Geotechnical Investigation Report Horse Mesa Bridge.pdf

To: Tomasz Kubicz, Project Manager, CFLHD, Lakewood CO

From: Brendan McGarity, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Devin T. Dixon, Acting Lead Geotechnical Engineer, CFLHD, Lakewood CO

Subject: Review of CSE Geotechnical Investigation Report and Additional Geotechnical

Recommendations for Horse Mesa Bridge, AZ FTBR SALT 80(1)

1.0 PURPOSE

This geotechnical memorandum presents a review of the Geotechnical Investigation Report provided by Copper State Engineering, Inc., additional geotechnical recommendations from

FHWA CFL, and a summary of shallow foundation design parameters for the Horse Mesa Bridge project. Largely, the CSE report is recommended to be used to inform the design of the project and should be referred to for further information on recommendations and construction considerations.

2.0 CSE GEOTECHNICAL INVESTIGATION REPORT

A Geotechnical Investigation Report compiled by Copper State Engineering Inc. on August 6, 2018, was provided to FHWA CFL to aid in the design of the Horse Mesa Bridge project. The report includes results of the subsurface exploration, laboratory testing results, and design recommendations. The report provides foundation, lateral earth pressures, and drainage recommendations, along with requirements and procedures for site preparation, grading, excavation, and material suitability.

All assumptions, discussions, and recommendations presented in the CSE Geotechnical

Investigation Report, apart from the items listed in section 3.0 of this memorandum, have been reviewed and are recommended to be incorporated into design.

3.0 FHWA CFL GEOTECHNICAL RECOMMENDATIONS

This section presents recommendations that deviate from the CSE report, shrink/swell parameters, and a summary of shallow foundation design recommendations.

Technical Memorandum

Date: July 18, 2022

3.1 Deviations from CSE Report

3.1.1 Cut Slopes

Cut slopes are anticipated for proposed structures and road construction. Max cut slope recommendations of 3H:1V in soil, and 1H:3V in weathered to intact bedrock, detailed in section

11.0 of the CSE report, should be adhered to with the following exceptions. In the preliminary

95% design plans dated June 2022, limited sections of cut propose slope angles ranging from

1V:1.5H to 1.5V:1H within the station ranges of 100+60 to 100+80 and 101+82 to 102+07. These oversteepened slopes may be incorporated into design as they are determined to be low risk due to the vertical height being constrained to approximately 5 feet or less, and the lateral extent limited to approximately 30 feet or less. Any slumping or failure of these slopes overtime is low consequence and would fall under the category of normal maintenance.

3.1.2 Material Requirements, Site Preparation, & Grading

Engineered fill specifications and site preparation and grading recommendations are presented in section 9.1 and 10.0 of the CSE report, respectively. For these items, instead, defer to the project plan set, standard specifications within the FP-14, and project specific SCRs.

3.1.3 Lateral Earth Pressures

Lateral earth pressures recommended for design are presented in section 8.2 of the CSE report.

CSE suggests an at-rest pressure of 60 pcf and an active pressure of 35 pcf for structural backfill.

Active pressure of 35 pcf and at rest pressure of 50 pcf were used in design of Horse Mesa Bridge, as shown on sheet number S2 in the preliminary 95% design plans dated June 2022. This reduced at rest pressure of 50 pcf is determined reasonable and representative and may be used in final calculations and design.

3.2 Shrink/Swell Recommendations

Based on the CSE report, on-site soils are assumed to be non-plastic and consist of silty sands with varying amounts of gravel and fines. It is estimated these soils will have a 10% shrink percentage, corresponding to a shrink/swell factor of 0.90. The recommended shrink/swell factors are based on a combination of standard tabled values for common materials in the FLH Technical Guidance

Manual (2006) and experience with other CFLHD projects in similar materials.

3.3 Shallow Foundation Design

The following subsection presents a summary of shallow foundation recommendations from the

CSE report. For further information on foundation recommendations and seismic design considerations, refer to sections 8.1 and 8.3 of the CSE report, respectively.

Additionally, special care should be taken to review compressional wave velocities (section 5.0 of the CSE report) and excavation recommendations (section 11.0 of the CSE report) to determine required equipment for ripping bedrock during structure excavation. CSE anticipates hard to very hard ripping three to five feet below the ground surface.

Table 1:- Summary of Spread Footing Design Information

Abutment Location Base of Spread Footing Elevation

(ft)1,3 Allowable Bearing Pressures2

Abutment 1 (South) 1812.33

3,000 psf (dense soil/weathered rock)

10,000 psf (less weathered rock)

Abutment 2 (North) 1806.51

3,000 psf (dense soil/weathered rock)

10,000 psf (less weathered rock)

1Spread footing elevations determined by FHWA 2Allowable bearing pressures recommended by CSE. Footings are anticipated to be founded on less weathered rock based on

FHWA elevations and CSE boring logs (10,000 psf).

3In the case of over-excavation due to variable surface or if bedrock is encountered below specified depths, refer to the “Alternate

Footing Embedment Detail” on sheet S8 of the plan set.

Table 2:- Resistance Factors for Abutment Design

Limit State

Resistance Factors

Bearing Resistance Shear Resistance to Sliding Passive Resistance to

Sliding

Strength 0.45 0.80 0.50

Service and Extreme Event 1.0 1.0 1.0

Table 3:- Summary of Seismic Design Considerations

Site Class Seismic Zone

B 1

4.0 DISCLAIMERS/LIMITATIONS CLAUSE

The Recommendations section of this report includes interpretations and recommendations developed by the Government in the process of preparing the design. These recommendations are intended as a supplement to the recommendations provided in the CSE Geotechnical Investigation

Report, dated August 6th, 2018. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

Attachments (1)

Geotechnical Investigation Report, by: Copper State Engineering, Inc., Project No. CSE-4387, dated: August 6, 2018.

Prepared by:

Brendan McGarity, E.I.

Geotechnical Engineer, CFLHD

Reviewed by:

______________________________________________ Date_________________

Devin T. Dixon, P.E.

Acting Lead Geotechnical Engineer, CFLHD

Approved by:

______________________________________________ Date_________________

Devin T. Dixon, P.E.

Acting Lead Geotechnical Engineer, CFLHD

5.0 REFERENCES

American Association of State Highway and Transportation Officials (AASHTO). “AASHTO

LRFD Bridge Design Specifications.” Eighth Edition, 2017.

Geotechnical Investigation Report, by: Copper State Engineering, Inc., Project No. CSE-4387, dated: August 6, 2018.

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2007).

“Geotechnical Technical Guidance Manual.”