1-George Washington Parkway - Geotechnical Report - 10-21-2025 - Reduced.pdf

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GEOTECHNICAL INVESTIGATION REPORT

Proposed George Washington Boulevard Extension Project

From near River Road to Proposed New Bridge Structure

St. George, Utah

Prepared for:

City of St. George

175 East 200 North

St. George, Utah 84770

Attention: Jay Sandberg, City Engineer

RA Project No. 11138-25-008

October 21, 2025

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

October 21, 2025 11138-25-008

City of St. George

175 East 200 North

St. George, Utah 84770

Attention: Jay Sandberg, City Engineer

SUBJECT: Geotechnical Investigation

Proposed George Washington Boulevard (1450 South) Extension Project

From near River Road to Proposed New Bridge Structure

St. George, Utah

Gentlemen:

Enclosed are the findings of a geotechnical Investigation conducted by Rosenberg

Associates (RA) for the proposed George Washington Boulevard (1450 South) Extension

Project in St. George, Utah. The portion of the proposed new roadway alignment evaluated by RA extends approximately 1-mile westward from near River Road to near the location of the proposed new bridge structure over the Virgin River. The objectives of RA’s services were to: (1) evaluate the general geologic conditions along the proposed alignment; (2) evaluate the nature and engineering properties of the on-site soils; (3) identify potential geologic hazards and constraints to construction; and (4) provide geotechnical recommendations for construction of the proposed new roadway improvements.

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 our previous geotechnical experience with similar soil conditions. 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 25R-031.G a Rosenberg Associates

Table of Contents

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

1.0 INTRODUCTION

1.1 General

1.2 Proposed Construction

2.0 SCOPE OF WORK

3.0 GEOLOGIC AND SEISMIC CONDITIONS

3.1 Site Geologic Setting

3.2 Potential Geologic Hazards

3.2.1 Flooding and Erosion Hazards

3.2.2 Seismic Ground Shaking Hazard

3.2.3 Liquefaction-Hazard

3.2.4 Other Geologic Hazards

3.3 Potential Geologic Constraints

3.3.1 Poorly Consolidated Soil Deposits

3.3.2 Shallow-Ground-Water Susceptibility

3.3.3 Pumping Subgrade Conditions

3.3.4 Soil Corrosion

3.3.5 Trench Excavation Stability

4.0 GENERAL SITE CONDITIONS

4.1 Surface Description

4.2 Subsurface Soil Conditions

4.3 Groundwater

5.0 GENERAL ENGINEERING EVALUATION

6.0 EARTHWORK

6.1 Site Preparation and Grading

6.1.1 Roadway Improvement Subgrade Preparation

6.1.2 Box Culvert Subgrade Preparation

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6.2 Subgrade Stabilization

6.2.1 Roadway Subgrade Stabilization

6.2.2 Box Culvert Subgrade Stabilization

6.3 Embankment and Structural Fill Material

6.3.1 Roadway Embankment Fill

6.3.2 Box Culvert Structural Fill

6.4 Excavations

6.5 Permanent Cut and Fill Slopes

6.6 Embankment Settlement Monitoring

7.0 ASPHALTIC CONCRETE PAVEMENTS

7.1 Traffic Loading Information

7.2 Structural Pavement Sections

8.0 B0X CULVERTS

8.1 Box Culvert Structures

8.2 Lateral Resistance

8.3 Lateral Earth Pressures

9.0 SOIL CORROSION

10.0 MOISTURE PROTECTION AND DRAINAGE

11.0 DESIGN REVIEW

12.0 CONSTRUCTION OBSERVATIONS

13.0 CLOSURE

13.1 Limitations

13.2 Closing

DRAWING NO. 1 –SITE PLAN

APPENDIX A - FIELD AND LABORATORY INVESTIGATIONS

APPENDIX B - DCP FIELD TESTING RESULTS

APPENDIX C - GEOPHYSICAL SURVEY RESULTS

APPENDIX D – TRAFFIC LOADING DESIGN INFORMATION

APPENDIX E – GEOTECHNICAL DATA FROM OTHERS

RA Project No. 11138-25-008 25R-031.G i Rosenberg Associates

EXECUTIVE SUMMARY

Rosenberg Associates (RA) has performed a geotechnical Investigations conducted by

Rosenberg Associates (RA) for the proposed George Washington Boulevard (1450

South) Roadway Extension Project in St. George, Utah. The portion of the proposed new roadway alignment evaluated by RA extends approximately 1-mile westward from the

Boulder Creek Crossing commercial development near River Road to the location of the proposed new bridge structure over the Virgin River. A Site Plan showing the location of the project site within the St. George area is provided as Drawing No. 1 following the body of this report.

RA initiated its field and laboratory investigations along the proposed roadway alignment; the results were presented in a Geotechnical Data Report dated February 28, 2023. A geotechnical investigation for the proposed George Washington Boulevard

Bridge Project was conducted by RA with the aid of a subconsultant Gerhart Cole (GC).

In addition to providing geotechnical recommendations for design of the bridge, roadway construction recommendations were provided for an approximate 1,000-foot reach of between the proposed west bridge abutment and Crosby Way (270 East Street).

