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REVIEWED BLD2023-0401+Geotechnical_Report+3.31.2023_5.22.33_PM+3457669OEOTECH CONSULTANTS, INC_ Bernard Shen & Catherine Federman 417 N.W. 32"d Street Seattle, Washington 98177 via email: bernard8shen(aMotmail.cam & cefer8(a-hotmail.com Subject: Geotechnical Report Proposed New Residence 23310 Humber Lane Edmonds, Washington Greetings: 2401 1 Oth Ave E Seattle, Washington 98102 (425)747-5618 January 19, 2023 J N 23011 RECEIVED Apr 13 2023 BLD2023-0401 ................................................. REVIEWED BY CITY OF EDMONDS BUILDING DEPARTMENT This report presents our geotechnical conclusions related to the planned redevelopment of the subject property. On January 18, 2023, the undersigned principal geotechnical engineer visited the subject site to assess both the surface and subsurface conditions. In addition to this work, we have reviewed published geologic maps, as well as the mapping by the Natural Resources Conservation Service (NRCS). Based on the information provided by Patricia Brennan Architects, the existing residence will be demolished and be replaced with a new home having a larger footprint. The west wall of the new house will be close to the western extent of the existing residence, and the new house's footprint will extend eastward from the existing. The new home will be two stories in height. No basement or deep below -grade spaces will be included. SITE CONDITIONS The existing house is one story in height and is centrally located on the lot. A paved driveway extends from Humber Lane through the southern portion of the property to the garage located in the south end of the current residence. The remainder of the property is covered primarily with grass, with some landscape beds. There are two small detached sheds located in the southwestern portion of the lot. There are no steep slopes on, or near, the subject property. Consistent with the neighboring properties, the natural ground surface slopes only gently downward toward the west from the edge of Humber Lane. It appears that a storm sewer and associated catch basins are located along the western edge of paved portion of Humber Lane. The neighboring properties are similarly developed with one-story homes. During our January 18, 2023 site visit, we assessed the subsurface conditions in two test holes located as indicated on the attached Site Exploration Plan. Logs of the test pits are also attached. Published geologic maps indicate that the site and surrounding area are underlain by glacial till, a glacially -compressed mixture of gravel, silt, and fine-grained sand. The natural soil conditions encountered in the test holes consist of topsoil overlying weathered, slightly gravelly, silty sand. GEOTECH CONSULTANTS, INC. Shen & Federman A 23011 January 19, 2023 Page 2 Beneath this thin weathered layer is dense, slightly gravelly, silty sand that has been glacially compressed, and which is referred to as glacial till. This is consistent with the mapped geology for the area. A layer of fill was encountered over the native soils in the western test hole (Test Pit 2). We expect that fill may be encountered elsewhere around the existing house, resulting from the excavation and grading that occurred during the original site development. The test holes were conducted following several months of wet weather. Shallow seepage perched on top of the glacial till, which is relatively impervious, was observed. This is a common occurrence in the Pacific Northwest on properties where glacial till or other impervious soils lie close to the ground surface. CONCLUSIONS AND RECOMMENDATIONS Based on the conditions observed in the test holes, it is our professional opinion that conventional foundations can be utilized for the proposed residence. All footing areas will have to be excavated down to the dense, native, glacially -compressed soil. It is possible that this may require overexcavation below the planned footing grades. We recommend that the footings be excavated using a smooth bucket, in order to prevent the subgrade disturbance that can result from the teeth on an excavator's bucket. Where overexcavation below the planned footing grades is necessary, the additional excavation can be backfilled to the planned footing grade using compacted quarry spalls or railroad ballast rock. In wet conditions, the footing subgrades should be protected with a layer of clean crushed gravel, in order to prevent disturbance and softening of the bearing soils during the placement of foundation forms and rebar. There are no geologically critical areas on, or near, the site. The potential for future slope movement is nonexistent and the glacially -compressed soils do not present a seismic hazard. No significant retaining walls are expected for the new project. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas until house excavation is are lowered below the surrounding grades. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. As with most projects, it may be necessary to implement additional erosion control measures to address site and weather conditions during the earthwork and construction. If crawl spaces or interior slab floors are lower than the surrounding grade, footing drains should be provided. A typical footing drain detail is attached. Given the impermeable nature of the dense soils, it is not uncommon for shallow seepage to enter crawl spaces. As a result, it would be prudent to provide a layer of gravel and perforated pipes under the vapor retarder/barrier, in addition to the perimeter footing drains. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site class within 100 feet of the ground surface is best represented by Site Class Type D (stiff soil). GEOTECH CONSULTANTS, INC. Shen & Federman JN 23011 January 19, 2023 Page 3 The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) be evaluated for the peak ground acceleration of the Maximum Considered Earthquake (MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring in a 50-year period). The dense, glacially -compressed soil that will support the foundations is not susceptible to seismic liquefaction under the ground motions of the MCE. CONVENTIONAL FOUNDATIONS The proposed residence can be supported on conventional continuous and spread footings bearing on undisturbed, dense soil, or on compacted crushed rock structural fill placed above this competent native soil. Prior to placing structural fill beneath foundations, the excavation should be observed by the geotechnical engineer or building inspector to document that adequate bearing soils have been exposed. