Attachment 4 - Geotech Investigation.pdf

Central Oregon Helibase Crook County | Prineville Airport

GEOTECHNICAL REPORT

15132 - Central Oregon Helibase Permit Set

January 8, 2019

023213 - 1

GEOTECHNICAL REPORT

SECTION 023213 - GEOTECHNICAL REPORT

PART 1 GENERAL

1.1. SECTION INCLUDES

A. Geotechnical Investigation - Prineville Airport USFS Airbase Prineville, Oregon - February 1, 2018.

B. Geotechnical Investigation - Memorandum - dated November 8, 2018.

END OF SECTION 023213

Geotechnical

Investigation

Prineville Airport

USFS Airbase

Prineville, Oregon

Prepared for:

Precision Approach Engineering, Inc.

Corvallis, Oregon

February 1, 2018

Foundation Engineering, Inc.

Professional

Services

Prineville Airport – USFS Airbase February 1, 2018

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GEOTECHNICAL INVESTIGATION

PRINEVILLE AIRPORT

USFS AIRBASE

PRINEVILLE, OREGON

BACKGROUND

A new U.S. Forest Service (USFS) Airbase facility is planned at the Prineville Airport.

Preliminary plans indicate the facility will be located northeast of the airport. The facility will include a building, a training yard with rappel towers, landside parking lots, and driveways, helipads, and taxilanes. Access to the new facility will be provided from a new road extending east from Tom McCall Road and a taxilane near the east end of

Runway 10-28. The project location is shown on Figure 1A (Appendix A). Figure 2A

(Appendix A) shows a layout of the site.

Precision Approach Engineering, Inc. (PAE) is the prime engineering consultant for the project and will provide airside pavement design and other civil design services.

Foundation Engineering, Inc. was retained by PAE to perform the subsurface investigation, conduct laboratory testing, and provide recommendations for subgrade preparation associated with construction of new pavement and foundation design for the new building and rappel tower.

Foundation Engineering completed a geotechnical investigation for the Runway 10-28 rehabilitation project in 2008 and an investigation in 2016 for new aprons and beacon tower. In addition, the current investigation is being completed in conjunction with the investigation for a tie-down apron between the USFS facility and Runway 10-28.

Information from those investigations has been used to supplement the current work, where appropriate. The following sections summarize our work.

FIELD EXPLORATION

We completed field explorations on December 13 and 14, 2017. The explorations included twenty-five borings (BH-6 through BH-30) drilled using a trailer-mounted drill rig equipped with ±4-inch and 8-inch diameter solid-stem augers. The drilling was completed concurrently with the explorations for the proposed tie-down apron and the borings were numbered sequentially with those explorations (i.e., BH-1 through

BH-5 were drilled for the apron). PAE surveyed the boring locations, which are shown on Figure 2A.

The borings extended to maximum depths ranging from ±1.3 to 10 feet below the ground surface. Borings that were drilled shallower than 10 feet were discontinued after reaching practical drilling refusal on the surface of the underlying bedrock.

Representative soils samples were obtained at select depths by collecting the drill cuttings. At BH-27, a sample was obtained at a depth of ±2.5 to 4 feet by driving a split-spoon sampler. The Standard Penetration Test (SPT), which is run when the

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split-spoon is driven, provides an indication of the relative stiffness or density of the soils. Upon completion, the borings were backfilled with the drill cuttings.

The boreholes were continually logged during drilling. The final logs (Appendix B) were prepared based on a review of the field logs, laboratory test results, and an examination of the soil samples in our office. Table 1B (Appendix B) provides a summary of the estimated ground surface elevations, drilling depths, and estimated bedrock surface elevations at each borehole location.

DISCUSSION OF SITE CONDITIONS

The following sections provide a summary of the surface and subsurface conditions encountered in the borings. More specific details of the subsurface conditions are provided on the summary logs (Appendix B).

Surface Conditions

Prineville Airport is located on a high plateau west of the city center. The terrain across the site is relatively flat. Available survey information indicates the surface elevation typically slopes very gently down to the east. Prior to our site visit, a

±12-foot tall mound of fill had been stockpiled east of BH-17. The approximate location is identified on Figure 2A. The stockpile consists of primarily sandy silt with trace gravel, which is similar to the shallow soils encountered across the site. We assume the fill was likely generated from grading work during previous airport projects.

The planned USFS Airbase facility will be located northeast of the airport. The project site is presently undeveloped exception for a few unpaved trails. The ground surface is primarily covered with short grass, fescue, sagebrush, and short juniper or pine trees. Scattered, cobble to boulder-sized basalt fragments were observed on the ground surface. Additionally, potential bedrock outcropping (or possible large, buried boulders) were scattered throughout the site. The site is underlain by bedrock at relatively shallow depths, as discussed below.

Subsurface Conditions

The explorations encountered similar subsurface conditions across the project area, which typically included a layer of silty sand to sandy silt underlain by basaltic bedrock. Additional details of the individual layers are provided below.

Sandy Silt to Silty Sand

The surficial soils across the site consist of primarily brown, stiff sandy silt. Medium dense silty sand was also encountered at varying locations and depths. The silt is typically non-plastic or has low plasticity. The sand is typically fine or fine to medium-grained. The soil was dry to damp at the time of the exploration.

