140L5722Q0009_Attachment_06_Geotechnical_Study.pdf

2650 north 180 east lehi, Utah 84043 Telephone number 801-766-3246

GEOTECHNICAL STUDY

Bald Mesa Communication Tower

Bald Mesa Grand County, Utah

PREPARED FOR:

Mr. Ryan Parsons Radio Program Manager

Bureau Land Management 440 West 200 South, Suite 600

Salt Lake City, Utah 84101

December 24, 2021

Project No. 21153

2650 North 180 East Lehi, Utah 84043 P. 801-766-3246

December 24, 2021

Mr. Ryan Parsons Radio Program Manager Bureau Land Management 440 West 200 South, Suite 600 Salt Lake City, Utah 84101

SUBJECT: GEOTECHNICAL STUDY

Bald Mesa Communication Tower Bald Mesa, Grand County, Utah

TG Project No. 21153

Dear Mr. Parsons:

Enclosed are the findings of the geotechnical engineering study performed for the above referenced property. Geotechnical engineering recommendations for foundation analysis are presented herein.

Please feel free to contact us should you or members of the design team have any questions or desire additional information. The opportunity to be of service on this project is appreciated.

Respectfully submitted, Taylor Geotechnical

Alanson O. Taylor, P.E.

Principal

TG Project No.21153

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SCOPE OF WORK

2.1 Subsurface Exploration

2.2 Laboratory Testing

2.3 Engineering Analyses and Report

3.0 SITE CONDITIONS

3.1 General

3.2 Geology

3.3 Subsurface Materials

3.4 Subsurface Water

3.5 Subsurface Variations

4.0 FOUNDATION ANALYSIS RECOMMENDATIONS

4.1 Tower Foundation Support

4.2 Foundation Support for Auxiliary Equipment

4.3 Estimated Settlement

4.4 Footing Excavations

4.5 Resistance to Lateral Loads

4.6 Seismicity

4.7 Structural Fill

4.8 Surface Drainage

5.0 CLOSURE

APPENDIX

Figure 1 Vicinity Map Figure 2 Site Plan and Site Photo Figure 3 Regional Geologic Map Figure 4 Test Pit No. TP-1 Log Figure 5 Test Pit Photos Figure 6 Seismic Output Data Sheet

TG Project No. 21153 1

1.0 INTRODUCTION

This report presents the results of the geotechnical engineering study performed for a proposed communication tower to be located on the west side of Bald Mesa, approximately 12 miles west northwest of Moab, on Forest Road 1649 in Grand County, Utah. The general location of the property is shown on Figure 1, Vicinity Map in the Appendix. The purpose of the study was to evaluate the nature and engineering properties of subsurface materials at the site and provide geotechnical engineering recommendations for the proposed foundations.

Based on the information provided by the Bureau of Land Management, the following aspects of the project were considered in our study:

1. Final grades will be approximately the same as existing grades.

2. The proposed tower will be constructed on a concrete mat foundation.

3. Tower will consist of a self supporting lattice tower about 40 feet in height.

4. A new accessory equipment building will be supported on square 10 ft. x 10 ft.

monolithic foundation and floor slab.

If aspects of the project vary from that described above, Taylor Geotechnical should be notified immediately so the applicability of the recommendations presented herein can be evaluated.

2.0 SCOPE OF WORK

This study included the following scope of work.

2.1 Subsurface Exploration

Two (2) exploratory test pits were excavated at the site to evaluate subsurface conditions. The exploratory test pits extended about 8 feet below existing ground surface for test pit TP-1 and 2.5 feet below existing ground surface for test pit TP-2. The test pit locations are shown on Figure 2, Site Plan in the Appendix.

Subsurface conditions observed in the exploratory test pit TP-1 were carefully documented at the time of excavation by Taylor Geotechnical field personnel. Test pit TP-2 was not logged due to encountering electrical cables at about 2 feet below the surface. The log of test pit TP-1 is provided on Figure 4 in the Appendix. The test pits were backfilled without compaction. If the foundation extends over the test pit areas, then the test pits backfill should be removed and replaced in compacted lifts.

TG Project No. 21153 2

2.2 Laboratory Testing

Representative soil samples obtained from the exploratory test pit were tested in the laboratory to aid in classification and to determine pertinent engineering properties. These tests included:

1. Natural Moisture Contents; and,

2. Gradations.

Results of the laboratory testing are presented on the test pit log TP-1 in the Appendix.

2.3 Engineering Analyses and Report

Data obtained from the exploratory test pit was evaluated. Geotechnical engineering analyses were performed which included preparation of this report presenting the findings and recommendations developed during the study.

