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REVIEWED nelson geo 10.16.20_MEMO_11584B20__Florez_Additional_ExplorationsNELSON GEOTECHNICAL ASSOCIATES. INC. 17311-135"' Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com MEMORANDUM DATE: October 15, 2020 TO: Mr. Jorge Florez Mrs. Tatiana Giraldo FROM: Khaled M. Shawish, PE Katelyn Brower, GIT RE: Comment Response for Additional Structures Florez Property Additional Explorations 16000 — 75th Place West Edmonds, Washington NGA File No. 115841320 This memorandum documents our opinions and recommendations regarding the additional existing structures at your residence located at 16000 — 75th Place West in Edmonds, Washington. INTRODUCTION We previously performed work on the site for the demolition of a partially constructed retaining wall system, with our opinions and recommendations outlined in a letter issued to you on February 27, 2020. The subject property is a trapezoidal -shaped parcel approximately 0.46 acres in area. The property is currently occupied by a gated single-family residence with associated paved driveway areas, a grass yard area and landscaping throughout the site. It is bordered to the east by 75th Place West, to the north and south by existing residential development, and to the western lower property line by the BNSF railroad. Localized steep slope hazards are mapped on the property by the City of Edmonds. The entire property steps down moderately westward toward the railroad tracks, with the subject pond, shed, and stairs structures situated above and to the east of the western property line. We understand that work related to these projects was not permitted according to the City of Edmonds code, and we have been requested to provide this evaluation of the current conditions of these structures to satisfy the City's requirements. Memorandum — Comment Response Florez Property Additional Explorations Edmonds, Washington SCOPE NGA File No. 11584620 October 15, 2020 Page 2 Based on our understanding of the planned development and site conditions, the services to be provided by NGA were to: 1. Review available soil and geologic maps of the area. 2. Explore the subsurface soil and groundwater conditions within the vicinity of the existing structures of the shed, pool, and stairs with hand auger explorations. 3. Qualitatively map the conditions of the site slopes in the vicinity of the structures. 4. Perform grain -size sieve analyses on soil samples, as necessary. 5. Provide opinions and conclusions on the construction of the present structures. 6. Provide recommendations for improving/replacing the structures, as needed or warranted. 7. Provide recommendations for site drainage and erosion control. 8. Document the results of our findings, conclusions, and recommendations in a written geotechnical letter. 9. Provide consultation and construction monitoring services throughout the duration of the project. SITE CONDITIONS Surface Conditions The property is generally a trapezoid -shaped parcel covering approximately 0.46 acres in area. The property is currently occupied by a gated single-family residence with associated paved driveway areas, a grass yard area, and landscaping throughout the site. The property is bordered to the east by 75tn Place West, to the north and south by existing residential development, and to the western lower property line by the BNSF railroad. Localized steep slope hazards are mapped on the property by the City of Edmonds, and the site itself is located within the Meadowdale Landslide Complex, as designated by the City of Edmonds. The entire property steps down moderately westward toward the railroad tracks, with the subject rockery, pond, shed, and stairs situated above the partially constructed retaining wall. The shed, pool and associated hardscape are situated immediately above an approximately 4 foot tall rockery and the stairs are situated between the partially constructed retaining wall and pathway along the bottom of the rockery. Based on discussions with the property owner, the unpermitted structures are 3 years old. NELSON GEOTECHNICAL ASSOCIATES, INC. Memorandum — Comment Response Florez Property Additional Explorations Edmonds, Washington NGA File No. 11584620 October 15, 2020 Page 3 The subject structures exist along the terraced slope that steps down below the residence, as shown in the representative cross-section of the slope, Cross -Section A -A' in Figures 3. We did not observe surface water ponding during our site visit on September 21, 2020. ADDITIONAL EXPLORATIONS Geology: The Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington (MF-1541), by James P. Minard, (USGS, 1983) was reviewed for this site. The site is mapped as Quaternary glacial deposits of Whidbey Formation (Qw), but is also located within the Meadowdale Landslide Complex, which can contain old landslide debris. The Whidbey Formation is described as medium bedded, brown to gray sand, silt, and clay. From our explorations, it appears that the onsite soils fit the description of the mapped Whidbey Formation glacial deposits. Explorations: The subsurface conditions within the site were explored with three additional hand auger explorations located in the general vicinity of the structures west of the existing residence, and east of the partially constructed retaining wall. