AZ NFSR 289 Tonto Creek Bridge Geotechnical Report.pdf

SUBMITTED TO:

Jacobs Engineering Group, Inc.

6312 South Fiddlers Green Circle, Suite 300N Greenwood Village, CO 80111-4927

BY:

Shannon & Wilson 5900 W 38th Ave Wheat Ridge, CO 80212

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL REPORT

AZ NFSR 289 Bridge Replacement

TONTO NATIONAL FOREST, GILA COUNTY, ARIZONA

July 2025 Shannon & Wilson No: 110781-002

Digitally signed by Gregory R Fischer DN: cn=Gregory R Fischer, o=Colorado, email=grf@shanwil.com Date: 2025.07.30 12:11:55 -06'00'

Gregory R Fischer

AZ NFSR 289 Bridge Replacement Geotechnical Report

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CONTENTS

CONTENTS

1 Introduction

2 Project and Site Description

3 Field Explorations and Laboratory Testing

3.1 Field Explorations

3.2 Geotechnical Laboratory Testing

4 Regional Geology and Subsurface Conditions

4.1 Regional Geology

4.2 Site Soil and Bedrock Observations

4.3 Subsurface Conditions

4.4 Subsurface Variation

5 Geologic Hazards

5.1 Corrosive Soil and Bedrock

5.2 Seismic Hazards

6 Geotechnical Recommendations

6.1 Seismic Ground Motion Design Parameters

6.2 Foundation Recommendations

6.2.1 Axial Resistance

6.2.2 Lateral Resistance

6.3 Permanent Slopes

6.4 Abutment and Wing Walls

6.4.1 Lateral Earth Pressures

6.4.2 Wall Drainage

7 Construction Considerations

7.1 Drilled Shaft Installation

7.1.1 Drilled Shaft Installation Methods and Equipment

7.1.1.1 Cobbles and Boulders

7.1.1.2 Rock Drilling

7.1.1.3 Excavation Support

7.1.2 Drilled Shaft Inspection and Observation

7.1.3 Concrete Placement

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7.1.4 Non-Destructive Integrity Tests

7.2 Earthwork

7.2.1 Site Preparation

7.2.2 Excavation Potential

7.2.3 Fill Placement and Compaction

7.3 Temporary Slopes and Shoring

7.4 Dewatering

8 Document Review and Final Design

9 Limitations

10 References

Exhibits Exhibit 4-1: General view of boulders exposed on face of slope on the east side of the roadway to the south of bridge (view looking east) Exhibit 4-2: General view of limestone bedrock exposed in the channel upstream of the proposed bridge crossing (view looking northeast along Horton Creek from the confluence with Tonto Creek) Exhibit 5-1: Corrosion Testing Results Exhibit 6-1: Seismic Design Ground Motion Parameters Exhibit 6-2: Recommended Earth Pressure Parameters for Backfill Materials

Tables Table 1: Deep Foundation Design Parameters

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Recommended P-Multipliers for Horizontally Loaded Pile or Drilled Shaft

Groups Figure 4: Recommended Surcharge Loading for Temporary and Permanent Walls

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Drilling Photographs Important Information About Your Geotechnical Report

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1 INTRODUCTION

This report presents geotechnical engineering recommendations for the proposed National Forest Service Road 289 (NFSR 289) Bridge Replacement Project (the Project). Our work consisted of completing subsurface explorations and geotechnical engineering analyses for the proposed replacement bridge carrying NFSR 289 over Tonto Creek in the Tonto National Forest, Gila County, Arizona (Figure 1). This report presents our interpretation and characterization of the subsurface conditions beneath the proposed Tonto Creek Bridge and provides geotechnical engineering recommendations. Our services were completed in accordance with our subcontracts with Jacobs dated December 6, 2023, and March 18, 2025.

Our scope of services included:

Coordinating a subsurface exploration program, which consisted of staking boring locations and clearing utilities with New Mexico 811, applying for the required permits, obtaining right-of-way access, coordinating traffic control services, and drilling seven geotechnical borings, including four borings for pavement design information (which are addressed in a separate report).

Performing geotechnical laboratory testing on select samples retrieved from the borings.

Conducting geotechnical engineering analyses to develop design parameters for bridge foundations, wing walls, and a potential cut slope.

Preparing this geotechnical design report.

The scope of our services did not include evaluating the presence of potentially contaminated soils or groundwater at or around the site. Our scope did not include well development, groundwater sampling, or hydrogeologic testing. If a service is not specifically indicated in this report, do not assume it was performed.

2 PROJECT AND SITE DESCRIPTION

Based on discussions with Jacobs, the Project consists of realigning NFSR 289 in the area of the existing bridge to improve pedestrian and vehicular safety. NFSR 289 is a two-lane, asphalt paved roadway providing access to residential homes, the Tonto Creek fish hatchery, a summer camp, a campground, and the Horton Springs Trailhead.

The existing bridge over Tonto Creek was constructed in 1956 and is two spans with a total length of approximately 51 feet, one-lane-wide with no sidewalks, and is a safety concern

110781-002 July 2025 due to the narrow structure width and limited sight distance with sharp curves in the approach roadway in both directions.

Based on the available plans provided by Jacobs (2025), the new bridge is expected to be a single-span structure with a total length of approximately 92 feet. The new bridge is indicated to have a width of approximately 32.5 feet with one 12-foot wide through lane in each direction and a 6-foot-wide sidewalk along the north side of the bridge. The bridge is shown to have concrete abutments with two 48-inch diameter drilled shafts at each abutment, and cantilevered wing walls with a maximum height of approximately 9 feet.

