (FILL) 1.0-6.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANICS (LOOSE, MOIST) (FILL) 6.0-7.0 BROWN -GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) SAMPLES WERE COLLECTED AT 3.0 AND 6.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER MET REFUSAL AT 7.0 FEET ON 10/12/10 HAND AUGER THREE 0.0-2.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANICS (LOOSE, MOIST) (FILL) 2.0-4.0 BROWN -GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER MET REFUSAL AT 4.0 FEET ON 10/12/10 HAND AUGER FOUR 0.0-1.5 BLACK, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANICS (LOOSE, MOIST) (FILL) 1.5-2,5 RED -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) SAMPLES NOT WERE COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER MET REFUSAL AT 2.5 FEET ON 10/12/10 LSB:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO: 833710 FIGURE 6