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19-233 834 Walnut St GeoRpt.pdfPanGEE) r Geotechnical & Earthquake Engineering Consultants July 29, 2019 Project No. 19-233 Ms. Nina Franey Edge Design & Build 2107 NW 201" Street Shoreline, WA 98177 Subject: Geotechnical Engineering Report Proposed Stairway and Retaining Wall Improvements 834 Walnut Street, Edmonds, Washington Dear Ms. Franey, As requested, PanGEO Inc. (PanGEO) completed a geotechnical engineering study to assist the design team for the proposed stairway and retaining wall improvements located at 834 Walnut Street in the City of Edmonds, Washington. This study was performed in general accordance with our mutually agreed scope of work outlined in our proposal dated July 12, 2019, and subsequently approved by you on July 15, 2019. Our service scope included reviewing readily available geologic and geotechnical data, excavating three hand borings, conducting a site reconnaissance, performing engineering analysis, and developing the conclusions and recommendations presented in this report. SITE AND PROJECT DESCRIPTION The project site is located at 834 Walnut Street in the City of Edmonds, Washington (see Figure 1, Vicinity Map). The subject property is an approximately 7,085 square foot, rectangular -shaped lot. It is bordered to the north by Walnut Street, and to the other three sides by existing single- family residences. The site is currently occupied by a one-story house with a daylight basement. The site grade generally slopes down from east to west with an average gradient of about 15 percent (see Plates I and 2). 3213 Eastlake Ave E, Ste B Seattle, WA 98102 Tel (206) 262-0370 Fax (206) 262-0374 Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 . . . . . . . . . . . Plate 1. View of the existing stairway and rockery on the Plate 2. View of the east side yard above the stairway, northeast comer of the house, looking south from Walnut looking south from northeast comer of the house. Street. We understand that you plan to remodel the interior of the house. We also understand that, as part of the remodel project, the concrete stairway and modular block wall located to the northeast comer of the house will be reconstructed. A new entry is also planned that will include new footings. We anticipate that temporary excavations for the proposed improvements will be up to about 4 feet deep. According to the City of Edmonds GIS maps, the site is within an erosion and landslide geologic hazards area. The conclusions and recommendations outlined in this report are based on our understanding of the proposed improvements, which is in turn based on the project information provided to us. If the above project description is substantially different from your proposed improvements, or if the project scope changes, PanGEO should be consulted to review the recommendations contained in this study and make modifications, if needed. SITE GEOLOGY Based on a review of the Composite Geologic Map of the Sno-King Area (Booth, et al. 2004) the primary geologic unit in the vicinity of the site is Advance Outwash (Map Unit Qva). Advance Outwash consists of a silt and sand soil unit deposited by meltwater streams emerging from an advancing glacier. 19-233 834 Walnut St GeoRpt Page 2 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 SUBSURFACE EXPLORATION AND CONDITIONS SUBSURFACE EXPLORATION Our subsurface exploration for the current study consisted of excavating three hand borings (HB- I through HB-3) at the site on July 17, 2019, using a hand auger. The approximate hand boring locations were taped in the field from on -site features, and are plotted on Figure 2. The hand borings were excavated to depths ranging from about 21/2 to 7 feet below the existing grade. SOIL The hand borings advanced at the site generally encountered about 3 feet of loose fill overlying medium dense to dense sand (Advance Outwash). The soils encountered in the hand borings are generally consistent with the mapped geology at the site. The following is a brief description of the soils encountered in the test borings drilled at the site. Please refer to the summary hand boring logs in Appendix A for additional details. UNIT 1: Fill — This unit was encountered in all three hand borings. The fill encountered generally consisted of loose sand with rounded to angular gravel and cobbles, and trace silt. This soil unit extended to about 3 feet below the existing ground surface in HB- I and HB-3, respectively, and to the refusal depth of 21/2feet in HB-2. UNIT 2: Advance Outwash (Qva) — Below the fill, HB-I and HB-3 encountered medium dense to dense, fine to medium sand with trace silt, that extended to the termination depth of 7 and 5 feet in HB-I and HB-3, respectively. This unit was not encountered in HB-2. We interpreted this unit as mapped Advance Outwash deposits. GROUNDWATER Groundwater was not observed in the hand borings within the depths excavated during our field exploration. It should be noted that groundwater elevations and seepage rates are likely to vary depending on the season, tidal fluctuation, local subsurface conditions, and other factors. Groundwater levels and seepage rates are normally highest during the winter and early spring (typically October through May). PREVIOUS SUBSURFACE EXPLORATION As a part of our study, we also reviewed readily available previous geotechnical explorations in the site vicinity. Specifically, the following previous exploration was reviewed: 19-233 834 Walnut St GeoRpt Page 3 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Logs of Hand Holes (HH-I and HH-2), 842 Walnut Street, Edmonds, Washington (The Galli Group, 2007). The previous hand holes were excavated on the adjacent property to the east. The hand holes encountered sand with silt and gravel, and bearing soil was encountered at about I and 2 feet in HH-1 and HH-2, respectively. In general, the results of the previous exploration are consistent with our current subsurface exploration- The previous geotechnical report is included as Appendix B for reference. Based on review of the previous hand holes and our experience in the area, the soil conditions are relatively uniform in the project area. GEOLOGICALLY HAZARDOUS AREAS ASSESSMENT Based on