Attachment_7_Gila_Box_Geotechnical_Evaluation.pdf

Geotechnical Evaluation

Gila Box Riparian National Conservation

Area Recreation Enhancements

Graham & Greenlee Counties, Arizona

GLHN Architects & Engineers, Inc.

2939 East Broadway Boulevard | Tucson, Arizona 85716

January 20, 2023 | Project No. 607468001

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witness

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS

1991 East Ajo Way, Suite 145 | Tucson, Arizona 85713 | p. 520.577.7600 | www.ninyoandmoore.com

Geotechnical Evaluation

Gila Box National Riparian Conservation Area

Recreation Enhancements

Graham & Greenlee Counties, Arizona

Ms. Donna Mertes, PE, LEED AP

GLHN Architects & Engineers, Inc.

2939 East Broadway Boulevard | Tucson, Arizona 85716

January 20, 2023 | Project No. 607468001

MJK/SDN/FFN/jom

Marek J. Kasztalski Principal Engineer

Steven D. Nowaczyk, PE Managing Principal Engineer http://www.ninyoandmoore.com/

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 i

CONTENTS

1 INTRODUCTION 1

2 SCOPE OF SERVICES 1

3 SITE DESCRIPTION 1

4 TOPOGRAPHIC MAP AND AERIAL PHOTOGRAPH REVIEW 2

5 PROJECT DESCRIPTION 3

6 FIELD EXPLORATION AND LABORATORY TESTING 3

7 GEOLOGY AND SUBSURFACE CONDITIONS 4

7.1 Subsurface Conditions 5

7.1.1 Alluvium/Residuum 5

7.1.2 Formational Material (Conglomerate) 6

7.2 Groundwater 6

8 GEOLOGIC HAZARDS 6

8.1 Land Subsidence and Earth Fissures 6

8.2 Faulting and Seismicity 7

9 CONCLUSIONS 8

10 RECOMMENDATIONS 8

10.1 Earthwork 9

10.1.1 Site Preparation 9

10.1.2 Excavations 9

10.1.3 Fill Materials and Reuse of On-site Soils 10

10.1.4 Subgrade Preparation 10

10.1.5 New Embankment Fill 11

10.1.6 Fill Placement and Compaction 12

10.1.7 Trench Backfill 12

10.1.8 Permanent Slopes 13

10.2 Seismic Design Considerations 13

10.3 Foundations 14

10.4 Slab-On-Grade 15

10.5 Aggregate Roadways 15

10.6 Corrosion 17

10.7 Concrete 18

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 ii

11 SITE DRAINAGE 18

12 CONSTRUCTION OBSERVATION AND TESTING 19

13 LIMITATIONS 19

14 REFERENCES 21

TABLES

1 – USDA Soil Units 5

2 – Compaction Recommendations 12

3 – International Building Code Seismic Design Criteria 14

4 – Aggregate Pavement Gradation 16

FIGURES

1 – Site Location

2A, 2B, and 2C – Boring Locations

APPENDICES

A – Boring Logs

B – Laboratory Testing

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 1

1 INTRODUCTION

In accordance with our proposal dated October 19, 2022, and your authorization, we have performed a geotechnical evaluation for the design and construction of proposed Gila Box

Riparian National Conservation Area (RNCA) Recreation Enhancements project in Graham and

Greenlee Counties, Arizona (Figure 1). The purpose of our evaluation was to assess the subsurface conditions at the project site in order to provide geotechnical recommendations for design and construction. This report presents the results of our evaluation and our geotechnical conclusions and recommendations regarding the proposed construction.

2 SCOPE OF SERVICES

The scope of our services for this project generally included:

• Reviewing available topographic information, soil surveys, geologic literature, and aerial photographs of the project area.

• Conducting a visual reconnaissance of the project area and marking out the boring locations.

• Notifying the Arizona 811 of the proposed boring locations prior to conducting our field work.

• Drilling, logging, and sampling eight exploratory soil borings to approximate depths ranging from 3 to 15 feet below ground surface (bgs). The boring logs are presented in Appendix A.

• Performing laboratory tests on selected samples collected from our borings to evaluate the in-situ moisture content and dry density, gradation, Atterberg limits, consolidation, and corrosivity characteristics (including pH, minimum electrical resistivity, and soluble sulfate and chloride contents). The results of the laboratory tests are included in Appendix B.

• Preparing this report presenting our findings, conclusions, and recommendations regarding the proposed design and construction.

Our scope of services did not include environmental consulting services such as hazardous waste sampling or analytical testing at the site. A detailed scope of services and estimated fee for such services can be provided upon request.

