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DIB 83-02 - 5.1 Problem Identification and Associated Repair for Culvert Barrels
DESIGN INFORMATION BULLETIN No. 83-02
CALTRANS SUPPLEMENT TO FHWA CULVERT REPAIR PRACTICES MANUAL
A discussion on joint requirements and performance is given in Topic 853.1 (2) and (3) of the HDM. Table 853.1C provides information to help the designer select the proper joint under most conditions. See Chapter 5- 8.1(a) and (b), FHWA Culvert Inspection Manual for a discussion on misalignment and joint defects. The joint repair strategy should be dependent on the specific type of problem associated with the defect present i.e., misalignment, exfiltration, infiltration, cracks, or joint separation. In addition, pipe diameter will be an important factor to be considered because human entry is usually limited to pipes 30 inches or larger.
See FHWA Culvert Repair Practices Manual Volume I, pages 3-32, 3-37 and 3-44. Misalignment may indicate the presence of serious problems in the supporting soil. If progressive settlement is present, joint repair should not be performed until a solution to stabilize the surrounding soil has been found. In some cases, reconstruction may be the only option. If so, where there is a need to withstand soil movements or resist disjointing forces, "positive" joints should be specified. Refer to Standard Specifications, Section 61 and Table 854.1 in the HDM.
If the misalignment is a result of leaking joints and undermining, a determination should be made whether the undermining is due to piping, water exfiltration or infiltration of backfill material and a combination of grouting the external voids and sealing the culvert joints may be warranted using chemical grouting or other joint repair methods that are described in index 188.8.131.52.3. In addition the joint should be specified "watertight" per Section 61 of the Standard Specifications. Further discussion on watertight joints is given in Topic 854.1 (2) of the HDM. Further discussion on piping is given in Topics 829.3 and 829.4 of the HDM.
Exfiltration occurs when leaking joints allow water flowing through the pipe to leak into the supporting material. Minor leakage may not be a significant problem unless soils are quite erosive. Where exfiltration has resulted in piping, measures should be taken for sealing culvert joints and making them “watertight” in addition to grouting for filling voids in the soil behind the joint as discussed in the previous section. The same techniques used to stop infiltration will also stop exfiltration. For storm drain systems with pipes less than 30 inches or less in diameter, grouting as described in 184.108.40.206.3, or some of the lining methods described under Index 6.1.3, such as cured in place, can be used to stop exfiltration. For larger diameters, internal grouting, PVC repair sleeves, grouting sleeves, internal steel expansion ring gasket joint sealing systems as described in 220.127.116.11.3, or other lining methods described in Index 6.1.3 such as sliplining or lining with CIPP will stop exfiltration.
Infiltration is the opposite of exfiltration. Many culverts are empty except during peak flows.
When the water table is higher than the culvert invert, water may seep into the culvert between storms. Infiltration can occur during flood events by suction from pressure differentials in inlet control culverts.
Infiltration can cause settlement and misalignment problems if it carries fine-grained soil particles from the surrounding backfill. See Index 18.104.22.168, Soil Migration. In such cases, measures should be taken to seal the joints to make them watertight. Internal grouting or some of the lining methods described in Index 6.1.3 such as sliplining, or lining with CIPP will stop infiltration. In general, for culvert repair work, Portland cement based grout, with and without special admixtures, is usually adequate and much less expensive than the foaming and chemical grouts that are used to resist high external and internal fluid pressures. Internal grouting can be specifically designed to stop infiltration at deteriorated, continuously leaking or open joints. See FHWA Culvert Repair Practices Manual Volume 1, pages 5-37, 6-11, 6-14, and Volume 2, Appendices B-30 and B-26 for procedures on grouting voids and sealing culvert joints. Also see Index 11.1.1.
Internal Chemical Grouted Joint
Chemical grouting is the most commonly used method for sealing leaking joints in structurally sound, sewer pipes that are under the groundwater table. It will not provide structural repair, and it is inappropriate for longitudinal or circumferential cracks, broken or crushed pipes. However, other methods such as using repair sleeves in combination with chemical grouting are appropriate for such repairs (see discussion towards end of this section). Attempting to seal joints that are not leaking or infiltrating during the sealing process has produced questionable results. Some types of chemical grouts have failed in arid regions where the grout has dried up during periods of low groundwater and in coastal regions where the ground is subject to tidal fluctuations. The long-term service life for chemical grouting is unknown. One study concluded the life expectancy for chemically grouted joints was no more than 15 years, other references indicate a 20 year service life, and it is known to last even longer in other applications such as sealing tunnels and dams.
In non-human entry pipes, grouting is generally accomplished using a sealing packer and a closed circuit television (CCTV) camera. The sealing packer and CCTV camera are pulled through the pipe with cables. Concurrently, air or water testing equipment is used to test the joint and determine the effectiveness of the sealing.
In pipes with large enough for human entry, pressure grouting is accomplished using manually placed inflatable pipe grout sealing rings or predrilled injection holes and a hand-held probe (see figure below):
The two basic groups of chemical grouting materials are gels and polyurethane foams. Polyurethane foam grout forms in place as a gasket and cures to a hard consistency but retains a rubber-like flexibility. The seal takes place in the joint and there is only minimum penetration outside the pipe. The service life of polyurethane foam is not moisture-dependent and therefore it can be considered for use in locations with wet-dry cycles. Gel grouts penetrate outside the pipe and infiltrate the soil surrounding the joint. The mixture cures to an impermeable condition around the joint area.
