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CA_geotech_report.pdfGary A. Flowers, P'LLC Geological & Geotechnical Consulting 1953212th Avenue NE Shoreline, WA 98155-1106 August 1.4, 2012 Project No. GF05079 ,. Mike & Anne Meiers 1901694 1h Ave West RECEIVED Edmonds, WA 98026 Subject: Critical Area Report AUG10 Meiers Residence/City of Edmonds R -O -W ^NEVE, M OPM 11. S RVIOTS 19016 94th Ave West �: 0U N � E1`1 Edmonds, Washington Dear Mike & Anne: This report presents information required by the City of Edmonds for construction or development projects to be located within a defined geologically hazardous area as per the Edmonds Critical Area Code. Mitigation of the landslide that occurred on your property and the City of Edmonds right-of-way for 190th Place SW during December 2007 will occur within a defined geologically hazardous area for both erosion hazard and landslide hazard. As such a Critical Area Report is a code requirement prior to issuance of a construction permit for the mitigation to occur. Documents reviewed for this study include reports by HWA GeoSciences, Inc. dated February 17, 2004, September 21, 2004 and November 22, 2004, the published geological map of the area titled "Geologic Map of the Edmonds East and part of the Edmonds West Quadrangles, Washington" by Minard, 1983, and the Edmonds Municipal Code, Chapter 23.80, Geologically Hazardous Areas. Existing Conditions Previous studies on the subject slope area by this firm and by HWA GeoSciences, Inc, consultant to the City of Edmonds, have determined that the overall slope angle of the subject site is on the order of 1 H:1 V (horizontal to vertical) and is greater than 10 feet in height. As such, as per the city code it is defined as a landslide hazard area. Due to the steeply sloping site and the presence of Alderwood and Alderwood/Everett series soil the site is also defined as an erosion hazard area. The subject site is not considered to be a seismic hazard area as the core of the slope is comprised of glacially consolidated sediments. Gary A. Flowers, PLLC. 19532 12'h Avenue NE Shoreline, Washington 98155-1106 206-417-7640 Meiers Property Geological/Geotechnical Assessment Edmonds, Washington Background On December 6, 2007 the landslide was reconnoitered by personnel from this firm. The slide scarp is 40 to 45 feet wide at the top, narrows to about 25 feet at the bottom and extends from the top of the slope to the bottom of the slope along 190th Place SW. The headwall scarp at the top of the slide ranges from 3 to 5 feet high. Overall slope height is approximately 25 feet. The slide scarp extends about 10 feet into the backyard area beyond the original top of slope. Debris run out from the landslide area was observed to extend across the roadway at the toe of the slope, down the roadway embankment, across a portion of the adjacent property to the northwest and pooled against a driveway embankment for an adjoining property. This covers a distance in excess of 100 feet and indicates that the soils were saturated and flowed down the hillside as a mud flow or debris flow event. Soils exposed in the slide scarp include 6 to 10 feet of fill soils comprised of gray, silty sand with gravel. This fill was likely placed at the time of the original house construction. Underlying the fill soils an 8 to 12 inch thick layer of relic topsoil was observed. These soils were in a loose to medium dense condition. Beneath the relic topsoil an approximate 3 foot thick layer of medium dense, brownish -gray silty sand to sandy silt was observed. This material is interpreted to be Vashon age recessional glacial drift. Underlying the glacial drift dense, gray, silty sand with gravel was observed. This material is interpreted to be Vashon age glacial till. The inclination of the landslide scarp can be broken down into five distinct areas. The exposed scarp headwall at the top of the slope is about 80° or near vertical. The next approximate 10 feet of scarp is inclined at 28° to 30°. These two areas are fully within the fill soil zone on the slope. At this point there is an oversteepened nick point that is only about 3 feet tall but is inclined at 55° to 60°. This nick point is immediately above the noted seepage zone on the slope and includes the base of the fill slope, the