TX_FTFW_LAAT_10(2)_100__Pavement_Recom_Tech_Memo.pdf

MEMORANDUM

Subject: TX FTFW LAAT 10(2) Date: 5/7/2014

Laguna Atascosa NWR

Pavement Recommendation

From: Scott Marshall, PE

HDR Engineering, Inc.

To: CFLHD

Brian Dobling, PE

Assumptions

0.75 inches of Surface Treatment

6 inches of Aggregate Base Course (3 inches may be credited from existing)

Attachments: Pavement Design Calculations

CFLHD Report of Soil Tests

The minimum recommended pavement structural section is:

The existing pavement section results in a Structural Number (SN) of 1.14. The recommended SN is 1.5‐2.0. Since the existing pavement has performed well over the past 12 years, provide at least the same strctural section of 0.75 inches of

Surface Treatment over 6 inches of Aggregate Base. If feasible, provide 0.75 inches of Surface Treatment over 9 inches of

Aggregate Base.

The proposed entrance road is already 20‐22' wide and will be reconditioned, primed, and a surface treatment applied.

Bayside Wildlife Drive will also be reconditioned and widened to a 22' top width utilizing the existing 6 inches of aggregate base and existing surface treatment. The Drive will then have an additional 3 inches of Aggregate Base along with a surface treatment. The two grade separated crossings will be constructed to protect the endangered Ocelot. This will include raising the road and placing precast concrete box structures. The route is generally flat with a low accident rate.

The ADT is not available, but is less than 400. The route design speed is 25 MPH.

Traffic data is unavailable for the roads at Laguna Atascosa. The Average Daily Traffic (ADT) was assumed to be 130 vehicles per day. It was also assumed that 10.5% of the traffic is truck traffic. An ESAL conversion factor can be used for

Low Volume Roads and Heavy Truck Routes, 0.4 and 0.5 respectively (IOWA DOT Pavement Design Standards). Using these assumptions results in 50,000 ESALs. The actual ADT and Truck percentage is probably half as much. However, per

Section 11.2.1.2 in the PDDM, 50,000 ESALS is the minimum that shall be used for designing paved roads.

CFLHD follows the AASHTO design processes. AASHOT recommended values and inputs were used to determine the proposed pavement section.

CFLHD tested two samples on Buena Vista Road and near the Monarco Blanco Trailhead on Bayside Wildlife Drive. The existing soils were classified to be CL/CH and A‐6, A‐7‐6.

The grade separated segment of the road will be constructed using unclassified borrow and MSE Wall Backfill, which will have an R‐Value greater than the in‐situ soil's R‐Value of 5. The R‐Value of the unclassified borrow should be above 10.

Also, Bayside Wildlife Drive will utilize the existing aggregate base for subgrade. The proposed pavement recommendations assume no existing subgrade disturbance and the existing aggregate base for roadbed soil.

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Gravel Road Thickness Design ‐ CFLHD Supplement 11.4.2‐1

Project: Laguna Atascosa NWR

Project No: TX FTFW LAAT 10(2)

Location: Harlingen, Texas

Designer: Scott Marshall, PE

HDR Engineering, Inc.

Introduction

Assumptions

1) Construction Year: 2015

2) Design Year: 2040

3) Road ADT: 130

4) % Trucks: 10.5%

5) ESAL Conversion: 0.4

The 18‐kip ESAL Repetitions were calculated as follows:

18‐kip ESAL Repetitions (W18) = (2‐1) x (3) x (4) x (5) x 365 days/year

18‐kip ESAL Repetitions (W18): 50,000

US Climatic Region Map

There are many factors that affect the surface performance of a gravel road during its design life. Some of the factors are:

axle load, which is referred to as the 18‐kip equivalent axle load (ESAL); cover aggregate properties; surface/subsurface drainage; freeze/thaw; subgrade properties; resilient modulus; and others.

The ESAL factor is considered vital to the gravel road thickness design. The American Association of State Highway and

Transportation Officials (AASHTO) recommends the use of a maximum of 100,000 ESAL applications, while the practical minimum is 10,000 ESAls during a single performance period.

Traffic data is unavailable for the roads at Laguna Atascosa. The Average Daily Traffic (ADT) was assumed to be 130 vehicles per day. It was also assumed that 10.5% of the traffic is truck traffic. An ESAL conversion factor can be used for

Low Volume Roads and Heavy Truck Routes, 0.4 and 0.5 respectively (IOWA DOT Pavement Design Standards).

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Gravel Road Thickness Design ‐ CFLHD Supplement 11.4.2‐1

Project No: TX FTFW LAAT 10(2)

Location: Harlingen, Texas

Designer: Scott Marshall, PE

HDR Engineering, Inc.

IV

Very Good

Traffic Level: Low

U.S. Climatic Region:

Roadbed Quality:

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Flexible Pavement Design

Project No: TX FTFW LAAT 10(2)

Location: Harlingen, Texas

Designer: Scott Marshall, PE

HDR Engineering, Inc.

IV

Very Good

Traffic Level: Low

March 2008 PDDM, Chapter 11

18‐kip ESALs Over Initial Performance Period: 50,000

Initial Serviceability: 4.2 Flexible Pavement; Per 11.2.1.1

Terminal Serviceability: 2.0 ADT<500; Per 11.2.1.1

Reliability Level: 75% ADT<2500; Per 11.2.2

Overall Standard Deviation: 0.49 Flexible Pavement(0.49), Rigid (0.39); Per 11.2.2

Roadbed Soil Resilient Modulus: 4000 psi

Layer

Material

Description Structural Coefficient

Drain

Coefficient

Thickness

(inches)

Calculated

Structural

Number (SN)

Structural

Number (%)

1 HACP 0.40 1 0.75 0.30 26%

2 Base Course 0.14 1 6 0.84 74%

Total 6.75 1.14

Layer

Material

Description Structural Coefficient

Drain

Coefficient

Thickness

(inches)

Calculated

Structural

Number (SN)

Structural

Number (%)

1 HACP 0.40 1 0.75 0.30 19%

2 Base Course 0.14 1 9 1.26 81%

Total 9.8 1.56

1993 AASHTO Guide for Design of Pavement Structures

U.S. Climatic Region:

Roadbed Quality:

Since the existing pavement section has performed well over the past 12 years on what appears to be existig soils; utilize 3" of existing agggregate base for the proposed pavement section. At least provide same section as existing(0.75' over 6" Aggregate Base). Determine: aggregate base thickness to acieve SN of 1.5.

Existing Pavement Section Design

Proposed Pavement Section Design (assume Existing 6" AB is spread to be 3" thick)

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