Within the project area, the sandy silt and silty sand typically extends to depths ranging from ±1.3 to 5 feet, where it is underlain by basaltic bedrock. Deeper soil deposits were encountered in BH-10, BH-18, BH-19, BH-20, and BH-21. At BH-19 and BH-21, Geotechnical Investigation 3 Project 2171109

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the soil stratum extended to ±8.5 feet. At BH-10, BH-18 ,and BH-20, the soil extended below ±10 feet (the maximum depth of the boring). Most of the borings where the deeper soil profile was encountered are located within the northeast quadrant of the property (see Figure 2A).

As noted above, the existing fill stockpile adjacent to BH-17 appears to consist of material consistent with the sandy silt to silty sand encountered elsewhere across the site.

Bedrock

All the explorations except for BH-10, BH-18, and BH-20 terminated on basaltic bedrock of the Deschutes Formation. Drilling encountered practical refusal on the bedrock surface. No rock samples were obtained from the borings. However, bedrock outcrops scattered across the site indicate the bedrock are typically slightly to moderately weathered, soft to medium hard (R2 to R3), and vesicular. The basalt surface was encountered at depths ranging from ±1.3 to 8.5 feet in the borings. The bedrock surface elevation of each boring location is summarized in Table 1B.

The Deschutes Formation basalt extends to depths ranging from tens to hundreds of feet below the ground surface within the vicinity of the airport. Deep borings

(extending to a maximum depth of ±90 feet) completed by others at the landfill north of the airport indicate the basalt is soft (R2) within the upper ±1 to 2.5 feet, then typically becomes less weathered and hard (R4).

Ground Water

No ground water was observed in the any of the current explorations for the USFS facility or the planned tie-down apron.

Most of our explorations completed for previous projects across the airport also did not encounter ground water. The only exception is one pavement core (completed in 2016) that was located southwest of Runway 10-28, near the recently-completed run-up apron. Ground water at that location was encountered at ±4.5 feet and was likely perched on the underlying bedrock.

Based on our current and previous exploration, we anticipate shallow ground water is not common. However, during periods of heavy rain, ground water may perch at shallow depths on the bedrock surface due to its low permeability. Ground water, if present, may also be localized in some areas due to the undulating surface of the underlying bedrock.

LABORATORY TESTING

Index Tests

The laboratory work included natural water content determinations, Atterberg limits tests, and mechanical sieve and hydrometer analyses to classify the soils and estimate their overall engineering properties. The results of the index tests are summarized on Table 1C and on Figures 1C and 2C (Appendix C).

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The Atterberg limits tests were completed on bulk samples obtained from TP-17

(S-17-1) and TP-22 (S-22-1). The test results for S-17-1 indicated the sample is non-plastic (i.e., PI value very close or equal to zero) with a resulting FAA/USCS classification of ML (i.e., low plasticity silt). The results for S-22-1 indicated the sample has a liquid limit (LL) of 50, a plasticity index (PI) of 16, and a resulting

FAA/USCS classification of ML. The PI value of sample S-22-1 indicates low to medium plasticity. In general, most other samples obtained during the exploration were non-plastic or have a low plasticity.

Mechanical sieve and hydrometer analyses were completed on bulk samples S-17-1 and S-22-1. The results are generally consistent with the Atterberg limits tests, indicating primarily fine-grained soils (i.e., silt and clay-sized particles) with ±35% to 45% fine to medium sand.

Moisture-Density and CBR Tests

Standard Proctor moisture-density curves (ASTM D698) were developed for bulk samples S-17-1 and S-22-1. The results are summarized in Figures 3C and 5C

(Appendix C). The test results for S-17-1 indicate a maximum dry density of

98.7 pcf at an optimum moisture content of 18%. The test results for S-22-1 indicate a maximum dry density of 76.4 pcf at an optimum moisture content of

30.5%. The current test results bound the range of results observed from previous testing at the airport.

California Bearing Ratio (CBR, ASTM D1883) tests were also completed on samples

S-17-1 and S-22-1 using the results from the compaction tests. The CBR results are summarized in Figures 4C and 6C (Appendix C). For S-17-1, a CBR value of 9.4 was recorded for the sample compacted to 95% relative compaction. For S-22-1, a CBR value of 4.5 was recorded for the sample compacted to 95% relative compaction.

Foundation Engineering previously completed moisture-density and CBR testing for a geotechnical investigation for the Runway 10-28 rehabilitation project in 2008 and for the apron and beacon tower project in 2016. We also completed testing for the current tie-down apron project that is located adjacent to the USFS facility. Results from those tests are summarized in Table 2C (Appendix C). The tests were completed on predominantly sandy silt (ML) with one test completed on silty sand

(SM). The test results indicated maximum dry densities ranging from ±76.4 to

98.7 pcf at optimum moisture contents ranging from ±18.0% to 30.5%. CBR values ranging from 1.5 to 9.4 were recorded for these samples.

The past and current CBR test results indicate a relatively wide range in values, even with similar sample material (i.e., sandy silt or silty sand). We attribute the variable results to the moisture-sensitive nature of the subgrade, particularly within the confines of the CBR testing procedure. Additional discussion and recommendations for a design CBR value are provided in the following section.

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DISCUSSION OF GEOTECHNICAL CONSIDERATIONS AND DESIGN REQUIREMENTS -

AIRSIDE PAVEMENTS

The following section provides pavement design recommendations associated with the new taxilanes and helipads servicing the USFS facility. PAE will complete the pavement design and determine the required pavement section thicknesses.