3 SITE DESCRIPTION

The Gila Box Riparian Conservation Area (GBRCA) is a national conservation area that is situated along the Gila River of Graham County and Greenlee County, approximately 15 miles northeast the City of Safford, Arizona. The nature preserve’s riparian ecosystem is located on approximately 23,000 acres of land administered by the Bureau of Land Management (BLM). At the time of our evaluation, the project’s southernmost recreational day-use sites (Figures 2A and

2B) were accessed by a two-lane asphaltic concrete (AC) paved roadway (Bonita Creek Road)

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 2 with five main low-water wash crossings and few recreation amenities. Several washes ran in a northwest to south east direction adjacent to the project area. The project’s northernmost recreational sites (Figure 2C, Owl Creek Campground) were connected by an unpaved roadway

(Black Hills Backcountry Byway) with seven campsites and multiple day-use sites. Both areas were surrounded by mostly undeveloped desert land, rolling hills with sparse desert vegetation, and were adjacent to perennial creeks with riparian vegetation and sandy beaches.

The Gila River was located within 500 feet of the project sites and the Coronado Trail (State

Highway 191) was located approximately 4 miles southeast and northeast of the project areas.

According to United States Geological Survey (USGS) stream monitoring site number

09448500, at the time of our evaluation the Gila River had a streamflow rate of approximately

800 cubic feet per second.

4 TOPOGRAPHIC MAP AND AERIAL PHOTOGRAPH REVIEW

According to the Gila Box Graham County and Guthrie Greenlee County, Arizona, 7.5-Minute

United States Geological Survey (USGS) Topographic Quadrangle Maps (2021) the ground elevations at our exploration locations varied between approximately 3,180 feet relative to mean sea level (MSL) near the southwestern portion of the site, and 3,460 feet MSL near the northeastern portion of the site. The topography of the area in the site vicinity slopes gently from northwest to southeast towards the Gila River.

Several historical aerial photographs from Historicaerials.com (Nationwide Environmental Title

Research [NETR]) and from Google Earth™ were reviewed for this project. Aerial images dated

1983 and 1984 depicted the Bonita Creek Road as an unpaved roadway and no other developments within the project area. A 1998 image showed development of recreational day use sites and campsites along Bonita Creek Road. An image dated 2005 showed improved drainage crossings and short paved sections along Bonita Creek Road. Aerials dated 2015 and later images depicted Bonita Creek Road as a paved roadway and the project sites generally similar to their current condition.

Images dated 1983 through 1997 showed the Black Hills Backcountry Byway as an unpaved roadway and the Old Safford Bridge in place. Photos from 2005 through 2019 show Owl Creek

Campground developed and the surrounding project area similar to its current condition.

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 3

5 PROJECT DESCRIPTION

We understand that the Bureau of Land Management (BLM) is considering recreation enhancements for the Gila Box Riparian National Conservation Area recreational day use sites and campgrounds in Graham and Greenlee Counties, Arizona. The project will include various new improvements along approximately 3 miles of the Gila River which has been a popular destination for off highway vehicle users, hikers and rafters. The proposed areas for enhancement are approximately 12 acres along the Gila River that will in general include:

• Large shade structure with shallow foundations and slab on grade (near Boring B-3):

• Small Ramadas at camp sites (near Borings B-7 and B-8);

• Single vault pit toilet (near Boring B-4);

• Aggregate pavement in overnight parking lot areas (near Boring B-5);

• Aggregate paved driveways (near B-6, B-7 and B-8);

• Walking paths with improved surfacing and railings (near Boring B-1);

• Site grading for various future improvements generally to raise the existing grades on the order of 3 feet or less (near Borings B-1, B-5, B-6, B-7 and B-8);

• Drainage crossings with high-density polyethylene (HDPE) culverts (near Borings B-7 and B-8);

• Picnic tables and information kiosks (near Borings B-2 and B-6); and

• Miscellaneous minor improvements.

Engineering plans for the proposed improvements were not available for our review at the time of our study. However, based on the provided information we understand that the new structures will be supported on shallow foundations and slab on grade. We further understand that the new construction may involve new aggregate pavement areas, and that earthwork cut and fills are assumed to be less than 5 feet deep and high; respectively.

6 FIELD EXPLORATION AND LABORATORY TESTING

On December 15th and 16th, 2022, Ninyo & Moore conducted a subsurface exploration at the site in order to evaluate the subsurface conditions and to collect soil samples for laboratory testing. Our evaluation consisted of the drilling, logging, and sampling of eight exploratory borings using a CME 75 truck mounted drill rig equipped with hollow-stem augers (Figures 2A through 2C). The borings extended to an approximate depth of 3 to 15 feet bgs. Bulk and relatively undisturbed soil samples were collected at selected depth intervals in our borings.

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 4

Ninyo & Moore personnel logged the borings in general accordance with the Unified Soil

Classification System (USCS) and American Society for Testing and Materials (ASTM) test method D 2488 by observing cuttings, drive (ring) samples and excavation spoils. Collected ring samples were trimmed in the field, wrapped in plastic bags, and placed in cylindrical plastic containers to retain in-place moisture conditions. Similarly, Standard Penetration Test (SPT) and bulk samples were sealed in plastic bags to retain their approximate in-place moisture. Detailed descriptions of the soils encountered are presented on the boring and test pit log in Appendix A.

