GRCA_318719_North_Rim_GIR_20250811_0004.pdf

Geotechnical Investigation Report

GRCA 318719: Rehabilitate North Rim Utility Infrastructure

Grand Canyon North Rim, Arizona

Submitted to:

HDR Engineering, Inc.

419 Canyon Avenue Fort Collins, CO 80521

Submitted by:

GEI Consultants, Inc.

350 Indiana Street, Suite 750 Golden, CO 80401 303.625.9502

August 11, 2025 Project No. 2305987

Nate C. Soule, P.E., P.G.

Principal Engineer

Matt P. Olson, P.E.

Project Manager www.geiconsultants.com GEI Consultants, Inc.

350 Indiana Street, Suite 750

Golden, CO 80401

303.625.9502

August 11, 2025

Project No. 2305987

VIA EMAIL: matt.gough@hdrinc.com

Matt Gough, P.E.

HDR Engineering, Inc.

419 Canyon Avenue

Fort Collins, CO 80521

Re: Geotechnical Investigation Report

GRCA 318719: Rehabilitate North Rim Utility Infrastructure

Grand Canyon North Rim, Arizona

Mr. Gough:

Submitted herewith is the Geotechnical Engineering Investigation for the North Rim Portion of the Grand

Canyon National Park Rehabilitate North Rim Utility Infrastructure project. This study was conducted in accordance with our contract between GEI Consultants, Inc. and HDR Engineering, Inc. This report contains the results of our findings, an engineering interpretation with respect to the available project characteristics, and recommendations to aid design and construction of earth-related phases of this project.

If you have any questions regarding the contents of this report, please contact the undersigned.

Sincerely, GEI Consultants, Inc.

Rob D. Johanson, P.E.

Project Engineer

R. Charlie Magill, P.G.

Project Geologist

Matt. P. Olson, P.E.

Project Manager

Nate C. Soule, P.E., P.G.

Principal Engineer http://www.geiconsultants.com/

GRCA 318719: Rehabilitate North Rim Utility Infrastructure Grand Canyon North Rim, Arizona

GEI Consultants, Inc i

Table of Contents

1. Introduction and Background 1

2. Geologic Setting 2

2.1. Regional Geology 2

2.2. Karst Geology 2

3. Geotechnical Investigation 4

3.1. Subsurface Investigation 4

3.1.1. Geotechnical Borings 4

3.1.2. Percolation Tests 5

3.1.3. Test Pit Investigation 6

3.1.4. Geotechnical Laboratory Testing 6

3.1.5. Corrosion Potential Testing 6

3.2. Geophysical Investigation 7

4. Subsurface Conditions 8

4.1. Subsurface Materials 8

4.1.1. Existing Fill 8

4.1.2. Residuum 8

4.1.3. Kaibab Formation Bedrock 9

4.1.4. Karst Features 9

4.2. Groundwater 10

5. Geotechnical Design Recommendations 11

5.1. Foundation Recommendations 11

5.2. Lateral Earth Pressures 12

5.3. Mechanically Stabilized Earth Wall Recommendations 13

5.4. Frost Protection 14

5.5. Pavement Design 14

5.6. Seismic Site Classification 15

5.7. Surface and Subsurface Drainage 15

5.8. Corrosion 15

5.9. Pipeline Recommendations 16

5.9.1. Differential Movement 16

5.9.2. Thrust Restraint 16

5.9.3. Modulus of Soil Reaction (E’) 16

6. Construction Considerations 18

6.1. Temporary Excavations 18

6.1.1. Shored Temporary Excavations 19

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6.1.2. Permanent Slopes 19

6.2. Site Grading and Earthwork 19

6.2.1. Site Preparation 20

6.2.2. Structure and Trench Backfill 20

6.2.3. Pipe Bedding 21

6.2.4. Fill Placement and Compaction 21

6.3. Bedrock or Oversized Material Excavation 22

6.4. Unstable Subgrade Conditions 22

6.5. Trench Plugs 22

7. Limitations 24

8. References 25

List of Tables Table 3-1. Geotechnical Boring Summary 5

Table 3-2. Geotechnical Laboratory Testing 6

Table 3-3. Corrosion Laboratory Testing 7

Table 5-1. Foundation Recommendations 11

Table 5-2. Static Lateral Earth Pressures 13

Table 5-3. Additional Lateral Earth Pressures due to Seismic Loading 13

Table 5-4. Retaining Wall Material Shear Strength Parameters 14

Table 5-5. American Concrete Institute Code 318-19 Water Soluble Sulfates 16

Table 5-6. Coefficient of Friction 16

Table 5-7. Modulus of Soil Reaction (E’) 17

Table 6-1. OSHA Temporary Excavation Slopes 19

Table 6-2. ADOT Class 4 Aggregate Gradation 21

Table 6-3. Fill Placement and Compaction 21

Figures Figure 1. Site Vicinity and Figure Location Map

Figure 2. Boring/Improvement Locations

Figure 3. Boring/Improvement Locations (cont.)

Figure 4. Boring/Improvement Locations (cont.)

Figure 5. Boring/Improvement Locations (cont.)

Figure 6. Boring/Improvement Locations (cont.)

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Appendices Appendix A Standard Geotechnical Boring Log Key and Boring Logs

Appendix B Rock Core Photographs

Appendix C Test Pit Logs

Appendix D Geotechnical Laboratory Testing Results

Appendix E Laboratory Corrosion Results

Appendix F Percolation Test Results Memo

RDJ/NCS:ah B:\Working\HDR\2305987 21186 GRCA North Rim Water Supply LR\Design\Reports\North Rim GIR\GRCA 318719 North Rim GIR FINAL.docx

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1. Introduction and Background

The National Park Service (NPS) is planning several upgrades to their existing water treatment infrastructure as part of the GRCA 318719: Rehabilitate North Rim Infrastructure Project. The purpose of the project is to rehabilitate existing drinking water supply infrastructure and includes improvements and upgrades to existing treatment facilities and replacement of the North Rim Waterline via directional drilling and installation of a cased bore from the North Rim to Roaring Springs. The NPS retained HDR

Engineering, Inc. (HDR) as the lead engineer for this project. HDR retained GEI Consultants, Inc. (GEI) to perform geotechnical investigation and design services for the various infrastructure improvements.