Pavement improvements are also included in the Project; however, our recommendations for pavement design are provided in a separate report.

3 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1 Field Explorations

Shannon & Wilson conducted a field exploration program from December 18 to 21, 2023, to evaluate subsurface conditions at the anticipated bridge abutment locations and for a potential cut slope. Borings SW-B-01 and SW-B-02 were drilled to approximately 27 feet and 13 feet below the ground surface (bgs), respectively, for bridge foundation design information. Boring SW-C-01 was drilled to approximately 19 feet bgs to obtain subsurface information for a potential slope cut. Locations of the borings are shown in Figure 2.

Appendix A describes the procedures used to complete the drilling and sampling of the geotechnical borings and presents the individual exploration logs and an explanation of the symbols and terminology used on the logs.

3.2 Geotechnical Laboratory Testing

Shannon & Wilson completed geotechnical laboratory testing to determine index and engineering properties of selected samples from the borings. Tests included natural water content, unit weight, grain size distribution, Atterberg limits, unconfined compressive strength, and corrosion. The laboratory test results are presented in Appendix B along with a brief discussion of the laboratory testing procedures. The natural water contents, Atterberg limits, and percent fines are also indicated on the individual boring logs in Appendix A.

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4 REGIONAL GEOLOGY AND SUBSURFACE

CONDITIONS

4.1 Regional Geology

The Project area is located on the southwest slope of the Mogollon Rim, downstream of the confluence of Tonto Creek and Horton Creek. Mapping completed by Wilson et al. (1959) shows the Project area is underlain by limestone, shale, and sandstone of the Pennsylvanian Naco Formation, Mississippian Redwall Limestone, and Devonian Limestone and Sandstone. The top of the Redwall Limestone unit is described as “karstic”; however, no karstic features are indicated in the Project area(Weisman and Wier, 1990).

4.2 Site Soil and Bedrock Observations

While onsite, we made generalized visual observations of soil and bedrock exposures adjacent to the roadway south of the bridge and in the channel near the proposed bridge crossing. This effort did not include performing detailed rock mass structure mapping.

Along the south approach roadway, we noted alluvial deposits with cobbles and boulders exposed in the cut face east of the roadway, including boulders up to about 3 to 4 feet in diameter (see Exhibit 4-1).

Exhibit 4-1: General view of boulders exposed on face of slope on the east side of the roadway to the south of bridge (view looking east).

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Bedrock observed in the channel near north and south abutments of the existing bridge consisted of sandstone. Sandstone was also exposed near the proposed south abutment location; however, the channel slope near the proposed north abutment consisted of alluvial deposits with cobbles and boulders up to about 2½ feet in diameter.

Upstream of the proposed bridge crossing, limestone was exposed along Horton Creek. The limestone was medium strong to strong and north to northwest dipping (see Exhibit 4-2).

The limestone outcrop extends into Tonto Creek, but it was not exposed in the vicinity of the proposed north abutment.

As discussed in Section 4.1, the Limestone is described as “karstic”; however, we did not observe evidence of karstificiation (i.e., sinkholes or caves) in the Project area.

Exhibit 4-2: General view of limestone bedrock exposed in the channel upstream of the proposed bridge crossing (view looking northeast along Horton Creek from the confluence with Tonto Creek).

4.3 Subsurface Conditions

Boring SW-C-01 was drilled from the top of the slope shown in Exhibit 4-1, near the proposed embankment cut, and encountered alluvium consisting of medium dense to very dense silty gravel with sand, cobbles, and boulders extending to the depth explored. This boring encountered refusal to auger penetration at 19 feet below the ground surface (bgs).

Based on drill action, auger refusal may have been on bedrock; however, bedrock was not exposed on the face of the slope near the boring.

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Boring SW-B-01 was drilled near the proposed south abutment location and encountered alluvium overburden materials consisting of medium dense, silty sand and very dense clayey gravel with sand and cobbles (inferred from drill action). Auger refusal occurred within the alluvium, likely due to boulders and cobbles, and we needed to offset and redrill several times. At a depth of 13 feet bgs, sandstone bedrock of the Naco Formation was encountered, which extended to the termination depth of the boring at approximately 26.7 feet bgs. The upper 7 feet of the bedrock was extremely weak and highly to completely weathered.

Boring SW-B-02 was drilled near the proposed north abutment location and encountered very dense alluvium consisting of clayey gravel with sand, cobbles and boulders and medium dense poorly graded sand with silt and cobbles. Auger refusal occurred within the alluvium, likely due to boulders and cobbles, and we needed to offset and redrill several times. Auger refusal occurred in what appeared to be Naco formation limestone bedrock at a depth of 13 feet bgs; however, due to schedule limitations, we were unable to penetrate bedrock at this location by coring methods.

Groundwater was not measured during drilling at the depths explored. At the time of our field work, there was a negligible amount of water flowing in the creek. In our opinion, the level of the creek should be fairly representative of the static groundwater level; however, fluctuations of groundwater levels at the site are possible and will depend on many factors, such as seasonal variations, local precipitation, and runoff.

4.4 Subsurface Variation

Our observations are specific to the locations, depths, and times noted on the logs and may not be applicable to all areas of the site. No amount of explorations or testing can precisely predict the characteristics, quality, or distribution of subsurface and site conditions.

Potential variation includes, but is not limited to:

The conditions between explorations may be different.

The passage of time or intervening causes (natural and human-caused) may result in changes to site and subsurface conditions.

