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CANOD.pdfCity of Edmonds Critical Area Notice of Decision Applicant:/ WIA �, M _(�. r Property Owner: /7 /&"- Critical Area File 004`7 Permit Number: 2, yo-/ Site Location: S/ t- (0 Parcel Number: 1/ 0 Oo e) 6 Project Description: .. . ... ...... —1 .. . ..... ... d e4,Yro 6/ilv" ....... ------ . . ..... ... . ..... . ..... c4t o,(,(),,e 0 Conditional Waiver. No critical area report is required for the project described above (J 1. There will be no alteration of a -Critical Area or its required buffer. / �' 1 76',- 2. The proposal is an allowed activity pursuant to ECDC 23,40,220, 23.50.220, and/or 23,80.040. 3, The proposal is exempt pursuant to ECDC 23.40.230. ❑ Erosion Hazard. Project is within erosion hazard area. Applicant must prepare an erosion and sediment control plan in compliance with ECDC 18.30. Critical Area Report Required. The proposed project is within a critical area and/or a critical area buffer and a critical area report is required. A critical area report has been submitted and evaluated for compliance with the following criteria pursuant to ECDC 23.40.160: I The proposal minimizes the impact on critical areas in accordance with ECDC 23.40:120, Mitigation sequencing; 2. The proposal does not pose an unreasonable threat to the public health, safety, or welfare on or off the development proposal site; 3. The proposal is consistent with the general purposes of this title and the public interest; Any alterations permitted to the critical area axe mitigated in accordance with ECDC 23,40.110, Mitigation requirements. The proposal protects the critical area functions and values consistent with the best available science and results in no net loss of critical functions and values; and —7 Z" The proposal is consistent with other applicable regulations and standards, E] Unfavorable Critical Area Decision. The proposed project is not exempt or' does not adequately mitigate its impacts on critical areas and/or does not comply with the criteria in ECDC 23,40.160 and the provisions of the City of Edmonds critical area regulations, See attached findings of noncompliance. I -.Favorable Critical Area Decision. The proposed project as described above and as shown on the attached site plan meets or is exempt from the criteria in ECDC 23.40,160, Review Criteria, and complies with the applicable provisions of the City of Edmonds critical area regulations. Any subsequent changes to the proposal shall void this decision pending re -review of the proposal, E] Conditions. Critical Area specific condition(s) have been applied to the permit number referenced above. See referenced permit number for specific condition(s). lh C�ry 't�L I C'fl Reviewer Signature Date Appeals: Any decision to approve, condition, or deny a developnient proposal or other activity based on the requirements of critical area regulations inay be appealed according to, and as part of, the appeal procedure, if any, for the permit or approval involved. Revised 12/16/2.010 Larry Rothmier 8534 — 210`x' PI SW Edmonds, WA 98026 Subject: Critical Areas Report and Geotechnical Recommendations Proposed Patio hoof and Deck 8534 — 210t" Place SW Edmonds, WA 98026 Dear Larry: July 10, 2014 Project 1767-1 n,u 2014 Sf.1NK,',1(" At your request we visited the above residence to evaluate the existing soil conditions. We understand that the proposed project will not enlarge the footprint of the existing residence but will include construction of a new roof over the existing concrete patio and a new deck over the existing concrete walkway. The patio roof will likely be post -supported and cover the existing concrete patio on the west side of the house. The existing site has been identified as located within or near a geologic hazard area due to potential landslide and erosion hazard. Because of this a Critical Areas Report is required along with geotechnical recommendations for mitigation of the hazard during construction. The purpose of our site visit was to evaluate the soil conditions in the vicinity of the proposed improvements and to provide recommendations for foundation support and mitigation of geologic hazards associated with the site. 1.0 PROJECT DESCRIPTION The project is located on the west facing slope west of 210r" Place and east of Pioneer Way (see Figure 1, Vicinity Map). The slope descends from the relatively :flat portion of the back of the lot approximately 30 to 35 feet at a declination estimated at 50 percent toward the homes along Pioneer Way below. The top of the slope is located about 36 feet from the existing residence and about 24 feet from the existing concrete patio. The proposed patio roof will cover the existing concrete patio and result in minimal increased impervious area. The stormwater runoff from the roof will be tied into the existing stormwater 503418th Avenue NF, Seattle, WA 98105 Phone: 206.525.5097 Fax; 206.525.5091 Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210"' Place SW Edmonds, WA 98026 collection system. The proposed deck and roof covering will be supported on conventional spread footing pads with post and pier construction. The slope itself is densely vegetated with brush, ivy, and blackberries, and supports mature Douglas Fir, Cedar, and Big Leaf Maple. There will be no change planned for the slope or for the lawn area in the back yard. The proposed improvements would be supported on foundations located approximately 25 to 26 feet away from the top of the slope at their closest point. Because the site has been designated as located within a geologic hazard, construction of the proposed deck might require a reduced buffer. In the report that follows we have supplied observations and an assessment that concludes that the proposed project will not adversely impact the slope or adjacent properties and that a reduced buffer of twenty feet would be permissible without increasing risk to the stability of the slope or slope system. 