a review of the City of Edmonds GIS map and the Edmonds Community Development Code (ECDC), the subject site is within a Geologically Hazardous Area for erosion and landslide hazards. The rockery and the slopes above it along the east side of the driveway and east property line is considered as a steep slope (greater than 40% slope). LANDSLIDE HAZARDS EVALUATION Based on review of the topographic survey map provided to us and our field observations, the site generally slopes down from the east to the west with an average slope gradient of about 15 percent. However, the rockery and slopes above the rockery along the east side of driveway, which is approximately 12 feet in height, appear to meet the steep slope definition (40% or greater slopes). Based on the soil data on the east adjacent site and presence of the rockery, it is our opinion that this steep slope is the result of the previous grading due to street and site development. It is also our opinion that the risk for the potential future landslide is considered low, and the site is not considered as a landslide hazard area. A site reconnaissance of the subject property was conducted on July 17, 2019. During our site reconnaissance, we did not observe obvious evidence of past slope instability or ground movement at the subject site. Based on our field observations and the results of subsurface data at the subject site and adjacent property to the east, in our opinion, the subject site appears to be globally stable in its current configuration. Furthermore, it is our opinion that the proposed stairway and wall improvements as currently planned is feasible from a geotechnical engineering 19-233 834 Walnut St GeoRpt Page 4 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 standpoint. It is our further opinion that the proposed improvements as currently planned will not adversely affect the overall stability of the site or adjacent properties, provided the recommendations outlined herein are followed and the proposed development is properly designed and constructed. EROSION HAZARDS EVALUATION According to USDA Soil Conservation Service Map, the site soil is the Alderwood Urban land complex. Based on the soils encountered in the hand borings and site topography, the near - surface site soils are anticipated to exhibit slight to moderate erosion potential. However, due to very minor ground disturbance and excavations planned, in our opinion, the potential erosion hazards at the site can be effectively mitigated with the best management practice during construction and with properly designed and implemented landscaping for permanent erosion control. During construction, the temporary erosion hazard can be effectively managed with an appropriate erosion and sediment control plan, including but not limited to installing silt fence at the construction perimeter, limiting removal of vegetation to the construction area, placing rocks or hay bales at the disturbed/traffic areas and on the downhill side of the project, covering stockpile soil or cut slopes with plastic sheets, constructing a temporary drainage pond to control surface runoff and sediment trap if needed, placing rocks at the construction entrance, etc. Permanent erosion control measures should include establishing vegetation, landscape plants, and hardscape established at the end of project. 23.80.060 DEVELOPMENT STANDARDS — GENERAL REQUIREMENTS According to ECDC 23.80.060, alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. 19-233 834 Walnut St GeoRpt Page 5 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Based on the site subsurface information and our understanding of the current plans, it is our opinion that the proposed project meets the above conditions, and will not have adverse impacts to the subject and surrounding properties during and after construction, provided that project is constructed in accordance with the approved plans and commonly accepted practice. 23.80.070 DEVELOPMENT STANDARDS — SPECIFIC HAZARDS The subject site is considered an erosion hazard area. However, based on the soil conditions and proposed improvements, it is our opinion that building setback and buffer distance are not required for the currently proposed improvements. ECDC 23.80.070 states that alterations of an erosion or landslide area hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties-, and c. Such alterations will not adversely impact other critical areas. In our opinion, the proposed stairway and wall improvements as currently planned meet the above conditions. GEOLOGIC HAZARDS MITIGATIONS Based on the results of our evaluation of the potential geologic hazards at the site, it is our opinion that no specific mitigation, other than the erosion measure discussed above, is required for the proposed improvements. GEOTECHNICAL DESIGN RECOMMENDATIONS SEISMIC DESIGN PARAMETERS The Table 1 on page 7 provides seismic design parameters for the site that are in conformance with the 2015 edition of the International Building Code (IBC), which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2015 USGS seismic hazard maps. The spectral response accelerations were obtained 19-233 834 Walnut St GeoRpt Page 6 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. Table 1 — 2015 IBC Seismic Design Parameters Spectral Spectral Design Spectral Site Site Acceleration Acceleration at Coefficients Response Class at 0.2 sec. (g) 1.0 sec. (g) Parameters Fa F, SDS SD1 Ss S1 D 1.269 0.497 1.00 1.50 0.846 0.498 FOUNDATIONS Based on the planned stairway, retaining wall, and entry footing elevations, it is our opinion that the proposed improvements may be supported on conventional footings bearing on the undisturbed native sand or compacted structural fill placed on the native sand. The foundation soils should be compacted to a firm/dense condition. Soil Bearing Pressure We recommend that an allowable soil bearing pressure of 2,000 pounds per square foot (psf) be used to size the new footings. The recommended allowable bearing pressure is for dead plus live loads. For allowable stress design, the recommended bearing pressure may be increased by one- third for transient loading, such as wind or seismic forces. Continuous and individual spread footings should have minimum widths of 18 and 24 inches, respectively. In designing the footings, the shape of footings will need to be considered in regard to the available space for temporary excavations. Where space may be limited for an unsupported open cut, it may be necessary to use L-shaped perimeter footings in order to conserve space and to allow the temporary excavations to be made within the property limits. Foundation Performance Footings designed and constructed in accordance with the above recommendations should experience total settlement of less than one inch and differential settlement of about 1/2 inch. Most of the anticipated settlement should occur during construction as dead loads are applied. 