If conditions that are different from those described herein are encountered during construction, we should review our description of the subsurface conditions and reconsider our conclusions and recommendations.

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5 GEOLOGIC HAZARDS

5.1 Corrosive Soil and Bedrock

The soil/rock encountered at the Project site can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, selected samples were tested for pH, resistivity, water-soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B and in Exhibit 5-1.

Exhibit 5-1: Corrosion Testing Results

Boring Sample Top

(feet) Bottom (feet) USCS pH

Resistivity (ohm-cm)

Sulfate Content

Chloride Content

SW-B-01

S-1 2.5 4.0 SM 8.0 10,000 0.010 0.024

S-5 14.0 15.5 - - 0.010 -

NOTES:

Water-soluble sulfate and chloride values are shown as percent weight in dry soil ohm-cm = ohm-centimeter, USCS = Unified Soil Classification System (See Figure A-1 in Appendix A for definitions)

Soils with resistivity values less than 2,000 ohm-centimeters (ohm-cm) are considered corrosive for pile foundations per AASHTO (2020). The resistivity measured in the sample tested was 10,000 ohm-centimeters. Based on correlations developed by Roberge (2012), this value suggests mildly to moderately corrosive subsurface conditions for metal in contact with subsurface materials across the site.

Soil or rock with sulfate concentrations more than 1,000 parts per million (0.1% by weight) are indicative of potential concrete degradation based on AASHTO (2020). The concentration of water-soluble sulfates measured in the samples was 0.010% by weight.

Based on classifications as defined by ACI-318-19 (ACI, 2022), these test results suggest an exposure class of S0 for concrete exposed to site soils.

The test results and above discussion are provided to assist the designer in the selection of project materials, concrete type, pile section size, or other features with respect to corrosion.

As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

5.2 Seismic Hazards

Based on a recent geologic map by the U.S. Geological Survey (USGS), the nearest fault to the Project is the Cottonwood Basin Fault, located approximately 40 miles to the northwest (Pearthree, 1998), with an estimated slip rate of less than 0.2 millimeters per year. Therefore, in our opinion, the potential for ground surface rupture at the Project site is low.

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Liquefaction may occur in loose, saturated, cohesionless soils when subjected to earthquake ground shaking, which can cause loss of soil shear strength and settlement of structures.

Similarly, seismic compression may occur when loose, granular soils above the groundwater table are rearranged into a tighter packing configuration during seismic ground shaking, which can also cause settlement. Based on the subsurface conditions encountered at the Project site and the relatively low peak horizontal ground acceleration for this area (see Section 6.1), it is our opinion that the risk of settlement from liquefaction and seismic compression is low.

6 GEOTECHNICAL RECOMMENDATIONS

The recommendations presented in the following sections are based on our understanding of the Project, conversations with Jacobs, and our subsurface exploration and laboratory testing program.

6.1 Seismic Ground Motion Design Parameters

Using the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO, 2020) criteria, and based on subsurface conditions encountered in our borings, we recommend Site Class C be used for the Project site.

Ground motion parameters were determined using the USGS Seismic Design Map Web Application (USGS, 2025), and procedures recommended by AASHTO (2020).

Recommended seismic ground motion parameters are summarized in Exhibit 6-1.

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Exhibit 6-1: Seismic Design Ground Motion Parameters

Ground Motion Parameters

Peak Ground Acceleration1 (PGAB) 0.071 g

Short-period Spectral Acceleration, SS 0.165 g

Long-period Spectral Acceleration, S1 0.050 g

Site Class C

Site Factor, FPGA 1.2

Site Factor, Fa 1.2

Site Factor, Fv 1.7

Peak Design Spectral Acceleration, As 0.085 g

Short-period Design Spectral Acceleration, SDS 0.198 g

Long-period Design Spectral Acceleration, SD1 0.085 g

Short-period Reference Time, T0 0.086 sec.

Long-period Reference Time, TS 0.43 sec.

Seismic Zone2 1

NOTES:

PGAB = peak ground acceleration for a site underlain by Site Class B material (soft rock) Seismic Zone from AASHTO (2020) Table 3.10.6-1 g = gravitational acceleration; PGA = peak ground acceleration; sec. = second

6.2 Foundation Recommendations

The following sections provide geotechnical recommendations for the proposed bridge. We anticipate that deep foundations will be used for support of the proposed bridge abutments.

As discussed in Section 2, we understand the proposed replacement structure will consist of a single-span bridge. In our opinion, micropiles or drilled shafts are feasible deep foundation types for support of the bridges. For either micropiles or drilled shafts, the bridge foundations will be designed to develop capacity in bedrock. Based on discussions with Jacobs and Central Federal Lands Highway Division staff, drilled shafts are preferred.

6.2.1 Axial Resistance

Drilled shafts can be designed for tip and side resistance in bedrock using the unit resistance parameters provided in Table 1. We recommend that drilled shafts penetrate a minimum of

1.5 shaft diameters into the strong bedrock layer, below the soil like bedrock layer. We recommend that side resistance from overburden soils be ignored. Additionally, we recommend that consideration be given to using permanent steel casing, due to the potential

110781-002 July 2025 to encounter cobbles, boulders, and caving soils overlying bedrock, and that the casing be embedded at least 2 feet into the soil like sandstone layer.

The drilled shaft axial resistance parameters presented in Table 1 are nominal values.

Resistance factors of 0.55 and 0.5 should be used for side and base resistance, respectively, based on the AASHTO guidelines (2020). The nominal axial resistance parameters do not require reduction due to shaft group action, provided the shafts are spaced a horizontal distance of at least 2 shaft diameters, center-to-center.

