9-Proposed Pedestrian Bridge Over GWB Geotechnical Report - Final 07-08-2026 - Reduced.pdf

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GEOTECHNICAL INVESTIGATION REPORT

Proposed Pedestrian Bridge Over Future George Washington

Boulevard, St. George, Utah

Prepared for:

City of St. George

City Hall, 61 South Main Street

St. George, UT, 84770

Attention: Cameron Cutler

RA Project No. 11138-25-008

July 8, 2026

352 East Riverside Drive, Suite A-2 St. George, Utah 84790 (435) 673-8586 Phone • (435) 673-8397 www.racivil.com

July 8, 2026 11138-25-008

City of St. George

City Hall, 61 South Main Street

St. George, UT, 84770

Attention: Cameron Cutler

SUBJECT: Geotechnical Investigation

Proposed Pedestrian Bridge Over Future George Washington Boulevard

St. George, Utah

Gentlemen:

Enclosed are the findings of a geotechnical Investigation conducted by Rosenberg

Associates (RA) for a proposed pedestrian bridge that will support a trail over the planned George Washington Boulevard extension Project in St. George, Utah. The pedestrian bridge will be constructed about 0.6 miles east of the new George

Washington Boulevard Bridge currently being constructed over the Virgin River. The objectives of Rosenberg Associates’ services were to: (1) evaluate the nature and engineering properties of the on-site soils and bedrock; (2) evaluate the general on-site geologic conditions; (3) identify potential geologic hazards and geotechnical construction constraints associated with the project site; and (4) provide geotechnical recommendations for design and construction of the proposed pedestrian bridge abutments. Rosenberg Associates led the geotechnical field investigations, conducted geologic studies, and prepared this geotechnical report. Gerhart Cole (GC), a subconsultant to RA, developed specific geotechnical design recommendations for the proposed project in accordance with AASHTO (2024) guidelines. IGES, a subconsultant to RA, prepared the design package for the planned mechanically stabilized earth (MSE) retaining walls.

Conclusions and opinions provided in the accompanying report are based on our review of readily available geologic documents covering the site, analysis of the data obtained from the field and laboratory investigations, and previous geotechnical experience with similar soil conditions.

RA Project No. 11138-25-008 26R-018.G B Rosenberg Associates

If you have any questions concerning the information contained in this report, please contact us at your convenience at (435) 673-8586.

Sincerely, ROSENBERG ASSOCIATES

David R. Black, P.E.

Principal Geotechnical Engineer

RA Project No. 11138-25-008 26R-018.G a Rosenberg Associates

Table of Contents

EXECUTIVE SUMMARY.………………………………………………………………………………..…………… i

1.0 INTRODUCTION

1.1 General

1.2 Project Overview

2.0 SCOPE OF WORK

3.0 GEOLOGIC CONDITIONS

3.1 Geologic Setting

3.2 Potential Geologic Hazards and Constraints

3.2.1 Seismic Activity

3.2.2 Liquefaction and Lateral Spreading

3.2.3 Flooding and Erosion

3.2.5 Unstable Over-Excavation Bottoms

3.2.6 Soil Corrosion

4.0 GENERAL SITE CONDITIONS

4.1 Surface Description

4.2 Subsurface Conditions

5.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

6.0 CLOSURE

6.1 Limitations

6.2 Closing

DRAWING NO. 1 – VICINITY MAP

DRAWING NO. 2 - SITE PLAN

APPENDIX A - FIELD INVESTIGATIONS

APPENDIX B - LABORATORY TESTING

APPENDIX C - GERHART COLE GEOTECHNICAL DESIGN RECOMMENDATIONS TM

APPENDIX D - IGES MSE RETAINING DESIGN PACKAGE

RA Project No. 11138-25-008 26R-018.G i Rosenberg Associates

EXECUTIVE SUMMARY

Rosenberg Associates (RA) has performed a geotechnical Investigations for a proposed pedestrian bridge that will support a trail over the planned George Washington

Boulevard extension Project in St. George, Utah. The proposed project site is located within undeveloped floodway and flood plain areas associated with the Virgin River. A

Vicinity Map showing the location of the proposed bridge is included as Drawing No. 1 following the body of this report. RA lead the geotechnical field investigations, conducted geologic studies, and prepared this geotechnical report. Gerhart Cole (GC), a subconsultant to Rosenberg Associates, developed specific geotechnical design recommendations for the proposed project in accordance with AASHTO (2024) guidelines. The mechanically stabilized earth (MSE) retaining wall design package was prepared by IGES, a subconsultant to RA.

The subsurface conditions encountered at the site generally consisted of about 1½ to 2 feet of clayey silt topsoils overlying unconsolidated, poorly graded, loose medium dense sand layers with varying concentrations of silt and gravel which extended down to about

18 feet. Soft to firm sandy silt deposits with intermittent lenses of sand and gravel were encountered between 18 and 24½ feet overly lying a firm silty clay layer which extended down to a depth of about 33 feet. Dense sandy gravel deposits with varying concentrations of cobbles was encountered between 33 and 40 feet where dense poorly graded sand layers with varying concentrations of gravel where encountered between

40 and 50 feet. Interbedded layers of clay and sand where encountered below 50 feet which transition to weathered mudstone bedrock at a depth of about 56 feet and extended down to the maximum depth explored (92 feet). Groundwater was encountered at a depth of 11.3 feet.

Based on the results of our investigations, it is our opinion (from a geotechnical viewpoint) that the proposed site is suitable for construction of the proposed pedestrian bridge provided that the geotechnical recommendations contained in this report are complied with. Specifically, GC prepared a Technical Memorandum (TM) with geotechnical design recommendations for the proposed bridge foundations and retaining walls at the bridge abutments. A copy of the GC TM is provided in Appendix C.

The executive summary is not intended to replace the information presented in the accompanying report. The executive summary should not be used separately from the report and is only provided as an overview to summarize conclusions and recommendations. The executive summary may omit a number of details, any one of which could be crucial to the proper interpretation and application of the report and implementation of the recommendations.

RA Project No. 11138-25-008 26R-018.G ii Rosenberg Associates

The pile foundations should be constructed using a specification that matches UDOT

Standard Specifications Section 02455 (UDOT, 2026). PDA testing should be completed by qualified geotechnical personnel for at least the initial pile driven at each abutment to confirm that pile yield stress is not exceeded as recommended in Appendix C.