2.0 SITE CONDITIONS No EKIIJ Ill W..Xu Wei ; : 30 A geotechnical engineer from The Galli Group visited the site on July 8, 2014, to provide a site reconnaissance. Observations from our site visit are provided below. The site is mapped as underlain by glacial till possibly feathering out farther down the slope into advance outwash (Geologic Alap of the Edmonds East and pant of the Edfnonds Rlrest quadrangles, Washington, Minard, 1983). Glacial till generally consists of unsorted mixtures of silt, sand, clay and gravel pushed over the existing topography thousands of years ago by the advancing glacier. It was subsequently compacted by tons of ice and is notable for its very dense consistency, relative impermeability, and ability to stand unsupported in near vertical relief. Glacial till slopes generally remain quite stable. Advance outwash consists of sorted layers and lenses or braided layers of sand and gravel deposited in meltwater environments in front of the advancing glacier. The sediment was subsequently overridden by ice and generally appears dense below the upper weathered zone. Advance outwash tends to readily convey water through the unit and toward daylight on steep slope exposures. Where underlain by impermeable units such as Lawton Clay the outwash steep slopes are somewhat notorious for shallow colluvial landslides. Native slopes protected by vegetation often appear inclined as steep as 80 percent in the region. On flatter slopes the landslide and erosion risks are primarily related to concentrated discharge of stormwater runoff and other human causes. ® The site appears to descend from the rear yard westerly a total vertical distance of about 30 to 35 feet in a horizontal distance of 70 feet or at a declination estimated at about 50 percent. A 1767 rothmier recon rpt.doc 2 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210"' Place SW Edmonds, WA 98026 rockery approximately 4 feet high is located inside the property line and at the top of the steep slope. We identified the rockery as the top of the steep slope in Figure 2, Site Features. The lot appears relatively flat, elevated slightly above street grade and then grades gradually toward the west property line. The rear yard is relatively flat from the house to the top of a slope that descends westerly. The rockery was comprised of 2 -man and 3 -man rock, and appeared in good condition. The rear yard consists primarily of lawn with perimeter planting beds. The west facing slope supported numerous mature Douglas Fir, Big Leaf Maple, and a couple Cedar trees. The understory consisted of blackberries, forest debris, and some ivy. We did not observe any indications of erosion on site or the slope. The rockery appeared to be in good condition with no evidence of seepage, or rotation. The slope appeared in stable condition at the time of our site visit with no evidence of erosion, sloughing, springs or seepage. The trees appeared to be straight and the young trees did not evidence much surficial creep. A geotechnical engineer from The Galli Group visited the site on July 8, to conduct a site reconnaissance and to advance a hand hole and probe in several locations in order to identify the near surface soil conditions. We advanced one hand hole to a depth of 4 feet at the location shown on Figure 2, Site Features. We probed with a T -probe to determine the depth to native, dense soil in several locations shown on Figure 2. We found native glacial till soils (silty SAND with trace of gravel and occasional cobbles) west of the house at the top of the slope in HH -1. The soil was loose to a depth of about 3 feet near the top of the slope (HH -1) and to depths varying from about 4 feet to 18 inches on the face of the slope (see Figure 3, Site Section). The hillside appeared comprised of medium dense to dense silty SAND with gravel, which we interpreted as glacial till. We did not observe any indications of seepage or perched water in the hand hole which remained open for about 30 minutes. The silty SAND appeared blanketed by about 18 inches of fill near the western edge of the lawn, but otherwise it appeared that dense soil was within 2 to 3 feet of the existing grade. Based upon original topography some of the excavated material might have been pushed toward the northwesterly corner of the lot. However, that portion of the lot is the most remote from the proposed patio roof and will be unaffected by the proposed improvements. Detailed logs of the exploratory hole and results of the probing are shown on Figure 3, Site Section. Based upon surfrcial characteristics, probing, and hand holes, we think that the site likely contains good bearing soil within the upper 2 feet in the vicinity of the proposed deck and patio cover footings. The depth to dense material on the slope appeared to be on the order of 3 or 4 feet at most. The overall inclination of the slope system (50 percent) does not appear steep 1767 rothmier recon rpt.doc 3 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210"' Place SW Edmonds, WA 98026 enough to warrant concern about slope movement provided stormwater drainage is adequately provided. 