20250630 - Geotechnical Report - Denver Basin Wells.pdf

Geotechnical Evaluation Parker Water & Sanitation District

Denver Basin Well Sites Phase II and Pipeline Project Parker, Colorado

Hazen and Sawyer 6251 Greenwood Plaza Boulevard, Suite 275 | Greenwood Village, Colorado 80111

June 30, 2025 | Project No. 503147001

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witness

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS

Nathaniel Boehler, EI Project Engineer

Serkan Sengul, PE, D.GE Principal Engineer

Geotechnical Evaluation Parker Water & Sanitation District Denver Basin Well Sites Phase II and Pipeline Project Parker, Colorado

Mr. Ryan Vevang, PE Hazen and Sawyer 6251 Greenwood Plaza Boulevard, Suite 275 | Greenwood Village, Colorado 80111

June 30, 2025 | Project No. 503147001

NJB/SS/mht

Distribution: (1) Addressee (via e-mail)

9707 East Easter Lane | Centennial, Colorado 80112 | p. 303.629.6000 | www.ninyoandmoore.com

06/30/2025 http://www.ninyoandmoore.com/

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 i

CONTENTS

1 INTRODUCTION 1

2 SCOPE OF SERVICES 1

3 SITE DESCRIPTION 2

4 PROPOSED CONSTRUCTION 2

5 FIELD EXPLORATION AND LABORATORY TESTING 2

6 GEOLOGY AND SUBSURFACE CONDITIONS 3

6.1 Geologic Setting 3

6.2 Subsurface Conditions 4

6.2.1 Fill Material 4

6.2.2 Alluvium 4

6.2.3 Dawson Formation Bedrock 5

6.3 Groundwater 5

7 GEOLOGIC HAZARDS 5

7.1 Expansive Soils and Bedrock 6

7.2 Compressible Fill Material 6

8 CONCLUSIONS 6

9 RECOMMENDATIONS 7

9.1 Demolition 8

9.2 Earthwork 8

9.2.1 Excavations 8

9.2.2 Temporary Excavations and Shoring 9

9.2.3 Site and Remedial Grading 10

9.2.4 Utility Installation 11

9.2.5 Re-Use of Site Soils 13

9.2.6 Fill Placement and Compaction 13

9.2.7 Imported Soil 14

9.2.8 Controlled Low Strength Material 14

9.3 Equipment Pad Foundations 15

9.4 Modulus of Soil Reaction, E’ 16

9.5 Thrust Restraint 16

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 ii

9.6 Lateral Earth Pressures 16

9.7 Pavement Restoration 17

9.8 Aggregate Surfaced Lot and Roadway 18

9.9 Corrosion Considerations 19

9.9.1 Concrete 19

9.9.2 Buried Metal Pipes 20

9.10 Construction in Cold or Wet Weather 20

9.11 Construction Observation and Testing 21

9.12 Plan Review 21

9.13 Pre-Construction Meeting 21

10 LIMITATIONS 21

11 REFERENCES 23

TABLES

1 – Passive Pressures for Thrust Block Design 2 – Recommended Lateral Earth Pressures 3 – Corrosion Potential to Steel

FIGURES

1A – Ridgegate Pipeline Site Location 1B – Rueter-Hess Pipeline Site Location 2A – Ridgegate Pipeline Boring Locations 2B – Ridgegate Pipeline Boring Locations 2C – Ridgegate Pipeline Boring Locations 2D – Ridgegate Pipeline Boring Locations 3 – Rueter-Hess Pipeline Boring Locations 4 – Cross Section Locations

APPENDICES

A – Boring Logs B – Laboratory Testing C – Site Photographs and Descriptions D – Cross Sections E – ESAL Estimate

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 1

1 INTRODUCTION

In accordance with your authorization and our proposal dated November 8, 2024, we have performed a geotechnical evaluation for the Parker Water and Sanitation District (PWSD)

Denver Basin Well Sites Phase II and Pipeline project in Parker, Colorado. The approximate locations of the sites are depicted on Figures 1A and 1B.

The purpose of our study was to evaluate the subsurface conditions and to provide design and construction recommendations regarding geotechnical aspects of the proposed project. This report presents the findings of our subsurface exploration program, results of our laboratory testing, conclusions regarding the subsurface conditions at the sites, and geotechnical recommendations for design and construction of this project.

2 SCOPE OF SERVICES

The scope of our services for the project generally included:

• Review of referenced background information, including aerial imagery, published geologic and maps, in-house geotechnical data, and available topographical information pertaining to the project site and vicinity.

• Performance of a geologic reconnaissance and mark-out of the boring locations at the project site and notification of Utility Notification Center of Colorado for mark-out of publicly owned utilities prior to drilling. Boring locations and elevations were established using a Trimble DA2 antenna with Trimble Catalyst Precision software.

• Drilling logging and sampling of 13 small-diameter exploratory borings along the proposed waterline alignments to depths ranging from approximately 14.5 to 15 feet below ground surface (bgs). Borings B-1 through B-11 were advanced at the proposed Ridgegate Well House Site Phase II and Badger Gulch Pipeline and Borings B-12 and B-13 were performed at the proposed Rueter-Hess Well House Site Phase II and Pipeline. The boring logs are presented in Appendix A. The locations of the borings are presented on Figures 2A-2D and Figure 3.