Traffic loading design information for George Washington Boulevard west of the proposed new bridge was provided by Horrocks (see Appendix D). The results of these previous studies were referenced during this investigation.

The subsurface soil conditions encountered along the proposed roadway alignment generally consisted of soft to firm, silty and clayey topsoils overlying very loose to medium dense interbedded layers of river sands and gravels. Medium dense to dense sands and gravels and interbedded stiff clays were encountered at depth. Within in the cultivated farm portion of the alignment, the topsoils consisted of porous, brown, lean clay and silty sand with roots and organics. Laboratory test results indicated the subgrade soils exhibited relatively low to moderate in-place dry densities, low to moderate potentials for hydro-collapse (rapid settlement under increased loading and moisture conditions), and are compressible. The clay interbeds exhibited low expansion potentials. Corrosivity (chemical) testing indicated that the on-site soils should be considered corrosive to buried metal and harmful to normal strength concrete.

Laboratory CBR values of the subgrade soils ranged from 5.1 to 16.3. Field DCP tests

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 25R-031.G ii Rosenberg Associates indicated in-place, unconsolidated CBR values ranging from approximately 3 to 10.

Groundwater was encountered within the explorations at depths ranging from about 7 to 13 feet below the existing site grade.

Based on the subsurface conditions encountered at the site, it is our opinion (from a geotechnical viewpoint), that the subject site will be suitable for the proposed construction project provided that the potential geologic hazards and construction constraints identified at the project site are properly addressed during design and construction, and that the recommendations contained in this report are complied with.

Specifically, it is our opinion that:

1. The near surface soils are not considered suitable for the support of the proposed roadway improvements and box culverts in their present condition and will require overexcavation and replacement with approved embankment and structural fill materials as discussed in Section 6.0.

2. 2. Within the planned roadway improvement areas, overexcavation depths on the order of 1½ feet below the aggregate base materials, or 1½ feet below the natural ground surface, whichever is greater are recommended. Excavated on-site soils free of roots, organic material, and debris are anticipated to be suitable for reuse and may be stockpiled for later use as compacted embankment fill.

3. Excavation of subgrade areas may encounter localized areas of very moist to wet native soils which may pump and be unstable under construction traffic. Stabilization of these soils will likely be required prior to placement of embankment fill as discussed in Section 6.2. Prior to placing embankment fill, the Geotechnical

Consultant should observe that the prepared overexcavation bottoms are suitable for support of embankment fill by having the Contractor proof roll the prepared surface using moderate-weight rubber-tired construction equipment over the surface.

4. At the planned box culvert locations, the exposed soils should be overexcavated to a depth of at least 2 feet below the planned bottom of box culvert elevation to allow the placement and compaction of at least 2 feet of imported granular structural fill as discussed in Section 6.3.2.

5. Unstable trench excavations and caving sidewall conditions should be anticipated.

Flowing sand conditions are anticipated within excavations where loose sands and gravels are encountered at or below the groundwater elevations as discussed in

Section 6.4.

RA Project No. 11138-25-008 25R-031.G iii Rosenberg Associates

6. Maximum embankment fill slopes should not be made steeper than 2½:1 (horizontal to vertical). Where steeper slopes are desired, retaining structures, reinforced slopes and/or additional analysis will be required.

7. Embankment fill heights greater than 4 feet may increase the stress in the underlying, compressible native soils and result in settlements greater than 1 inch. If the site grade will be raised more than 4 feet, settlement monuments and monitoring are recommended during construction to observe that at least 95% of the total anticipated settlements have occurred prior to the construction of pavements and utilities.

8. Three (3) minimum pavement section alternatives for both a 20-year design life and

25-year design life are provided for your consideration for the new roadway improvements. Alternative 1 (preferred) includes a geo-grid (Tensar NX750-FG) reinforced subgrade to reduce the overall required aggregate base thickness.

Alternative 2 consists of no geo-grid reinforcement with a full depth of Type II aggregate base. Alternative 3 consists of no geo-grid reinforcement with a combination of Type II aggregate base and Type I sub-base. The minimum recommended minimum structural pavement section alternatives for a 20-year design life and a 25-year design life are presented in Section 7.0.

This report presents geotechnical recommendations for general earthwork, asphaltic concrete pavements, box culverts, soil corrosion, moisture protection and drainage, design review, and construction observations.

RA Project No. 11138-25-008 25R-031.G 1 Rosenberg Associates

1.0 INTRODUCTION

1.1 General

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

Associates (RA) for the proposed George Washington Boulevard (1450 South) Roadway

Extension Project in St. George, Utah. The portion of the proposed new roadway alignment evaluated by RA extends approximately 1-mile westward from the Boulder

Creek Crossing commercial development near River Road to the location of the proposed new bridge structure over the Virgin River. A Site Plan showing the location of the project site within the St. George area is provided as Drawing No. 1 following the body of this report. The objectives of RA’s services were to: (1) evaluate the general geologic conditions along the proposed alignment; (2) evaluate the nature and engineering properties of the on-site soils; (3) identify potential geologic hazards and/or constraints to construction; and (4) provide the results of our geotechnical investigations in this report.