The service life of the non-urethane type gels discussed below is moisture-dependent, and therefore these types should not be considered for use in locations with wet-dry cycles. Urethane gel however, is different from the acrylamide, or acrylate gels in that water is the catalyst and they may be used in locations with wet-dry cycles to form either an elastomeric collar within the pipe joint as well as filling the voids in the soil outside the joint.
Generally the foam grouts are more expensive and difficult to install.
The most commonly used gel grouts are of the acrylamide, acrylic, acrylate and urethane base types. Acrylamide base gel is significantly more toxic in its pre-gelled form than the others but grout toxicities are of concern only during handling and placement or installation and EPA has now withdrawn a long standing proposal that sought to ban the use of acrylamide grouts. Due to its very low viscosity, acrylamide has long been the material of choice to repair underground structures in the sanitary sewer industry. The non-toxic urethane base gels are EPA approved for potable water pipelines because they use water as the catalyst rather than other chemicals. Because of soil and moisture variability, formulating the correct mixture is largely dependent on trial and error on a case-by-case basis, and is difficult to accurately specify in design.
As of this writing, there are no Caltrans specifications for internal chemical grouting. It is a good idea to contact a chemical grouting manufacturer and/or contractor for further information. See Appendix F.
To seal leaking culvert joints with excessive infiltration and exfiltration, if the pipe is round and large enough for human entry and the external hydraulic head pressure (groundwater) is low (i.e., external head pressure does not exceed 15' above pipe invert at any point) and the internal head pressure (hydraulic grade line) does not exceed 20' above the invert at any point, it may be possible to use an internal EPDM rubber pipe joint sealing system comprising an EDPM rubber membrane, backing plates, spacers, shims, clips, and set screws for the securing rubber membrane across pipe joints. Depending on each individual situation, supplemental grouting may also be needed to fill any voids in the soil behind the joint. See FHWA Culvert Repair Practices Manual Volume 2, pages B-111 to B-116.
These seals may be used in CMP, RCP and plastic (both PVC and HDPE). If the host pipe is corrugated metal, the EPDM seal must have a smooth surface on both sides with no extrusions. For all host pipe types, any joint gaps, low areas and deep imperfections of the pipe periphery are filled with cement mortar or epoxy and on each side of the joint where the seating surface band location of the seal is to be located and rendered flush with the surrounding joint surface in compliance with the manufacturer's recommendations.
If abrasion protection is needed, it may be necessary to cover the steel expansion ring with concrete, shotcrete or other authorized material. Section 15-6.05 includes specifications for installing EPDM rubber pipe joint seals onto the interior of an existing pipe or culvert, creating a circumferential leak resistant seal at joints.
Manufacturer’s claims regarding the range of application for internal joint seals varies widely. For the purposes of design, the following recommendations are suggested for Caltrans facilities. Situations beyond these suggested ranges may be discussed with the District Hydraulic Unit or Headquarters Office of Highway Drainage.
Internal mechanical pipe joint seals should not to be used where the joint gap exceeds 2" for tongue and groove RCP (i.e., physical separation of pipe ends) or where the joint gap exceeds 3" for all other joint types (including bell/spigot plastic and RCP). However, the maximum allowable gap may be increased by 1-1/2" for both indicated situations if a back-up plate used. Furthermore, internal mechanical pipe joint seals should not to be used where the joint offset exceeds 1/2" for pipe 36" diameter and below, or 3/4" for pipe larger than 36" diameter.
AMEX-10/WEKO-SEAL Internal Joint Sealing System examples above (Courtesy of Miller Pipeline).
Another method for sealing joints uses a jacked-in-place PVC repair sleeve combined with O-rings and annular space chemical or cementitious grouting. PVC repair sleeves range from 36 inches to 100 inches in diameter.
A third option may be to use “grouting” sleeves ranging from 12 inches to 54 inches in diameter. Grouting Sleeves have a stainless steel core surrounded by an absorbent gasket which is soaked in an expanding Polyurethane foam grout which bonds the repair sleeve to the host pipe upon contact with water or air by filing the annular space between the structural stainless steel core and the host pipe. At each end is a closed cell Polyethylene End Sealer. Both of these repair sleeves are discussed in FHWA Culvert Repair Practices Manual Volume 2, pages B-155 to B-159, however, the information above supercedes the size range and grouting information presented. For manufacturer contact information, see Appendix F.
Examples of Internal Joint Sealing Systems include: the HYDRA-Tight Seal by Hydra-Stop, In-Weg Internal Seals by J. Fletcher Creamer and Son, Depend-O-Lok by Brico Industries, Link-Pipe PVC Sleeve and Link-Pipe Grouting Sleeve. All of these systems are non-structural. Also see Index 6.1.2 for grouting voids in the soil envelope.