Using good installation techniques and equipment based on criteria described in the 2018 FHWA Manual “Drilled Shafts: Construction Procedures and LRFD Design Procedures” (Brown and others, 2018), the total settlement for a single drilled shaft will be approximately ½ inch or less, under service conditions. This settlement value does not include elastic compression of the shaft under the service load. We anticipate that differential settlement between adjacent piers will be about 50% of this value. About 75% of the settlement is anticipated to occur as the load is applied with the remaining settlement occurring within the first year following construction.

6.2.2 Lateral Resistance

Lateral loads acting on the structure from wind, seismic, traffic, and other loadings are typically resisted by the passive earth pressure against the caps, and the lateral resistance provided by the deep foundation members. The lateral behavior of drilled shafts is highly dependent on the degree of fixity of the top of the substructure elements.

In our opinion, frictional sliding resistance at the base of the cap should be ignored because a deep foundation-supported structure may not transmit load directly to the soil beneath the cap. The degree of movement required to initiate passive pressure and pile cap movement will depend on the degree of fixity of the deep foundation element.

To determine the lateral resistance of an individual drilled shaft, and the deflection, shear, and moment along the shaft, we have provided input parameters for the commercial software LPILE (Ensoft, 2022) in Table 1. Group action can be analyzed using p-multipliers within LPILE. Recommended values of p-multipliers (i.e., group reduction factors) for loading perpendicular and parallel to a row of piles or shafts should be applied in accordance with AASHTO LRFD Bridge Design Specifications (2020) Table 10.7.2.4-1 and Figure 10.7.2.4-1, which are summarized in Figure 3.

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6.3 Permanent Slopes

Final cross-sections of the proposed site grading were not available at the time we prepared this report. However, based on our understanding of the proposed construction, we anticipate that minor cuts and fills will be required to construct the proposed structures and for the proposed roadway realignment. We recommend that permanent excavated slopes in native soils be constructed no steeper than 2 horizontal to 1 vertical (2H:1V). We recommend fill slopes constructed with properly compacted granular backfill material be constructed at 2H:1V or flatter.

6.4 Abutment and Wing Walls

Based on the 70% plans (Jacobs, 2025), the wing walls are indicated to be about 8 feet long, up to approximately 7.5 feet tall, and cantilevered from the bridge abutments. The following sections provide additional recommendations related to retaining walls.

6.4.1 Lateral Earth Pressures

The lateral earth pressures acting on concrete walls depend on many factors, including surcharge loads, the soil type/consistency, and whether the top of the wall is free to yield or deflect laterally during and after excavation. If the top of the wall is free to yield a minimum of 0.1% of the wall height (i.e., 0.001 times the height [H]), the soil pressures will generally be less (i.e., active condition) than if the wall is not able to yield this amount due to stiffness or resistance of the wall (i.e., at-rest condition).

On-site granular soils or imported granular fill may be used to backfill retaining walls.

Recommended soil parameters for on-site granular soils are provided in Exhibit 6-2 below.

The provided static earth pressure coefficients assume the structure walls are backfilled with properly compacted granular material in a 1H:1V zone extending upward from the base of the structure from a point 1.5 feet behind the edge of the foundation. These pressures also assume a vertical wall face and a flat backslope. The parameters provided in Exhibit 6-2 assume a free-draining backfill condition. Surface water behind the wall should not be allowed to discharge or accumulate directly into the wall backfill materials.

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Exhibit 6-2: Recommended Earth Pressure Parameters for Backfill Materials

Design Parameter Recommended

Value3

On-site Granular Soils

Total Unit Weight (pcf) 135

Effective Friction Angle (deg) 34

Effective Cohesion (psf) 0

Horizontal At-Rest Earth Pressure Coefficient, K0 0.44

At-Rest Equivalent Fluid Pressure, above groundwater (psf/ft) 60

At-Rest Equivalent Fluid Pressure, below groundwater (psf/ft) 95

Horizontal Active Earth Pressure Coefficient, KA 0.28

Active Equivalent Fluid Pressure, above groundwater (psf/ft) 38

Active Equivalent Fluid Pressure, below groundwater (psf/ft) 83

Horizontal Passive Earth Pressure Coefficient, KP 3.74

Passive Equivalent Fluid Pressure, above groundwater (psf/ft) 500

Passive Equivalent Fluid Pressure, below groundwater (psf/ft) 330

NOTE:

Total unit weight is the moist unit weight of soil above the groundwater table.

Differential water pressure should be included unless drainage measures are included behind the walls.

Ignore passive pressure in front of walls because of sloping ground conditions.

Lateral earth pressures reflect horizontal slope conditions.

Passive earth pressure values reflect limited lateral movement, consistent with that expected for drilled shaft movement.

pcf = pounds per cubic foot; deg = degrees; psf = pounds per square foot)

Surcharge loads, such as those from motor vehicles and construction equipment, will induce lateral loads on retaining walls and buried structures. Consistent with AASHTO (2020) criteria, we recommend using a live load traffic surcharge of 250 psf for areas subject to motor vehicle loading. Lateral loads due to various types of surcharges can be calculated using the earth pressure coefficients provided in Exhibit 6-2 and the loading diagrams provided in Figure 4, along with appropriate load factor(s).

