10-Attachment K Geotechnical Accessible Companion.docx

Attachment K — Geotechnical Engineering Study Parker Mountain Communications Tower Latitude 38.324557 degrees north, Longitude 111.892708 degrees west. Piute County, Utah.

Notice — Authoritative Source Document. This document is an accessible companion to the sealed geotechnical engineering study prepared by Taylor Geotechnical, Project Number 26019, dated April 22, 2026, and digitally signed by Alanson O. Taylor, P.E., Utah Registered Professional Engineer Number 183650. The sealed PDF report remains the authoritative engineering deliverable for all design, permitting, and construction purposes. This companion document reproduces the textual content of the sealed report in a structure compatible with screen readers and other assistive technologies; it is not itself a sealed engineering document and is not a substitute for the original. Where any discrepancy exists between this companion and the sealed report, the sealed report governs.

Document Information

Document Contents

1. Section 1 — Introduction

1. Section 2 — Scope of Work

1. Section 3 — Site Conditions

1. Section 4 — Foundation Analysis Recommendations

1. Section 5 — Closure

1. Appendix Figures Section 1 — Introduction This report presents the results of the geotechnical engineering study performed for a proposed communications tower located approximately 1,300 feet southwest of Parker Mountain peak and 14.5 miles southwest of Loa, Utah, in Piute County.

The purpose of the study was to evaluate the nature and engineering properties of subsurface materials at the site and to provide geotechnical engineering recommendations for the design of the proposed communications tower foundation.

Based on information provided by the Utah Communications Authority, the following project aspects were considered:

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

1. The proposed tower will be constructed on a concrete mat foundation with the bottom of the foundation located below the existing grade.

1. The tower will consist of a self-supporting lattice tower approximately 100 feet in height.

1. A new accessory equipment building will be located adjacent to the tower. The building will be supported on a square monolithic foundation or spread footings with a slab-on-grade floor.

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

Section 2 — Scope of Work Section 2.1 — Subsurface Exploration One exploratory test pit, designated Test Pit Number 1 (TP-1), was excavated at the site to evaluate subsurface conditions. The exploratory test pit extended approximately 1.5 feet below the existing ground surface. The test pit location is shown in Figure K-2, Site Plan and Site Photograph.

Subsurface conditions observed in exploratory test pit TP-1 were documented at the time of excavation by Taylor Geotechnical field personnel. The log of test pit TP-1 is provided in Figure K-4. The test pit was backfilled without compaction. The test pit backfill should be removed and replaced in compacted lifts if it affects the foundation.

Section 2.2 — Laboratory Testing Due to the nature of the subsurface materials, which consist of bedrock, no laboratory testing was completed for this study.

Section 2.3 — Engineering Analyses and Report Data obtained from the exploratory test pit was evaluated. Geotechnical engineering analyses were performed, including preparation of this report presenting the findings and recommendations developed during the study.

Section 3 — Site Conditions Section 3.1 — General The surface of the proposed communications tower site slopes downward to the southwest at approximately a 14 percent grade. West of the site the surface slopes steeply downward to the west, with a vertical face further west. Andesite is exposed at the surface at the tower site, and conglomerate bedrock outcrops are exposed west of the site. Vegetation is minimal, consisting of evergreen trees, native weeds, and grasses. Access to the site is from a small single-lane dirt road located southwest of the site. The site is situated at approximately elevation 9,760 feet above mean sea level.

Section 3.2 — Geology The Utah Geological Survey has mapped the surface of the site as map unit Tla, defined as volcanic rocks of Langdon Mountain, alluvial facies, Oligocene in age. The unit is described as poorly to moderately resistant, mostly dacitic but locally andesitic volcanic mudflow breccia, also called lahar deposits, with subordinate sandstone and conglomerate. The dominant lithology of the clasts has low to moderate phenocrysts of plagioclase and generally subordinate conspicuous hornblende, with mostly lesser pyroxene, in a glassy to aphanitic groundmass.