• Performance of laboratory tests on selected samples obtained from the borings to evaluate engineering properties including in-situ moisture content and dry density, Atterberg limits, percent materials finer than the No. 200 sieve, swell and consolidation potential, direct shear, unconfined compressive strength (UCS), and soil corrosivity characteristics (including pH, resistivity, water-soluble sulfates, and chlorides). The results of the laboratory testing are presented on the boring logs and in Appendix B.

• Compilation and analysis of the data obtained. Site photographs associated with our February 26, 2025 initial site assessment are presented in Appendix C. Cross-sections of the subsurface conditions encountered are presented in Appendix D. Approximate cross-section locations are presented on Figure 4.

• Preparation of this report presenting our findings, conclusions, and geotechnical recommendations regarding design and construction of the project.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 2

3 SITE DESCRIPTION

The project will include the development of two sites located in Parker, Colorado. The proposed

Ridgegate Well Site Phase II and Badger Gulch Pipeline (Ridgegate Pipeline) is located north to northeast of the intersection of Ridgegate Parkway and Highnote Avenue. The proposed

Rueter-Hess Well Site Phase II and Pipeline (Rueter-Hess Pipeline) is located southeast of the intersection of Chambers Road and Stroh Road. Topography of the Ridgegate Pipeline varies and ungulates along the proposed alignment, while topography of the Rueter-Hess Pipeline generally decreases from a high point near Stroh Road.

Past storm line construction activities were evident at the Ridgegate Pipeline and grading operations were underway at the Rueter-Hess Pipeline for surrounding residential developments.

Ninyo & Moore performed an initial site assessment on February 26, 2025, and site photographs and descriptions from this visit are presented in Appendix C.

4 PROPOSED CONSTRUCTION

The proposed project will include the design and construction of approximately 6,500 feet of 8-inch diameter raw waterline connecting the Ridgegate Well Site Phase II with the existing

Ridgegate Well House (Hazen and Sawyer, 2025b). The project will also include approximately

500 feet of 8-inch diameter raw waterline connecting the Rueter-Hess Well Site Phase II to existing waterline in Stroh Road (Hazen and Sawyer, 2025a). Proposed construction at each site will also consist of electrical equipment and transformer pads, an aggregated surfaced parking area and access road, a blowdown pond and outflow, and an approximately 12 foot by

20 foot below-grade vault (Hazen and Sawyer, 2025a). The proposed vaults will bear approximately 10 feet bgs.

We assume the proposed waterline will be located between depths of approximately 5 to 7 feet bgs. Utility construction will consist of standard open cut trenching and will not require trenchless operations.

5 FIELD EXPLORATION AND LABORATORY TESTING

On March 7, 10, and 11, 2025, Ninyo & Moore conducted subsurface exploration services along the subject alignments to evaluate the existing subsurface conditions and to collect soil and bedrock samples for visual observation and laboratory testing. The evaluation consisted of drilling, logging, and sampling of 13 small-diameter exploratory borings along the proposed

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 3 alignments. Borings B-1 through B-11 were advanced at the Ridgegate Pipeline and Borings B-

12 and B-13 were performed at the Rueter-Hess Pipeline. The borings were advanced using a truck-mounted drill rig equipped with four-inch diameter, continuous-flight, and solid-stem augers. The borings were advanced to depths ranging from approximately 14.5 to 15 feet bgs.

The approximate boring locations at the Ridgegate Pipeline are presented on Figures 2A through 2D and boring locations at the Rueter-Hess Pipeline are presented on Figure 3. Boring coordinates and ground elevations were measured in the field using a Trimble Model DA2-BT survey unit with a global navigation satellite system output of NAD83 (2011) and referencing

Geoid model GEOID18. Boring coordinates and elevations are included on the boring logs in

Appendix A. Relatively undisturbed and disturbed soil samples were collected at selected intervals. Photographs and descriptions of drilling activities are presented in Appendix C.

The soil and bedrock samples collected from our drilling activities were transported to the Ninyo

& Moore laboratory for geotechnical laboratory analysis. Selected samples were analyzed to evaluate engineering properties including in-situ moisture content and dry density, Atterberg limits, percent fines passing the No. 200 sieve, direct shear, UCS, swell/consolidation, and soil corrosivity characteristics (pH, resistivity, water-soluble sulfates, and chlorides). The results of the in-situ moisture content and dry density tests are presented on the boring logs in Appendix

A. A description of each laboratory test method and the remainder of the laboratory test results are presented in Appendix B.

6 GEOLOGY AND SUBSURFACE CONDITIONS

The geology and subsurface conditions at the site are described in the following sections.

6.1 Geologic Setting

The sites are located in Parker, Colorado, approximately 16 miles east of the Rocky Mountains, within the Colorado Piedmont section of the Great Plains Physiographic Province. The Laramide

Orogeny uplifted the Rocky Mountains during the late Cretaceous and early Tertiary Periods.

Subsequent erosion deposited sediments east of the Rocky Mountains. As a result of regional uplift approximately 5 to 10 million years ago, streams and rivers down-cut and excavated into the Great Plains forming the Colorado Piedmont section (Trimble, 1980).