Geotechnical instrumentation to track subgrade settlements are recommended during and following construction of the MSE walls and embankments for the project. The MSE retaining wall design package as prepared by IGES is provided in Appendix D. A final report of compliance, including all test results, should be prepared upon completion of the project.

We recommend that a pre-construction meeting be held to discuss the project plans and requirements. The City, Civil Engineer, Geotechnical Consultants (RA and GC), and

Contractor representatives should attend this meeting.

RA Project No. 11138-25-008 26R-018.G 1 Rosenberg Associates

1.0 INTRODUCTION

1.1 General

This report presents the results of a geotechnical Investigation conducted by Rosenberg

Associates (RA) for a proposed pedestrian bridge that will support a trail over the planned

George Washington Boulevard (GWB) extension Project in St. George, Utah. A Vicinity

Map showing the location of the proposed bridge is included as Drawing No. 1 following the body of this report. The objectives of Rosenberg Associates’ services were to: (1) evaluate the nature and engineering properties of the on-site soils and bedrock; (2) evaluate the general on-site geologic conditions; (3) identify potential geologic hazards and geotechnical construction constraints associated with the project site; and (4) provide geotechnical recommendations for design and construction of the proposed pedestrian bridge abutments.

RA led the geotechnical field investigations, conducted geologic studies, and prepared this geotechnical report. Gerhart Cole (GC), a subconsultant to Rosenberg Associates, developed specific geotechnical design recommendations for the proposed project in accordance with AASHTO (2024) guidelines. IGES, a subconsultant to RA, prepared the design package for the planned mechanically stabilized earth (MSE) retaining walls. The recommendations contained herein are subject to the limitations presented in Section 6.1.

In 2022 and 2025, RA conducted a Geotechnical Investigation for the proposed George

GWB Extension Project from near River Road to the new George Washington Bridge over the Virgin River. These prior investigations included two explorations, one test pit and one shallow boring, near the planned trail crossing location. Pertinent data from these prior investigations was referenced as part of this study.

1.2 Project Overview

St. George City plans a new, single span pedestrian bridge that will support a pedestrian trail over the planned GWB extension. The new bridge will be approximately 95 feet in length and the embankments at the bridge abutments will be supported by mechanically stabilized earth (MSE) retaining walls that will be about 16 feet in height. The proposed bridge will be located in geologically recent (Holocene), unconsolidated, alluvial deposits that may be susceptible to liquefaction and load induced settlements. Therefore, driven

H-piles are the preferred foundation type for the proposed structure.

RA Project No. 11138-25-008 26R-018.G 2 Rosenberg Associates

2.0 SCOPE OF WORK

Our scope of work included subsurface exploration, soil sampling, laboratory testing, engineering evaluation, and preparation of this report. The following tasks were included in our scope of work.

1. Readily available geological maps and geotechnical reports covering the general vicinity of the project site were referenced as part of this study. The following documents were reviewed.

a. Christenson, Gary E. and Deen, Roy D., April 1983. "Engineering Geology of the St.

George Area, Washington County, Utah," Utah Geological and Mineral Survey

Special Studies No. 58.

b. Hayden, Janice M. and Willis, Grant C., 2011, Geologic Map of the St. George 7.5’on

7.5’ Quadrangle, Washington County, Utah: Utah Geological Survey Map 251DM, scale 1:24,000.

c. Lund, W.R., Knudsen, T.R., Vice, G.S., and Shaw, L.M., 2008, “Geologic Hazards and

Adverse Construction Conditions, St. George – Hurricane Metropolitan Area, Washington County, Utah,” Utah Geological Survey Special Study 127.

d. Rosenberg Associates, “Geotechnical Investigation Report, Proposed George

Washington Boulevard Extension Project From near River Road to Proposed New

Bridge Structure, St. George, Utah.” RA Project No. 11138-25-008, dated October

21, 2025. Pertinent exploration logs and laboratory tests results are presented in

Appendix A.

e. Rosenberg Associates, “Geotechnical Report, Proposed George Washington

Boulevard Bridge Project, St. George, Utah.” RA Project No. 11138-21-003, dated

December 20, 2023.

2. A site reconnaissance was conducted to evaluate the general conditions at the site and locate the exploration locations. The explorations were located by field survey prior to conducting the subsurface investigations.

4. The subsurface soil conditions at the proposed abutment locations were explored through a combination of drilling one exploratory boring to a depth of 92 feet and conducting one Cone Penetration Test (CPT) down to a depth of 37.14 feet. A temporary piezometer was installed in the exploratory boring to monitor the groundwater depth. The locations of the explorations are shown on Drawing No. 2.

The subsurface conditions encountered during the exploration program were logged by field personnel. A description of the equipment and procedures used, as well as a log of the exploratory boring and the CPT results are presented in Appendix A.

RA Project No. 11138-25-008 26R-018.G 3 Rosenberg Associates

Representative soil samples of the subsurface soils were obtained from the boring using standard penetration (SPT) sampling equipment. After bedrock was encountered, the boring wase advanced using coring techniques. The samples were packaged, labeled, and transported to the laboratory for additional evaluation.

In 2022 and 2025, one exploratory test pit and one exploratory boring were conducted near the proposed trail crossing location as part of the referenced Geotechnical

Investigations for the proposed GWB Extension Project. The approximate locations of these two prior explorations are shown on Drawing No. 2. Logs of the two prior explorations are included in Appendix A.

5. Laboratory testing was conducted by RA and GC on representative soil and bedrock samples to evaluate the pertinent engineering properties. Tests included unit weight and moisture content determinations, unconfined compression, point load, pocket penetrometer, modified consolidation, Atterberg limits, sieve analyses, and chemical testing. The results of the laboratory tests are presented in Appendix A on the exploration logs, in the summary tables in Appendix B, and included in the GC .

Technical Memorandum (TM) provided in Appendix C. Laboratory testing associated with the two prior explorations are included in Appendix B.

6. Supplemental subsurface data was collected by GC using geophysical testing to identify seismic site class for design. The geophysical survey results are included the in the GC Technical Memorandum (TM) provided in Appendix C.