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System presented in Figure 4. The logs of our explorations are attached to this letter and presented as Figure 5. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the exploration logs should be reviewed. At the surface of the hand augers above the rockery, Hand Augers 101 and 103, near the pool and shed, we encountered 2.0 feet of dry dark brown silty fine to medium sand with organics, which we interpreted as undocumented fill. At 2.0 feet, we encountered brown to dark gray silty fine to medium sand with gravel in a medium dense or better condition that we interpreted as native glacial soils. In Hand Auger 102 below the rockery, we encountered 2.0 feet of undocumented fill before encountering dark gray silty fine to medium sand in a loose to medium dense condition at depth. In the hand augers done above the rockery, Hand Augers 101 and 103, the soils were dry to moist. In Hand Auger 102 below the rockery, soils were moist to wet. Hand augers were terminated between 2.2 and 3.5 feet of depth below the existing ground surface. NELSON GEOTECHNICAL ASSOCIATES, INC. Memorandum — Comment Response Florez Property Additional Explorations Edmonds, Washington NGA File No. 11584620 October 15, 2020 Page 4 Hydrogeologic Conditions Heavy groundwater seepage was encountered within Hand Auger 102 below the rockery and pool. We interpreted this water to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils, and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. CONCLUSIONS AND RECOMMENDATIONS Based on our observations and general explorations of the site, as well as the previous explorations of the site, we believe the additional unpermitted structures, consisting of the stairs, pool and associated hardscape, and pumphouse/shed, are stable in their current state and should not pose a significant danger or significant impact to the current slope conditions. According to discussions with the property owner, these structures are approximately 3 years old. In our opinion, from a global stability standpoint, we do not believe the minor grading associated with the construction of these structures has significantly changed the loading of the slope or the drainage picture. The hardscape around the pool does not show visible cracking or settling, but the rockery below the shed, hardscape, and pool does show some minor soil erosion through the rocks. A rockery is generally not considered a retaining structure and acts more as an erosion control feature. Therefore, some soil movement and/or settlement is to be expected to require some maintenance and should be expected with time. Provided the pond/pool is lined with a flexible membrane, this should not be a concern. Even if minor seepage from the pond/pool exists or has a potential to exist, it should not pose a significant issue for the current site and slope conditions, provided that drainage systems are maintained on the site. We recommend the existing subject structures remain in place so as to avoid further unnecessary disturbance of the site slopes. Significant seepage was encountered below the rockery, which is below the pond and shed. The soils above the pond on the slope were dry to moist, with no perched water. As a result, we recommend that the drainage practices around the pond be investigated and improved. This could entail installing a level spreader to help disperse runoff over a larger area. Under no circumstances should water be allowed to flow freely over and concentrate on the site slopes. NELSON GEOTECHNICAL ASSOCIATES, INC. Memorandum — Comment Response Florez Property Additional Explorations Edmonds, Washington NGA File No. 115841320 October 15, 2020 Page 5 As noted in our previous letter, the soils on the site are extremely moisture sensitive and disturb easily when wet. Although no grading should be required to retain the current structures on the site, if grading is to occur for previously evaluated projects, it is our recommendation that no grading or earthwork be performed during the wet season between October 1st and April 30th. If grading is to take place during this time period, additional expenses are to be expected and could include, but are not limited to, the placement of rock spalls to protect native soils, installation of temporary drainage, and covering bare soils with erosion resistant materials. CONSTRUCTION MONITORING