Att G - GEOTECHNICAL REPORT S-10900-00016730.pdf

CONTRACT DOCUMENTS

FOR

OREGON DEPARTMENT OF AVIATION

MULINO STATE AIRPORT

HANGAR AND WATERLINE PROJECT

ATTACHMENT G – HANGAR AND WATERLINE PROJECT

GEOTECHNICAL REPORT

OWNER:

Oregon Department of Aviation

3040 25th St. SE Salem, OR 97302

ENGINEER:

Century West Engineering

5500 Meadows Rd., Suite 250

Lake Oswego, OR 97035

Phone: 503.419.2130

APRIL 2026

This Page Left Intentionally Blank

Geotechnical Investigation

Mulino State Airport Hangar and Waterline Project

Mulino, Oregon

April 22, 2026

Prepared for

Century West Engineering Corporation 5500 SW Meadows Road, Suite 250

Lake Oswego, OR 97035

Prepared by

16520 SW Upper Boones Ferry Road, Suite 100

Tigard, OR 97224-7661

(503) 641-3478 | www.gri.com

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report

TABLE OF CONTENTS

1 INTRODUCTION

2 PROJECT DESCRIPTION

3 SITE DESCRIPTION

3.1 General

3.2 Geology

4 SUBSURFACE CONDITIONS

4.1 General

4.2 Subsurface Conditions

4.3 Groundwater

4.4 In-Situ Infiltration Tests

5 FINDINGS AND RECOMMENDATIONS

5.1 General

5.2 Earthwork

5.3 Hangar Foundation and Structural Slab Support

5.4 Seismic Considerations

5.5 Taxilane Pavements

6 DESIGN REVIEW AND CONSTRUCTION SERVICES

7 LIMITATIONS

8 REFERENCES

TABLES

Table 5-1: Recommended Seismic Design Parameters (ASCE 7-22) Table 5-2: Multi-Period MCER and Design Spectral Values (ASCE 7-22)

FIGURES

Figure 1: Vicinity Map Figure 2: Site Map Figure 3: Footing Overexcavation Detail

APPENDICES

Appendix A: Field Explorations and Laboratory Testing Appendix B: Geoprofessional Business Association Guidance Document

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 1

1 INTRODUCTION

GRI completed a geotechnical investigation for the proposed Mulino State Airport Hangar and Waterline Project in Mulino, Oregon. The project location is shown on the Vicinity Map, Figure 1. This report provides a summary of our investigation, which included subsurface explorations, field and laboratory testing, coordination and interpretation of laboratory testing performed by others, and development of geotechnical design and construction recommendations for the proposed project.

2 PROJECT DESCRIPTION

The Oregon Department of Aviation is planning improvements at the Mulino State Airport that will include construction of a new T-hangar and taxilane extension near the northeast corner of the airport. The proposed T-hangar will extend north of an existing hangar building, completing the third row of hangars in this portion of the airport. A new stormwater infiltration basin is also planned east of the new hangar building. An aerial image showing the existing conditions at the project site is shown on the Site Map, Figure 2.

We anticipate that the proposed T-hangar structures will be designed in accordance with the 2025 Oregon Structural Specialty Code (OSSC), which references the 2024 International Building Code (IBC) and the American Society of Civil Engineers (ASCE) Document 7-22, Minimum Design Loads for Buildings and Other Structures (ASCE 7-22). We anticipate the proposed pavement improvements will be evaluated in accordance with Federal Aviation Administration (FAA) Advisory Circulars 150/5320-6G, Airport Pavement Design and Evaluation; 150/5320-5D, Airport Drainage Design; and 150/5370-10H, Standard Specifications for Construction of Airports. We understand that stormwater improvements will also follow guidelines provided in Clackamas County’s 2023 Water Environment Services Stormwater Standards.

3 SITE DESCRIPTION

3.1 General

As noted above, the project site is located near the northeast corner of the airport. The site is relatively flat and primarily covered with short grass. Available topographic data indicates the ground surface elevation is approximately 249 feet (North American Vertical Datum of 1988 [NAVD 88]). Existing development adjacent to the project site includes taxilanes and hangars that are similar to the proposed improvements.

3.2 Geology

Local geology maps indicate the project site is mantled with Missoula flood deposits, commonly referred to in the area as the Willamette Silt Formation. This unit typically includes interlayered silt, clay, and sand. A review of local well logs and previous

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 2 explorations identified clayey silt to silty clay at shallow depths. These conditions are similar to the conditions encountered in the current explorations discussed below.

4 SUBSURFACE CONDITIONS

4.1 General

Subsurface materials and conditions at the project site were investigated on January 5, 2026, with four geotechnical borings, designated B-1, B-2, I-1, and I-2. The locations of I-1 and I-2 also coincided with infiltration tests, which are described below. The approximate locations of the explorations are shown on Figure 2.

The explorations were advanced using solid-stem auger drilling techniques and extended to a maximum depth of about 16.5 feet below the ground surface. Disturbed and relatively undisturbed samples were collected, typically at 2.5-foot intervals of depth in each boring.

Standard Penetration Tests (SPTs) were conducted during collection of disturbed samples from the borings. The Standard Penetration Resistance (SPT N-value) provides a measure of the relative density of granular soils and the relative stiffness, or consistency, of cohesive soils.