3.0 CRITICAL AREAS DISCUSSION A review of the Edmonds Community Development Code (ECDC) indicates that site might be governed by Critical Areas regulations. Below we have discussed the elements that apply to the project site with reference to ECDC code requirements. 3.1.1 Erosion hazard Areas The ECDC defines Erosion Hazard Areas as areas possessing steep slopes in excess of 40 percent (see below.) Erosion hazard areas include: "areas of the city of Edmonds that may experience severe to very severe erosion hazard. This group of soils includes, but is not limited to, the following when they occur on slopes of 15 percent or greater: a. Alderwood soils (15 to 25 percent slopes); b. Alderwood/Everett series (25 to 70 percent slopes); c. Everett series (15 to 25 percent slopes)." (ECDC 23.80.020 A(1) The slope on the west side of the house is inclined at about 50 percent below the rockery. The Soil Conservation Service maps the area as underlain by Alderwood gravelly sandy loam. Because of these topographic and mapping conditions the project site would be designated an Erosion Hazard Area. 3.1.2 Landslide hazard Areas The inclination of the slope at the west side of the residence and steepest portion of the slope exceeds 40 percent for a height of about 25 to 30 feet. The slope has a 4 -foot high rockery at the top that defines the upper limits of the slope. Section 23.80.020B defines "Landslide Hazard Areas" as follows: Landslide hazard areas are areas potentially subject to landslides based on a combination of geologic, topographic, and hydrologic factors. Within the city of Edmonds landslide hazard areas specifically include: "any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet except areas composed of consolidated rock." (ECDC 23.80.020B(2). The western portion of the project site qualifies for designation as "Landslide Hazard Area" due to topographic features. 1767 rothrrier recon rpt.doc 4 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210`x' Place SW Edmonds, WA 98026 3.1.3 Seismic Hazard Area "Seismic hazard areas" are areas subject to severe risk of damage as a result of earthquake - induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting." (ECDC 23.80.020C) The project site appears underlain by dense glacially consolidated soil, or glacial outwash at depth. This dense material does not present a risk of deep-seated slope movement, seismic liquefaction, lateral spreading, or surface rupture. Provided the new foundations are supported on native undisturbed soil, the risk of seismic -induced settlement is not significant. Our slope stability assessment concluded that the risk of slope failure due to seismic ground shaking is likely limited to shallow slumps in the loose surficial soil rather than deep-seated slope failures or failures that would impact the residence or planned improvements. In our opinion the site does not represent a severe risk of damage due to seismic induced ground shaking. In the report sections that follow we have described the site soil conditions and the subsurface geologic conditions. The site appears underlain by dense glacially consolidated sediment that appeared stable in its current condition. The project site contains slopes in excess of 40 percent that are well vegetated but still present risks of erosion if drainage is not properly maintained. In our opinion it does not present a significant risk of seismic liquefaction, landslides, or erosion if conventional Best Management Practices are followed during site improvements, and our recommendations are followed during project development. The existing residence is located about 36 feet away from the top of the steep slope (see Figure 3, Section A -A'). The existing patio is currently located 24 feet from the top of the slope and within the recommended minimum buffer plus building setback (ECDC 23.80.070 A(la and lb). The proposed improvements are confined to previously developed flat areas on the site but are close to being located within the standard 10 -foot minimum hazard buffer plus 15 foot building setback. And the total setback is less than the height of the slope. Therefore a reduced buffer will be necessary. The proposed improvements will be located 25 feet away from the top of the steep slope. Because of the dense underlying stable soil it is our professional opinion that the standard buffer may be reduced to accommodate the proposed deck and patio roof. Alterations to areas within buffers or steep slope areas must meet the following requirements in order to be permitted: 