The surficial geology of the site vicinity, which lies in the Parker Quadrangle, is mapped by

Maberry and Lindvall (1972) as Pleistocene loess and alluvium. Alluvial deposits mapped within the site vicinity are generally considered to be some of the oldest surficial units in the quadrangle. Loess is mapped overlying the older alluvial deposits consisting of silt and clay, Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 4 commonly mixed with fine sand. Based on site observations, Ninyo & Moore believes that most of the near surface native soils within the project vicinities were deposited by alluvial processes.

Bedrock mapped as the Dawson Formation underlies the surficial soils within the site vicinity.

The Dawson Formation in the site vicinity is composed of coarse arkosic sandstone and contains common beds of greenish-gray-to olive green sandy claystone.

6.2 Subsurface Conditions

Our understanding of the subsurface conditions at the project site is based on our field exploration, laboratory testing, review of published geologic maps, historic aerial imagery, and our experience with the general geology of the area. The following sections provide a generalized description of the subsurface materials encountered. More detailed descriptions are presented on the boring logs in Appendix A. Cross Sections A-A’, B-B’ and C-C’, presented in

Appendix D, illustrate changing subsurface conditions between Borings B-1 and B-4, Borings B-

4 and B-10, and Borings B-10 and B-11, respectively. The approximate cross section locations are presented on Figure 4.

6.2.1 Fill Material

Fill was encountered at the surface in Borings B-1, B-6, B-7, B-12, and B-13. The fill materials encountered generally consisted of light to brown, dry to moist, lean clay with various amounts of sand and gravel.

Varying thickness of fill may be encountered along the proposed alignments in areas that were not explored. Information regarding ground preparation, remedial excavation, and the degree of compaction during placement, is unknown to this firm. As a result, the fill material is considered undocumented.

6.2.2 Alluvium

Alluvial deposits were encountered at the surface in Borings B-2, B-3 through B-5, and B-8 through B-11 and below fill in Borings B-1, B-6, B-7, B-12 and B-13. The alluvium extended to the borings’ termination depths of approximately 15 feet bgs in Borings B-1 through B-4, B-6 through B-8, and B-13. In the remaining borings, bedrock was encountered below alluvium and the alluvial deposits extended to depths ranging from approximately 2 to 14 feet bgs.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 5

The alluvium encountered generally consisted of various shades of brown, dry to moist, stiff to hard, fat to lean clay with various amounts of sand as well as loose to medium dense, clayey to relatively clean sand. Alluvium was also found to have varying amounts of gravel.

Based on the results of the laboratory testing, selected samples of the alluvial deposits had in-place moisture contents between approximately 3.3 and 26.4 percent, and dry densities ranging between approximately 80.5 and 120.3 pounds per cubic foot (pcf).

6.2.3 Dawson Formation Bedrock

Bedrock mapped as the Dawson Formation was encountered in Boring B-5 as well as

Borings B-9 through B-12. Bedrock was encountered at depths of approximately 2 to 14 feet bgs and extended to the borings’ termination depths of approximately 14.5 to 15 feet bgs. Bedrock depths encountered in the Borings B-5 and B-9 through B-12 at the

Ridgegate Pipeline correspond to approximate bedrock elevations ranging between approximately 5,975.2 and 6,023.8 feet, while bedrock depth encountered in Boring B-12 at the Rueter-Hess Pipeline corresponds to an approximate bedrock elevation of 6,183.3 feet.

The Dawson Formation generally classified as weakly to strongly cemented, sandstone and moderately soft to hard, claystone.

Based on the results of the laboratory testing, selected samples of the bedrock had in-place moisture contents between approximately 8.2 and 32.1 percent, and dry densities ranging between approximately 86.1 and 102.9 pcf.

6.3 Groundwater

Groundwater was not encountered during drilling, however, groundwater levels can fluctuate due to seasonal variations in precipitation, irrigation, groundwater withdrawal or injection, and other factors. In addition, perched water can develop within higher permeability soils following periods of heavy or prolonged precipitation or at the overburden and bedrock interface. The possibility of groundwater level fluctuations and perched water should be considered when developing the design and construction plans for the project.

7 GEOLOGIC HAZARDS

The following sections describe potential geologic hazards at the site including expansive soils and bedrock and compressible and collapsible soils.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 6

7.1 Expansive Soils and Bedrock

One of the more significant geologic hazards in the Front Range area is the presence of swelling clays in bedrock or surficial deposits. Wetting and drying of bedrock or surficial deposits containing swelling clays can result in expansion and collapse of those units, which can cause major damage to structures. A review of a Colorado Geological Survey map delineating areas based on their relative potential for swelling in the Front Range by Hart (1973-1974) indicates that the soil and bedrock in the site vicinity can exhibit up to high swell potential.

To assess the swell potential of the subsurface soils and bedrock, we collected relatively undisturbed samples for one-dimensional swell/consolidation tests, conducted at surcharge pressures that closely resemble representative field conditions, in accordance with guidelines outlined in the American Society for Testing and Materials (ASTM) D4546.