relic topsoil layer and the top of the glacial drift layer. At the base of the nick point is a scoured area that is 12 to 18 inches deep, 2 to 4 feet wide and extends about 6 feet down the slope entirely within the recessional glacial drift. The area below the nick point and within the glacial drift soils is inclined at 22° to 28°. Near the contact with the glacial till, and immediately above the roadway, the slope inclination increases to 40°. A cross section down the landslide scarp is attached as Figure 1. The published geological map of the area titled "Geologic Map of the Edmonds East and part of the Edmonds West Quadrangles, Washington " by Minard, 1983, indicates that the slope is located close to a contact between glacial till and advance outwash. Our findings indicate that glacial till is present underlying the fill. The glacial drift sediments observed overlying the till are likely a recessional drift unit. Our findings are in agreement with the published geology map of the area. Linear cracks in the ground at the top of the slope were observed along a portion of the top of slope adjacent to the concrete driveway and the trellis work on the north side of the house. The Gary A. Flowers, PLLC. 1953212 Ih Avenue NE Shoreline, Washington 98155-1106 206-417-7640 2 Meiers Property Geological/Geotechnical Assessment Edmonds, Washington cracks were on the order of 8 to 12 feet long and about I/2 inch wide. The slope along this area is very steep but only about 10 feet tall. Ground water was observed seeping from the silty sand to sandy silt glacial drift soils that overlie the glacial till soils. This seepage zone is near the midpoint of the slope. Supersaturated soil was observed oozing from several mountain beaver holes adjacent to the landslide scarp and within the glacial drift soils. During previous reconnaissance of the slope no evidence of ground water seepage or hydrophilic vegetation was ever observed. Geological Hazards Assessment Soil characteristics for the steep slope are defined above based on our reconnaissance of the landslide scarp in 2007 which exposed the soils throughout the slope face. These soils consist of fill soils overlying recessional glacial drift soils overlying glacial till soils. At the base of the slope a series of three (3) exploration borings were drilled and sampled in January 2012. The borings were located within the asphalt roadway of 190th Place SW. A copy of the borings is attached in the Soldier Pile Wall Design Memorandum dated May 18, 2012. These borings encountered 3 to 11 feet of fill soils overlying dense glacially consolidated soils (glacial till and advance outwash deposits). Across 190th Place SW from the project site is another steep slope area. This steep slope includes the roadway embankment for the street. To the best of our knowledge this steep slope area has not been impacted by any landslides other than the debris runout from the Meier's slide that flowed across the street and down this steep embankment. Both of the steep slopes (above and below 190th Place SW) are heavily vegetated and not subject to erosion under normal conditions. No ground water seepage has been observed on the subject slope or the nearby embankment slope except immediately following the landslide in 2007. No ground water seepage has been observed within 200 feet of the subject site. No known or recorded landslides are located within 200 feet of the subject site. The core of the slope is comprised of glacially consolidated sediments which are known to have a slope stability factor of safety greater than 2 for static conditions and greater than 1.2 for pseudostatic conditions. Fill soils near the top of the Meiers' slope and fill soils within the roadway embankment will have a lower factor of safety. Modeling of the slopes to determine actual stability factors of safety are not considered to be necessary for this project as the purpose of the project is to construct a soldier pile wail to stabilize the slope above the roadway. Construction of the wall will not have any adverse impact to the steep slope below the roadway and will protect the lower slope area from any future landslide runout. Gary A. Flowers, PLLC. 19532 12i6 Avenue NE Shoreline, Washington 98155-1106 206-417-7640 3 Meiers Property