401 - Culverts
The inspector’s preliminary investigation should determine if the culvert is correctly staked and the pipe is the type, size and class as called for on the plans, or as allowed by specification. The pipe must be accompanied by the manufacturer’s certification, tested stock reports, or other requirements as specified in the Materials Quality Assurance Manual.
All culvert, regardless of type (i.e. concrete, corrugated metal, smooth lined corrugated plastic pipe culvert, etc.), must be clearly marked with the following three pieces of information:
- The pipe class and specification designation.
- The date of manufacture.
- The name or trademark of the manufacturer.
Other culvert requirements that are type specific are:
Circular concrete pipe with elliptical reinforcement must have lift holes cast in the pipe which will indicate the top of the pipe as installed. These lift holes are to be sealed with concrete plugs and waterproofed after installation.
Joints for smooth lined corrugated plastic pipe (CPE) shall be provided with homing marks to indicate correct joining of the pipe and the joint material during field installation.
The inspector should visually inspect the culvert pipe periodically and should reject any damaged pipe, even if it is certified by the manufacturer, or is from tested stock. Damage incurred during shipment, unloading at the project site, or handling en route to its placement location can be determined only by inspection just prior to installation.
Some common culvert deficiencies, which are specific to concrete, metal and plastic pipe and which may be cause for rejection prior to installation are listed below.
- Fractures or cracks passing through the shell, except for a single end crack that does not exceed the depth of the joint.
- Defects that indicate imperfect proportioning, mixing or molding.
- Defects which indicate honeycombed or open texture.
- Damaged or cracked ends where such damage would prevent making a satisfactory joint.
- Exposed circumferential steel reinforcement that would indicate misalignment of the reinforcing.
- Any continuous crack having a surface width of 0.01 inch (0.25 mm) or more and extending for a length of 12 inches (300 mm) or more, regardless of position in the wall of the pipe.
- Small imperfections in manufacture or damage during handling. These may be repaired as long as repairs are sound and properly finished and cured.
- Bruised or broken zinc coating. Must be repaired by Contractor at own expense in accordance with subsection 716.03 of the Standard Specifications for Construction.
- Out-of-round shape.
- Deviation from a straight alignment.
- Dents or bends in the metal.
- Loose, poorly formed or missing rivets.
- Out-of-round shape.
- Blemishes in the extrusion.
- No homing marks (as previously described) to indicate correct joining of the pipe and joint material during field installation.
- Unprotected/uncovered gaskets which may cause them to dry out or become dirty.
When only the size and class of culvert is specified, the Contractor may furnish any of the alternates permitted in Table 401-1 of the Standard Specifications for Construction. Additional materials are now allowed for culverts to be used on state trunkline projects based on design life and proposed pipe use. Design life is 25 years for driveway culverts, 50 years for culverts and 70 years for sewers.
When a particular type of culvert material is required in lieu of the classes designated in Table 401-1 of the Standard Specifications for Construction, the type and size of the culvert will be specified in the proposal or plans. A higher strength or greater thickness of culvert may be substituted for the minimum strength or minimum thickness of culvert specified.
With corrugated metal pipe, rivets shall have full hemispherical heads and shall completely fill the holes. Rivets shall be spaced not more than 6 inches (150 mm) apart circumferentially. In 12 inch (300 mm) diameter pipe, six rivets are usually sufficient. One rivet in the valley of each corrugation is required longitudinally. For corrugated metal pipe greater than 42 inches (1050 mm) in diameter, the longitudinal seam shall be double riveted.
Dissimilar types of base metal (steel or aluminum alloy) or dissimilar types of coatings on steel (zinc or aluminum) shall not be used in a single line of pipe. The only exception to this is that zinc coated steel end sections may be used with aluminum coated steel pipe. All coupling bands shall be of the same base metal and coating metal as the pipe.
Culverts should be placed on suitable foundation material. Soft or spongy materials do not provide a firm bed for the pipe culvert. Excavation for culverts will be in accordance with subsection 206.03 of the Standard Specifications for Construction, except that the culvert bedding shall be constructed using Granular Material Class IIIA according to the details of the plans.
