Category:724 Pipe Culverts
The purpose of this article is to overview the construction and materials requirements related to culverts and sewer systems. Design information for these areas can be found in EPG 748 Hydraulics and Drainage and EPG 750 Hydraulic Analysis.
724.1 Pipe Selection and Sizing
MoDOT utilizes a wide variety of pipe materials and generally considers the design life of a culvert pipe to be 50 years. Sec 724 and EPG 750.7.2 Types contain information that applies to all culvert pipe types and instruction on how to select appropriate pipe types. MoDOT contracts prescribe either a specific pipe type or a group of pipe types for the contractor to choose amongst. Pipe use differing from the groups defined in Sec. 724 should not be allowed.
Complicating the issue is different pipe types have differing flow characteristics depending on whether the interior wall is smooth or corrugated. To ensure the pipe flow is sufficient, often two different diameters may be specified in the contract to account for the difference in flow characteristics.
Also affecting the contractors options are the allowable fill heights from the standard plans. MoDOT has performed the engineering to populate the fill height tables and the contractor and inspectors should ensure that the limits are observed.
MoDOT considers a single run of pipe that is open on both ends to be a culvert. A bank of pipes meeting this definition is considered to be one culvert.
A storm sewer carries rain water and consists of a network of culvert pipes joined together by drainage structures such as drop inlets and manholes.
Sanitary sewers can carry wastewater as well as rain water and consist of a network of culvert pipes and drainage structures that are designed to be completely water tight. Sanitary sewers usually have special joint details and may include pressure testing or other means to ensure the soundness of the system.
The male end of the pipe is the spigot end and the female end is the bell.
724.3 Rigid versus Flexible
Reinforced concrete pipe and vitrified clay pipe are considered to be rigid pipes while metal and thermoplastic pipes are considered to be flexible. Rigid pipes are considered to be their own structure while flexible pipes rely on the combination of the pipe and surrounding backfill to form a structural system. In general, rigid pipes are less susceptible to poor installation methods than flexible pipes with the most common problem for rigid pipe being joint fitment. Flexible pipes will generally require a wider trench and more careful installation to ensure success.
724.4 Installation and Bedding
For all types, proper bedding and installation greatly affect the long term performance of the pipe. The inspector should take the necessary steps to ensure the bedding is according to the standard plans.
The contractor may choose the type of backfill to be used and that choice greatly affects the allowed fill heights on flexible pipes. A-1-a soil type is typically 1” clean material and A-1-b is a course sand, but still must be clean. Any backfill with significant fines such as Type 5 base will likely fall into the A-2-x soil types and the allowed fill height will be lower.
The backfill must be properly compacted to ensure long term performance of the pipe. According to AASHTO standards, it can be assumed that a Type A-1-a soil (1” clean) will be 90% compacted just by dumping. All other cases require backfill material to be tested for adequate compaction.
The alignment of a pipe should be straight and at the correct slope to ensure hydraulic performance. In some cases where pipes are installed on top of new embankment, pipes are installed with a camber purposefully to ensure the settling of the embankment does not create a sag in the flow line.
In general, pipe joints should be soil tight as defined in AASHTO. When evidence is found that water is entering or leaving the culvert pipe at a joint, the situation should be investigated. Water entering or leaving at a pipe joint may erode the surrounding soils which are supporting the pipe and/or roadway. For this reason, joints that aren’t performing can be detrimental to the long term performance of the pipe and should be corrected.
Construction traffic over newly placed culvert pipes should be considered carefully as the type of construction traffic expected may need to affect the type of backfill used or the type of pipe material chosen. In some cases, the specifications and standard plans include details on what is allowable but ultimately the risk is on the contractor not to damage the new culvert.
724.5 Contractor Inspection
Contractors are required to provide post installation inspection of culverts as detailed in Sec 724. Various technologies may be used and in all cases the purpose is to ensure the pipe is sound, soil tight, and has not deflected during installation to a point that may affect long term stability. All Group A pipes are to be inspected regardless of pipe material type. The Resident Engineer should notify the contractor which Group B and Group C pipes to inspect based off of the rates found in the specification and any concerns they have.
There are two inspection methods listed in the Sec 724, manual inspection and remote inspection. Manual inspection involves a person actually entering the culvert and for that reason it is not recommended by AASHTO LRFD Bridge Construction Manual for culverts less than 24” in diameter. For both types of inspection the results should be provided electronically by the contractor per specification including a video recording of the inspection.
Joint widths and cracks should be measured and recorded on video by the contractor as outlined in Sec 724.
Deflection can be measured manually using a variety of methods. When performing remote inspection, deflection is measured by either a mandrel or a laser profiler. Deflection is measured against the actual diameter of the pipe, not the nominal diameter of the pipe. Some pipe types may have an as produced actual diameter that varies several percent from the nominal diameter. As a result, the use of an adjustable mandrel may help, but is not necessary. Use of a fixed mandrel is allowed so long as it is sized equal to or larger than the actual diameter minus 5% (DIAmandrel ≥ DIApipe actual x 0.95).
Additional details are available in each specific pipe section.
Pipe can be produced in either English or metric units at the manufactures choice and the prequalified list contains the unit selection for each plant. When reporting quantities, the units will always be English. When testing pipe, the as produced units may be needed to determine compliance with the specifications.