The URL can be used to link to this page

Home My WebLink About20251105_PJA25-0059_Infiltration Rate Report PanGE0 C O R P O R A T E D Geotechnica/&Earthquake Engineering Consultants July 301h 2025 File No. 25-253 Steve Hansen stevehgishproperties.com Thunderbird Hangar,LLC 8383 158th Avenue North, Suite#200 Redmond, WA 98052 Subject: Infiltration Test Vacant Lot North of Airport Office Arlington Municipal Airport, Washington Dear Steve: As requested, we have completed a Small Pilot Infiltration Test(PIT) designated as PIT-1 at the vacant lot located immediately north of the airport office at the Arlington Municipal Airport in Arlington, Washington. The approximate location of the site is shown in Figure 1 attached at the end of this report, and the approximate location of the infiltration test is shown in Figure 2. The site is an open field that is practically level. Plate 1,below, shows the site conditions after the infiltration test was completed. Details of our testing are discussed below. J" Plate 1. Site of the infiltration test location 3213 Eastlake Ave E,Ste B Seattle,WA 98102 Tel(206)262-0370 www.pangeoinc.com Infiltration Test Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington July 30, 2025 SUBSURFACE EXPLORATION We observed and logged the excavation of one test pit (PIT-1) on July 24, 2025, at the approximate location shown in Figure 2. The field exploration was overseen by an engineer with our firm who logged and sampled the soil encountered in the test pits. The test pit was initially completed at a depth of approximately 4 feet for infiltration testing. After the infiltration test was completed, PIT-1 was over-excavated to about 8 feet deep to check for possible groundwater mounding. The soils exposed in the test pit were logged in general accordance with the system summarized on Figure A-1 of Appendix A, Terms and Symbols for Boring and Test Pit Logs. The summary test pit log is included in Appendix A of this report provides descriptions of the materials encountered, depths to soil contacts, and depths of seepage or caving, if present. The relative in-situ density of cohesionless soils, or the relative consistency of fine-grained soils, was estimated from the excavating action of the excavator, and the stability of the test pit sidewalls. Where soil contacts were gradual or undulating, the average depth of the contact was recorded on the logs. The test pit excavated for this study was backfilled with the excavated soils. The backfill was tamped with the backhoe bucket and the ground surface leveled. The backfill was not compacted to a dense condition for the structural support. During construction of the project, the earthwork contractor should locate the test pits, remove the loose backfill and replace it with properly compacted structural fill if the tests are located in the load-bearing structural areas. SUBSURFACE CONDITIONS SITE SOIL CONDITIONS The following is a summary description of the soil conditions encountered in the test pit. For a detailed description of the subsurface conditions encountered, please refer to the attached test pit log. The stratigraphic contacts indicated on the test pit log represent the approximate depth to boundaries between soil units. Actual transitions between soil units may be more gradual or occur at different elevations. The descriptions of groundwater conditions and depths are likewise approximate. 25-253 Arlington Airport Infiltration Rpt.docx 2 PanGEO, Inc. Infiltration Test Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington July 30, 2025 In summary, the test pit encountered several inches of topsoil at the surface, in turn underlain by loose to medium dense,fine to medium dense sand with silt to the termination depth of the test pit at about 9 feet deep. Groundwater was not encountered in the test pit at the time of testing. The designers and contractors should be aware there will be fluctuations in groundwater conditions depending on the season, amount of rainfall, surface water runoff, and other factors. Generally, the water level is higher and seepage rates are greater in the wetter, winter months (typically October through May). INFILTRATION TESTING The field infiltration tests were conducted in general accordance with the procedure for the small PIT as outlined in the Washington Department of Ecology Stormwater Management Manual for Western Washington (WDOE, 2019). In general, the test consisted of the following procedure: • Excavate a pit to the approximate design bottom of the proposed infiltration facility with a minimum bottom area of at least 12 square feet. • Pre-soak the bottom of the pit by maintaining a water level of at least 6 inches above the bottom of the pit for at least 6 hours. • At the end of the pre-soak period, a flow meter was used to monitor the amount of water needed to maintain a constant head of 6 inches for at least one hour and until at least a point at which a constant volume of water per time unit was achieved. • At the end of the constant head test, we measured the falling head infiltration rate by shutting off the water flow and recording the drop in water level over regular time intervals for one hour or until all the water has infiltrated. 