MT_North_of_Moose_Creek_North_Pave_Memo_FINAL.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

TO: Keith Wong, PM Kevin Harrison, HDM Kimber Miller, Designer Diann Morehouse, Geotechnical Engineer

In Reply Refer to:

HFL-17

FROM:

Stephen Henry, Assistant Materials Engineer

DATE: January 16, 2018

SUBJECT: INFORMATION:

Final Structural Pavement Recommendation North of Moose Creek North

MT DOT 569(3)

Background In a previous memo dated August 26, 2016 preliminary pavement recommendations were made for the North of Moose Creek North project in Deer Lodge County, Montana. To facilitate project design and cost estimation, a memorandum dated August 10, 2017 provided intermediate pavement structure recommendations for the project. Due to changes in the project design, recommendations provided in previous memos are no longer relevant. The recommendations in this memorandum supersede and replace all previous recommendations on this project.

Subgrade Evaluation Geotechnical investigations were conducted from May 9 to May 11, 2017 as well as on June 23, 2017. Broadly speaking, the existing roadway materials and underlying subgrade can best be described by dividing them into two distinct sections.

From STA 314+75 (BOP) to STA 386+00 (California Creek Bridge), the existing asphalt concrete pavement is approximately 4 inches thick, and is in poor condition. Beneath the existing pavement layer is a gravel fill layer 2 to 4 feet thick, AASHTO classification A-1-a. This gravel layer is well graded and can be considered good to very good in quality. Below the gravel fill layer is a clayey sand layer of native subgrade, AASHTO classification A-7-6.

Beyond STA 386+00 (California Creek Bridge) to STA 495+00 (EOP) existing asphalt concrete pavement varies in thickness from 2 to 7 inches, and is in very poor condition. Beneath the existing pavement layer is a gravel fill layer 1 foot thick. This gravel layer is well graded and is of good quality, AASHTO classification A-1-b. Below the gravel fill layer is a clayey sand layer of native subgrade, AASHTO classification A-7-6.

Testing of the native subgrade materials returned R-values of 3 and 6. These are extremely low values and indicate that the subgrade will not provide meaningful support to the roadway. For design purposes a subgrade resilient modulus of 1000 psi will be used.

To prevent distress in the existing pavement from reflecting through to an overlay, it is recommended that the existing pavement be removed.

Design Considerations Due to the low strength and inadequate drainage of the native subgrade, it is critical to the long term performance of the road that a sufficient high strength free draining structure separate the pavement structure from the native subgrade.

The Geotechnical Engineer has identified the native subgrade as having a moderate to very high potential to swell. This has the potential to cause differential movement in the roadway, which can result in high severity distress to the road surface. Historically, the road has experienced repeated maintenance issues likely linked to the swelling subgrade. This risk can be minimized by surcharging the subgrade with sufficient weight, and by preventing an increase in moisture content in the subgrade.

Historic performance of the road indicates that areas of isolated distress and an accelerated maintenance schedule are potential risks associated with differential movement. Widespread distress or impacts to the long-term performance of the pavement structure as a whole are unlikely. To eliminate this risk entirely would require extensive subexcavation along the entire length of the project, or importing several feet of additional crushed aggregate to provide additional surcharge weight. Neither of these options would be cost effective or within the scope of the project.

To minimize risk of differential movement, the following is recommended: From STA 314+75 (BOP) to STA 386+00 (California Creek Bridge), minimize lowering of the road profile to the greatest extent possible, and do not lower the profile more than 18-inches from the existing surface.

From STA 386+00 (California Creek Bridge) to STA 495+00 (EOP), do not lower the road profile.

Wherever possible, provide additional fill to increase surcharge weight on the subgrade.

During construction, sequence removal of existing pavement and base material to minimize risk of rain or other water infiltration into the subgrade. Where widening or realigning the roadway, place new embankment material as early as practical, to maximize settlement time of the underlying subgrade prior to paving.

Traffic Data Traffic Data was provided by MDT. The AADT is 190. Historic growth is 3%. The percent trucks is 4%. The traffic loading design projects the 18-kip ESALs for a 20-year design life to be approximately 100,000.

Recommended Pavement Structure The specified layer designs were determined from the 1993 AASHTO Pavement Design Guide. The design life for this pavement is 20 years.

The following is recommended for the roadway structure:

3 inches – Asphalt Concrete Pavement, Gyratory Mix, 1/2-inch or 3/4-inch nominal maximum aggregate size, 0.3 to <3 million ESAL (Item 40101-5600, estimated @ 1.94 tons/cuyd)

12 inches – Aggregate Base Grading D (Item 30101-2000, estimated @ 1.97 tons/cuyd)

Additional Bid Items required for this roadway segment:

Antistrip additive, type 3 (Item 40105-3000, estimated @ 1% by weight of Item 40101-5600)

Tack coat (Item 41201-0000, estimated at 0.10 gal/yd2, 240 gal/ton)

Specifications Pavement roughness will be evaluated as Type III, per Subsection 401.16.

The asphalt concrete binder grade is designated as PG 58-28.

Materials will provide any required SCRs for inclusion in the plan-in-hand package.

Should new information develop that impacts the information and assumptions made in this recommendation, the pavement design should be reexamined.

cc: Materials/Pavement File