Category HIGHWAY ENGINEERING HANDBOOK

Welded wire fabric may be used in conjunction with shotcrete to rehabilitate deteriorated portions of either metal or concrete culverts. The welded wire should be anchored to the in-place pipe either through the use of drilled dowels, if the pipe is concrete, or by welding to either the corrugations (of metal pipe) or to previously welded studs. The shotcrete can then be placed by the use of high-pressure hoses. The repair can be designed to restore structural integrity, with little loss of hydraulic capacity.

5.12.1 Grouting Soil Voids

Regardless of the method of repair or rehabilitation chosen, the possible need for grouting potential voids in the soil envelope surrounding the pipe should be addressed. Portland cement grout may be pressure-injected from the interior of the culvert through drilled holes located toward the bottom of the suspected voids. Drilled holes located toward the top of the pipe then allow for the trapped air and water to exit prior to the grout. Grouting of the voids is necessary to complete the structural rehabilitation of the culvert and to reduce the possibility of any future piping of the culvert backfill.

Where conventional grouting will fill the large voids adjacent to the culvert created by infiltration or piping, compaction grouting will densify the soil. The equipment, method of injection, and makeup of the grout will all be different from what is required for conventional grouting. Compaction grouting in addition to or in lieu of conventional grouting may be necessary. However, the benefit of knowing that there is a well-compacted, stabilized soil in the vicinity of the culvert may not outweigh the expense involved in the process of compaction grouting.

Relining of either rigid or flexible culverts may be done either by slip lining or by installing a flexible liner. Slip lining is merely the insertion of a prefabricated pipe inside an existing pipe. The most common insertion pipes are either corrugated metal or plastic. Obviously, the cross-sectional area of the pipe will be reduced. This will likely affect the hydraulic capacity of the culvert. If a smooth plastic pipe is utilized, the velocity of the flow may be increased, in part offsetting the reduction in the capacity due to the decreased area. Should this be the case, the downstream end of the culvert should be investigated to prevent additional erosion. Regardless of the type of pipe inserted, the annular space between the inside of the existing pipe and the outside of the inserted pipe should be grouted. The final product is comparable to a new culvert, placed with little disruption to the traveling public.

Another method of relining consists of inserting a flexible tube inside the damaged pipe. The flexible tube will generally consist of a resin liner, which, after being inserted inside the subject pipe, is expanded to fit the full cross-section of the pipe. It spans irregularities such as joints that may have opened. Flexible reline pipes are available in a variety of materials such as HDPE, PVC, and thermal-set resins. They are installed using a variety of methods such as fold and form, and inversion processes. The products are specified and designed on a project-by-project basis. Many of the rehabili­tation products can be designed to provide additional structural supprt to the existing pipe. The manufacturer of the material should be consulted as part of the overall design process.

Flexible metal pipes may need rehabilitation wherever there is a loss of section or where large deflections (greater than 5 percent) are present. Where the culvert has
undergone a loss of its structural section due to corrosion or erosion, the amount of loss should be noted and a determination made whether the culvert needs to be strengthened or only protected. The loss of section in metal culverts usually occurs at the invert due to the abrasive conditions of the water flow and/or the corrosive effects of the water. If the loss of section is not significant, it may be adequate to protect the invert with a coating to prevent future erosion or corrosion. The reason for the loss of section should be determined. If the loss is due to corrosion, the application of an asphalt paved invert material should provide protection against future corrosion. However, the asphalt coating does not withstand abrasion well. If the loss is due to erosion, paving the lower quadrant with portland cement concrete will be adequate. Either of these methods is applicable as long as there is no significant loss of structural section that would reduce the structural capacity of the culvert. Where the loss of section is considerable, the structural integrity may be maintained by the addition of welded wire fabric to the concrete paving of the invert. The wire mesh may be welded to the invert corrugations of the metal culvert and then the portland cement concrete placed to provide a smooth channel for the water. Figure 5.47 illustrates rehabilitation with concrete paving. Should the culvert have major structural defects, it may be necessary to replace or reline it or place reinforced concrete around the complete periphery.