6.4.2 Wall Drainage

Fluctuations in the water level of Tonto Creek could potentially result in differential hydrostatic water pressure in wall backfill (i.e., water levels in the backfill may rise with the water level in the creek and then remain elevated after the creek level decreases, depending on wall drainage measures, the duration of elevated water levels in the creek, and how quickly the creek levels recede). The earth pressure parameters provided in Exhibit 6-3 provide pressures that can be used in the event that unbalanced hydrostatic pressures develop in the retained zone behind walls.

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In general, materials with greater than about 3% fines content are not considered free draining; therefore, the onsite granular soils are not free draining and an unbalanced condition could occur following flood events, i.e., water could be trapped in the backfill after flood waters recede. This condition could be addressed by designing the wall for appropriate water pressures or by including internal drainage features to reduce the potential for water to accumulate in the backfill. Appropriate drainage features could include:

Placement of a 12-inch-thick drainage layer behind the walls with a suitable discharge system (e.g. weep holes or a perforated collector pipe, daylighting to a suitable discharge point). Gravel backfill within the drainage layer should meet the requirements of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-24 Section 703.03 Underdrain Backfill (FHWA, 2024).

Limiting the maximum fines content of the backfill material to 3% and providing weepholes.

7 CONSTRUCTION CONSIDERATIONS

The applicability of the design recommendations provided in this report is contingent on good construction practice. Poor construction techniques may alter conditions from those on which our recommendations are based, resulting in reduced foundation capacity and increased settlement. The following sections present additional construction and material considerations for this Project.

7.1 Drilled Shaft Installation

7.1.1 Drilled Shaft Installation Methods and Equipment

Specifications and installation methods should be in general accordance with our recommendations and guidelines in the 2018 FHWA Manual, "Drilled Shafts: Construction Procedures and Design Methods" (Brown and others, 2018).

7.1.1.1 Cobbles and Boulders

Installation of drilled shafts will require advancing the excavation through cobbles and boulders. Cobbles and boulders can sometimes be excavated by conventional augers, but modified single-helix augers, designed with a taper and sometimes with a calyx bucket mounted on top of the auger, a.k.a. boulder rooters, are generally more successful at extracting small boulders (Brown and others, 2018). The extraction of large boulders and rock fragments can cause considerable difficulty and significantly reduce drilling production. Boulders that are solidly embedded can likely be cored, while coring through

110781-002 July 2025 boulders loosely embedded in soil may be ineffective. The removal of loosely embedded boulders may require breaking the boulder in the hole with percussion methods or a rock breaker tool (or other appropriate methods). The specifications should indicate the potential for boulders to be encountered.

7.1.1.2 Rock Drilling

Drilled shafts will obtain their capacity within the bedrock and construction of drilled shafts will require drilling sockets in relatively high strength rock. Unconfined compressive strength testing indicated strengths of 7,180 and 9,430 psi, and drilling in these materials may require specialized techniques, such as coring, full-face rotary drilling, or down-the-hole impact hammers, to advance the excavation. Use of these techniques will decrease production rates and increase costs. The specifications should require the drilled shaft contractor to demonstrate experience in this formation to confirm proposed methods and expected production.

7.1.1.3 Excavation Support

Overburden soils at the site generally consist of medium dense to very dense sand and gravel with varying amounts of silt, cobbles, and boulders. Groundwater was not observed in the borings during drilling; however, groundwater will likely be encountered in shafts installed below the creek level.

During drilled shaft installation, we recommend that permanent casing be considered and embedded at least 2 feet into the bedrock, to help prevent raveling, caving, and flowing conditions in the overburden. Where casing is used, it should be pushed, rotated, vibrated, or driven into the bedrock. The inside diameter of the casing should be equal to or larger than the specified drilled shaft dimensions. The use of casings larger than the diameter of the specified casing must have prior approval from the Engineer.

Loss of drilling fluid may occur in areas with cobbles, boulders, and fractured bedrock. To overcome this issue, the contractor could construct the shafts using an oscillator to simultaneously advance casing with the excavation. In this procedure, the shaft casing is equipped with cutting teeth or a cutting shoe and installed by either rotating or oscillating the casing. Such methods are advantageous in that they can advance through cobbles, boulders, obstructions, and rock when combined with appropriate methods to drill within the casing.

Where casing is sealed into the bedrock bearing zone, bedrock penetration to achieve capacity should begin at the bottom of the casing. If significant penetration (more than 3 feet) of casing into bedrock is anticipated, we should be notified so we may provide

110781-002 July 2025 appropriate reduced side resistance parameters. Even with casing, groundwater can infiltrate into drilled shaft excavations from perched water or within fractured or more permeable zones within the bedrock. Hence, the contractor should be prepared for underwater concrete placement (tremie pipes).

If slurry methods are required to stabilize the excavation, we recommend the use of a polymer slurry in the bedrock. Uncontrolled slurries should not be permitted.

Additionally, the drilled shaft contractor should not be permitted to use mineral (e.g., bentonite) slurry in the bedrock. Mineral slurries may reduce the side resistance in the bedrock below the values provided herein. Construction of drilled shafts using wet methods (i.e., slurry) is more difficult than constructing shafts using dry methods. Because a wet excavation cannot be easily observed, good construction practices are critical to constructing shafts that perform adequately. Wet installation methods and specifications should be in accordance with the 2018 FHWA Manual (Brown and others, 2018).

7.1.2 Drilled Shaft Inspection and Observation

A geotechnical engineer familiar with the subsurface conditions at the site should observe drilled shaft installation to determine the top of rock elevation and shaft penetration into rock. Because the drilled shafts will develop a significant portion of their axial resistance in end bearing, it will be critical to confirm that the base of the drilled shaft is clean and firm.

