Geotechnical Report - Batch 3 Traffic Signal Rebuilds - Final_stamped_090925.pdf

Geotechnical Engineering Report Centennial Traffic Signal Replacement Centennial, Colorado

March 2025

David S. Crotsley, PG Senior Geologist

Will N. Brandenberger, PE, PG Geotechnical Engineer

HDR

1670 Broadway, Suite 3400 Denver, CO 80202

09/09/2025

Geotechnical Engineering Report Centennial, CO

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Contents

1 Purpose and Scope

2 Project Description

3 Information Search and Findings

3.1 Physiographic Location

3.2 Soils

3.3 Geology

4 Subsurface Exploration

5 Geotechnical Evaluation

5.1 Foundation Recommendations

5.2 Construction Considerations

6 Limitations

7 References

Appendices

Appendix A: Field Boring Logs

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1 Purpose and Scope This Geotechnical Engineering Report (Report) presents the results of HDR Engineering, Inc.’s (HDR’s) geotechnical evaluation for the proposed Span Wire to Mast Arm Traffic Signal Rebuilds for the intersections of E. Arapahoe Road and S Franklin Street (Site 1), E Dry Creek Road and South Franklin Street (Site 2), and E Dry Creek Road and S Clarkson Street (Site 3) in Centennial, Colorado. The work completed has been performed under HDR’s Scope of Work agreement with the City of Centennial (Client), executed on May 17, 2024.

The following tasks were performed as part of the geotechnical evaluation and development of this report:

• Perform a desktop review of existing information for the project area,

• Perform a site visit to evaluate the on site conditions and oversee geotechnical borings,

• Log on site soils,

• Prepare this report.

2 Project Description The intersections at Sites 1 through 3 identified above are currently constructed span wire traffic signals. The goal of this project is to provide a more modern, aesthetically pleasing design at these intersections, which includes installation of mast arms. The traffic signals are anticipated to have mast arms from 30 to 55 feet in length and the foundation design will be following City of Centennial, Traffic Signals, Signal Pole Foundation design. Site grading and earthwork is anticipated to be minimal with any potential cut or fill less than two (2) feet in thickness.

3 Information Search and Findings

3.1 Physiographic Location

The project site is located within the Colorado Piedmont Subprovince of the Great Plains Physiographic Province (Finneman and Johnson, 1946). In general, the province extends into eastern Colorado and is characterized by flat to rolling prairie with scattered hills and bluffs.

The Great Plains are bordered by the Rocky Mountains to the west, and the Central Lowlands to the east.

3.2 Soils

A review of the local soils per the United States Department of Agriculture’s (USDA) Natural Resources Conservation Service (NRCS) web soil survey, the project area contains approximately eight (8) soil types, with two (2) main soil types mapped at the proposed intersections:

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• Fondis Silt Loam, 1-3% slopes – Well drained, with typical depth to water exceeding 80 inches.

• Renohill-Buick Loams, 3-9% – Found within drainageways, well drained, depth to water exceeding 80 inches.

3.3 Geology

According to the Colorado Geologic Map Data from the United States Geological Survey, the project site is underlain by the Piney Creek Alluvium (Qp) of Holocene Age and Loess of the Holocene and Pleistocene Ages (Ql) (Maberry and Lindvall, 1977).

The Piney Creek Alluvium consists of a light gray to light brown clay, silt, sand and gravel. The formation is underlain by older gravelly alluvium or bedrock, with some colluvium on hillsides.

Generally occupying valley floors can also be found along low terraces adjacent to modern stream channels. Commonly 5-15 feet thick.

The Loess formation is typically light brown to tan sandy silt deposited by wind. Upper part may contain small amounts of clay and silty clay. It occurs on upland surfaces and exhibit s columnar structure. Can be slightly calcareous and hard when dry to slightly sticky when wet.

The formation is generally less than 10 feet thick.

4 Subsurface Exploration For the subsurface exploration at the above referenced intersections, two (2) structure borings were performed approximately in opposite corners of the intersection. Borings were designated as FA-1 and -2 for the Franklin and Arapahoe intersection, FD-1 and -2 for the Franklin and Dry Creek Intersection, and DC-1 and -2 for the Dry Creek and Clarkson Intersection. Drilling was performed by Vine Laboratories of Commerce City, CO, and was performed on September 5 and October 16, 2024. Interval Standard Penetration Testing (SPT) was performed in the borings, with solid stem auger advance to a total depth of 25 feet below existing ground surface (bgs)in each boring. Boring locations are shown in Figures 1 and 2, below. A copy of the Field Boring Logs are included in Appendix A.

Borings for Site 1, FA-1 and FA-2, generally encountered clayey soils overlying a poorly graded sand at depth. Boring FA-1 also had a near surface fill seam consisting of sand with some silt.