Welded wire

over reinforcement

Section A-A

FIGURE 5.47 Example of invert paving of metal culverts with reinforced concrete. (From Highway Design Manual, California Department of Transportation, with permission)

Rigid culverts with invert wear may be rehabilitated by paving the lower quadrant of the culvert. Where there is no reduction in the structural capacity of the culvert, the invert may be protected from further erosion by placing portland cement concrete or by using shotcrete. Welded wire mesh may be used to strengthen the culvert where it is necessary to do so. For an unreinforced concrete pipe, this will be the case where there is either significant invert wear or longitudinal cracking. Strengthening of a rein­forced concrete pipe may be deemed necessary where there is significant longitudinal cracking, invert wear, or spalling. Dowels should be drilled into the member to be repaired, to provide anchorage for the welded wire fabric.

Cracks and spalls caused by flexural distress may be repaired in rigid culverts by sealing and patching. Spalls may be patched with a mortar – or cement-based material, a procedure that is inexpensive and requires little resource allocation. Cracks may be sealed with either a flexible or a nonflexible sealant. If the crack is continuing to move, and if there will be no loss in the structural capacity of the culvert if it continues to do so (circumferential cracks may be an example), a flexible sealant may be used. If the crack has stabilized, or if additional movement is not acceptable, a nonflexible sealant such as a cement mortar may be appropriate. However, the sealant itself will not prevent additional movement. The underlying cause of the cracking must be dis­covered and appropriate measures such as pressure grouting applied.

Cracking caused by shear failure of the wall section is a cause for immediate rehabilita­tive efforts. The replacement or structural relining of the affected pipe will be necessary.

If reinforced concrete pipes separate at the joints and infiltration or exfiltration occurs, not only must the joint be repaired, but the surrounding embankment must be stabilized. The concrete pipe joint may be sealed by the use of an expansion ring gasket and band to prevent further infiltration or exfiltration. Stabilizing the embankment may be accomplished by pressure grouting.

The appropriate method to be used for culvert rehabilitation depends upon the type and size of the culvert, its condition, and hydraulic and economic considerations.

Hydraulic and economic considerations bear on the issue of repair versus replace­ment. If the hydraulic capacity of the culvert is in question, or if a rehabilitation method that would reduce its capacity—either by reducing the waterway or by increas­ing its roughness—is under consideration, a hydraulic analysis is required. In addition, if there will be additional highway construction in the area or if there are plans to widen the roadway in the future, these considerations should be included in the deci­sion of rehabilitation versus replacement.

Pipe replacement is the only method applicable to all pipe types regardless of defects. It is also the most disruptive to the traveling public if done using an open-trench method of construction. As has been previously discussed, jacking or tunneling, at an increased cost, may eliminate this disruption. The advantage is that the hydraulic capacity may be increased and, at the present time, replacement is comparable in cost to relining. However, other methods of rehabilitation will often suffice, as discussed below.

Inspection of reinforced concrete pipe should focus on problems with alignment, joints, and the wall.

The alignment of the culvert may be inspected visually. Misalignment may be caused either by poor installation practices or by subsequent settling of the pipe or the backfill. In any case, the pipe should be periodically monitored to ensure that the con­dition does not worsen. Close inspection of the joints may reveal conditions that will lead to an increase in the misalignment of the structure.

Joints should be inspected for cracks, separation, exfiltration, and infiltration. Cracks and separation of joints are detrimental to the culvert only insofar as they increase the possibility of infiltration and exfiltration. Infiltration is the inflow of water and the accompanying fines during times of high groundwater when the flow in the pipe itself is low. If the inspection is made during this time period and infiltration is occurring, it will be evident. If the inspection is made during a period when high groundwater is not present, but infiltration has occurred, there may be evidence of residual fines and silt at the joints. Infiltration can cause the loss of backfill and even­tually lead to a failure of the roadway above as shown in Fig. 5.45.

Exfiltration is the outflow of water from the pipe into the surrounding backfill. This may cause piping, a loss of backfill material carried away by the outflowing water. This can create problems both with the roadway above and with the culvert itself, which can lose structural integrity because of the loss of side support. If exfil­tration is occurring, it may be observed when the flow is relatively low by inspection of the joints. In addition, there may be some evidence of piping at the outlet end of the culvert, where undermining and the deposition of fines may be present.

Whereas loss of backfill support would be evidenced by excessive deflection in a flexible culvert, rigid culverts will not exhibit this condition. Despite the loss of back­fill support, there may be little or no sign of distress in the wall of the culvert.

