Examples of such non-standard loadings would be all-terrain vehicle trails or logging/mining roads which are subjected to significantly more or less than the standard HS 20-44 loading. In these cases, design engineers must understand that the intent is to assure that the section is firm and unyielding when subjected to the design load and make their decisions accordingly, exercising engineering judgment. Standard AASHTO design tables should be used whenever practical to determine the section beneath the articulated concrete block (References 1).ĪASHTO design criteria are sometimes difficult to implement for a project due to remote site locations, availability of construction materials and the like.

The most common vehicular loading used for low-water crossings design is AASHTO HS 20-44.Īs with most vehicular load or bearing capacity designs, the strength of site soils defines the thickness of the recommended section beneath the armoring or traveling surface layer. Roadway section design is a well established civil engineering practice which enables loading characteristics and standard loading sections to be applied to low-water crossings. This will depend on what kind of vehicle and associated wheel loads that will be traveling on the system, as well as the frequency of use. The first step with any vehicular loading design is to determine the load to which the roadway will be subjected. The articulated concrete block mattresses serve a dual purpose of erosion control armoring and as a flexible roadway surface. These designs consist of a properly-designed subgrade to support the anticipated loads, and articulated concrete block mattresses installed for erosion control and sized for hydraulic stability. This article focuses on equipment or vehicular loading as it relates to the use of articulated concrete block mattresses in low-water crossing designs. The Natural Resources Conservation Service (NRCS) in its 'Conservation Practice Standard for Stream Crossings' (References 5), defines a low-water crossing as 'a stabilized area or structure constructed across a stream to provide a travel way for people, livestock, equipment or vehicles'. Access roads in need of drainage remediation both new and old often are located in rural areas with relatively low traffic volume making low-water crossings a viable and cost-effective option to consider. In addition, roads continually are being built to provide access to new sites for recreational development and commercial expansion for which drainage structures are required. There are thousands of roadways providing back-country access across the North American continent where existing drainage structures, such as bridges or culverts, are in need of replacement. This article focuses on the three critical design elements of a successful low-water crossing project: design and construction of the load-bearing subgrade, hydraulic stability of the crossing and its surroundings, and the use of articulated concrete block as a flexible roadway surface.

When properly designed, an Articulated Concrete Block, also called ACB, low-water crossing can provide a hydraulically-robust load-bearing system while also facilitating fast-paced design and construction schedules.

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