19-233 834 Walnut St GeoRpt Page 7 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Lateral Resistance Lateral loads on the structures may be resisted by passive earth pressure developed against the embedded faces of the foundation system and by frictional resistance between the bottom of the foundation and the supporting subgrade soils. For footings bearing on the firm native soil or compacted sand/structural fill, a frictional coefficient of 0.3 may be used to evaluate sliding resistance developed between the concrete and the compacted subgrade soil. Passive soil resistance may be calculated using an equivalent fluid weight of 250 pcf, assuming properly compacted structural fill will be placed against the footings. The above values include a factor of safety of 1.5. Unless covered by pavements or slabs, the passive resistance in the upper 12 inches of soil should be neglected. Footing Excavation and Subgrade Preparation All footing subgrade should be carefully prepared. The footing subgrade should be compacted to a dense and unyielding condition prior to concrete pour. Depending on the footing subgrade, foundation over -excavation may be needed to remove the existing fill and disturbed outwash sand. Any loose/soft footing subgrade soil or subgrade soil that cannot be compacted to a dense condition should be removed from the footing subgrade and backfilled with lean -mix concrete, Control Density Fill (CDF) or structural fill. Foundation excavations and subgrade conditions should be observed by PanGEO to confirm that the exposed subgrade is consistent with the expected conditions and adequate to support the proposed building. CONCRETE SLAB -ON -GRADE Conventional slab -on -grade concrete floors, if needed, may be used for this project. We recommend that floor slabs be supported on the recompacted sand or structural fill placed on recompacted on -site sand. If loose or soft soils are encountered at the slab subgrade elevation that cannot be adequately compacted, the loose or soft soil should be over -excavated to competent native and replaced with compacted structural fill, such as WSDOT Gravel Borrow or approved equivalent. The concrete slab -on -grade floors should be underlain by at least 4 inches of capillary break, which consists of free -draining, clean crushed rock or well -graded gravel compacted to a firm and unyielding condition. The capillary break material should have no more than 20 percent 19-233 834 Walnut St GeoRpt Page 8 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 passing the No. 4 sieve and less than 5 percent by weight of the material passing the U.S. Standard No. 100 sieve. We also recommend that a minimum I 0-mil polyethylene vapor barrier be placed below the proposed basement slab. RETAINING AND BASEMENT WALL DESIGN PARAMETERS Retaining and basement walls should be properly designed to resist the lateral earth pressures exerted by the soils behind the wall. Proper drainage provisions should also be provided behind the walls to intercept and remove groundwater that may be present behind the walls. Our geotechnical recommendations for the design and construction of the retaining/basement walls are presented below. Lateral Earth Pressures Concrete cantilever walls should be designed for an equivalent fluid pressure of 35 pcf for level backfills behind the walls assuming the walls are free to rotate. If walls are to be restrained at the top from free movement, such as basement walls, equivalent fluid pressures of 45 pcf should be used for level backfills behind the walls. Walls with a maximum 2H: 1 V backslope should be designed for an active and at rest earth pressure of 45 and 55 pcf, respectively. Permanent walls should be designed for an additional uniform lateral pressure of 7H psf for seismic loading, where H corresponds to the buried depth of the wall. The recommended lateral pressures assume that the backfill behind the wall consists of a free draining and properly compacted fill with adequate drainage provisions. Surcharge Surcharge loads, where present, should also be included in the design of retaining walls. We recommend that a lateral load coefficient of 0.3 be used to compute the lateral pressure on the wall face resulting from surcharge loads located within a horizontal distance of one-half wall height. Lateral Resistance Lateral forces from seismic loading and unbalanced lateral earth pressures may be resisted by a combination of passive earth pressures acting against the embedded portions of the foundations and by friction acting on the base of the foundations. Passive resistance values may be determined using an equivalent fluid weight of 250 pcf. This value includes a factor of safety of 1.5, assuming the footing is poured against dense native sand, re -compacted on -site sandy soil or 19-233 834 Walnut St GeoRpt Page 9 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 properly compacted structural fill adjacent to the sides of footing. A friction coefficient of 0.3 may be used to determine the frictional resistance at the base of the footings. The coefficient includes a factor safety of 1.5. Wall Drainage Provisions for wall drainage should consist of a 4-inch diameter perforated drainpipe behind and at the base of the wall footings, embedded in 12 to 18 inches of clean crushed rock and pea gravel wrapped with a layer of filter fabric. Where applicable, in -lieu of conventional footing drains, weep holes (2" diameter of 10 feet on center) may be used for site retaining walls. A minimum 18-inch wide zone of free draining granular soils (i.e. pea gravel or washed rock) is recommended to be placed adjacent to the wall for the full height of the wall. Alternatively, a composite drainage material, such as Miradrain 6000, may be used in lieu of the clean crushed rock or pea gravel. The drainpipe at the base of the wall should be graded to direct water to a suitable outlet. Wall Bac4fill In our opinion, the clean sand at the site may be used as wall backfill, provided it can be compacted to a dense condition. Imported wall backfill, if needed, should consist of free draining granular material, such as WSDOT Gravel Borrow. In areas where the space is limited between the wall and the face of excavation, pea gravel or clean crushed rock may be used as backfill without compaction. Wall backfill should be moisture conditioned to within about 3 percent of optimum moisture content, placed in loose, horizontal lifts less than 8 inches in thickness, and systematically compacted to a dense and relatively unyielding condition and to at least 95 percent of the maximum dry density, as determined using test method ASTM D 1557. Within 5 feet of the wall, the backfill should be compacted with hand -operated equipment to at least 90 percent of the maximum dry density. CONSTRUCTION CONSIDERATIONS SITE PREPARATION Site preparation for the proposed project includes removing existing stairway, stripping and clearing of surface vegetation and excavations to the design subgrade. All stripped surface 19-233 834 Walnut St GeoRpt Page 10 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 materials should be properly disposed off -site or be "wasted" on site in non-structural landscaping areas. Following site clearing and excavations, the adequacy of the subgrade where structural fill, foundations, slabs, or pavements are to be placed should be verified by a representative of PanGEO. The subgrade soil in the improvement areas, if recompacted and still yielding, should also be over - excavated and replaced with compacted structural fill or CDF/lean-mix concrete. TEmpoRARY EXCAVATIONS As currently planned, the proposed construction will require excavations up to 4 feet below the existing grade. We anticipate the excavations to mainly encounter loose to dense sand. All temporary excavations should be performed in accordance with Part N of WAC (Washington Administrative Code) 296-155. The contractor is responsible for maintaining safe excavation slopes and/or shoring. All temporary excavations deeper than a total of 4 feet should be sloped or shored. Based on the soil conditions at the site, for planning purposes, it is our opinion that temporary excavations for the proposed construction may be sloped IH: IV (Horizontal:Vertical) or flatter. Based on our current understanding of the anticipated building layout and finished floor elevation, it appears that sufficient space is available for unsupported open cuts. Where space may be limited, the use of L-shaped footings may be required to conserve space for the temporary cuts. In event that sufficient space is not available for unsupported open cuts, temporary shoring will be needed to support the temporary excavations. The temporary excavations and cut slopes should be re-evaluated in the field during construction based on actual observed soil conditions, and may need to be flattered in the wet seasons and should be covered with plastic sheets. We also recommend that heavy construction equipment, building materials, excavated soil, and vehicular traffic should not be allowed within a distance equal to 1/3 the slope height from the top of any excavation. MATERIAL REUSE In the context of this report, structural fill is defined as compacted fill placed under footings, concrete stairs and landings, and slabs, or other load -bearing areas. In our opinion, the on -site sand is poorly graded and will be difficult to compact to a dense condition. As such, on -site sand is not suitable to be used as structural fill, but can be used as wall backfill and general fill in the 19-233 834 Walnut St GeoRpt Page I I PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 non-structural areas. Structural fill, if needed, should consist of imported, well -graded, granular material, such as WSDOT Gravel Borrow, or approved equivalent. Well -graded recycled concrete may also be considered as a source of structural fill. Use of recycled concrete as structural fill should be approved by the geotechnical engineer. If use of the on -site sofl is planned, the excavated soil should be stockpiled and protected with plastic sheeting to prevent softening from rainfall in the wet season. STRUCTURAL FILL PLACEMENT AND COMPACTION Structural fill should be moisture conditioned to within about 3 percent of optimum moisture content, placed in loose, horizontal lifts less than 8 inches in thickness, and systematically compacted to a dense and relatively unyielding condition and to at least 95 percent of the maximum dry density, as determined using test method ASTM D 1557. Depending on the type of compaction equipment used and depending on the type of fill material, it may be necessary to decrease the thickness of each lift in order to achieve adequate compaction. PanGEO can provide additional recommendations regarding structural fill and compaction during construction. WET WEATHER EARTHWORK In our opinion, the proposed site construction may be accomplished during wet weather (such as in winter) without adversely affecting the site stability. However, earthwork construction performed during the drier summer months likely will be more economical. Winter construction will require the implementation of best management erosion and sedimentation control practices to reduce the chance of off -site sediment transport. Some of the site soils contain a high percentage of fines and are moisture sensitive. Any footing subgrade soils that become softened either by disturbance or rainfall should be removed and replaced with structural fill, Controlled Density Fill (CDF), or lean -mix concrete. General recommendations relative to earthwork performed in wet conditions are presented below: Site stripping, excavation and subgrade preparation should be followed promptly by the placement and compaction of clean structural fill or CDF; The