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand. Depending on the encountered soil conditions, overexcavation may be required below the footings to expose competent native soil. Unless lean concrete is used to fill an overexcavated hole, the overexcavation must be at least as wide at the bottom as the sum of the depth of the overexcavation and the footing width. For example, an overexcavation extending 2 feet below the bottom of a 2-foot-wide footing must be at least 4 feet wide at the base of the excavation. If lean concrete is used, the overexcavation need only extend 6 inches beyond the edges of the footing. An allowable bearing pressure of 2,500 pounds per square foot (psf) is appropriate for footings supported on competent native soil. A one-third increase in this design bearing pressure can be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil will be less than one inch, with differential settlements on the order of one -half -inch in a distance of 25 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, well -compacted fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE Coefficient of Friction 0.40 Passive Earth Pressure 300 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the Equivalent Fluid Density. GEOTECH CONSULTANTS, INC. Shen & Federman JN 23011 January 19, 2023 Page 4 If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. The above ultimate values for passive earth pressure and coefficient of friction do not include a safety factor. EVALUATION OF INFILTRATION FEASIBILITY The dense soil known to underlie this area, and which was observed in the test holes is glacially compressed. Our review of published geologic maps confirms that the near -surface geologic unit in this area is glacial till (aka hardpan), which is consistent with the observed soil conditions. There are no large or continuous pore spaces in the glacial till soils that can transmit water. This soil is essentially impermeable, preventing downward percolation of water, which often causes a perched water table to form following extended heavy rainfall. A 1997 study published by U.S. Geologic Survey (USGS) in cooperation with the Washington Department of Ecology (WDOE) determined the infiltration capacity of various Washington till soils to vary between 0.0005 and 0.005 inches/hour. We have found similar extremely low infiltration rates in Pilot Infiltration Tests (PITS) our firm has conducted in glacial till soils. Often, the impermeable nature of the glacial till causes a shallow seasonal perched water table to form where the ground surface is not covered by an impervious layer. This is a common problem in the wet season throughout the Pacific Northwest, and shallow seepage is evident in the test pits conducted on the site. Our review of the Web Soil Survey website run by the Natural Resources Conservation Service (NRCS) indicates that the soil mapping is Alderwood-Urban land complex. This is consistent with the soil conditions observed in the test pit. The following information is from the Web Soil Survey, and also indicates that the limiting layer has a very low permeability, far below what is allowed for design of infiltration systems. 5—Alderwood-Urban land complex, 2 to 8 percent slopes Map Unit Composition • Alderwood and similar soils: 60 percent • Urban land: 25 percent • Minor components: 15 percent • Estimates are based on observations, descriptions, and transects of the mapunit. Properties and qualities • Slope: 2 to 8 percent • Depth to restrictive feature: 20 to 40 inches to densic material • Drainage class: Moderately well drained • Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) • Depth to water table: About 18 to 36 inches • Frequency of flooding: None • Frequency of ponding: None • Available water supply, 0 to 60 inches: Low (about 3.0 inches) Considering the observed soil conditions and the presence of seasonal shallow perched groundwater, it is our professional opinion that infiltration of concentrated storm water runoff is infeasible for this site. Attempting to infiltrate or disperse stormwater on the site would only increase the potential for surface and subsurface drainage problems on neighboring properties. GEOTECH CONSULTANTS, INC. Shen & Federman January 19, 2023 Please contact us if you have any questions regarding this report. Respectfully submitted, GEOTECH CONSULTANTS, INC. ��-G R- McGI,�, e OF WASy� '1'lcP AL 1 /19/2023 Marc R. McGinnis, P.E. Principal Attachments: Vicinity Map, Site Exploration Plan, Test Pit Logs, Footing Drain Detail cc: Patricia Brennan Architects via email. office(a�.patriciabrennanarchitects.com MRM:kg J N 23011 Page 5 GEOTECH CONSULTANTS, INC. GEOTECH CONSULTANTS, INC. (Source: Microsoft MapPoint, 2013) VICINITY MAP 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan.2023 I 1 Legend: TP-1 Test Hole Location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan. 2023 No Scale 1 2 TEST PIT 1 Depth feet Soil Description 0 — 1.0 Crushed Rock over Topsoil 1.0 — 2.0 Orangish-bro.°, n. slightly gravelly. silty SAND. fine-grained, very moist. loose 2.0 — 3.0 Grayish-bro�,,.rn, slightly gravelly, silty SAND, fine-grained, very moist, dense Glacial Till) Test Hole v,as terminated at a depth of 3.0 feet on January 17, 2023. Perched ground ,ater seepage ,%as observed at 2.0 feet. TEST PIT 2 Depth feet Soil Description 0-2.0 Bro-:,,m, slightly gravelly, silty SAND. fine-grained. very moist. loose i FILLI 2.0 — 2.5 Old Topsoil 2.5 — 3.5 Orangish-bro,.,:,n, slightly gravelly, silty SAND, fine-grained, very moist. loose 3.5 Grayish-bro,.^:,n, slightly gravelly, silty SAND, fine-grained, very moist, dense iGlacial Till) Test Hole ✓as terminated at a depth of 3.5 feet on January 18, 2023. Perched ground,• ater seepage was encountered at a depth of 3.5 feet. GEOTECH CONSULTANTS, INC. TEST PIT LOGS 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan.2023 1 1 3 Slope backfill away from foundation. Provide surface drains where necessary. 4" min. Tightline Roof Drain (Do not connect to footing drain) Backfill (See text for requirements) O c� Nonwoven Geotextile Filter Fabric O LL Possible Slab oo.a�-000.a� 000.a�0 .00a ° o°< 0 0 0 0 0 "Oo� oO oO oO oO oOo oo.00a 0'0� o O ° Q' aQ o 0 O L�IL)00(� �-o o� 000 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) a.o.o 0 o.a.o-a o a.o:o-o 0 o.a:o-o 0 o.a 00° ` oO' 00 ` oO' 0o0 ` 00, 0000°° boa 0'o a�oa 0 0000�00 0 000�00 0 000�c o,00q °o ° Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. TYPICAL FOOTING DRAIN 23310 Humber Lane Edmonds, Washington Job No: Date: Plate. 4 23011 Jan.2023