The overall map unit is brown or grayish brown. East of Otter Creek Reservoir, where the unit overlies deep reddish-brown and yellowish-brown altered trachyte lava flows of the Lake Creek unit, the deposits include volumetrically significant lava flows that are commonly hydrothermally altered. The deposits were laid down principally as lahars sourced from one or more stratovolcanoes in the central Sevier Plateau. These stratovolcanoes were likely destroyed by development of the Monroe Peak caldera.

The unit is reported to be in excess of 2,600 feet, or approximately 800 meters, thick at Parker Mountain on the west-central flank of the Awapa Plateau. This thickness suggests that stratovolcano sources are nearby. Deposition of the map unit spans the age of the Sevier gravity slide, but without deformed rocks from the slide, any contact or unconformity separating pre-slide and post-slide rocks has not been identified.

Reference: Robert F. Biek, 2023. Geologic Map of the West Half of the Loa 30-Minute by 60-Minute Quadrangle, Garfield, Piute, and Wayne Counties, Utah. Utah Geological Survey publication M-292DM, at a scale of 1 to 62,000.

A regional geologic map is provided as Figure K-3. No active faults cross the subject area.

Section 3.3 — Subsurface Materials The subsurface soils consisted of 9 inches of topsoil underlain by 9 inches of weathered gray andesite. At 18 inches below the existing ground surface, the andesite could not be excavated, and the test pit was terminated. A more detailed description of the subsurface conditions is provided in Figure K-4, the test pit log for TP-1. Photographs of the test pit are shown in Figure K-5.

Section 3.4 — Subsurface Water Groundwater was not observed at the time of the 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, including seasonal recharge, regional and local irrigation, and periods of heavy precipitation. Detailed evaluation of these factors is beyond the scope of this study.

Section 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 the soils documented at the site, care should be taken in interpolating or extrapolating subsurface conditions beyond the exploratory test pit.

Section 4 — Foundation Analysis Recommendations Proposed foundation recommendations are provided in this section. The proposed communications 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.

Section 4.1 — Tower Foundation Support The mat foundation should have minimum dimensions as required to resist downward and overturning loads imposed by the tower. Estimated foundation widths range from 7 to 15 feet. The foundation may be supported directly on undisturbed competent bedrock or on compacted structural fill extending to bedrock. Foundations placed on non-frost-susceptible material do not need to extend to frost depth but should extend below the weathered bedrock zone to prevent moisture from entering between the bedrock and concrete interface.

Recommended maximum allowable bearing capacities for the tower mat foundation:

These allowable bearing values are for total dead load and frequently applied live loads. They may be increased by 33 percent for short durations of loading, including the effect of wind or seismic forces.

Section 4.2 — Foundation Support for Auxiliary Equipment A maximum allowable bearing pressure of 4,000 pounds per square foot is recommended for foundation design of the auxiliary equipment building using strip footings or a monolithic foundation supported on weathered bedrock, competent bedrock, or compacted structural fill extending to bedrock.

The proposed footings may also be supported on the following non-frost-susceptible fill section, from top down:

1. Six (6) inches of granular road base, compacted to at least 95 percent of the modified Proctor maximum dry density as per ASTM Standard D1557.

1. Twenty-four (24) inches of 100 percent crushed gravel with a maximum particle size of 1.5 inches and containing less than 5 percent passing the Number 40 sieve. The crushed gravel should be compacted during placement with at least 4 passes of a vibratory plate compactor.

1. Filter fabric placed between the native soils and the gravel fill.

Individual or spread column footings for the auxiliary equipment pads should be at least 18 inches wide and should extend at least 40 inches below grade for frost protection unless supported directly on competent bedrock. The 33 percent short-duration loading increase applies as described in Section 4.1.

Section 4.3 — Estimated Settlement Using the anticipated bearing values indicated above, the estimated maximum post-construction settlement will be less than 1 inch for the tower foundation and for conventional shallow spread footings or continuous footings. The 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.