7. Results of the field explorations and laboratory testing were evaluated, and geotechnical engineering analyses were performed by GC to develop engineering design recommendations for the proposed bridge foundations and retaining walls at the bridge abutments. GC’s geotechnical design recommendations are provided in a

TM included as Appendix C.

8. IGES was retained by RA to design and detail the proposed MSE retaining walls. The

MSE retaining wall design package is presented In Appendix D.

9. This geotechnical report was prepared to present the results of the geotechnical investigations, engineering analyses, and geotechnical recommendations for design and construction of the proposed pedestrian bridge foundations and abutments.

RA Project No. 11138-25-008 26R-018.G 4 Rosenberg Associates

3.0 GEOLOGIC CONDITIONS

3.1 Geologic Setting

The subject site lies within the southern portion of the St. George Basin. The St. George

Basin is a topographic and structural depression along the border between the Basin and

Range and the Colorado Plateau physiographic provinces at the confluence of the Virgin and Santa Clara Rivers. The Virgin River is the primary drainage course for the basin. Local tributaries such as the Santa Clara River feed the Virgin River System, and along with other ephemeral streams, have cut valleys and built gently sloping floodplains and alluvial fans in the lowlands. Bedrock in the St. George area consists of Upper Permian to Lower

Jurassic sandstone, shale, mudstone, limestone, gypsum, and conglomerate. These beds are folded in the southeast into the north trending Virgin Anticline but elsewhere dip gently to the north and northeast. Upper Tertiary and Quaternary basalt flows, and alluvial deposits overlie these rocks in over 50% of the area (Christenson and Deen, 1983).

As indicated on the following Figure 1, Geologic Map, quaternary alluvial deposits consisting of interbedded layers of sands and gravels with minor silt and clay interbeds are mapped along the Virgin River and its associated floodplain. Bedrock underlying the alluvial deposits at depth generally consists of weathered, varicolored mudstone bedrock associated with the Petrified Forest Member of the Chinle Formation, locally known as

“Blue Clay.” Swelling, bentonitic mudstone and claystone are common throughout this formation which is typically about 400 to 650 feet thick (Hayden and Willis, 2011).

FIGURE 1: GEOLOGIC MAP (Hayden and Willis, 2011).

RA Project No. 11138-25-008 26R-018.G 5 Rosenberg Associates

3.2 Potential Geologic Hazards and Constraints

Based on the anticipated geologic and subsurface conditions described above, the potential geologic hazards and constraints considered for the proposed site consist of: (1) seismic activity; (2) liquefaction and lateral spreading; (3) flooding and erosion; (4) compressible native soils; (5) unstable over-excavation bottoms; and (6) soil corrosion.

Each of these potential hazards/constraints are described in greater detail in the following subsections.

3.2.1 Seismic Activity

The St. George Basin lies at the southern end of a zone of moderate earthquake activity, identified as the Intermountain Seismic Belt, that extends from southwestern

Utah to northwestern Montana. Our review of available literature did not suggest the presence of visually active faults transecting the proposed project site. However, seismic activity associated with active faults in the area has the potential to generate moderate to strong ground shaking at the site. The nearest known faults with pronounced seismic activity are the Washington fault and Hurricane fault, located approximately 5 miles and 15 miles east of the project site, respectively, and the Grand

Wash fault located approximately 11 miles west of the site (Christenson and Deen, 1983). Based on the results of the field investigations, the seismic site classification was found to be Site Class D. Site-specific seismic design recommendations are presented the GC’s TM provided in Appendix C.

3.2.2 Liquefaction and Lateral Spreading

The UGS has classified the area of the proposed site as having a very high potential for liquefaction-induced settlement that may occur during a significant seismic event.

Liquefaction is a phenomenon where a soil loses grain to grain contact due to an increase in soil pore water pressures during a dynamic event such as an earthquake.

This causes the soil to behave as a viscous fluid rather than a solid. Among other effects such as lateral spreading, liquefaction can result in densification of such deposits causing settlements of overlying layers after an earthquake as excess pore water pressures are dissipated. The primary factors affecting liquefaction potential of a soil deposit are: (1) level and duration of seismic ground motions; (2) soil type and consistency; and (3) depth to groundwater. The results of GC’s site-specific liquefaction evaluation are presented the GC’s TM provided in Appendix C.

3.2.3 Flooding and Erosion

The areas within and immediately adjacent to the Virgin River are mapped in the limits of "generalized flood prone areas" and “erosion hazard areas” (UGS; Lund and others, 2008). Prior hydrologic studies and detailed erosion protection design

RA Project No. 11138-25-008 26R-018.G 6 Rosenberg Associates recommendation have been provided separately by RA for the Roadway Extension

Project.

3.2.4 Compressible Native Soils;

Embankment fills up to 16 feet in height will increase the stress in the underlying, compressible native soils and may result in excessive settlements greater than 2 inches.

The results of GC’s site-specific settlement evaluation are presented the GC’s TM provided in Appendix C.

3.2.5 Unstable Over-Excavation Bottoms

The project alignment extends over poorly consolidated stream deposits which are generally loose and/or soft to firm in consistency and exhibit relatively low in-place densities. The engineering properties of these soils can generally be improved through a combination of over-excavation, moisture conditioning, and recompaction.

Excavation of subgrade areas may encounter very moist native soils which may pump and be unstable under construction traffic. Stabilization of these soils may be required prior to placement of embankment fill. Site-specific subgrade preparation recommendations are presented the GC’s TM provided in Appendix C.

3.2.6 Soil Corrosion

On site soils and groundwater may be corrosive to buried metal and harmful to normal strength concrete. Corrosion, cement type, and other considerations are addressed by

GC in the TM provided in Appendix C.

4.0 GENERAL SITE CONDITIONS

4.1 Surface Description

As shown in Drawings No. 1 and 2, the project site is located within undeveloped floodway and flood plain areas associated with the Virgin River. The ground surface was relatively flat and covered with moderate to heavy brush vegetation. Approximately 900 feet to the west and north from the proposed bridge location, the Virgin River flows in a northeast to southwest trend. The existing paved Virgin River South Trail is located about 65 to 125 east of the proposed bridge abutment locations.

