Soldiers roll out Mo-Mat to make a roadway over a sandy beach.

Building Roads on Soft and Sandy Soils

by Rosa L. Santoni and Jeb S. Tingle

 

        Joint logistics over-the-shore (JLOTS) exercises simulate military sealift deployment operations and provide military forces with realistic lessons that can be applied anywhere in the world. During these exercises, equipment and personnel are transported from a ship to a bare beach and moved to the area of operations. However, JLOTS operations usually are conducted on soils that are not strong enough to support aircraft or ground vehicles.

    When loose sand, marshes, or swamps are encountered, transferring personnel, equipment, and materials can be delayed because the ground lacks the strength to support military truck traffic. Heavy trucks traveling over sandy and soft soil can produce ruts up to 14 inches deep. Sometimes vehicles become stuck in the ruts, resulting in delays that could jeopardize both the mission and soldiers' safety.

    To solve the problem of moving vehicles over soft and sandy soils, the Army Engineer Research and Development Center (ERDC) in Vicksburg, Mississippi, was tasked to develop new materials and matting systems for rapid construction of airfields and roadways. ERDC conducted an enhanced coastal trafficability demonstration that investigated using a combination of geosynthetics, mat systems, and strong soil layers to surface roadways. Geosynthetics are planar, polymeric (synthetic or natural) materials used in contact with soil, rock, or any other geotechnical material in civil engineering applications

    Since 1996, ERDC has identified discrete fibers as a potential material for constructing roads and airfields over sandy soils; developed several structural matting systems to support operations over sandy soils; and evaluated several structural mats and geosynthetic-reinforced pavement systems to support heavy-truck traffic over soft soil.

Existing Airfield and Roadway Systems

    The Army has several mats and a sand grid system available to construct airfields and roadways over sandy soil.

    Mats are divided into two major categories—airfield mats and roadway mats—based on the primary use of the mat system. These mats were developed in the 1960s and 1970s.

    Airfield mats are made of aluminum and steel for quick airfield construction. They support the high gross loads and tire pressures associated with aircraft. Airfield mats include a light-duty steel mat (M8A1), three types of medium-duty aluminum mats (XM18, M19, and AM-2), and a heavy-duty truss web aluminum mat. These mats were developed to support aircraft when placed on a California bearing ratio-4 subgrade. (California bearing ratio (CBR) is a standardized testing procedure for determining the strength of soils by measuring the soil's penetration resistance and comparing it to the penetration resistance of crushed rock. For example, you will sink up to your knees in a CBR of less than 0.2, you will sink up to your ankles in a CBR of 0.5, your heels will sink about ¼ inch in a CBR of 1, and a spike heel will make a slight indentation in a CBR of 4.)

    Three existing types of mats are available to create roadways over sandy soils. The first, Mo-Mat®, consists of semirigid panels of fiberglass-reinforced resin material that are rolled out, bolted together, and anchored in place to form temporary roadways and parking and storage pads. The second, the M8A1 light-duty airfield mat, works well for large turning area pads and straight roadway sections. The third roadway mat, the Uni-Mat®, is a patented interlocking mat made from hardwood lumber. Two layers of Uni-Mat create a heavy-duty roadway over sand or wet soil.

    These existing mat systems have several limitations that prohibit their use in many military applications. The Mo-Mat requires extensive maintenance and is difficult to replace because it is no longer available commercially. The M8A1 requires significant maintenance when used in curved roadway sections. The last known military purchase of the M8A1 mat was in the late 1960's for airfield construction during the Vietnam War. Only limited supplies of the Mo-Mat and M8A1 exist. The heavy-duty truss web aluminum mat has never been purchased for military use because its weight makes it difficult to transport. SOLOCO, Ltd., purchased the Uni-Mat patent and stopped making the original Uni-Mat design. SOLOCO now manufactures another wood mat in place of the Uni-Mat.

