Polymeric marine mattresses

Case studies highlight
marine mattress applications

By Jeff Fiske

Introduction

Marine mattresses are rock-filled containers constructed with advanced UV-stabilized co-polymer geogrid materials. Geogrid panels are laced together to form mattress-shaped baskets that are filled with small stones similar to the construction of gabions. Generally they are filled off-site and installed by lifting into place (Figure 1). While they are usually filled with small stone, they can also be filled with recycled concrete aggregate to form an effective protective layer against scour and erosion. Marine mattresses can be integrated with vegetation for a green solution in high energy environments.

The use of marine mattresses in high energy environments dates back to 1993 when the Massachusetts Port Authority (Massport) included marine mattresses as a corrosion resistant erosion control system in an environmentally sensitive area at the end of Runway 27L at Boston Logan International Airport (Figure 2). As part of a project to smooth and protect the transition slope from the runway into Boston Harbor, the erosion control system selected had to meet certain criteria including containment of stone fill, resistance to uplift from storm waves and attenuation of wave energy. The proposed polymeric marine mattress system, similar to a wire “reno mattress” met the requirements and provided a level of corrosion resistance not offered by the other systems available at the time.

Since their initial use, marine mattresses have been utilized in applications such as foundations for rubble mound structures (breakwaters, jetties, groins, etc.); channel linings and bridge scour protection; causeways, levees, dikes and bridge approach projects; protection for cables and pipelines; and in situ capping of contaminated sediments.

Specification of polymeric marine mattresses

In the early years of production, polymeric marine mattresses were fabricated from a punched and drawn uniaxial geogrid consisting of high-density polyethylene (HDPE). Other key properties of the geogrid included:

• Aperture dimensions of 0.7in. × 6in. provided enough open surface area to reduce reflection of wave activity while still retaining stone as small as a 1.5-in. diameter.
• Minimum of 2% carbon black to prevent ultraviolet (UV) degradation. The geogrid used for polymeric marine mattresses has been certified to retain 100% of original test strength at 500 hours per ASTM D4355.
• High tensile strength of the grid allowed for a high factor of safety for a 12in.-thick (nominal) mattress up to 35ft long to be safely hoisted after filling with aggregate. The mattress thickness is controlled by the length of the apertures of the grid used for the baffles (0.7in. × 6in.).
• HDPE was considered the ideal material—it does function well in this application but the tendency of the material to become brittle in subfreezing temperatures caused problems during construction on some projects in the early 2000s. During the filling and handling of the mattresses when temperatures dipped below 25 degrees longitudinal cracks propagated along the ribs of the geogrid and even through the transverse bars in some cases. A copolymer uniaxial geogrid was developed to improve the ductility of the geogrid in colder temperatures while retaining all other physical characteristics of the original product. Today’s marine mattresses are constructed from geogrid consisting of both HDPE and polypropylene.

Case history:
Fish Haul breakwaters

In 2006, a series of breakwaters were constructed in an estuary off the coast of Hilton Head Island, S.C. The subgrade where the structures were to be constructed was too weak to support a riprap structure without some degree of confidence.

This led the project engineer to specify a 6in.-thick marine mattress as a stable foundation for the rock structures. Marine mattresses have been used in such applications since the mid 1990s when a 12in.-thick marine mattress system was specified as a foundation for the T-head groins at the southern end of Tybee Island, Ga., just south of Hilton Head.

In most cases the mattresses act as a bedding layer and scour protection but in the case of a soft soil application the mattresses also help to distribute the load of the overlying structure, leading to a reduced risk of differential settlement.

For this project a geotextile was attached to the bottom of the mats allowing for simultaneous placement of the bedding stone and geotextile. Additional geotextile panels extended beyond the perimeter of the marine mattresses on one side and one end to allow adjacent mattress units to overlap and provide 100% coverage beneath the structure. The geotextile is supported by a stiff biaxial geogrid so it will maintain position even when placed under water.

Monitoring on the project has indicated that the structures are performing to expectations. Additionally it has been noted that the stone filled marine mattresses provide an excellent substrate for oyster growth. This unintended benefit to the project has led to the specification of marine mattresses as substrate for oyster beds in various locations around the Gulf Coast.

Case history: Kennedy Space Center bridge scour protection

Part of the allure of a revetment system, as opposed to conventional riprap is the potential to reduce the thickness of the revetment. This has obvious implications for the amount of materials used, but also impacts the project in more subtle ways such as reduced flood plain impact and excavation requirements.

In 2010, all of these factors influenced the engineers’ decision to specify a polymeric marine mattress system as scour protection around four bridges at the Kennedy Space Center in Cape Canaveral, Fla.

The engineering team initially considered riprap for the erosion protection. The 3.5-ft section thickness would have required significant excavation and dredging in order to meet navigational depth requirements. Articulating concrete block (ACB) revetment systems were also considered but the ACB systems were ruled out due to higher cost and the difficulty associated with trying to custom fit the system around the bascule piers.

As shown in Figure 3 and Figure 4 there is a 2-ft to 5-ft gap between the marine mattress revetment system and the structures. This gap is protected with riprap lying on top of a geogrid/geotextile extension that connects the marine mattress system to the structure. The intent is to prevent channelization between the concrete structure and the mattress system that could lead to scour. Figure 3 also indicates the dredge template that would have been required if riprap had been used for this project.

Conclusion

Marine mattresses have been around for more than 20 years filling a niche in the coastal engineering marketplace. What started out as a potential competitor to steel wire gabion mattresses and reno mattresses has become a tool for coastal and waterway engineers for a variety of applications including shoreline revetment, riverbank protection, pipeline cover, foundation for coastal structures, channel lining in high velocity applications, and more.

On the Orleans Landbridge shoreline protection project (2011–2013), engineers in Louisiana found that a polymeric marine mattress could be filled with recycled concrete to provide a relatively lightweight, high-performance shore protection system.

The growth of this geosynthetic-based product in the market has been built on that adaptability of the product itself and the track record of success that has been demonstrated for more than 20 years.