Typical defects that affect BCMS include joint defects, invert deterioration, corrosion, shape distortion and soil migration. The cause of the defect can be a result of the construction process, in-service loading or environmental conditions. These defects should be identified as part of a Level 2 structural inspection. The following points explain the most common defects:
(a) Helical structure
The most common problems associated with helical steel joints are misalignment, water exfiltration, backfill infiltration and joint separation.
Misalignment of the joints may be a sign of settlement in the supporting soil structure. This settlement may have occurred during construction and stabilisation. The more serious problem is if progressive settlement is continuing to occur while in-service. Misalignment can also lead to undermining of the BCMS, water exfiltration or infiltration of backfill material.
Exfiltration occurs when leaking joints allow water flowing through the culvert to leak into the supporting material. Exfiltration can result in piping where supporting soil material is easily eroded.
Infiltration is the opposite problem to exfiltration and occurs where water from the backfill material is seeping through the culvert joints. Infiltration can cause settlement and misalignment problems if the water carries fine-grained soil particles from the backfill material.
Joint separation may also occur due to external loads and changing soil conditions and this
(b) Bolt defects
Joint defects for multi-plate structures occur at the bolt lines typically from construction damage. The bolt lines are weaker than the plate itself, and some construction specifications call for the joint to be offset in each ring to avoid a line of weakness. If this was not done, the backfilling operation could put high bending moments on the bolted joint lines, causing local cracking in the plates where excessive tension occurs (Figure 4.1).
This is a defect that should be avoided in construction; however, once the BCMS is completed and backfilled, the cracked joint should go into compression and not be a long-term failure initiator.
It is essential, though, that the cause of such defects be determined. If they are due to continuing vertical loading they may indicate the start of structural failure.
Figure 4.1: Cracks in metal plate probably caused by excessive side pressures during backfill (c) Invert deterioration
Invert deterioration is usually due to a combination of corrosion and abrasion. Once the galvanising layer is abraded from material carried by the flow of the water, corrosion then attacks the bare steel and is accelerated by further abrasion that constantly removes the protective oxide layer formed by corrosion. The continuation of this action will ultimately lead to the loss of the invert and the creation of scour holes under the BCMS. Continued
deterioration could result in the complete washout of the structure. It should be noted that the progression of scour holes to full washout can occur in a matter of minutes.
For BCMS to withstand significant fill loads and heavy repetitive live loads, an effective soil structure interaction is necessary. This composite behaviour uses the compressive strength of the structure wall, with the compressive or bearing strength of the well-compacted soil surrounding the structure. As loads are applied to the structure, the flexible structure attempts to deflect, with the vertical diameter decreasing and the horizontal diameter increasing. The change in horizontal diameter is resisted by the lateral soil pressure and results in the relatively uniform radial pressure around the structure that creates a compressive thrust in the structure walls, hence ring compression theory can be assumed (Conn DOT 2000).
As described in Section 2.1 the metal structure behaviour is reliant on uniform radial pressure around the pipe, and loss of the invert may result in severe distortion and collapse of the BCMS.
(d) Corrosion
Corrosion of the BCMS can ultimately cause failure by reducing the material thickness. This can occur at the invert level due to the removal of the galvanising through abrasion and standing water, or on the external face of the metal structure from chloride or sulphate attack due to the content of the backfill material. As the external face of the metal structure is not visible, the determination of metal thickness is important to establish the aggressiveness of the environment and the necessary repairs. This can be undertaken using destructive
methods (hole drilling) or non-destructive methods such as ultrasonics. If holes are drilled as part of testing program, each hole must be fitted with a galvanised screw.
(e) Shape distortion
The construction process for BCMS requires care and attention to the backfilling procedure.
During construction, metal structures are flexible and will distort if excessive earth pressure or construction loads are applied. In extreme cases, the metal structures can collapse or be severely distorted during construction (Figure 4.2).
In assessing a BCMS in-service, it is essential to consider the cause of any distortion (out-of-roundness). Construction damage should be assessed separately to overloading, distortions or soil movements under current service conditions. Significant distortions occurring during construction should be recorded in construction files or earlier inspection reports. Unfortunately, construction records may be difficult to access after several decades.
Determining when damage occurred may require structural engineering advice.
As part of a structural management plan, a geometry survey plan should be undertaken as part of a commissioning inspection undertaken by the team responsible for future inspection and management of the culvert.
Source: TMR (2010).
Figure 4.2: New culvert damaged at joint during backfill, probably due to construction overload (f) Soil migration
Soil migration occurs when there is a loss of backfill support due to water eroding fine material from the trench side walls. For migration to occur the backfill material must be erodible and there must be a flow path for the water. Due to the fact that granular material is typically used as BCMS bedding, any perforation of the structure by corrosion can lead to floodwater washing the bedding sand out. Any significant flow of water behind the metal
(g) Crimping of the wall (CSPI 2007)
Crimping is local bucking of the shell into a large number of waves, each of relatively small length. It predominately occurs in the compression zone of the wall where the culvert undergoes large bending deformations. This type of crimping usually takes place in culvert wall segments of relatively small radius of curvature. It indicates that the soil behind the segment is not dense enough to prevent excessive bending deformations. Crimping can also occur throughout an entire culvert wall section subject to excessive thrust while being
supported by a very well compacted backfill. This situation is rare but can occur in culverts built on relatively yielding foundations. In this instance the long-term foundation settlement is thought to induce negative arching thus subjecting the wall to greater thrust loads than assumed in design, resulting in buckling of the wall.
(h) Distortion of bevelled ends (CSPI 2007)
Bevelled ends are particularly vulnerable to damage by horizontal pressures due to the inability to develop ring compressions. They are also vulnerable to heavy pieces of equipment falling on them or impact from debris.
(i) Excessive silt build-up
Excessive silt can result in reduced flow volumes through the culvert and provides a medium to ensure that moisture remains in contact with the metal structure for longer periods of time.
This results in accelerated corrosion of the culvert invert. Such build-ups need to be removed as part of ongoing maintenance and assessments need to be made as to why it is occurring.
It might be the sign of a more pressing problem. Mitigation strategies should also be suggested.
(j) Surrounding soil condition
Erosion and undermining of the culvert need to be identified including the condition of the various forms of end treatments.