    The sand grid, also known as Geocell, consists of a plastic material designed to confine sand or other cohesionless materials to produce a load-distributing base layer. The plastic grids are manufactured and shipped in collapsed 4-inch thick, 110-pound sections. Each expanded grid section is 8 feet by 20 feet and contains a honeycomb arrangement of cells.

New Soil Stabilization Systems

    ERDC has developed three new rapid-road-construction methods to overcome the deficiencies of truck operation over sandy soils. The road construction methods include stabilizing sand with geofibers and surfacing the soil with two types of mats. A fourth method—a combination of mat, geosynthetics, and structural material—provides an innovative solution for constructing roads over soft soils.

    Geofibers. Sand-fiber road stabilization technology involves mixing hair-like, 5-centimeter-long polypropylene fibers into moist sand with a self-propelled rotary mixer. The sand-fiber layer is compacted with a smooth-drum vibratory roller. A wearing surface is added by spraying a resin-modified emulsion or an emulsified asphalt onto the road surface to bond the sand grains with the fiber filaments and protect the sand-
fiber surface.

    Using this method, military supply roads can be constructed quickly at remote sites over the beach or across desert sands with less equipment, manpower, and materials than other road-building methods require. Experiments conducted at ERDC indicate that roads constructed with this new technology can carry over 10,000 passes of heavy military supply traffic with very little or no maintenance required. Sand-fiber stabilization uses existing military construction equipment and requires no special construction skills. It can be used on a wide variety of sands and silty soils found around the world.

    Fiberglass-reinforced mat. This mat is a spin-off of the fiberglass-reinforced mat developed by the Air Force as part of its rapid runway repair project. The mat is made of polyester resin reinforced with four plies of woven chopped fiberglass. The weight of a 6-foot by 6-foot panel is approximately 115 pounds. Each panel has an 8-inch underlap with a downward fold for connecting panels together. A quick-install connector pin bolts the panels together. The connector pin is placed in overlapping rectangular holes and tightened. As the pin connector is tightened, a bottom plate rotates 90 degrees, locks in place, and pulls the two panels tightly together. When installed over sand, the mat will support over 5,000 truck passes with very little rutting. The mat may flex significantly under heavy wheel loads; however, it springs back to a flat position and usually suffers no damage.

    Hexagonal mat. These lightweight interlocking mat panels were designed for quick installation to create parking areas and access roads. The installation rate ranges from 600 to 900 square feet per man-hour. The panels do not deteriorate from exposure to ultraviolet light and are made from recycled, high-density polyethylene. Each panel weighs 7 pounds and has a surface area of approximately 3 square feet. The factory-recommended maximum wheel load is 13,000 pounds per panel when installed over a gravel base. The panel's hexagonal form permits road angles of 30, 60, and 90 degrees.

    DURA-BASE® mat. The DURA-BASE interlocking mats were designed for temporary roadway systems and construction platforms placed over soft soils and environmentally sensitive areas. These plastic mats are made by bolting together two high-density polyethylene sheets and heat-welding the periphery of the mats. Each plastic mat weighs 1,050 pounds. The mat size is 8 feet by 14 feet with a thickness of 4¼ inches. Each panel has a tread pattern that improves traction for load-bearing vehicles and equipment. A small crane or forklift and two or three laborers are needed to install the mats. Over very soft soil, two layers of mat plus a geotextile separator are required.

Sandy Soil System Comparison

    Several logistics issues must be considered for these new mats and composite roadway systems. The table on page 25 shows a logistics analysis of the systems based on the construction of a 24-foot-wide and 1-mile-long section of road on sandy soil. For this analysis, it was assumed that 20-foot International Organization for Standards (ISO) containers would be used to ship the roadway systems. This size container has a capacity of 1,666 cubic feet, weighs 6,060 pounds, and can hold 68,890 pounds. The container's weight and dimensions set limits on what type, and how much, material can be transported.

   

Left, geofibers are mixed with sand to create a stable base for a resin-modified emulsion or emulsified asphalt surface. Right, hexagonal mats are designed for quick installation to create parking areas and access roads.