Cracked joints are more than likely not watertight even if gaskets were used. However, if no other problems are evident, such as misalignment, and the cracks are not open or spalling, they may be considered a minor problem to only be noted in inspection. Severe joint cracks are similar in significance to separated joints. Separated joints are often found when severe misalignment is found. In fact either problem may cause or aggravate the other. Embankment slippage may also cause separations to occur. An attempt should be made to determine whether the separations are caused by improper installation, undermining, or uneven settlement of fill. If undermining is determined, an attempt should be made to determine whether the undermining is due to piping, water exfiltration, or infiltration of backfill material. It may also be necessary to test the density of the surrounding soil.
Refer to the previous discussion under misalignment, exfiltration and infiltration for joint repair considerations. See Indices 22.214.171.124.1, 126.96.36.199.2 and 188.8.131.52.3.
For culverts that have been newly installed and backfilled, cracks should not exceed 0.01 inch in width in severely corrosive environments (pH of 5.6 or less, water containing vegetal or animal wastes, seawater, or other water with high concentration of chlorides). Conversely, for culverts installed in a non-corrosive environment (neutral pH close to 7, low concentrations of salt, vegetal or animal wastes), cracks of up to 0.1 inch in width of the installed pipe are acceptable if they are not excessive in number.
For all culverts cracks less than 0.01 inch in width are minor and only need to be noted. Cracks greater than 0.01 inch in width but less than 0.1 inch should be noted as possible candidates for routing a 0.25 inch wide minimum by 0.5 inch deep maximum V-Grind, then patching or sealing (see Appendix E). Cracks greater than 0.1 inch in width may indicate a serious condition and the Underground Structures Unit within the Division of Engineering Services should be contacted.
Typical locations for longitudinal cracking can be found in the crown and invert.
See FHWA Culvert Repair Practices Manual Volume 1, page 3-45.
Longitudinal cracking in excess of 0.1 inch in width may indicate overloading or poor bedding. If there is no soil loss associated with cracking in excess of 0.1 inch, rehabilitation may be considered.
See Figures 3.16 and 3.17 in FHWA Culvert Repair Practices Manual Volume 1 for the results of poor and good side support, the deformation of cracked pipes, the cause of the deformation and the visible effects. It should be noted that reinforced concrete pipe may fail (see Index 184.108.40.206.1) but will rarely "collapse".
There is a choice of materials that may be used to repair cracks. The materials generally may be categorized as either flexible crack fillers or rigid materials that are more permanent that may create a structural repair. The latter group includes both Portland cement-based mortar (for cracks greater than 0.01 inch which must first be routed out) and structural adhesives that provide tensile and shear strength including epoxy systems that may be filled with a powder or unfilled. See Appendix E, and FHWA Culvert Repair Practices Manual Volume 2, Appendix B-25 for information on crack sealing with cement mortar or epoxy adhesive. Other options for repair may be to use one of the repair sleeves or chemical grouting using a hand held probe as described in Index 220.127.116.11.3.
See Caltrans Standard Specification Section 95: Epoxy, in conjunction with "Repair by Injection of Epoxy Adhesive" guidelines given in above-referenced Appendix B-25.
Pipe diameter will be an important factor to be considered when repairing individual cracks because human entry is usually limited to pipes 30 inches or larger. For smaller diameter, non human-entry pipes, consideration should be made for the use of a slip liner. See FHWA Culvert Repair Practices Manual Volume 1, page 6-24 and Volume 2, Appendix B-39, B-40, Index 18.104.22.168 of this D.I.B. for general sliplining procedures and Caltrans Standard Special Provision No. 15-6.10 for sliplining using Plastic Pipe Liners”.
If diameter reduction is a concern, lining options may include use of a cured in place resin-impregnated pipeliner (pipes 12 inches to 108 inches diameter) or HDPE pipeliner (reformed/deformed) for pipes under 24 inches in diameter.
It should be noted that regardless of the lining method chosen, the lining itself does not need to provide load carrying ability or independent structural support; if the host pipe is not capable of doing this, it should be replaced. See Index 6.1.1, Caltrans host pipe structural philosophy. Replacement due longitudinal cracking should be considered as a final option and will be dependent on consultation with the Division of Underground Structures within the Division of Engineering Services. See Indices 22.214.171.124 and 11.1.1.
Poor bedding and/or poor installation may cause transverse cracks. Cracks may occur across the top of pipe when settlement occurs and rocks or other areas of hard foundation material near the midpoint of a pipe section are not adequately covered with suitable bedding material. For repairs of transverse cracks, the same discussion of crack sealing and lining and other options for repairs as outlined under longitudinal cracks in Index 126.96.36.199.1 apply to repairing transverse cracks.
See FHWA Culvert Repair Practices Manual Volume 1, page 3-47.
Spalls (fractures) often occur along the edges of either longitudinal or transverse cracks when the crack is associated with overloading or poor support rather than tension cracking. For Spalls associated with cracks, the cause of cracking should first be determined.
If the cause is construction overloading, clean around the spall and apply a mortar patch. See FHWA Culvert Repair Practices Manual Volume 2, Appendix B-28 for more information on patching concrete. Also rout out the crack (if over 0.01 inch) to a depth of at least 0.5 inch and grout the crack. If the cracking is due to post construction loading, either the loading must be reduced, or the pipe should be replaced by another, which is capable of supporting the applied load.