3.0 SITE CONDITIONS

Existing surface and subsurface conditions associated with the property are presented in this section.

3.1 General

The surface of the proposed communication tower area slopes downward to the north and south at a 10 percent grade from the tower location with the gradient increasing with distance from the site. The grade is fairly level to the east and west. The general area currently contains a several communication towers and supporting equipment buildings. An existing equipment building and 30 foot latter tower occupies the study area. The current tower is supported on a concrete mat foundation. The area is sparsely vegetated with native weeds and sage brush. The surface of the proposed tower location has been disturbed from previous grading activities. The site is off of Forest Road 1649 that is 3.6 miles from the intersection of Forest Road 1649 and Forest Road 0063. The said intersection is 2.8 miles from the intersection of Forest Road 0063 and La Sal Loop Road. The site is situated at about elevation 8,574 feet above mean sea level and is located at geographic coordinates North 38.5288° latitude, West 109.3248° longitude.

3.2 Geology

The Utah Geologic Survey1 has mapped the surface of the site as QTaf, defined as older alluvial-fan deposits comprised of sand, silt, granules, pebbles, cobbles, and sparse boulders deposited at the foot of the La Sal Mountains. The thickness ranges from 200-300 ft and ) and was deposited

1 Doelling, H. H., Geologic Map of The Moad and Eastern Part of the Sand Rafael Desert 30’ x 60’ Quadrangles, Grand and Emery Counties, Utah, and Mesa County, Colorado, Utah Geological Survey, 2002.

TG Project No. 21153 3 early Pleistocene to Pliocene(?). A regional geologic map is shown on Figure 3 in the Appendix.

No active faults cross the subject area. The soils encountered during our field investigation as described in section 3.3 Subsurface Materials, is consistent with the regional geologic map.

3.3 Subsurface Materials

The subsurface soils generally consisted of 8 inches of gravelly topsoil underlain 2 feet of dense clayey sand with gravel and cobbles (SC) followed by medium dense well graded gravel with silt, sand, cobbles and boulders (GW-GM) extending beyond the depth investigated of 8 feet.

Boulders were up to 18 inches in diameter. A more detailed description of the subsurface conditions can be found on Figure 4, Test Pit Log TP-1 in the Appendix. Photograph of the test pit is shown on Figure 5 in the Appendix.

At the location of Test Pit TP-2, an electrical cable (3-way) was encountered at about two feet below grade. The alignment of the cable is shown on Figure 2 and on Figure 5 in the Appendix.

It is unknown if the cable was actively feeding power to any equipment in the area. The utility was not marked by the utility locating services during or after a site meet just 3 days prior to excavation activities.

3.4 Subsurface Water

Groundwater was not observed at the time of our subsurface exploration and is not expected to impact construction. Seasonal fluctuations in groundwater conditions do occur, and groundwater fluctuations of several feet are not unusual. Numerous factors contribute to groundwater fluctuations such as seasonal recharge, regional and local irrigation, and periods of heavy precipitation. The detailed evaluation of these and other factors which may be responsible for groundwater fluctuations is beyond the scope of this study.

3.5 Subsurface Variations

Based on the results of the subsurface exploration and our experience, variations in the continuity and nature of subsurface conditions should be anticipated. Due to the nature and depositional characteristics of soils documented at the site, care should be taken in interpolating or extrapolating subsurface conditions beyond the exploratory test pit.

4.0 FOUNDATION ANALYSIS RECOMMENDATIONS

Proposed foundation recommendations are provided in this section. The proposed communication tower may be supported on a large mat foundation. The proposed auxiliary equipment building may be supported on shallow strip footings or a monolithic foundation..

Details for foundation support follow.

TG Project No. 21153 4

4.1 Tower Foundation Support

The mat foundation should have a minimum dimension as required to resist downward and overturning loads imposed by the tower. The foundation may be supported directly on competent undisturbed native soils or compacted structural fill extending to competent undisturbed native soils. Existing fill, if encountered at the tower foundation location should be removed and replaced with compacted fill as required for grading purposes. The bottom of the foundation should be located at least 4 feet below grade for frost protection. The foundation may be designed using a maximum allowable bearing capacity of 6,000 psf if supported on undisturbed native soils or 3,000 psf if supported on compacted structural fill. This bearing value is for total dead load and frequently applied live loads, and may be increased 33% for short durations of loading which will include the effect of wind or seismic forces.