The hole should be cleaned of loose material and observed by the geotechnical engineer prior to pouring concrete. The drilling and concrete placement process should be relatively continuous with minimal stoppage of work between the completion of drilling, cleaning the excavation, and the placement of concrete after setting the reinforcing cage.

7.1.3 Concrete Placement

Groundwater inflow into drilled shaft excavations from fractured or more permeable zones is possible, even when casing is extended into bedrock. Tremie placement should be used if wet methods are used to excavate the shafts or if water cannot be controlled by pumping or bailing such that more than 3 inches of water is present when concrete is placed. The contractor should be prepared to address these issues.

We recommend concrete be designed and placed with a slump of 4 to 6 inches if placed in the dry (with no casing to be pulled), 5 to 7 inches if casing is to be pulled or the shaft is heavily reinforced, and 8 to 10 inches (with maximum aggregate size of 0.75 inches) when pumping and/or tremie placement is used. When casing and/or tremie concrete placement methods are used, a minimum head of concrete of 5 feet above the bottom of the tremie pipe and/or casing should be maintained at all times.

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Drilled shaft defects in cased shafts are frequently the result of inadequate head of concrete, particularly when combined with marginal or low slump concrete. If a truck-mounted pump is used to tremie concrete, pull-out of the pipe can occur if a pressure surge causes upward boom movement. Adequate methods should be established to measure and confirm that minimum head requirements are met throughout the concrete placement process.

7.1.4 Non-Destructive Integrity Tests

We recommend that non-destructive tests be completed on select drilled shafts for the Project. If permanent casing is used, consideration could be given to reducing the testing requirements. In our opinion, cross-hole sonic logging (CSL) will provide the best evaluation of the integrity of the drilled shafts, particularly where casing is used. CSL is a non-destructive testing method that requires steel (preferred for durability and to avoid delaminating from the concrete) or plastic tubes installed in the drilled shaft and tied to the reinforcing cage. The permeant tubes are attached to the interior of the rebar cage and cast in the shaft. After the concrete has cured, a sound source and receiver are lowered, maintaining a consistent elevation between source and sensor. A signal generator transmits a sonic pulse from the emitter, which is recorded by the sensor. Relative energy, waveform, and differential time are recorded and logged. This procedure is repeated at regular depth intervals throughout the shaft. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the soundness of the concrete throughout the drilled shaft can be interpreted.

We recommend installing access tubes for CSL in all shafts in case uncertainty arises during installation regarding the integrity of the shaft, and that CSL testing be completed on at least one shaft per abutment. If permanent steel casing used on all shafts, in our opinion, CSL testing may not be required.

7.2 Earthwork

7.2.1 Site Preparation

Prior to site grading, construction areas should be cleared to a depth necessary to remove all surface and subsurface structures associated with current development of the site, including all foundations, pavements, underground utilities, and other deleterious material. Further, any existing surficial topsoil and soil containing organics should be stripped and removed from all areas. Topsoil and organic-rich soils are not considered suitable for reuse as fill and should be removed from the site or stockpiled for use as topsoil material. Any existing pavement that is stripped from the Project area is not suitable for reuse as fill material and should be removed from the site.

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Earthwork, including site preparation and subgrade preparation, should conform to the requirements provided in FP-24, Division 200 (FHWA, 2024).

7.2.2 Excavation Potential

We anticipate that excavation for the proposed construction will generally occur in existing granular overburden containing boulders and cobbles, except for excavations within the existing channel where bedrock is exposed.

We anticipate that excavation of the overburden materials can be accomplished with conventional excavating equipment, such as dozers, tracked excavators, and front-end loaders. The removal of large boulders may require breaking the boulder with percussion methods or a rock breaker tool (or other appropriate methods). Removal of boulders may also result in overbreak (removal of material beyond the planned excavation limits), potentially leading to quantity over-runs.

Medium strong to strong rock was exposed in the channel, with estimated uniaxial compressive strength (UCS) values ranging from approximately 3,600 to 14,500 psi, based on our observations and interpretation of published correlations by Brown (1981). Where highly fractured bedrock is exposed in the excavation, techniques and equipment such as ripping, hydraulic rock breakers, and the use of percussive drilling to break up rock in the excavation may be considered. Less fractured zones of bedrock may also be encountered as the depth of the rock excavation increases, and slow, difficult rock excavation should be expected.

7.2.3 Fill Placement and Compaction

Subgrade material and fill should be placed in accordance with FP-24, Section 204.11

(FHWA, 2024).

7.3 Temporary Slopes and Shoring

The safe slope for the excavation of subsurface materials depends on many factors, including: (a) the presence and depth of groundwater; (b) the type, density, and shear strength of the subsurface materials; (c) the depth of excavation; (d) the presence of adjacent facilities; (e) surcharge loading adjacent to the excavation (including stockpiled excavated material, existing dead or live loads, and construction equipment); and (f) duration of construction. Considering these factors, unshored, temporary excavation slopes may be possible in areas around the site. However, we recommend that unshored, temporary excavation slopes only be utilized above the groundwater level. Constructing excavation

110781-002 July 2025 slopes below the groundwater level may cause slope instability due to the seepage of groundwater into the excavation.