Selected samples were tested for swell/consolidation against surcharge pressures of 500 pounds per square foot (psf). The selected samples tested exhibited swell potentials up to 3.4 percent. Based on these results, the soils expected to be encountered during project development are considered to have a low to moderate swell potential. Remedial grading recommendations are provided in this report to reduce the impacts of the site soils with low to moderate swell potential.

7.2 Compressible Fill Material

Compression of fill material encountered in our borings could occur due to new loads imparted by equipment pads, aggregate surfaced parking and drive lanes, and other improvements. As a result, remedial grading recommendations are provided in this report to reduce the impacts of the existing fill material.

8 CONCLUSIONS

Based on the results of the subsurface evaluation, laboratory testing, and data analyses, it is our opinion that the proposed project is feasible from a geotechnical standpoint, provided the recommendations presented herein are implemented and appropriate construction practices are followed. Geotechnical design and construction considerations for the proposed project include the following:

• Fill was encountered at the surface in Borings B-1, B-6, B-7, B-12, and B-13. The fill materials encountered generally consisted of light to brown, dry to moist, lean clay with various amounts of sand and gravel. The fill material is considered undocumented.

• Alluvial deposits were encountered at the surface in Borings B-2, B-3 through B-5, and B-8 through B-11 and below fill in Borings B-1, B-6, B-7, B-12 and B-13. The alluvium extended

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 7 to the borings’ termination depths of approximately 15 feet bgs in Borings B-1 through B-4, B-6 through B-8, and B-13. In the remaining borings, bedrock was encountered below alluvium and the alluvial deposits extended to depths ranging from approximately 2 to 14 feet bgs. The alluvium encountered generally consisted of various shades of brown, dry to moist, stiff to hard, fat to lean clay with various amounts of sand as well as loose to medium dense, clayey to relatively clean sand. Alluvium was also found to have varying amounts of gravel.

• Bedrock mapped as the Dawson Formation was encountered in Boring B-5 as well as Borings B-9 through B-12.

o Bedrock was encountered at depths of approximately 2 to 14 feet bgs and extended to the borings’ termination depths of approximately 14.5 to 15 feet bgs. Bedrock depths encountered in the Borings B-5 and B-8 through B-11 at the Ridgegate Pipeline correspond to approximate bedrock elevations ranging between approximately 5,975.2 and 6,023.8 feet, while bedrock depth encountered in Boring B-12 at the Rueter-Hess Pipeline corresponds to an approximate bedrock elevation of 6,183.3 feet.

o The Dawson Formation generally classified as weakly to strongly cemented, sandstone and moderately soft to hard, claystone.

• Groundwater was not encountered during drilling; however, perched water can develop within higher permeability soils following periods of heavy or prolonged precipitation or at the overburden and bedrock interface. The possibility of groundwater level fluctuations and perched water should be considered when developing the design and construction plans for the project.

• Our evaluation of the excavation characteristics of the on-site materials is based on the results of the subsurface exploration, our site observations, and our experience with similar materials. The on-site surface and near-surface soils may generally be excavated with moderate- to heavy-duty earthmoving or excavation equipment in good operating condition.

Excavation of Dawson Formation bedrock may be difficult and heavy-duty excavating equipment and the use of special equipment (i.e. single-shank rippers) or other rock breaking means may be needed.

• Remedial grading should be performed below site improvements and aggregate surfaced roadways as described in Section 9.2.3.

• Based on our laboratory data and our experience with similar materials at adjacent sites, the sulfate content of the tested soils presents a negligible risk of sulfate attack to concrete.

• Based on our laboratory data and our experience with similar materials at adjacent sites, the soils at the site may present severe corrosion potential to ferrous metals. Therefore, special consideration should be given to the use of heavy gauge, corrosion-protected, underground steel pipe, if any are planned. As an alternative, plastic pipe could be considered. A corrosion specialist should be consulted for further recommendations.

9 RECOMMENDATIONS

Based on our understanding of the proposed construction, the following sections present our geotechnical recommendations for design and construction of the proposed pipeline alignments.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 8

Ninyo & Moore should be notified and given an opportunity to reevaluate our recommendations if the proposed construction is altered.

9.1 Demolition

The subject project will include partial demolition of existing pavements at utility crossings and potentially abandoned utilities. Remnants from the demolition activities should be removed from the site. Demolition of the existing improvements should include rerouting, removal, or in-place abandonment of underground utilities. Utilities should be adequately capped or rerouted at the in accordance with the requirements of the governing authorities and the recommendations of the geotechnical consultant at the time of demolition. Abandoned underground utility pipes under the proposed pipeline alignments should be removed from the site, or, if the pipes are left in place, they should be filled with flowable fill, such as grout or controlled low strength material

(CLSM). The contractor should take adequate precautions to reduce the potential for damage to existing utilities that are to remain in service.

Though not anticipated, if existing underground utility pipes will be left in place under the proposed pipeline alignment, these should be discussed on a case-by-case basis with the project Geotechnical Engineer and the design team. Additional site-specific evaluations and data analysis will be needed to address these conditions.

9.2 Earthwork

The following sections provide our earthwork recommendations for this project. In general, PWSD and/or project specific specifications are expected to apply unless otherwise noted.