Spalling can also be caused by corrosion of the steel reinforcing when corrosive water is able to reach the steel through cracks or shallow cover. As the steel corrodes, the oxidized steel expands and causes the concrete covering the steel to spall. It must be determined where the corrosive material is coming from (i.e., interior or exterior or both). If it is coming from the interior only, chip back around the spall and sandblast steel to remove the rust and apply mortar patch. In strongly acidic environments, such as drainage from mines or caustic water, various applied coatings (thermoplastic flame sprays, for example) or full-length sliplining may be warranted. See Index 188.8.131.52.1 for pipe liner references.
If the corrosive material is coming from both the interior and exterior, patch as indicated for the interior, but monitor the culvert to determine the rate of degradation for timing of future replacement.
If the spalls are caused by debris (logs, boulders, etc.), it is recommended to clean around the spalled area and apply a mortar patch, assuming no other damage is present.
See FHWA Culvert Repair Practices Manual Volume 1, page 3-47.
The terms slabbing, shear slabbing, or slab shear refer to a radial failure of concrete that occurs from straightening of the reinforcement cage. It is characterized by large slabs of concrete "peeling" away from the sides of the pipe and a straightening of the reinforcement due to excessive deflection or shear cracks. Slabbing is a serious problem that may occur under high fills with reinforced concrete pipe of inadequate D-load strength and/or an inadequate depth of bedding on a rock foundation. It may also occur under poor consolidation/backfill conditions with a high water table.
If it is determined that the culvert is structurally stable, the primary concern is protection of the inner (and exposed) layer of steel reinforcing against corrosion.
Clean around the damaged area, chip back and sandblast steel to remove the rust and apply mortar patch. In strongly acidic environments, such as drainage from mines or caustic water, various applied coatings (thermoplastic flame sprays, for example) or full- length sliplining may be warranted. See Index 184.108.40.206.1 for pipe liner references. See FHWA Culvert Repair Practices Manual Volume 2, Appendix B-28 for more information on patching concrete.
If the slabbing is due to post construction loading, either the loading must be reduced, or the pipe should be replaced with one capable of supporting the applied load. Refer to Standard Plans A62D and A62DA for the allowable minimum classes of RCP and D-load verses cover, and Section 19-3.03D (Foundation Treatment) of the Standard Specifications when solid rock or other unyielding material is encountered. See FHWA Culvert Repair Practices Manual Volume 1, page 3-47.
The inverts of precast concrete culverts are normally quite durable to damage. However, abrasion can be a serious problem in mountain areas where moderate-to-large sized rock is carried by fast moving water. When the water velocity that is generated by the 2-5 year return frequency flood is greater than 10 ft/s, and the upstream channel has a course aggregate or large diameter bed load, abrasion related problems can be expected. See table in Index 220.127.116.11.
Deteriorated inverts in precast concrete culverts generally require paving to restore them to an acceptable functional condition. In order to accomplish this, and to dry the invert, it will be necessary to divert any flows present and/or perform the work during the summer for non-perennial streams and channels. For human entry pipes, guidelines for shotcrete/gunite paving, lining, and repairs, and invert paving are provided in appendices B-11 and B-29 of FHWA Culvert Repair Practices Manual Volume 2. Also, see Section 53 of the Standard Specifications and Abrasion Table in Index 18.104.22.168. Where abrasion is present, a harder aggregate than the channel bedload should be substituted for fine aggregate. Contact District Materials for aggregate sources and specification criteria. For smaller diameter precast concrete culverts with invert deterioration, trenchless robotic applicators for cement mortar with a polypropylene fiber mesh additive and concrete hardener could be considered. See Index 22.214.171.124.2 for a general discussion of cement mortar lining.
Failed crown in Reinforced Concrete Box Culvert
Precast concrete culverts may sustain damage in their crown section due to the depth of cover being too shallow to adequately support and distribute vehicle live loads. The result may be cracking, spalling and distortion in the crown area. Some information On procedures that may be used to repair such problems is provided in appendix B-37 of FHWA Culvert Repair Practices Manual Volume 2. For severe cases of crown deterioration (see photo), replacement may be necessary.
The primary conditions that affect corrugated metal pipe (CMP) and pipe arch culverts are: (1) joint defects, (2) invert deterioration, (3) shape distortion, (4) soil migration, (5) corrosion, and (6) abrasion. See Indices 126.96.36.199.1, 188.8.131.52.2, 2.1.2, and 184.108.40.206-3 for material characteristics, coatings and service life discussion relative to the deteriorating factors to metal pipe.
At steel pipe sites where abrasion is present, once the galvanizing layer has been worn away, corrosion will occur, followed by eventual perforation of the invert and loss of surrounding backfill soil. This in turn may lead to shape distortion depending on the compromise to the soil-pipe interaction resulting from the migration of backfill fines.
Aluminum corrodes differently than steel and is not susceptible to corrosion attack within the acceptable pH range of 5.5-8.5. Abrasion potential is dependent upon, volume, velocity, size, shape and hardness of bedload. Culvert flow velocities that frequently exceed -5 ft/s are only allowable for low volumes of smaller, rounded bedload. In non-corrosive environments, Aluminum pipes may abrade quicker than steel and are not recommended in an environment where the velocity frequently exceeds 5 ft/s and if angular or large sized bedload material is present. See Indices 220.127.116.11, 18.104.22.168 and HDM Index 854.4(2)(a) through (f), prior to selecting aluminum as an allowable alternative.