Discussion of Subgrade Conditions

The explorations typically encountered subgrade consisting of sandy silt with

FAA/USCS classification ML. Silty sand (FAA/USCS classification SM) was also encountered in some areas. Appendix A of the 2016 FAA Pavement Advisory

(AC 150/5320-6F) indicates that soil classified as SM or ML is fair to good foundation material when not subject to frost action. Appendix A indicates the potential for frost action can range from slight to very high and drainage can range from fair to poor. Frost-susceptibility and drainage are discussed in subsequent sections of this report.

Basaltic bedrock was typically encountered at relatively shallow depths (i.e., ±1.3 to 5 feet) across most of the site. The bedrock surface was deeper in the northeast quadrant of the site (i.e., ±8.5 feet to greater than ±10 feet). Depending on the final grades and the thickness of the new pavement section, bedrock may comprise a portion of the pavement subgrade for the planned improvements.

Subgrade Preparation and Parameters for New Pavement Design

The pavement subgrade will consist predominantly of silty sand to sandy silt, and may include areas where the pavement section is underlain directly by bedrock. The minimum pavement section will be predicated on the properties of the sandy silt.

Where bedrock is encountered at the subgrade elevation, the base rock thickness may be reduced to limit the need for bedrock excavation. However, we recommend providing at least 4 inches of base rock between the bedrock and AC wearing course.

The laboratory testing indicated CBR values of 4.5 and 9.4 for subgrade samples compacted to 95% relative compaction (based on ASTM D698). Previous tests on similar subgrade at the site indicated CBR values ranging from 1.5 and 6.7. As discussed above, we believe the range in CBR values is reflective of the moisture-sensitive nature of the soil. With proper moisture control and compactive effort, the subgrade performance should be significantly better than what is indicated by the lower-bound test results. Based on the current and previous test results and observation of the subgrade materials, we recommend assuming a CBR value of 4 for designing new pavements. This value corresponds to a k-value of ±84 pci for rigid pavement design, based on correlations provided in the 2016 FAA Pavement

Advisory (AC 150/5320-6F).

Subgrade preparation for new pavements should follow all necessary requirements for Item P-152, as summarized in the most recent FAA Standard Specifications for construction of Airports Circular (AC no. 150/5370-10). The recommended CBR design value provided herein assumes the subgrade will be compacted to a minimum

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of 95% relative compaction (based on the maximum dry density of ASTM D 698) prior to placing subbase and base rock. The predominantly silt subgrade is moisture-sensitive. Therefore, we recommend compacting it at or slightly dry of optimum moisture to reduce the risks of pumping. The prepared subgrade should be backfilled with base rock or subbase as soon as practical to limit moisture fluctuations (i.e., wetting or drying).

Where bedrock is exposed at the subgrade elevation, subgrade compaction will not be necessary. Subgrade preparation in these areas should include excavating the rock as required to provide a minimum of 4 inches of base rock beneath the AC and removing any soft soil from the rock surface. Preparation should also include close inspection of the bedrock surface and confirmation that no large voids (e.g., large vesicles or lava tubes) are present that would affect the performance of the new pavement.

Frost Considerations

The silty sand to sandy silt subgrade corresponds to a FAA/USCS soil classification of SM to ML. The percentage of soil finer than 0.02 mm is greater than 15%.

Therefore, A FG-4 FAA frost group classification is appropriate. FG-4 subgrade is considered highly frost-susceptible.

Complete mitigation of the risk of detrimental frost heave typically requires overexcavation and replacement of frost-susceptible soils below the depth of frost penetration. The local building code in Crook County indicates a frost penetration depth of ±18 inches. FAA (2016) indicates limited subgrade frost penetration may be used where FG-4 soil is present. This includes providing non-frost-susceptible material to at least 65% of the frost penetration depth (±12 inches).

In some portions of the site, excavations to this depth may extend to bedrock. The bedrock is not considered frost-susceptible. Therefore, excavating below the bedrock surface to mitigate frost concerns is not necessary. Providing drainage to prevent the accumulation of water in the subgrade is also beneficial to reduce the risk of frost heave.

Site Drainage

The pavement will be underlain by predominantly silty sand to sandy silt with

FAA/USCS classification ranging from SM to ML, followed by moderately weathered basalt. The 2016 FAA Pavement Advisory (AC 150/5320-6F), Appendix A, indicates the drainage characteristics of these soils have fair to poor drainage characteristics.

The 2013 FAA Surface Drainage Design Advisory Circular (AC 150/5320-5D), Figure G-3, suggests a coefficient of permeability, k, for sandy silt in the range of

10-8 cm/sec, which corresponds to practically impermeable soil. Other texts report a larger range and generally higher permeability. We anticipate the permeability of the sandy silt will be significantly higher than 10-8 cm/sec. However, the underlying bedrock is expected to be practically impermeable (unless large cracks or vesicles

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are present in the bedrock). Therefore, we believe it is reasonable to assume a k value of 10-8 cm/sec for evaluating drainage alternatives.

Construction Considerations

Site Stripping

For construction of the new pavements, we recommend assuming a nominal stripping depth of 2 inches to remove the bulk of the organics. Deeper stripping depths may be required in isolated areas to remove deeper roots or other deleterious materials.

Construction Timing

We recommend completing the earthwork during the dry summer months (typically

July through September), when it should be practical to adjust the moisture content of the soil to near optimum and compact the subgrade. The contractor may still experience pumping problems and have difficulty achieving adequate compaction in the summer if the soils have not adequately dried.

The silty sand to sandy silt will be moisture-sensitive and will soften considerably when wet. Compaction of the subgrade will not be practical during wet weather.

