NM FS 141(1) S. Fork Negrito Bridge - Pavement V1 Tech Memo - Final.pdf

Subject: Pavement & Materials Preliminary Recommendations NM FS 141(1) South Fork Negrito Creek Bridge

From: Mike Voth Pavement & Materials Team Leader, CFLHD

To: Mike Daigler, Project Manager, CFLHD Erin Jessee, Highway Designer, CFLHD

INTRODUCTION

This memorandum provides preliminary pavement and material recommendations for the approaches, diversion, and other items for the South Fork Negrito Creek Bridge. The existing bridge will be replaced as a part of this project. A temporary diversion will need to be constructed as the route cannot be closed during construction. The length of new asphalt pavement is 200 feet, which includes two 100 foot paved approaches, but due to a grade raise of the bridge structure the reconstruction limits will be beyond the 100 foot paved approaches and will require the addition of gravel surfacing. The project is in the Gila National Forest in southwestern New Mexico.

This memorandum also documents the assumptions used to develop the preliminary recommendations. No pavement investigation was completed but preliminary data from the geotechnical borings for foundations and earthwork will be used to estimate the subgrade conditions. The geotechnical field investigation was completed a couple weeks ago. Lab testing submittals have been submitted, but test results are not expected until November 2020. The final recommendations memo will include lab test reports and validation of recommendations. Traffic data was estimated from discussions with staff from Gila National Forest.

FIELD INVESTIGATIONS

The geotechnical investigations, as mentioned above, took place during the week of September 21, 2020. In addition to completing borings for foundation analysis, geotechnical staff measured surface gravel thickness at the bridge approaches. They also photographed, reviewed, and collected grab samples from multiple potential borrow sources. This information along with data from the Forest Service and scoping documents were used to develop the preliminary recommendations. The following is a bullet list of key results from the field investigation and other data sources:

Technical Memorandum

Date: October 9, 2020

MICHAEL D VOTH Digitally signed by MICHAEL D VOTH Date: 2020.10.13 12:43:39 -06'00'

Based on preliminary notes and visual appearance, the upper 2 to 3 feet of subgrade is expected to be of good quality. It consisted of silty sands with gravels and cobbles. An R-Value of 40 and resilient modulus of 10,000 psi can be conservatively assumed.

The existing thickness of the surface gravel at the approaches was reported to be 4 to 8 inches.

No defects or deficiencies in the roadway gravel at the approaches were reported. Staff indicated that the performance and function of the gravel roadway was adequate.

From feedback from the Forest Service, the ADT is less than 200. The Forest Service indicated that there is a potential for future timber sales which would add about 10 trucks/day in each direction.

Geotechnical staff reviewed four borrow sources that were close to the project site and previously cleared by Forest Service from a compliance basis. The borrow sources appear viable from an access and material quality standpoint.

DESIGN RECOMMENDATIONS

The preliminary pavement structural section recommendation for the bridge approaches is as follows:

3 inches ACP (403 asphalt, type 1, placed in two lifts) 6 inches of aggregate base (302 minor aggregate, method 2)

For the graveling surfacing beyond the paved approaches suggest the following:

9 inches of aggregate surface course (302 minor aggregate, method 2)

The 9-inch thickness matches the pavement structural section and would provide ease of grading and finishing of subgrade.

Note: Given very small quantities and length, no minimum plasticity index requirements will be specified.

For the diversion, the recommendations are as follows:

6 inches of aggregate surface course (302 minor aggregate, method 2)

Note that the contractor is required to maintain the diversion at a good serviceability level throughout the duration of use.

The following borrow sources were investigated:

Deep Creek Pit (108.669, 33.512): Recent activity (stockpiles). Easy access. Sample collected. Material appears to meet unclassified borrow requirements. Farthest source from bridge site.

Rainy Mesa Pit (108.62, 33.547): Recent activity (stockpiles). Easy access. Sample collected. Material appears to meet unclassified borrow requirements.

Hail Canyon Pit (108.597, 33.53): Recent activity (stockpiles). Easy access. Sample collected. Material appears to meet unclassified borrow requirements. Cannot excavate below ground surface per Forest Service (need to clarify).

Beaverdam Pit (108.569, 33.534): No recent activity in pit. Easy access. Material appears to meet unclassified borrow requirements.

Recommend reviewing all four sources for further viability. At 30% review meeting, CFT should discuss potential issues such as source development, quantity limits, and restoration requirements from the Forest Service, if any. If CFT concurs with use of this borrow source, an SCR for Section 105.02 would be provided.

Additional material recommendations:

Provide tack coat (41201-0000) applied between asphalt pavement lifts.

Given very small paving scope and limited risk, recommend not including fog seal and prime coat on this project.

CONSTRUCTION CONSIDERATIONS & SPECIAL CONTRACT PROVISIONS

Our standard 403 SCR will be adjusted to allow for multiple NM DOT asphalt mixes to be used (with expectation for reduced bid risk and lower costs). If the borrow source is incorporated in the project, an SCR for Section 105.02 will be provided. Numerous SCR examples from similar projects are available. The plasticity index requirement of our standard 703 SCR will be adjusted as discussed above.

With the expected prefabricated steel superstructure bridge, quantities for concrete will be relatively small. Haul distances for concrete are expected to be two hours or more with material possibly coming from the Springerville, AZ or Silver Spring, NM area. If a concrete or timber deck is not selected as the deck wearing surface, support for asphalt surfacing will be provided, as needed. A gravel surfaced diversion will be constructed and carry traffic for the duration of the project.