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Home My WebLink About20251105_PJA25-0059_GeotechReport Memo PanGE0 Geoh=ch?jr al&cac'hgi ake Engmeermc_.l Consultants August 71h 2025 File No. 25-253.200 Steve Hansen steveh do ishproperties.com Thunderbird Hangar,LLC 8383 1581" Avenue North, Suite #200 Redmond, WA 98052 Subject: Geotechnical Report for Foundation Design Vacant Lot North of Airport Office Arlington Municipal Airport, Washington Dear Steve: As requested, we have completed a geotechnical evaluation for the proposed hangar at the Arlington Municipal Airport, at the vacant lot located immediately north of the airport office. The approximate location of the site is shown in Figure 1 attached at the end of this report. The proposed hangar will have a plan dimension of about 150 feet by 75 feet and will be located in the approximately western one-third of the site. At-grade parking will be provided on the east side of the hangar. We anticipate that the proposed hangar will be one-story, lightweight high-bay metal building. The layout of the proposed development is shown in Figure 2. The site is an open field that is practically level. 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 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 Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 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. 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. This unit appears to be consistent with the recessional outwash mapped in the area. Groundwater was not encountered in the test pit at the time of testing. Based on a review of nearby monitoring wells at 17825 501h Ave NE(near the northwest corner of the Airport) and Arlington Facility South Landfill (just east of the airport), groundwater at the project site appears to be less than 20 feet deep. 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). 25-253 Arlington Airport GwRpt docx 2 PanG EO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 LIQURFACTION EVALUATION Based on the site geology and our understanding of the groundwater depth in the site vicinity, it is our opinion that liquefaction could occur at the site. Liquefaction occurs when saturated, predominately sand and silt are subjected to cyclic loading during a strong seismic event. This causes the porewater pressure to increase in the soil, thereby reducing the inter-granular stresses. As the inter-granular stresses are reduced, the shearing resistance of the soil decreases. If pore pressures develop to the point where the effective stresses acting between the grains become zero, the soil particles will be in suspension and behave like a viscous fluid.Typically, loose, saturated, sand and silt that have a low enough permeability to prevent drainage during cyclic loading have the greatest potential for liquefaction. Soil liquefaction may cause the temporary loss/reduction of foundation capacity and settlement. A detailed evaluation of soil liquefaction is beyond the scope of our current scope of work. However, based on the relatively light-weight nature of the proposed building, it is our opinion that conventional footings are adequate in the event of soil liquefaction. SEISMIC SITE CLASS We anticipate the project design will follow the 2021 edition of the International Building Code (IBC). Both editions of IBC specify a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and both IBC reference ASCE 7-16 for site class determination. Because the site soil is considered prone to soil liquefaction, the site would be considered Site Class F in accordance with the IBC. For Site Class F, Section 11.4.8 of ASCE-7-16 states that "A site response analysis shall be performed in accordance with Section 21.1 for structures on Site Class F sites, unless the exception to Section 20.3.1 is applicable." The exception in Section 20.3.1 (1) of ASCE 7-16 states that "For structures having fundamental periods of vibration equal to or less than 0.5 s, site response analysis is not required to determine spectral accelerations for liquefiable soils. Rather, a site class is permitted to be determined in accordance with Section 20.3 and the corresponding values of F, and F, determined from Tables 11.4-1 and 11.4-2." In other words, for structures with a period of vibration equal to or less than 0.5 second and situated on liquefiable soils, 25-253 Arlington Airport GeoRpt docx 3 PanGEO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 the IBC/ASCE 7-16 exception allows the values of Fa and F,, for liquefiable soils be taken equal to the values of site class determined without regard to soil liquefaction. We anticipate the fundamental period of the structure will be less than 0.5 second, and the seismic site coefficients should be determined based on a Site Class D. The fundamental period of the proposed buildings should be confirmed by the structural engineer. FOUNDATION It is our opinion that the proposed structure may be supported on conventional