The focus of this report is to present the findings of our geotechnical investigation at the North Rim and to provide geotechnical recommendations for design and construction of the various improvements proposed on the North Rim. The report includes the initial Phase 1 and Phase 2 investigations completed in October through November 2022, the Phase 3 investigation completed in November 2023, and the

Phase 4 investigation completed in November 2024. Geotechnical reports covering the proposed inner canyon improvements and the proposed bore are provided under separate cover.

The project site is located along Arizona State Route 67 in the North Rim of Grand Canyon National Park as shown on Figure 1 and includes an existing Wastewater Treatment Plant (WWTP), proposed new

Water Treatment Plant (WTP), and proposed pipeline alignment. Proposed upgrades/additions to the existing and proposed structures include:

1. Additions to the WWTP including a secondary treatment/UV disinfection building, headworks/EQ building, retaining walls, and ancillary structures as shown in Figure 2.

Renovations to the existing control building are not anticipated to require geotechnical recommendations and are therefore not discussed in this report.

2. Addition of a new diesel fuel tank adjacent to the existing generator building east of the WWTP as shown in Figure 3.

3. Construction of a new WTP building and two above-grade raw water tanks as shown in Figure 4.

4. Installation of several new utilities from the proposed WTP to the proposed bore location including a new 1.5-inch PVC waterline, 3.5-inch steel raw waterline, and 2-inch fiber optic line, all of which will be installed in a single open-cut trench as shown in Figure 4 through Figure 6.

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2. Geologic Setting

2.1. Regional Geology

Soils at the project site are described as residual materials developed from limestone and other carbonate rocks by Soller et al. (2009). This description is supported by the US Department of

Agriculture’s Web Soil Survey (USDA, 2022), who describes the surficial deposits as the following:

⚫ Consisting of cobbly fine sandy loam, gravely sandy loam, or clay to sandy clay loam depending on the member of the Kanabownits-Kippers-Kaiparowits complex encountered.

⚫ Soils derived from cherty limestone or calcareous sandstone parent material.

Bedrock at the project site consists of the Kaibab Formation, as mapped by Huntoon et al. (1996).

Descriptions of the Kaibab formation have changed over time, as the formation is comprised of several different interbedded rock types and several new members and distinctions within the formation have been added. The uppermost member of the Kaibab Formation is the Harrisburg Member, described by

Bassler and Reeside (1921) as 160 ft thick and consisting primarily of cream to gray limestone separated by shale, sandstone, and gypsum. The Harrisburg Member is further defined by Sorauf and Billingsley

(1991) as containing interbedded limestone, gypsum, dolomite, chert, and sandstone. Both Sorauf and

Billingsley (1991) and McKee (1938) note that in the eastern portion of the Grand Canyon and further to the east the Kaibab Formation transitions to being mostly sandstone, calcareous sandstone, or sandy limestone with red shale beds and gypsum also being noted. The lower, Fossil Mountain Member, meanwhile, contains fossiliferous limestone, white chert, and a yellow sandstone bed (Sorauf and

Billingsley, 1991). Altogether, the Kaibab formation is approximately 350 ft thick at the North Rim (Sorauf and Billingsley, 1991; Huntoon et al., 1996) and is underlain by gypsum, shale, and sandstone of the

Toroweap Formation.

2.2. Karst Geology

Carbonate rocks (limestone and dolomite) and evaporites (halite and gypsum) are known lithologies that commonly experience dissolution when in contact with water resulting in the formation of karst features. Common karst features include sinkholes, depressions, pinnacles/towers, and dissolution voids

(e.g., caves). There are numerous lithologies within the Kaibab formation that are susceptible to karstification, including limestone and gypsum. The Arizona Geologic Survey has documented numerous surficial karst features in the Kaibab Formation on the north rim of the Grand Canyon (Harris, 2002).

These include enlarged joints and shallow sinkholes formed through the dissolution of gypsum and, to a lesser extent, limestone. Furthermore, numerous enlarged joints and voids within the shallow subsurface are noted in the Kaibab formation, particularly within the Holbrook Basin located approximately 140 miles southeast of the Grand Canyon. These enlarged joints are commonly encountered when drilling and often result in the loss of circulation down the drill hole, indicating that drilling fluids are entering and following the joints rather than returning to the surface.

GEI performed a limited desktop review to screen for the presence of sinkholes associated with large karst features at the subject sites. GEI reviewed historical aerial imagery available on Google Earth and

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HistoricAerials.com dating back to 1954 but was unable to distinguish any closed topographic depressions due to the tree cover at the sites. GEI also reviewed LiDAR-based mapping of karst sinkhole features on the Kaibab presented in Jones et al. (2017). In general, the Jones et al. (2017) sinkhole map was at too coarse a resolution to completely rule out existing sinkhole features at the subject sites, as several sinkhole features were mapped in the area.