Therefore, if construction during wet weather is necessary, it will likely require completely removing the overlying soils where bedrock is shallow. Where rock is deeper, ±18 to 24 inches of subbase and/or base rock over a separation geotextile would likely be required to prevent subgrade pumping. Geogrid could also be used to help reinforce the pavement section over soft soils. The actual recommended fill thickness and use of geogrid would depend on the subgrade density and moisture content, and the depth to bedrock.

Bedrock Excavation

Basalt was encountered across the site at varying depths. The subsurface explorations indicate an undulating rock surface with bedrock depths ranging from

±1.3 to greater than ±10 feet below the current ground surface. Observed bedrock depths and estimated elevations are summarized in Table 1B.

Our explorations using a small, trailer-mounted drill encountered practical drilling refusal at the bedrock surface. The basalt surface is typically slightly to moderately weathered and vesicular. The relative hardness of the rock near the surface appeared to range from soft to medium hard (R2 to R3). However, we anticipate the rock will become less weathered and harder with depth. We believe it would be prudent to assume a rock hardness of R4 for the bedrock below the weathered surface. The rock description sheet included in Appendix B provides a summary of rock hardness classifications.

If rock excavation was required as part of recent construction projects at the airport, the details from that excavation work should be made available for contractors bidding on this project to better identify the range of expected rock hardness and appropriate excavation techniques (e.g., digging, ripping and/or blasting).

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Fill Materials and Compaction

The observed on-site fill includes a ±12-foot tall soil stockpile located immediately east of BH-17. The fill consists of predominantly sandy silt with trace gravel. This material may be used to raise the site grade, if required, during dry weather conditions only. This material should not be considered as a substitute for a subbase material. The fill should be placed and compacted as described below.

Base rock for the new pavements should consist of ¾ or 1-inch minus, clean

(i.e., less than 5% passing the No. 200 sieve), well-graded, crushed gravel or rock conforming to FAA P-209 requirements. Subbase material should consist of free-draining sand, gravel, rock, asphalt grindings, or mixtures of the above that are free of plastic clay and organic matter that conforms to FAA P-154 requirements.

All fill should be placed in level lifts not exceeding 12 inches and compacted to a minimum of 95% relative compaction. The maximum dry density of ASTM D 698 should be used as the standard for estimating relative compaction. The moisture content of the fill should be adjusted to within ±2% of its optimum value prior to compaction. Efficient compaction of fine-grained soils (where appropriate) will typically require the use of a padfoot or kneading roller to achieve the required compaction. Granular fill (sand, rock or gravel) and sand or non-plastic silt subgrade will compact most efficiently with a smooth drum, vibratory roller. Field density tests should be run frequently to confirm adequate compaction of the base rock, 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.

ENGINEERING ANALYSIS AND DESIGN RECOMMENDATIONS - USFS FACILITY

The following sections summarize our engineering analyses and provides design recommendations for the USFS facility, including building structures and landside pavements. The site is underlain at shallow depths by basaltic bedrock. Therefore, we assume the new airbase building and rappel tower will be supported on shallow foundations (i.e., spread footings or mat).

Seismic Design Criteria

A design spectral acceleration response spectrum for the site was developed in accordance with the Oregon Structural Specialty Code (OSSC) 2014, which is based on International Building Code (IBC) 2012 Section 1613. The design maximum considered earthquake ground motion maps in IBC 2012 are based on modified

USGS 2008 maps with a 1% probability of exceedance in 50 years

(i.e., a ±4,975-year return period) for design spectral accelerations. The modifications include factors to adjust the spectral accelerations to account for directivity and risk.

Based on subsurface profile encountered in the explorations, we have concluded a

Site Class B (Rock) is appropriate for design. The seismic design parameters and

OSSC response spectrum are shown on Figure 3A (Appendix A).

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We believe the risk of liquefaction is negligible due to the presence of shallow bedrock, the stiffness, and limited thickness of the surficial soil, and the absence of persistent ground water.

Building Foundations

Bearing Capacity and Settlement

Borings BH-28 and BH-29 were located within the footprint of the planned USFS building, and BH-27 was located near the proposed rappel tower. The subsurface profile encountered at these locations consists of stiff sandy silt to a depth of ±2 to

4 feet, followed by bedrock. We did not encounter high plasticity (i.e., expansive) clay, organic soil, construction debris, or other deleterious materials within the explorations.

Based on the subsurface conditions encountered, we recommend assuming an allowable bearing pressure of 2,500 lbs/ft2 (psf) for designing shallow, spread or continuous footings and/or mat foundations. This bearing pressure is predicated on the estimated bearing strength of the sandy silt, and may be increased significantly, if needed, for footings that extend to bedrock. The design bearing pressure assumes a typical factor of safety of 3. A one-third increase in this allowable bearing pressure may be assumed for transient (i.e., wind and seismic) loading. Settlement associated with the allowable bearing pressure is expected to be ±½ inch or less.

For all footings underlain by native soils (i.e., sandy silt or silty sand), we recommend the footings be underlain by a minimum of 6 inches of compacted crushed gravel or rock meeting the requirements for Base Rock, as defined below. The granular fill should also extend at least 6 inches beyond the footprint of the foundations.

Foundation excavations should be completed with an excavator equipped with a smooth-edged bucket, operating outside of the excavation to limit disturbance to the foundation soils. Base Rock beneath the footings may be eliminated where the footing excavations extend to bedrock.

Sliding Coefficient and Passive Resistance

A sliding coefficient of 0.5 is recommended for footings poured on compacted granular fill or directly on bedrock.