spread footings. The following sections summarize our recommendations for foundation design and subgrade preparation. Allowable Bearing Pressure We recommend a maximum allowable soil bearing pressure of 2,000 pounds per square foot(psf)to size the 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; however, the one-third increase should not be applied for seismic forces due to risk of soil liquefaction. For frost protection considerations, the footings should be placed at a minimum 12 inches below final exterior grade. Lateral Resistance Lateral loads on the foundation elements may be resisted by passive earth pressure developed against the embedded portion of the foundation system and by frictional resistance between the bottom of the foundation and the supporting subgrade soils. A frictional coefficient of 0.4 may be used to evaluate sliding resistance developed between the foundation and the compacted structural fill subgrade. Unless covered by pavements or slabs, the passive resistance in the upper 12 inches of soil should be neglected. Passive soil resistance may be calculated using an equivalent fluid weight 350 pcf, assuming foundations are backfilled with properly compacted structural fill and level ground surface. 25-253 Arlington Airport Ge Rpt docx 4 PanGEO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 The above values include a factor of safety of 1.5. Compaction of Footing Subgrade Improvement All footing subgrade should be carefully prepared. We recommend that the exposed footing subgrade soil be compacted with a jumping jack to a dense and unyielding conditions before placing reinforcing steel. Any softened or disturbed soils that cannot be compacted should be removed from the footing excavations and replaced with structural fill. The adequacy of footing subgrade should be observed and verified by PanGEO. Estimated Settlement Total and differential settlements under service loads are anticipated to be within tolerable limits for footings designed and constructed as discussed above. Under static loads, we anticipate the footings to settle less than one inch and differential settlement should be less than about '/2 inch. Additional settlements could occur during the IBC design seismic event. However, based on our estimate,we anticipate that differential settlement from liquefaction will be about 1 to inches or less. Footing Drain For at-grade buildings, footing drains are optional. SLAB Where the slab will be subject to heavy loads from the aircrafts or heavy equipment, a structural slab should be used. In lightly loaded areas, a conventional slab on-grade is considered adequate. 25-253 Arlington Airport GeoRpt docx 5 PanGEO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 Subgrade Preparation for Structural Slab The structural slab, where used, should be supported on at least one foot of structural fill. All loose and unsuitable soils should be removed before placing the structural fill. The soils exposed at the bottom of the excavation should be compacted to a firm and unyielding condition before placing the structural fill. Any soft/loose and pumping subgrade soil detected during compaction should be removed and replaced with structural fill. Design Parameters for Structural Slab For design of the structural slab, we recommend the use of a modulus of subgrade reaction, ks, of 90 pounds per cubic inch (pci). Capillary Break In spaces where moisture may be sensitive, the slabs should be constructed on a minimum 4-inch-thick capillary break. Where needed, the capillary break should consist of open-graded, free-draining, crushed rock compacted to a firm and unyielding condition. The capillary break material should have no more than 10 percent 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 10-mil polyethylene vapor barrier be placed below the entire slab. STRUCTURAL FILL Unless otherwise noted, structural fill should consist of imported, well-graded, granular material, such as Gravel Borrow (Section 9.03.14 (1) of the 2025 WSDOT Standard Specifications), Seattle Mineral Aggregate Type 17, Crushed Surfacing Base Course (CSBC), or other approved equivalent. Structural fill should be moisture conditioned to near its optimum moisture content, placed in loose, horizontal lifts less than 12 inches in thickness, and systematically compacted to 25-253 Arlington.Airport Gt,oRpt docx 6 PanGEO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 a dense and relatively unyielding condition. If density tests will be performed, the test results should indicate 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. 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. 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. 