size and type of construction equipment used may have to be limited to prevent soil disturbance; 19-233 834 Walnut St GeoRpt Page 12 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 • The ground surface within the construction area should be graded to promote run-off of surface water and to prevent the ponding of water; • Geotextile silt fences should be strategically located to control erosion and the movement of soil; • Structural fill should consist of less than 5% fines; and • Excavation slopes should be covered with plastic sheets. SURFACE DRAINAGE AND EROSION CONSIDERATIONS Surface runoff can be controlled during construction by careful grading practices. Typically, this includes the construction of shallow, upgrade perimeter ditches or low earthen berms in conjunction with silt fences to collect runoff and prevent water from entering excavations or to prevent runoff from the construction area from leaving the immediate work site. Temporary erosion control may require the use of hay bales on the downhill side of the project to prevent water from leaving the site and potential storm water detention to trap sand and silt before the water is discharged to a suitable outlet. All collected water should be directed under control to a positive and permanent discharge system. Permanent control of surface water should be incorporated in the final grading design. Adequate surface gradients and drainage systems should be incorporated into the design such that surface runoff is directed away from structures. Potential problems associated with erosion may also be reduced by establishing vegetation within disturbed areas immediately following grading operations. ADDITIONAL SERVICES To confirm that our recommendations are properly incorporated into the design and construction of the proposed addition, PanGEO should be retained to conduct a review of the final project plans and specifications, and to monitor the construction of geotechnical elements. The City of Edmonds, as part of the permitting process, will also require geotechnical construction inspection services. PanGEO can provide you a cost estimate for construction monitoring services at a later date. We anticipate that the following additional services will be required: Review final project plans and specifications Verify implementation of erosion control measures; 19-233 834 Walnut St GeoRpt Page 13 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Verify soil bearing; Confirm the adequacy of the compaction of structural backfill; and 9 Other consultation as may be required during construction. Modifications to our recommendations presented in this report may be necessary, based on the actual conditions encountered during construction. CLOSURE We have prepared this report for Ms. Nina Franey and the project design team. Recommendations contained in this report are based on a site reconnaissance, a subsurface exploration program, review of pertinent subsurface information, and our understanding of the project. The study was performed using a mutually agreed -upon scope of work. Variations in soil conditions may exist between the locations of the explorations and the actual conditions underlying the site. The nature and extent of soil variations may not be evident until construction occurs. If any soil conditions are encountered at the site that are different from those described in this report, we should be notified immediately to review the applicability of our recommendations. Additionally, we should also be notified to review the applicability of our recommendations if there are any changes in the project scope. The scope of our work does not include services related to construction safety precautions. Our recommendations are not intended to direct the contractors' methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. Additionally, the scope of our work specifically excludes the assessment of environmental characteristics, particularly those involving hazardous substances. We are not mold consultants nor are our recommendations to be interpreted as being preventative of mold development. A mold specialist should be consulted for all mold -related issues. This report has been prepared for planning and design purposes for specific application to the proposed project in accordance with the generally accepted standards of local practice at the time this report was written. No warranty, express or implied, is made. This report may be used only by the client and for the purposes stated, within a reasonable time from its issuance. Land use, site conditions (both off and on -site), or other factors including advances in our understanding of applied science, may change over time and could materially 19-233 834 Walnut St GeoRpt Page 14 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 affect our findings. Therefore, this report should not be relied upon after 24 months from its issuance. PanGEO should be notified if the project is delayed by more than 24 months from the date of this report so that we may review the applicability of our conclusions considering the time lapse. It is the client's responsibility to see that all parties to this project, including the designer, contractor, subcontractors, etc., are made aware of this report in its entirety. The use of information contained in this report for bidding purposes should be done at the contractor's option and risk. Any party other than the client who wishes to use this report shall notify PanGEO of such intended use and for permission to copy this report. Based on the intended use of the report, PanGEO may require that additional work be performed and that an updated report be reissued. Noncompliance with any of these requirements will release PanGEO from any liability resulting from the use this report. We appreciate the opportunity to be of service. Sincerely, John A. Manke, L.G. Staff Geologist Attachments: Figure I Vicinity Map Figure 2 Site and Exploration Plan Appendix A — Summary Hand Boring Logs H. Michael Xue, P.E. Senior Geotechnical Engineer Figure A- I Terms and Symbols for Boring and Test Pit Logs Figure A-2 Log of Hand Boring HB- I Figure A-3 Log of Hand Boring HB-2 Figure A-4 Log of Hand Boring HB-3 Appendix B — Previous Geotechnical