1. "The development will not increase the surface water discharge or sedimentation to the adjacent properties beyond predevelopment conditions." (ECDC 23.80.070 2a). Construction of the deck will not increase the rate of runoff from the site provided the deck is not finished with an impervious deck surface. If the deck is constructed with an impervious deck 1767 rothmier recon rpt.doc 5 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210"' Place SW Edmonds, WA 98026 surface all runoff must be captured in a gutter system and connected to the existing storm drain. The patio roof will not significantly increase the amount of impervious area on the site and would actually decrease the amount of runoff added to the soil above the top of the steep slope. We recommend the following mitigation measures: The collected stormwater runoff from the patio roof must be directed via a tightline pipe to the existing storm drain so that collected runoff does not contribute to the slope system. ® Where areas under the proposed deck will not be existing concrete, we recommend keeping the area thickly mulched to avoid forming rivulets or erosion. ® All disturbed areas during construction should be mulched and replanted prior to the wet season (October 1). 2. "The development will not decrease the slope stability on adjacent properties." (ECDC 23.80.070 2b) Our assessment indicates that the lightly loaded deck footings supported in the dense underlying soil will have no adverse impact on the slope or adjacent sites. We recommend the following measures to help maintain slope stability: ® All disturbed areas must be mulched and/or blanketed with mulch prior to the wet season. Permanent erosion control should consist of amending the disturbed soil and replanting the area within the combined vegetated buffer and building setback. 3. "Such alterations will not adversely impact other critical areas." (ECDC 23.80.070 2c) The proposed deck will be supported on native, undisturbed, medium dense to dense outwash soils about 25 feet from the top of the slope. The proposed improvements will not impact other critical areas. No additional mitigation measures beyond those described above are needed to protect adjacent sites or critical areas. 19IZOL119re Ilii reliz1 Based upon our geotechnical reconnaissance and limited subsurface exploration, it appears the existing house is supported on native, undisturbed, dense glacial till. We described the hillside as comprised of medium dense to dense silty SAND and interpreted the material as glacial till. In areas planned for the new foundations, the underlying soil was described as medium dense to dense silty SAND within the upper 2 feet of soil. The existing soil will provide excellent foundation support for conventional spread footings. Reducing the landslide hazard buffer to 10 feet would allow the proposed improvements to be safely constructed outside of the landslide hazard buffer plus building setback (total of 25 feet). The building foundation would remain in its existing condition. 1767 rothrrier recon rpt.doc 6 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210"' Place SW Edmonds, WA 98026 Best Management Practices commonly observed should be employed during construction. We don't anticipate a lot of export or import of materials for the project. We anticipate these will include the following: 1. Maintain the street free of sediment during excavation and hauling and when mobilizing equipment to and from the site. Mud and silt tracked from the site should be removed or cleaned by the contractor daily or more often if needed. 2. Maintain the vegetation between the patio and the fence during construction. A minimum of 15 feet of lawn area should be maintained to capture sediment from exposed soil and to help attenuate surface water runoff in the unlikely event of a summer downpour. 3. Disturbed soils on the site should be protected with mulch once the excavation and backfill are completed. Mulching may be used until vegetation is established. 4. Excavated soil must not be deposited on the slope or within the 10 -foot hazard buffer. 5. Stockpiles must not remain uncovered for more than 2 days during the wet season. The table below provides soil parameters used in the analyses for this project. Soil design parameters used in determination of lateral earth pressures Soil Unit Allowable Passive Active Type Weight Searing Resistance Earth Y, pcf Capacity (EFW) Pressure (psf) (EFW) m. dense silty SAND or SAND 120 2000 350pcf 35 pcf w/silt Dense, silty SAND 125 3500 350 pcf 35 pcf Compacted 125 2000 350 pcf 35 pcf Fill (EFW) = Equivalent Fluid unit Weight in pounds per cubic foot Excavation for the proposed footings must be at least 24 inches below the existing grade. Footings must bear on the native undisturbed silty SAND as observed in our test holes at about 2 1767 rothmier recon rpt.doc 7 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210t" Place SW Edmonds, WA 98026 feet below existing ground surface. We recommend using an allowable bearing capacity of 2000 psf for design of footings supported on the medium dense to dense silty SAND. Lateral resistance for the footings should be ignored in the uppermost 12 inches of embedment. Foundations for the deck and patio roof will consist of spread footings supported on the undisturbed silty SAND unit. We anticipate that this unit will be encountered at depths on the order of 2 to 21/2 feet from existing grade in the vicinity of the proposed deck, and at about 2 feet below grade near the existing patio. 