The field and laboratory programs conducted to evaluate the physical engineering properties of the materials encountered in the explorations are described in more detail in Appendix A. Logs of the explorations are provided on Figures 1A through 4A. The terms and symbols used to describe the materials encountered in the explorations are defined in Table 2A and on the attached legend.

4.2 Subsurface Conditions

For the purpose of discussion, the materials disclosed by our investigation have been grouped into the following categories based on their physical characteristics and engineering properties:

a. Silty CLAY (Alluvium / Flood Deposits)

b. Silty CLAY (Possible Decomposed Basalt)

The following paragraphs provide a description of the units encountered in the explorations and a discussion of the observed groundwater conditions at the site.

a. Silty CLAY (Alluvium / Flood Deposits) Alluvial silty clay soils were encountered in each boring, extending below the ground surface to depths of approximately 15 feet and 15.5 feet. The silty clay is typically gray to light brown-orange, has medium plasticity, and is moist grading to wet below depths ranging from about 5 feet to 7.5 feet. Organics consisting of scattered fine roots and possible agricultural till zones were observed in borings B-1 and B-2 extending below the

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 3 ground surface to depths ranging from about 0.8 feet to 1.3 feet. SPT N-values and Torvane results recorded in the borings typically indicated medium stiff to stiff conditions at shallow depths, becoming soft to medium stiff below depths of about 5 feet to 7.5 feet, which also coincided approximately with the depths where wet soil conditions were observed.

One-dimensional consolidation tests were completed on samples from borings B-1 and B-2, obtained at depths of approximately 5.25 feet and 10.8 feet, respectively. The tests indicate that the alluvial silty clay stratum is overconsolidated and exhibits slight compressibility in the preconsolidation range of pressures and high compressibility in the normally consolidated range of pressures. The test results summarized in Figures 10A and 11A suggest that the preconsolidation pressure of the samples is at least 2 tons per square foot (tsf). Additional information regarding the laboratory testing is provided in Appendix A.

b. Silty CLAY (Possible Decomposed Basalt) Below a depth of approximately 15 feet, the explorations encountered silty clay soils that varied in consistency relative to the shallower soils. The silty clay encountered below 15 feet was typically gray to gray-brown, had medium to high plasticity, and was moist.

SPT N-values recorded in this stratum indicated medium stiff to stiff conditions.

Based on the consistency of the samples and our previous experience near the project location, we have interpreted the deeper silty clay as possible decomposed basalt. Nearby explorations north of the airport encountered similar silty clay soils that become harder and more rock-like with depth.

4.3 Groundwater

Groundwater was observed during drilling at depths ranging from about 5.5 feet to 7 feet below the ground surface. The holes were allowed to stay open for up to several hours after drilling, and we observed water rising to depths ranging from about 1.5 feet to 3 feet.

We anticipate that groundwater levels across the project area fluctuate seasonally but remain relatively shallow throughout the year. The shallowest conditions will be encountered in the winter to early spring, when precipitation is highest. Perched groundwater may also approach the ground surface during periods of heavy or extended rainfall.

4.4 In-Situ Infiltration Tests

Soil infiltration tests were completed adjacent to borings I-1 and I-2 to evaluate the infiltration rate of the soil where a stormwater infiltration basin is planned. Tests at I-1 and I-2 were completed at depths of approximately 3 feet and 2 feet, respectively. A shallower

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 4 test depth of 2 feet was completed in I-2 after groundwater seepage was encountered at about 3 feet prior to initiating the test. The soil at the test depths consisted of medium plasticity silty clay, as described above. Additional description of the test procedure is provided in Appendix A.

The tests were performed over several hours. During the final incremental period of observation, we measured uncorrected infiltration rates of about 0.2 inches per hour (in/hr) at I-1 and 0.1 in/hr at I-2. Clackamas County’s 2023 Water Environment Services Stormwater Standards recommend applying a factor of safety of 3.0 to the uncorrected infiltration rate from encased falling head tests to determine the design infiltration rate.

This corresponds to a design infiltration rate of approximately 0.03 in/hr based on the slower infiltration test results. We believe the test results are generally consistent with the observed soils at the test depths.

FAA Advisory Circular No. 150/5320-5D also uses hydraulic conductivity, k, to evaluate drainage design for pavements. The hydraulic conductivity is a measure of the ease with which a fluid flows through the soil. Calculations to estimate the k value account for the hydraulic gradient (or head) of the fluid and the dimensions of the medium through which the fluid is passing. Using the data collected, we calculated an average k value of approximately 0.08 in/hr (5.9x10-5 centimeters per second [cm/sec]) at I-1 and 0.02 in/hr (1.4x10-5 cm/sec) at I-2.

5 FINDINGS AND RECOMMENDATIONS

5.1 General

The explorations completed for this investigation indicate that the site is mantled by alluvial silty clay. Deeper silty clay interpreted to be decomposed basalt was encountered below a depth of about 15 feet. At the time of our investigation, groundwater was observed at depths ranging from about 1.5 feet to 3 feet below the ground surface. We anticipate that perched groundwater approaches the ground surface during the wet winter months and periods of extended rainfall.

In our opinion, the primary geotechnical considerations associated with the project include the presence of moisture-sensitive, fine-grained soils and the presence of shallow groundwater. The following sections of this report provide our conclusions and recommendations for use in the design and construction of the proposed improvements.

General earthwork considerations and foundation design recommendations are provided first, followed by geotechnical pavement design parameters.