RA initiated its field and laboratory investigations along the proposed roadway alignment;

the results were presented in a Geotechnical Data Report dated February 28, 2023. A geotechnical investigation for the proposed George Washington Boulevard Bridge Project was conducted by RA with the aid of a subconsultant Gerhart Cole (GC). In addition to providing geotechnical recommendations for design of the bridge, roadway construction recommendations were provided for an approximate 1,000-foot reach of between the proposed west bridge abutment and Crosby Way (270 East Street). Traffic loading design information for George Washington Boulevard west of the proposed new bridge was provided by Horrocks (see Appendix D). The results of these previous studies were referenced during this investigation.

This report was prepared for use only by the Client, only for the purposes stated, and within a reasonable time from its issuance. The opinions and recommendations contained herein are subject to the limitations presented in Section 13.1 of this report.

1.2 Proposed Construction

It is our understanding the proposed roadway extension will be classified as a “Minor

Arterial’’ roadway with a right-of-way width of 90 feet. Maximum grading cuts and fills along most of the proposed alignment are anticipated to be on the order of 2 to 4 feet respectively; however, localized fills may be up to 8 feet near the bridge abutment and at the eastern end of the project alignment near the Boulder Creek Crossing commercial development.

RA Project No. 11138-25-008 25R-031.G 2 Rosenberg Associates

2.0 SCOPE OF WORK

RA’s scope of work included a document review, site reconnaissance, 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 reviewed. The following documents were reviewed.

a. AGEC 2021, “Geotechnical Investigation, Boulder Creek Crossing, Phase 1: Lots 3-5 and 13, Phase 2: Lots 10-12, St. George, Utah.” AGEC Project No. 2211698, dated

October 20, 2021. Select data from this report is provided in Appendix E.

b. AGEC 2023, “Preliminary Subsurface Investigation and Grading Recommendations, 4.76-Acre Parcel-Taylor Property, St. George, Utah.” AGEC Project No. 2230107, dated June 15, 2023. Select data from this report is provided in Appendix E.

c. 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.

d. GC 2023, “Pavement Design and Preliminary Geotechnical Recommendations, George Washington Boulevard, West of Proposed GW Bridge, Saint George, UT,” dated July 11, 2023. Traffic loading design information, as provided by Horrocks, is provided in Appendix D.

e. 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.

f. 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.

g. RA 2023a, “Geotechnical Data Report, Proposed George Washington Boulevard

Extension, from near River Road to Proposed New Bridge Structure, St. George, Utah.” RA Project No. 11138-21-003, dated February 28, 2023.

h. RA 2023b, “Geotechnical Report, Proposed George Washington Boulevard Bridge

Project, St. George, Utah.” RA Project No. 11138-21-003, dated December20, 2023.

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2. A site reconnaissance was conducted to evaluate the general surface and geologic conditions at the site and to select the exploration locations.

3. The subsurface soil conditions along the proposed roadway alignment were explored by excavating 9 test pits to depths ranging from 6 to 12½ feet and drilling one (1) boring to a depth of 20 feet below the existing site grade. The locations of the explorations are shown on the enclosed Site Plan, Drawing No. 1. Continuous logs of the subsurface conditions, as encountered in the explorations, were recorded by our field engineer A description of the equipment and procedures used, as well as logs of the subsurface conditions, are presented in Appendix A.

4. Relatively undisturbed ring soil samples and/or disturbed bulk samples were obtained from the explorations and transported to our laboratory for further evaluation and testing. Laboratory tests included unit weight and moisture content determinations, modified consolidation, classification, clay expansion, modified Proctor, CBR, and chemical testing. The results of the laboratory testing are presented in Appendix A.

5. Dynamic cone penetrometer (DCP) testing conducted adjacent to each of the test pit locations along the alignment. The DCP test results are provided in Appendix B.

6. A geophysical survey was conducted by Sage Earth Science at the proposed new bridge alignment and across the proposed roadway alignment near St. James Park as indicated on Drawing No. 1. A copy of the geophysical survey results is provided in

Appendix C.

7. Results of the field and laboratory investigations were evaluated, and engineering analyses were performed to develop geotechnical recommendations for design and construction of the proposed new roadway project.

8. This geotechnical report was prepared to present the results of our geotechnical investigation.

RA Project No. 11138-25-008 25R-031.G 4 Rosenberg Associates

3.0 GEOLOGIC AND SEISMIC CONDITIONS

3.1 Site Geologic Setting

Surficial soils along the proposed roadway alignment are mapped as Quaternary alluvium deposits (Qal1) along the Virgin River as indicated on the Geologic Map below. The stream alluvium deposits generally consist of stratified, moderately to well-sorted, poorly consolidated clay, silt, sand, and gravel deposits in large, active drainages which are estimated to be up to feet thick (Hayden and Willis, 2011).

GEOLOGIC MAP

(Hayden and Willis, 2011).

3.2 Potential Geologic Hazards

Potential geologic hazards identified at the project site are: (1) flooding and erosion; (2) seismic ground shaking; and (3) liquefaction during a significant seismic event. These potential geologic hazards are discussed in the following subsections.

Approx. Alignment

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3.2.1 Flooding and Erosion Hazards

According to Washington County Flood Insurance Rate Maps (FIRM, 2009), areas immediately adjacent to the Virgin River flood plain are mapped in the limits of flood prone areas, Zone AE and subject to inundations by the 1% annual chance flood. The

UGS flood-hazard category for this area is “Very High.”