9.2.1 Excavations

Our evaluation of the excavation characteristics of the on-site materials is based on the results of the subsurface exploration, our site observations, and our experience with similar materials. The site soils should generally be excavated with medium- to heavy-duty earthmoving or excavation equipment in good operating condition. Excavation of Dawson

Formation bedrock may be difficult and heavy-duty excavating equipment and the use of special equipment (i.e. single-shank rippers) or other rock breaking means may be needed.

The excavation difficulties and wear and tear on the equipment will increase as the cementation of the sandstone bedrock layers and lenses increase. The earthwork contractor should anticipate encountering difficult excavation conditions, including increased wear and tear on equipment, within the bedrock at this site.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 9

Equipment and procedures that do not cause significant disturbance to the excavation bottoms should be used. Excavators and backhoes with buckets having large claws to loosen the subgrade material should be avoided when excavating the bottom 6 to 12 inches of excavations as such equipment may disturb the excavation bases.

If the excavation subgrade becomes disturbed, it should be scarified, moisture-conditioned, and compacted or stabilized before placing pipe bedding material.

Groundwater was not encountered within the borings advanced for this study, and therefore is not anticipated to be a constraint during site excavations. However, groundwater levels will fluctuate due to seasonal variations from precipitation, irrigation, groundwater withdrawal or injection, and other factors. In addition, perched groundwater may develop or be encountered within higher permeability alluvium or at the overburden soil and bedrock interface, especially after prolonged precipitation or snow-melt events.

The contractor should provide safely sloped excavations or an adequately constructed and braced shoring system, in compliance with Occupational Safety and Health Administration

(OSHA, 2005) guidelines, for employees working in an excavation that may expose employees to the danger of moving ground. If material is stored or equipment is operated near an excavation, stronger shoring should be used to resist the extra pressure due to superimposed loads.

9.2.2 Temporary Excavations and Shoring

The near-surface soils encountered in the exploratory borings were comprised predominantly of interlayered cohesive and cohesionless soils. In our opinion, temporary slopes in the near-surface fill and alluvial soils should be stable at a slope ratio of approximately 1:1 (horizontal[H]:vertical[V]) or flatter. Temporary slopes should be evaluated in the field by a competent person in accordance with OSHA guidelines. The near-surface soils are considered Type B soils, as evaluated by OSHA. Temporary excavations should conform to OSHA regulations.

The formational materials (Dawson Formation bedrock) at the site should generally be considered Type A soil, where inclinations of 3/4(H):1V or flatter can be considered.

The contractor should provide safely sloped excavations or an adequately constructed and braced shoring system, in compliance with OSHA regulations, for employees working in excavations that may expose them to the danger of moving ground. Reducing the inclination of the sidewalls of the excavations, where feasible, may increase the stability of

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 10 the excavations. If construction or earth material is stored, or equipment is operated near an excavation, flatter slope geometry or shoring should be used during construction.

In areas where the proximity of the existing improvements limit the construction of temporary slopes, design and construction of temporary shoring or bracing systems will be needed to complete the pipeline excavations. Shoring or bracing systems should be designed for the contractor by a professional engineer registered in the State of Colorado.

In addition to lateral earth pressures, shoring design should include surcharge loads exerted by adjacent existing structure foundations, construction equipment, construction traffic, material stockpiles, etc. located within a 1:1 (H:V) plane extending upward from the toe of the excavation. Shoring design should discuss the anticipated top deflection of the shoring components. The deflection of the temporary shoring components will translate into a triangular-shaped wedge of settlement perpendicular to the shoring. Depending on the anticipated top deflection of the shoring components, settlement of buried utility lines and exterior flatwork located within close proximity (approximately 10 feet or more) of the temporary shoring should be anticipated.

9.2.3 Site and Remedial Grading

Prior to grading, the ground surface in proposed improvement areas should be cleared of any surface obstructions, debris, asphalt, topsoil, organics (including vegetation), and other deleterious degradable and decomposable material. Materials generated from clearing operations should be removed from the site and disposed of at a legal dump site.

Obstructions that extend below finish grade, if present, should be removed and the resulting holes filled with compacted soil or cement slurry, in accordance with the recommendations of the geotechnical consultant.

Equipment pads may be placed on 24 or more inches of moisture conditioned and compacted engineered fill, with 12 or more inches of moisture-conditioned and compacted engineered fill below the turn-down edges. The limits of this moisture-conditioned and engineered fill should extend laterally 12 or more inches beyond the turndown edges.

The below-grade vaults may be placed on 12 or more inches of moisture conditioned and compacted engineered fill. The limits of this moisture-conditioned and engineered fill should extend laterally 12 or more inches beyond the vault.

Aggregate surfaced parking and roadways may be placed on 12 or more inches of moisture conditioned and compacted engineered fill.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 11

Engineered fill and aggregate surfaced roadways should be moisture-conditioned, placed, and compacted in accordance with the recommendations set forth in Section 9.2.6.

Depending on the time of year construction occurs, unstable subgrade conditions may be encountered during general construction operations. Subgrade conditions should be observed by Ninyo & Moore during construction. Where unstable subgrade conditions develop, stabilization measures will need to be employed. Stabilization methods should be provided by the grading contractor, as needed, and may include the use of geogrids, geotextiles, pushing oversized rock into the subgrade, and chemical stabilization. The subgrade stabilization methods proposed should be discussed with the geotechnical engineer prior to implementation. The stabilization method selected should consider the effects of such stabilization on future utility installation and/or repair work.