Passive resistance of the soil in front of the footings was calculated as an equivalent fluid density equal to *Kp, where is the unit weight of the soil and Kp is the passive earth pressure coefficient. We calculated an equivalent fluid density of 350 pcf for the ultimate passive resistance (assuming drained conditions).

The base sliding resistance will develop with very small translational movement.

However, development of the full passive resistance in front of the buried footings would likely require excessive translation of the footings. Assuming small movements (on the order of a fraction of an inch), we recommend using one-third of the ultimate passive resistance in combination with the base friction. Therefore, a coefficient of friction of 0.5 and an allowable passive resistance of ±115 pcf are recommended for design.

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Slab-on-Grade

If slab-on-grade floors are planned for the new building, we recommend the structure be underlain by a minimum of 12 inches of compacted granular fill. If designed and constructed as recommended herein, a modulus of subgrade reaction of 200 lb/in3

(pci) is recommended for floor slab design.

Landside Pavements – Access Roads and Parking Lot

New landside pavements will include an access road extending west from

Tom McCall Road and parking lot to service the new USFS facility. We understand the access road is being designed per City of Prineville requirements for an

Industrial Street. The standard pavement section for an Industrial Street includes

4 inches of AC over 10 inches of compacted Base Rock. We anticipate this section will be adequate for the anticipated traffic, provided it is constructed over subgrade prepared to the same requirements as the airside pavements.

Based on the City of Prineville minimum requirements, Crook County, and the Oregon

Department of Transportation Pavement Design Guide (ODOT, 2011), the following pavement section and mix design is recommended for the access road and parking lot:

• 4 inches of Level 2, ½-inch Dense-Graded HMAC Wearing Course with

PG 64-28 binder. The AC should be placed in two equal lifts.

• 10 inches of Base Rock. Base Rock should conform to the material requirements of either ODOT 2018 Standard Specifications for Construction for ¾-inch minus Base Aggregate (ODOT Section 02630.10) or FAA Item

P-209.

For cost considerations, the design team may consider reducing the AC thickness in area with limited traffic (e.g., parking stalls or areas with no truck access). For these areas, 3 inches of AC should suffice.

If bedrock is encountered shallower than 14 inches below the planned AC surface, the base rock thickness should be reduced, as needed, to limit rock excavation.

However, we recommend providing a minimum of 4 inches of Base Rock beneath the AC.

CONSTRUCTION RECOMMENDATIONS – USFS AIRBASE & RAPPEL TOWER

Earthwork Materials and Compaction Recommendations

1. Base Rock should consist of durable, ¾ or 1-inch minus, clean (i.e., less than 5% passing the #200 U.S. Sieve), well-graded, crushed gravel or rock.

The Base Rock defined herein should conform to the material requirements of the ODOT 2018 Standard Specification for

Construction, Section 02630.10. Alternatively, it may be specified based on FAA Item P-209 standards.

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2. Drain Rock should consist of 1 to 2-inch, clean (i.e., less than 2% passing the #200 sieve), open-graded gravel or rock.

3. Separation Geotextile should consist of a woven geotextile with Mean

Average Roll Value (MARV) strength properties meeting the requirements of an AASHTO M 288-17 Class 2 geotextile. The geotextile should have a permittivity greater than 0.1 sec.-1 and an

Apparent Opening Size (AOS) less than 0.6 mm.

4. Filter Fabric should consist of a non-woven geotextile meeting the requirements of an AASHTO M 288-17 Class 2 Subsurface Drainage

Geotextile. The fabric should have a maximum average roll value apparent opening size (AOS) of 0.22 mm and a permittivity greater than

0.1 sec-1 based on soil with greater than 50% fines.

5. Compact all fill in loose lifts not exceeding 12 inches. Thinner lifts

(maximum 6 inches) may be required if light or hand-operated equipment is used. Compact all fill to a minimum of 95% relative compaction, unless otherwise specified. The maximum dry density of ASTM D 698 should be used as the standard for estimating relative compaction, unless otherwise specified. The moisture content of the fill should be adjusted to within ±2% of its optimum value prior to compaction.

Field density tests should be run frequently to confirm adequate compaction of the fill and subgrade. Fill that is too coarse or variable for density testing should be proof-rolled using a loaded, 10-yd3 dump truck or other approved vehicle. Areas of pumping or deflection observed beneath the truck wheels should be moisture-conditioned

(during dry weather only) and recompacted. If moisture-conditioning is impractical, loose or soft subgrade soils should be overexcavated and replaced with compacted Base Rock and proof-rolled again.

Foundation Design

6. Place the base of all footings at least 18 inches below the exterior finish grade. Provide a minimum of 6 inches of compacted Base Rock beneath the footings underlain by stiff sandy silt or bedrock. The Base Rock may be eliminated beneath footings that extend to bedrock.

7. Design all footings using an allowable bearing pressure of 2,500 psf with a factor of safety of 3. This value may be increased by one-third for short-term seismic loads.

8. Assume the foundations could experience total settlements of up to

±½ inch. This estimate assumes all disturbed (i.e., loose or soft) soils will be removed from beneath new footings and slabs. A representative of Foundation Engineering should be on-site at the time of construction to confirm the suitability of the foundation conditions.

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9. Design the new structures assuming the seismic parameters provided in

Figure 3A (Appendix A).

10. Use a modulus of subgrade reaction, ks, of 200 lb/in3 (pci) for floor slab design. This value assumes the floor slab will be constructed on a building pad consisting of at least 12 inches of compacted Base Rock underlain by compacted, native soils (i.e., sandy silt or silty sand) or bedrock. Reinforce the floor slabs to resist cracking and warping.