The walls of concrete pipe should be inspected for longitudinal and transverse cracks and spalls and wearing of the invert. Longitudinal cracks at the pipe crown or invert (cracks that run lengthwise down the culvert) are indicative of high flexural stresses in the pipe. As the pipe is loaded, it tends to deflect downward and outward. These deflections cause the inside of the pipe at the crown and invert to be in tension as well as the outside of the pipe at the springlines. If the pipe is subjected to a high load, longitudinal cracks may develop at these locations. Because the pipe is buried, inspection of the longitudinal cracks located at the springline on the outside of the pipe is not possible. However, the longitudinal cracks at the crown and the invert will be evident if they exist. Cracks 0.01 in (0.25 mm) or less in width are consid­ered to be hairline cracks and are of minor importance. Larger cracks should be noted and monitored.

Longitudinal cracks located between the crown or invert and the springline are usu­ally caused by shear failure of the wall section. If this type of cracking is visually observed, it is imperative that the cause of the cracking be investigated further. If a shear-type failure is determined to be the cause of the cracking, a rehabilitation or replacement strategy needs to be implemented immediately since the load-carrying capacity of the pipe has been compromised.

Transverse cracks (cracks extending around the circumference of the pipe) are caused by differential settlement along the length of the pipe. This can be caused by either unsuit­able foundation material or poor installation practices. These cracks are usually not struc­tural in nature but can lead to spalling or subsequent corrosion of the reinforcing steel. Figure 5.46 illustrates transverse cracking resulting from improperly prepared bedding.

Invert wear on a reinforced concrete pipe or box culvert will be indicated by rutting of the surface or rust stains on the surface. In the extreme case, there will be exposed reinforcement. All of these conditions lead to a reduction in the structural adequacy of the culvert. Where the reinforcing is exposed, the bond is broken between it and the concrete and the reinforcing is not able to carry the intended stresses.

Unreinforced concrete pipe, whether cast in place or precast, should be inspected for invert wear and cracking. Because the concrete itself must take the flexural stresses, any reduction in thickness due to abrasive wear is of concern. For that reason, if rutting of the invert is evident, an attempt should be made to determine the amount of loss of section. The culvert should be reanalyzed for its structural capacity using this changed section to determine whether or not rehabilitation or replacement is necessary. If lon­gitudinal cracks are present in unreinforced concrete pipe, the modulus of rupture has been met or exceeded and the flexural capacity of the pipe has been reached. As previ­ously mentioned, only those cracks at the crown and the invert may be easily detected.

(See “Culvert Inspection Manual,” Report No. FHWA-IP-86-2, Federal Highway Administration.)

A flexible structure should be checked to ensure that the cross-sectional shape it was designed for is intact. If the flexible culvert, whether it is a round pipe, a pipe arch, an arch, a horizontal ellipse, or any other structural shape, deflects from its design shape, it is not receiving the required support from the backfill. It is assumed in the design of flexible structures that moment in the structure is negligible and that due to the thrust forces, the structure is in compression throughout. If the deflection is large enough to cause a flattening of the structure, these assumptions will not hold true and the struc­ture may collapse. Larger structures with large top radii, such as long-span structures, can withstand a smaller percentage of deflection before reverse curvature occurs than can round structures.

Visual observations of the culvert shape may reveal only large distortions and deflections; deformations may not be readily apparent until they reach approximately 10 percent. For this reason, if excessive deflections in the cross-sectional shape are sus­pected, physical measurements should be taken and documented with changes over time. Reference points should be permanently marked, and for a corrugated structure, measurements should be taken to inside corrugations for consistency. General deflec­tions of round pipe greater than 5 percent should be investigated and monitored; reversal of curvature is expected at 20 percent for a metal culvert, but it may occur at a lesser value for a large structure. Localized flat spots or reversals of curvature are matters of special concern. It is necessary to determine, over time, if the structure deflection has stabilized. This information is critical in determining how serious the deflection is, if rehabilitation is necessary, and in determining if the rehabilitiation method needs to offer additional structural support to the culvert. A computer program is available to aid in the investigation and evaluation of multiple-radius metal structures. (See D. C. Cowherd et al., “Application of the Program MULTSPAN/SOILEVAL to Analyze Problem Structures,” Proceedings of the Second Conference on Structural Performance of Pipes, Ohio University, Athens, Ohio, 1993, A. A. Balkema, Rotterdam, 1993.)