The soil samples collected from our exploratory activities were transported to the Ninyo & Moore laboratory in Tucson, Arizona for geotechnical laboratory testing. The testing included in-situ moisture content and dry density, gradation, Atterberg limits, consolidation, and corrosivity characteristics (including pH, minimum electrical resistivity, and soluble sulfate and chloride contents). The results of the in-situ moisture content and dry density testing are presented on the boring logs in Appendix A and a description of each laboratory test method and the remainder of the test results is presented in Appendix B.

7 GEOLOGY AND SUBSURFACE CONDITIONS

The project site is located in the Transition Zone of the Basin and Range physiographic province, which is typified by broad alluvial valleys separated by steep, discontinuous, subparallel mountain ranges. The mountain ranges generally trend north-south and northwest-southeast. The basin floors consist of alluvium with thickness extending to several thousands of feet.

The basins and surrounding mountains were formed approximately 18 million years ago during the mid- to late-Tertiary age. Extensional tectonics resulted in the formation of horsts

(mountains) and grabens (basins) with vertical displacement along high-angle normal faults.

Intermittent volcanic activity also occurred during this time. The surrounding basins were filled with alluvium from the erosion of the surrounding mountains as well as from deposition from rivers. Coarser-grained alluvial material was deposited at the margins of the basins near the mountains.

The surficial geology of the site consists of native alluvium deposits underlain by Pliocene to

Middle Miocene (2 to 16 Ma) formational material consisting of moderately to strongly consolidated conglomerate and sandstone deposited on basin floors during and after late

Tertiary faulting. This formation includes lesser amounts of mudstone, siltstone, limestone and gypsum. These deposits are generally light gray or tan. They commonly form high rounded hills and ridges in modern basins, and locally form prominent bluffs. Deposits of this unit are

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 5 exposed widely in the distressed basins of southeastern and central Arizona (Pearthree, Richard, Reynolds, Spencer, 2000).

Our review of the United States Department of Agriculture (USDA), Soil Conservation Service, Soil Survey of Graham and Greenlee Counties, indicates that the soils at these sites are classified as various different units described in Table 1. Loam is an agricultural soil classification that refers to a soil comprised of a mixture of clay, silt, and sand.

Table 1 – USDA Soil Units

UNIT NAME SOIL TYPES WITHIN UNIT PROFILE

Peloncillo Extremely cobbly sandy clay loam

Extremely cobbly sandy clay loam, very gravelly clay loam, cemented material

Peloncillo- Orthents -Pinaleno complex

Extremely cobbly sandy clay loam, very gravelly clay loam, cemented material

Cave gravelly sandy loam Gravelly sandy loam, cemented material

Continental- Pinaleno complex Gravelly sandy loam, gravelly clay loam, cobbly clay, gravelly clay, cemented material

7.1 Subsurface Conditions

Our knowledge of the subsurface conditions at the project site is based on our field exploration, laboratory and include, and our general understanding of the geology of the area. The following paragraphs provides a generalized description of the materials encountered. More detailed stratigraphic information is presented on the boring logs in Appendix A. The logs contain our field and laboratory test results, as well as our interpretation of conditions believed to exist between actual samples retrieved. Therefore, these logs contain both factual and interpretive information. Lines delineating subsurface strata on the logs are intended to group soils having similar engineering properties and characteristics. They should be considered approximate, as the actual transition between soil types (strata) may be gradual. A key to the soil symbols and terms used on the boring logs is provided in Appendix A.

7.1.1 Alluvium/Residuum

Native alluvial and residual soil deposits were encountered at the surface of our borings, and continued to formational material or the boring termination depths in Borings B-1, B-2, B-4, B-5, and B-6. In general, these deposits in our borings varied in relative density from loose to very dense, and consisted of silty sand, clayey sand, and sandy silt with varying percentages of gravel, as well as poorly graded gravel with silt, silty gravel and silty, clayey

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 6 gravel with varying percentages of silt and sand. Zones of cobbles, possible boulders, and carbonate cementation were encountered at various depths and locations.

7.1.2 Formational Material (Conglomerate)

Formational material consisting of Conglomerate was encountered below the alluvial and residual deposits in our borings B-3, B-7, and B-8. This formational material was encountered at depths ranging from 2.5 to 3.0 feet bgs and extended to the termination depths of approximately 4.4 to 9.3 feet bgs in our borings.

7.2 Groundwater

Groundwater was not encountered in our exploratory borings. Based on well data provided by the United State Geological Survey (USGS) monitoring location 325224109305001 which is associated with a well in Graham County, Arizona, groundwater has been historically measured at a depth on the order of 30 feet bgs. However, it should be noted that groundwater levels near the site can fluctuate due to seasonal variations, flows in the Gila River and other nearby washes, irrigation, groundwater withdrawal or injection, and other factors.

8 GEOLOGIC HAZARDS

The following section provides a discussion regarding potential geologic hazards such as land subsidence, earth fissures, faulting and seismicity.

8.1 Land Subsidence and Earth Fissures

Groundwater depletion, due to groundwater pumping, has caused land subsidence and earth fissures in numerous alluvial basins in Arizona. It has been estimated that subsidence has affected more than 3,000 square miles and has caused damage to a variety of engineered structures and agricultural land. From 1948 to 1983, excessive groundwater withdrawal has been documented in several alluvial valleys where groundwater levels have been reportedly lowered by up to about 500 feet. With such large depletions of groundwater, the alluvium has undergone consolidation resulting in large areas of land subsidence (Schumann and Genualdi, 1986).