Provide a suitable vapor barrier under the slab that is compatible with the proposed floor covering and the method of slab curing.

Site Preparation

11. Strip the existing ground ±2 to 4 inches or as required to remove roots and sod. Deeper grubbing may be needed under any existing trees.

Dispose of all strippings outside of construction areas, including pavements.

12. Grade the building footprint as required to accommodate a minimum

12-inch thick building pad. Where bedrock is encountered at the foundation elevation, the Base Rock thickness may be reduced to limit the need for bedrock excavation. Prior to placing fill, moisture-condition and compact the subgrade as recommended in Item 5.

Subgrade compaction will not be practical if site preparation occurs during wet weather. In that case, the building pad thickness should be increased to at least 18 inches (or to bedrock) and should be placed over undisturbed (i.e., uncompacted) subgrade. The actual recommended fill thickness will depend on the stiffness and/or density of the subgrade and the presence of shallow bedrock. A representative of Foundation Engineering should be present at the time of construction to confirm the subgrade conditions and determine if any additional excavation is needed.

13. Proof-roll the subgrade using a 10-yd3 dump truck or approved heavy construction vehicle prior to constructing the building pad. Any subgrade observed to be soft or pumping should be moisture-conditioned and recompacted (during dry weather only) or overexcavated and replaced with compacted Base Rock.

14. Use Base Rock to construct the building pad. A minimum building pad thickness of 12 inches is recommended for dry weather construction.

This should be increased to at least 18 inches for wet weather construction. Place the fill in compacted lifts as recommended in

Item 5.

15. Trench as required for the footings. All footing excavations should extend at least 6 inches below the bottom of the footings to allow placement of a leveling course of compacted Base Rock. Where bedrock

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is encountered at the foundation elevation, the Base Rock thickness may be reduced to limit the need for bedrock excavation.

The Base Rock should extend at least 6 inches beyond the edges of the footings. A representative of Foundation Engineering should be present at the time of construction to confirm the suitability of the foundation soils and determine if any additional excavation is needed.

16. Grade the finished ground surface surrounding the building to promote runoff away from the foundations.

Drainage for Buildings

17. Install a foundation drain along the perimeter of the new buildings. The drain should consist of a 3 or 4-inch diameter, perforated or slotted, PVC pipe. The flowline of the pipe should be set near the base of the footings. The pipe should be bedded in at least 4 inches of Drain Rock and backfilled with Drain Rock to within 6 inches of the ground surface.

The entire mass of Drain Rock should be wrapped in Filter Fabric that laps at least 12 inches at the top.

18. Provide clean-outs at appropriate locations for future maintenance of the drainage system.

19. Discharge the water from the drain system into the nearest catch basin, manhole or storm drain.

Subgrade Preparation and Pavement Construction – Landside Parking and Driveways

20. Strip the existing ground ±2 to 4 inches or as required to remove roots and sod. Deeper grubbing may be needed under any existing trees.

Dispose of all strippings outside of construction areas.

21. Grade the pavement areas to the planned subgrade elevation.

Moisture-condition and compact the subgrade as recommended in

Item 5 prior to placing fill.

22. Place a Separation Geotextile on the prepared subgrade that meets the requirements specified in Item 3. The Separation Geotextile should be laid smooth, without wrinkles or folds, in the direction of construction traffic. Overlap adjacent rolls a minimum of 2 feet. Pin fabric overlaps or place the granular fill in a manner that will not separate the overlap during construction. Seams that have separated will require removal of the granular fill to establish the required overlap.

23. Place the Base Rock in loose lifts. Compact the Base Rock as recommended in Item 5.

24. Proof-roll the completed Base Rock with an approved construction vehicle prior to paving. Overexcavate any areas of Base Rock pumping or deflection and replace with additional Base Rock.

Geotechnical Investigation 14 Project 2171109

Prineville, Oregon Precision Approach Engineering, Inc.

25. Place and compact the AC to the requirements of either ODOT

Section 00744, FAA P-401, or P-403 (based on recommendations from

PAE).

DESIGN REVIEW/CONSTRUCTION OBSERVATION/TESTING

We should be provided the opportunity to review all drawings and specifications that pertain to site preparation and fill placement. Site preparation for new pavements and foundations will require field confirmation of the subgrade conditions. Mitigation of pumping subgrade and fill will also require engineering review and judgment. That judgment should be provided by one of our representatives. We recommend we be retained to provide the necessary construction observations.

VARIATION OF SUBSURFACE CONDITIONS, USE OF THIS REPORT AND WARRANTY

The analysis, conclusions and recommendations contained herein assume the soil and bedrock profiles encountered in the borings are representative of the site conditions. The above recommendations assume we will have the opportunity to review final drawings and be present during construction to confirm the assumed subgrade conditions. No changes in the enclosed recommendations should be made without our approval. We will assume no responsibility or liability for any engineering judgment, inspection or testing performed by others.

This report was prepared for the exclusive use of Precision Approach Engineering, Inc. and their design consultants for the Prineville Airport – USFS Airbase project in

Prineville, Oregon. Information contained herein should not be used for other sites or for unanticipated construction without our written consent. This report is intended for planning and design purposes. Contractors using this information to estimate construction quantities or costs do so at their own risk. Our services do not include any survey or assessment of potential surface contamination or contamination of the soil or ground water by hazardous or toxic materials. We assume that those services, if needed, have been completed by others.