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3. Geotechnical Investigation

GEI conducted a total of four phases of geotechnical investigation at the project site. GEI conducted the first phase of investigation between October 18, 2022, and October 20, 2022, the second phase between

November 1, 2022, and November 7, 2022, the third phase between November 7, 2023, and November

13, 2023, and the fourth phase on November 4, 2024. The geotechnical investigation included site observations, geotechnical drilling, percolation testing, a test pit investigation, and a geotechnical laboratory testing program. GEI completed a total of 5 percolation tests (Phase 1), 1 test pit (Phase 1), and 32 geotechnical borings (17 borings in Phase 2 and 15 borings in Phase 3) to investigate the general subsurface conditions at the WWTP, WTP and pipeline alignment. The percolation test locations, test pit location, and boring locations are shown in Figure 2 through Figure 6 at the end of this report.

3.1. Subsurface Investigation

3.1.1. Geotechnical Borings

Geotechnical borings were advanced to depths between 10.4 feet and 41.5 feet below ground surface

(bgs). GEI subcontracted Geomechanics Southwest Inc. (GSI) to drill the borings using a CME-55 truck-mounted drill rig for Phase 2, and a CME-85 truck-mounted drill rig for Phase 3. Drilling and sampling procedures were conducted in general accordance with ASTM D1586 – Standard Test Method for

Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM D 3550 – Standard Practice for

Thick Wall, Ring-Lined, Split Barrel, and Drive Sampling of Soils, and ASTM D2113 Standard Practice for

Rock Core Drilling and Sampling of Rock for Site Investigation. Continuous-flight, hollow-stem augers were used to advance the borings through soil terminating in bedrock. During advance of the hollow-stem augers, alternating Modified California samples (2.0-inch inner diameter) and Split Spoon samples

(1.4-inch inner diameter) were obtained at intervals of 2.5 to 5 feet. The number of blows by a 140-pound hammer falling 30-inches required for 12 inches of sampler penetration (recorded in 6-inch increments) are presented on the boring logs. Blow counts with less than 6 inches of penetration are presented showing the number of blows for the resulting depth of penetration (50/2” = 50 blows to drive the sampler two inches). After auger refusal was encountered in bedrock, HQ wireline rock coring methods were used to obtain rock core samples until the required depth was achieved. Boring logs are included in Appendix A. Table 3-1 below summarizes the geotechnical borings completed and associated upgrade/improvement.

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Table 3-1. Geotechnical Boring Summary

Boring Name Depth (ft) Proposed Upgrade/Improvement

GB 1 41.5 WWTP Secondary Treatment Building/UV Disinfection Building

GB 2 40.1 Headworks/EQ Building

GB 3 41.5 WTP Foundation - Alternate Location

GB 4 41.0 WTP Foundation and Waterline

GB 5 10.4 WTP Foundation and Waterline

GB 6 15.3 Waterline

GB 7 10.8 Waterline

GB 8 15.2 Waterline

GB 9 11.5 Waterline

GB 10 16.0 Waterline

GB 11 10.4 Waterline

GB 12 11.3 Waterline

GB 13 16.4 Waterline

GB 14 23.0 Waterline and Bore Feasibility Assessment

GB 15 40.0 WWTP Secondary Treatment Building/UV Disinfection Building

GB 16 40.0 WWTP Secondary Treatment Building/UV Disinfection Building

GB 17 35.0 WWTP Secondary Treatment Building/UV Disinfection Building

GB 18 40.0 WWTP Secondary Treatment Building/UV Disinfection Building

GB 19 40.0 WTP Raw Water Tanks

GB 20 40.0 WTP Raw Water Tanks

GB 21 40.0 WTP Foundation and Waterline

GB 22 41.5 WTP Foundation and Waterline

GB 23 40.0 WTP Foundation and Waterline

GB 24 40.0 WTP Foundation and Waterline

GB 25 15.3 Waterline

GB 26 16.0 Waterline

GB 27 15.8 Waterline

GB 28 16.0 Waterline

GB 29 13.1 Waterline

Ballfield 25.0 Waterline and Potential Bore Drill Pad Foundation

Tank Farm 16.5 Waterline and Potential Bore Drill Pad Foundation

North Rim Lodge 15.3 Waterline and Potential Bore Drill Pad Foundation

3.1.2. Percolation Tests

GEI performed five percolation tests to determine the percolation rate at several potential leach field locations around the WWTP. Percolation tests were conducted using the EPA’s falling head percolation test guidelines (EPA, 1980). The location of these percolation tests is shown in Figure 2. Results of the percolation testing are included in Appendix F.

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3.1.3. Test Pit Investigation

A test pit investigation was conducted at one location (TP-1) utilizing a mid-size Cat backhoe loader with an approximately 2-foot bucket and extended to a depth of approximately 66 inches below existing grade. GEI provided full time oversight of the test pit investigation and documented encountered subsurface conditions as excavation progressed. The location of the test pit is shown in Figure 2. The resulting test pit log is presented in Appendix C.

3.1.4. Geotechnical Laboratory Testing

A geotechnical laboratory testing program was developed by GEI and conducted by Martinez Associates and Advanced Terra Testing on representative samples collected during the subsurface investigation. A laboratory summary table and graphical testing results are provided in Appendix D. Laboratory tests conducted in general accordance with associated ASTM standards are presented in Table 3-2 below.

Table 3-2. Geotechnical Laboratory Testing

Test Standard

Moisture Content ASTM D2216

Dry Density ASTM D2937

Grain Size Distribution ASTM D422/ASTM C136

#200 Wash ASTM D1140

Atterberg Limits ASTM D4318

Swell/Consolidation ASTM D4546

UCS (Soil) ASTM D2166

UCS (Rock) ASTM D7012 – Method C

3.1.5. Corrosion Potential Testing

In addition to the geotechnical laboratory testing, selected soil samples were tested for corrosion potential. Corrosion laboratory testing was performed by Project X Corrosion Engineering. Laboratory corrosion results are presented in Appendix E. Corrosion testing conducted in general accordance with associated ASTM standards are presented in Table 3-3 below.