According to the St. George City Erosion-Hazard Zone Map, the areas immediately adjacent to the Virgin River are located within an erosion-hazard zone as defined by

CH2MHILL (1997) as a “land area adjoining a body of water or adjacent to or located partially or wholly within a delineated flood plain which due to soil instability, is likely to suffer flood-related erosion damage”.

3.2.2 Seismic Ground Shaking Hazard

The St. George Basin lies at the southern end of a zone of strong to very strong earthquake activity that extends from southwestern Utah to northwestern Montana.

Prominent faults, with suspected Quaternary and/or Holocene displacement, trend generally north-south along the western, central, and eastern portions of the St.

George Basin. These are the Grand Wash fault, the Washington fault, and the Hurricane fault, respectively. RA’s review of the referenced geologic maps did not suggest the presence of known faults transecting the proposed alignment. The nearest known fault with pronounced seismic activity is the Washington fault located about 4½ miles east of the project site (Christenson and Deen, 1983).

Historic earthquakes have not exceeded magnitude 6.5 in southwest Utah; however, geological studies indicate that faults in the region may be capable of producing earthquakes of magnitude 7 to 7.5. The project design should consider the potential for strong to very strong ground shaking (Lund and others, 2008).

3.2.3 Liquefaction-Hazard

In conjunction with the seismic ground shaking as discussed in Section 3.2, the UGS has classified the poorly consolidated stream deposits (Qal1) along the Virgin River as having a “Very High” susceptibility, 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.

RA Project No. 11138-25-008 25R-031.G 6 Rosenberg Associates

The liquefaction analysis conducted for the proposed bridge indicated that based on the lack of lateral continuity of soil strata at the project site, together with the relative thinness of strata in which triggering of liquefaction was calculated, the lateral spread hazard appears to be minimal, and no mitigation measures were anticipated.

3.2.4 Other Geologic Hazards

Review of the referenced geologic maps show no evidence of surface-fault-rupture, landslide, or rockfall hazards on or near the project site (Hayden and Willis, 2011 and

UGS; Lund and others, 2008).

3.3 Potential Geologic Constraints

Potential geologic construction constraints identified within the project site are related to the presence of: (1) poorly consolidated soil deposits; (2) shallow groundwater susceptibility; (3) local pumping subgrade conditions; (4) soil corrosion; and (5) utility trench excavation stability. These potential geologic construction constraints are discussed in the following subsections.

3.3.1 Poorly Consolidated Soil Deposits

The roadway alignment extends over poorly consolidated stream deposits which are very loose to loose and soft to firm in consistency, exhibit relatively low in-place densities, and are subject to hydrocollapse and consolidation. The engineering properties of these soils can be improved through a combination of overexcavation, moisture conditioning, and recompaction. Embankment fill heights greater than 4 feet may increase the stress in the underlying, compressible river deposits and result in settlements greater than 1 inch.

3.3.2 Shallow-Ground-Water Susceptibility

The UGS classifies the project site as “SGW1”, having naturally wet, poorly drained soils where permanent shallow ground water may be present at depths less than 10 feet.

Groundwater was encountered during our investigations within the one boring and within 5 of the 9 test pits at depths ranging from 7 to 13 feet below the existing ground surface. Groundwater levels are projected to fluctuate as much as 2 to 4 feet seasonally. The measured depths to groundwater are presented in Section 4.3.

3.3.3 Pumping Subgrade Conditions

Excavation of subgrade areas may locally encounter very moist to wet native soils which may pump and be unstable under construction traffic. Stabilization of these soils will be required prior to placement of embankment fill and/or structural fill.

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3.3.4 Soil Corrosion

Laboratory test results, and our experience in the area, indicate that the soils on this site contain sulfates in sufficient concentration to be considered severely corrosive to metal and concrete.

3.3.5 Trench Excavation Stability

Caving sidewall soil conditions were encountered within the test pits excavated for this study and are anticipated within excavations required for buried utilities and/or box culverts, etc. Flowing sand conditions are anticipated within excavations where loose sands and gravels are encountered at or below the groundwater elevations.

4.0 GENERAL SITE CONDITIONS

4.1 Surface Description

At the time of our field investigation, the project site consisted of undeveloped flood plain areas located along the southern side of the Virgin River. The eastern end of the alignment was being utilized for agricultural purposes with cultivate fields. The surface was relatively flat and covered with either cultivated fields or natural vegetation consisting of thick brush and occasional trees. At the eastern end of the proposed alignment, there is an approximate 8-foot elevation differential and an existing rockery retaining wall near the connection location to the existing 1450 South Street improvements within the Boulder

Creek Crossing commercial development. As shown on Drawing No. 2, the alignment is bordered on the north by the Virgin River and associated floodway. On the south, the alignment is bordered by farmland, undeveloped flood plain areas, a paved pedestrian trail with a trailhead and the St. James Park facilities at the western end of St. James Lane.