Our work was done in accordance with generally accepted soil and foundation engineering practices. No other warranty, expressed or implied, is made.

Geotechnical Investigation 15 Project 2171109

Prineville, Oregon Precision Approach Engineering, Inc.

REFERENCES

AASHTO, 2017; Geosynthetic Specification for Highway Applications, American

Association of State Highway and Transportation Officials (AASHTO)

M288-17.

ASTM, 2014, Standard Test Methods for CBR (California Bearing Ratio) of Laboratory-Compacted Soil: American Society for Testing and Materials (ASTM), Standard D1883, vol. 04.08.

ASTM, 2012, Standard Tests Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400ft-lbf/ft3 (600kN-m/m3)): American Society for Testing and Materials (ASTM), Standard D698, vol. 04.08.

City of Prineville, 2013, Standards and Specifications.

FAA, 2013, Subsurface Drainage Design Advisory Circular: AC No. 150/5320-5D, Federal Aviation Administration.

FAA, 2014, Standards for Specifying Construction of Airports Advisory Circular:

AC No. 150/5370-10G, Federal Aviation Administration.

FAA, 2016, Airport Pavement Design and Evaluation Advisory Circular:

AC No. 150/5320-6F, Federal Aviation Administration.

Oregon Department of Transportation, 2018; Oregon Standard Specifications for

Construction, Highway Division.

Appendix A

Figures

4,0002,0001,000

SCALE IN FEET

SITE

Prineville

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FIGURE NO.

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7857 SW CIRRUS DRIVE, BLDG 24

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BUS. (503) 643-1541

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PROFESSIONAL GEOTECHNICAL SERVICES

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BEAVERTON, OR 97008

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PROJECT NO.

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REVIS.

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DWN.

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APPR.

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DATE

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FILE NAME:

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FOUNDATION ENGINEERING INC.

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VICINITY MAP

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PRINEVILLE AIRPORT

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USFS AIRBASE

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PRINEVILLE, OREGON

AutoCAD SHX Text 1A

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JAN. 2018

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AutoCAD SHX Text 2171109

AutoCAD SHX Text Fig 1A - Vicinity Map

BH-1

BH-2

BH-3

BH-4

BH-5

BH-30

BH-7

BH-6

BH-9

BH-10

BH-13

BH-15

BH-8

BH-11

BH-12

BH-14

BH-16

BH-17

BH-29

BH-28

BH-24

BH-26

BH-25

BH-23

BH-22

BH-18

BH-19

BH-20

BH-21

BH-27

R a p e l

T o w e r s

USFS

Building

New Tie-Down Apron

Soil Stockpile (Approximate)

USFS AIRBASE

PRINEVILLE, OREGON

2171109

SCALE IN FEET

0 100' 200' 400'

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FIGURE NO.

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820 NW CORNELL AVENUE

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BUS. (541) 757-7645 FAX (541) 757-7650

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PROFESSIONAL GEOTECHNICAL SERVICES

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CORVALLIS, OR 97330-4517

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PROJECT NO.

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REVIS.

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DWN.

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APPR.

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DATE

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FILE NAME:

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FOUNDATION ENGINEERING INC.

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SITE LAYOUT AND BOREHOLE LOCATIONS

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PRINEVILLE AIRPORT

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JAN 2018

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FIG 2A

AutoCAD SHX Text Notes: 1. Base map with surveyed borehole locations was provided by PAE. 2. BH-1 through BH-5 were completed for the 2018 New Tie-Down Apron project. BH-6 through BH-30 were completed for the 2018 USFS Airbase project. 3. See report for a discussion of subsurface conditions.

Notes:

1. The Design Response Spectrum is based on IBC 2012 Section 1613.

2. The following parameters are based on the modified USGS 2008 maps provided in IBC 2012/OSSC 2014:

Site Class= B Damping = 5% SS = 0.36 Fa = 1.00 SMS = 0.36 SDS = 0.24

S1 = 0.18 Fv = 1.00 SM1 = 0.18 SD1 = 0.12

3. SS and S1 values indicated in Note 2 are the mapped, risk-targeted maximum considered earthquake spectral acclerations for 1% probability of exceedence in 50 years.

4. Fa and Fv were established based on IBC 2012, Tables 1613.3.3(1) and 1613.3.3(2) using the selected SS and S1 values. SDS and SD1 values include a 2/3 reduction on SMS and SM1 as discussed in IBC 2012 Section 1613.3.4.

5. Site location is: Latitude 44.2887, Longitude -120.8941.

Prineville, Oregon

FIGURE 3A

Prineville Airport - USFS Airbase

IBC 2012/OSSC 2014 SITE RESPONSE SPECTRUM

Project 2171109

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 0.5 1 1.5 2 2.5 3

S p ec tr al A cc el er at io n

S a (g

Period (seconds)

IBC 2012/OSSC 2014

Response Spectrum

Appendix B

Boring Logs

Prineville Airport – USFS Airbase Project 2171109

Table 1B. Summary of Bedrock Depths and Elevations

Exploration 1Surface Elevation

(feet)

2Depth to Bedrock (feet)

3Bedrock Elevation

(feet)

Location Notes

BH-6 ±El. 3233.8 ±2.0 ±El. 3231.8 Access Road (proposed)

BH-7 ±El. 3232.7 ±2.4 ±El. 3230.3 Taxilane (proposed)

BH-8 ±El. 3232.0 ±1.6 ±El. 3230.4 Taxilane (proposed)

BH-9 ±El. 3232.4 ±3.7 ±El. 3228.7 Helipad (proposed)