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Table 3-3. Corrosion Laboratory Testing

Test Standard

Soil Resistivity ASTM G187 pH ASTM G51

Sulfate ASTM D4327

Redox Potential ASTM G200

Sulfide SM 4500-S2

Ammonia ASTM D4327

Nitrate ASTM D4327

Fluoride SM 2320B

Phosphate ASTM D4327

Lithium ASTM D4327

Sodium ASTM D4327

Potassium ASTM D4327

Magnesium ASTM D4327

Calcium ASTM D4327

Bicarbonate SM 2320B

3.2. Geophysical Investigation

GEI subcontracted Collier Geophysics to perform a geophysical investigation using various geophysical methods to further characterize the subsurface conditions at the project site. GEI summarized the results and interpretation of the geophysical investigation in the North Rim Geophysical Study letter provided to

HDR. Collier’s report and other supporting information is included with the summary letter.

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4. Subsurface Conditions

Subsurface conditions were assessed based on the findings of the geotechnical investigation described in

Section 3 above. Soil and rock descriptions noted on the boring logs and below are in general accordance with ASTM D 2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil

Classification System) and D 2488 – Standard Practice for Description and Identification of Soils (Visual-

Manual Procedure).

4.1. Subsurface Materials

Primary materials encountered during the subsurface investigation include fill, residuum and sandy limestone of the Kaibab Formation. Boring logs and a supplementary boring log key explaining boring log details are provided in Appendix A.

4.1.1. Existing Fill

Existing fill was encountered in proximity to the existing WWTP in borings GB 1, GB 15, GB 16, GB 17, and GB 18 at ground surface and extended to depths of 2 feet to 16.5 feet. Encountered existing fill was described as the following in accordance with the Unified Soil Classification System (USCS):

⚫ Lean Clay with Sand (CL)

⚫ Sandy Lean Clay with Gravel (CL)

⚫ Clayey Sand (SC)

⚫ Clayey Sand with Gravel (SC)

Existing fill was primarily comprised of clay and sand with varying gravel content with uncorrected SPT blow counts ranging from 1 to 17 blows per foot (bpf) indicating a relative consistency of very soft to very stiff for fine soils and a relative density of very loose to medium dense for coarse soils.

4.1.2. Residuum

Residuum generated from in situ weathering of limestone bedrock was encountered in all borings at depths between ground surface or below the existing fill (where present) and extended to depths of between 5 feet bgs and the depth of exploration. For the purposes of this investigation, GEI generally classified the weathered bedrock as soil material where auger drilling was feasible. GEI generally classified the subsurface material as bedrock below the depth where auger refusal occurred. Residuum classifications in accordance with USCS are presented on the boring logs in Appendix A.

Residuum consisted of a wide variety of soil types with varying clay, silt, sand, and gravel content.

Uncorrected SPT blow counts in the residuum ranged from 0 bpf to 50 blows for 1 inch of sampler penetration (50/1”) indicating a relative consistency of very soft to hard for fine soils and a relative density of very loose to very dense for coarse soils. Two unconfined compressive strength tests were performed on the residuum with results ranging from 10 to 76 psi as shown in Appendix D.

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Swell/collapse testing on five samples of residuum resulted in swell/collapse percent between -0.5% and

0.5% when inundated at a confining pressure of 500 psf as shown in Appendix D.

4.1.3. Kaibab Formation Bedrock

Limestone bedrock of the Kaibab Formation was encountered in 14 of the 32 borings below the residuum at depths ranging from 5 to 40 feet bgs and extended to the depth of exploration. Bedrock was generally described as soft to medium hard, fresh to highly weathered, light brownish gray to light gray, very fine to fine grained sandy limestone with thin to thick bedding, very close to wide joint spacing, with hard chert nodules throughout. Unconfined compressive strength (UCS) tests were performed on

12 samples of bedrock resulting in UCS values ranging from 1,460 to 9,273 psi as shown in Appendix D.

Low UCS values may be attributed to fractures within the rock mass and associated planes of weakness in the tested sample.

4.1.4. Karst Features

Though no shallow sinkholes or other surficial karst features were observed at the project site, Phase 2 and Phase 3 geotechnical investigation results indicated the potential presence of karstic voids associated with the Kaibab Formation. During Phase 2 of the geotechnical investigation, boring GB 4 encountered a 2-foot-thick void followed by soils with relatively low blow counts. Relatively softer materials within the residuum and bedrock were encountered in several other borings during the Phase

3 investigation at the WTP and WWTP sites as indicated by low blow counts, zones of no recovery during rock coring activities, and loss of drilling fluid circulation during rock coring.

To better understand the presence, extent, and size of potential karst features in the vicinity of the proposed WTP improvements, a geophysical ERT survey was performed as summarized in the North Rim

Geophysical Study Letter. ERT data from the geophysical investigation conducted at the WTP does not indicate the presence of large, laterally extensive void structures as indicated by relatively low electrical resistivity measurements throughout the site (approximately 10 to 8,000 Ohm-m). Were large void structures present at the site, associated high electrical resistivity readings would be expected to be about 100,000 Ohm-m which is significantly higher than shown in the ERT data. Overall, the geophysical investigation did not produce evidence for the presence of significant karst voids within approximately

100 feet of the ground surface.

GEI interprets the structure encountered in Boring GB-4 at a depth of 20 to 22 feet bgs to represent an enlarged bedrock joint where minor dissolution has occurred over time. The size of this structure is smaller and generally beyond the limits of detection with the ERT methods used. It is likely that other similar size bedrock joints exist within the subsurface at the subject sites.

GEI interprets the significantly softer residuum soils (with blow counts less than approximately 10 blows per foot) to be a result of the in-situ weathering of the bedrock through networks of fractures resulting in large, clay-filled fracture zones within the bedrock. This interpretation is consistent with observations of nearby exposures of the Kaibab Formation, where zones of more heavily jointed and thin-bedded rock are typical and presumably more prone to weathering.