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 5

5.2 Earthwork

5.2.1 General

As noted above, groundwater may be encountered at shallow depths beneath the project site. The fine-grained alluvial soils will be prone to disturbance and softening if overly wet and subject to construction traffic. Therefore, it is our opinion that earthwork can be completed most economically during the drier summer months, which typically extend from June to mid-October. Regardless of the time of year, the contractor should anticipate moist to wet soil conditions if the existing pavements are removed.

The contractor must use construction equipment and procedures to prevent disturbance and loosening of the subgrade soils. To reduce disturbance, any mass site grading should be completed using track-mounted hydraulic excavators. Finished excavations should generally be completed using a smooth-edged bucket to produce a firm, undisturbed surface. Granular haul roads and work pads constructed concurrently with the excavation may also be necessary to minimize subgrade disturbance, particularly if construction occurs during wet ground conditions. If the subgrade is disturbed during construction, loosened, disturbed soils should be overexcavated to firm soil and backfilled with structural fill.

The borings encountered evidence of an agricultural till zone extending to depths ranging from approximately 0.8 feet to 1.3 feet. Where new structures or pavements are planned, it should be confirmed that previously tilled or otherwise disturbed soils are excavated and replaced with structural fill. Any soil that remains in place beneath structures should also be relatively free of organics.

5.2.2 Structural Fill

We recommend that structural fill consist of imported crushed gravel or rock with a maximum particle size in the range of ¾- to 1½-inch-minus and with less than 5% passing the No. 200 sieve (washed analysis). Gravel particles should exhibit a minimum of two fractured faces. The granular fill should be placed in lifts and compacted with vibratory equipment to at least 95% of the maximum dry density, as determined in accordance with ASTM International (ASTM) D698.

Alternate gradation and compaction specifications may be necessary to meet FAA requirements for pavements. Material properties and gradation for aggregate base (AB) and subbase beneath new pavements should meet the requirements of FAA Advisory Circulars 150/5320-6G and 150/5370-10H. Further discussion is provided in Section 5.5.7 of this report.

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 6

Appropriate lift thicknesses will depend on the type of compaction equipment used. For example, if hand-operated vibratory plate equipment is used, the lift thickness should be limited to about 6 inches to 8 inches. Based on field-density testing, if smooth-drum vibratory rollers are used, lift thicknesses up to about 12 inches may be appropriate; if backhoe- or excavator-mounted vibratory plates are used, lift thicknesses of up to about 2 feet may be acceptable.

On-site, fine-grained soils and site strippings free of debris may be used as fill in non-structural landscaped areas. These materials should be placed at about 90% of the maximum dry density, as determined by ASTM D698. The moisture content of soils placed in landscaped areas is not as critical, provided construction equipment can effectively handle the materials.

5.2.3 Site Preparation for New Structures

Most of the project area is currently covered with short grass or similar vegetation. A smaller portion of the project area is surfaced with existing pavements. The ground surface within all building areas and areas to receive structural fill should be stripped of existing vegetation, surface organics, and loose surface soils. Where surface vegetation is present, we anticipate that stripping to a depth of about 4 inches will be required to remove the bulk of the organics. Organic strippings should be disposed of off-site or stockpiled on-site for use in landscaped areas. Deeper excavation may be required to remove soft soil within the previous tilled zone, as described above.

Site improvements within developed areas may encounter undocumented or poorly documented fills, improvements, and infrastructure. Demolition of existing improvements within the areas designated for new structures or hardscaping should include the removal of existing pavements, underground utilities, and any unsuitable backfill encountered.

Excavations made during demolition to remove existing improvements should be backfilled with compacted structural fill.

A GRI representative should evaluate the exposed subgrade prior to backfilling with structural fill. Proof rolling with a loaded dump truck or potholing the subgrade may be part of this evaluation. Any soft areas or areas of unsuitable material disclosed by the evaluation should be overexcavated to firm material and backfilled with additional structural fill. We recommend the contract documents provide unit costs for overexcavation and structural backfill.

5.2.4 Trenching and Backfilling

All excavations should be shored or sloped to protect workers from sloughing or caving soils. Occupational Safety and Health Administration (OSHA) Type C soil should be assumed within all excavations based on the subsurface conditions encountered and the

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 7 depth to groundwater. OSHA requires that open excavations in soils classified as Type C be sloped at 1.5H:1V (horizontal to vertical) or flatter. Excavations should be laterally supported or provided with stable side slopes in accordance with all applicable local, state, and federal regulations. In our opinion, adequate lateral support may be provided by common methods, such as the use of a trench shield or hydraulic shoring.

Depending on the depth of below-grade utilities or other excavations, the groundwater level may occur within the anticipated maximum excavation depths. Groundwater seepage, running soil conditions, and unstable trench sidewalls or soft trench subgrades, if encountered during construction, will require dewatering of the excavation and trench sidewall support. The impact of these conditions can be reduced by completing trench excavation during the summer months, when groundwater levels are lowest, and by limiting the depth of the trenches.

We anticipate that groundwater inflow, if encountered in shallow excavations, can generally be controlled by pumping using sumps. To facilitate dewatering, it will be necessary to overexcavate the trench bottom to permit installation of a granular working blanket. We estimate that the required thickness of the granular working blanket will be on the order of 1 foot or as required to maintain a stable trench bottom. The actual required depth of overexcavation will depend on the conditions exposed in the trench and the effectiveness of the contractor’s dewatering efforts. The thickness of the granular blanket must be evaluated based on field observations during construction. We recommend the use of relatively clean, free-draining material, such as 2- to 4-inch-minus crushed rock, for this purpose. The use of a nonwoven geotextile fabric between the trench-bottom subgrade and the crushed rock will assist in trench-bottom stability and dewatering.