BH-10 ±El. 3230.0 >10 n/a Helipad (proposed)

BH-11 ±El. 3227.9 ±2.2 ±El. 3225.7 Taxilane (proposed)

BH-12 ±El.3226.7 ±4.0 ±El. 3222.7 Taxilane (proposed)

BH-13 ±El.3229.7 ±5.0 ±El. 3224.7 Helipad (proposed)

BH-14 ±El. 3226.5 ±4.5 ±El. 3222.0 Taxilane (proposed)

BH-15 ±El. 3233.9 ±2.0 ±El. 3231.9 Helipad (proposed)

BH-16 ±El. 3228.2 ±3.9 ±El. 3224.3 Helipad (proposed)

BH-17 ±El. 3225.6 ±3.8 ±El. 3221.8 Taxilane (proposed)

BH-18 ±El. 3222.8 >10 n/a Helipad (proposed)

BH-19 ±El. 3221.9 ±8.5 ±El. 3213.4 Access Road (proposed)

BH-20 ±El. 3222.2 >10 n/a Access Road (proposed)

BH-21 ±El.3223.8 ±8.5 ±El. 3215.3 Access Road (proposed)

Notes: 1. Ground surface elevations were surveyed by Precision Approach Engineering, Inc.

2. Drilling refusal in the borings occurred at the bedrock surface. Bedrock was not encountered in BH-10, BH-18, and BH-20.

3. Bedrock elevations were estimated based on the estimated surface elevation and the measured depth of the bedrock surface.

Table 1B. Summary of Bedrock Depths and Elevations (Continued)

Exploration 1Surface Elevation

(feet)

2Depth to Bedrock (feet)

3Bedrock Elevation

(feet)

Location Notes

BH-22 ±El. 3227.5 ±2.7 ±El. 3224.8 Access Road (proposed)

BH-23 ±El. 3229.9 ±2.0 ±El. 3227.9 Access Road (proposed)

BH-24 ±El. 3232.5 ±2.6 ±El. 3229.9 Access Road (proposed)

BH-25 ±El. 3231.0 ±1.9 ±El. 3229.1 Parking (proposed)

BH-26 ±El. 3227.9 ±1.3 ±El. 3226.6 Parking (proposed)

BH-27 ±El. 3225.4 ±4.0 ±El. 3221.4 Rappel Towers (proposed)

BH-28 ±El. 3229.3 ±2.0 ±El. 3227.3 USFS Building (proposed)

BH-29 ±El. 3229.2 ±2.5 ±El. 3226.7 USFS Building (proposed)

BH-30 ±El. 3235.2 ±3.2 ±El. 3232.0 Parking (proposed)

Notes: 1. Ground surface elevations were surveyed by Precision Approach Engineering, Inc.

2. Drilling refusal in the borings occurred at the bedrock surface. Bedrock was not encountered in BH-10, BH-18, and BH-20.

3. Bedrock elevations were estimated based on the estimated surface elevation and the measured depth of the bedrock surface.

mkrawczyk Highlight

S-6-1

Sandy SILT, trace gravel (ML); brown, non-plastic, dry, stiff, fine sand, fine subrounded gravel.

Practical drilling refusal encountered at ±2 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3233.8 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH- 6 Prineville Airport USFS Airbase Prineville, OregonDecember 13, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

Comments

S-7-1

Sandy SILT, trace gravel (ML); brown, non-plastic, dry, stiff, fine sand, fine subrounded gravel.

Practical drilling refusal encountered at ±2.4 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3232.7 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH- 7 Prineville Airport USFS Airbase Prineville, OregonDecember 13, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

Comments

Last edited: 1/29/18 WG

S-8-1 Sandy SILT (ML); brown, non-plastic, dry, stiff, fine sand.

Practical drilling refusal encountered at ±1.6 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3232.0 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH- 8 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

Comments

S-9-1

Sandy SILT (ML); brown, non-plastic, dry, stiff, fine sand.

Practical drilling refusal encountered at ±3.7 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3232.4 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH- 9 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

S-10-1

S-10-2

S-10-3

Sandy SILT (ML); brown, non-plastic, dry, stiff, fine sand.

Silty SAND (SM); brown, non-plastic silt, damp, medium dense, fine sand.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3230.0 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH-10 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

Comments

S-11-1

SILT, some sand (ML); brown, low plasticity, dry, stiff, fine sand.

Practical drilling refusal encountered at ±2.2 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3227.9 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH-11 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

S-12-1

S-12-2

Silty SAND (SM); brown, non-plastic silt, dry, medium dense, fine sand.

SILT, trace sand (ML); brown, low plasticity, damp, stiff, fine sand.

Practical drilling refusal encountered at ±4 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3226.7 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH-12 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti o n

C la ss S ym b o l

W at er T ab le

C , T

S F

S ym b o l

Comments

S-13-1

S-13-2

Silty SAND (SM); brown, non-plastic silt, dry, medium dense, fine sand.

SILT, some sand (ML); brown, low plasticity, damp, stiff, fine sand.

Practical drilling refusal encountered at ±5 feet on basalt.

BOTTOM OF BORING

No seepage or ground water encountered to the limit of exploration.

3229.7 feet (Approx.)

Project No.:

Surface Elevation:

Date of Test Pit:

2171109 Boring Log: BH-13 Prineville Airport USFS Airbase Prineville, OregonDecember 14, 2017

Soil and Rock Description

D ep th

F ee t

S am p le

L o ca ti…