4.2 Foundation Support for Auxiliary Equipment

A maximum allowable bearing pressure of 3,000 psf is recommended for foundation design using strip footings or monolithic foundation placed on competent undisturbed native soils or compacted structural fill extending to competent undisturbed native soils. The proposed footings may be supported on undisturbed native soils at frost depth or may be supported by 6 inches of granular road base underlain by 42 inches of 100 percent crushed gravel with a maximum particle size 1.5 inches and contain less than 5 percent passing the No. 40 sieve. The depth of gravel is required in order to provide a non-frost susceptible fill. The crushed gravel should be compacted during placement with at least 4 passes of a vibratory plate compactor. The road base should be compacted to at least 95 percent of the modified Proctor as per ASTM D 1557. A filter fabric should be placed between the native soils and gravel fill.

Individual or spread (column) footings for the auxiliary equipment pads should be at least 18 inches wide and extend at least 4 feet below grade for frost protection.

The alloawable bearing value is for total dead load and frequently applied live loads, and may be increased 33% for short durations of loading which will include the effect of wind or seismic forces. This bearing value may be increased 33% for short durations of loading which will include the effect of wind or seismic forces.

4.3 Estimated Settlement

Using the anticipated bearing values indicated above, estimated maximum post-construction settlement will be on the order of 1-inch for conventional shallow spread and/or continuous footings. Maximum differential settlement between foundation elements is estimated to be approximately one-half of the total settlement. Estimated settlements are based on the assumption that foundation subgrades are prepared as specified in Section 4.4 Footing Excavations.

TG Project No. 21153 5

4.4 Footing Excavations

Due to the granular nature of the native soils, the base of the excavation should be compacted to a firm unyielding surface using a vibratory compactor or four blows with the end of the excavator bucket. We recommend that Taylor Geotechnical observe all footing excavations prior to placement of structural fill, steel, and concrete to verify that all deleterious and/or soft materials have been removed; that footing subgrades consist of undisturbed native soil and that the recommendations presented herein have been appropriately interpreted.

4.5 Resistance to Lateral Loads

Resistance to lateral loads can be provided by friction acting at the base of foundations and by passive earth pressure against the foundations. For footings supported on native soil or compacted structural fill, we recommend using a coefficient of friction of 0.50. The coefficient of friction times the normal load may be assumed to act horizontally at the base of the footing.

Passive pressure from properly compacted granular backfill against the sides of footings can be calculated on the basis of an equivalent fluid weighing 460 pcf. The passive pressure should be reduced using the appropriate factor of safety.

The recommended coefficient of friction and passive pressure values are based on mobilization of the ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case of passive resistance.

The top 30 inches of backfill should be neglected for passive resistance calculations.

4.6 Seismicity

The site is situated in a seismically active region. Ground shaking resulting from earthquakes associated with nearby and distant faults will occur. During the life of the project, seismic activity associated with active faults in the area has the potential to generate moderate to strong ground shaking at the site.

Based on experience with similar sites in Utah and limited subsurface exploration, the site is likely best represented by Site Class C and by Seismic Design Category B (2018 International Building Code).

The following spectral design response spectrum coefficient should be used for seismic design:

Short Period Spectral Response Design Acceleration, SDS = 0.168g One Second Period Spectral Design Acceleration, SD1 = 0.062g

4.7 Structural Fill

If required for construction of the mat foundation or auxiliary equipment foundations, import structural fill should have a maximum particle size of 4 inches and less than 20 percent passing the No. 200 sieve. The fill should be compacted to at least 95 percent of the modified proctor as

TG Project No. 21153 6 per ASTM D1557 in 8 inch lifts. Compaction testing using a nuclear gauge should be completed a minimum of every other lift.

The native soils may be used as structural fill if the rocks larger than 4 inches in diameter are screen out of the fill prior to placement and compaction.

Structural fill used for foundation support should extend horizontally an additional one foot horizontally beyond the perimeter of the foundation for every vertical foot of fill thickness.

4.8 Surface Drainage

Providing positive site drainage during development will be fundamental to the stability and long term performance of the proposed structures. Adequate positive drainage for water flowing onto and off of the site is critical and should be provided.

Positive drainage should be provided away from the foundation areas to prevent infiltration beneath the structures and to reduce percolation of water into foundation soils. The ground surface surrounding the exterior of the structures should be sloped to drain away from the structure in all directions.

5.0 CLOSURE

The findings and recommendations of this report were prepared in accordance with generally accepted professional geotechnical engineering principles and practice in this area of Utah at this time. There is no other warranty, either express or implied.

The conclusions and recommendations presented herein are based on the results of limited subsurface exploration and laboratory testing, combined with interpolation and extrapolation of subsurface conditions between and beyond exploration locations. Subsurface conditions may differ in some locations from the conditions observed in the exploration, and may require additional analyses and possibly modified recommendations.