The Project team may choose to limit where the contractor can make temporary excavation slopes based on the need to limit impacts to the roadway and adjacent property and or to avoid utilities. If it is possible to slope the excavation, we recommend the temporary slopes be consistent with the Occupational Safety & Health Administration (OSHA) guidelines contained in 29 CFR 1926, Subpart P (1989). For cost estimating and planning purposes only, we anticipate that temporary excavations in cohesionless soils can be made at slopes of 1.5H:1V and in bedrock at 0.75H:1V. Flatter slopes should be anticipated if groundwater is encountered in the excavation. Temporary slopes should be consistent with the Occupational Safety and Health Administration (OSHA) guidelines contained in 29 CFR 1926, Subpart P (OSHA, 1999).

Consistent with conventional practice, the contract documents should require the contractor to be responsible for the actual temporary excavation slopes, including methods, sequence, and schedule of construction. The contractor is able to observe the nature and conditions of the subsurface materials encountered and should evaluate the factors discussed above. If instability is observed, slopes should be flattened or shored. All excavations should be accomplished in accordance with local, state, and federal safety regulations.

Due to geometric considerations, temporary slopes may not be feasible at all locations of the site and temporary shoring may be required. Feasible shoring methods for the subsurface conditions present at the site include driven sheet piling, soldier pile wall with lagging, or temporary soil nail walls. However, depending on the depth of shoring, it may not be feasible to achieve adequate embedment of sheet piles into the rocky ground or bedrock.

Nail installation through rocky soils may also be difficult in some areas. Therefore, a soldier pile wall may be preferable.

The final selection of shoring type, temporary shoring design, and method of construction should be the responsibility of the Contractor. Further, it should be the Contractor’s responsibility to monitor the stability of shored excavations and adjacent structures and take corrective action if any potentially unstable conditions are encountered. We recommend the Project specifications stipulate that any shoring design be performed by an Arizona-licensed Professional Engineer. All temporary walls should include wall drainage measures and should be designed with appropriate surcharge loads.

7.4 Dewatering

Groundwater was not encountered in the borings; however, excavations for abutments and wing walls that extend near to or below the creek channel may encounter groundwater and

110781-002 July 2025 experience instability of open-cut excavations and subgrades. Groundwater levels will likely be influenced by the stages and seasonal changes of Tonto and Horton Creeks, and the Contractor should consider these seasonal conditions in planning the work and groundwater control measures.

We recommend that the groundwater level should be drawn down prior to excavating and should be maintained in a dewatered state until the abutments and wing walls are constructed and backfilled.

Consistent with typical practice, the Contractor will be responsible for control of surface water and groundwater during construction, including the design of dewatering and diversion features. In this regard, slope protection, ditching, sumps, dewatering wells, diversion, and other measures should be employed as necessary, to direct water away from excavations, to prevent ponding of water next to the work zone, and to permit completion of the work.

8 DOCUMENT REVIEW AND FINAL DESIGN

We recommend that we be retained to review the geotechnical aspects of the plans and specifications before construction begins to determine that they are in accordance with our recommendations. Although this phase of a project is often neglected, it has been our experience that substantial savings in future cost overruns during construction can be avoided by a geotechnical review of these documents prior to construction.

9 LIMITATIONS

This report was prepared for the exclusive use of Jacobs and Central Federal Lands Highway Division for the purpose of providing geotechnical engineering recommendations for the Project. This report should not be used without our approval if any of the following occurs:

Assumptions stated in this report have changed.

Project details change or new information becomes available such that our analysis and recommendations may be affected.

A substantial period of time has passed since the date of this report.

If any of these occur, we should be retained to review the applicability of our analyses and recommendations.

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Within the limitations of scope, schedule and budget, the analyses, conclusions and recommendations presented in this report were prepared in accordance with generally accepted professional geotechnical and geological principles and practice in this area at the time this report was prepared. We make no other warranty, either express or implied.

Shannon & Wilson has prepared the attached document, "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our reports.

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10 REFERENCES

American Concrete Institute (ACI), 2022, Building code requirements for structural concrete and commentary, Farmington Hills, Mich., ACI 318-19.

AASHTO, 2020, AASHTO LRFD bridge design specifications, customary U.S. units, 9th edition: Washington, D.C., AASHTO.

Brown, E.T., ed., 1981, Rock characterization, testing & monitoring: ISRM suggested methods: Oxford, Pergamon Press, 211 p.

Brown, D.A., Turner, J.P., Castelli, R.J., and Loehr, J.E., 2018, Drilled Shafts: Construction Procedures and Design Methods, NHI Course No. 132014, Federal Highway Administration Geotechnical Engineering Circular No. 10, Washington, D.C., Report No. FHWA-NHI-18-024.

Ensoft, Inc., 2022, User's Manual for LPILE 2022, a program to analyze deep foundations under lateral loading, Austin, Texas.

Jacobs, 2025, U.S. Department of Transportation Federal Highway Administration, Plans for Proposed AZ FS 289(1) AZ NFSR 289 Bridge Replacement, Tonto Creek Bridge Replacement, 100% Plan Set, July.

Occupational Safety & Health Administration (OSHA), 1999, 29 CFR 1926, Subpart P.

Roberge, P.R., 2012, Handbook of Corrosion Engineering, McGraw-Hill, P. 103.

Pearthree, P.A., compiler, 1998, Fault number 1020, Cottonwood Basin fault, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 4/16/2025.

U.S. Geological Survey (USGS), Quaternary fault and fold database for the United States, accessed April 16, 2025 from USGS web site:

https://www.usgs.gov/programs/earthquake-hazards/faults.

USGS, 2025, U.S. Seismic Design Maps Web Application, accessed March 18, 2025 from USGS web site: https://earthquake.usgs.gov/ws/designmaps/aashto-2009.html.

