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DIB 83-02 - 7.1 Influencing Factors
DESIGN INFORMATION BULLETIN No. 83-02
CALTRANS SUPPLEMENT TO FHWA CULVERT REPAIR PRACTICES MANUAL
Urbanization is the most dominant factor in modifying the calculated runoff of a watershed. Other factors include logging and cultivation, hydraulic roughness (natural and man-made channels), and updated climatic data. All of these factors should be reviewed for changes and accounted for when replacing or rehabilitating a culvert. Refer to Topics 803 and 812 through 815 in the HDM and FHWA Culvert Repair Practices Manual Volume 1, pages 2-1 and 2-2 for factors affecting runoff, and for Departmental procedures for upgrading existing drainage facilities.
Debris, if allowed to accumulate either within a culvert or at its entrance, can adversely affect the hydraulic performance of the facility. Refer to Index 813.8, and Topic 822 in the HDM for a discussion on Debris Control and Bulking. Vegetation, if allowed to accumulate at the downstream end of a culvert will raise the tail water. If the culvert is operating under inlet control, it may be better not to remove the vegetation since it will not significantly affect the capacity and may serve to create a lower outlet velocity. Under inlet control, the cross sectional area of the culvert, inlet geometry and elevation of the headwater at the entrance are of primary importance. However, even though the roughness of the culvert barrel has minimal impact to the headwater elevation, increasing the roughness will serve to reduce velocity. On the other hand, if the culvert is operating under outlet control, the vegetation may need to be removed since it resists flow to the point of affecting the culvert capacity. Other factors affecting tail water include backwater in the vicinity of a confluence downstream, and tidal influences. At these locations, aggradation or deposited sediments may lessen channel and culvert capacity and increase headwater depth and flood heights. Outlet control involves the additional consideration of tail water elevation, and the slope, roughness and length of the culvert barrel. These two types of control are important hydraulic concepts to be considered when choosing the type of lining method or impacting entrance and/or exit conditions. Refer to Index 825.2 in the HDM and FHWA Culvert Repair Practices Manual Volume 1, pages 2-3 to 2-6 for a discussion on Culvert Flow. Outlet velocity is another factor to be considered when relining or changing the roughness of the culvert barrel. Refer to Topic 827 in the HDM for a discussion on Outlet Design.
Refer to Index 110.12 in the HDM for a discussion on safety for jacking and tunneling and tunnel classifications in relation to potential flammable gas or vapor. Refer to Topic 309.1 in the HDM for a discussion on horizontal clearances (e.g. existing headwall and end wall location on rural 2-lane highways). Other safety considerations will be dependent on the scope of the rehabilitation and ADT of the highway. For example, using a trenchless technology method to replace a culvert may result in a reduced number of construction related traffic accidents. Workers are less exposed to traffic and there is usually less disruption to traffic. In addition, there are fewer (but more specialized) workers needed for most trenchless technology jobs that should enhance overall project safety. Consideration should always be made for safety to the traveling public when considering the ability of a deteriorated pipe to support roadway and traffic loads. See Index 11.1.1.
Repair, rehabilitation, or retrofit projects must be developed that will balance biological, engineering, and hydraulic considerations. Examples of this may include but not limited to;
a) Water quality considerations for compaction grouting where groundwater may be present.
b) Omission of certain pipe lining methods (such as water heated cured in place) in biologically sensitive areas where construction residue may contaminate the stream with styrene residue. Also, stream flow will be interrupted during installation of many rehabilitation/replacement techniques.
c) Chemical grouting to stop infiltration at deteriorated, leaking or open joints in small diameter (24 inches or less) pipes. The most commonly used gel grouts for this are of the acrylamide, acrylic, acrylate and urethane base types. Acrylamide base gel is significantly more toxic than the others. Grout toxicities are of concern only during handling and placement or installation, however, EPA has now withdrawn a long-standing proposal that sought to ban the use of acrylamide grouts.
The modification of an existing culvert to facilitate the movement of fish to spawn can introduce several problems in the operation of an installation. Culverts are generally designed to operate under inlet control, which can be detrimental to fish passage. See the picture below for an example where the outlet scour hole created a jump too high for fish passage.
If a culvert is modified to operate under outlet control, or modifications are made to the barrel, there may be a decrease in efficiency, and related increase in water depth and sedimentation. Refer to FHWA Culvert Repair Practices Manual Volume 1, pages 3-58 to 3-61, 5-39 to 5-50 and Volume 2, Appendix B-23, for a discussion on Fish Passage and Fish Passage Devices. Most recently, the California Department of Fish and Game and NOAA Fisheries have each published guidelines on fish passage. Refer to the DRAFT Culvert Criteria for Fish Passage and the Guidelines for Salmonid Passage at Stream Crossings by these agencies.
When using “tight fitting” rehabilitation methods (i.e., no annular space between the host pipe and the liner, e.g., cured in place or deformed/reformed HDPE) in small diameter host pipes, it is essential to inspect the existing pipe by physically entering the pipe or with a remote controlled camera. See Index 3.1.1. It may also be necessary prior to construction to verify dimensions and remove protrusions with the use of a proofing pig. A pig is a bullet shaped device made of hard rubber or similar material that is pulled through the host pipe. This technique has low mobilization costs and low to moderate overall costs.