The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations, and care should be taken to maintain the moisture content of the subgrade prior to construction of equipment pads, vaults, aggregate surfaced roads, or other improvements. If the subgrade should become desiccated, saturated, frozen, or disturbed, the affected material should be removed or these materials should be scarified, moisture conditioned, and recompacted prior to equipment pad or structure construction.

The geotechnical consultant should be retained to observe the remedial excavations, and the elevations of the excavation bottoms should be surveyed by the project civil engineer prior to fill placement.

9.2.4 Utility Installation

The contractor should take care to achieve and maintain adequate compaction of the backfill soils around valve risers and other vertical pipeline elements where settlements commonly are observed. Additionally, excavation bottoms should be firm and unyielding prior to pipe bedding material and pipe placement. Use of CLSM should be considered in lieu of compacted soil backfill for areas with low tolerances for surface settlements. This would also reduce the permeability of the utility trenches.

Trench width, pipe bedding materials, placement, and compaction should meet the specifications of the pipe manufacturer and applicable municipal standards.

Notwithstanding, the trench width should be a minimum of 2 feet and 4 inches wide to allow for bedding placement and embedment placement (PWSD, 2017) and trench subgrade conditions should be observed by Ninyo & Moore prior to bedding and pipe placement.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 12

Bedding shall be at least 6 inches thick below the pipe and 12 inches above the pipe

(PWSD, 2017). PWSD standards specify that bedding material should consist of squeegee sand for pipes with diameters less than 20 inches. Squeegee should have 100 percent passing the No. 3/8-inch sieve and between 0 and 3 percent passing the No. 200 sieve

(PWSD, 2016). Materials proposed for use as pipe bedding should be tested for suitability prior to use.

Trench backfill, regardless of location, should be moisture-conditioned, placed, and compacted in accordance with the recommendations set forth in Section 9.2.6.

Special care should be exercised to avoid damaging the pipe or other structures during the compaction of the backfill. In addition, the underside (or haunches) of the buried pipe should be supported on bedding material that is compacted as described above. This may need to be performed with placement by hand or small-scale compaction equipment.

Surface drainage should direct water away from utility trench alignments. Where topography, site constraints or other factors limit or preclude adequate surface drainage or as added precaution, the granular bedding materials should be surrounded by non-woven filter fabric (e.g., TenCate Mirafi® 140N or the equivalent) to reduce migration of fines into the bedding which can result in severe, isolated settlements.

Development of site grading plans should consider the subsurface transfer of water in utility trenches and the pipe bedding. Coarse-grained pipe bedding materials with little or no fines can function as efficient conduits for re-distribution of natural and applied waters in the subsurface. Cut-off walls in utility trenches or other water-stopping measures should be considered to reduce the rates and volumes of water transmitted along utility alignments and toward buildings, pavements and other structures where excessive wetting of the underlying soils will be damaging. Incorporation of water cut-offs and/or outlet mechanisms for saturated bedding materials into development plans could be beneficial to the project.

These measures also will reduce the risk of loss of fine-grained backfill soils into the bedding material with resultant surface settlement. Cut-off walls could be constructed using

2 or more feet thick clay earth material or 1 or more feet thick CLSM extending 12 or more inches into undisturbed soil on each side and bottom of the trench, and extending 2 or feet above the utility pipe. The placement locations for such cut off walls should be decided by the Project Civil Engineer based on susceptibility of the utility bedding to water intrusion.

Cut off walls could be considered for the Ridgegate Pipeline at Stations 40+00, 44+75, 50+00, and 64+00 to reduce velocity of subsurface water transmitted along the utility alignments (Hazen and Sawyer, 2025c).

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9.2.5 Re-Use of Site Soils

Site soils generated from on-site excavation activities that are free of deleterious materials and organic matter, and do not contain particles larger than 3 inches in nominal diameter, can generally be used as engineered fill and trench backfill.

If excavated formational materials are to be used as engineered fill, significant processing

(i.e. disking in an open area) of these materials will be needed to break-down the bedrock to 3-inch or smaller fragments prior to placement.

Fragments of rock, cobbles, and inert construction debris (e.g., concrete or asphalt) larger than 3 inches in diameter may be incorporated below the anticipated utility installation depths. A Geotechnical Engineer should be consulted regarding appropriate recommendations for usage of such materials on a case-by-case basis when such materials have been observed during earthwork. Care should be taken to avoid nesting of oversized materials during placement. Recommendations provided in Section 203 of the current the Colorado Department of Transportation (CDOT) Standard Specifications for

Road and Bridge Construction should be followed during the placement of oversized material.

Additional evaluation and laboratory testing should be performed during earthwork activities to better evaluate the suitability of the on-site soils for re-use as backfill. An evaluation of the potential for contamination by hazardous materials was beyond the scope of this study and the possibility of restrictions on full material importation and re-use due to environmental factors was not studied.