All utility trench excavations within the project areas should be backfilled with relatively clean on-site or imported granular material with less than 5% passing the No. 200 sieve (washed analysis), as described in Section 5.2.2 of this report. The bottoms of all excavations should be thoroughly cleaned to remove loose material prior to backfilling, and the utilities should be underlain by a minimum 6-inch thickness of bedding material.

The granular backfill material should be compacted to at least 95% of the maximum dry density, as determined by ASTM D698. The use of hoe-mounted vibratory plate compactors is usually most efficient for this purpose. Flooding or jetting as a means of compacting the trench backfill should not be permitted.

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 8

5.3 Hangar Foundation and Structural Slab Support

5.3.1 General

Foundation loads for the proposed hangar building were not available at the time this report was prepared. However, we anticipate the loading for the structure to be relatively light, with maximum column loads of less than 50 kips and wall loads less than 5 kips/foot.

In our opinion, the structural loads can be supported on conventional slab-on-grade with thickened-edge footings or isolated column and wall footings in accordance with the following design criteria. However, if the structural loads are heavier than those assumed, GRI should re-evaluate the foundation recommendations.

5.3.2 Footing Subgrade Preparation

The base of all footings should be established at a minimum depth of 12 inches below the lowest adjacent finished grade to accommodate the design frost depth per OSSC Table 1809.5. Excavations for all foundations should be made using a trackhoe equipped with a smooth-edged bucket, and the foundation subgrade should be evaluated by a representative of GRI prior to backfilling and foundation construction.

Foundation excavations are expected to encounter primarily medium stiff, silty clay soils.

If soft, loose, organic-rich, or otherwise disturbed soil is encountered at the subgrade elevation in the footing excavations, the material should be overexcavated and replaced with imported structural fill as described in Section 5.2.2 of this report. The structural fill should be placed within the limits indicated on the Footing Overexcavation Detail, Figure 3. For all footing excavations, the approved subgrade should be capped with a minimum 6-inch-thick layer of compacted structural fill surfacing to reduce the risk of disturbance during installation of concrete formwork and rebar.

5.3.3 Allowable Bearing Pressure and Settlement

Allowable bearing pressure is based on the assumed loads, the proposed construction location, and the field investigation. Foundations established in accordance with the above criteria can be designed based on an allowable soil bearing pressure of 2,000 pounds per square foot (psf). The recommended allowable bearing pressure is appropriate for typical load combinations provided in ASCE 7-22 Section 2.4. The allowable bearing pressure may be increased by one-third when evaluating alternative load combinations that include wind and seismic, such as those included in IBC Section 1605.2.

Results from laboratory consolidation testing suggest the shallow alluvial soils are moderately overconsolidated, with a preconsolidation pressure of at least 2 tsf. Using an allowable bearing pressure of 2,000 psf for design is intended to keep the allowable bearing pressure below the preconsolidation pressure of the foundation soils and limit total and differential settlements. In this case, total settlements are estimated to be less

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 9 than 1 inch. The differential settlement between adjacent similarly loaded footings (or along the length of the wall footings) may be assumed to be approximately one-half of the total settlement. Foundation settlements will occur after the footings are loaded and the soil consolidates over time.

5.3.4 Lateral Forces

Horizontal shear forces can be resisted partially or completely by frictional forces developed between the base of the footings and the underlying granular fill or foundation soil and by soil passive resistance. The total frictional resistance between the footing and the soil is the normal force times the coefficient of friction between the soil and the base of the footing. We recommend an ultimate value of 0.45 for footings cast on compacted granular structural fill. The normal force is equal to the sum of the vertical forces (dead load plus real live load). If additional lateral resistance is required, passive earth pressures against embedded footings can be computed based on an equivalent fluid. For horizontal backfill conditions, allowable footing passive earth pressures can be computed by having a unit weight of 200 pounds per cubic foot. This passive resistance includes a factor of safety and assumes an allowable lateral movement of less than 1 inch. The allowable passive resistance may be increased by one-third when evaluating alternative load combinations in accordance with IBC Section 1806.1.

5.3.5 Subdrainage / Floor Support

To provide a capillary break and reduce the risk of damp floors, slab-on-grade floors established at or above the adjacent final site grades should be underlain by a minimum of 10 inches of free-draining, clean, angular rock such as ¾- to 1½-inch crushed rock with less than 2% passing the No. 200 sieve (washed analysis). This rock should be placed in one lift and compacted to at least 95% of the maximum dry density, as determined by ASTM D698, or until well keyed. To improve workability, the drain rock should be capped with a 2-inch-thick layer of compacted granular structural fill, as defined in Section 5.2.2 of this report. Therefore, the recommended minimum total thickness of granular backfill beneath slabs is 12 inches.

Prior to backfilling, the subgrade should be evaluated by a qualified geotechnical engineer.

Areas of loose or disturbed soil should be moisture-conditioned and compacted to the required density or overexcavated and backfilled with additional structural fill. A coefficient of subgrade reaction (k) of 100 pounds per cubic inch is appropriate for evaluating point loads on slab-on-grade floors. This value is based on a review of the subsurface conditions and the anticipated loads.