A discussion on joint requirements and performance is given in Topic 853.1 (2) and (3) of the Highway Design Manual. Table 853.1C provides information to help the designer select the proper joint under most conditions. See Chapter 5- 4.2 (b), FHWA Culvert Inspection Manual for a discussion on joint defects. The joint repair strategy should be dependent on the specific type of problem associated with the defect present i.e., misalignment, exfiltration, infiltration, and joint separation. Most of the concerns and repairs that are outlined in this D.I.B. under the joint repair section for precast concrete pipe also hold true for flexible pipe (i.e., misalignment, exfiltration, infiltration, and joint separation). Joint defects and associated repairs specifically for CMP and pipe arches are discussed in FHWA Culvert Repair Practices Manual Volume 1, pages 6-14 and 6-15. Also see Indices 22.214.171.124.2, and 126.96.36.199.3. Once again, pipe diameter will be an important factor to be considered because human entry is usually limited to pipes 30 inches or larger.
A variety of external loads and changing soil conditions may cause joints to open allowing backfill infiltration and water exfiltration, however, this is unlikely if the proper bands are used. Key factors in the inspection of joints are indications of backfill infiltration and water exfiltration causing erosion of surrounding soil resulting in surface holes or pavement deflections. See Index 11.1.1.
Sink hole damage (location unknown)
Loss of backfill fines
Abrasion of the pipe wall occurs through the action of materials carried in flow (bedload) impacting on the pipe wall. It is affected by the frequency of heavy loads in the flow and velocity of the flow (5 ft/s or greater). Obviously the amount, type and size of material carried in the flow have a significant impact on the life expectancy of the pipe, as does the material composition of the pipe itself.
Example of abrasive, angular, quartz - sand bedload
One of the most common problems with corrugated metal culverts is deterioration of the invert, usually due to a combination of corrosion and abrasion once the galvanizing layer has been worn away. It is for this reason that corrugated steel culverts are frequently coated with an asphaltic or other type of protective coating. However, with the exception of polyethylene (CSSRP), towards the upper end of the flow velocity range for moderate abrasion (depending on bedload angularity), and for the severe abrasion level, these coatings are generally ineffective and alternative invert materials are recommended. See Indices 188.8.131.52, 184.108.40.206, and 220.127.116.11, for corrosion and abrasion influences that must be included in any estimation of service life. If these influences have been overlooked or inadequately addressed during the original design, eventually the coatings are abraded or broken away, and corrosion that attacks the bare steel is accelerated by abrasion that constantly removes the somewhat protective oxide layer formed by corrosion.
Continuation of this action, if unchecked, will ultimately lead to loss of the invert and the creation of scour holes under the culvert (see pictures below).
Corrosion that attacks the bare steel is accelerated by abrasion
Worn invert on leading edge (to flow) of corrugations
Since a corrugated metal pipe is classified as a flexible structure that requires interaction with soil for stability, loss of the invert may result in severe distortion and collapse of the culvert (see Index 11.1.1).
Thus, repairs for severely deteriorated inverts in metal culverts must include:
- Structural repairs that restore the structural capacity of the culvert to resist circumferential thrust loads
- Re-establishing the connection between the soil and the pipe by filling voids immediately on the backside of culverts with low strength pressure grout mix. This will tend to crack rather than build an undesirable 'block'. Refer to Index 6.1.2 and page B-135 of FHWA Culvert Repair Practices Manual Volume 2, for procedures for grouting voids behind and under culverts.
See Appendix H for case studies of structural repairs and filling voids on the backside.
Many types of repairs and corrective action may be taken to alleviate or minimize future invert durability problems. In most cases, the material selection should be both abrasion and corrosion resistant. Plastic slipliners are an effective rehabilitation method primarily for smaller pipe sizes in both abrasive and corrosive environments; they are available in a broad range of dimensions and joint type selections. See Index 18.104.22.168.1. Other abrasion and corrosion resistant sliplining materials for consideration may include:
- Centrifugally cast glass fiber reinforced polymer mortar (RPMP) and
- Fiber Reinforced Plastic (FRP). See Indices 22.214.171.124.3.1 and 126.96.36.199.3.5.
If access is limited, or the reduction of pipe cross sectional area resulting from sliplining is unacceptable, it may be necessary to use an alternative lining method such as cement mortar lining with a polypropylene fiber mesh reinforcement additive, lining with a machine wound PVC liner, or CIPP. See Indices 188.8.131.52.2, 184.108.40.206 and 220.127.116.11.
In general, for pipes large enough for human entry with invert durability problems, sliplining should not be the first choice and most of the work may be classified in two categories: (1) invert paving, to restore or replace weakened inverts, (2) steel armor plating, to provide increased resistance to abrasion and impact damage.
A summary of materials/invert protection recommended for various levels abrasion is presented in a table in Index 18.104.22.168.
One of the most effective ways to rehabilitate corroded and severely deteriorated inverts of CMP is by paving them with reinforced concrete using Class 3 or Minor Concrete or shotcrete. If abrasion is present, the aggregate source should be harder material than the streambed load and have a high durability index (consult with District Materials Branch for sampling and recommendation). Consideration should be given for using a higher strength concrete mix with a nominal strength of 6000 psi or higher. See Standard Specifications; Sections 90, 51, 53, and HDM Index 853.6.