9.2.6 Fill Placement and Compaction

Fine-grained, cohesive soils (CL or CH) used as engineered fill for site improvements or trench backfill should be moisture-conditioned to moisture contents between optimum and 3 percent over optimum moisture content. Granular soils (SC, SP-SC, SP, and imported soils) used as engineered fill should be moisture-conditioned to moisture contents within 2 percent of optimum moisture content. Fill should be compacted to a relative compaction of

95 percent or more as evaluated by ASTM D698.

The fill should be compacted by appropriate mechanical methods. Lift thickness for fill will be dependent upon the type of compaction equipment utilized. Backfill should be placed in lifts not exceeding 8 inches in loose thickness in areas compacted by other-than hand

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 14 operated machines. Backfill should be placed in lifts not exceeding 6 inches in loose thickness in areas compacted by hand operated machines.

Fill materials should not be placed, worked, rolled while they are frozen, thawing, or during poor/inclement weather conditions.

Compaction areas should be kept separate, and no lift should be covered by another until relative compaction and moisture content within the recommended ranges are obtained.

9.2.7 Imported Soil

Imported soil to be used as trench backfill should be free of organic material and other deleterious materials should have a very low to low expansion potential (less than 1 percent against a surcharge pressure of 500 psf when remolded at optimum moisture content).

Imported fill should have between approximately 15 and 40 percent passing the No. 200 sieve and should have a plasticity index of 15 or less.

Import soil in contact with ferrous metals should have low corrosion potential. Import material in contact with concrete should have soluble sulfate content less than 0.1 percent.

We further recommend that proposed import soils be evaluated by the project’s geotechnical consultant at the borrow source for its suitability prior to importation to the project site. Import soil should be moisture-conditioned and placed and compacted in accordance with the recommendations set forth in Section 9.2.6.

9.2.8 Controlled Low Strength Material

PWSD may require use of flowable fill in roadway cuts. Use of CLSM should be considered in lieu of compacted fill for areas with low tolerances for surface settlements, for excavations that extend below the groundwater table and in areas with difficult access for compaction equipment. CLSM consists of a fluid, workable mixture of aggregate, Portland cement, and water. CLSM should be placed in lifts of 5 feet or less with a 24-hour or more curing period between each lift.

The use of CLSM has several advantages:

• A narrower excavation can be used where shoring is present, thereby minimizing the quantity of soil to be excavated and possibly reducing disturbance to the nearby traffic;

• Compaction requirements do not apply;

• There is less risk of damage to improvements since little compaction is needed to place

CLSM;

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 15

• CLSM can be batched to flow into irregularities in excavation bottoms and walls; and

• The number of workers needed inside the trench excavation is reduced.

The CLSM mix design should be submitted for review prior to placement. The 28-day strength of the material should be no less than 50 pounds per square inch (psi) and no more than 150 psi. CLSM should be observed and tested by the geotechnical consultant.

9.3 Equipment Pad Foundations

The at-grade equipment pads may be supported on raft slabs with turndown edges. Soils underlying foundations should be moisture conditioned, and compacted in accordance with the recommendations presented in Section 9.2.3 of this report.

The at-grade equipment pads and below-grade vaults may be designed using an allowable bearing pressure of 2,000 psf for static conditions. The bearing capacity may be increased by one-third when considering loads of short duration such as wind or seismic forces. Foundations may be designed using an ultimate coefficient of friction of 0.35, where the total frictional resistance equals the coefficient of friction times the dead load.

For slab design, a design modulus of subgrade reaction, k, of 125 pounds per square inch per inch of deflection (psi/inch) may be used for the subgrade soils in evaluating deflections for the slab. This value is based on a unit square foot area and can be adjusted for large slabs.

Adjusted values of the modulus of subgrade reaction, Kb, can be obtained from the following equation for slabs of various widths (where B is the width of the foundation in feet):

Kb = k[(B+1)/2B]2 (psi/inch)

Perimeter slab turndowns should extend 36 or more inches below the lowest exterior finished grade (for frost protection). The total and differential settlements under static loading conditions corresponding to the allowable bearing capacity are estimated to be approximately 1-inch and

½-inch, respectively.

Joints should be observed periodically, particularly during the first several years after construction. Utility penetrations should be provided with positive bond breaks that allow 2 or more inches of differential movement. The Structural Engineer should account for this potential differential settlement through use of sufficient control joints, appropriate reinforcing, or other means.

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 16

9.4 Modulus of Soil Reaction, E’

The modulus of soil reaction (E’) is an empirical value used to evaluate initial flexible pipe horizontal deflection caused by the weight of the backfill over the pipe, and is related to the nature and degree of compaction of the pipe bedding and embedment material and the stiffness of the native soils (Bureau of Reclamation, 1977). The depth of cover is assumed to be approximately 5 to 7 feet over the pipe crown, and the soils encountered at these depths generally consisted of interlayered clay and sand. For flexible PVC pipe, Ninyo & Moore recommends an E’ value of 1,000 psi. This value assumes pipe embedment is compacted to a relative compaction of 95 percent or more as evaluated by ASTM D698.

9.5 Thrust Restraint

Thrust forces will develop at the bends in the waterline when alignment changes occur. Thrust blocks or restrained joints can be considered for thrust restraint.

Assuming a negligible amount of movement at the pipe bends, passive earth pressures listed in the following table for fine-grained soil, coarse-grained soil, and bedrock along the proposed alignment may be used for thrust block design.