In Arizona, earth fissures are generally associated with land subsidence and pose an on-going geologic hazard. Earth fissures generally form near the margins of geomorphic basins where significant amounts of groundwater depletion have occurred. Reportedly, earth fissures have also formed due to tensional stress caused by differential subsidence of the unconsolidated

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 7 alluvial materials over buried bedrock ridges and irregular bedrock surfaces (Schumann and

Genualdi, 1986).

Based on our field reconnaissance and review of the referenced material, there are no known earth fissures at the surface of the subject site. Based on fissure maps published by the Arizona

Geological Survey (AZGS, 2014), the closest reported unconfirmed earth fissures to the site are located approximately 35 miles to the southeast of the project area near the Town of Bowie.

However, the project sites are located within the Upper Gila Land Subsidence Feature. The

Envisat Satellite Interferometric Synthetic Aperture Radar (InSAR) data that compared satellite passes between February 24, 2017 and January 17, 2022 indicated that up to 4 centimeters (or approximately 1.6 inches) of subsidence has been measured in the project area within the last approximately 5 years. Historic subsidence may have also occurred prior to ADWR’s 2017 measurements.

Continued groundwater withdrawal in the area may result in subsidence and the formation of new fissures or the extension of existing fissures. While the future occurrence of land subsidence and earth fissures cannot accurately be predicted, these phenomena are not expected to be a constraint to the construction of this project.

8.2 Faulting and Seismicity

The site lies within the Sonoran zone, which is a relatively stable tectonic region located in southwestern Arizona, southeastern California, southern Nevada, and northern Mexico

(Euge et al., 1992). This zone is characterized by sparse seismicity and few Quaternary faults.

Based on our field observations and on our review of readily available published geologic maps and literature, there are no known active faults underlying the subject site or adjacent areas.

The closest known Quaternary fault to the site is the Buena Vista Fault Zone, located approximately 4 miles southwest of the project area. The Buena Vista Fault Zone is situated along the southwestern piedmont of the Black Hills. The fault zone is a series of northeast-striking normal faults that extends to about 2.5 miles in length. The most recent movement along this fault was approximately 750,000 years ago during the Middle to Late Quaternary. The slip-rate category of this fault is less than 0.2 millimeters per year (Pearthree, 1998). Seismic parameters recommended for the design of the proposed improvements are presented in

Section 10.2.

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 8

9 CONCLUSIONS

Based on the results of our subsurface evaluation, laboratory testing, and data analysis, the proposed construction is feasible from a geotechnical standpoint, provided the recommendations of this report are incorporated into the design of the project, as appropriate.

Geotechnical considerations include the following:

• Near surface alluvial and residual soil deposits can generally be excavated or ripped using heavy-duty earthmoving or excavation equipment. However, zones of very dense, gravelly materials, and possible cobble/boulder deposits should be anticipated at depth, which may result in difficult and/or slower excavation rates. In addition, relatively shallow formational material (Conglomerate) should be anticipated at some locations and will call for more aggressive excavation techniques including blasting.

• Alluvial and residual soils of variable relative densities encountered in our borings may be compressible upon saturation.

• Imported soils and soils generated from on-site excavation activities, that exhibit a relatively low plasticity index (PI) can generally be used for engineered fill. Based on the results of our study, many of the on-site soils will be suitable for re-use as engineered fill.

• The planned improvements should be constructed on a zone of compacted engineered fill as detailed in this report

• Groundwater was not observed in our borings. Based on nearby well data, the regional groundwater table has been historically measured at depths on the order of 30 feet bgs.

However, groundwater levels near the site can fluctuate due to seasonal variations, flows in the Gila river, washes, irrigation, groundwater withdrawal or injection, and other factors. In general, groundwater is not expected to be a constraint to the design and construction of this project.

• No documented geologic hazards are present underlying or immediately adjacent to the site.

However, the site is located within the Upper Gila Land Subsidence Feature. The InSAR data that compared satellite passes between February 24, 2017 and January 17, 2022 indicated that up to 4 centimeters (or approximately 1.6 inches) of subsidence has been measured in the project area within the last approximately 5 years.

Corrosivity test results indicate that subgrade soils at the site are corrosive to ferrous metals, and the sulfate content of the soils present a negligible sulfate exposure to concrete.

10 RECOMMENDATIONS

The following sections present our geotechnical recommendations for the project design and construction. If the proposed construction is changed from that discussed in this report

Ninyo & Moore should be contacted for additional recommendations.

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 9

10.1 Earthwork

The following sections provide our earthwork recommendations for this project. In general, the earthwork specifications contained in the Maricopa Association of Governments (MAG), Standard Specifications for Public Improvements (Standard Specifications) or as amended by

Graham or Greenlee Counties are expected to apply unless specifically noted.