U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA), 2008, Federal Lands Highway project development and design manual (PDDM):

U.S. Department of Transportation and Federal Highway Administration.

Available from: http://flh.fhwa.dot.gov/resources/manuals/pddm/.

https://earthquakes.usgs.gov/hazards/qfaults

110781-002 July 2025

U.S. Department of Transportation (USDOT), Federal Highway Administration (FHWA), 2024, Federal standard specifications for construction of roads and bridges on Federal Highway projects, FP-24 English Units: U.S. Department of Transportation and Federal Highway Administration. Available from:

http://flh.fhwa.dot.gov/resources/pse/specs/.

Weisman, M. K., and Weir, G. W., 1990, Geologic Map of the Pine 7.5’ Quadrangle, Coconino and Gila Counties, Arizona, U.S. Geological Survey, Department of the Interior, Miscellaneous Field Studies Map MF-2123.

Wilson, E. D., Moore, R. T., and Peirce, W. H., 1959, Geologic Map of Gila County, Arizona, 1:375,000 scale, Arizona Bureau of Mines, University of Arizona.

http://flh.fhwa.dot.gov/resources/pse/specs/

Nominal Side Resistance qs

Nominal Base Resistance qb p-y Curve Total Unit

Weight γ11

Drained Friction Angle ϕ' RQD Strain

Factor krm

Unconfined Compressive

Strength qu

Rock Mass Modulus, EM

Top Bottom (ksf) (ksf) (%) (pcf) (deg) (%) (in./in.) (psi) (ksi)

0 7 -- -- Sand (Reese) 125 38 -- --

7 13 -- -- Sand (Reese) 125 38 -- --

13 23 5 -- Sand (Reese) 150 38 -- -- 23 26 (BOE) 20 300 Weak Rock 150 -- 50 0.0005 8,000 120 0 5 -- -- Sand (Reese) 125 38 -- --

5 13 -- -- Sand (Reese) 125 32 -- --

13 20 5 -- Sand (Reese) 150 38 -- -- 20 26 (BOE) 20 300 Weak Rock 150 -- 50 0.0005 8,000 120

NOTES:

1 Resistance factors of 0.55 and 0.5 should be used for side and base resistance, respectively, based on AASHTO (2020).

2 For Load and Resistance Factor Design, the factored shaft end bearing should be calculated by multiplying the nominal unit end bearing by the area of the shaft and the given resistance factor. Factored side resistance should be calculated by multiplying the nominal unit side friction by the surface area of the sides of the shaft and the given resistance factor. Total factored axial compressive capacity for the shaft is determined by summing the factored end bearing resistance and factored side friction resistance.

3 Side resistance is neglected in overburden. Side resistance should be ignored where disturbed by drill casing (start rock socket below casing).

4 The nominal shaft resistance parameters above are based on a single shaft and do not consider group action of closely spaced shafts (closer than 2 diameters, center to center).

5 The LPILE parameters are for a horizontal ground surface on the side of the foundation resisting lateral loading. Sloping ground surface modifications should be included as per Ensoft, Inc.'s recommendations for the LPILE program as necessary.

6 The LPILE parameters do not consider group effects. P-multipliers should be applied in accordance with AASHTO LRFD Bridge Design Specifications (2020) Table 10.7.2.4-1 and Figure 10.7.2.4-1.

7 Adjustments to the design parameters may be required to account for elevation changes due to final grading.

8 The lateral loading analysis should consider the potential loss of material from scour.

9 We recommend utilizing the Ensoft default values for initial P-Y modulus (k) in LPILE. The default values may be utilized by entering "0" in the input field in the program.

10 We recommend utilizing the Weak Rock p-y curve for Strong Bedrock, due to the brittle failure mode exhibited by the Strong Rock p-y curve.

11 Groundwater should be taken as the flood elevation and the unit weight adjusted for buoyant conditions.

Table 1 - Deep Foundation Design Parameters

Structure Location and

Representative Borings

Drilled Shaft Axial Resistance Parameters1,2,3,4

Approximate Depth (feet) Generalized Soil/Rock Description pcf = pounds per cubic foot; deg = degrees; psi = pounds per square inch; pci = pounds per cubic inch; ksf = kips per square foot

LPILE Parameters5,6,7,8,9,10

Tonto Bridge

North Abutment (based on SW-B-02)

South Abutment (based on SW-B-01)

Clayey GRAVEL with SAND, COBBLES, and BOULDERS

Poorly graded SAND with SILT, COBBLES and BOULDERS

Silty SAND Clayey GRAVEL with SAND, COBBLES, and BOULDERS

Soil-like Bedrock Strong Bedrock

Soil-like Bedrock Strong Bedrock

110781-002 Page 1 of 1 110781_T1_Deep Foundation.xlsx - 6/6/2025

SHANNON & WILSON, INC.

Geotechnical and Environmental Consultants

Flagstaff

Phoenix

Tucson

Payson17

0 1 2

Miles

AZ NFSR 289 Bridge Replacement Gila County, Arizona

VICINITY MAP

FIG. 1

110781-002

N

Project Location

Project Location

Esri, NASA, NGA, USGS, Esri, TomTom, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, Bureau of Land Management, EPA, NPS, USDA, USFWS

12 miles to Payson

July 2025

SHANNON & WILSON, INC.

Geotechnical and Environmental Consultants

0 300 600

Approximate Scale In Feet

SITE AND EXPLORATION PLAN

FIG. 2

Horton Creek

To nt o C re ek