4.2 Subsurface Conditions

The subsurface conditions encountered at the site generally consisted of about 1½ to 2 feet of clayey silt topsoils overlying unconsolidated, poorly graded, loose medium dense

RA Project No. 11138-25-008 26R-018.G 7 Rosenberg Associates sand layers with varying concentrations of silt and gravel which extended down to about

18 feet. Soft to firm sandy silt deposits with intermittent lenses of sand and gravel were encountered between 18 and 24½ feet overly lying a firm silty clay layer which extended down to a depth of about 33 feet. Dense sandy gravel deposits with varying concentrations of cobbles was encountered between 33 and 40 feet where dense poorly graded sand layers with varying concentrations of gravel where encountered between 40 and 50 feet. Interbedded layers of clay and sand where encountered below 50 feet which transition to weathered mudstone bedrock at a depth of about 56 feet and extended down to the maximum depth explored (92 feet). The mudstone bedrock was associated with the Petrified Member of the Chinle Formation, locally known as “Blue Clay”.

Groundwater was encountered at a depth of 11.3 feet. The subsurface conditions are described in more detail on the exploration logs enclosed in Appendix A.

The Cone Penetration Test (CPT) soundings and soil behavior types were relatively consisted with the results of the exploratory boring. Exploration refusal was met within dense sand and gravel deposits at a depth of 37.14 feet below the ground surface, and groundwater was estimated at a depth of about 14 feet at the CPT location. The results of the CPT testing are provided in Appendix A.

The engineering properties of the on-site soils and bedrock are summarized in Table No.

4.2 with more detailed laboratory test results included in Appendix B.

Table 4.2: Summary of Soil & Bedrock Properties

Properties Range of Results

Classification

- Sands

- Silts

- Clays

- Gravels

- Bedrock

SP/A-3, SP-SM/A-3. SM/A-4

ML/A-4, MH/A-7-5

CL-ML/A-4, CH/A-7-66

GP-GM/ A-3

Weathered Mudstone

*Rock Strength of Mudstone Bedrock

- Point Load Test

- Unconfined Compressive Strength

162 psi to 289 psi

5.8 psi to 95.2 psi

Soluble Sulfate 39 ppm to 184 ppm

Resistivity 770 Ω-cm to 4,700 Ω-cm pH 8.2 to 8.5

RA Project No. 11138-25-008 26R-018.G 8 Rosenberg Associates

5.0 ENGINEERING ANALYSIS AND RECOMMENDATIONS

Based on the results of our investigations, it is our opinion (from a geotechnical viewpoint) that the proposed site is suitable for construction of the proposed pedestrian bridge provided that the geotechnical recommendations contained in this report are complied with. Specifically, GC prepared a Technical Memorandum (TM) with geotechnical design recommendations for the proposed bridge foundations and retaining walls at the bridge abutments. A copy GC’s TM is provided in Appendix C. The pile foundations should be constructed using a specification that matches UDOT Standard Specifications Section

02455 (UDOT, 2026). PDA testing should be completed by qualified geotechnical personnel for at least the initial pile driven at each abutment to confirm that pile yield stress is not exceeded as recommended in Appendix C.

Geotechnical instrumentation to track subgrade settlements are recommended during and following construction of the MSE walls and embankments for the project. The MSE retaining wall design package as prepared by IGES is provided in Appendix D. A final report of compliance, including all test results, should be prepared upon completion of the project.

We recommend that a pre-construction meeting be held to discuss the project plans and requirements. The City, Civil Engineer, Geotechnical Consultants (RA and GC), and

Contractor representatives should attend this meeting.

6.0 CLOSURE

6.1 Limitations

Our assumptions, conclusions, recommendations, and opinions contained in this report are: 1) based on our review of the referenced documents; 2) based on the findings of the referenced field and laboratory investigation programs; 3) based on geotechnical experience with similar soil conditions; 4) based on our understanding of the proposed construction; 5) subject to confirmation of the conditions encountered during construction, and 6) based upon the assumption that sufficient observation and testing will be provided during construction. If the actual construction changes from the assumptions presented in this report or if any conditions are encountered at this site which are different from those described in this report, our firm should be immediately notified so that the recommendations presented herein can be re-evaluated for applicability to the new conditions.

N O R

TH

PROJECT

LOCATION

AutoCAD SHX Text 352 East Riverside Drive Suite A2 St. George, Utah 84790 - (435) 673-8586

VIRGIN

RIVER

26-BH-1

WALKING

PATH

PROJECT

LOCATION

LIZZIE LANE

ST JAMES LANE

CPT-1

N O

R

TH

Approximate Boring Location

Key

Approximate CPT Location

22-TP-3

Approximate Test Pit Location

25-BH-1

RA Project No. 11138-25-008 26R-018.G Rosenberg Associates

APPENDIX A

FIELD INVESTIGATIONS

APPENDIX A

FIELD INVESTIGATIONS

The subsurface soil conditions at the proposed abutment locations were explored through a combination of drilling one exploratory boring to a depth of 92 feet and conducting one Cone Penetration Test (CPT) down to a depth of 37.14 feet. Additionally, prior explorations (25-BH-1 and 22-TP-3) from prior RA Investigations for the proposed

George Washington Boulevard Extension Project were referenced as part of this study.

The locations of the explorations are shown on the enclosed Site Plan, Drawing No. 2.

Exploratory Borings

The borings were completed by AGEC (a subcontractor to RA) using a truck-mounted

CME rig equipped with standard hollow-stem augers for soil sampling. Temporary piezometer pipes were installed in the borings to allow monitoring of the groundwater levels. Logs of the subsurface conditions, as encountered in the explorations, were recorded by a field engineer. The subgrade soils were visually classified in accordance with the Unified Soil Classification System. Logs of subsurface conditions are presented in this appendix. A key to the soil symbols and terms is presented following the boring logs.