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Further explanation of the methods, results, and conclusions of the geophysical investigation are presented in the North Rim Geophysical Study Letter and Collier’s report.

4.2. Groundwater

Groundwater was not encountered in any of the borings during the geotechnical investigation.

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5. Geotechnical Design Recommendations

The following sections are included primarily for the engineer performing design. If additional geotechnical design recommendations are necessary, GEI should be contacted to provide the required information.

5.1. Foundation Recommendations

Based on data presented in this report, our experience, and the proposed foundation elevations, GEI recommends the foundation schemes shown in Table 5-1 below for the various structures proposed:

Table 5-1. Foundation Recommendations

Proposed Structure

Representative Borings

Assumed Foundation Depth1

Recommendation2

Secondary Treatment/UV Disinfection Building

GB 1, GB 15,

GB 16, GB 17,

GB 18

13 feet Existing fill is present below proposed structure and should be removed. Support mat or spread footing foundation on a minimum 5-foot zone of Select Fill extending to residuum.

Headworks/EQ Building

GB 2

At Grade to 15 feet

Residuum anticipated at structure bottom. Support mat or spread footing foundation on a minimum 1-foot zone of Select Fill extending to residuum. Use CLSM in place of Select Fill if bedrock is encountered during foundation excavation. Structurally tie-in headworks footing to EQ tank.

Retaining Walls Various At Grade

Based on results from WWTP borings and visual inspection, residuum anticipated below retaining walls. Support retaining wall foundations on a minimum 1-foot zone of Select Fill extending to residuum.

Ancillary Structures

Various 10 feet to 15 feet

Based on results from WWTP borings and visual inspection, residuum anticipated below foundations. Support mat foundations on a minimum 1-foot zone of Select Fill extending to residuum.

Diesel Fuel Tanks None At Grade

Based on results from WWTP borings and visual inspection during Phase 4 of the investigation, residuum anticipated at pad bottom. Support mat foundation on a minimum 1-foot zone of Select Fill extending to residuum. Remove all organic material from base of excavation prior to fill placement.

North Rim WTP Building

GB 4, GB 5,

GB 21, GB 22,

GB 23, GB 24

At Grade Residuum anticipated at structure bottom. Support mat or spread footing foundation on a minimum 1-foot zone of Select Fill extending to residuum.

Raw Water Tanks GB 19, GB 20 At Grade Residuum anticipated at structure bottom. Support mat or ringwall footing foundation on a minimum 1-foot zone of Select Fill extending to residuum.

Notes:

1. Assumed foundation depth is as measured from existing ground surface.

2. Select Fill (see Section 6.2.2) zone dimensions are the thickness below the bottom of the foundation.

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Based on our desktop review, subsurface investigation, and the results of the geophysical investigation, we do not regard karst-related subsidence as a geologic hazard for the proposed structures. Therefore, we recommend the structures be supported on shallow foundations.

Existing fill was encountered in five borings during the subsurface investigation at the WWTP.

Foundations constructed on existing fill may experience unacceptable differential settlement and therefore such foundation footprints should be overexcavated to remove the existing fill to firm residual soil and be backfilled with Select Fill.

Additionally, the residuum encountered during the subsurface investigation was generally stiff with high sand and gravel content which is generally favorable for support of shallow foundations. However, due to the potential for boulder-size bedrock particles or bedrock pinnacle features within the residuum, there is a risk of point loading effects on the proposed structure foundations and unacceptable differential settlement. Therefore, we recommend a minimum 1-foot overexcavation and backfill with Select Fill to remove potential boulder-size bedrock particles and bedrock pinnacles from below structure foundation depths.

Shallow foundations are recommended for the proposed structures. Shallow foundations may consist of conventional spread footing or mat foundations with overexcavation depths as described in Table 5-1 above. Overexcavations should extend laterally beyond the structure footprints in all directions a distance equal to the overexcavation depth. The proposed structures may be then founded on conventional spread footing or mat foundations bearing on Select Fill material meeting the requirements outlined in Section 6.2.2. GEI recommends the following criteria be utilized in design of the spread footings or mat foundation elements:

⚫ The foundations should be designed using a maximum allowable bearing pressure of 3,000 psf

This value includes appropriate allowable-stress-design (ASD) safety factors.

⚫ The foundations should be designed to withstand a total settlement of 1 inch and differential settlements of 0.5 inches.

⚫ It is acceptable to increase the net allowable bearing capacity by one-third if the load combination utilized considers infrequent loads such as wind or earthquake. Snow loading is not considered an infrequent load condition.

⚫ Sliding friction between anticipated subgrade and foundation materials should be evaluated using a coefficient of 0.35.

⚫ The coefficient of subgrade reaction (k) is typically used for flexible foundation analysis and describes the load intensity per unit of displacement. For compacted Select Fill, a k value of 125 psi/in is recommended.

5.2. Lateral Earth Pressures

Lateral earth pressures presented in Table 5-2 and Table 5-3 below should be considered during the structural design process for foundation elements extending below grade. Lateral earth pressure values are a function of the properties and the geometry of the retained fill, soil, and/or bedrock and anticipated magnitude of lateral deflection. Groundwater was not encountered within the geotechnical

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Additional lateral earth pressure due to seismic loading was assessed using a site modified peak ground acceleration of 0.2g as determined using the ASCE 7 hazard tool with inputs consisting of the ASCE 7-22

Standard, Site Class C, and a Structure Risk Category of III.