Where rock or unyielding hardpan is encountered in the culvert trench, it should be excavated to a minimum of 6 inches (150 mm) below the proposed bottom of the pipe, backfilled with Granular Material Class IIA and compacted. The Standard Plan R-82 series provides details for bedding and filling around pipe culverts and should be strictly followed. The backfill should be thoroughly tamped under the haunches to provide all the bearing possible.
Compaction can be checked using a stake. If the stake can be pushed into the bedding material under the haunches, further compaction is required. A pneumatic tamp may be required instead of a vibratory compactor. In these confined working areas, density tests farther out from the culvert cannot be assumed to reflect the compaction under the pipe. Therefore, the efforts of the inspector are often the only reliable assurances of adequate compaction in these areas.
All culverts shall be laid true to the lines and grades given, bells or grooves upgrade, ends fully and closely jointed, and each section shall have full, firm bearing throughout its length. All pipe joint assemblies for use in culverts shall be selected from the Qualified Products List for Watertight Sewer and Culvert Joint Systems. A field assembly diagram shall be provided to the Engineer. All culverts with diameters greater than 24 inches (600 mm) shall have pipe joints wrapped with a non-woven geotextile fabric. The fabric shall have a minimum width of 36 inches (900 mm) and shall be centered on the joint.
If circular concrete pipe with elliptical reinforcement is installed, it should be installed with lift holes on top of the pipe. The manufacturer’s marks designating the top and bottom of the pipe shall not be more than 5 degrees from the vertical plane through the longitudinal axis of the pipe. After installation, the lift holes shall be sealed with suitable concrete plugs and waterproofed.
Reinforced concrete elliptical pipe shall be installed with the long axis placed horizontally unless otherwise specified.
Wedge lock dimple bands are not allowed for corrugated metal pipe. Angle bolt dimple bands are allowed only on the first connection when extending an existing corrugated metal pipe with a corrugated metal pipe.
Flow velocities are a definite factor in the self-cleaning ability of corrugated metal pipe (CMP). A desirable minimum slope of 0.5 percent is suggested for CMP culverts. Every means should be taken to provide at least this minimum slope. Otherwise, the culvert may soon cease to function and will become a continual maintenance problem.
It is important to backfill the pipe as soon as possible after placement so that runoff is directed through and not alongside the pipe. This will reduce the possibility of undercutting the pipe and displacing it. When the culverts are within the influence of the roadbed as shown on the plans, all Granular Material Class II, III, or IIIA backfill material (as specified) shall be placed in layers not more than 10 inches (250 mm) in thickness; and each layer is to be compacted to 95 percent of the maximum unit weight. Backfill for culverts outside the influence of the roadbed shall be placed as detailed on the plans and compacted thoroughly. Backfill should be brought up along both sides of the culvert at the same time so as to not displace the pipe.
The backfill material for corrugated plastic pipe will be Granular Material Class III or IIIA to a minimum of 24 inches (600 mm) above the pipe and as detailed on the plans, except that no stones larger than 1 inch (25 mm) in diameter are allowed within 6 inches (150 mm) of the pipe. The backfill is placed in layers not more than 10 inches (250 mm) or half the diameter of the pipe, whichever is less. Staking or other methods to restrain the pipe may be necessary during the backfilling operation to maintain the line and grade of the culvert. The costs of restraining the pipe are included with the appropriate item of work and not paid for separately.
To prevent damage to the pipe, a minimum of 3 feet (1 meter) of cover must be maintained at all times, except when trimming for final grades. The cost of providing temporary cover will be included with the appropriate item of work and not paid for separately.
Pipe culverts jacked-in-place are used when it is desired to place a drainage feature without disturbing the roadbed or railroad above the pipe. The excavation should not be more than approximately 1 inch (25 mm) greater than the outside diameter of the pipe. The pipe should be advanced with the hole to prevent caving and subsidence of the grade. A steel cutting edge or shield may be attached to the front section of pipe to form and cut the required opening for the pipe.
