Culverts & Highway Drainage
Culverts, Subdrains, Ditch Enclosures
Culverts, Subdrains, Ditch Enclosures
Used beneath roadways and even under railroad tracks, culverts are usually short in length with both ends of the pipe open to direct water. But they need to be strong to support overburden embankments and heavy traffic loads. Culverts often provide access to commercial developments, homes, farms or livestock facilities, or beneath haul roads used for mining or forestry.
Depending on the diameter, corrugated HDPE pipe meets or exceeds broadly accepted standards such as AASHTO M252 or M294 and ASTM F667, F2306, or F2648 and is listed in the AREMA manual for use in railroad projects. Similarly, corrugated polypropylene pipe meets or exceeds the standards set by AASHTO M330 and/or ASTM F2881. Both Type C (corrugated interior) and Type S (smooth interior) are widely available. Corrugated plastic pipe is approved by most state Departments of Transportation, local municipalities, and regional agencies for use as culverts and in other storm drainage applications.
Corrugated HDPE is also an approved culvert material within CSA and BNQ standards in Canada.
Besides culverts, common applications for highway drainage are median drains (between lanes on a divided highway) and edge drains (along the pavement edge). Median drains use larger pipe, usually without perforations, and collect water from surface flows and edge drains. Edge drains use small perforated pipe to drain runoff from beneath the pavement.
Corrugated pipe with a smooth interior has excellent flow efficiencies, allowing sediment to be flushed out of the system while keeping drainage at capacity. With such excellent hydraulics, pipe can often be reduced by at least one diameter from its corrugated interior counterpart. This downsizing can reduce the pipe cost and allows for a narrower trench, which reduces excavation and backfill requirements and reduces installation costs.
Most state transportation departments, regional agencies, and local municipalities have approved corrugated plastic pipe for culverts, median and edge drains and other applications. HDPE corrugated pipe meets AASHTO M252 or M294, depending on the diameter. Most suppliers produce both a corrugated interior (Type C) and a smooth interior (Type S). The pipe is available with or without perforations. Generally, plain pipe is used for median drains. Pipe is available with a complete line of fittings and accessories and joint qualities, including watertight joints.
A subdrain system is an underground network of piping used to remove water from an area that collects or retains surface water or groundwater. The network can be small and designed to drain a limited area, or large enough to drain a sizeable number of acres.
Surface water can be collected into the subdrain system by installing a surface inlet or catch basin. Groundwater is collected by allowing water into the pipe through perforations. Both surface water and groundwater can be discharged to an appropriate outlet such as a nearby storm sewer, pond or other waterway.
The effectiveness of a subdrain system is determined by the native soil, spacing, and depth of the pipe, as well as its diameter. It is important to determine specifically why a site needs improved drainage:
Answers to such questions help determine how deep the pipe should be buried, how close together the laterals should be placed, and the pipe size and slope. The native soil is also a major factor in the system layout. Tight clay soils do not release groundwater as readily as looser sandy soils so a network needs to be designed for local conditions.
Subdrain system requirements vary tremendously, often even on the same site. Corrugated plastic pipe producers manufacture pipe in diameters from 2 to 60-in (50 to 1500 mm), with or without a smooth interior, perforations and geotextile wrap; and a full line of fittings to meet just about any layout configuration. Most pipe is compatible with a variety of inlet structures.
Deep burials are not typical for subdrain systems. However, there may be occasions when the pipe will experience major soil loads especially if the ground is hilly or rolling. Corrugated plastic pipe can meet cover requirements of 20 ft (6 m) or more with proper backfill.
A ditch enclosure involves installing a pipe where there was originally an open ditch, a common irrigation application in the western United States. The primary reason for enclosing irrigation channels is to mitigate water loss due to evaporation and reduce infiltration. But they also improve safety, the aesthetics of the area, and increase hydraulic performance versus an open ditch.
Pipe used in a ditch enclosure is usually shallow and can experience traffic loading if installed at road crossings. They are often backfilled to support vehicle loading. The flexibility of plastic pipe provides superior durability than rigid concrete or steel products. For irrigation ditch enclosures only corrugated plastic pipe designed for this application and watertight joints should be used.