All metal culverts should be investigated for evidence of corrosion and erosion. With a general loss of section there will be an accompanying loss of structural capacity. Wear will first be noted by a loss of the galvanized or other coating. If this occurs, then the unprotected metal may be expected to deteriorate more rapidly because of the erosive effects of the bedload. Corrugated metal pipe should be checked to ensure joint integrity (see Art. 5.11.4).

Bolted longitudinal seams of structural-plate culverts should be inspected for cocking, cracking, and bolt tipping. Cocking occurs where the structure deflects inward at the seam, causing a significant change in the structure’s shape or appearance. This may be caused by improper erection or fabrication of the plates and can result in loss of back­fill due to piping and a reduced allowable compression strength of the structure due to the distortion. Cracking may occur where there is excessive deflection at the seam. This could ultimately lead to a disjointing, which would result in loss of ring thrust.

Bolt tipping is rare; it occurs where the plates slip because of high compressive forces. However, if the structure is under high fill and the plates slip, the bolt holes could become elongated, with the result that the bolt is eventually pulled through the plate.

Plastic pipe should be inspected for excessive deflection, joint integrity (see Art. 5.11.4), and cracking.

An inspection of the culvert should include the approach roadway, the embankment, the headwalls and wingwalls, the waterway, and the culvert barrel.

Roadway. The roadway over the culvert should be inspected for sags and cracks in the pavement that are the result of settlement. These may be evident in both the road­way itself and adjacent guiderail. The settlement may be the result of poorly compacted material adjacent to the culvert piping (infiltration or transportation of fines by water flowing through the backfill), or settlement of the culvert itself. The structural integrity of the culvert itself may or may not have been compromised. An inspection of the culvert must be made.

Embankment, Headwalls, and Wingwalls. The embankment, headwalls, and wing – walls at the inlet and outlet ends of the culvert should be inspected for signs of erosion, undermining, and settlement. If there is erosion at the ends, the structural integrity of the culvert will not necessarily be immediately compromised, but the hydraulic capacity will be affected. Any erosion or undermining will only worsen, and corrective action should be scheduled. If there is separation between the culvert and the endwalls, there could be a loss of supporting soil somewhere along the length of the culvert, which would affect structural capacity.

Waterway. The waterway should be inspected directly upstream and downstream for changes in the drainage. The culvert may have effected the changes in this drainage, and conversely, the changes in the drainage may have an effect on the culvert. An example of the former is where the velocity of the water is increased because of the channeling effect of the culvert. This velocity change could then cause either scour or accretion downstream. An example of the latter is accretion affecting the backwater up to the culvert, which can alter the subsequent performance of the culvert. In addi­tion, the waterway should be inspected for accumulations of debris and sediment at both the inlet and the outlet and within the culvert itself.

Culvert Barrel. The barrel or structure of the culvert should be inspected for defects, distortions, and deflections. The nature of these will depend upon the type of culvert being inspected.

Many storm drains and highway culvert systems have in the past been and are presently designed for a 50-year life span. The local roadway and state highway and interstate systems have in large part reached this age or soon will. Consequently, rehabilitation and repair of existing storm sewers and highway drainage culverts are presently requiring more and more attention and resources from the responsible agencies. It is generally less expensive to rehabilitate or repair an existing underground structure than to replace it. In addition, the cost of repair to the facility after a catastrophic failure

greatly exceeds the cost of rehabilitating the structure and preventing that failure. The key, of course, is being able to identify those structures that are in jeopardy of failing.

5.11.1 General Considerations

Failure of a culvert can be defined as any condition that could reasonably lead to the collapse of the roadway above or the inability of the culvert to carry the design flow. Failure of the roadway above may be a direct result of the collapse of the structure, or may be caused by a loss of the fill due to piping and the infiltration of fines. Excessive seepage through open joints can cause loss of the backfill material as illustrated in Fig. 5.45.