In areas where floor coverings will be provided or moisture-sensitive materials are stored, it would be appropriate to install a vapor-retarding membrane. The membrane should be installed as recommended by the manufacturer. In addition, a foundation drain should be

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 10 installed around the building perimeter to collect water that could potentially infiltrate beneath the foundations and should discharge to an approved storm drain.

5.4 Seismic Considerations

5.4.1 Design Acceleration Parameters

We anticipate that seismic design for the new hangar structures will be completed in accordance with the 2025 OSSC and ASCE 7-22. The design acceleration parameters for the site were obtained from the ASCE 7 Hazard tool by inputting the approximate latitude and longitude coordinates, which are 45.2207°N and 122.5876°W, respectively, along with the Site Class derived from the subsurface data. The SS and S1 bedrock-mapped spectral response acceleration parameters for the project site are 0.79 g and 0.31 g, respectively.

ASCE 7-22 generally defines Site Class based on the average shear wave velocity, VS, profile from the ground surface to a depth of 100 feet. The Site Classes are summarized in ASCE 7-22 Table 20.2-1. When measured VS data are not available, an estimated VS profile may be determined using suitable correlations with SPT blow counts, shear strength measurements, overburden pressures, cone penetration testing tip resistance, or similar geotechnical data. When using correlation models, ASCE 7-22 also recommends including upper and lower bounds of 30% relative to the estimated VS value and selecting the most critical Site Class if the range of estimated VS results in multiple Site Classes per ASCE 7-22 Table 20.2-1.

A review of nearby previous geotechnical explorations and local water well logs suggests that the silt and/or clay soils may be relatively deep beneath the project site. However, the deeper fine-grained soils appear to be decomposed basalt that we believe will become stiffer and/or harder with depth. Borings completed for a previous geotechnical investigation northeast of the current project site encountered decomposed basalt below a depth of about 22 feet, with recorded SPT N-values ranging from 36 blows per foot (bpf) to 59 bpf below a depth of about 30 feet. Using this supplemental information in conjunction with current on-site explorations, we estimated VS values consistent with a Site Class D profile. However, depending on the thickness of soft to medium stiff soils at shallower depths, it is possible that a Site Class DE profile is more appropriate. Therefore, spectral acceleration values and associated parameters for both Site Class D and Site Class DE are provided below. The more severe loading condition for the design period of the structure should be used.

The recommended Risk-Targeted Maximum Considered Earthquake (MCER) and design spectral response parameters are summarized in Table 5-1, below. The multi-period MCER and design spectral values are summarized in Table 5-2 for Site Classes D and DE. The

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 11 design-level response spectra are calculated as two-thirds of the ground-surface MCER spectra.

Table 5-1: RECOMMENDED SEISMIC DESIGN PARAMETERS (ASCE 7-22)

Seismic Parameter Recommended

Value Recommended

Value

Site Class D DE

MCER 0.2-Second Period Spectral Response Acceleration, SMS

1.02 g 1.09 g

MCER 1.0-Second Period Spectral Response Acceleration, SM1

0.68 g 0.92 g

Design-Level 0.2-Second Period Spectral Response Acceleration, SDS

0.68 g 0.73 g

Design-Level 1.0-Second Period Spectral Response Acceleration, SD1

0.45 g 0.61 g

Abbreviations: ASCE = American Society of Civil Engineers; MCER = Risk-Targeted Maximum Considered Earthquake

Table 5-2: MULTI-PERIOD MCER AND DESIGN SPECTRAL VALUES (ASCE 7-22)

Period, T, seconds

Spectral Acceleration, Sa, g Site Class D Site Class DE

MCER Design MCER Design

0 0.47 0.31 0.49 0.32

0.01 0.47 0.31 0.49 0.32

0.02 0.48 0.32 0.49 0.33

0.03 0.48 0.32 0.49 0.33

0.05 0.52 0.34 0.51 0.34

0.075 0.64 0.43 0.63 0.42

0.1 0.79 0.52 0.77 0.51

0.15 0.96 0.64 0.95 0.63

0.2 1.06 0.70 1.06 0.71

0.25 1.09 0.73 1.12 0.75

0.3 1.13 0.75 1.19 0.79

0.4 1.12 0.75 1.21 0.81

0.5 1.04 0.69 1.17 0.78

0.75 0.83 0.55 1.00 0.66

1 0.68 0.45 0.84 0.56

1.5 0.49 0.32 0.63 0.42

2 0.38 0.25 0.49 0.33

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 12

Period, T, seconds

Spectral Acceleration, Sa, g Site Class D Site Class DE

MCER Design MCER Design

3 0.25 0.17 0.33 0.22

4 0.19 0.13 0.25 0.17

5 0.19 0.10 0.20 0.14

7.5 0.093 0.062 0.13 0.084

10 0.075 0.05 0.099 0.066

Abbreviations: MCER = Risk-Targeted Maximum Considered Earthquake; ASCE = American Society of Civil Engineers

5.4.2 Liquefaction and Other Seismic Hazards

Liquefaction is a process through which saturated, granular materials, such as sand and nonplastic or low plasticity silts, temporarily lose strength during and immediately after a seismic event. Liquefaction-induced settlement occurs as the elevated pore-water pressures dissipate and the soil consolidates after the earthquake. The cyclic behavior of fine-grained material is generally different from that of granular material; therefore, the term “cyclic softening” is used for fine-grained materials to differentiate from liquefaction.