Frequent checks of the line and grade should be made to make sure that the pipe is going in the proper direction. Minor changes in direction can be made by a competent jacking crew.
Full circular pipe reinforcement is required when concrete pipe is used. The watertight joint requirement does not apply to jacked-in-place culverts. Joints are to be thoroughly wetted, filled with mortar and wiped smooth. Corrugated steel pipe sections are riveted together or joined with couplings furnished by the manufacturer. Voids between the excavation and the pipe are filled using approved grout and grouting methods.
The face of excavation must be bulkheaded whenever work is shut down, especially during the passage of trains, when jacking under a railroad. Sheeting and bracing plans of jacking pits are to be submitted for railroad company approval through the MDOT Engineer. Excavation of the jacking pit in conjunction with jacking under a railroad may not start until this approval has been received.
Culvert sections showing signs of settlement or poor alignment shall, as directed by the Engineer, be taken up and relaid at the Contractor’s expense.
Any damage caused by grading equipment or other construction equipment must be corrected as soon as possible, at the Contractor’s expense.
Experience has shown that occasionally sign posts, guardrail posts and other miscellaneous items are driven through the pipe. Culverts should be inspected periodically after installation and prior to acceptance of the project to ensure that no damage has occurred due to the Contractor’s operations.
All dirt or sediment that has washed into the pipe shall be cleaned out before final acceptance of the work.
If smooth lined corrugated plastic pipe (CPE) is installed on a project, at least 50 percent of the installed length of each size pipe shall be tested for deformation by the Contractor, using a nine point mandrel. The arms of the mandrel extend to a diameter equal to 95 percent of the nominal pipe diameter. The Contractor is to provide both the mandrel and a proving ring. The proving ring should be used by the inspector to verify that the diameter of the mandrel is correct before it is used.
The Contractor is to perform the mandrel testing after the required backfilling of the trench is achieved, but between 5 and 10 days prior to pavement surfacing or completion of the final grade, except as otherwise approved by the Engineer. For enclosed pipe, the Contractor accesses the pipe from one drainage structure and pulls the mandrel through to the next drainage structure. If the mandrel pulls through the pipe without snagging, the pipe has retained its roundness and has an acceptable diameter (the diameter has been reduced less than 5 percent).
If the mandrel gets hung up on any section of pipe as the Contractor pulls it through, it indicates that the pipe has not retained its roundness after installation and does not have an acceptable diameter (the diameter has been reduced by 5 percent or more). CPE pipe with a diameter reduced by 5 percent or more must be removed and either reinstalled (if the pipe is not damaged) or replaced at no cost to MDOT. The Contractor is responsible for all expenses and delays due to the replacement of deformed or damaged pipe. No re-rounding of CPE pipe with a diameter reduced by 5 percent or more is allowed.
The Contractor must conduct a video inspection of all types of installed culverts with diameters from 12 to 36 inches (300 to 900 mm) with the following exceptions.
- Driveway culverts.
- Culvert extensions of less than 50 feet (15 m).
- Extension of existing catch basin leads of less than 20 feet (6 m).
- New culverts of less than 50 feet (15 m).
The purpose of the video inspection is to determine any damage or installation defects to the culvert, including pipe deformation, cracking, joint separation, corrosion, perforation, excessive siltation, excessive ponding of water, or any other discernible feature observed in the video. For corrugated plastic pipe, videotaping of all pipe 12 to 36 inches (300 to 900 mm) in diameter, unless an exception as listed above, is required after installation, regardless of whether the same pipe is mandrel tested by specification. The sole purpose of the mandrel test is to determine deformation.
For culverts installed under the pavement, the inspection must be conducted after the required backfill compaction of the trench has been achieved, and between 5 and 10 working days prior to pavement surfacing or completion of final grade, except as otherwise approved by the Engineer. For culverts not under pavement, the video inspection is conducted as close to the project completion as possible, while still allowing sufficient time for corrective action. All video inspection is conducted as described in subsection 402.03 of the Standard Specifications for Construction.