10.1.1 Site Preparation

Construction areas should be cleared of deleterious materials, if any are present, construction debris, vegetation, and any other material that might interfere with the performance or progress of the work. These materials should be disposed of at a legal dumpsite. Existing features that call for relocation or removal and extend below finished grade, if present, should be removed, and the resulting excavations backfilled with compacted engineered fill as discussed in this report.

10.1.2 Excavations

Our evaluation of the excavation characteristics of the on-site soils is based on the results of our exploratory borings, site observations, and experience with similar soils. Generally, excavation of the near-surface alluvial and residual materials can generally be accomplished using heavy-duty earthmoving equipment. However, zones of gravel and cobbles were encountered at a relatively shallow depth in our borings. In addition, zones of carbonate cementation and Formational material (Conglomerate) should be anticipated. As such, some excavations may call for more aggressive excavation techniques (i.e.

machinery equipped with ripping teeth, pneumatic hammering, etc.) or blasting. Specifically, we anticipate that more aggressive excavation techniques may be needed for improvements located near our borings B-1, B-3, B-7 and B-8 should the construction involve excavation extending into the formational material.

Equipment and procedures should be used that do not cause disturbance to the excavation bottoms. Excavators and backhoes with buckets having large claws to loosen the soil should be avoided when excavating the last 6 to 12 inches. Such equipment will probably disturb the excavation bases. If wet or saturated soils are encountered at the excavation bases, these soils may soften under the action of light equipment and foot traffic.

The contractor should provide safely sloped excavations or an adequately constructed and braced shoring system, in compliance with Occupational Safety and Health Administration

(OSHA) regulations, for employees working in an excavation that may expose them to the

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 10 danger of moving ground. For planning purposes and according to OSHA soil classifications, a "Type C" soil should be considered for this project for the alluvial and uncemented colluvial deposits. This corresponds to a temporary slope inclination no steeper than 1.5:1 (horizontal to vertical [H:V]). If material is stored or equipment is operated near an excavation, stronger shoring should be used to resist the extra pressure due to superimposed loads. During excavation, soil classification and excavation performance should be evaluated in the field by Ninyo & Moore in accordance with the

OSHA regulations.

10.1.3 Fill Materials and Reuse of On-site Soils

On-site and imported soils that exhibit relatively low plasticity indices and very low to low expansive potential are generally suitable for re-use as engineered fill. Relatively low plasticity indices are defined as a PI value of 15, or less, as evaluated by ASTM D 4318.

Based on laboratory test results, the shallow-depth (upper 5 feet) on-site soils are characterized by PI values ranging from 0 (non-plastic) to 6. As such, it is our opinion that many of the on-site soils will be suitable for re-use as engineered fill during construction.

The Contractor should perform additional testing prior to or during construction to better delineate the soil conditions at the site.

In addition, clay lumps, construction debris and soil/rock particles should not be larger than

4 inches in dimension. Any unsuitable material should be disposed of off-site or in non-structural areas.

Engineered fill materials in contact with ferrous metals should also have low corrosion potential (minimum resistivity more than 2,000 ohm-cm, chloride content less than 25 parts per million [ppm]). Engineered fill material in contact with concrete should have a soluble sulfate content of less than 0.1 percent.

10.1.4 Subgrade Preparation

As stated previously, the near-surface soils observed in our borings generally consisted of loose to very dense, and consisted of silty sand, clayey sand, and sandy silt with varying percentages of gravel as well as poorly graded gravel with silt, silty gravel and silty, clayey gravel with varying percentages of silt and sand in our borings. Based on our laboratory test results and experience with similar subsurface conditions, the near-surface alluvial and colluvial material may exhibit collapse potential upon saturation. Accordingly, we recommend that new shallow foundations (spread footings) be supported on a zone of engineered fill that extends 1 foot below the bottom of the new foundation. The engineered

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 11 fill should be placed as discussed in Section 10.1.6. This overexcavation zone should extend a horizontal distance from the edge of the new foundation that is equal to the depth of the overexcavation.

In addition, we recommend that any new fill embankments, slab-on-grade, and roadways be supported on 6 inches of moisture-conditioned and compacted engineered fill, as measured from the bottom of the aggregate base course or leveling layer. This can be achieved by overexcavation or in-place scarification. The fill thickness should be measured from the bottom of the aggregate base (AB) layer, where applicable. This subgrade improvement should extend laterally 1 foot beyond the slab/pavement footprint. If competent bedrock is encountered prior to the above recommended overexcavation depth, we should be notified.

After the overexcavation described above is finished and prior to the placement of engineered fill, exposed surfaces from excavations should be carefully evaluated by Ninyo

& Moore for the presence of soft, loose, or wet soils that were not removed as part of the improvement process. This evaluation should consist of probing and visual observation of the excavation bottom. Based on this evaluation, additional remediation may be needed.

This could include further scarification of the exposed surface. This additional remediation, if needed, should be addressed by the geotechnical consultant during the earthwork operations.

10.1.5 New Embankment Fill

Based on the results of our study, many of the on-site alluvial/residual soils will be suitable for use in the new embankment construction. However, soil/rock particles larger than 4 inches in dimension should not be used for the new construction and screening of the material may be needed.