Two soil samplers, a Standard Penetration Test (SPT) sampler and a Modified California ring sampler, were used to collect soil samples from the borings at select intervals and depths. Soil samplers were driven using a 140-pound hammer lifted automatically 30-inches and released. The SPT sampler has a length of 18 inches, an O.D. of 2 inches, and an I.D. of 1⅜ inches and is driven without inner rings. The Modified California sampler has a length of 18 inches, an O.D. of 3 inches, an I.D. of 2⅜ inches and is driven with 1-inch tall inner rings. The sampler type is noted on the boring logs for each sample collected. Resistance to sampler penetration was recorded in the field as hammer blows required to drive the sampler through six-inch intervals. The blows required to drive the sampler through the six-inch intervals are recorded on the enclosed logs at the respective sample depths. The corrected (N1)60 number of blows per foot are recorded on the logs. After bedrock was encountered, the borings were advanced using core drilling techniques. Representative samples of the soil and bedrock were obtained at selected intervals and transported back to the laboratory for further evaluation and testing.

CPT Testing

Cone penetration Test (CPT) soundings were conducted by AGEC to depths ranging of

37.14 feet (refusal) using a Geoprobe 3230DT Direct Push and Rotary, track mounted rig equipped with a Nova type cone. The CPT soundings were performed to assist in evaluating liquefaction potential, lateral spread, and deep foundation design parameters. CPT results are provided herein following the boring logs.

It should be noted that the lines designating boundaries between materials on the test hole logs should be considered approximate as we anticipate that actual transitions between subsurface materials may be gradual or vary in the field. Additionally, when interpreting soil type from CPT soundings, it is important to recognize that the soil behavior type (SBT) index shown on the CPT log is based on correlation, and actual soil types may vary; the soil behavior type index is not the same as a Unified Soil

Classification System (USCS) soil type which is typically shown on test hole logs.

Exploratory Test Pit(s)

Trenching was performed with a rubber-tired backhoe. Relatively "undisturbed" ring soil samples, and disturbed bulk samples, were obtained using hand-sampling equipment.

Representative portions of the soils were packaged and transported to the laboratory for further evaluation and testing. Logs of the subsurface conditions, as encountered in the test pits, were recorded by a field engineer.

Sheet 1 of 5

2-5-7 (12)10

SILT (ML/A-4) Sandy, Firm to Stiff, Light Brown, Porous, Slightly Moist, Trace Organics, With Thin Seams of Red-Brown Clay

4-7-8 (15)10

6-6-9 (15)10

2-6-7 (13)10

2-1-2 (3)10

SAND (SP/A-3) Trace Silt, Pink, Loose, Slightly Moist, Trace Roots

SAND (SP/A-3) Trace Silt, Trace Gravel, Pink, Medium Dense, Slightly Moist

D ep th (f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

11138-25-008 St. George, Utah George Washington Parkway Pedestrian Bridge

JTT - ASH

11.3

5/21/26 DRB

2550.8

AGEC Truck Mounted, CME 55 4" Auger / Christensen Bit

Log of Boring 26-BH-1

SAND (SP-SM/A-3) Some Silt, Pink, Medium Dense, Slightly Moist

SAND (SM/A-4) Silty, With Intermittent Lenses of Gravel, Yellowish Brown, Loose, Wet

Groundwater 11.3'SAND (SP-SM/A-3) Some Silt, Trace Gravel, Pink, Medium Dense, Wet

F=4.7%

4.1

1.1

1.9

18.4

22.7

AL (LL=NP, PI=NP)

F=55.0%

AL (LL=NP, PI=NP)

F=8.8%

AL (LL=NP, PI=NP)

AL (LL=NP, PI=NP)

F=5.7%

AL (LL=NP, PI=NP)

F=45.4%

1.1 95.1

AL (LL=NP, PI=NP)

F=4.7%1.9

18.0

14.3

SAND (SP-SC/A-2-4) With Gravel, Some Clay, Pink, Medium Dense, Wet

F=5.2%

AL (LL=NP, PI=NP)

F=9.7%

Sheet 2 of 5

2-4-2 (6)13

4-16-19 (35)9

CLAY (CL-ML/A-4) Sandy, Some Gravel, Yellowish Brown, Trace Organics, Firm, Wet

GRAVEL (GP-GM) Sandy, Dense, Red Gray, Wet

D ep th (f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

11138-25-008 St. George, Utah George Washington Parkway Pedestrian Bridge

Log of Boring 26-BH-1

JTT - ASH

5/21/26 DRB

2550.8

AGEC Truck Mounted, CME 55 4" Auger / Christensen Bit11.311.3

23.9

13.0

AL (LL=26, PI=7)

F=53.5%

--A-3

--With Cobbles

AL (LL=NP, PI=NP)

F=8.3%

AL (LL=35, PI=13)

F=49.6%22.2 111.1

AL (LL=35, PI=16)

F=98.0%24.1 96.9

SAND (SM/A-4) Silty, With Intermittent Lenses of Gravel, Yellowish Brown, Loose, Wet

SAND (SC/A-6) Clayey, With Gravel, Yellowish Brown, Loose to Medium Dense, Wet

CLAY (CL) Silty, Trace Sand, Yellowish Brown, Stiff, Wet

--A-6

Sheet 3 of 5

Sand (SP-SM/A-3) Gravelly, Dense, Brown, Wet15-14-17 (31)10

14-10-18 (28)12

14-15-16 (31)11.5

17-19-27 (36)19

57 95

Clay (CH/A-7-6) Sandy, With Gravel, Red Brown, Very Stiff, Wet Sand (SP-SM) Gravelly, Trace Silt, Trace Cobbles, Red Brown, Dense, Wet

SILT (MH) Clayey With Veins of Gypsum, Reddish Brown, Trace Gravel, Very Stiff, Wet, (Residual)

D ep th (f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

11138-25-008 St. George, Utah George Washington Parkway Pedestrian Bridge

JTT - ASH

5/21/26 DRB

2550.8

AGEC Truck Mounted, CME 55 4" Auger / Christensen Bit

Log of Boring 26-BH-1

11.3

13.0

16.7

23.6

--Transitions to With Gravel

MUDSTONE BEDROCK, Weathered, Soft to Moderately Hard, Brown, Wet

AL (LL=NP, PI=NP)

F=6.6%

AL (LL=NP, PI=NP)

F=5.4%

AL (LL=50, PI=16)

AL (LL=50, PI=27)

F=57.1%

13:51

AL (LL=57, PI=31)