GeologicallGeotechnical Assessment Edmonds, Washington As per EMC Chapter 23.80, Part IV, Development Standards in Geologically Hazardous Areas, 22.80.060 Development standards, 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. The project, i.e. construction of a soldier pile wall at the toe of the subject slope, meets all of the above requirements. As per EMC 22.80.070, Development Standards for Specific hazards a buffer shall be established from all edges of landslide and erosion hazard areas. Alterations of an erosion or landslide hazard area or buffer may only occur for activities for which a hazards analysis is submitted and certifies that 1) the development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; 2) the development will not decrease slope stability on adjacent properties, and 3) such alternations will not adversely impact other critical areas. The soldier pile wall construction meets all of the requirements for this code section and will be placed within the hazard area. In addition, the proposed development does not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for pseudostatic or dynamic conditions and in fact dramatically increases these factors of safety for the subject slope. Construction is anticipated to be completed prior to the October 1 deadline for dry season grading. Vegetation will only be removed in the area of the planned wall construction and backfilling. Backfill will primarily be quarry spalls which have no potential for erosion. Drainage from behind the wall will be placed into the city storm water system via a catch basin located about 20 feet from the end of the soldier pile wall. Drainage will in no way promote erosion or increase saturation of the slope. CONCLUSIONS AND RECOMMENDATIONS The project as planned and designed meets all the requirements of the Edmonds Municipal Code for construction in geologically hazardous areas and should proceed as planned. Gary A. Flowers, PLLC. 19532 121' Avenue NE Shoreline, Washington 98155-1106 206-417-7640 4 Meiers Property Geological/Geotechnical Assessment Edmonds, Washington Thank you for the opportunity to be of service on this project. Should you have any questions regarding this matter, please do not hesitate to contact our office. Sincerely, A 'Y a Gary A. Flowers Gary A. Flowers, P.G., P.E.G. Principal Engineering Geologist Attachment: May 18 Wall Design Memorandum and Exploration Logs Gary A. Flowers, PLLC. 19532 12°i Avenue NE Shoreline, Washington 98155-1106 206-417-7640 Gary A. Flowers, PLLC 19532 12th AvenueLTE Shoreline, WA 98155-1106 May 18, 2012 Project No. 05079 To: Anne and Mike Meiers Phil Williamson — City of Edmonds From: Gary Flowers, P.E.G. and Jamey Battermann, P.E. Ile: Meiers Property and 190th Place SW Slope Stabilization 19016 94t11 Avenue West, Edmonds, WA This memo presents the results of our recent exploration borings and resultant geotechnical design parameters for the planned soldier pile wall along the east side of 190th Place SW in Edmonds. The soldier pile wall is required in order to repair a landslide that occurred on the steep slope to the east of 190th Place SW and to stabilize the remainder of the slope in this area. Other methods of retainage for the slope have been proposed and discussed with a soldier pile wall at the toe of the slope being the preferred option of all parties. Stability of the slope has been an issue since about 2003. The steep slope above the roadway is owned in common between the Meiers and the City of Edmonds. Summary of Field Exploration and Results A total o1`3 exploration borings were placed along the east side of 1908' Place SW on January 24, 2012. The approximate location of the borings is shown on Figure 1, Site & Exploration Plan. The borings were advanced to depths ranging between 16 t/2 to 31 "/2 feet. Borings were drilled by Geologic Explorations, Inc. using a DeepRock XL drill rig. Samples were obtained at either 2.5 or 5.0 foot increments as deemed appropriate at the time in the field. Samples were obtained via the Standard Penetration Test methodology. Boring EB -1 encountered approximately 2 feet of medium dense, blackish -brown, silty sand with gravel (road fill) overlying dense to very dense, silty sand with gravel to medium to coarse sand with trace silt (glacially consolidated