25-253 Arlington Airport Ge Rpt docx 7 PanGEO, Inc. Geotechnical Report for Hangar Foundation Design Vacant Lot North of Airport Office, Arlington Municipal Airport, Washington August 7, 2025 Sincerely, w,3 ti � o �0 �-/Sly SS✓ONAL�NG August7, 2025 Siew L. Tan, P.E. Principal Geotechnical Engineer (STan@pangeoinc.com) 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 GwRpt docx 8 PanGEO, Inc. Port4Qe Creek w z j�977iS1 NE Cemetery Rri a w w z � z a Project Site 188th SI NE Feld 1881th St NE Quake Pa►k - - Arlington z - Municipal p Airport n ul York . . z Patk �76 > Syr <t � a C vwsbDn n. High , 172na S1 NE School �n .7ltit St NE 1591h SyNE - - i I IV Not to Scale Base Map:ESRI Topographic FbnGProposed EO Hangar VICINITY MAP 18204 59th Ave NE, I N C O R P O R A T E D Arlington, WA 98233 project N0 25-253 Figure No I 6 U N 3N anb 4169 Q InO Q N � m J X II N m a O _ Z Q T a O Q a in J a d w X W LO G_ Z N co ' 3N as ul69 m co 0 . . . . . • •-� r+ t i s a s' . CM 1� • • ��•• • • • • • . . • . • • • • • • • h § •I �z GO T ' •' ^ V a 2 Q . : : . . . . . . . . . . . . . . • . . . - . . . . o o :I oo c T Y � a, •i O 04 CL co At N F- 04 z C O LL O cn 6 MR L Xu X X AW • — i nui zzn:w ,o — Q a Q Q J APPENDIX A SUMMARY TEST PIT LOG RELATIVE DENSITY/CONSISTENCY TEST SYMBOLS for In Situ and Laboratory Tests SAND/GRAVEL SILT/CLAY listed in"Other Togs"column. SPT Approx.Relative ConsistencySPT Approx.Undrained Shear ATT Atterberg Limit Test Density N-values Density(%) N-values Strength(psi) 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 DO 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) '' ...•'••••••• "'"' TXC Triaxial Compression 50%or more of the coarse 3' GP : Poorly-graded GRAVEL - ° ••••••• ••........................ UCC Unconfined Compression fraction retained on the#4 ' � •��- • sieve.Use dual symbols leg. o a GM: Silty GRAVEL S GP-GM)for 5%to12%fines. GRAVEL(>12%fines) ........... SYMBOLS GC: Clayey GRAVEL ...........I......................................... amplelln Situ test types and intervals SW. Well-graded SAND Sand SAND(<5%fines) `. - ••••••...I ......•• -- 2-inch OD Split Spoon,SPT SP : Poorly-graded SAND 140-lb,hammer,30"drop) 50%or more of the coarse ':-; - __.• ( fraction passing the#4 sieve. SM Silty SAND Use dual symbols(eg.SP-SM) SAND >12%fines for 5%to 12%fines. ( ) 3.25-inch OD Spilt Spoon ................................ .............................. SC Clayey...SAND..... .............................-..... (300-lb hammer,30"drop) ML: SILT .................................1.......... ...... .,, i Liquid Limit<50 ! CL : Lean CLAY Non-standard penetration test(see boring log for details) Silt and Clay == OL : Organic SILT or CLAY 50%or more passing#200 sieve i 5 .................................. — .. .:...-. .. ..... .... . ............................... MH Elastic SILT , Thin wall(Shelby)tube ........l a s t s IL........... ..........,............. Liquid uid Limit>50 CH i Fat CLAY .................................................. OH: Organic SILT or CLAY .........................y.......................................... . ................. ..._.-.............................. Grab Hi hl Or anic Soils PT PEAT n15 N otes: 1. Soil exploration logs contain materal descriotions based on visual observation and field tests using a system modified from the Unr:orm Sod Classification System(USCS).Where necessary laboratory tests have peen Rock core conducted(as noted in the"Other Tests"column),unudescriptions may include a classification.Please"efer to the discussions in the report text for a more complete description of the subsurface conditions 2. The graphic syr:bols 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 a Groundwater Level at Laminated: Layers of soil typically 0.05 to 1 mm thick,max.1 cm Bloc Angular soil lumps that resist breakdown time of drilling r Lev Blocky: 9 P � Static 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 1 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 T 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 $ Gravel Medium Sand: #10 to#40 sieve(2.0 to 0.42 mm) Dry Dusty,dry to the touch c� i Coarse Gravel: 3 to 314 inches Fine Sand: #40 to#200 sieve(0.42 l0 0.074 mm) Moist Damp but no visible water a Fine Gravel: 3/4 inches to#4 sieve Silt 0.074 to 0.002 mm Wet Visible free water c? Clay > <0.002 mm 0 0 30 Terms and Symbols for PanGEO V. N C G R P O R A T E o Boring and Test Pit Logs Figure A-1 o Phone: 206.262.0370 Test Pit Logs Project No: 25-253 Project Name: Proposed Hangar Project Location: 18204 59`h 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 Death ft Material Description 0—'A [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 ALI t� rtr:3 � 1 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 Pa1"ZGE i Figure A-2 I N C O R P O R A T E D Oobtlnisfl{!*sNpwb Enp6w�Yp Cwu/Ynb PanGEO , o R r o a n r e o ,technical&tarthquake Engineering Consultants July 301h 2025 File No. 25-253 Steve Hansen stevehajshproperties.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. t f Kw Plate 1. Site of the infiltration test location 1�11ulk'. vX v isi�a I A i'oo)2o-1-0 -0 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 In Filtration 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 (CFm)and site variability(CF„). 