The new embankment fill should be placed in lifts the thickness of which will depend on the compaction equipment used, and should be re-compacted to 95 percent or more relative compaction, as evaluated by ASTM D 698 at a moisture content generally within 2 percent of the optimum moisture.

Settlements of the new embankment fills as recommended above should be anticipated and can generally be estimated to be on the order of 1 to 1-½ percent of the fill height. Due to the generally unsaturated and granular nature of the on-site soils majority of settlement is

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 12 anticipated to occur during construction. No appreciable long-term settlements are anticipated.

Where new fill is to be placed on existing slopes, we recommend that horizontal benches angled slightly into the slope be cut into the native soils prior to placing fill. Benches should roughly parallel slope contours. These benches should extend 4 feet or more into competent material at vertical distances of approximately 4 feet. Bench recommendations may need revision during construction to account for field conditions

10.1.6 Fill Placement and Compaction

Engineered fill soils should be moisture-conditioned within the moisture range shown below in Table 2 and mechanically compacted to the percent compaction shown. Engineered fill should generally be placed in 8-inch-thick loose lifts such that each lift is firm and non-yielding under the weight of construction equipment.

Table 2 – Compaction Recommendations

Engineered Fill Description

Percent Compaction per

ASTM D698

Moisture Content

Under footings, fill embankments, roadways, and slab-on-grade

95 percent ±2 percent of optimum

Aggregate Base (AB) and aggregate surface course

100 percent ±2 percent of optimum

Trench Backfill – within 2 feet below pavements

100 percent ±2 percent of optimum

Trench Backfill – deeper than 2 feet below pavement

95 percent ±2 percent of optimum

An earthwork (shrinkage) factor of 5 to 15 percent is estimated. This shrinkage factor range represents an average of the material tested and assumes that materials excavated from the site will be placed as fill. Potential bidders should consider this in preparing estimates and should review the available data to make their own conclusions regarding excavation conditions.

10.1.7 Trench Backfill

Trench backfill should be mechanically compacted to a relative compaction as detailed in

Section 10.1.6 above. Lift thickness for backfill will be dependent upon the type of compaction equipment utilized, but should generally be placed in lifts not exceeding

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 13

8 inches in loose thickness. Special care should be exercised to avoid damaging the pipe or other structures during the compaction of the backfill. In addition, the underside

(or haunches) of the buried pipe should be supported on a well-graded, compacted bedding material. This area may need placement by hand or small-scale compaction equipment.

10.1.8 Permanent Slopes

Based on our experience with similar projects, we recommend that permanent cut slopes within native soils (alluvium and colluvium) be constructed no steeper than 2H:1V.

Permanent fill embankment slopes should be no steeper than 2. 5H:1V.

Surface erosion may occur on unprotected embankments from runoff during rain events, and from burrowing animals. This can result in features such as gullies, boils, and burrow holes. These features can ultimately reduce the embankments stability post construction, and after significant rain events. As such, erosion protection is recommended for earthen slopes. We recommend the following treatments be considered:

o Provide rock mulch consisting of ¾-inch or greater aggregate applied to the surface of the embankment.

o Apply relatively dense shallow rooted vegetation to the surface of the embankment to reduce potential for erosion.

o Implement an animal control strategy to mitigate colonization of burrowing animals.

Fill slopes should be constructed in a manner such that the degree of compaction is achieved to the finished slope face (e.g., overfilling and cutting to grade). We recommend that the overfill width be 12 inches or more; however, the overfill width may vary depending on the fill soil properties, compaction equipment used, and other factors.

10.2 Seismic Design Considerations

Design of the proposed improvements should be performed in accordance with the requirements of the governing jurisdictions and applicable building codes. Table 3 presents the seismic design parameters for the site in accordance with International Building Code (IBC) guidelines and adjusted maximum considered earthquake (MCE) spectral response acceleration parameters evaluated using the California’s Office of Statewide Health Planning and Development (OSHPD) Seismic Design Maps (web-based).

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Table 3 – International Building Code Seismic Design Criteria

Site Coefficients and Spectral Response Acceleration Parameters Values

Site Class C

Site Coefficient, Fa 1.3

Site Coefficient, Fv 1.5

Mapped Spectral Response Acceleration at 0.2-second Period, Ss 0.251 g

Mapped Spectral Response Acceleration at 1.0-second Period, S1 0.08 g

Spectral Response Acceleration at 0.2-second Period Adjusted for Site Class, SMS

0.326 g

Spectral Response Acceleration at 1.0-second Period Adjusted for Site Class, SM1

0.12 g

Design Spectral Response Acceleration at 0.2-second Period, SDS 0.217 g

Design Spectral Response Acceleration at 1.0-second Period, SD1 0.08 g

10.3 Foundations

Based upon our review of field exploration and laboratory test results, we are providing recommendations for shallow foundations. We understand that the foundation system used will be selected by others based on loading conditions, settlement tolerances of the structures, as well as associated costs and constructability.