Table 5-2. Static Lateral Earth Pressures

Backfill Material Type

Unit Weight (pcf)

Friction Angle (degrees)

Static Earth Pressure Coefficients Static Equivalent Fluid Pressure (pcf)

Active At-Rest Passive Active At-Rest Passive1

Imported ADOT Class 4 Aggregate

130 34 0.28 0.44 3.54 37 57 230

Select Fill 130 31 0.32 0.48 3.12 42 63 200

Notes:

1. Values shown are half of theoretical calculated value due to large strains required to mobilize passive earth pressure.

Table 5-3. Additional Lateral Earth Pressures due to Seismic Loading

Backfill Material Type

Point of Application of Additional Loading2

Seismic Earth Pressure Coefficients Static Equivalent Fluid Pressure (pcf)

Active At-Rest Passive Active At-Rest Passive1

Imported ADOT Class 4 Aggregate 0.6H

0.06 0.10 -0.19 8 13 -12

Select Fill 0.07 0.10 -0.18 9 14 -12

Notes:

1. Values shown are half of theoretical calculated value due to large strains required to mobilize passive earth pressure.

2. As measured from the base of the wall where H = total wall height.

For the lateral earth pressures presented above to remain applicable, backfill material placement adjacent to below-grade walls should be in accordance with procedures outlined in this report. In addition, backfilled material must be placed within a 1:1 (H:V) backfill geometry, up and away from the base of the structure. If the excavation is steeper than 1H:1V, the Select Fill values outlined above should be used. Consideration has not been given to vertical loads applied to the backfill surfaces during or after construction as a result of traffic and/or other surcharge loads.

5.3. Mechanically Stabilized Earth Wall Recommendations

A Mechanically Stabilized Earth (MSE) retaining wall is anticipated to be constructed adjacent to the proposed North Rim WWTP access road with a maximum height of 10 feet. Design of the retaining wall should consider the lateral earth pressures presented in Section 5.2 above as well as all anticipated surcharge loading including but not limited to construction stockpiles, construction equipment, and all temporary and permanent traffic loading. GEI Recommends the following criteria be used in the design of MSE walls.

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1. MSE walls should be founded on shallow foundations consisting of spread footings with design criteria including bearing capacity, settlement, sliding friction, and coefficient of subgrade reaction as outlined in Section 5.1 above. MSE wall foundation should be excavated and prepared in accordance with the recommendations outlined in Section 6.2.4 prior to construction of the retaining wall.

2. Reinforced backfill for MSE walls should consist of free-draining imported material such as an

ADOT Class 4 aggregate or equivalent. MSE walls should incorporate a minimum 1-foot zone of free-draining uniform aggregate with a nominal size of 3/4-inch immediately behind the wall face to facilitate drainage. Shear strength parameters for retaining wall materials are provided in

Table 5-4 below.

Table 5-4. Retaining Wall Material Shear Strength Parameters

Material Type Unit Weight (pcf) Friction Angle (degrees) Cohesion (psf)

Foundation (Native) 125 28 500

Reinforced Backfill (ADOT Class 4) 130 34 0

Drainage Aggregate (3/4-inch) 130 36 0

3. MSE wall reinforcement should consist of a uniaxial geogrid such as Tensar UX series, Mirafi

Miragrid, or equivalent with tensile strength as determined during retaining wall design. Geogrid should be placed between modular blocks at a vertical spacing of approximately 1 to 2 feet with the exact vertical spacing to be determined during retaining wall design. Geogrid should be oriented such that the direction of highest tensile strength is perpendicular to the wall face.

Geogrid length should be determined through retaining wall design calculations.

5.4. Frost Protection

In colder climates, certain soils can freeze and can cause heave resulting in larger than expected differential movement. Typically, clays and clean sands or gravels have negligible frost susceptibility whereas silts, silty sands, and very fine sand have high frost susceptibility. Additionally, as moisture contents increase so does the potential for frost heave. We recommend that perimeter shallow foundations and shallow foundations in non-heated enclosed areas have a minimum cover of 30 inches for frost protection per Coconino County Building Codes, Ordinances, and Design Criteria. Frost depth recommendations for buried pipelines should be provided by the pipeline designer.

5.5. Pavement Design

On-site pavement for access roads and parking areas should consist of 6 inches of full-depth asphalt concrete (without base course) or 4 inches of asphalt concrete over 8 inches of moisture conditioned and compacted Arizona DOT Class 2 aggregate base. In high traffic areas and where semi-trucks make tight turns, a minimum of 6 inches of concrete over minimum 8 inches of moisture conditioned and compacted Arizona DOT Class 2 aggregate base should be used. The pavement sections should be constructed atop a minimum 1-foot-thick zone of moisture conditioned and compacted soil consisting of reworked existing on-site soil. The resulting subgrade should be proof-rolled to verify stability and if the proof-roll reveals soft or loose soils, the subgrade should be stabilized. If performed, proof rolling should

GEI Consultants, Inc 15 utilize a fully loaded 10,000-gallon water truck or equivalent, with acceptable rutting criteria as deemed appropriate by the Owner's onsite geotechnical engineering representative. Stabilization can be accomplished by over excavating existing soil and replacing with at least 12 inches of compacted, moisture conditioned Select Fill. If the subgrade is still not stabilized, geotextile stabilization fabric meeting the requirements of an AASHTO Class 2 nonwoven stabilization fabric should be placed under the 12 inches of Select Fill.

The asphalt concrete pavement shall consist of a bituminous plant mix composed of a mixture of high-quality aggregate and bituminous material, which meets the requirements of a job-mix formula established by a qualified engineer. The asphalt concrete material used should be based on a SuperPave

Gyratory Design Revolution of 75. The base lift should be constructed using an asphalt concrete mix designation 3/4-inch (per MAG, 2021 gradation) and asphalt cement binder grade PG 58-22, with a thickness of 2.5 to 4 inches. The top lift should be constructed using an asphalt concrete mix designation

1/2-inch (per MAG, 2021 gradation) and asphalt cement binder grade PG 58-22, with a thickness of 1.5 to 3 inches. An intermediate lift of 1/2-inch asphalt concrete may be utilized if desired.