This report was written for the exclusive use of the Bureau of Land Management and only for subject project described herein. Taylor Geotechnical is not responsible for technical interpretations by others of exploratory information which has not been described or documented in this report. Specific questions or interpretations concerning the findings and conclusions presented herein may require written clarification to avoid possible misunderstandings. The opportunity to be of service on this project is appreciated.

TG Project No. 21153

APPENDIX

FIGURES

Proposed Bald Mesa Radio Tower

La Sal Loop Road Moab, Utah

VICINITY MAP

Figure No. 1

Taylor Geotechnical Appendix Page 1 of 6 TG Project No. 21153

Photo Exhibit 1, Subject Site Looking East

SITE PLAN AND SITE PHOTO

TP-2

TP-1

Figure No. 2

0 36 ft. 18 ft.

SCALE Reference: Google Earth (2021)

Proposed Bald Mesa Radio Tower La Sal Loop Road

Moab, Utah

Underground Wires

Taylor Geotechnical Appendix Page 2 of 6 TG Project No. 21153

REGIONAL GEOLOGIC MAP

0 2,500 ft. 1,250 ft.

SCALE

Jmb: Brushy Basin Member of Morrison Formation, consisting of variegated (purple, green, white, orange) mudstone interbedded with gray, white, or brown conglomeratic sandstone, conglomerate, nodular limestone, and gritstone.

Kdbc: Dakota Sandstone and Burro Canyon Formation, undivided.

Qms: Slumps and slides consisting of coherent to broken and jumbled masses of bedrock that have moved downslope due to gravity. Holocene to late Pleistocene.

Qmt: Talus and colluvium, rock-fall blocks, boulders, smaller angular gravel, sand, and silt; deposited on slopes below cliffs and steep slopes; only larger deposits mapped. Holocene to late Pleistocene.

QTaf: Older alluvial-fan deposits consisting of sand, silt, granules, pebbles, cobbles, and sparse boulders deposited at the foot of the La Sal Mountains; thickness (200-300 ft); early Pleistocene to Pliocene(?).

SITE

Kdbc

Figure No. 3

Jmb

Proposed Bald Mesa Radio Tower La Sal Loop Road

Moab, Utah

Qmt

Reference: Doelling (2002)

Taylor Geotechnical Appendix Page 3 of 6 TG Project No. 21153

CLIENT:

SITE LOCATION:

JOB NO.:

m

PROJECT:

GROUNDWATER LEVEL:ELEVATION: NORTHING: EASTING:

TEST PIT NO.:

Lehi, Utah 84043 Telephone: 801-766-3246 Fax: 801-769-3336

DEPTH

ft.

G ra ph ic

L og

SOIL DESCRIPTION

D ry

D en si ty

(p cf

M oi st ur e

Sa m pl e

G ra ve l (

In de x Pl as tic ity

Li m it P la st ic

Li m it Li qu id

S an d

Fi ne s

Gradation Atterberg Limits

FIELD DATE:

EQUIPMENT:

LOGGED BY:

2650 North 180 East

0 30

U

SC

S

Grab Sample

TY

P

E S

A M

P

LE

California Sampler

Dames and Moore

SPT sampler

3.25" O.D./2.42" I.D. "U" Sampler

NOTES:

Figure No.:

Shelby

Bald Mesa Radio Tower 21153 Bald Mesa, La Sal Loop Road, Moab, Utah Bureau of Land Management

12-2-21 Kubota KX018-4 Mini Ex.

A. Taylor

Not Encountered

TP-1

9.1

4.0

3.4

2.5

2.7

TOPSOIL: 8 inches, sandy silt with organices, mmedium dense, moist, brown.

CLAYEY SAND: With gravel, cobbles and boulders up to 12 inches in diameter, medium dense, moist, brown.

WELL GRADED GRAVEL WITH

SILT AND SAND: Cobbles, boulders up to 18 inches, medium dense, caving at 5.5 feet, more sandy, dry to moist, light brown.

Bottom of Test Pit at 8 feet.

OH

SC

GW-GM

Taylor Geotechnical Appendix Page 4 of 6 TG Project No. 21153

Photo Exhibit 1, Test Pit-1

Photo Exhibit 2, Location of Test Pit-2 and Alignment of Underground Wires

TEST PIT PHOTOS

Figure No. 5

Proposed Bald Mesa Radio Tower La Sal Loop Road

Moab, Utah

Taylor Geotechnical Appendix Page 5 of 6 TG Project No. 21153

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Taylor Geotechnical Appendix Page 6 of 6 TG Project No. 21153

Text2: 12-24-2021