The City of Miami Beach is committed to protect the safety and general welfare of its residents and visitors. One way the City meets this goal is by preserving and improving all transportation facilities such as roadways and sidewalks. In the case of roadways, the City implements minimum requirements for pavement design for new construction and pavement rehabilitation.

Pavement design is primarily a matter of sound application of acceptable engineering criteria and standards.

At the February 18, 2020, Land Use and Sustainability Committee (LUSC) meeting, Mr. Papadopoulos, Florida International University's Extreme Events Institute and founder and Chairman of Resiliency Action Fund's (RAF), documented his views of resilience in an essay entitled, "Eleven Ideas to Improve Miami Beach's Resilience" (attached).

The article further references using full depth recycled asphalt for public roads being reconstructed to make them more resistant to flooding while recycling the existing worn-out asphalt into the new base thereby reducing waste.  FDOT recently utilized this technique for the Alton Road reconstruction project from 5^th Street to Michigan Avenue.

At its March 18, 2020 meeting, the City Commission approved a referral to the LUSC to review the City of Miami Beach's experience and perspective on full depth asphalt (FDA) and identify potential opportunities for use.

The referral asks the City to advise at Committee on its experience with this technique and identify potential avenues/projects for its use.

Asphalt pavement is comprised of three main layers: top, base and subbase. The top layer, the strongest layer, is designed to distribute the traffic loadings to the base and then to the subbase, the weakest layer.

Asphalt pavement can be categorized as two major types: FDA pavement and conventional asphalt pavement. The major difference between the two types is the former has asphalt within the top and base layers while the latter has asphalt only within the top layer. The subbase layer for both types is compacted dirt and the base for the conventional pavement is fully compacted limerock material.

Regardless of the type, asphalt pavements are designed with sufficient strength to counter traffic loading. One major parameter in the design of asphalt pavement is the proximity of the water table to the bottom of the base, what is commonly referred to as base clearance. The base clearance is the vertical distance between the bottom of the base layer and the Seasonal High Ground Water Table (SHWT). As the base clearance decreases, so does the strength of the pavement.

In low lying areas, where the SHWT is at 1-3 feet from the surface, the construction of pavement becomes very challenging due to difficulties in achieving the compaction require for the compacted dirt and limerock material. Florida Department of Transportation (FDOT) Pavement Design Manual recognizes this as major problem. As a result, FDOT recommends the use thicker asphalt layers. Such case occurred with the Alton Road reconstruction project between 5^th Street and Michigan Avenue where FDA was provided for the entire corridor due to the reduced base clearance (just below 2.5 feet in average).

In the recent report prepared by Jacobs Engineering with respect to the road elevation strategy, Jacob’s recommendation for the pavement design is consistent with the FDOT Pavement Design Manual with the evaluation of different design options performed by the engineer responsible of the pavement design for each specific roadway project.

Specifically, Jacobs recommends that when the base clearance is 1 foot or higher throughout the 30 year design life (accounting with the Sea Level Rise and increasing depth of SHWT), then FDA may be unnecessary. However, if the base clearance is less than 1 foot, then thicker asphalt layers may need to be evaluated. 

Results from the 2019 Resident Survey show that 50% of residents rated capital improvements completed by the City as very satisfied/satisfied.  In order to continue maintaining excellent standards in this area, the City is open to receiving resilience initiatives for consideration.

The use of FDA pavements may be required on roads where the base clearance is less than one foot. However, the use of FDA may not be optimal in every application and can present additional concerns from a technical and fiscal perspective.

Additionally, FDOT recommends the use of lime rock base and FDA with a three foot and a two-foot base clearance, respectively, for a proper design. Following these guidelines would increase the elevation of roadways that are now designed with one-foot of base clearance within the City.

Recommendation

The Administration recommends maintaining design standards in alignment with Jacob’s Engineering recommendation. The standard would specify FDA in cases where the base clearance is less than one-foot and specify limerock base in cases where base clearance is in excess of one-foot, unless specific site conditions warrant a deviation from the standard. This standard would apply to newly constructed and rebuilt roadways. In cases where the roadway is being milled and resurfaced, the base course would not be changed.