25-253 Arlington Airport Infiltration Rpt.docx 3 PanGEO, Inc. Infiltration Test Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington July 30, 2025 UNCORRECTED FIELD INFILTRATION RATE The field infiltration rate is then calculated based on the final measured volume per time unit and the surface area of the pit. In summary, our test results yield an uncorrected infiltration rate of 20 and 18 inches per hour for the constant head and falling head conditions, respectively. DESIGN INFILTRATION RATE The small pilot infiltration test provides an uncorrected, saturated hydraulic conductivity (Ksat) of the soil. To provide a long-term design infiltration rate, the Ksat value is factored by applying a series of correction factors (CF) outlined in the WDOE Manual. As discussed below, the correction factors account for the test method (CFt), influent control (CF,n)and site variability(CFI). The value of each of these correction factors are discussed below. Test Method The correction factor for the test method (CFt) is used to account for differences between the test method and in-situ infiltration testing. WDOE Manual specifies a CFt value of 0.5 based on the use of the small PIT method. This value was incorporated in our calculation. Influent Control The influent control correction factor (CFm) is intended to account for a reduction in infiltration capacity due to clogging from siltation and the build-up of biological material. An influent control factor of 0.9 was used in our calculation, assuming that when the infiltration systems lose 10 percent of their infiltration capacity due to clogging, the system will be maintained or cleaned. Site Variability The correction factor for site variability(CFI) is intended to correct for the number of locations sampled and the consistency of the underlying soil conditions. The value for CFI ranges from 0.33 to 1.0. Based on the number of exploration locations, relatively uniform soil conditions encountered at our exploration locations and our 25-253 Arlington Airport Infiltration Rpt.docx 4 PanGEO, Inc. Infiltration Test Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington July 30, 2025 experience and engineering judgment, we assigned a correction factor of 0.75 for site variability. Total Correction Factor The total correction factor (CF = CFv x CFt x Um = 0.33) is then applied to the infiltration rate summarized in Table 2 to obtain a corrected infiltration rate appropriate for long term design purposes. The total correction factor of 0.33 is then applied to the field rate to estimate a long-term design infiltration rate. In summary, we recommend that a design infiltration rate of 6 inches per hour to size he infiltration facility. TEST PIT BACKFILL The test pit was backfilled with the excavated soils. The backfill was tamped with the excavator bucket and the ground surface leveled. The backfill was not compacted to the requirements of structural fill. During construction of the project, the earthwork contractor should locate the test pits,remove the loose backfill and replace it with properly compacted structural fill. LIMITATIONS We have prepared this report for use by Thunderbird Hangar, LLC and the project 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. 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 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. 25-253 Arlington Airport Infiltration Rpt.docx 5 PanGEO, Inc. Infiltration Test Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington July 30, 2025 Within the limitation of scope, schedule, and budget, PanGEO engages in the practice of geotechnical engineering and endeavors to perform its services in accordance with generally accepted professional principles and practices at the time the Report or its contents were prepared. No warranty, express or implied, is made. We trust that the information outlined in this letter meets your need at this time.Please call if you have any questions. Sincerely, LAN T �;ygti ti :-. o 30342 0 FG/s ss/ ONA1.�NG`a July 30, 2025 Siew L. Tan, P.E. Principal Geotechnical Engineer Enclosures: Figure 1 Vicinity Map Figure 2 Site and Exploration Plan Figure A-1 Terms and Symbols for Boring and Test Pit Logs Figure A-2 Log of Test Pit PIT-1 25-253 Arlington Airport Infiltration Rpt.docx 6 PanGEO, Inc. Creek i Z �997hSt NE Cemetery Rd Q - —- - CD w _ z > I Q z 1 V - - I v CD Project Site vans 188th St NE Field 118th St NE Quake Pa rk Arlington z Municipal p Airport L u� LU York 4- z Park �� w rry > L_ y LL M z 0 C y o Q a 5 x Weston � High 331 'E 1 72nd St NE School (n 171s7 St NE 169th S/NE ro„ w � Not to Scale Base Map:ESRI Topographic PanGEO Proposed Hangar VICINITY MAP 18204 59th Ave NE, I N C O R P O R A T E D Arlington, WA 98233 Project No. 25-253 Figure No. 1 46 MK PAID 22� • exam n . s 141 75 a7= I ;1 �i [D 3 ` a PIT-1 ;y:_ _- Z � � � - � � •�-=�••-••-•PROPOSED LEASE•AR EA•�•••+••• / •� �y .'±48,805 Sq. Feet. 8 1 W NEW PEMB - LU .f1.1204 Acres.• •�.- ,�/ice 18M" 4i Z /r t11,250"Sq. Feet LO h ❑z - — LO I : : : : : : : : : : : : : : : : : ::• • • • • • • • • • • • • • • • •; .;. 