Shallow foundations consisting of spread or continuous footings should be supported at a depth of 18 inches or more below finished grade, bearing on engineered fill in accordance with recommendations presented in Section 10.1.4. Continuous footings should have a width of 16 inches or more, and isolated column footings should have a width of 24 inches or more.

Footings may be designed using the allowable net bearing pressure of 2,500 pounds per square foot (psf) for static conditions and the footing depth of 18 inches or more below finished grade.

The allowable soil bearing pressure may be increased by one-third when considering total loads including loads of short duration such as wind or seismic forces.

Total and differential settlement of 1-inch and 1/2-inch over a horizontal distance of 40 feet, respectively, may occur. These settlement estimates are based on the estimated loading conditions, the available soil boring information, and our experience with similar soils. These settlements are contingent on the preparation of soils underlying the footings in accordance with the recommendations contained in Section 10.1.4 of this report.

Foundations bearing on engineered fill and subject to lateral loadings may be designed using an ultimate coefficient of friction of 0.40 (total frictional resistance equals the coefficient of friction

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 15 multiplied by the dead load). An ultimate passive resistance value of 360 psf per foot of depth may be used up to a value of 3,600 psf. The ultimate lateral resistance can be taken as the sum of the frictional resistance and passive resistance, provided that the passive resistance does not exceed one-half of the total allowable resistance.

The passive resistance may be increased by one-third when considering loads of short duration such as wind or seismic forces. The foundations should preferably be proportioned such that the resultant force from lateral loadings falls within its kern (i.e., middle one-third).

10.4 Slab-On-Grade

The design of the slab-on-grade is the responsibility of the structural engineer. Placement of the reinforcement in the slab is vital for satisfactory performance. The slabs should be underlain by

4 or more inches of aggregate base material in general accordance with the Standard

Specifications. We recommend that the slab-on-grade be supported on engineered fill as described in Section 10.1.4 of this report.

The slab-on-grade should either be constructed so that it “floats” independent of the foundations or be designed to be structurally connected to the foundations. Fill soils under slabs should be maintained in a moist condition until the overlying slab is constructed. Joints should be constructed at intervals designed by the structural engineer to help reduce random cracking of the slab.

10.5 Aggregate Roadways

Based on the information provided by your office, new asphalt concrete (AC) roads will not be constructed within the project site; however, a new aggregate surfacing will be installed to access Owl Creek Campground and a new aggregate paved parking lot for Overnight parking nearby the Boat Put-in.

It is assumed that the traffic loading for the roads will be primarily recreational vehicles, passenger cars and light pick-up trucks. However, daily traffic volume estimates were not available.

Aggregate surface roadway design was conducted in general accordance with the Army Corps of Engineers (Corps), Technical Manual TM-5-822-12, Design of Aggregate Surface Roads and

Airfields (September, 1990). Our design was based on the assumed Design Index 3 and the

California bearing Ratio (CBR) value of 10 or more for the subgrade soils. We have assumed that the gravel (or crushed rock) surfacing will be constructed on prepared subgrade soils, which

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 16 meet the requirement for engineered fill. We anticipate that many of the on-site soils will meet the requirements for engineered fill.

We anticipate that the existing on-site soils will provide adequate foundation support for the proposed gravel roadways provided the following recommendations in Section 10.1.4 above.

However, due to variability of site soils, some areas of the subgrade may need over-excavation and re-compaction to mitigate soft, loose, or disturbed soil conditions. Any over-excavation and replacement needed to address soft or disturbed conditions should be limited to depth of one foot unless otherwise specified by the Geotechnical Engineer.

The aggregate surface course should be of 5 inches or more in depth, constructed above the prepared subgrade. The aggregate surface course should be compacted to 100% or more of the maximum dry density within 2 percent of the optimum moisture content (ASTM D698). The aggregate surface course should have a plasticity index (PI) of between 6 and 12, and meet gradation specified in Table 4 below.

Table 4 – Aggregate Pavement Gradation

US Standard Sieve Designation Gradation Designations in Percent Passing

1 - inch 100

No. 4 40-60

No. 8 30-50

No. 30 16-35

No. 200 6-15

The design recommendations provided have been presented for construction of a relatively low-maintenance aggregate roadway. Designing for a low-maintenance roadway does not eliminate the need for a standardized program for inspection and maintenance of the roadway.

Regardless of the design, gravel roadways will display varying levels of wear and deterioration in different areas based on traffic loads and weather and site drainage conditions. Deviation from the recommended surface course gradation and plasticity may also lead to additional maintenance requirements.

Accumulation of surface water on the roadway should be minimized to reduce moisture damage. Ponding water on the roadway surface increases the potential for absorption, which may result in a loss of soil strength and increased maintenance. The surface of the roadway should be designed and maintained to provide drainage along the roadway alignment. Drainage measures should include constructing the roadway with a crowned or continuous cross-slope

Ninyo & Moore | Gila Box Riparian National Conservation Area, Graham and Greenlee Counties, Arizona | 607468001 | January 20, 2023 17 surface. In addition, the roadway should be constructed at an elevation above surrounding areas to promote positive drainage. Designing the proposed roadway grades should take into account the potential for any regrading that may be performed during long term maintenance.