The maximum grading indicated (1.5 inch) for coarse aggregate may need to be modified if the concrete must be pumped. The abrasion resistance of cementitious materials is affected by both its compressive strength and hardness of the aggregate. There is a correlation between decreasing the water/cement ratio, increasing compressive strength and increasing abrasion resistance. Therefore, where abrasion is a significant factor, the lowest practicable water/cement ratios and the hardest available aggregates should be used.
A typical design detail for invert paving is shown above for situations with minimal loss of the invert (i.e., some perforations, but not complete invert loss) that do not require an extensive structural connection between the invert paving and the CMP. Paving thickness will range from 3 inches to 6 inches depending on abrasiveness of site, and paving limits typically vary from 90 to 120 degrees for the internal angle.
For situations where there is significant loss of the pipe invert (see picture on page 34), it is necessary to tie the concrete to the more structurally sound portions of the pipe wall in order to transfer compressive thrust of culvert walls into the invert slab to create a “mechanical” connection using the following general procedures.
During cleaning and preparing the host pipe for lining, if there are just a few strands of steel remaining, it is preferable to keep as much of the remaining pipe material in the invert as possible – regardless of its condition. Therefore, it is not necessary to remove it. However, if necessary, small openings can be cut in the invert to expose void pockets beneath the culvert. Filling voids with slurry will restore lost bedding material beneath the culvert. A typical design is to use: 4 x 4 - W4.0 x W4.0 WWF with a 6 inch invert concrete pad and to tack weld WWF @12" oc each way. Note this is the new 'W-Number designation' for WWF. Welded wire fabric is installed to control cracking in the concrete invert paving. To provide a bond between the concrete and culvert wall, and as a shear mechanism to transfer thrust into slab (“mechanical” connection), angle iron may be welded to the pipe wall. Shear connector welding studs (welded headed studs) can be used as an alternative to welding angle iron. However, regardless of which of these thrust transfer methods is selected, it should only be spot welded to walls that are still in excellent condition. To develop full fastener strength, a general minimum ratio of plate thickness to stud weld base diameter is 1:5. To develop full fastener strength, however, plate thickness should be a minimum of about 1/3 the weld base diameter. Figure 4A on page 19 of “Complete Data for Stud Welding, Nelson Stud Welding Systems for Construction” lists recommended steel plate thickness in relation to weld base diameter. A welded headed stud manufacturer representative should be consulted for technical support and installation instructions for a particular project.
When a mechanical connection is used, paving limits may vary up to 180 degrees for the internal angle.
Metal culvert with paved concrete invert
Example of either inadequate concrete mix design or poor quality control for invert paving in an abrasive environment
Wear cones (colored concrete cones) can be placed to monitor wear. See Appendix B-11, FHWA Culvert Repair Practices Manual Volume 2, for procedures for shotcrete/gunite paving, lining, and repairs (all human entry). See Appendix B-29, FHWA Culvert Repair Practices Manual Volume 2, for procedures for invert paving. There are advantages to both types of materials and application methods that are discussed in these appendices.
Large diameter invert repairs 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.
See Appendix H for some invert paving case studies.
This method should be limited to large diameter (10 ft or greater) that are located on (hydraulically) steep slopes and operate under inlet control. Lining the culvert barrel invert with concreted RSP can provide an effective countermeasure to abrasion and increase barrel roughness thus decreasing velocity within the barrel. A nominal strength of 4500 psi may be used in the concrete. The rock size may vary, however, it is imperative to achieve adequate embedment into the concrete. At the culvert ends, a smooth transition back to the channel bed profile should be provided with adequate embedment to prevent undermining.
In locations with severe abrasion (see Index 22.214.171.124) a viable option to invert paving with concrete may be to armor plate the invert with steel plates (thickness between 0.25 inch and 0.75 inch). This method is used in large diameter pipes that can accommodate a reduction in waterway area. The smooth, wide invert spreads wear over a greater area and is less of an impediment to flow than corrugated metal. It is important to securely attach steel armor plates to the host pipe. See 03-Nev-49 pictures below of 0.375 inch thick steel armor plate example at Shady Creek that replaced a concrete invert lining.
Finished 0.375-inch thick steel plate invert
Workers Placing steel plates (see detail below)
Example Steel Armor Plating Detail
Several other materials that have been successfully used to plate inverts subject to abrasion include guardrail elements, railroad rails and bridge deck grating.
The single most important feature to observe and measure when inspecting corrugated metal culverts is the cross-sectional shape of the culvert barrel. The corrugated metal culvert barrel depends on the backfill or embankment to maintain its proper shape and stability. The culvert will deflect, settle or distort when the backfill does not provide the required support. See Index 126.96.36.199, for a general discussion on flexible pipe behavior.
Flexible piping must utilize the soil to construct an envelope of supporting material around the pipe so that the deflection is maintained at an acceptable level. The extent to which the pipe depends on this enveloping soil is a function of the depth of cover, surface loading and the ring stiffness of the pipe. The deflection of flexible pipe is the sum total of two major components: the "installation deflection", which reflects the technique and care by which the pipe was handled and installed, and the "service deflection", which reflects the accommodation of the constructed pipe-soil system (pipe and compacted backfill) to the subsequent earth loading and other loadings. Overloading or soil movement may cause distortion.