5.6. Seismic Site Classification

Based on the International Building Code and the American Society of Civil Engineers Minimum Design

Loads for Buildings and other Structures, our subsurface investigation, and our experience and knowledge of seismic conditions in the area, a Site Classification of C is recommended for seismic design.

5.7. Surface and Subsurface Drainage

Construction details should ensure that surface water does not penetrate into utility trenches and migrate beneath the structures. Also, the upper few feet of backfill adjacent to structures should consist of low permeability soils to reduce the potential for surface water to migrate into foundation wall backfill. Vegetation should not be located within a distance of 5 feet from buildings. Irrigation systems should be arranged so that water under pressure does not spray within 5 feet of foundation walls.

Current schematic design drawings show a designated area for snow piling within the vicinity of the proposed North Rim WTP and the proposed raw water tanks. Due to the potential for snow melt water to migrate beneath building foundations, we recommend this area is relocated away from the structure footprints. Alternatively, the snow piling area may be paved and proper drainage installed such that snow melt drains away from the building foundations.

5.8. Corrosion

Pipe installation trenches are anticipated to be backfilled with properly compacted granular pipe bedding material meeting the requirements outlined in Section 6.2.3. Depending on the specific pipe type selected, the corrosion potential of soils and water surrounding trench backfill should be assessed.

Water soluble sulfate testing was conducted on 12 representative samples with test results ranging from

0.0015 to 0.0081% as shown in Appendix D which would characterize the concrete risk as “Not

Applicable” as shown in Table 5-5 below.

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Table 5-5. American Concrete Institute Code 318-19 Water Soluble Sulfates

Exposure Class Water-Soluble Sulfate (SO4) in Soil, Percent by Mass Risk

S0 SO4 <0.10 Not Applicable

S1 0.10< SO4 <0.20 Moderate

S2 0.20< SO4 <2.00 Severe

S3 SO4 >2.00 Very Severe

5.9. Pipeline Recommendations

5.9.1. Differential Movement

Pipelines excavated through various soils and bedrock will be susceptible to differential movements up to 1 inch. Pipe penetrations into structures should be designed to tolerate a total of at least 1 inch of upward or downward relative movement. Existing and proposed structures are expected to function as relatively rigid structures, therefore, flexible joints and/or connections between lines and structures are recommended. Allowance for differential movement should be expected and planned for where pipelines penetrate structures.

5.9.2. Thrust Restraint

Buried, pressurized pipes experience thrust forces at various pipe configurations including tees, reducers, dead ends, valves, bends, and wyes. In order to balance internal transient pressures, thrust blocks or restrained joints are recommended for pipe restraint. For thrust block design, an allowable lateral bearing pressure of 1,500 psf is recommended. A minimum of 1 foot of cover over thrust blocks is recommended and thrust blocks should be oriented such that the passive pressure influence zones do not overlap for adjacent blocks and do not impact other subsurface utilities. If restrained joints are selected, various coefficients of friction for potential pipe materials against anticipated trench backfill material are presented in Table 5-6 below.

Table 5-6. Coefficient of Friction

Pipe Type Coefficient of Friction

Steel 0.30

PVC 0.25

5.9.3. Modulus of Soil Reaction (E’)

The modulus of soil reaction (E’) is an empirically derived coefficient which is related to the stiffness of the native soils and the nature and degree of compaction of the pipe bedding and backfill material.

Various design guidelines specific to pipe material provide recommended values for E’. For full embedment of pipe in compacted granular pipe bedding (meeting the requirements outlined in Section

6.2.3), where the trench is excavated in soils that are loose/medium stiff or better (typically with blow counts greater than 4 blows per foot), and where the trench width measured at the pipe elevation is at least two times the pipe diameter, an E’ value as shown in Table 5-7 below should be used.

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Table 5-7. Modulus of Soil Reaction (E’)

Pipe Type E’ (psi)

Steel 1,500

PVC 2,000

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6. Construction Considerations

The following sections are intended for the Engineer producing specifications and the contractor constructing the proposed project. Construction considerations include temporary excavations, site grading and earthwork, backfill materials, fill placement and compaction, bedrock or oversize material excavation, unstable subgrade conditions, trench plugs, and drilled shaft construction.

6.1. Temporary Excavations

General site safety including temporary excavations are the sole responsibility of the contractor performing construction. GEI is providing temporary excavation information strictly as an informational benefit to the project team, specifically the general contractor. An Occupational Safety and Health

Administration (OSHA) defined competent person should be identified by the contractor to be in charge of temporary excavations. In general, the contractor’s competent person should have experience or training in determining soil types, benching and shoring, and have the ability to detect potential temporary slope stability and protective system issues.

OHSA defines an excavation as a man-made cut, trench, or depression formed by the removal of earth. A trench is a specific type of narrow excavation with a geometry including a greater depth than width and a width of 15 feet or less. Trenches 5 feet deep or greater should be sloped, retained with shoring, or shielded appropriately. Shielding most commonly includes trench boxes. In general, shoring can include inclined, horizontal, or vertical systems depending on the excavation geometry and availability of retention alternatives. Sloping and benching should be in accordance with OSHA recommendations.

Benching should include a maximum 4-foot vertical face for each bench and an overall excavation geometry less than or equal to the OSHA defined slope.

A professional engineer registered in the State of Arizona should approve the contractor’s approach for trenches greater than or equal to 20 feet in depth. In addition, shoring for trench excavations should be designed by a professional engineer or be based on tabulated data prepared or approved by a registered professional engineer in accordance with OSHA 1926.652(b) and (c).