4 . • • • • - • - • •: : : : : : : : : : : : : : : : : : : : : : : : : : : : . . a Approx. Scale NB9'460/T 2N.50' 0 20' 40' n Legend: QwR LINE W CONNECNNG hi4NNOES Note: Base map modified Site Plan by Magellan Architects. Approx. Infiltration Test Pit Locations (PanGEO, July 2025) PmGE& Proposed Hangar SITE AND EXPLORATION PLAN Approx. Building Footprint 18204 59th Ave NE, INCORPORATED Arlington, WA98233 r1 J Approx. Site Boundary Project No. 25-253 Figure No. 2 APPENDIX A SUMMARY TEST PIT LOG RELATIVE DENSITY/CONSISTENCY TEST SYMBOLS for In Situ and Laboratory Tests SAND/GRAVEL SILT/CLAY listed in"Other Tests"column. Density SPT Approx.Relative Consistency SPT Approx.Undrained Shear ATT Atterberg Limit Test N-values : Density(/o) N-values Strength(psf) Comp Compaction Tests Very Loose <4 <15 Very Soft <2 <250 Con Consolidation Loose 4 to 10 15-35 : Soft 2 to 4 250-500 DID Dry Density Med.Dense 10 to 30 35-65 Med.Stiff 4 to 8 500-1000 DS Direct Shear Dense 30 to 50 65-85 : Stiff 8 to 15 1000-2000 %F Fines Content Very Dense >50 85-100 Very Stiff 15 to 30 2000-4000 GS Grain Size Hard >30 >4000 Perm Permeability PP Pocket Penetrometer UNIFIED SOIL CLASSIFICATION SYSTEM R R-value MAJOR DIVISIONS GROUP DESCRIPTIONS SG Specific Gravity •�' GW: Well-graded GRAVEL TV Torvane Gravel GRAVEL <5%fines •' .....:...................... ............................... ( ) GP : Poorl raded GRAVEL TXC Triaxial Compression 50%or more of the coarse o o Y-g fraction retained on the#4 "' """............""""""" UCC Unconfined Compression sieve.Use dual symbols(eg. GM: Silty GRAVEL GRAVEL(>1 2%fines) oo . .......................................................... GP•GM)for5%to12%fines. ; GC: Clayey GRAVEL SYMBOLS ..................................... ............................. ......:....................................................... Samplelin Situ test types and intervals SW: Well-graded SAND Sand SAND(<5/°fines) :'r 2-inch OD Split Spoon,SPT SP; Poorly-graded raded SAND 50%or more of the coarse Y-g (140-lb.hammer,30"drop) fraction passing the#4 sieve. Use dual symbols e .SP-SM ": SM: Silty SAND Y 1 9 ) SAND(>12%fines) ......................................................... for 5%to 12%fines. B 3.25 inch OD Spilt Spoon SC : Clayey SAND P P .................................:.................................. ....:....................................................... (300-lb hammer,30"drop) ML : SILT Liquid Limit<50 CL Lean CLAY Non-standard penetration ...................................................... Silt and Clay —= OL : Organic SILT or CLAY test(see boring log for details) 50%or more passing#200 sieve MH: Elastic SILT...................................... , Thin wall(Shelby)tube Liquid Limit>50 CH Fat CLAY . OH : Organic SILT or CLAY ......................g..y.. ....:................................. ....:....................................................... Grab Highly Organic Soils — PT : PEAT Notes: 1. Soil exploration to 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 Rock core 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. M Vane Shear 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 MONITORING WELL composition from material units above and below Slickensided: Fracture planes that are polished or glossy SZ Groundwater Level at Laminated: Layers of soil typically 0.05 to 1 mm thick,max.1 cm Blocky: Angular soil lumps that resist breakdown time of drilling(ATD) Y 9 P 1 time Groundwater Level Lens: Layer of soil that pinches out laterally Disrupted: Soil that is broken and mixed Cement/Concrete Seal 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 Bentonite grout/seal Homogeneous: Soil with uniform color and composition throughout BCN: Angle between bedding plane and a plane Silica sand backfill normal to core axis COMPONENT DEFINITIONS Slotted tip a COMPONENT SIZE/SIEVE RANGE COMPONENT SIZE/SIEVE RANGE Slough Boulder: >12 inches Sand Bottom of Boring Cobbles: 3 to 12 inches Coarse Sand: #4 to#10 sieve(4.5 to 2.0 mm) MOISTURE CONTENT O Gravel Medium Sand: #10 to#40 sieve(2.0 to 0.42 mm) Dry Dusty,dry to the touch w c� Q Coarse Gravel: 3 to 3/4 inches Fine Sand: #40 to#200 sieve(0.42 to 0.074 mm) Moist Damp but no visible water Fine Gravel: 3/4 inches to#4 sieve Silt 0.074 to 0.002 mm Wet Visible free water Clay <0.002 mm 0 0 J W Y PanGEO Terms and Symbols for 0N C O R P O R A T E D Boring and Test Pit Logs Figure A-1 2 Phone: 206.262.0370 Test Pit Logs Project No: 25-253 Project Name: Proposed Hangar Project Location: 18204 591h Ave NE,Arlington,WA Excavated: 7/24/2025 Test Pit No. TP-1 Location: 48.161275, -122.151633 (WGS84) Approximate ground surface elevation:N/A Depth ft Material Description 0_1�4 [Topsoil] Loose,brown, silty SAND with gravel,moist,roots and rootlets [Marysville Sand Member-Qvrm] 1/4-4 Loose to medium dense,reddish brown to brown,fine to medium SAND with silt,moist; some gravel and cobbles ---------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 4- 9 Medium dense,grayish brown to gray, SAND with silt,moist; some gravel y j Image of soils encountered approximately 9 feet below the existing ground surface.PIT-1 was terminated approximately 9 feet below grade. Groundwater was not encountered at the time of exploration. Logged by: R. Cooter PanGEO Figure A-2 I N C O R P O R A T E O Geofxh mM B E-hQ-*1 EWo—fing Consu/hots