4.2 Subsurface Soil Conditions

The subsurface soil conditions encountered within the explorations performed by RA along the proposed roadway alignment consisted of about 0 to 4 feet of soft to firm, silty and clayey topsoils overlying very loose to medium dense interbedded layers of river sands and gravels. Medium dense to dense sands and gravels and interbedded stiff clays were encountered below depths of about 9 to 18 feet. Laboratory test results indicated the subgrade soils exhibited relatively low to moderate in-place dry densities, low to moderate potentials for hydro-collapse (rapid settlement under increased loading and moisture conditions), and are compressible. The clay interbeds exhibited low expansion potentials. Corrosivity (chemical) testing indicated that the on-site soils should be considered corrosive to buried metal and have sulfate concentrations high enough to be

RA Project No. 11138-25-008 25R-031.G 8 Rosenberg Associates harmful to normal strength concrete. The subgrade soils exhibited CBR values ranging from 5.1 to 16.3. Laboratory test results as well as logs of the explorations are presented in Appendix A. The DCP field tests indicated in-place, unconsolidated, CBR values generally ranging from 3 to 10. DCP test results are presented in Appendix B.

Within the cultivated farmland portion of the roadway alignment, the referenced 2023

AGEC report for the 4.76-Acre Taylor Property indicates that the subsurface conditions consist of about 1 to 2 feet of overlying interbedded layers of river sand, silt, and gravel deposits. The cultivated topsoils consisted of porous, brown, lean clay with sand (CL) and silty sand (SM) with roots and organics. The sands and gravels were loose to dense in consistency. The silts were soft, moist to wet, and red-brown in color. Select logs of AGEC’s explorations as well as their laboratory test results are presented in Appendix E.

Stratification lines shown on the exploration logs represent the approximate boundary between soil types. The actual in-situ transition may be gradual. Due to the nature and depositional characteristics of the native soils, care should be taken in interpolating subsurface conditions between and beyond the exploration locations.

4.3 Groundwater

Groundwater was observed within the one exploratory boring and within 5 of the 9 test pits performed by RA at depths ranging from about 7 to 13 feet below the existing ground surface. The measured depths to groundwater are presented in Table 4.3.

Table 4.3: Depths to Groundwater

Exploration Location Depth to GW (ft)

22-TP-2 10

22-TP -3 10½

22-TP -5 7

22-TP -6 8½

22-TP -7 8

25-BH -1 13

Within the cultivated farmland area, AGEC reported groundwater depths ranging from about 7 feet down to 9 feet (AGEC, 2023). Groundwater levels are anticipated to fluctuate as much as 2 to 4 feet seasonally. Numerous other factors such as heavy precipitation, irrigation of neighboring land, and other unforeseen factors, may also influence ground water elevations at the site. The detailed evaluation of these and other factors, which may be responsible for ground water fluctuations, is beyond the scope of this study.

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5.0 GENERAL ENGINEERING EVALUATION

Based on the subsurface conditions encountered at the site, it is our opinion (from a geotechnical viewpoint), that the subject site will be suitable for the proposed construction project provided that the potential geologic hazards and construction constraints identified at the project site are properly addressed during design and construction, and that the recommendations contained in this report are complied with.

Specifically, it is our opinion that:

1. The near surface soils are not considered suitable for the support of the proposed roadway improvements and box culverts in their present condition and will require overexcavation and replacement with approved embankment and structural fill materials as discussed in Section 6.0.

2. 2. Within the planned roadway improvement areas, overexcavation depths on the order of 1½ feet below the aggregate base materials, or 1½ feet below the natural ground surface, whichever is greater are recommended. Excavated on-site soils free of roots, organic material, and debris are anticipated to be suitable for reuse and may be stockpiled for later use as compacted embankment fill.

3. Excavation of subgrade areas may encounter localized areas of very moist to wet native soils which may pump and be unstable under construction traffic. Stabilization of these soils will likely be required prior to placement of embankment fill as discussed in Section 6.2. Prior to placing embankment fill, the Geotechnical Consultant should observe that the prepared overexcavation bottoms are suitable for support of embankment fill by having the Contractor proof roll the prepared surface using moderate-weight rubber-tired construction equipment over the surface.

4. At the planned box culvert locations, the exposed soils should be overexcavated to a depth of at least 2 feet below the planned bottom of box culvert elevation to allow the placement and compaction of at least 2 feet of imported granular structural fill as discussed in Section 6.3.2.

5. Unstable trench excavations and caving sidewall conditions should be anticipated.

Flowing sand conditions are anticipated within excavations where loose sands and gravels are encountered at or below the groundwater elevations as discussed in

Section 6.4.

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6. Maximum embankment fill slopes should not be made steeper than 2½:1 (horizontal to vertical). Where steeper slopes are desired, retaining structures, reinforced slopes and/or additional analysis will be required.

7. Embankment fill heights greater than 4 feet may increase the stress in the underlying, compressible native soils and result in settlements greater than 1 inch. If the site grade will be raised more than 4 feet, settlement monuments and monitoring are recommended during construction to observe that at least 95% of the total anticipated settlements have occurred prior to the construction of pavements and utilities.

8. Three (3) minimum pavement section alternatives for both a 20-year design life and

25-year design life are provided for your consideration for the new roadway improvements. Alternative 1 (preferred) includes a geo-grid (Tensar NX750-FG) reinforced subgrade to reduce the overall required aggregate base thickness.