Fortunately, the complete collapse of a culvert is a rare occurrence. Culverts that are overstressed, either because of loss of the surrounding soil support or because of over­loads, tend to redistribute those stresses in many cases. For example, the loss of support or the effect of excessive live loads may not occur over the complete length of the struc­ture. Consequently, as one section becomes overstressed, it may deflect more than the adjacent sections and transfer loads to those stiffer sections. In addition, underground structures that show distress, such as a concrete pipe that cracks excessively or a flexible pipe that deflects excessively, may reduce the loads upon themselves by the very act of deflecting. For instance, flexible pipe that overdeflects may have a reduced over­burden load on it because the complete prism of earth above the structure may not necessarily move downward with the deflection; competent soils will have a tendency to arch over the pipe and support some of the load. The concrete pipe that cracks may form hinges and redistribute loads within the structure; the concrete pipe may now have more of a tendency to act as a flexible structure with reduced moments and increased compression forces. However, this discussion should not give the false impression that structural distress can be ignored. Catastrophic failures have occurred and caused fatalities when vehicles plunged into the void left by the collapse. Large structures with low covers are probably the most susceptible to structural failures and should be evaluated carefully.

Even if complete collapse does not occur, structural distress can affect the adjacent soil and accelerate failure. Piping and infiltration that cause loss of adjacent soil support may proceed at an increasing rate and cause failure of the roadway above. In some cases, enough fill may be lost through piping to create a sinkhole with the structure below showing no signs of severe structural distress.

The National Bridge Inspection Program requires that all structures with span greater than 20 ft (6 m) be inspected every 2 years. That is, all structures with spans

greater than 20 ft (6000 mm) when measured along the centerline of the roadway are classified as bridges for purposes of inspection. Two important points should be mentioned here. First, the measured distance is along the centerline of the roadway. That means a structural-plate pipe or a reinforced concrete box culvert with a 15-ft (4500-mm) span on a 42° skew will be classified as a bridge for inspection purposes and included in the bridge inspection program, even though the span is 15 ft (4500 mm) for hydraulic and structural design purposes. The second point is that multiple pipes are considered to be a bridge for inspection purposes when the out-to-out distance between the first and last pipes is 20 ft (6000 mm) or greater and there is a maximum of one-half diameter of the smaller pipe between them. For example, two 102-in­diameter (2550-mm) pipes separated by 51 in (1290 mm) would qualify as a bridge (102 + 51 + 102 = 255 in, or 21.25 ft, or 6375 mm).

Culverts that do not qualify for inspection under the bridge program should never­theless be given consideration for inclusion in a regular inspection program. To avoid repetition of inspections, some coordination between the engineers responsible for the two programs is necessary. Although the ideal would be to inspect all culverts, obvi­ous constraints, with regard to both physically inspecting the culverts and the costs of doing so, place limits on any program of culvert inspection. It may be less expensive to replace small culverts that are located beneath lightly traveled roads and have little fill on them than it would be to maintain them in an inspection program with rehabili­tation prior to failure as a goal. Conversely, where some culverts may not warrant inspections absent obvious signs of distress, others may require frequent inspections. Large structures that carry high flows during major storms or have a history of structural deficiencies, such as cracking (in concrete) or corrosion (in metal), should be inspected more frequently and especially after periods of storms.

Where high embankments are placed on original ground, the fill may compress and consolidate the foundation soil. Thus, culverts constructed on or near the original ground surface tend to undergo some settlement. The amount of settlement varies with fill height and the consolidation characteristics of the foundation soil. Because the amount of settlement varies with the fill height, the culvert will tend to settle more toward the center than at the ends. If the culvert is built upon a straight grade between the inlet and outlet elevations, a sag will develop. The sag may create a low point in the culvert, or may cause accumulation of debris and silt and opening and leaking of joints. These in turn may lead to a reduced waterway capacity and the possibility of loss of stability to the embankment through piping of fines at the joint. As illustrated in Fig. 5.44, these dangers may be avoided by cambering the culvert so that after settle­ment occurs, the culvert grade line will be at or close to that desired. Almost any type of culvert that is not cast in place may be cambered. These include precast concrete pipes and box culverts, corrugated metal pipes, structural-plate steel or aluminum pipes, and plastic pipes. The amount of camber required can be determined by a soils engineer.

5.10.2 Jacking and Tunneling

Should open-trench construction prove uneconomical or the disruption to the traveling public too great, either jacking or tunneling may prove to be more efficient. Either

Cambered Pipe

Camber

Final grade after settlement

FIGURE 5.44 Illustration of camber to allow for settlement of culvert under high fill.