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 (CFv) is intended to correct for the number of locations sampled and the consistency of the underlying soil conditions. The value for CF„ 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 Arlinglon 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 CFm = 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, N P 9 30342 oc� FGiS ONA L tiNG 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. C PertaOe C�eett z 9`�7j1S1 NE Cr'±metely Rd c w z v � a z rz V Project Site vans 188Ih SI NE Field 188th St NE T L_t •l+ -.. I Quake F41tk Arkngton -Z - talunrcip J p c_ Auporl - �r? 4� w York m_ z _ Q 0 a = 5! y V%Sbn _ x High 531 c 172nd St NE Se hoof a `n 5 7it1 S1 NE 169th,g/NE 3 I IV Not to Scale Base Map:ESRi Topographic / Proposed Hangar VICINITY MAP F�nGEXD 18204 59th Ave NE, I N C O R P O R A T fl 0 Arlington, WA 98233 Protect No 25-253 Figure No 1 N U N 3N and 4169 (6 C Z N O 11 N 0. z Q� T 0 Q Q y 0 - 0 a x w E Q M LD a z N m -- — E Q N LU 9N as 4469 c co U) a d y, o a g o a,lfl Ina. q.r-e -r .r.. 71. 3..... Z J i � 8•R: xh•- •I M :�: . . . . . . . • . . . . . . . . . . w'[ ...•.•. . . . . . . . Ana: . . . I os�, . a 4-1 �� 0 C = �� a I: Q co o p o :I 'I ,r It ;Iti —� s .» �c r •'ri � c W ) z r n Bn — C m •. ❑ a ••3• (D N O \ O N O s ca pp APFW�I I srt rtx any 1 1 •�+= • X W X X O L7 O .�• .. .. . . . . . . . . . . . . .n O Q _ - - Q Q Q J 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(%) 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 IDS 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 .....:.....,......._...................I. ................. ( 1 TXC Triaxial Compression 50%or more of the coarse � GP: Poorly-graded GRAVEL fraction retained on the#4 '"......""......................... UCC Unconfined Compression . .................... sieve.Use dual symbols leg. GM: Silty GRAVEL d intervals GP-GM)for 5%to12%fines. GRAVEL(>12%fines) -- .- -......I.......................... SYMBOLS GC: Clayey GRAVEL ...... Samplelln Situ test types an SW: Well-graded SAND Sand SAND(<5%fines) — ... .•.... ..••••••............••~......•••... . 2-inch OD Split Spoon,SPT "'" SP : Poorl - raded SAND 50%or more of the coarse :' N--,.•;,_,,,,,Y9, (140-Ib.hammer,30"drop) fraction passing the#4 sieve. Use dual symbols(eg.SP-SM) ° SM: Silty SAND SAND >12/°fines ..:...................................................... for 5%to 12%fines. ( ) 3.25 inch OD Spilt Spoon SC Clayey SAND ........................... .......................................... ...................................................... (300-lb hammer,30"drop) MIL: 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 ........lastic ......................................... Liquid Limit>50 CH Fat CLAY ...................................................... OH: Organic SILT or CLAY ....................................................................... ....... ...... ................................ Grab Highly Organic Soils PT : PEAT Notes: 1. Soil exploration togs contain material descriptions based on visual oeservation and field tests using a system modfied from the Uniform Sol Classification System(USCS)-Where necessary laboratory tests have been H Rock core conducted(as noted in the"Other Tests"column),umt descriptions may include a classification.Please refer to the discussions in the report text for a more complete description of the s0surface conditions. 2. The graphic symbols given above are not inclusive of all symbols that may appear on the borehole logs. ® 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 disnnguisheo by color and/or Fissured: Breaks along defined planes MONITORING WELL composition from matehal units above and below Slickensided: Fracture planes that are polished or glossy Q Groundwater Level at Laminated: Layers of soil typically 0.05 to 1 mm thick,max.1 cm time of drilling(ATD) Blocky: Angular soil lumps that resist breakdown t 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 I Silica sand backfill normal to core axis #• COMPONENT DEFINITIONS Slotted tip COMPONENT SIZE/SIEVE RANGE COMPONENT SIZE I 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 z � Coarse Gravel: 3 to 314 inches Fine Sand: #40 to#200 sieve(0.42 to 0.074 min) Moist Damp but no visible water a a Fine Gravel: 3/4 inches to#4 sieve Silt 0.074 to 0.002 min Wet Visible free water r? Clay i <0.002 mm 0 0 J m YPanGE& Terms and Symbols for N C O R P O R A T E D Boring and Test Pit Logs Figure A-1 o Phone: 206.262.0370 Test Pit Logs Project No: 25-253 Project Name: Proposed Hangar Project Location: 18204 59'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— '/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 '—'--"TMedium dense, grayish brown to gray, SAND with silt,moist; some gravel ti r; ;`off �.�� -J 's �.� `-� A�,•;JT. 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 PmGE& Figure A-2 I M C O R P O R A T i 0 OroPo V"&E..We 8,0—yCamulYnb