GEO Report Addendum WY FLAP JOH 14(1).pdf

To: Julia Dietsch, P.E., PMP., Project Manager, CFLHD, Lakewood CO

From: Devin T. Dixon, P.E., Lead Geotechnical Engineer, CFLHD, Lakewood CO

Through: Devin T. Dixon, P.E., Lead Geotechnical Engineer, CFLHD, Lakewood CO James M. Arthurs, P.E., PhD., Sr. Geotechnical Engineer, CFLHD, Lakewood CO

Subject: Geotechnical Addendum for the North Fork Crazy Woman Creek Bridge Final Geotechnical Report, Johnson County, Wyoming, WY FLAP JOH 14(1)

This memorandum serves as an addendum to the final geotechnical report for the North Fork Crazy Woman Creek Bridge replacement project, titled “North Fork Crazy Woman Bridge, Bighorn National Forest, Johnson County, WY, WY FLAP JOH 14(1), FINAL Geotechnical Report”, and dated April 2023. This addendum primarily addresses changes to Section 4 of the FINAL geotechnical report. Recommendations provided in the referenced report that are not specifically superseded herein should be followed.

The report provided recommendations for a drilled shaft foundation system for the new bridge abutments. The project Cross Functional Team (CFT) determined at the 95% site visit in late August 2023 that construction access for drilled shaft equipment from both sides of the bridge was not feasible due to geometric constraints as well as timing and funding issues for the replacement of load posted structures on U.S. Forest Service land. Two options were selected to improve constructability:

1. Redesign the proposed abutments using micropile foundations.

2. Add a temporary bridge and/or bridge strengthening methods into an SCR for the contractor to determine (probable increased bid prices would follow).

The use of micropiles for the bridge foundations selected as the preferred option because it requires smaller equipment capable of crossing the existing load posted structure prior to demolition. The project manager proposed a change control in early September 2023 to incorporate the adjustment from the previously proposed drilled shaft foundations to micropile foundations.

Technical Memorandum

Date: October 11, 2023

1 GEOTECHNICAL AXIAL RESISTANCE AND LATERAL LOAD ANALYSIS

The following table and bulleted text present the summarized micropile design information as determined by the combined team of CFLHD bridge, hydraulics, and geotechnical engineers:

TABLE 1: Summary of Micropile Design Information

ABUTMENT

LOCATION

MICROPILE

DIAMETER

(inch)

ANTICIPATED CASED

LENGTH

(feet)

MINIMUM UNCASED

BOND LENGTH

(feet)

ESTIMATED TIP

ELEVATION

(feet)

East Abutment (Abut. 2) 9.0 25.0 15.0 5,843.46

West Abutment (Abut. 1) 9.0 25.0 15.0 5,842.98

Note: Refer to S-sheets in plan set for further details.

AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017 procedures for micropile design were used to estimate the minimum bond length required to resist the applied loads at the strength limit state with a resistance factor (φqs) of 0.55 for side resistance from AASHTO LRFD Table 10.5.5.2.5-1.

A nominal micropile grout-to-ground bond strength, αb, of 4.5 ksf was assumed within the highly weathered to completely weathered sandstone encountered in boring BH22-02 at the east abutment based on typical grout-to-ground bond strength values for preliminary micropile design listed in Table C10.9.3.5.2-1 of AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017.

Type A (grout placed under gravity head only) micropiles are recommended for this project. Micropiles at both abutments should be cased from top of micropile elevation to 25 feet beneath the bottom of the abutment cap (see sheet S5 in the plans). Ten-inch outside diameter (OD) casing should be used for the permanent casing.

A continuous-thread, minimum #14 reinforcing steel bar with suitable centralizers connected to the bar and spaced per plans along the length of the micropile should be used to assist in transferring axial loads to the uncased bond zone. The bar should extend the entire length of the micropile and through a steel plate installed on top of the casing.

Overburden depth was only confirmed at the east abutment (abut. 2) in boring BH22-02 and not at the west abutment (abut. 1). The geotechnical engineer should be notified if actual subsurface conditions do not meet anticipated conditions.

Recommended parameters for use with LPILE are presented in Table 4.3 of the FINAL Geotechnical Report. The CFL Bridge Team completed lateral load analysis of the proposed micropile foundations in September 2023. Cased lengths of the micropiles were based on lateral load requirements and the depth of scour susceptible material as presented in the Final Hydraulics Report dated April 2023.

2 MICROPILE GROUT

A flowable, high strength cementitious grout mix is recommended. Type I or Type II cement should be used for the construction of all micropiles with a water to cement ratio between 0.4 and

0.5 by weight and a minimum compressive strength of 4,500 pounds per square inch.

It is essential to the integrity of the micropile that upon completion of the grouting operation there is no significant loss of grout from any part of the micropile that will be relied upon for load bearing or corrosion protection. This can be achieved by grouting until the grout overflows through the top and no additional grout can be added after one day during construction. Grout losses can be assumed if the estimated ratio between actual and theoretical grout volume exceeds 1.5. If highly fractured bedrock materials are encountered during drilling, pre-grouting and pre-drilling, the use of admixes/sand to reduce grout viscosity, or grout socks may be required.

3 SETTLEMENT

Elastic settlements are estimated to be less than ½ inch. Settlements in the micropiles will occur immediately and be essentially complete at the end of construction. Differential or group settlements are not anticipated.

4 VERIFICATION & PROOF TESTING

It is recommended that one verification test at the west abutment (Abut. 1) and one proof test at each abutment be performed. The reasoning behind this decision is because only one boring was drilled at the east abutment (Abut. 2) and no drilling was performed at opposite abutment. Test according to the procedures in Section 567 of FP-14.

5 CONSTRUCTION CONSIDERATIONS

The contractor should visit the site before bidding on the project. This will allow the contractor to perform a surface investigation of the site to identify potential construction concerns, such as equipment access and selection, presence of water or soft areas, and potential excavation difficulties (i.e. cobbles and boulders).

6 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations presented in this memorandum are based on the data obtained from exploratory borings, field review, and laboratory test results as described in the referenced FINAL Geotechnical Report. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not be evident until excavation is performed. The recommendations presented in this report include interpretations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

Prepared by:

Devin T. Dixon, P.E.

Lead Geotechnical Engineer, CFLHD

Reviewed by:

James M. Arthurs, P.E., Ph.D.

Senior Geotechnical Engineer, CFLHD

Approved by:

Devin T. Dixon, P.E.

Lead Geotechnical Engineer, CFLHD