UCS=41 psi

42 --(MH/A-7-5) Transitions to Weathered Mudstone Bedrock, Very Stiff, Wet

Sheet 4 of 5

60 100

54 90

60 100

54 92

57 95

--Transitions to Brownish Red to Light Gray in Layers

--Transitions to Dark Brown Mottled with Gray

D ep th (f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

11138-25-008 St. George, Utah George Washington Parkway Pedestrian Bridge

JTT - ASH

5/21/26 DRB

2550.8

AGEC Truck Mounted, CME 55 4" Auger / Christensen Bit

Log of Boring 26-BH-1

MUDSTONE BEDROCK, Veins of Gypsum, Light Gray, Moderately Hard, Wet

11.3

14:04 14:14 UCS=37 psi

14:28 14:38

UCS=34 psi

UCS=32 psi

15:00 PL=162 psi 15:15

15:52 UCS=95 psi 16:05

Sheet 5 of 5

54 92

End of Boring 92.0'

D ep th (f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

11138-25-008 St. George, Utah George Washington Parkway Pedestrian Bridge

JTT - ASH

5/21/26 DRB

2550.8

AGEC Truck Mounted, CME 55 4" Auger / Christensen Bit

Log of Boring 26-BH-1

11.3

16:46

17:24

UCS=8 psi

UCS=6 psi

PL=289 psi

MUDSTONE BEDROCK, Veins of Gypsum, Light Gray, Moderately Hard, Wet

CLAYEY SILT (ML/A-4) With Sand, Dark Brown, Porous, Slightly Moist, Soft

Log of Boring 25-BH-1 D ep th

(f ee t)

Ty pe

Re co ve ry

(f ee t)

(N 1)

Gr ap hi c

Lo g

M oi st ur e

Notes

RQ

D

Sa m pl in g Re sis ta nc e

(U nc or re ct ed

N

Project:

Project Location:

Project Number:

Date(s) Drilled

Excavation Contractor

Apparent Groundwater Depth (feet)

Logged By

Excavation Equipment

Checked By

Approximate Depth (feet)

Ground Surface Elevation (feet)

Material Description The following is a summary of subsurface conditions encountered at the time of exploration. Subsurface conditions may differ at other locations and may vary at this location with the passage of time. The data contained in this log is a simplification of actual conditions. D ry

D en sit y

(p cf

AGEC - KW

20'

9/29/25 DRB

End of Boring at 20'

AGEC CME 55

4.25" ID HSA

George Washington Boulevard St. George, Utah 11138-25-008

GRAVELLY SAND (SP/A-3) Trace Silt, Red-Gray, Wet, Very Loose

6-6-11

5-14-14

3-1-2 (3)

7-7-12 (19)

5-10-15

SAND (SP-SM/A-3) Trace Silt, Red-Brown, Slightly Moist, Medium Dense(17)

(25)

(28)

9-19-25 (44)

1.4 94.5

2.5 90.1

101.3

1.7 100.9

15.4 105.8

14.2 108.2

13.0'

--(SP/A-3) With Gravel

--Moist

--Very Moist, Very Loose

--Wet

--Dense

Groundwater 13.0'

SILTY SAND (SM/A-2-4) With Clay, Light Brown, Slightly Moist, Loose

SUMMARY OF SUBSURFACE CONDITIONS

Log of Trench No. T-

Groundwater:

Project Title:

AP

PR

O

VE

D B

Y

O

N

3 10

Date Trenched:

G R

AP

H

IC

LO

G

D

EP

TH

(f t.)

D

EP

TH

(m

Logged By:

Project No.

End of Trench at Feet

D R

IV

E

BU

LK

M O

IS

TU

R E

C O

N

SI

ST

EN

C Y

Ground Surface Elevation:

FI

EL

D M

O

IS

TU

R

E

FI

EL

D D

EN

SI

TY

(p cf

9.5'

George Washington Boulevard

11-22-22

JTT/JLW

11138-21-003

11.5

22-TP-3

CLAYEY SILT (ML) Light Brown, Some Sand, Trace Gypsum, Slightly Porous

FINE SILTY SAND (SM/A-4) Light Red Brown

--Trace Clay

Coarse Sand and Gravel (GP-GM)

SILTY CLAY (CL/A-6) Red Gray, Trace Sand

End of Trench at 11.5'

3.1 85.8

Slightly Moist

Soft to Firm

Wet Medium Dense

1.5 91.6

30.5

Stiff

Loose

5.1

Very Moist

Very Loose

--Caving Sidewall Conditions

AutoCAD SHX Text contained in this log is a simplification of actual

AutoCAD SHX Text encountered at the time of exploration. Subsurface

AutoCAD SHX Text The following is a summary of subsurface conditions

AutoCAD SHX Text at this location with the passage of time. The data

AutoCAD SHX Text conditions may differ at other locations and may vary

AutoCAD SHX Text conditions.

LIMESTONEFILL

MUDSTONEGYPSUM

SAND GRAVEL

SILT

PERCENT BY WEIGHDESCRIPTIVE TERM

FIELD IDENTIFICATION TESTDESCRIPTIVE TERM

Scratched with knife with with hammer blows.

Knife scratch leaves dust trace, will withstand a few hammer blows before breaking.

difficulty, difficult to break

DESCRIPTIVE TERM

readings or by direct shear tests.

CONSISTENCY AND HARDNESS

RECOVERY

NO

BULK

CORE

OR

PITCHER

SPT

MODIFIEDSHELBY

CLAY

SOIL MOISTURE

SAMPLER TYPES

SOIL TYPE KEY

SIZE PROPORTIONS

LEGEND OF LABORAT

ROCK:

FINE GRAINED SOILS:

BLOW COUNTS 1FT (N1)60DESCRIPTIVE TERM

COARSE GRAINED SOILS:

TERMS DESCRIBING CONDITION,

KEY TO SOIL SYMBOLS AN

Hard Very Stiff Stiff Firm Soft Very Soft

Greater than 50 31 to 50 11 to 30 5 to 10 0 to 4

Very Dense Dense Medium Dense Loose Very Loose

From low to high the soil moisture is indicated by:

>30 12 to 30 5 to 12 0 to 5

-ly With Some Trace

P - Compaction Sol - Solubility T - Triaxial C - Consolidation N - Chemical Heave CH - Chemical

U - Unconfined PP - Pocket Penetrometer A - Liquid & Plastic Limits DS - Direct Shear S - Swell G - Grain readily under light hammer blows.

with knife and will crumble Friable, can be gouged deeply crushed by fingers.