The cohesive soils were medium stiff and the cohesionless soils were medium dense.

Groundwater was not encountered in the borings.

Borings performed for Site 2, FD-1 and FD-2, generally encountered clayey soils with some silt and little sand overlying a stiff to very stiff clay to depth. Soil consistencies were stiff to very stiff, with medium dense cohesionless soils. A seam of very soft gavels was encountered in Boring FD-1; however, the layer appears to be consistent with bedding soils and is therefore not considered representative of the in situ soils. Water was encountered in this layer, but was not present in FD-2.

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Borings performed for Site 3, DC-1 and DC-2, generally encountered clayey soils to depth.

Boring DC-1 had a layer from 10.0-17.0 feet below ground surface (bgs) of clayey sand. The clays were typically medium stiff to stiff overall, and the sand layer was medium dense. Neither boring encountered groundwater.

Figure 1 – Boring Locations for Clarkson and Franklin Intersections on Dry Creek

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Figure 2 – Boring Locations for Franklin and Arapahoe Intersection

5 Geotechnical Evaluation

5.1 Foundation Recommendations

The proposed construction includes the installation of Traffic Signal Poles at the corners of the intersections, with the mast arm lengths between 30 and 55 feet. Therefore, the construction is anticipated to be per City of Centennial Standard SD-24.7 – Signal Pole Foundation detail.

Recommendations of caisson diameter and embedment based on the standard detail are provided below for the different mast arm lengths.

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Table 1: Caisson Diameter and Embedment

Relevant Intersection and Number of Mast

Arms (#)

Mast Arm length (ft) Caisson Diameter (in) Caisson Embedment (ft)

E. Arapahoe Rd & S. Franklin St (2) 30 36 15

E. Dry Creek Rd & S. Franklin St (1) / E. Dry Creek Rd & S. Clarkson St (2)

40 36 15

E. Dry Creek Rd & S. Franklin St (1) / E. Dry Creek Rd & S. Clarkson St (1)

45 36 15

E. Arapahoe Rd & S. Franklin St (2) / E. Dry Creek Rd & S. Franklin St (2) / E. Dry Creek Rd & S. Clarkson St (1)

55 42 20

Based on the notes on the Standard Detail, the minimum required soil conditions shall be 110 pounds per cubic foot (pcf) unit weight, a minimum of 750 pounds per square foot (psf) medium stiff cohesive soils or 30 degrees for medium dense cohesionless soils. Based on the corrected N-values from the borings, the soils encountered at the site meet or exceed these minimum requirements, therefore the standard caisson dimensions are suitable for construction provided the other conditions are met from the Construction Notes on SD-24.7, as identified in Figure 3, below.

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Figure 3 – Centennial Standard Drawing SD-24.7

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5.2 Construction Considerations

Foundation drilling should be inspected by the City’s Traffic Engineer to confirm soil conditions are consistent with those encountered during the drilling program. Deviations from those conditions should be brought to the attention of HDR, to verify if the current design is suitable.

Concrete used in the caisson shall conform to CDOT Class BZ with a 28-Day minimum compressive strength of 4,000 pounds per square inch (psi) and shall be vibrated.

During construction, the Contractor shall maintain a safe and clean project site. They shall grade the project area and excavation to maintain sufficiently dry conditions, including limiting surface runoff around the caisson area. No bedrock is anticipated during caisson drilling;

however, if bedrock is encountered, blasting is not permitted to excavate the material.

6 Limitations The recommendations presented in this report have been based upon the available geotechnical information. The borings only depict the soil conditions at the specific locations at which they were drilled. The soil conditions at other locations may differ significantly from those occurring at the boring locations. Any revisions to the plans for the proposed structures, made after the date of this report, should be brought to the attention of the Geotechnical Engineer, so that a determination whether any changes in foundation recommendations are necessary. If deviations from the noted foundation conditions are encountered during construction, they should also be brought to the attention of the Geotechnical Engineer.

We have endeavored to complete the services identified herein in a manner consistent with that care and skill ordinarily exercised by members of the profession currently practicing in the same locality and under similar conditions as this project. No other representation, express or implied, is included or intended, and no warranty or guarantee is included or intended in this report, or other instrument of service.

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7 References

• Natural Resource Conservation Service, October 21, 2024. “Web Soil Survey”. United

States Department of Agriculture, http://websoilsurvey.sc.egov.usds.gov

• Mayberry, John O. and Lindvall, Robert M., “Geologic Map of the Highlands Rach Quadrangle, Arapahoe and Douglas Counties, Colorado”, 1977

• City of Centennial, “Roadway Design & Construction Standards”, 2024

• Colorado Department of Transportation, “Standard Specifications”, 2023 http://websoilsurvey.sc.egov.usds.gov/

Appendix A

Field Boring Logs