The installation of a pipe culvert is a systematic process. Following the process will help to reduce oversight and mistakes. The proper installation of a culvert is absolutely essential for its performance. If damage occurs due to improper handling or storage, pipe sections may not properly fit up in the field. If trenches are not properly excavated, bedding and backfill placement cannot be completed adequately. If bedding or backfill materials are not compacted properly, settlement will occur which could lead to failures in the roadway. If pipe joints are not correctly sealed, exfiltration or infiltration may occur.
Prior to any excavation, it is the contractor’s or NCDOT's responsibility to call a utility locator service, utility company, municipality, telephone company, and/or cable company to mark lines that need to be relocated. Inspectors are responsible for knowing that utility relocation is required prior to excavation and must be aware of all utilities located on this project.
2. INSTALLATION TYPES
There are two main types of pipe installations: Trenched installations are those where pipe is placed in natural ground or a constructed fill section with a specified trench width. Embankment installations are those where pipe is placed on natural ground and then covered by a constructed embankment.
For both types of installations, correct placement and compaction of the material surrounding the pipe are crucial. This makes the need for following the specified methods of foundation conditioning, laying pipe, and backfilling of the pipe essential. In addition, for trenched installations, the load transmitted to the pipe is directly related to the trench width. Therefore, if the actual trench width is not within the specified limits, the load transmitted to the pipe will be greater than designed and distress in the pipe may occur. Details on methods of installation can be found in the NCDOT
Roadway Standard Drawings 300.01, 300.02, 300.03, and 300.04
-- see Appendix A of this Manual.
Other types of pipe installations include Jacking and Boring and Tunneling. These two types are used whenever conventional open excavation is not practical or deep installations are needed. The advantages of these installations include: no interruption in traffic on the overlying roadway, cutting and patching of existing pavement is not necessary, and depth of installation is of no concern.
3. PREPARATION OF PIPE FOUNDATION
Prepare the pipe foundation in accordance with the applicable method shown on the plans, true to line and grade, and uniformly firm. A firm, uniform foundation is a necessity because a pipe culvert performs both a hydraulic and a structural function. Distortion of the foundation under load may result in pipe failure and ultimately embankment failure.
During the preparation of the pipe foundation, unsuitable material or rock may be encountered. One must use sound judgment in determining whether a foundation contains suitable or unsuitable material. A probe rod can be used to confirm suspicions of unsuitable material. If the rod indents under firm pressure, the foundation is probably satisfactory. If the rod easily penetrates under a firm push, the foundation needs adjustment.
No pipe culvert shall rest on rock foundations or foundations that cannot be reasonably expected to support the anticipated dead and live loads. Where material is found to be of rock or of poor supporting value and when the Engineer cannot make adjustment in the location of the pipe, undercut existing foundation material within the limits established on the plans or as shown in NCDOT
Roadway Standard Drawings 300.01, 300.02, 300.03, or 300.04
, as applicable.
In areas of unsuitable material, undercut to depth as directed by the Engineer. Ideally, this depth would be to some lower strata of material capable of supporting the load. In this case, the foundation conditioning could consist of any earth material considered acceptable for embankment. Many times, however, it is not practical to undercut to a depth that a suitable strata of foundation material can be obtained. In this case, the Engineer should direct the depth to be that as considered sufficient to "bridge over" the unsuitable material.
During the undercutting of the material, the pipe inspector will need to do a visual check for water (seepage and standing) in the trench foundation, especially in the mornings. A small trickle can be handled with foundation conditioning material (crushed stone or gravel) or underdrains. Read
Section 300-8 and 300-9 in the Standard Specifications
and the Construction Manual for the method of measuring and basis of payment for foundation conditioning material.
If more than a trickle is observed, other methods should be employed to remove standing water. Sump pumps and wellpoints are used frequently to keep trenches dry while excavation or work activities are ongoing.
b. Foundation Conditioning Material
Once the sufficient undercut depth is reached, it may be possible to bridge with local material meeting the requirements of embankment material. However, if a wet condition is encountered, it will normally require a more granular type of backfill. When other than local material is used, the material must meet the requirements of
for Class II or III.
Regardless of the foundation conditioning material used, it shall be compacted to a degree that will afford a firm, uniform foundation.
Where pipe culverts are placed on compressible material, camber should be placed in the grade of the pipeline to compensate for settlement. The amount of camber used depends on the load imposed on the foundation materials and the compressibility of the material. Since these factors vary, judgement is required in selecting the amount of camber to be used. In an effort to provide some guidance in the selection of values for camber, a chart of these values is included in Appendix D of this Manual. Unless more precise information is available, the values taken from the chart should be used.
d. Preparation of Bedding
Once the foundation for the pipe is completed the actual surface on which the pipe is laid must be prepared. This bedding surface distributes the resistive load of the foundation around the bottom circumference of the pipe and reduces the resulting point loading. See the Standard Drawings for more details on proper bedding requirements. The shape of the bedding should be carefully checked by template to insure the formation of a proper cradle for the pipe. The pipe should fit snugly in the cradle. Any gaps between the pipe and the cradle which are large enough to poke fingers into are too large and should be fixed. When a bell and spigot type of pipe is used, the foundation must be excavated to a sufficient depth and width to accommodate the bell such that the entire length of joint is resting snugly in the foundation. If the excavation is too large, the joint will not have the necessary foundation support. The cradle must also be straight. The line and grade of the bedding should be checked using either an offset string line, batter boards, or laser. When a string line is used, it should be supported at intervals not to exceed 50 feet so as to prevent sag. A good "visual examination" down the string line will assist in removing sags and/or errors made in establishing the string line. When the Contractor elects to utilize laser methods to establish line and grade, NCDOT personnel should insure that the instrument is properly set up and functioning correctly. The bedding should be smooth and free of large rocks or other protrusions which may also cause point loading. No material larger than 2" in size should come into direct contact with the pipe.