Cyclic softening describes a relatively gradual and progressive increase in shear strain with load cycles. Settlement due to post-seismic consolidation can occur, particularly in lower-plasticity silts, although not generally to the degree of sand soils. Large shear strains can develop, and strength loss related to soil sensitivity may be a concern.

Based on the relative consistency of the soils encountered in the explorations, we believe there is a low to negligible risk for liquefaction. This opinion is based on the observed stiffness of the predominantly silty clay soils, the limited sand content, and our experience with the performance (e.g., dynamic testing) of similar soils in the region. We believe that there is low to moderate risk of limited cyclic softening in the soft to medium stiff clay encountered below groundwater. However, the risk of significant soil strength loss at the site is considered to be low during a code-based earthquake. Estimated post-seismic settlements at the site are expected to be less than 1 inch of total settlement and ½ inch of differential settlement.

Based on the subsurface conditions and topography, the risk of lateral spreading and/or slope instability affecting the proposed improvements is negligible. The U.S. Geological Survey map of Quaternary faults indicates that the nearest potentially active fault is the Canby-Molalla fault, located approximately 2 miles west to southwest of the project site.

It is our opinion that the risk of ground rupture at the site is low unless it occurs on a previously unmapped or unknown fault. Risks associated with earthquake-induced damage by a tsunami and/or seiche are absent, based on the site location.

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 13

5.5 Taxilane Pavements

5.5.1 General

Taxilane extensions and/or widening are planned to facilitate access to the new hangar.

The following provides a discussion of pertinent design parameters and construction considerations for the pavements.

5.5.2 Discussion of Subgrade Conditions

The shallow subgrade encountered within the explorations consists of medium plasticity, silty clay with a Unified Soil Classification System classification of CL. Appendix A of FAA Advisory Circular 150/5320-6G indicates that soil with the CL classification is “fair to good” as a foundation material when not subject to frost action. Appendix A also indicates that the potential for frost action can range from medium to high, that the on-site soils are typically considered practically impervious for drainage considerations, and that the shrink/swell potential is medium. Further discussion of frost, drainage, and shrink/swell considerations is provided below.

5.5.3 Frost Considerations

Based on FAA Advisory Circular No. 150/5320-6G, Table 2-2, the FAA frost group classification for the silty clay subgrade is FG-3, suggesting moderate frost susceptibility.

Complete mitigation of the risk of detrimental frost heave generally requires overexcavation and replacement of frost-susceptible soils below the depth of frost penetration. The local building code in Clackamas County indicates a frost penetration depth of 12 inches in the area below 2,500 feet elevation, which includes the city of Mulino (as referenced in OSSC Table 1809.5). Therefore, we assume that design for frost mitigation is not a concern for new pavement sections thicker than 12 inches. Providing drainage to prevent the accumulation of water in the subgrade is also beneficial to reduce the risk of frost heave.

5.5.4 Site Drainage

As noted above, the in-situ infiltration testing completed adjacent to explorations I-1 and I-2 indicated k-values ranging from approximately 5.9x10-5 cm/sec to 1.4x10-5 cm/sec.

These values are in the range of low to very low permeability soils according to Figure G-3 in FAA Advisory Circular No. 150/5320-5D, which is consistent with the soils encountered on site.

The evaluation and design of subsurface drainage systems for pavements should follow the guidelines provided in FAA Advisory Circular No. 150/5320-5D. Based on the anticipated groundwater conditions discussed above and existing surface grades, we do not anticipate the need for interceptor drains to draw down the static groundwater table.

However, a subsurface drainage system to remove surface infiltration is likely necessary

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 14 for new pavements. At a minimum, we anticipate that the drainage system for new pavements will include a drainage layer within the pavement section to allow horizontal drainage to edge drains. The need for supplemental drainage will be dictated by site grading (e.g., slope of the drainage path) and the corresponding length of drainage paths beneath the pavements. Appendix G of FAA Advisory Circular No. 150/5320-5D indicates that the drainage system should be designed so the drainage layer attains 85% drainage within 24 hours for runways and taxiways and within 10 days for airfield parking aprons and other pavement areas receiving only low-volume, low-speed traffic.

5.5.5 Swelling/Shrinking Soils

FAA Advisory Circular No. 150/5320-6G Section 3.9 indicates that pavements constructed on swelling soils may be subject to differential movements resulting in surface roughness and cracking. Soils exhibiting shrink or swell behavior typically include clay soils with liquid limits above 40 and plasticity indices (PIs) above 15. Furthermore, soils exhibiting a swell greater than 3% when tested in combination with the laboratory California bearing ratio (CBR) test (ASTM D1883) require treatment to prevent or reduce the effects of soil volume changes.

In general, the subgrade encountered across the site has medium plasticity and PI values that exceed 15 (see Figures 6A and 7A, Appendix A). However, the laboratory CBR tests on combined bulk samples (see Figures 13A and 15A) indicated swelling of significantly less than 3% for the varying levels of soil compaction. Based on these results, we believe the risk of swelling subgrade soils is low and does not require mitigation.