Alternative 2 consists of no geo-grid reinforcement with a full depth of Type II aggregate base. Alternative 3 consists of no geo-grid reinforcement with a combination of Type II aggregate base and Type I sub-base. The minimum recommended minimum structural pavement section alternatives for a 20-year design life and a 25-year design life are presented in Section 7.0.

The following sections of this report present our geotechnical recommendations for general earthwork, asphaltic concrete pavements, box culverts, soil corrosion, moisture protection and drainage, design review, and construction observations. We recommend that the Geotechnical Consultant be allowed to review the final grading plans when prepared to evaluate the compatibility of these recommendations.

6.0 EARTHWORK

All earthworks including clearing, grubbing, excavation, grading, fill materials, and fill placement and compaction should be performed in accordance with the current St.

George City Standard Specifications unless otherwise recommended in this report. The

Geotechnical Consultant should observe the site grading operations to observe that unsuitable soils are identified and treated as recommended.

6.1 Site Preparation and Grading

Within the areas to be graded, any existing vegetation and debris should be removed and hauled off the site. Any undocumented fill soils and/or disturbed soils should be excavated. After clearing, and the required site grading cuts have been made, the exposed

RA Project No. 11138-25-008 25R-031.G 11 Rosenberg Associates soils should be over-excavated as outlined below to allow the replacement with approved structural fill. The Geotechnical Consultant should observe the site prior to the continuance of grading to observe that required removals have been made and any unsuitable materials are identified and treated as recommended.

6.1.1 Roadway Improvement Subgrade Preparation

Following the stripping operations, and after any required grading cuts have been made, the exposed native soils should be overexcavated to a minimum depth of 1½ feet below the planned aggregate base materials, or 1½ feet below the existing site grade, whichever is greater. Suitable materials free of roots, organic material, and debris may be stockpiled for later use as compacted embankment fill. Site preparation operations should extend laterally at least 2 feet beyond roadway improvement areas, or to a distance equal to the depth of embankment fill, whichever is greater.

Except for soils requiring subgrade stabilization, the native soils exposed after the required overexcavations have been accomplished should be scarified to a depth of at least 8 inches, moisture conditioned (air dried or watered as necessary) to obtain a moisture content within 2% of optimum, and recompacted to at least 90% of the maximum dry density as determined by ASTM D-1557. The excavation bottoms should be observed by the Geotechnical Consultant prior to the continuance of grading to observe that the recommended removals have been made and whether the exposed soils are suitable for the support of embankment fill by having the Contractor proof roll the prepared surface using moderate-weight rubber-tired construction equipment over the surface.

Excavation of subgrade areas may encounter native soils which may pump and be unstable under construction traffic. Stabilization of these soils will be required prior to placement of embankment fill as discussed in Section 6.2.

The site may then be brought to rough grade with embankment fill as described in

Section 6.3. Subgrade materials supporting roadway improvements should be kept moist and not be allowed to dry out and crack. If this occurs, or if the subgrade is disturbed, the exposed soils should be scarified, moisture-conditioned and re-compacted as outlined in Section 6.3.

6.1.2 Box Culvert Subgrade Preparation

Within the planned box culvert areas, the exposed soils should be overexcavated to a depth of at least 2 feet below the planned bottom of box culvert elevation to allow the placement of at least 2 feet of imported granular structural fill. Laterally the overexcavations should extend at least 2 feet beyond the box perimeters. Excavation

RA Project No. 11138-25-008 25R-031.G 12 Rosenberg Associates of subgrade areas may locally encounter very moist to wet native soils which may pump and be unstable under construction traffic. Stabilization of these soils may be required prior to placement of embankment fill as discussed in Section 6.2. The geotechnical consultant should observe and document that the recommended removals have been made and whether the exposed soils are suitable for the support of structural fill.

6.2 Subgrade Stabilization

Excavation of subgrade areas may encounter very moist to wet native soils which may stabilization prior to placement of embankment fill.

6.2.1 Roadway Subgrade Stabilization

Where subgrade stabilization is required, localized soft spots may be overexcavated and replaced with imported granular embankment fill. Where more extensive stabilization is required, crushed rock, or a geotextile such as Tensar NX750-FG, or equivalent, may be required for initial subgrade stabilization. Geotextiles should be installed in accordance with the manufacturer's recommendations directly on the exposed subgrade prior to placing and statically compacting at least 9 inches of fractured (angular) aggregate materials with a maximum particle size of 2 inches and a minimum of 50% fines passing a 1½-inch sieve (crushed concrete or road base is preferred). The Geotechnical Consultant should observe any required stabilization operations and observe that the prepared subgrade is suitable for support of embankment fill by proof rolling moderate-weight rubber-tired construction equipment over the surface at least twice.

If additional stabilization is required, a second layer of geogrid and aggregate (9 inches) should be then installed, and the aggregate again statically compacted. The top of the stabilization blanket should then be proof rolled. Laterally, the stabilization operations should extend at least 2 feet beyond the new roadway improvements, or to the toe of the embankment, whichever is greater. Structural fill may then be placed on the stabilized subgrade to bring the site to grade.