Report by The Gatti Group 19-233 834 Walnut St GeoRpt Page 15 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 REFERENCES Booth, D.B., Cox, B.F., Troost, K.G., and Shimel, S.A., 2004, Composite Geologic Map of the Sno-King Area, Central Puget Lowland, Washington, Seattle- Area Geologic Mapping Project (SGMP), University of Washington, and the United States Geological Survey (USGS), Scale 1:24,000. international Code Council, 2015, International Building Code. The Galli Group, 2007, Geotechnical Reconnaissance, Proposed Deck Addition, 842 Walnut Street, Edmonds, Washington. United States Geological Survey, Earthquake Hazards Program, Interpolated Probabalisitic GroundMotionfor the Conterminous 48 States byLatitude andLongitude, 2008 Data, accessed via: http://earthquake.usgs.gov/designmgps/�s/gpplication.php WSDOT, 2016, Standard Specificationsfor Road, Bridge and Municipal Construction, M 41-10, Washington State Department of Transportation. 19-233 834 Walnut St GeoRpt Page 16 PanGEO, Inc. .00, Picnic Point -North Lynnwood > Martha I 17 > PERRINVILLE Alderwood Mall 0 qW 196th St SW C52 - Edmonds 0 Community College 0 Edmonds Main St Lynnwood Transit Center Aldervwood Manor f 0 Woodway > f,6 I # Esperance _j 228th St SVI I 2281h st SW �Tourntlakep - ,e race Brier S� CD �D > > z Shoreline Lake Forest Park > z Y'j M M Not to Scale Proposed Stairway and Retaining Wall Improvements PmGE(D 834 Walnut Street I N C 0 R P 0 R A T E D Edmonds, WA 19-233 Reference: Google Terrain Map I VICINITY MAP 4� WALNUT,ISTREET Neq'wwt 629.M' s s -4-2L-s —s —s —4—s —s — --�—s —s —s —�-s —s —s '9)S-2 316T1 IB� SWMH SHE MW.- IWL /IV 0" 240 05o CV SIDEWA�K' '..J: p96m, o.24W orlpRop. ON ONE e COR., I M SEr REIM 1.0* , OF PROP. COR. T REMNING IMA ROCKERY DUE 70 WALL 75 —00 '10.00 '54 -258 44. 238- 4. 1' D057M &NOM . H18�11 CUMNO AR64!11D � �.IAV SQFr zv'z"O' i0o r AREA-7o&5, SQT HB-2 20.1, js HB 3 Approx. Scale 2 E/C 1 20' - 29�� -- N90*00100"w 64.97� U9 12' A IFOUND 518" REW-1 1 LS 119631' .45 W OF PROP. COR.. OON LAE Legend: Approx. Hand Boring Locations Note: Base map modified from Topographic Survey prepared by Crones Surveying, dated May 27, 2019. Proposed Stairway and P MGEe Retaining Wall Improvements SITE AND EXPLORATION PLAN 834 Walnut Street I M C 0 R P 0 R A T E D Edmonds, Washington Project No. Figure No. 19-233 2 APPENDixA SUMMARY HAND BORING LOGS RELATIVE DENSITY / CONSISTENCY SAND / GRAVEL SILT CLAY SPT Approx. Relative SPT Approx. Undrained Shear Density N-values Density Consistency N-values Strength (psf) Very Loose <4 <16 Very Soft <2 <260 Loose 4 to 10 16-36 Soft 2 to 4 260-600 Mecl 10 to 30 36-66 Med. Stiff 4 to 8 600-1000 Dense 30 to 60 66-86 Stiff 8 to 16 1000 -2000 Very Dense >60 86-100 Very Stiff 16 to 30 2000 -4000 Hard >30 >4000 UNIFIED SOIL CLASSIFICATION SYSTEM MAJOR DIVISIONS GROUP DESCRIPTIONS GW: Well -graded GRAVEL Gravel GRAVEL (<6% fines) ..... ...................................................... 50% or more of the coarse GP Poorly -graded GRAVEL fraction retained on the #4 sieve. Use dual symbols leg. ..................................... ..... ...................................................... GM Silty GRAVEL GP -GM) for 5% to 12% fines. GRAVEL (>12% fines) ..... ...................................................... GC Clayey GRAVEL ...................................................................... .............................................................. SW Well -graded SAND Sand SAND (<6% fines) ...................................................... . . . . . . 50% or more of the coarse SP Poorly -graded SAND fraction passing the #4 sieve. Use dual symbols (eg. SP-SM) ........................................... ..... ...................................................... SM Silty SAND .... for 5% to 12% fines. SAND (>12% fines) ............................................................. SC : Clayey SAND . ...................................................................... ... .......................................................... ML SILT Liquid Limit < 60 ...................................................... CL ...................................................... Lean CLAY Silt and Clay OL Organic SILT or CLAY 50%or more passing #200 sieve MH Elastic SILT Liquid Limit > 60 ... CH .................................................. Fat CLAY ........ OH ............................................. Organic SILT or CLAY ..................................................................... Highly Organic Soils ............................................................. PT : PEAT Notes: 1. Soil exploration lo s contain material descriptions based on visual observation and field tests using a system modified from the Uniform Soil Classification System (USCS). Where necessary laboratory tests have -been conducted (as noted in the "Other Tests" column), unit descriptions may include a classification. Please refer to the discussions in the report text for a more complete description of the subsurface conditions. 2. The graphic symbols given above are not inclusive of all symbols that may appear on the borehole logs. Other symbols may be used where field observations indicated mixed soil constituents or dual constituent materials. DESCRIPTIONS OF SOIL STRUCTURES Layered: Units of material distinguished by color and/or Fissured: Breaks along defined planes composition from material units above and below Slickensided: Fracture planes that are polished or glossy Laminated: Layers of soil typically 0.05 to 1mm thick, max. 1 cm Blocky: Angular soil lumps that resist breakdown Lens: Layer of soil that pinches out laterally Disrupted: Soil that is broken and mixed Interlayered: Alternating layers of differing soil material Scattered: Less than one per foot Pocket: Erratic, discontinuous deposit of limited extent Numerous: More than one per foot Homogeneous: Soil with uniform color and composition throughout BCN: Angle between bedding plane and a plane normal to core axis COMPONENT DEFINITIONS COMPONENT SIZE / SIEVE RANGE COMPONENT SIZE / SIEVE RANGE Boulder: > 12 inches Sand Cobbles: 3 to 12 inches Coarse Sand: #4 to #10 sieve (4.5 to 2.0 mm) Gravel Medium Sand: #10 to #40 sieve (2.0 to 0.42 mm) Coarse Gravel: 3 to 3/4 inches Fine Sand: #40 