It is quite common to have at least some symmetrical or unsymmetrical distortion in corrugated metal culverts. A flexible pipe has been defined as one that will deflect at least 2 percent without structural distress. It is also common that the culvert is stable in that distorted shape; that is, it is not continuing to distort. Therefore, it is important to determine by measurement and monitoring whether the culvert is stable in its distorted shape or whether it is continuing to become distorted. Usually 85-90% of deflection occurs within the first month of construction. This is the time that it takes for the soil to settle and stabilize. However, if there is instability in the backfill, the pipe will continue to change shape. In general, deflections of more than 7-8% (either horizontal or vertical) should be noted and may lead to structural problems. Beyond 10%, even joints designed to be watertight will be prone to leakage and the associated potential for soil migration/piping. Seam separation and/or buckling may occur for deflections greater than 15%. If deflection is identified, the location, by distance from the inlet and degrees from invert, should be noted and the length of the horizontal and vertical axes of the culvert barrel should also be recorded. Unless water-tightness is an issue, monitoring deflections below 10 - 12% is typically the appropriate course of action so that a determination can be made of whether the conditions are worsening. Beyond 10 - 12%, it is recommended that plans for rehabilitation/replacement be undertaken.
Example of shape distortion caused by soil movement
The overall condition of the culvert should be assessed, as well as the soil-pipe conditions that caused the deformation to occur. Symmetrical deflection of the crown may be indicative of problems with support of the bottom of the culvert or insufficient backfill/cover over the top of the culvert. Unsymmetrical deformation of the top of the culvert may be the result of loss of soil support on one side of the bottom (potentially from problems due to infiltration, infiltration and piping at joint(s) or perforated invert) or improper compaction of the backfill on one side of the culvert. Thus the shape may not be symmetrical for either the entire length of the culvert or individual sections of it. Therefore, the conditions that caused the deformation must be assessed and the rehabilitation plan must include correcting the underlying problem. See Appendix B-34, FHWA Culvert Repair Practices Manual Volume 2, for procedures for repair at a distorted section.
The decision to repair (re-compact embedment material, grout voids, repair joints or line invert) verses replace the culvert by trenching and cover or by other trenchless methods such as jacking or microtunneling, is dependent in part on the structural integrity of the culvert. If the culvert must be replaced, the decision to replace by trench and cover versus other trenchless methods will be influenced by cost, the need to maintain traffic during construction and possibly other environmental concerns. Relining by sliplining or other methods that are outlined in this D.I.B. should not be used on host culverts with excessive (generally greater than 15-20 %) deflection because the host pipe must be structurally sound and capable of withstanding all loads. See Index 6.1.1 for Caltrans host pipe structural philosophy. However, if the host pipe can be adequately stabilized, stopping further distortion, and the soil-pipe interaction re-established, it may be feasible to rehabilitate pipes with deflections beyond 10-12%. See Index 11.1.1.
When the pipe is located beneath the ground water level, consideration must be given to the possibility of loss of side support through soil migration (the conveying by groundwater of finer particle soils into void spaces of coarser soils). Generally, migration can occur where the void spaces in the pipe backfill are sufficiently large enough to allow the intrusion of eroded fines from the trench sidewalls. For migration to occur, the in-situ soil must be erodible, and there must be a flow path for the water. Normally, erodible soils are fine sands and silts and special clays. This situation is exacerbated where a significant gradient exists in the ground water from the outside of the trench towards the inside, i.e., the trench must act as a drain, and/or the pipe joints are not watertight (see Highway Design Manual 853.1 (3) - Joint Performance - Watertight Joint).
As a remedial measure for such anticipated conditions, depending on the amount of shape distortion, grouting, or a combination of expansion rings (refer to previous discussion for sealing culvert joints with expansion ring gaskets or repair sleeves under Index 188.8.131.52.3.2), and Slurry Cement pressure grouted backfill in lieu of Structure Backfill, or a combination of Structure Backfill with Filter Fabric (only if external access is feasible) is recommended. Also see appendices B-26 and B-34, FHWA Culvert Repair Practices Manual Volume 2 for procedures for sealing culvert joints and repair at a distorted section.
There are several main types of corrosion leading to failure in pipes - atmospheric, microbiological and galvanic corrosion. Any of these types of corrosion are influenced by the structure of the soil, but the most commonly used criteria to indicate relative corrosivity to steel are the pH or hydrogen iron concentration, the specific electrical resistance, and the chloride and sulfate content of both soil and water. Other factors that can influence the corrosion rates are the effects of industrial effluents from either commercial or residential sources or stray electrical currents in close proximity to the pipe. Stray current sources include electricity transmission lines, electrified rail lines and the like.
In general, in areas of high rainfall, the soils tend to be acidic and of high electrical resistivity. Acid soils are generally regarded to be corrosive, while a high electrical resistivity is indicative of low corrosivity. Some typical values of the resistivity of soils and waters are shown in Appendix D (Table 5-1). Table 5-2 in Appendix D shows a rating of the soils corrosivity as determined by specific electrical resistance. Visual indications of the relative corrosivity of various soil types are shown Table 5-3 of Appendix D.