5.5.6 Subgrade Preparation and Parameters for Pavement Design Laboratory testing indicated CBR values of 2.3 to 3.2 for the selected subgrade samples compacted to 95% relative compaction (based on ASTM D1883), as summarized in Table 1A and on Figures 13A and 15A. FAA pavement design guidelines require subgrade improvement through stabilization or replacement with more suitable material when the mean subgrade strength is less than a CBR of 3. FAA Advisory Circular No. 150/5320-6G identifies chemical stabilization (e.g., cement or lime amendment) and mechanical stabilization (e.g., bridging soft soils) as mitigation alternatives. Based on the project scope and discussion with Century West Engineering Corporation (Century West), it is our opinion that mechanical stabilization in the form of overexcavation and replacement of weak subgrade soils can provide a cost-effective mitigation approach.

We understand that pavement analysis for the taxilane extension using FAA Rigid and Flexible Iterative Elastic Layered Design (FAARFIELD) software and representative aircraft loads selected by Century West with a subgrade CBR of 3 resulted in a minimum design section of 3 inches of asphalt concrete (AC) (P-401/403) over 6 inches of AB course (P-208/209) and 4 inches of subbase (P-154). When the design subgrade CBR value is

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 15 reduced to 2, FAARFIELD calculated a revised minimum pavement section of 3 inches of AC over 6 inches of AB course and 6 inches of subbase. Considering this range of results and recognizing the need to construct over weak subgrade conditions, we recommend using a minimum pavement section consisting of 3 inches of AC over 6 inches of AB course and 12 inches of subbase. This recommended section is intended to provide mechanical stabilization by replacing the upper approximately 8 inches of subgrade with additional granular subbase. A separation geotextile fabric should be installed beneath the granular subbase to prevent contamination and maintain long-term pavement performance.

Depending on the condition of the subgrade at the time of construction, it may also be prudent to replace the typical P-154 subbase with stabilization fill. Further description of the fill materials is provided in Section 5.5.7 of this report. The contractor should also be prepared to place up to 18 inches of stabilization fill in areas where softer soils are encountered.

Subgrade preparation for new pavements should generally follow the requirements for Item P-152 as summarized in the most recent FAA Circular No. 150/5370-10. However, because the subgrade is relatively weak and moisture-sensitive, subgrade compaction may be impractical, particularly during wet weather. In that case, the subgrade should remain relatively undisturbed, and excavation should be completed using an excavator equipped with a smooth-edged bucket working outside of the finished subgrade area, as described in Section 5.2.1. The recommended thicker pavement section with mechanical stabilization noted above is intended to accommodate construction over undisturbed subgrade.

5.5.7 Pavement Surface Course and Base Course Materials

AB course for new pavements should consist of well-graded crushed gravel or rock conforming to FAA P-208 or P-209 requirements. Subbase should consist of free-draining sand, gravel, rock, asphalt grindings, or mixtures of the above that conform to FAA P-154 requirements and are free of plastic clay and organic matter.

Granular stabilization fill, if required to mitigate soft and/or overly wet soil conditions, should consist of 3-inch-minus, clean (i.e., less than 5% passing the No. 200 sieve), well-graded, angular, crushed (quarry) rock. Stone Embankment Fill, as defined in the Oregon Standard Specifications for Construction, Section 00330.16 (ODOT, 2024), may be specified with the alteration that a maximum particle size of 3 inches is required. The gradation of the stabilization fill meets the requirements for Subbase Course (Item P-154) and may be substituted for subbase where overexcavation is required. The percentage fractured faces for the angular material should meet the requirements for Item P-208 but may be confirmed by visual observation.

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 16

A separation geotextile placed over the finished subgrade will facilitate placement of the base materials and limit migration of fine-grained subgrade into the AB or subbase for long-term protection. The separation geotextile should consist of a woven or nonwoven geotextile with Mean Average Roll Value (MARV) strength properties meeting the requirements of an American Association of State Highway and Transportation Officials (AASHTO) M 288-17 Class 2 geotextile. The geotextile should have MARV hydraulic properties meeting the requirements of AASHTO M 288-17, Table 3, with a permittivity greater than 0.05 sec.-1 and an apparent opening size less than 0.6 millimeters (maximum average roll value).

All fill should be placed in level lifts and compacted based on FAA requirements for the selected material type, as defined in FAA Advisory Circular No. 150/5370-10H. The moisture content of the fill should be adjusted to within about 2% of its optimum value prior to compaction. Field-density tests should be run frequently to confirm adequate compaction of the AB, subbase, and subgrade. Adequate compaction of fill materials, which are too coarse or too variable for density testing, should be evaluated by observation of the compaction method and proof rolling with a loaded dump truck or other approved heavy construction vehicle.

6 DESIGN REVIEW AND CONSTRUCTION SERVICES

We welcome the opportunity to review and discuss construction plans and specifications for this project as they are being developed. In addition, GRI should be retained to review all geotechnical-related portions of the plans and specifications to evaluate whether they are in conformance with the recommendations provided in our report. To observe compliance with the intent of our recommendations, the design concepts, and the plans and specifications, we are of the opinion that all construction operations pertaining to earthwork should be observed by a GRI representative. Our construction-phase services will allow for timely design changes if site conditions are encountered that are different from those described in our report. If we do not have the opportunity to confirm our interpretations, assumptions, and analyses during construction, we cannot be responsible for the application of our recommendations to subsurface conditions different from those described in this report.