to #200 sieve (0.42 to 0.074 mm) Fine Gravel: 3/4 inches to #4 sieve Silt 0.074 to 0.002 mm Clay <0.002 mm TEST SYMBOLS for In Situ and Laboratory Tests listed in "Other Tests" column. ATT Atterberg Limit Test Comb Compaction Tests Con Consolidation DID Dry Density DS Direct Shear %F Fines Content GS Grain Size Perm Permeability PP Pocket Penetrometer R R-value SG Specific Gravity TV Torvane TXC Triaxial Compression UCC Unconfined Compression SYMBOLS Sample/In Situ test types and intervals OD Split Spoon, SPT H2-inch (140-lb. hammer, 30" drop) OD Spilt Spoon H3.25-inch (300-lb hammer, 30" drop) Non-standard penetration test (see boring log for details) Thin wall (Shelby) tube Grab Rock core MVane Shear MONITORING WELL Groundwater Level at time of drilling (ATD) Static Groundwater Level Cement / Concrete Seal Bentonite grout / seal Silica sand backfill Slotted tip Slough Bottom of Boring MOISTURE CONTENT Dry Dusty, dry to the touch Moist Damp but no visible water Wet Visible free water r-1 PanGEO Terms and Symbols for ' m c 0 R P 0 R A T Boring and Test Pit Logs Figure A-1 Phone: 206.262 .0370 1 Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Figure A-2: Summary Log of Hand Boring HB-1 Approximate ground surface elevation: 253 feet Depth (ft) Material Description Unit 1: Loose, brown, fine to medium SAND; occasional roots with rounded and angular gravel and cobbles; moist (Fill) 0-3 - Rounded and angular gravel and cobbles from surface to I 1/2feet - Trace gravel below I 1/2feet Unit 2: Medium dense, light brown, fine to medium SAND with silt and trace 3-7 gravel; moist (Advance Outwash) - Becomes dense at 5 feet Notes: 1. HB-1 was terminated at 7 feet below ground surface. 2. Groundwater was not observed in the hand boring. 44 Plate 1: Cuttings from about 4 feet below surface in HB-1. 19-233 834 Walnut St GeoRpt Figure A-2 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Figure A-3: Summary Log of Hand Boring HB-2 Approximate gro d surface elevation: 253 feet Deptb (ft) Material Description Unit 1: Loose, light brown, fine to medium SAND; occasional roots with 0-21/2 rounded and angular gravel and cobbles; dry (Fill) - Becomes moist at 13/4feet Notes: 3. HB-1 was ternii'nated at 2V2feet below ground surface due to refusal on gravel/cobbles. 4. Groundwater was not observed in the hand bon*ng. Plate 1: Cuttings from about I 1/2feet below surface in HB-2. 19-233 834 Walnut St GeoRpt Figure A-3 PanGEO, Inc. Ms. Nina Franey Proposed Stairway and Retaining Wall Improvements — 834 Walnut Street, Edmonds, WA July 29, 2019 Figure A-4: Summary Log of Hand Boring HB-3 Approximate gro d surface elevation: 253 feet Dep b (ft) Material Descrjpji�on Unit 1: Loose, brown, fine to medium SAND; with rounded and angular gravel and cobbles; moist (Fill) 0-3 - Rounded and angular gravel and cobbles from surface to 2 feet - Trace gravel below 2 feet Unit 2: Medium dense, light brown, fine to medium SAND with silt and 3-5 trace gravel; moist (Advance Outwash) - Becomes dense at 5 feet Notes: 5. HB-1 was terminated at 5 feet below ground surface. 6. Groundwater was not observed in the hand boring. NOW -4 Plate 1: Cuttings from about 3 feet below surface in HB-3. 19-233 834 Walnut St GeoRpt Figure A-4 PanGEO, Inc. APPENDix B Previous Geotechnical Report by The Galli Group N6 40 THMIHM? Geotechnical Consulting Jeff Quinn 842 Walnut Street Edmonds, WA 98020 0 S-ubject: Geotechnical Reconnaissance Proposed Deck Addition 842 Walnut Street Edmonds, Washington Dear Jeff- — 0'"' 40 March 13, 2007 Project 1458-01 Revised March 31, 2007 D L��31 AR PIN 06 23r,,1Y On March 7, 2007, a geotechnical engineer from The Galli Group visited the above site to perform a preliminary geotechnical site reconnaissance. The purpose of our visit was to identify the near surface soils on the project site and to assess the impact of the proposed deck addition on the steep slopes west and north of your house. This letter summarizes our research, observations and conclusions. Site Conditions The project lot is located on the south side of Walnut Street in Edmonds, Washington (see Figure 1, Vicinity Map.) The existing residence is situated on one of a series of lots created by terracing the southern side of Walnut Street as it descends westerly toward the downtown part of Edmonds. The lot is separated from the adjacent lots by rockeries and steep banks. The north side of the lot is separated from the right-of-way by a rockery varying in height from about 2 feet to about 8 feet. The total height of the rockeries or steep bank does not exceed ten feet except the northwestern comer of the lot where the height of the rockery and cut slope together is about 14 feet high. The existing two-story residence consists of a two-story structure with a daylight basement opening out to the yard on the westerly side of the residence. The house is set back approximately 32 feet from the top of the cut bank along Walnut Street and about 38 feet from the top of the bank along the western lot line (see Site Features, Figure 2). The western bank descends at a declination of about I H: IV toward the west and is well vegetated with ivy and a 5034 18t'Avenue NE, Seattle, Washington 98105 a Phone 206.525.5097 - Fax 206.525.5091 0 0 Jeff Quinn — 842 Walnut Street March 13, 2007 Revised March 31, 2007 few blackberries. A terraced yard is situated at the toe of the bank, approximately 7 vertical feet below the top of the bank. All exposed soils are either vegetated or covered with thick mulch. We did not observe any iridications of slope movement or erosion during our site visit. The existing rockery appeared to be in good condition. The site appeared stable in its current condition. Proposed Improvements The proposed elevated deck is designed to extend along the western side of the existing residence. We understand that it will extend approximately 16 feet from the existing residence and that it will be supported by joists on a post and beam foundation system. The deck posts will be supported on isolated spread footings located no more than 12 feet from the existing residence. The deck plans also include a hot tub situated at the southern end of the proposed deck. Conventional deck framing is planned to support the deck. The deck footings