sediments). Ground water was not observed in the boring. Borings EB -2 and EB -3 encountered approximately 10 feet of very loose to loose sand with silt and occasional fine gravel to silty sand with gravel and sandy gravel (road fill) overlying dense to very dense silty sand with gravel to medium to coarse sand with trace silt (glacially consolidated sediments). Ground water was not observed in these borings. Gary A. Flowers, PLLG. 19532 12th Avenue NL Shoreline, Washington 98155-1106 206-417-7640 190`h Place SW Slope Stabilization Geotechnical Set -vices Memorandum Edmonds, Washington Although no free ground water was observed within the borings, mottling of the soils was observed below a depth of 10 feet in EB -2. The mottling indicates that ground water is often present within these soils. Asphalt thickness on the roadway was observed to range from 5.5 to 7.0 inches in 2 to 3 lifts. Since this is not a main arterial it would seem to indicate that the road has bad settlement/movement issues in the past which has necessitated adding additional layers of asphaltic pavement. Upon auger withdrawal from the borings EB -1 caved in to a depth of 8 feet. Borings EB -2 and EB -3 experienced only minor caving that filled in 2 to 3 feet of the bottom of the bore hole. Our evaluation indicates that a soldier pile wall that is approximately 125 feet long will stabilize the areas of concern along the slope face. The wall should begin at approximate street elevation of 84 feet (undated survey by Reid Middleton for City of Edmonds and provided by City of Edmonds) and extend southerly to an approximate street elevation of 66 feet. Due to the presence of a storm drain pipe along the edge of the roadway the soldier piles should be installed about 2 feet behind the pipe. This will require some excavation at the toe of the slope to prepare the drill pad and base of wall. • Based on the survey information it appears that wall heights ranging from 6 to 10 feet will be required. ® Quarry rock backfill should have a maximum slope inclination on the order of 2H: IV in order to provide suitable support to the upper portions of the slope and to restore the yard area on the Meiers' property that was lost by the landslide. ® Poor soil conditions in the upper portion of the borings will have an adverse affect on the allowable soil design parameters and require either larger beam sizes or tiebacks. ® The wall should be designed for an active equivalent fluid pressure of 45 pef. ® A uniform surcharge of 8H should be applied for seismic loading. • Lateral resistance to toe kickout can be developed from passive resistance on the front of the pile portion that is embedded into the ground. The upper 2' should be ignored. From 2' to 7' passive resistance of 100 pef may be used and applied over 2 pile diameters. • Passive resistance below a depth of 7' will be 500 pcf and should be applied over 2 pile diameters. ® Tie back anchors, if required, should extend into the dense glacial sediments a sufficient distance to provide the required tensile forces for wall restraint. A shear strength value of 1,500 psf around the perimeter of the grouted anchor may be used for design purposes. This value must be confirmed by 200% load testing performed on at least two anchors during installation of the soldier pile anchor system. Performance testing to 130% must be performed on the remaining anchors. The shear value typically begins outside of the Gary A. Flowers, PLLC:. 19532 12°i Avenue NF shoreline, Washington 98155-1106 206417-7640 190ih Place SW Slope Stabilization Geotechnical Services Memorandum Edmonds, Washington no load zone which starts 6 feet behind the wall and extends upwards at an angle of 60 degrees. In the area of the landslide the shear value begins a minimum of 10 feet into the exposed sediments behind the wall. Anchor orientation should be approximately 15 to 20 degrees from the horizontal. ® A perforated drain pipe should be installed at the base of the wall that connects into the existing storm system. The drain pipe should be sewer quality (ASTM 3034) due to the depth of burial with quarry spalls. ® Prior to backClling, earthwork contractor to remove all vegetation down to ground surface and cover the slope with landscape fabric to prevent regrowth. Quarry