7 LIMITATIONS

This report has been prepared for use by Century West and should not be relied upon by any other entity without the written authorization of an authorized representative. The scope is limited to the specific project and location described within this report, and our description of the project represents our understanding of the significant aspects of the project relevant to the evaluation, design, and construction of the proposed improvements. In the event that any changes in the design and location of the project

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 17 elements as outlined in this report are planned, we should be given the opportunity to review the changes and modify or reaffirm the conclusions and recommendations of this report in writing.

The conclusions and recommendations submitted in this report are based on the data obtained from the explorations made at the locations indicated on Figure 2 and other sources of information discussed in this report. In the performance of subsurface investigations, specific information is obtained at specific locations at specific times.

However, variations in soil conditions may exist between exploration locations. This report does not reflect any variations that may occur between these explorations. The nature and extent of variation may not become evident until construction. If subsurface conditions during construction differ from those encountered in the explorations, we should be advised at once so we can observe and review these conditions and reconsider our recommendations where necessary.

We have included as Appendix B the Geoprofessional Business Association guidance document “Important Information about This Geotechnical-Engineering Report” to assist you and others in understanding the use and limitations of this report. We recommend you read this document.

Submitted for GRI, Jonathan C. Huffman, PhD, PE, GE

Associate

This document has been submitted electronically.

RENEWS: 06/2027

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page 18

8 REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2017, AASHTO M 288-17: Standard specification for geosynthetic specification for highway applications: Washington, D.C.

American Society of Civil Engineers (ASCE), 2022, ASCE 7-22: Minimum design loads and associated criteria for buildings and other structures: Reston, Virginia.

ASTM International (ASTM), 2021, ASTM D698: Standard test methods for laboratory compaction characteristics of soil using standard effort.

ASTM, 2021, ASTM D1883: Standard test method for California Bearing Ratio (CBR) of laboratory-compacted soils.

Clackamas County Water Environment Services, 2023, Water Environment Services stormwater standards.

Federal Aviation Administration (FAA), 2013, Airport drainage design: Advisory Circular No. 150/5320-5D.

FAA, 2018, Standards for specifying construction of airports: Advisory Circular No. 150/5370-10H.

FAA, 2021, Airport pavement design and evaluation: Advisory Circular No. 150/5320-6G.

International Code Council (ICC), 2024, International building code: ISBN: 978-1-959851- 62-2.

Oregon Building Codes Division, 2025, Oregon structural specialty code: Salem, Oregon.

Oregon Department of Transportation (ODOT), 2024, Oregon standard specifications for construction: Salem, Oregon.

Personius, S. F., compiler, 2002, Fault number 716, Canby-Molalla fault: Accessed from Quaternary Fault and Fold Database of the United States, U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults.

CENTURY WEST ENGINEERING CORPORATION

MULINO STATE AIRPORT

HANGAR AND WATERLINE PROJECT

VICINITY MAP

JOB NO. 7161-AAPR. 2026 FIG. 1

PROJECT SITE

0 0.5 1

1 INCH = 0.5 MILES

BASEMAP PROVIDED BY USGS, 2026

B-2

B-1 I-2

I-1

SITE MAP

JOB NO. 7161-AAPR. 2026 FIG. 2

0 150 300

1 INCH = 150 FEET

CENTURY WEST ENGINEERING CORPORATION

MULINO STATE AIRPORT

HANGAR AND WATERLINE PROJECT

LEGEND:

APPROXIMATE LOCATION OF BORING COMPLETED BY GRI

APPROXIMATE LOCATION OF INFILTRATION TEST COMPLETED BY GRI

BASEMAP PROVIDED BY GOOGLE EARTH, 2026

S MULINO RD

S M

ULIN

O

RD

LANDING WAY

S A

IR

PO

RT

R

D

FOOTING

1 1

NOT TO SCALE

GRANULAR FILL WITH LESS THAN 5%

PASSING THE NO. 200 SIEVE (WASHED

ANALYSIS), COMPACTED TO AT LEAST 95%

OF THE MAXIMUM DRY DENSITY, AS

DETERMINED BY ASTM D698

VARIES

FOOTING

OVEREXCAVATION DETAIL

CENTURY WEST ENGINEERING

CORPORATION MULINO STATE AIRPORT

HANGAR AND WATERLINE PROJECT

APR. 2025 JOB NO. 7161-A FIG. 3

APPENDIX A

Field Explorations and Laboratory Testing

GRI 7161-A – Mulino State Airport Hangar and Waterline Project Geotechnical Report Page A-1

APPENDIX A

FIELD EXPLORATIONS AND LABORATORY TESTING

A.1 FIELD EXPLORATIONS

A.1.1 General

Subsurface materials and conditions at the project site were investigated on January 5, 2026, with four drilled borings, designated B-1, B-2, I-1, and I-2. The approximate locations of the explorations are shown on the Site Map, Figure 2. The approximate locations of I-1 and I-2 also coincided with infiltration tests, which are described below. The explorations extended to a depth of approximately 16.5 feet below the ground surface.

An experienced geologist from GRI observed the explorations; coordinated the drilling and sampling depths; maintained logs of the type, thickness, and condition of the materials and conditions encountered during the drilling operations; and coordinated and completed the infiltration testing. The exploration and field and laboratory testing programs completed for this project are described below.

A.1.2 Drilled Borings The drilled borings were completed using a Little Beaver LS 300trailer-mounted drill rig with solid-stem auger drilling techniques, provided and operated by Dan J. Fischer Excavating, Inc., of Forest Grove, Oregon.

Disturbed and relatively undisturbed soil samples were obtained from the…