We also recommend that a program be established for inspection of the pavements at six-month intervals and immediately following the first two to three periods of heavy precipitation following construction. Development of potholes and rutting of the roadway surface are typical occurrences that will call for occasional maintenance. Options for improving ruts or potholes include scraping and regrading, or placing and compacting additional aggregate which meets the requirements for the surface course material as specified above. We recommend that the pavement surface be scarified to a depth of 4 inches or more, moisture conditioned, and recompacted in areas where additional surface material is needed.

10.6 Corrosion

The corrosion potential of the on-site materials was analyzed to evaluate its potential effect on the foundations and structures. Corrosion potential was evaluated using the results of laboratory testing of soil samples obtained during our subsurface evaluation that were considered representative of soils at the subject site.

Laboratory testing consisted of pH, minimum electrical resistivity, and chloride and soluble sulfate contents. The pH and minimum electrical resistivity tests were performed in general accordance with Arizona Test 236c, while sulfate and chloride tests were performed in accordance with Arizona Test 733 and 736, respectively. The results of the corrosivity tests are presented in Appendix B.

The soil pH value of the tested samples ranged between 7.8 and 8.3, which is considered to be alkaline. The minimum electrical resistivity measured in the laboratory varied between 1,475 and 3,350 ohm-cm, which is considered to be corrosive to ferrous metals. The chloride content of the samples tested varied between 10 and 130 parts per million (ppm), which is also considered to represent a corrosive environment to ferrous metals. The soluble sulfate content of the soil samples tested ranged between approximately 0.005 and 0.021 percent by weight, which is considered to represent negligible sulfate exposure for concrete.

The results of the laboratory testing indicate that the on-site materials may be corrosive to ferrous materials. It is possible that soils with variable corrosivity characteristics may be encountered at the site and should be evaluated during construction. A corrosion specialist should be consulted for further recommendations.

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10.7 Concrete

Laboratory chemical tests performed on collected soil samples indicate sulfate content of approximately 0.005 to 0.021 percent by weight. Based on American Concrete Institute (ACI), the on-site soils should be considered to represent negligible sulfate exposure to concrete.

We recommend the use of Type II cement for construction of concrete structures at this site.

Due to potential uncertainties as to the use of reclaimed irrigation water, or topsoil that may contain higher sulfate contents, pozzolan or admixtures designed to increase sulfate resistance may be considered.

The concrete should have a water-cementitious materials ratio of no more than 0.50 by weight for normal weight aggregate concrete. The structural engineer should select the concrete design strength based on the project specific loading conditions. Higher strength concrete may be selected for increased durability and resistance to slab curling and shrinkage cracking.

We recommend that concrete cover over reinforcing steel for foundations be in accordance with the recommendations of the structural engineer. The structural engineer should be consulted for additional concrete specifications.

11 SITE DRAINAGE

Surface drainage should be provided to divert water away from the structures and off of paved surfaces. Surface water should not be permitted to drain toward the structures or to pond adjacent to footings or on flatwork or pavement areas. Positive drainage for this project is defined as a slope of 2 or more percent for a distance of 5 or more feet away from the structures. Roof gutters should be installed on structures. Downspouts should discharge to drainage systems away from structures and pavements.

Further drainage recommendations include the following:

• Roof drain downspouts should be tight-lined to an appropriate outlet such as a storm drain or the street. If tight-lining of the roof drains is not practicable, they should discharge approximately 5 feet away from the structure or onto flatwork (with flexible sealant in its joints), which slopes away from the structure. Roof drains should not be allowed to discharge onto the ground surface near the building foundations.

• If planters are constructed adjacent to the building, we recommend that these planters be waterproofed and equipped with drains tight-lined to an appropriate drainage outlet.

• We recommend that low-water-use (desert-type) landscaping be utilized on site, particularly within 5 feet of the building and hardscaped areas.

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Beneath the perimeter of any structure, utility trenches should be backfilled with either compacted non-pervious fill material (pea gravel or clean sand backfill should be avoided in these areas) or lean concrete to reduce water infiltration into the interior of the building. Special care should be taken during installation of sub-floor water and sewer lines to reduce the possibility of leaks.

12 CONSTRUCTION OBSERVATION AND TESTING

During construction operations, we recommend that Ninyo & Moore perform observation and testing services for the project. These services should be performed to evaluate exposed subgrade conditions, including the extent and depth of overexcavation, to evaluate the suitability of the on-site materials for use as fill, and to observe placement and test compaction of fill soils.

Qualified subcontractors utilizing appropriate techniques and construction materials should perform construction of the proposed improvements.

13 LIMITATIONS

The field evaluation, laboratory testing, and geotechnical analyses presented in this geotechnical report have been conducted in general accordance with current practice and the standard of care exercised by geotechnical consultants performing similar tasks in the project area. No warranty, expressed or implied, is made regarding the conclusions, recommendations, and opinions presented in this report. There is no evaluation detailed enough to reveal every subsurface condition.