(From Handbook of Steel Drainage and Highway Construction Products, American Iron and

Steel Institute, 1994, with permission)

method removes from consideration the possible disruption of traffic. In addition, for deep fills, these methods can be economically competitive with the open-trench method. The designer should be cautioned that when jacking or tunneling is used, small differences in anticipated geologic conditions may lead to large changes in the method by which the contractor solves the problem. For example, the difference between “running” and “flowing” ground can be not only very costly, but disastrous as well. If unanticipated geologic conditions are encountered by the tunneling or jacking contractor, the cost of the contract could increase dramatically. For this reason, if geo­logic conditions are in doubt, the designer is advised to obtain adequate geotechnical information through borings.

Jacking. Jacking of underground structures requires that the structure being jacked be able to withstand the large compressive forces acting on it. This generally limits the possibilities to reinforced concrete pipe, reinforced concrete boxes, and solid wall steel pipes. The first step is to adequately provide for a jacking pit, or to design a thrust wall if the jacking is to take place above ground. The jacking force and the ade­quacy of the structure itself to withstand that force are often left to the contractor. The jacking force required is dependent upon the type and diameter or span of the structure, the type of soil, the amount of overfill, and the jacking distance. Table 5.28 provides values of frictional resistance on reinforced concrete pipe determined from past jacking projects. These values may be reduced if a lubricant such as bentonite slurry is injected into the void created by the overcut. If the frictional resistance is too high for the thrust blocks or the jacks, intermediate jacking stations may be necessary.

Tunneling. Tunneling through soft ground is accomplished by pushing a shield forward and erecting a liner inside of it. The shield is then pushed off the liner as the tunneling progresses, so that there is no limit to the length that may be tunneled. The initial liner may consist of precast concrete sections, steel tunnel liner plates, or steel ribs with either wood or steel lagging. After the liner is erected within the shield and the shield is jacked forward, the void created between the liner and the ground due to overcut may or may not need to be grouted. The grouting of this area depends upon the judgment of the engineer and the type of liner. Tunnel liner plates may not be expanded once they are erected. Because of this, the void caused by the overcut is generally grouted. Precast concrete sections and steel ribs may be expanded to contact the earth once the shield is jacked forward. In this case it is left to the judgment of the engineer whether or not grouting is necessary. After the tunnel is completed, the carrier pipe is placed inside the liner and the void between the two is generally filled with either sand or grout.

Microtunneling. Microtunneling is a term used to describe a method of horizon­tally boring pipes approximately 36 in (900 mm) in diameter and smaller, using highly sophisticated remotely controlled equipment. The use of lasers allows for extremely accurate placement of the pipe in both grade and alignment. The pipe is jacked from a jacking pit as the tunnel is being bored and the spoils are removed.

Directional Drilling. Directional drilling is similar to microtunneling except that where microtunneling is a one-stage process, the directional drilling method consists of first drilling a pilot hole, reaming it to the proper diameter, and then pulling the pipe through. Because of this methodology, no jacking pit is required. This method has a high degree of precision in location of grade and may be used where the pipe diameter is 42 in (1050 mm) or smaller and the length to be placed is less then 5000 ft (1.5 km).

Stabilization Methods for Tunneling. As previously stated, tunneling may be required where it is necessary to keep a roadway or rail line open. This may occur where there is little fill over the crown of the excavation, or where there is adequate fill but it is lacking in stiffness or cohesive strength. When this happens, the soil above the excavation cannot, by itself, develop an arching effect that will adequately support the roadway. This situation necessitates unusual solutions such as chemical grouting, compaction grouting, ground freezing, and the use of spiles. The applicable method of increasing the support depends upon the site and soil conditions. Chemical grouting, compaction grouting, and ground freezing are all methods of stabilizing the soil. Spiles are horizontally drilled small-diameter holes extending from one side of the proposed tunnel to the other and surrounding the tunnel, generally in an arch shape. The holes, after being drilled, have a steel pipe placed in them, which is subse­quently filled with concrete. The spile diameter is commensurate with the size of opening to be excavated, and the spacing is reliant upon the amount of coverage and cohesiveness of the soil. After the spiles are in place, the tunnel excavation may begin with steel arch supports placed as necessary.