6.2.2 Box Culvert Subgrade Stabilization

Subgrade stabilization at the proposed box culvert locations should consist of a high strength geogrid (Tensar NX750-FG or equivalent), overlying a filter fabric (Mirafi

160N or equivalent) installed directly on the exposed subgrade prior to placing and statically compacting 12 inches of fractured (angular) aggregate materials with a maximum particle size of 2 inches and a minimum of 50% fines passing a 1½-inch sieve (crushed concrete or road base is preferred). A second layer of geogrid and

RA Project No. 11138-25-008 25R-031.G 13 Rosenberg Associates aggregate (12 inches) should be then installed, and the aggregate again statically compacted. Laterally, the stabilization operations should extend at least 2 feet beyond the box culverts.

6.3 Embankment and Structural Fill Material

As discussed in Section 6.0, the Geotechnical Consultant should observe the prepared subgrade prior to the placement of embankment and structural fill materials to observe that the recommended removals have been accomplished and whether the exposed soils are stable and suitable for the support of embankment and structural fill.

6.3.1 Roadway Embankment Fill

Except where subgrade stabilization is required (see Section 6.2), all fill placed for the support of roadway improvements may consist of approved excavated on-site soils or pre-approved imported soils. Embankment fill consisting of on-site soils should be free of roots, organic material, and debris and contain no inert materials larger than 4 inches in nominal size. Imported structural fills should be granular, well graded, and have a minimum CBR value of 10. Imported materials proposed for use as embankment fill should be approved prior to importing.

Embankment fill should be placed in maximum 8-inch loose lifts and compacted on a horizontal plane, unless otherwise approved by the Geotechnical Consultant. Soils in compacted fills should be compacted to at least 95% of the maximum dry density as determined by ASTM for granular soils, and to at least 92 percent for approved fine-grained soils. The moisture content should be within 2 percent of optimum for granular soils and at least 2 percent above optimum for fine-grained soils.

6.3.2 Box Culvert Structural Fill

All fill placed for the support of box culverts should consist of imported granular structural fill. Granular structural fill should be well graded, free of vegetation and debris, and contain no inert materials larger than 4 inches in nominal size. Imported materials proposed for use as structural fill should be approved prior to importing.

Where subgrade stabilization is required, structural fill should consist of fractured

(angular) aggregate materials with a maximum particle size of 2 inches and a minimum of 50% fines passing a 1½-inch sieve (crushed concrete or road base is preferred) as discussed in Section 6.2.

RA Project No. 11138-25-008 25R-031.G 14 Rosenberg Associates

Structural fill should be placed in a maximum of 8-inch loose lifts and compacted to at least 95% of the maximum dry density as determined by ASTM D-1557. The moisture content should be within 2 percent of optimum.

6.4 Excavations

The subgrade materials encountered at the site consisted of soft to firm, silty and clayey topsoils overlying very loose to medium dense layers of river sands and gravels. These subgrade soils should be readily excavatable with conventional excavation equipment;

however, unstable excavations and caving sidewall conditions should be anticipated within excavation required for buried trenches and box culverts. Flowing sand conditions are anticipated within excavations at or below the groundwater elevations. Groundwater was encountered at depths ranging from about 7 to 13 feet below the existing ground surface. The measured depths to groundwater are presented in Section 4.3.

Temporary trench excavations should be laid back to safe slopes or properly sheeted, braced, or shored to protect the workers while the work is in progress. Contractors should meet OSHA health and safety standards. Safety of personnel during construction is the responsibility of the Contractor.

6.5 Permanent Cut and Fill Slopes

It is recommended that in general, the maximum permanent cut and fill slopes should not be made steeper than 2½:1 (horizontal to vertical). These requirements should be adequate for overall stability; however, flatter slopes may be desired for erosion control.

Also, to reduce the potential for erosion, all drainage above the slopes should be directed away from the slope face. Where steeper slopes are desired within the development, retaining structures, reinforced slopes and/or additional analysis will be required.

6.6 Embankment Settlement Monitoring

Embankment fill heights greater than 4 feet will increase the stress in the underlying, compressible native soils and result in settlements greater than 1 inch. If the site grade will be raised more than 4 feet, settlement monuments and monitoring are recommended during construction to observe that at least 95% of the total anticipated settlements have occurred prior to the construction of pavements and utilities.

RA Project No. 11138-25-008 25R-031.G 15 Rosenberg Associates

7.0 ASPHALTIC CONCRETE PAVEMENTS

7.1 Traffic Loading Information

Design traffic loading information for George Washington Boulevard west of the proposed new bridge was provided by Horrocks as part of the bridge design project in

2023. The traffic loading information provided by Horrocks (GC, 2023) is presented in

Appendix D. The average annual daily traffic (AADT) for year 0 was 12,000 (total for both directions, all lanes), adjusted to 5,072 in the design lane (reflecting two principal lanes of traffic in each direction) for the start of the pavement life, with approximately 8% trucks.

An annual, compounding growth rate of 3.077% (equivalent to a straight-line average growth rate of 4.2%) was used in conjunction with vehicle load factors as defined by UDOT for axle classifications consistent with an “urban minor collector” to yield a total ESAL

(Equivalent Single Axle Load) value of approximately 3.29 million over a 20-year design life for the pavement. Using the same traffic loading information, the total ESAL value for a 25-year design life would be approximately 4.63 million.