might conflict with the existing side sewer. Plans call for redirecting the side sewer on the property so that it will not be adversely impacted by the new footings. Site and Soil Conditions Geologic maps of the area indicate that the lot is likely underlain by advance outwash (Geologic A-4ap ofEdmonds East and Part of the Edmonds West Quadrangles, James P. Minard, 1983). Advance outwash deposits tend to consist of sand and pebbly gravel deposited in meltwater streams in front of the advancing glacier. These outwash deposits were subsequently overridden by the advancing ice and tend to appear compact and relatively stable except where exposed on steep hillsides. Where left exposed to concentrated runoff they present significant risks of erosion. We excavated two shallow hand holes to verify the depth to native soil. In each of the hand holes we encountered silty SAND with gravel, representative of the advance outwash unit. Native soil suitable for bearing was encountered at a depth of about 12 inches near the existing residence. At the top of the bank, the depth to native dense soil was about 24 inches. The existing bank on the west side of the lot appears comprised of native dense soil within the upper few feet. Logs of the hand holes are provided in the table below: TABLE 1 Logs of Hand Holes Hand Hole Soil Description Depth to Groundwater Comments Bearing HH-1 SAND with gravel; 12 inches N/A native outwash; no weathered zone moist HH-2 SAND with silt and 24inches NA near top of slope; slope likely a cut I gravel I slope; not comprised of fill 1457 JQuinn Deck Rev 0331 2 of 4 The Galli Group Jeff Quinn — 842 Walnut Street March 13, 2007 Revised March 31, 2007 A representative cross section of the bank showing the relative location of the proposed deck is shown on Figure 3. Based upon our subsurface hand holes and site observations, the bank on the west side of the lot appears comprised of native outwash soil, not fill soil. The slope face appears stable against erosion and densely vegetated with ivy and groundcover. Edmonds Community Development Code According to the Development Standards for Edmonds Community Development Code (ECDC 23.80.070 the standard 50 foot buffer may be reduced provided that the activity: 1) Will not increase the threat of the geological hazard to adjacent properties beyond the predevelopment conditions; 2) Will not adversely impact the other critical areas; 3) Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4) Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. The intent of our report has been to communicate that the proposed deck will not increase the threat to adjacent properties beyond existing conditions; that it will not adversely impact the existing slope or slope system; that hazard will remain unchanged following the deck construction, and that the impact from the proposed improvements will be maintain the current safe stability of the slope. EDCD 23.80.070 further states that the buffer may be reduced to a minimum of 10 feet, provided that the "reduction will adequately protect the proposed development, adjacent developments and uses and the subject critical area." The underlying soil is very dense within 2 feet of the surface at the top of the slope. The core of the slope is composed of dense granular soil with no evidence of seepage. The slope is stable in its current condition and shallow foundations situated more than 20 feet from the top of the slope will not at all impact the existing slope. We therefore recommend that the buffer be reduced to 10 feet. Conc(usions The site appeared stable at the time of our geotechnical -reconnaissance. We did not observe any indications of recent slope movement. Conventional spread footings can be used to support the proposed deck. The proposed development of the lot can safely proceed generally as planned without introducing significant risk to the stability of the site. Tempora?y Erosion Control Recommendations Best Management Practices commonly observed should be employed during construction. We anticipate these will include the following: I . Maintain the street free of sediment during excavation and when mobilizing equipment to and from the site. Mud and slit tracked from the site should be removed or cleaned by the contractor. 1457 JQuinn Deck Rev 0331 3 of 4 The Galli GrouD 0 0 Jeff Quinn — 842 Walnut Street March 13, 2007 Revised March 31, 2007 2. A silt fence can be erected along the southern side of the lot to prevent migration of sediment over the slope. Alternatively, the contractor can simply maintain a vegetated path of at least 15 feet along the top of the bank to prevent silt -laden runoff from impacting the slope. Foundation Recommendations I . We recommend supporting the deck posts on conventional concrete footings bearing on native, undisturbed glacial soils. The soils must be free from organics and undisturbed by previous excavation activity (avoid the existing sewer trench.) For planning purposes the contractor should assume that these soils will be exposed at depths on the order of about 2 feet from existing grade in most locations on site. 2. An allowable bearing of 2,000 psf may be used for design of the footings. Lateral passive earth pressures of 350 pcf may be used for determining the lateral resistance. Limitations and Additional Services The above recommendations are based upon soil conditions observed during our limited geotechnical reconnaissance and subsurface investigation. Soil conditions can vary significantly especially where the soil has been disturbed due to grading activities. If conditions are encountered during construction that vary significantly from those anticipated we should be advised so that we can revise our recommendations if necessary. This report is not intended as a warranty of the subsurface conditions. We trust that this helps you move forward in your plans for the project. Please do not hesitate to contact us if you have any questions about this report or the procedures we have recommended. Sincerely, THE GALLI GROUP Wr Paul L. Stoltenberg, P.E. Senior Geotechnical Engineer Attached: Figure 1: Vicinity Map Figure 2: Site Features Figure 3: Slope Cross Section $To Z "�&29339 f C5g 1457 JQuinn Deck Rev 0331 4 of 4 The Galli Group