Can be dug by hand and

Very Hard

Hard

Moderate Hard

Soft

Hardness is related to field identification procedures described below.

Includes gravels, cobbles, rock, caliche and bedrock materials.

according to shearing strength as indicated by penetrometer or silty clays, and (3) clayey silts. Consistency is rated

(1) inorganic and organic silts and clays (2) gravelly, sandy Major portion passing No. 200 sieve. Includes:

as determined by laboratory test.

gravelly sands. Consistency is rated according to relative density,

(1) clean gravels, (2) silty or clayey gravels and (3) silty, clayey or Major portion retained on No. 200 sieve. Includes:

distributions are generally in accordance with the Unified Soils Classification System.

Terms used in this report for describing soils according to their texture or grain size

N R

BLOW COUNTS 1FT (N1)60

Greater than 30 16 - 30 9 - 15 5 - 8 2 - 4 Less than 2

CALIFORNIA

RING

-Absence of Moisture, Dusty, Dry to Touch -Apparent Moisture but well below optimum Moisture Content -Damp, but no visible water; at or near optimum Moisture Content -Above optimum moisture content -Visible Free Water; Substantially above optimum moisture content; at or above liquid limit

Dry Slightly Moist

Moist

Very Moist Wet

Project: 2260859

AGEC

600 W. Sandy Parkway Sandy, UT 84070 Total depth: 37.14 ft, Date: 5/26/2026

Surface Elevation: 0.00 ft, Est. GWL: 14.00 ft

Cone Type: NOVA

Cone Operator: Ty Gwilliam and J. Staker

CPT: CPT-1

Location: Rosenberg Bridge

Cone resistance qt

Tip resistance (tsf) 5004003002001000

D e p th ft

4 0

Cone resistance qt

Refusal

Sleeve friction

Friction (tsf) 543210

D e p th ft

4 0

Sleeve friction

Refusal

Pore pressure u

Pressure (psi) 403020100

D e p th ft

4 0

Pore pressure u

Refusal

Friction ratio

Rf (%) 6543210

D e p th ft

4 0

Friction ratio

Refusal

Soil Behaviour Type

SBT (Robertson, 2010) 181614121086420

D e p th ft

4 0

Soil Behaviour Type

Sand & silty sand

Silty sand & sandy silt

Sand & silty sand

Silty sand & sandy silt

Sand & silty sand

Silty sand & sandy silt Silty sand & sandy silt

Sand & silty sand

Silty sand & sandy silt

Sand & silty sand

Silty sand & sandy silt Silty sand & sandy silt Silty sand & sandy silt

Sand & silty sand

Silty sand & sandy silt

Clay Sand & silty sand Clay

Sand & silty sand

Silty sand & sandy silt Sand Sand

Sand & silty sand

Sand

Sand & silty sand

Sand

CPeT-IT v.3.9.7.6 - CPTU data presentation & interpretation software - Report created on: 5/27/2026, 3:55:17 PM 0

APPENDIX B

LABORATORY TESTING

APPENDIX B

LABORATORY TESTING

Soil samples were tested in the laboratory by both RA and GC to develop the pertinent engineering properties. Atterberg Limits and sieve analyses were performed for soil classification purposes. Unit weight and moisture content determinations were performed to evaluate the in-place moisture and density conditions of the on-site soils.

Modified consolidation (collapse) tests were conducted to evaluate the potential for settlement under increased loading and moisture conditions. Unconfined compression, point load, pocket penetrometer, and other strength tests were conducted on select bedrock samples to evaluate rock properties. Chemical testing was performed to evaluate the corrosion characteristics of the subgrade soils. Test results are presented in the following summary tables, on Drawings B-1 through B-5 in this appendix, and on the exploration logs in appendix A.

Table B-26-1: Laboratory Test Results Summary

George Washington Blvd. Pedestrian Bridge

Project No.: 26-2064 6 9 13 6 7 8 9 10 11 12 13 14 15 16

T e s t

H o le

D e p th ft

L L

P L

P I

G R

A V

E L

(N o .4

S A

N D

(N o .2

-N o .4

F

IN

E S

N o .2

-i n m m

.5

-i n

.5 m m

/4

-i n m m

/8

-i n

.5 m m

N o

.4

.7 m m

N o

.1 m m

N o

.2

.8 m m

N o

.4

.4 m m

N o

.6

.2 m m

N o

.1

.1 m m

N o

.2

.0 m m

P e a k s h e a r s tr e s s

(s -s p s f)

C o n fi n in g s tr e s s

(p s f)

U n ia x ia l c o m p re s s iv e s tr e n g th d u c p s i) :a

B re a k T y p e

C o rr e c te d U n ia x ia l

C o m p re s s iv e S tr e n g th

(p s i)

D e s c ri p ti o n o f B re a k

R e s is ti v it y

(o h m

-c m p H

S o lu b le c h lo ri d e p p m

S o lu b le s u lf a te p p m

B-26-1 2.5-4.5 ft 1.1 95.1 96.2 0 95 5 100 100 100 100 100 100 100 99 75 22 4.7

B-26-1 7.5-9.5 ft 1.9 NP NP NP 2 96 2 100 100 100 100 98 96 94 84 53 10 2.4 4,700 8.5 43 39

B-26-1 11.5-13.5 ft 18.0 1 94 5 100 100 100 100 99 97 89 65 15 6.8 5.2

B-26-1 16.5-18.5 ft 14.3 23 13 10 29 61 10 100 100 95 81 71 61 53 44 24 13 9.7

B-26-1 21.5-23.5 ft 22.2 111.1 135.8 28 15 13 13 37 50 100 100 100 94 87 80 73 68 59 53 50 770 8.2 85 184

B-26-1 30-32 ft 24.1 96.9 120.3 35 19 16 0 2 98 100 100 100 100 100 100 100 100 99 99 98 1257 2520 CON, COL (x2)