5.4.1 Seismic Design Parameters The site is underlain by glacially consolidated silty SAND. Based upon the density of the underlying soil we do not think seismic liquefaction or lateral spreading will be a significant risk factor to site development. We recommend using site Class D for this project site. Lateral acceleration of backfill against walls may be estimated as a uniform horizontal force acting against the wall equivalent to 8H where H is the height of the retained soil. 5.4.2 Spread Footings For spread footings supporting the proposed deck we recommend the following: 1. An allowable bearing pressure of 2000 psf may be used for footings bearing on undisturbed SAND with silt. This may be increased by 1/3 for temporary loads such as wind loads or seismic loads. 2. The passive resistance for the footings may be calculated at 350 pcf in the native undisturbed soil. The contribution of the upper 12 inches should be ignored. 3. A coefficient of friction of 0.4 may be used for the interface between the bottom of the footing and the soil. 4. The footing area must be free from loose, organic -rich, or wet soil prior to placing reinforcing or pouring concrete. 5. Deck or porch footings should bear on native undisturbed soils to avoid settlement. These can be provided by overexcavating, pouring a footing, and extending the support to grade using a concrete pier. The site appears underlain by glacial till soils consisting of silty fine to medium SAND with a trace of gravel. Generally, provided the soils remain vegetated and concentrated runoff is avoided, the soils will remain stable and erosion can be avoided. Construction erosion control measures were discussed in section 5.1 above the following: For permanent erosion control we recommend 1767 rothmier recon rpt.doc 8 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210'1' Place SW Edmonds, WA 98026 All water collected from the patio roof should be routed to the existing downspout collection system where it is conveyed to the storm drain. Although the 150 square foot area is minimal, we recommend avoiding the use of splash blocks in the event that concentrated discharges begin to make their way toward the slope. No infiltration of storm water runoff should be permitted near the top of the slope. 2. Avoid deposition of yard debris over the fence and onto the face of the steep slope. This material can become saturated and begin to translate down slope during the winter. We recommend use of isolated paving stones or gravel instead of concrete for pathways topographically above the steep slope area. The intent is to avoid increasing impervious area above the slope. This geotechnical investigation was planned and conducted in accordance with generally accepted engineering standards practiced presently within this geographic area for a site report with limited scope. Geotechnical investigations performed by these standards reveal with reasonable regularity soils that are representative of subsurface conditions throughout the site under consideration. Recommendations contained in this report are based upon the assumption that soil conditions encountered in explorations are representative of actual conditions throughout the building site. However, inconsistent conditions can occur between exploratory borings or test pits and not be detected by a geotechnical study. If, during construction or subsequent exploration, subsurface or slope conditions are encountered which differ from those anticipated based upon results of this investigation, The Galli Group should be notified so that we can review and revise our recommendations where necessary. If conditions change prior to the proposed construction, we should be consulted so that we may alter our recommendations if necessary. This report is prepared for the exclusive use of the owner or the owner's consultants for specific application on this project at this particular site. Copies of this report should be made available to the design team, and should be included with the contract drawings issued to the contractor. Our report, conclusions, and interpretations should not be construed as a warranty of the subsurface conditions on the site and should not be applied to neighboring sites. No warranty, expressed or implied is made. The report was prepared anticipating geotechnical observation and testing provided during the construction phases to verify that the recommendations provided in this report are incorporated into the actual construction. 1767 rothmier recon rpt.doc 9 The Galli Group Geotechnical Site Reconnaissance & Critical Areas Report Rothmier Residence 8534 210th Place SW Edmonds, WA 98026 We trust that this helps you move forward. Please contact the undersigned if you have any questions about our assessment. Regards, THE GALLI GROUP Paul L. Stoltenberg, P.E. 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