We should be retained to provide construction monitoring services during the earthwork phase of the project and during any wet season work, to evaluate slope conditions, any excavations, and drainage system installation. CLOSURE We trust that this memorandum should satisfy your needs at this time. Please do not hesitate to contact us should you require anything further. Five Figures Attached • • • NELSON GEOTECHNICAL ASSOCIATES, INC. Lunds Gulch creek Tiffany Hansen Sales. O Project S't VICINITY MAP Not to Scale Bernard Ford d N Meado'Po E uCatl On Services 16Dth St SW d/P Hry Q 4 v MEADOWDALE s 2�m 161 St _ V Q O ny 1621d PI SW �G SGI G� h N� Meadowdale Beach Park Q Crozier Photograph ,s,th 9 � a g. n 764th St SW Northwest Cane Supply 1&4thP1,.,, Meadowdale Q Q c Community Clubhouse r ` n1651h PI SW ?atment ® 3 D `m 16Mh PI SW Meadowdale PlayfieldsQ -- a p < Deb the noWalker 0 Residential Locks Edmonds WA a Meadowdale 0 Middle School Edmonds, WA I"th St SW 164th P1 SW 1661h PI SA m 168th St SW 168th St o Meadow a Meadowdale High School 0 Neighbor L o` Park o .1� Q y Project Number NELSON GEOTECHNICAL No. Date Revision By CK o Florez Additional ASSOCIATES, INC. 11584B20 NGA 1 9/28/20 Original DPN KSB Explorations etL GEOTECHNICAL ENGINEERS & GEOLOGISTS E Vicinity Map W°tlle °e East Wenatchee orr2 a Figure 1 17311-13535[hAAve. NE, A-500 5526 Industry Lane, it J Woodinville, WA 98072 East Wenatchee, WA 98802 (425) 486-16691 Fax:481-2510 ww—lsongeotech.com (509) 665-76961 Fax: 665-7692 = m0 �- N CD o D -0 a) Q o r: v o (n v sv g m I MOST E"LY -L HA - RR RAIL qNE POND v a N n 00, - - A n Z Ilf KEYSTONE WAL� TO RR TIE WAL TO S I A= "= S a p, o �� z D m Z m o i m O z 3 M 0 �m 3 N M d m� mM �n m � OM G0i 0 Z m> N n > -I N r N w4304 W, z LEGEND HA-101 — — Property line y I 0 HA-1 : : Approximate location W of cross-section I NI� Reference: Site plan based on a plan dated August 1 k\HILL\company\2020 NGA Project Folders\115846-20 Florez Addition Explorations Meadowdale\Draftin Site Plan ., VOL. 5, PC 38 Pv- Number and approximate location of hand auger (9-21-2020) Number and approximate location of hand auger (2-17-2020) 117 titled "Florez Residence Retainina Wa ,g 0 30 60 Scale: 1 inch = 30 feet II," prepared by CG En 0 West 40 T 30 a) c 20 c� Patio Wood Retaining Wall —\ Rockery a� �m a� w HA-101 ? ,¢D M Pool HA-102 n > Z Q 10 —? m m z U) r�, - �' m o Fill m O z 00 3 �m -� mo 0 m� Nm A' N,o V m I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I \ 0 Z Z 0 10 20 30 40 50 60 70 80 T��q m'F0 y Distance (feet) rn r Exploration m Hand Auger Designation ---> HA-1 o � m East 40 30 20 10 I N NOTES: Groundwater Level � 1 During Exploration 1) Stratigraphic conditions are interpolated between the explorations. Actual conditions may vary. Geologic Contact ? _ —? 2) Elevations are arbitrary. Z moo (approximate) I Reference: Cross Section is based on field measurements using a hand-held clinometer and 100-ft tape measure. . 0 X AHII I \rmmnnn,Aq qn NrA Prniori Fnlrior \11SRdR_gn Fin— Aririitinn F—In-fin— NA—inrIM7\nl ftinnV(.q riwn UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests is based on ASTM D 2488-93. Moist - Damp, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 11584B20 Florez Additional ASSOCIATES, INC. NGA Explorations GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 9/28/20 Original DPN KSB Figure 4 Soil Classification Chart W°35thAv..NE,°e Ea=1We"at°heeoff2 811-1%[h Ave.NE,A-500 Industry Lane,88 Woodinville, WA 98072 East Wenatchee, WA 98802 East W (425) 486-1669 / F_ 481-2510 www. nelsongeotech.con, (509) 665-7696 1 Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION HAND AUGER 101 0.0 - 2.0 GRASS, UNDERLAIN BY DARK BROWN SILTY FINE TO MEDIUM SAND WITH ORGANICS AND GRAVEL (LOOSE, DRY) (FILL) 2.0 - 2.2 BROWN GRAY SILTY FINE TO MEDIUM SAND WITH ROCK SPALLS (MEDIUM DENSE TO DENSE, MOIST) NO SAMPLES WERE COLLECTED NO GROUNDWATER SEEPAGE WAS ENCOUNTERED NO HAND AUGER CAVING WAS ENCOUNTERE HAND AUGER WAS COMPLETED AT 2.2 FEET ON 9/21/2020 HAND AUGER 102 0.0 - 2.0 GRASS, UNDERLAIN BY DARK BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, (LOOSE, MOIST TO WET) (FILL) 2.0 - 3.5 SM-ML DARK GRAY SILTY FINE TO MEDIUM SAND WITH SILT LENSES (MEDIUM DENSE, WET) NO SAMPLES WERE COLLECTED SLIGHT TO HEAVY GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 2.0 FEET NO HAND AUGER CAVING WAS ENCOUNTERED HAND AUGER WAS COMPLETED AT 3.5 FEET ON 9/21/2020 HAND AUGER 103 0.0-2.0 BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, ORGANICS, AND WOOD DEBRIS (LOOSE, DRY) (FILL) 2.0 - 3.0 SM BROWN GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, DRY -MOIST) NO SAMPLES WERE COLLECTED NO GROUNDWATER SEEPAGE WAS ENCOUNTERED NO HAND AUGER CAVING WAS ENCOUNTERED HAND AUGER WAS COMPLETED AT 3.0 FEET ON 9/21/2020 KSB: NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 115841320 FIGURE 5