B-26-1 57 ft 57 26 31

B-26-1 59 ft 40.8 Vertical Fractures

B-26-1 62.5 ft 37.3 Vertical Fractures

B-26-1 68 ft 34.0 Vertical Fractures

B-26-1 69 ft 31.8 Vertical Fractures

B-26-1 71.5 ft 162 Perpendicular

B-26-1 76.3 ft 95.2 Vertical Fractures

B-26-1 80 ft 7.5 Conical Fracture

B-26-1 82 ft 5.8 Vertical Fractures

B-26-1 87 ft 289 Perpendicular

Notes:

1) CON - One-Dimensional Consolidation Properties of Soils, COL - One-Dimensional Collapse / Swell Properties of Soils

Other Tests /

Notes

Corrosion

Unconfined Compressive

Strength of Rock

(ASTM D7012 Method C)

M o is tu re c o n te n t

Atterberg

Limits

D ry u n it w e ig h t

(p c f)

Grain-Size

Point Load Test

(ASTM D 5731)

M o is t

S a t.

u n it w e ig h t

(p c f)

Grain-Size Analysis (Percent Finer) Strength

Parameters

Consolidation Test

LOAD (psf)

C O

N S O

L

10000

Water Added:

Depth (ft):

Exploration No:

Project Title:

Moisture Content (%):

Soil Description:

Drawing No.

Project No.

Dry Density (pcf):

1000 psf

1450 South Extension - George Washington Parkway

2.5

25-BH-1 90

Silty Sand

11138-25-008

B-1

2.5

C O

N S O

L

10000

Water Added:

Depth (ft):

Exploration No:

Project Title:

Moisture Content (%):

Soil Description:

Drawing No.

Project No.

Dry Density (pcf):

1000 psf

1450 South Extension - George Washington Parkway

6.5

25-BH-1 101

Silty Sand

11138-25-008

B-2

2.6

C O

N S O

L

10000

Water Added:

Depth (ft):

Exploration No:

Project Title:

Moisture Content (%):

Soil Description:

Drawing No.

Project No.

Dry Density (pcf):

1000 psf

1450 South Extension - George Washington Parkway

9.0

25-BH-1 92

Silty Sand

11138-25-008

B-3

2.7

C O

N S O

L

10000

Water Added:

Depth (ft):

Exploration No:

Project Title:

Moisture Content (%):

Soil Description:

Drawing No.

Project No.

Dry Density (pcf):

1000 psf

George Washington Boulevard

22-TP-3 83.6

Clayey Silt

11138-21-003

B-4

3.7

C O

N S O

L

10000

Water Added:

Depth (ft):

Exploration No:

Project Title:

Moisture Content (%):

Soil Description:

Drawing No.

Project No.

Dry Density (pcf):

1000 psf

George Washington Boulevard

22-TP-3 90.4

Silty Sand

11138-21-003

B-5

2.4

APPENDIX C

GERHART COLE GEOTECHNICAL DESIGN

RECOMMENDATIONS TECHNICAL MEMORANDUM

PO Box 880 Midvale, Utah 84047

801.849.0055 gerhartcole.com

TECHNICAL MEMORANDUM

To: Dave Black, PE Rosenberg Associates 352 East Riverside Drive, Suite A2 St George, UT 84790

From: Nick Langford, PE

Reviewed by: Ryan Maw, PE, BCGE

Date: July 7, 2026

Subject: George Washington Blvd. Pedestrian Overpass

Geotechnical Design

BACKGROUND AND PURPOSE

St. George City (City) plans a new, single span pedestrian bridge that will support a trail over the proposed George Washington Boulevard (GWB) extension. The new bridge will be approximately 95 feet in length and the embankments at the bridge abutments will be supported by Mechanically Stabilized Earth (MSE) retaining walls. This proposed structure will replace an originally planned box culvert below the future roadway that was made after a utility conflict was identified. Rosenberg Associates (RA) will be leading the project delivery including the Civil and Structural design of the project. Additionally, RA will develop a project geotechnical report based on geologic, field, and laboratory studies performed by RA. RA has requested geotechnical design support from Gerhart Cole (CG) that includes providing design recommendations for the bridge foundations and retaining walls at the bridge abutments. RA also asked GC to perform supplemental laboratory testing on soil and rock samples collected by RA as well as to collect supplemental subsurface data collection using geophysical testing to identify seismic site class for design. IGES has also been retained by RA to design and detail the MSE walls. The project area is highlighted in a vicinity map in Figure 1.

EXISTING FACILITIES AND GEOTECHNICAL INFORMATION

There are no existing structures within the footprint of the project. The paved ‘Virgin River South’ trail will be realigned as part of construction of George Washington Blvd with the proposed bridge structure providing the pedestrian crossing over George Washington Blvd.

GEOLOGIC SETTING AND HAZARDS

Surficial Site Geology

Near the structure, the valley floor of the St. George Basin consists of Holocene to lower Pleistocene aged stream alluvium (Qal1) to alluvial-terrace deposits (Qat2-Qat5). These are characterized by fine-grained to boulder sized materials (Hayden and Willis, 2022).

Approximately 1,000 feet to the west and north from the proposed structure, the Virgin River flows in a northeast to southwest trend. The river bottoms are composed of well sorted clay, silt, sand, and gravel stream alluvium (Qal1). The Virgin River is an active river channel that is prone to flooding and stream meandering during heavy precipitation events.

Along the east side of the St. George basin, the St. George Fault runs in a north trend. This fault is approximately 0.85 miles east of the proposed structure. This fault is widely considered a late Cenozoic extensional normal fault, which has offset strata approximately 400 feet (Lund et al., 2008). Note that the St. George Fault is overlain by 1.4-million-year-old Middleton Lava flows which have not been offset by the fault, which is an indication that activity of this fault is unlikely (Lund et al., 2008).

Landslides

No landslide deposits are mapped near or directly adjacent to the planned structure. Hayden and Willis (2022) have mapped landslide deposits approximately 7700 feet to the west and approximately 5600 feet to the northeast.

Seismicity

The project is located within a seismically active area near the St George Fault of the Washington Fault Zone. Seismic related hazards are discussed later in this document.

Debris Flows and Flooding

Mapping completed by Hayden and Willis (2022) indicates that the structures at the site are underlain by stream alluvium fan deposits. Further investigations of the geologic hazards of the site are considered to have an elevated flood and debris-flow hazard.