Table 1 – Passive Pressures for Thrust Block Design

Soil Condition Passive Pressure (pcf)

Fine-grained Soil (CH, CL) 100

Coarse-grained Soil (SC, SP-SC, SP) 150

Dawson Formation Bedrock 300

If restrained joints are considered, coefficient of friction (μ) between the soil and the pipe can be used for restrained joint design. The μ will depend on the type of pipe, or fitting, and the material surrounding the pipe. Ninyo & Moore suggests a μ ranging from 0.2 to 0.3 depending on pipe smoothness. However, the pipe manufacturer should be consulted for this parameter.

9.6 Lateral Earth Pressures

Earth pressures are used to compute the lateral forces acting on below-grade walls. These pressures can be classified as at-rest, active, and passive. The direction and magnitude of the soil/wall movement just before failure affects the resulting pressure condition. At-rest conditions exist when there is no movement, such as for a restrained wall. Active stresses are exerted when the wall moves out and the soil moves toward the wall away from the soil mass, thereby

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 17 mobilizing the shear strength of the soil. Passive stresses exist when the wall moves toward the soil mass.

The recommended equivalent fluid pressures in Table 2 assumes an angle of internal friction (φ) of 28 degrees and a unit weight of 120 pcf for engineered fill. The values listed below are for static conditions and horizontal backfill.

Table 2 – Recommended Lateral Earth Pressures

Soil Condition Active Pressure (pcf)

At-rest Pressure (pcf)

Passive Pressure (pcf)

Engineered Fill (Horizontal) 40 64 330

The use of heavy compaction equipment adjacent to below-grade walls could result in lateral earth pressures well in excess of those predicted in Table 2. We recommend that the upper 24 inches of soil that is not protected by pavement or a concrete slab, be neglected when calculating passive resistance. This zone, where applicable, should be backfilled with cohesive soils to minimize infiltration of surface water into the backfill. For passive resistance, a value of

330 psf per foot of depth may be used, up to a maximum passive pressure of 2,000 psf. This is assuming the ground is horizontal for a distance of 10 feet or three times the height generating the passive pressure. For frictional resistance to lateral loads, we recommend that an ultimate coefficient of friction of 0.35 be used between site soils and concrete.

Measures should be taken so that moisture does not build up behind the below-grade walls.

Below-grade walls should be dampproofed or waterproofed in accordance with the recommendations of the project architect and structural engineer. To reduce the potential for water- and sulfate/salt-related damage to the foundation walls and efflorescent development, particular care should be taken in selection of the appropriate type of waterproofing material to be utilized and in the application of this material.

9.7 Pavement Restoration

Where pavement patching is needed over the proposed waterline, new asphaltic concrete pavement should match or exceed the existing section, but should be no less than 6 inches thick (PWSD, 2017). Aggregate base course (ABC) encountered during waterline installation at existing roadways should match or exceed the existing thickness.

The AC pavement shall consist of a bituminous plant mix composed of a mixture of high-quality aggregate and bituminous material, which meets the requirements of a job-mix formula and

Ninyo & Moore | Denver Basin Well Sites Phase II and Pipeline Project, Parker, Colorado | 503147001 R01 | June 30, 2025 18

Town of Parker requirements established by a qualified engineer. The asphalt material used should be based on a SuperPave Gyratory Design Revolution of 75. Lower lifts should be constructed using an asphalt mix Grading S or SG and asphalt cement binder grade PG 58-28.

The top lift should be constructed using an asphalt mix Grading SX and asphalt cement binder grade PG 64-22. Pavement layer thickness should be between 2 and 4 inches for the lower lifts and 1½ to 2 inches for the top lift. The Geotechnical Engineer should be retained to review the proposed pavement mix designs, grading, and lift thicknesses prior to construction.

Subgrade soils below AC pavement and ABC should be firm and unyielding. Subgrades that are exposed to the elements may need additional moisture conditioning and compaction, prior to pavement or ABC placement.

The ABC material placed beneath pavements should meet the criteria of CDOT Class 6 aggregate base. Requirements for CDOT Class 6 aggregate base can be found in Section 703 of the current CDOT Standards and Specifications for Road and Bridge Construction (2023).

The ABC should be moisture-conditioned to moisture contents within 2 percent of optimum moisture content and compacted to a relative compaction of 95 percent or more as evaluated by

ASTM D698.

9.8 Aggregate Surfaced Lot and Roadway

We understand that aggregate is being placed at the well sites and for the access roadways.

Lots and roadways constructed with aggregate are subject to seasonal moisture fluctuations, which affects the stability and strength of the subgrade soils.

Specific traffic loadings for these areas were not available at the time of this report preparation.

However, we understand the aggregate surfaced roadway and equipment lot will service standard automobile traffic, as well as occasional use by heavy vehicles such fire trucks and dump trucks for maintenance and emergency access. As a result, an equivalent 18-kip single-axle (ESAL) load value of 10,000 for a 20-year design life was assumed. This estiamte includes occasional fire truck and dump truck traffic. The associated estimated traffic volume and ESAL calculation is presented in Appendix E.

The ABC structural section was developed in general accordance with the AASHTO…