7.2 Structural Pavement Sections

Asphaltic concrete pavement sections should receive adequate support from properly prepared subgrade as recommended in Section 6.0. A flexible pavement analysis was conducted based on the AASHTO 93 design methods using the traffic loading information presented in Section 7.1 and assuming that the subgrade would consist of a minimum of

18 inches of properly placed and compacted embankment fill materials having a minimum

CBR value of 5.0. If these values are not considered appropriate, our office should be notified as soon as possible.

Based on the results of the analysis, three (3) minimum pavement section alternatives for both the 20-year design life and 25-year design life are provided for your consideration for the new roadway improvements. Alternative 1 (preferred) includes a geo-grid (Tensar

NX750-FG) reinforced subgrade to reduce the overall required aggregate base thickness.

Alternative 2 consists of no geo-grid reinforcement with a full depth of Type II aggregate base. Alternative 3 consists of no geo-grid reinforcement with a combination of Type II aggregate base and Type I sub-base. The minimum recommended minimum structural pavement section alternatives for a 20-year design life and a 25-year design life are summarized on the following page in Tables 7.2a and 7.2b.

RA Project No. 11138-25-008 25R-031.G 16 Rosenberg Associates

Table 7.2a: 20-Year Design Life Minimum Pavement Sections

Traffic

Conditions

Roadway

Class

Design

ESALs

(Million)

Asphaltic

Concrete

(in.)

Type II

Base

(in.)

Geogrid

Type I

Sub-Base

(in.)

Embankment

Fill

Minor Arterial III

3.29

6½

*NX750-FG

* For Alternative 1, a layer of Tensar NX750-FG geogrid should be installed on top of the prepared subgrade in accordance with the manufacturer’s recommendations prior to placing the aggregate base materials.

Table 7.2b: 25-Year Design Life Minimum Pavement Sections

Traffic

Conditions

Roadway

Class

Design

ESALs

(Million)

Asphaltic

Concrete

(in.)

Type II

Base

Geogrid

Type I

Sub-Base

(in.)

Embankment

Fill

Minor Arterial III

4.63

*NX750-FG

* For Alternative 1, a layer of Tensar NX750-FG geogrid should be installed on top of the prepared subgrade in accordance with the manufacturer’s recommendations prior to placing the aggregate base materials.

Base and asphaltic concrete materials should conform to St. George City specifications for gradation, quality, and compaction. Collection and diversion of drainage away from roadway improvements is important to the satisfactory performance of the pavement surface. Proper pavement maintenance should be provided to extend pavement life.

RA Project No. 11138-25-008 25R-031.G 17 Rosenberg Associates

8.0 B0X CULVERTS

8.1 Box Culvert Structures

Box culverts should receive adequate bearing support from thickened slab foundations as part of a box culvert structure, founded on a 2-foot minimum zone of properly placed and compacted structural fill materials as recommended in Section 6.0. Areas requiring stabilization procedures should be observed by the Geotechnical Consultant to observe that the stabilization has been accomplished and that the area is suitable for the support of concrete structures.

8.2 Lateral Resistance

Horizontal loads acting on foundations formed in open excavations will be resisted by friction acting at the base of foundations (see Table 8.2) and by passive earth pressures.

The nominal sliding resistance is the total vertical force (including the weight of the wall and footing plus the weight of backfill and pavement over the wall footing), multiplied by the base sliding coefficient, tan δ (with a 1.5 factor of safety). Passive earth pressure parameters for use in determining lateral resistance are provided in Section 8.3.

Table 8.2: Sliding Resistance Coefficients

Material

Friction

Angle δ

Base Sliding

Coefficient (tan δ)

Granular Structural fill 32˚ 0.4

8.3 Lateral Earth Pressures

Box culverts will retain backfill resulting in an at-rest earth pressure state. The associated wing walls will be subjected to active earth pressures. For the purposes of analysis, the pavement section and traffic loading conditions were modeled as a uniform surcharge load. The active, at-rest lateral pressure, and passive lateral earth pressure parameters are based on level ground conditions.

Table 8.3 on the following page presents the lateral earth pressure criteria for both static and seismic conditions. The seismic lateral earth pressure values are based on peak

(horizontal) ground accelerations (PGA) which indicated a PGA of 0.22g and 0.14g for respective 3% and 7% probabilities of exceedance in 75 years at the site. The acceleration values were reduced by 50% to obtain the horizontal coefficients used in the calculations.

RA Project No. 11138-25-008 25R-031.G 18 Rosenberg Associates

Table 8.3: Equivalent Fluid Density for Lateral Earth Pressure Calculations

Backfill

Type

Assumed

Backfill

Properties

Case

Equivalent Fluid Density (pcf)

Static 2PE50yrs

Granular

Structural fill ɸ = 32˚

C = 0 psf γ = 125 pcf

Active 38 7

At-Rest 59 30

Passive 400 650

The equivalent fluid density values provided above assume backfill materials do not become saturated. If backfill material other than that described above is used as backfill immediately adjacent to the wall of the structure, the equivalent fluid densities must be modified.

In calculating lateral earth pressures acting on the box structure, we recommend the following approaches:

a. The static earth pressure for the active and at-rest cases is calculated using the above equivalent fluid density values assuming a triangular distribution that increases with depth from the top of the wall.