    Although the new roadway systems were developed for both sandy and soft soils, the ERDC analysis only compared the new systems recommended for sandy soils—geofiber stabilization, fiberglass-reinforced mat, and hexagonal plastic mat—with the existing systems for sandy soils—Mo-Mat, M8A1, and Geocell—
because the Army has no existing systems that can support heavy-truck traffic over very soft soil. The potential soft soil systems—plastic DURA-BASE mats and wood SOLOCO mats—may work for sandy soils as well.

    The table shows the requirements in terms of weight, volume, cost, number of containers, and placement rates for each system to construct the 1-mile section of road. When the existing technologies are compared to the new road systems, several advantages of the new systems can be identified. The fiberglass-reinforced mat has less weight and volume and requires fewer containers to ship enough mat panels to build a 1-mile section of road than the M8A1 mat. The fiberglass- reinforced mat also has a faster placement rate than the M8A1. These reductions represent a construction cost saving of over 35 percent. The fiberglass mat is 50 percent lighter, requires 60 percent fewer containers, and can be installed 100 percent faster than the Mo-Mat. While each Geocell grid is smaller and lighter than the fiberglass-reinforced mat, it takes 6 times longer to install.

Geocell, or sand grid, is used to provide a load-distributing base layer for a road.

Fiberglass-reinforced mats are bolted together to provide a resilient road surface. DURA-BASE mats are very heavy and require a forklift or crane for installation.

    Using geofibers for roadway construction reduces cost, weight, and containers by 87 percent, 39 percent, and 45 percent, respectively, when compared to the M8A1 and Mo-Mat. Geocell is 38 percent more expensive and 4 percent heavier than the geofibers. The hexagonal plastic mat costs 50 percent less and installs 225 percent faster than Mo-Mat.

Soft Soil Systems

    Large quantities of plastic DURA-BASE mats or wood SOLOCO mats are needed to construct a road over soft soils. A geotextile should be placed over the soft soil subgrade to avoid mud intrusion. For a soft soil subgrade condition (CBR less than 1), two layers of DURA-BASE or SOLOCO mats are needed. (Two layers of SOLOCO mats also are needed for sandy soils.) Consequently, 2,252 DURA-BASE panels and 123 containers would be required to construct a 1-mile section of road over a very soft soil. The change of the subgrade condition to a soft soil represents a 100-percent increase in the weight, volume, number of containers, and cost of the proposed solutions over what would be needed for a sandy soil (see table above). Also, DURA-BASE and SOLOCO mats require a crane or rough-terrain forklift for installation and handling because of their weight and size. The placement rates for these mats when placed over sands, as shown in the table, will decrease for soft soils.

    The soil condition will dictate which system can be used for expedient road construction. Fiberglass-reinforced mats, hexagonal mats, and geofiber stabilization are recommended alternatives to the existing Geocell technology for sandy soils. For sandy soils, the fiberglass-reinforced mat should be used for small roadway sections (less than ½ mile) and geofibers should be used for large roadway sections (longer than ½ mile). These roadway systems are lighter, faster, and more tactically mobile. DURA-BASE and SOLOCO mats should not be used to construct roads over sandy soils because of their cost, logistics requirements, and the availability of more suitable solutions. However, for soft soil, the DURA-BASE and SOLOCO mats are the only existing alternatives that have performed successfully when tested under heavy truck traffic.  ALOG

    Rosa L. Santoni is a research civil engineer for the Army Engineer Research and Development Center, Geotechnical and Structures Laboratory, in Vicksburg, Mississippi. She holds bachelor's and master's degrees in industrial and civil engineering from the University of Puerto Rico at Mayagüez and a master's degree in civil engineering from Mississippi State University.

    Jeb S. Tingle is a research civil engineer for the Army Engineer Research and Development Center, Geotechnical and Structures Laboratory. He holds bachelor's and master's degrees in civil engineering from Mississippi State University.