Refer to Indices 184.108.40.206.1 and 220.127.116.11.2 for a discussion on how service life is estimated relative to pH and coatings for metal culverts.
Refer to FHWA Culvert Repair Practices Manual Volume 1, pages 2-12 to 2-14, 6-13 and Volume 2, Appendix B-31, for a discussion on the corrosion process and procedures for cathodically protecting metal culverts.
Aluminum corrodes differently than steel and is not susceptible to corrosion attack within the acceptable pH range of 5.5-8.5. See Indices 18.104.22.168 and 22.214.171.124 when considering abrasion potential.
Since they too are flexible structures that are made from metal, they suffer from the same types of problems, as do corrugated metal and pipe arch culverts. In addition, they also suffer from problems that are unique to their style of construction, which is assembly with individual pieces of metal that are fastened together with bolts.
The longitudinal seams of structural steel plate culverts are subject to displacement and cracking due to incorrect assembly of the plates and differential soil pressures.
Repairs are made by splicing, re-bolting or welding with reinforcing steel to the inside corrugation valleys at the location of seam distress. See Appendix B-38, FHWA Culvert Repair Practices Manual Volume 2.
Longitudinal or transverse seams in structural plate assemblies may deteriorate due to sheared or corroded connector bolts, lost or corroded nuts, or plate tears.
Except in rare scenarios, the deficient seam must be welded. This can be costly and can raise safety concerns due to the toxicity of fumes from melted zinc (galvanizing). Often the introduction of reinforcing bars welded to the base structural plate metal is the only practical way to repair the deteriorated seam.
Whatever repair is made must be structurally sufficient to accommodate the load thrust, which will be present in the shell of the conduit. Therefore, a simple "tack" weld may not be adequate.
If the seams are to be repaired using shotcrete or gunite, brackets, firmly attached to the structural plates, must be incorporated to anchor the concrete mix to the plates. Because of the inherent low thrust resistance in such repair, this type of seam repair may be useful only for transverse seams.
Again, the conditions that caused the seam defects must be assessed and the rehabilitation plan must include repairing the seams and correcting the underlying problem and/or stabilizing the soil envelope if necessary.
The same discussions outlined under invert durability and shape distortion for corrugated metal pipe also apply to structural steel plate. There are ordinarily no joints in structural plate culverts, only seams. Distress in circumferential seams is rare and can result from severe differential deflection caused by a foundation or soil failure - usually as a result of invert failure (see photos in Index 126.96.36.199). Depending upon the degree of deflection, it may be possible to rehabilitate the invert, however, contrary to the recommendation under "Joint Defects" on page 6-19 of FHWA Culvert Repair Practices Manual Volume 1, and Appendix B-26 of Volume 2, internal steel expansion ring gasket joint sealing systems are not recommended for circumferential seam repairs. If it is not possible to rehabilitate the invert, and there is severe differential deflection present, replacement is recommended.
Plastic pipe culverts are a relatively new form of culvert in sizes ranging from 12 inches to 48 inches for new pipe and potentially up to 120 inches for use as a liner with headquarters approval. Refer to Indices 188.8.131.52.2, and 2.1.2 for discussion of material and service life factors.
Although plastics are not subject to corrosion and show good resistance in abrasive environments, they are still part of the "flexible" pipe materials family and therefore most of the discussion and repair procedures that are outlined under the joint repair, shape distortion and soil migration for metal pipe will also apply to plastic. See Indices 184.108.40.206, 220.127.116.11.4 and 18.104.22.168. However, there are some issues that are unique to plastic; including stress cracking and problems associated with exposure to ultraviolet rays at the ends and being flammable. Cracks in high-density polyethylene (HDPE) pipes are most typically going to occur at a seam. In reference to HDPE, it is worth noting that since it is a relatively new culvert product, both the material qualities and physical design are undergoing continuous change. Pipe made today has a different profile, different corrugation (annular instead of helical or spiral) and is made with revised resin compounds as the industry upgrades its products. Given that our standards for placement have been relatively constant, we are more likely to see cracking and other problems in older pipes.
Profile of pipe: Note wall buckling and obvious oval shape. This 42 inch diameter pipe was installed in 1994. The pipe is 82 feet long and has a maximum cover of about 10 feet. Separations of the joints ranged from 1 to 3 inches. Rippling of the sidewalls is apparent throughout the length of the pipe (see below).
Small crack and wall rippling
Compared to other pipe materials, plastic may have a higher potential for damage from improper handling, and a higher potential for damage from improper backfilling procedures including wall cracks, excessive deflection, bulges, joint separation, excessive joint overlap caused by longitudinal expansion and wall rippling and buckling.
Some of the problems that have been outlined for plastic pipe may be monitored, such as deflection (see Index 22.214.171.124.4). However, pipes with excessive deflection will need to be replaced or lined with a rigid material that is capable of supporting all ground and traffic loads. See Index 11.1.1.
Depending on the problem, excluding excessive deflection, other possible choices for repair not discussed in the previously referenced indices include, lining with cured in place pipe, machine wound PVC or replacement. See Index 6.1 and 9.1.
This page last updated December, 6, 2012