spall backfill to be placed atop fabric. Attachments: Site and Exploration Plan Exploration Logs xc: Lucia Engineering, 12014 86th Ave NE, Kirkland, WA 98034 Gary A. Flowers, PLLC. 19532 12'h Avenue NE Shoreline, Washington 98155-1106 206-417-7640 94V AVEW VIPWPRM� waft-blb*m- m 3nN3Av ONZ:6 MN -IAV Hlk6 94V AVEW VIPWPRM� waft-blb*m- Number EB=1 Approximate Elevation 83' Subsurface Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by geologic Interpretations, engineering analysis, and judgment. They are not necessarily representative of other fimes and locations. We will not accept responsibility for the use or interpretation by others of information presented on this log. METERS RESIDENCE 19016 94TH AVENUE WEST EDMONDS, WASHINGTON JANUARY 24, 2012 PROJECT NO. 05-079 W Q cl� STANDARD PENETRATION SEDIMENT DESCRIPTION C a � � RESISTANCE W ®B BlowslFoot ® (9 cU) 10 20 30 40 ACP in 3 lifts __... - - -- -- ------------ Loose, moist, blackish -gray, silty SAND with gravel (fill) i 27 92 - 42 Very dense, moist, brownish -gray, silty SAND with 50 gravel (glacial till) ------------------------------------------------------------------ Very dense, moist, brown, medium to coarse SAND, 19 100+ trace to some silt (advance outwash deposits)50/3 10 -- i 50/3 100+ 1 100/5* 15 19-- 24 61 37 BOH @ 16.5' - No free water observed. During auger withdrawal, hole caved in so only upper 8' - remained open. f *bouncing; blow count overstated i 30 i --- -- Subsurface Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by geologic Interpretations, engineering analysis, and judgment. They are not necessarily representative of other fimes and locations. We will not accept responsibility for the use or interpretation by others of information presented on this log. METERS RESIDENCE 19016 94TH AVENUE WEST EDMONDS, WASHINGTON JANUARY 24, 2012 PROJECT NO. 05-079 Number EB -2 EXPLORATION BORING LOG -7Approximate Elevation 74' STANDARD PENETRATION -J 2!W RESISTANCE SEDIMENT DESCRIPTION® Q Blows/Foot U) (� 10 20 30 40 6" ACP in 2 lifts - - --- ------ -..----- ----- ---Very Veryloose to loose, moist, brown and gray SAND 8 10 with silt, occasional fine gravel (fill) 7 A 3 5 5 2 - - A4 2 0 A2 Asphalt chips at 10' 10 - 1 Some mottling of soils A 5- - 4 Very dense, moist, gray, silty SAND with gravel 45 95 (glacial till) 50 Becomes less silty; some mottling 50/5 100+ --------------------------------------------------------------- - 20 —� Very dense, moist, gray to brown SAND, trace silt, significant mottling in upper portion of unit (advance outwash deposits) _ I 50/6 100+ 25 - 50/6 i 100+ i - - ----------- ----------- --- BOH @ 29' 30 -- -- - No free water observed. Minor caving following auger withdrawal. Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by geologic Interpretations, engineering analysis, and judgment. They are not necessarily representative of other times and locations. We will not accept responsibility for the use or interpretation by others of information presented on this log. EXPLORATION BORING LOG Number EB -3 Approximate Elevation 64' STANDARD PENETRATION. W RESISTANCE SEDIMENT DESCRIPTION Blows/Foot U) 10 20 30 40 5.5" ACP in 2 lifts ------------- -------- - ---- ---------------- Loose, moist, interbedded, gray, brown, and black, gray, 8, A SAND silty SAND with gravel, medium SAND with silt, and 5 sandy GRAVEL (fill) 3 8 A5 3 2 3 9 4 Al 5 10 6 48 A 18 Dense to very dense, moist, brown and gray, silty 30 SAND with gravel (glacial till) 15 50/6 C - 100+ Thin silt interbed at 20' --- 20 41 1 Mottling from 20' to 25' 50/4 25 1100+ 50/5* ------------------------------------------------------------------ Very dense, moist, brown, medium to coarse SAND (advance outwash deposits) -- 30 21 77 --------- - - ------ ------------- - ----------- --------- 43 BOH @ 31' 34 No free water observed. Minor caving following auger withdrawal. *bouncing; blow count overstated Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by geologic Interpretations, engineering analysis, and judgment. They are not necessarily representative of other times and locations. We VAII not accept responsibility for the use or interpretation by others of information presented on this log. % METERS RESIDENCE 19016 94TH AVENUE WEST EDMONDS, WASHINGTON JANUARY 24, 2012 PROJECT NO. 05-079