GEI has evaluated observed soil conditions likely to be penetrated by the proposed construction. Based on the OSHA determined soil Types, Table 6-1 below presents maximum recommended temporary excavation slopes to be utilized during construction.

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Table 6-1. OSHA Temporary Excavation Slopes

Backfill Material Type OSHA Classification Maximum Recommended Slope (H:V)1,2

Existing Fill Type C 1 1/2:1

Residuum Type B 1:1

Bedrock Type A 3/4:1

Notes:

1. H:V is an abbreviation for Horizontal:Vertical.

2. Valid for trench excavations less than 20 feet in depth.

During construction, heavy equipment, excavated material stockpiles, or construction material stockpiles should be kept away from excavation edges to the extent possible. It is recommended that heavy equipment or excavated material stockpiles be kept at least 10 feet from the edges of excavations.

Existing underground utilities should be fully understood and documented prior to initiating excavations including information regarding location, depth, type, and size. Finally, the contractor’s competent person should inspect trenches and excavations routinely for signs of instability including sliding, toppling, subsidence, bulging, heaving, squeezing, boiling, and/or other visual concerns.

6.1.1. Shored Temporary Excavations

The contractor may elect to use a temporary shoring system to limit excavation extents, particularly if other site infrastructure may be impacted. For excavations greater than 20 feet deep, the contractor will be required to submit a design prepared by a professional engineer registered in the state of Arizona to demonstrate that the shoring system maintains both local and global slope stability for the duration of the open excavation.

Typically trench boxes are used to support lateral loads during excavation and construction in soils that have sufficient standup time to excavate the trench, install the trench box, complete installation and remove the trench box without full contact of the soils. Tight shoring typically involves full contact of the soil with the shoring system. Typical tight shoring systems include sheet pile walls, soldier pile and lagging walls, secant pile walls, soil nail walls, and slide rail systems.

6.1.2. Permanent Slopes

GEI recommends a maximum slope of 3:1 for permanent cuts and fills. Erosion control may be required for slope stabilization. There are several options regarding erosion control including erosion control blankets, fiber rolls, reinforcement mats, and vegetation.

6.2. Site Grading and Earthwork

Appropriate site preparation, material placement and compaction, and backfill selection can reduce the risk of post-construction settlement and potential issues related to lateral earth pressures. General site grading and earthwork considerations are presented in the following sections.

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6.2.1. Site Preparation

Areas supporting backfill and pipe bedding should be properly prepared. Once the rough grade has been established in excavated areas, and prior to placement of backfill, the exposed subgrade should be carefully inspected via proofrolling, probing and testing, as determined necessary by the geotechnical engineer and as outlined in Section 6.2.4. If performed, proofrolling should utilize a fully loaded 10,000-gallon water truck or equivalent, with acceptable rutting criteria as deemed appropriate by the Owner’s onsite geotechnical engineering representative. At the time of such observation, it may be necessary to make hand auger borings and/or use a hand penetration device in the base of the foundation excavation to confirm that the soils below the base are satisfactory for foundation support. The necessary depth of penetration will be established during observation. Frozen, wet, soft, or loose soil, as well as any other undesirable material should be removed. Once suitable soil conditions are achieved, the subgrade should be scarified and compacted prior to fill placement. The suitable exposed soil materials should be scarified and moistened or dried, as necessary, to a minimum depth of 8 inches below the proposed construction. Scarified material should be compacted to the minimum specifications defined in Section

6.2.4.

6.2.2. Structure and Trench Backfill

Select Fill should be used for backfill adjacent to structures, backfill beneath spread footings and mat foundations, trench backfill, and support of structure floor slabs. Select Fill should consist of imported material or processed site-generated material having a maximum particle size of 3 inches, a maximum of

60 percent fines (passing No. 200 sieve), a minimum of 30 percent passing the No. 4 sieve, a maximum liquid limit of 40, and a maximum Plasticity Index of 20. Select Fill should not contain any frozen debris, trash, cobbles greater than 3 inches in diameter, boulders, organic material, wood or plant debris, or other deleterious materials. It is anticipated that Select Fill may be generated from on-site soil, provided it meets the criteria presented above. GEI anticipates some of the native soil will require blending and processing for use as Select Fill. If another material is considered for any of the presented applications, GEI should be contacted to review submitted particle size distribution and plasticity testing results.

ADOT Class 4 aggregate may be considered for imported structural backfill against foundation walls in order to use the lower earth pressures presented in Section 5.2. ADOT Class 4 Aggregate also meets requirements for Select Fill. For ADOT Class 4 aggregate, the maximum PI is 5 and the gradation is as shown in Table 6-2 below:

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Table 6-2. ADOT Class 4 Aggregate Gradation

Sieve Size Percentage Passing

3 inch 100

1 1/2 inch -

1 inch - 3/4 inch - 1/4 inch 35-70

#8 -

#200 0-10

6.2.3. Pipe Bedding

Piping for the proposed pipelines is anticipated to be within residuum deposits. Piping should be bedded in a clean granular material from a depth of 6 inches below the pipe to 6 inches above the pipe. To achieve the design E’ values, the contractor must adequately compact the pipe bedding, especially under the haunches of the pipe. Bedding can consist of ASTM C33 No. 67 coarse aggregate with less than 5 percent fines. It is not recommended that the on-site soil be used for pipe bedding.

6.2.4. Fill Placement and Compaction

GEI recommends fill placement occurs in maximum 8-inch loose lifts for fill beneath structures and backfill adjacent to structures. Lift thickness for pipe bedding should not exceed 6 inches. Minimum recommended compaction specifications are outlined in Table 6-3 below.

Table 6-3. Fill Placement and Compaction

Backfill Material Type Moisture and Compaction…