Upper Mill Creek Canyon Road Improvements_Pavement Addendum_20240508 - signed.pdf

6952 South High Tech Drive, Suite B

Midvale, UT 84047

P (801) 545-8500

F (801) 545-8600

Terracon.com

May 8, 2024

DJ&A, P.C.

2000 Maple Street

Missoula, MT 59808

Attn: Mr. Jon Pederson, P.E.

P: (406) 549-6371

E: jonathan@djanda.com

Re: Pavement Report Addendum — Thousand Springs

Upper Mill Creek Canyon Road Improvements, UT FLAP SLA 10(1)

Salt Lake County, Utah

Terracon Project No. 61215174A

Dear Mr. Pederson:

Terracon Consultants Inc. (Terracon) is providing this pavement addendum to the Final

Pavement Report Revision 1, dated January 6, 2023, to address pavement section design changes at the Thousand Springs area. This letter summarizes coordination between the

DJ&A design team and Central Federal Lands (CFL) Project Support Team (PST), and revised pavement section recommendations.

Overview

For the pavement geotechnical report, Terracon completed pavement field explorations, a laboratory testing program, and a pavement section analysis design for this project and provided a design pavement section of 3 inches of Asphalt Concrete (Item 401) over 6 inches of Crushed Aggregate Base (Item 301) bearing on firm and stable subgrade. On further review and discussion with the design team and CFL PST, team members expressed concern regarding the existing subgrade in the Thousand Springs area and potential for destabilizing the subgrade during design pavement section construction.

The Thousand Springs area is approximately 840 feet long, extending from Station 187+20 to 197+65. The existing roadway has Mill Creek on the left (when looking up station) and a marsh area fed by a nearby spring on the right. During project site visits it was noted that in some areas the existing road surface appeared to only be 2 to 3 feet above the creek and marsh water level. The existing roadway is narrow, allowing for just two lanes, and minimal or no shoulders, before sloping moderately to nearly vertical to the creek or marsh. The mailto:jonathan@djanda.com mailto:jonathan@djanda.com pavement surface appeared to be in moderately good condition, though some localized areas of alligator cracking were noted, suggesting poor subgrade conditions may exist in these areas.

During the exploration phase we completed a boring, PB-07-TS, at approximate station

192+00, within the Thousand Springs area. Soil conditions consisted of 2.5 feet of silty gravel fill with clay layers underlain by medium dense gravel with varying amounts of clay, silt, and sand. An organic odor, indicative of possible decaying organic material, was noted during drilling. Water was encountered 3 feet below the pavement surface during drilling. All samples collected, beginning at 6 inches below the surface, were noted as being wet.

The 70 percent design profile through this area was approximately at or slightly above the existing road surface. Constructing the design pavement section would result in excavating down a minimum of 9 inches below the existing road surface. Team members felt this represented a significant risk during construction to try and create stable subgrade so close to water and where there was the potential to encounter soft organic materials. Efforts to stabilize the subgrade, usually consisting of overexcavating existing subgrade and backfilling with import soils, would only extend the excavation closer to, or below, water and potentially deeper into poor subgrade soils.

The design team and CFL reviewed several alternative pavement sections for this area including the following:

■ a full depth reconstruction using the design section

■ pavement preservation approach including an asphalt overlay in combination with a fiberglass grid

■ reduced pavement section reconstruction

■ full-depth reclamation with cement and pulverization

■ cement-treated base course

■ mill and fill

■ rigid pavement

After initial review, the consensus was to complete a 4-inch overlay using a fiberglass grid, and complete localized spot repair in areas of unstable subgrade as necessary. But upon further review, the roadway design team reduced the design roadway section width, updated the safety edge to 1V:3H, and added two rock armored slopes. These modifications will allow the construction of the full design pavement section above the existing roadway surface. This will limit construction risk to exposing and destabilizing the existing subgrade, while still providing a full pavement section thickness. Localized soft spot repair may still be required to improve unstable subgrade. Nonwoven geotextile, in combination with

Stabilization Geogrid will also be placed below the Crushed Aggregate Base to provide additional support to this pavement section through the Thousand Springs area.

Design Recommendations

The existing asphalt pavement should be removed from within the Thousand Springs limits

(approximate Station 188+00 to 196+40). To reduce the risk of destabilizing the pavement subgrade due to construction traffic and inclement weather, Crushed Aggregate Base should be placed and properly compacted no later than 2 weeks following existing pavement removal. Asphalt Concrete should be placed no later than 2 weeks after Aggregate Base

Course placement.

The exposed subgrade should be proofrolled in the presence of the geotechnical engineer or

Contracting Officer (CO) to identify unstable subgrade. Deflecting or unstable subgrade should be repaired by removing and replacing with Roadway or Borrow Excavation backfill to create a firm and unyielding surface. Stabilization Fill, in combination with Stabilization

Geogrid and nonwoven geotextile, may be required to create a working platform on which to place and compact roadway fill or the design pavement section. Stabilization Fill should meet the requirements specified in the Final Pavement report.

For preliminary planning purposes, the following areas may require soft spot repair:

■ 188+75 to 189+00 — Full roadway width

■ 190+20 to 190+80 — Full roadway width

■ 191+60 to 191+80 — Half roadway width

■ 192+20 to 193+00 — Half roadway width

■ 192+40 to 192+80 — Half roadway width

■ 194+20 to 194+60 — Half roadway width

These areas are based on limited visual observations during site visits and review of aerial imagery. Actual conditions may vary and should be verified by proofrolling.

Following proofrolling and preparing the roadway subgrade, place Class 2, Type E, nonwoven geotextile (FP-14 Section 714.01, Table 714-1) directly on exposed subgrade to act as a separation fabric. Place Stabilization Geogrid (FP-14 Section 714.03, Table 714-4) on top of the separation fabric. Nonwoven geotextile and Stabilization Geogrid should be placed following manufacturer’s specifications on the exposed subgrade the full length of the

Thousand Springs section, and the full width of the proposed roadway. The design pavement section should then be placed and compacted per project specifications.

Where Crushed Aggregate Base Course or any fill will be placed on top of rock armored slope areas, the nonwoven geotextile and Stabilization Geogrid should extend out over the top of the armored slope material.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this pavement addendum or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Jeff Gilbert. P.E. Rick Chesnut, P.E., P.G.

Senior Engineer Senior Principal https://na3.documents.adobe.com/verifier?tx=CBJCHBCAABAA7mbM5xbtgAiiruPaV5iggASXXi3HXwfU https://na3.documents.adobe.com/verifier?tx=CBJCHBCAABAA7mbM5xbtgAiiruPaV5iggASXXi3HXwfU