Section 4.4 — Footing Excavations Rock excavation techniques such as rock chipping, ripping, or grinding will be required. The base of the excavation should be cleared of all loose bedrock materials prior to placement of concrete. Taylor Geotechnical recommends that the firm observe all footing excavations prior to placement of structural fill, steel, and concrete to verify that all deleterious or soft materials have been removed, that footing subgrades consist of undisturbed native soil, and that the recommendations presented in this report have been appropriately interpreted.

Section 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 bedrock, the recommended coefficient of friction is 0.65. The coefficient of friction multiplied by 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 may be calculated on the basis of an equivalent fluid weighing 450 pounds per cubic foot. 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 that 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.

Section 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 the limited subsurface exploration performed for this study, the site is best represented by Site Class B as defined in the 2024 International Building Code.

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

These values are derived from the ASCE 7-22 Standard for the site latitude and longitude. Full data sheets are provided in Figure K-6.

Section 4.7 — Structural Fill If required for construction of the mat foundation or the auxiliary equipment foundations, imported structural fill should have a maximum particle size of 4 inches and less than 20 percent passing the Number 200 sieve. The fill should be compacted to at least 95 percent of the modified Proctor maximum dry density as per ASTM Standard D1557, placed in 8-inch lifts. Compaction testing using a nuclear gauge should be completed at a minimum of every other lift.

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

Structural fill used for foundation support should extend horizontally an additional 1 foot beyond the perimeter of the foundation for every vertical foot of fill thickness.

Section 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 off of the site is critical and should be provided. Positive drainage should be directed 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.

Section 5 — 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 in this report 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 Utah Communications Authority and only for the subject project described in this document. 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 in this report may require written clarification to avoid possible misunderstandings.

Appendix — Figures The figures referenced in this report are reproduced below from the sealed source PDF. Each figure is accompanied by a detailed text description in the alt text and caption. For engineering use, the sealed PDF figures are authoritative.

Figure K-1 — Vicinity Map

Figure K-1. Vicinity map showing the route from State Route 24 mile marker 41 to the Parker Mountain communications tower site. Source: Taylor Geotechnical, Project 26019, Figure 1.

Figure K-2 — Site Plan and Site Photograph

Figure K-2. Site plan showing the location of test pit TP-1, with a ground-level photograph showing the excavator at the subject site looking east. Source: Taylor Geotechnical, Project 26019, Figure 2.

Figure K-3 — Regional Geologic Map

Figure K-3. Regional geologic map showing the site located within the Tla unit, volcanic rocks of Langdon Mountain. Source: Taylor Geotechnical, Project 26019, Figure 3, after Biek 2023.

Figure K-4 — Test Pit TP-1 Log

Figure K-4. Test pit log for TP-1 showing 9 inches of topsoil over 9 inches of weathered andesite, with refusal on bedrock at 1.5 feet. Source: Taylor Geotechnical, Project 26019, Figure 4.

Figure K-5 — Test Pit Photographs

Figure K-5. Test pit TP-1 photographs showing the southeast wall (upper) and northwest wall (lower), with topsoil over weathered andesite bedrock. Source: Taylor Geotechnical, Project 26019, Figure 5.

Figure K-6 — Seismic Design Output Data

Figure K-6a. ASCE 7-22 seismic design data for the site, page 1 of 2. Source: Taylor Geotechnical, Project 26019, Figure 6a.

Figure K-6b. ASCE 7-22 seismic design data underlying values, page 2 of 2. Source: Taylor Geotechnical, Project 26019, Figure 6b.

Accessibility Statement This document is an accessible companion to the sealed geotechnical engineering study. It has been formatted for compatibility with screen readers and other assistive technologies, following Web Content Accessibility Guidelines (WCAG) 2.1 Level AA. Headings use proper outline levels, lists use semantic list formatting, tables include designated header rows, and all figures include descriptive alt text and visible captions. Engineering symbols such as subscripts and Greek letters have been written out in plain language to support assistive-technology pronunciation.

Reminder: The original sealed PDF from Taylor Geotechnical, Project Number 26019, dated April 22, 2026, signed by Alanson O. Taylor, P.E., remains the authoritative engineering document. Where any difference exists between this companion document and the sealed report, the sealed report governs.

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