4. Invert Elevations
Invert elevations for cross pipe drainage must be set so that aquatic life movement is not substantially disrupted. To this end, inverts of cross pipe with diameters from 18" through 48" shall be buried below the streambed a minimum of 20% of the diameter of round pipe or the rise of arch pipe. Pipe diameters of 54" and larger shall be buried a minimum of 1.0 foot. Maximum burial depth will be as indicated on the plans.
Invert elevations of storm drainage systems shall be in accordance with the NCDOT Roadway Design Manual - Part I, Section 5-3, and must meet minimum cover requirements as noted above in Chapter 4.
Invert elevations for TIP projects let under the 1995 Standard Specifications shall be as directed by the Engineer. TIP projects let under the 2002 NCDOT Standard Specifications shall be as specified in
of the Specifications.
B. LAYING PIPE
Problems can develop for pipe if the joints are not connected properly. The problems can range from minor to serious in nature. Typical joint defects include leakage (exfiltration and infiltration), cracks, and joint separation.
Exfiltration occurs when leaking joints allow water flowing through the pipe to leak into the supporting material. Minor leakage may not always be a significant problem unless soils are quite erosive. Leaking joints may be detected during low flows by visual observation of the joints and by checking around the ends of the culvert.
Infiltration is the opposite of exfiltration. Many culverts are essentially empty except during peak flows. When the water table is higher than the culvert invert, water may seep into the culvert between storms. This infiltration can cause settlement and misalignment problems if it carries fine grained soil particles from the surrounding backfill. Infiltration may be difficult to detect visually in its early stages, although it may be indicated by open joints, staining at the joints on the sides and top of the culvert, deposits of soil in the invert, or by depressions over the culvert.
Cracks in the joint area may be caused by improper handling during installation, improper joint material/gasket placement, and movement or settlement of the pipe sections.
Joint separations may be caused by settlement, undermining, or improper installation. Joint separations are significant because they accelerate damage caused by exfiltration and infiltration resulting in the erosion of the backfill material.
During installation, care should be taken when joining the pipes to prevent damage by mechanical equipment. Blunt forces will not be allowed, such as hitting the end of the pipe with a backhoe bucket, to push the joints of pipe together. Batter boards or other sufficient material should be used to cushion the blow, when this type of equipment is used. Damage to the pipe ends will be cause for rejection.
Ensure the first section of pipe is not disturbed as the next section is being joined to it. A major disturbance of the pipe will result in the pipe being out of line and/or grade, and it will need to be reset. Lay the pipe on the prepared foundation, bell or groove end upgrade with the spigot or tongue fully inserted to make a soil-tight joint.
Make sure that the sections are compressed together properly. Any sections that are dropped or dragged should be inspected immediately for damage. Check each joint for alignment and grade as the work proceeds.
When installing multiple lines of pipe, leave a clearance equal to one-half the diameter of the pipe between each line to provide adequate space for proper compaction of the backfill material.
C. BACKFILLING PIPE
As stated before, correct placement and compaction of the material envelope surrounding pipe is crucial to the performance of the culvert. Well-compacted material surrounding the pipe serves three main purposes:
1) Provides the lateral pressures needed to support the pipe structurally,
2) Prevents settlement that can induce unwanted stresses on the pipe, which can ultimately cause pipe failure or failure of the overlying roadway.
3) Reduces the movement of groundwater around the outside of the pipe, which can undermine the pipe and possibly lead to failure.
The type of material used for backfill is dependent on the fill height requirements specified:
Fill Height is 30 feet or less:
An "approved backfill material" is required and may be any material which is considered acceptable for embankment. The backfill material should be suitable local material if available. Backfill material must be kept free of rocks greater than 2", frozen lumps, chunks of highly plastic clay, organic materials like tree stumps, and other objectionable material.
Fill Height is over 30 feet through 40 feet:
Select backfill material is to be used meeting the requirements of Class II select material as shown in
Fill Height is over 40 feet through 50 feet:
Select backfill material is to be used meeting the requirements of Class III select material as shown in
Fill Height is over 50 feet:
Select backfill material is to be used meeting the requirements of Class IV select material as shown in
Backfill should be placed around the pipe in accordance with the applicable method shown on the plans or the Standard Drawings, in layers not to exceed six (6) inches loose, unless otherwise permitted. Approval for increasing layer depths above six (6) inches should be granted only when the backfill meets the criteria for Class I or Class II select material as shown in
No subsequent layers shall be placed until the preceding layer has been satisfactorily compacted. The lifts of backfill shall be kept approximately equal in height on each side of the pipe to prevent distortion of alignment and pipe cross section by the compactive effort. Care also should be taken during backfill and compaction operations to prevent damage to the joints.
Periodic density tests shall be made on pipe backfill to verify the fact that the method of compaction is giving satisfactory results. This is especially important at the start of backfilling operations to allow the Engineer to get the "feel" of satisfactory compaction. This "feel", whether found by using one's thumb, heel , or a probing rod, may be used as a guide when density tests are not being made. Density testing is also very important whenever the type of compactor being used changes. Wherever possible, density tests should be made on pipe backfill placed in a trench section where the top of the trench represents the subgrade line. It is in this area of pipe backfill that most pavement irregularities and failures occur due to a lack of proper backfill compaction.