Although plastic pipe sizes in excess of 60 inches are available (not in all styles), any re-lining project that proposes to utilize such large diameters should be treated as a special design and consultation with the Headquarters Office of Highway Drainage Design within the Division of Design and the Underground Structures unit in the Division of Structures within the Division of Engineering Services (DES) is advised. For any plastic liner or slipliner, if the diameter exceeds 60 inches, headquarters approval will be required.
126.96.36.199 Headquarters Assistance/Approval for Pipe Replacement using Trenchless Excavation Construction (TEC) Methods
It is strongly advised to contact the above-referenced headquarters units along with the Headquarters Office of Permits and Geotechnical Design within the Division of Engineering Services (DES) for assistance when considering replacement using the trenchless construction (TEC) methods that are referenced in Index 188.8.131.52. Many of the TEC methods and pipe materials will need Headquarters approval by the Office of Highway Drainage Design
The following steps summarize the general process that should be followed to coordinate with Geotechnical Services:
Geotechnical Services will not typically be the initial contact for culvert related problems. For most instances, District Maintenance will have either already conducted an initial inspection as part of their culvert management system, or may have conducted an inspection if the problem (typically a sinkhole or other surface depression) was identified by Maintenance forces. Where information from Maintenance does not already exist, but Design desires an inspection as part of a programmatic upgrade/rehabilitation to existing facilities, the designer should contact District Maintenance and schedule the inspection of culverts that are suspected of needing repair. This information, either pre-existing or via specifically scheduled inspection, needs to occur early in the PID phase of the project in order to generate appropriate repair strategies and their associated cost estimates.
After the initial inspection has been performed by Maintenance, involvement by Geotechnical Services may be necessary if any of the following factors are present or suspected and documented (documentation either by Maintenance forces or by the designer, based on the input from the maintenance inspection):
- Soil infiltration
- Obvious piping
- Sinkholes or significant depressions
- Voids or loose soils beyond the immediate area of the invert
If involvement by Geotechnical Services is requested (usually by Design or Maintenance), then a field reconnaissance shall be coordinated with Geotechnical Services, District Maintenance and the District Project Engineer. At the field reconnaissance or earlier, Geotechnical Services shall be provided any available historical/maintenance information, As-Built records, maps, etc. Following the reconnaissance, the requesting office shall address in writing the scope of work requested from Geotechnical Services. Depending on the scope of work and the request letter, Geotechnical Services will develop a memo or Geotechnical Design Report to document both findings and recommendations. If none of the above factors are present or suspected, the Designer should coordinate with District Hydraulics for repair or replacement options to the culvert.
- During contract preparation the designer coordinates with the District Office Engineer, Geotechnical Services and District Hydraulics on developing specifications, bid items and quantities.
Note: In some cases (e.g. emergency projects) it may not be feasible to coordinate a more detailed investigation by Geotechnical Services and for them to develop a Geotechnical Design Report as outlined above. In these situations, Geotechnical Services should be contacted early on during contract preparation by the designer for guidance on how to include void detection and grouting or other mitigating measures within the construction contract.
Any proposal to insert a liner by pushing must also consider the issue of stresses on the face of the pipe being pushed. Pipes typically used as liners are not typically used for jacking and as such are not designed to have large compressive force applied to the end of the pipe. The maximum push distance is a function of weight, strength of the material, coefficient of friction between the liner and the host pipe and area of the pushing face. Most relatively short lengths (200-300 ft) of smooth wall liner in smaller diameters will rarely pose a problem. It is recommended that the designer consult with a manufacturers representative to obtain input on maximum push distance for various liner pipe diameters.
For example, using a coefficient of sliding friction equal to 1, (which is conservative) to line an existing 96 inch diameter CMP with a 14 Gage spiral ribbed pipe, the maximum push distance is approximately 270 feet, whereas for 12 Gage, it is approximately 400 feet. Skids are typically used to reduce the sliding friction during insertion when CMP is used as a liner for another CMP. For metal pipe, re-rolled ends with an external band, usually works best along with a bolt bar and strap connectors with the pieces of the "extra" bolt cut off after first being tightened to avoid catching the host pipe during insertion.
Another option is to use no-rolled ends with alignment tabs. If no-rolled ends are used, it is recommended to use alignment tabs on the exterior and a flat band with flat gasket on the interior for grouting (this should be removed after the line is grouted). This option has the advantage of not having to jack the full length of pipe all at one time; instead each piece (say a 20 ft length), or the maximum that can be can be inserted from one end and slid into place (there may not be access from both ends). In addition, using shorter individual pieces allows the flexibility of using a lighter gage. Once the pipes are in place the internal bands are removed after grouting the annular space.
Similar concerns would be raised with plastic pipe liners, depending upon type, size, etc., and can be an issue requiring either pushing the liner in from both ends of very long host pipes and then using an internal coupling to hold the pipe ends together until the annular space grout has cured, or using a combination of both pulling and pushing on the liner. If it is anticipated that pulling will be used, the designer must only specify liner pipes that have tensile strength at the joint sufficient to withstand the force of the pull. Where tension resistance is needed, plastic pipe will be limited to types with solvent or thermally fused joints. Generally the installation method using a backhoe shown in Index 184.108.40.206.1.6 is applicable for smaller diameter plastic pipe and does not apply to larger diameter plastic liners.
This page last updated August 20, 2011