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DIB 83-02 - 11.1 Other Considerations
DESIGN INFORMATION BULLETIN No. 83-02
CALTRANS SUPPLEMENT TO FHWA CULVERT REPAIR PRACTICES MANUAL
|11.1 OTHER CONSIDERATIONS|
|11.1.1 Supporting Roadway and Traffic Loads|
|11.1.2 Compaction Grouting|
|11.1.3 Future Rehabilitation|
Our understanding of the final stages that lead to pipe or roadway prism collapse is still limited. Collapse/catastrophic failure normally originates where an initial, often minor, defect allows further deterioration to occur. Such defects may include:
- Cracking or deflection caused by excessive vertical load or bad bedding,
- Poor construction practice,
- Leaking joints or perforated invert,
- Damage caused by third parties;
Therefore it is not possible to predict when a pipe and/or the roadway prism will collapse.
However it is possible to judge whether a pipe has deteriorated sufficiently beyond its maintenance free service life (see HDM Topic 852) for collapse of the pipe and/or the roadway prism to be likely. As previously discussed (see Index 22.214.171.124.1) it should be noted Reinforced Concrete Pipe will fail but rarely “collapse”.
Collapse is often triggered by some random event that may not be related to the cause of deterioration, perhaps a storm or an excavation nearby. Serious defects do not always lead quickly to collapse; in one study of pipe collapses there were many minor defects compared to the number of collapses that occurred.
The following general discussion on risk of ground loss and voids on cohesionless and cohesive soils should be considered in context with the assumption that an existing pipe (either rigid or flexible) has been placed in accordance with the Standard Plans and Standard Specifications as referenced in Indices 126.96.36.199.1 and 188.8.131.52.
When evaluating the potential for soil loss or soil arching, the engineer must understand that imported material placed as either structure backfill or roadway embankment may differ significantly from native soils. The following discussion on soil behavior must be viewed within the context of the various soil properties which may exist in close proximity to the culvert - i.e., perforations or other discontinuities which might allow for soil migration may lead to soil reactions that vary significantly from the reaction of native soils depending upon the specific nature of structure backfill material and any other soil material placed above the pipe.
Risk of ground loss from subsurface erosion during storm flows is generally low for most soil types except cohesionless soils (silts and silty fine sands). However, for pipes with defects larger than 3/8th inch, any soil type can be affected by severe ground loss. If infiltration occurs, even if there is no hydraulic surcharging, almost all silts and sands will be highly susceptible to ground loss through large defects. Only well graded sandy gravels whose coarser part includes gravel particles of at least medium size will not be susceptible to ground loss. For smaller defect sizes well-graded sandy fine gravels would also be resistant. Silts and sands without gravel in the grading are likely to migrate even through minor defects.
If hydraulic surcharge does occur all cohesionless soils apart from well-graded sandy, medium to coarse gravels are likely to be highly susceptible to migration through minor defects.
Cohesive Soils: If infiltration occurs, then clay (invisible particles less than 0.0002 inch in diameter) backfills with a plasticity index (PI - an indicator of the “clayeyness” of a soil determined by the difference between the liquid limit and plastic limit per AASTHO T-90-00) lower than about 15 are susceptible to migration through severe and large defects irrespective of whether hydraulic surcharging takes place. If the PI exceeds about 15 then it is probable that ground loss will only occur through severe defects; ground loss in these circumstances is sensitive to the head of ground water present.
Water flow through the voids in clay backfill tends to erode the soil and high heads of water due to high ground water tables accelerate this process. Clays containing coarse particles (such as fill and many glacial tills) are more prone to erosion because the soil particles tend to induce more turbulent conditions. Undisturbed clays normally have a low percentage of voids, which reduces the risk of erosion even if the plasticity is low. Thus pipes constructed by tunnelling in clay are unlikely to suffer ground loss from the virgin ground but the material around the pipe will behave like trench fill.
Voids above the water table can remain stable, through capillary suction in cohesionless soils and through tensile strength in cohesive soils. Below the water table large voids can only be stable in cohesive soils. If a large void exists in a cohesionless soil above the water table, any wetting of the surface caused by hydraulic surcharge will destroy the capillary suction and the void will tend to collapse. This will produce a zone of loosened soil next to the pipe, which may be lost through defects. The void may migrate upwards away from the pipe. In a cohesive soil above the water table surcharge can cause progressive softening of the soil around the void, which can lead to further loss of soil and to the void increasing in size. Below the water table a void in a cohesive material will act as a drainage path and softening and erosion can also lead to an increase in size. Voids in cohesive soils both above and below the water table can also collapse and migrate away from the pipe leaving a zone of loose soft ground. In the fieldwork undertaken by others, voids or evidence of them was found at a number of the collapse sites studied.
Voids that develop around culverts which have been in place for a long time are similar to voids around newly installed jacked pipes and tunnels; They may go undetected until the overlying ground collapses into the void loosening this material. This loosened material, which supports the roadway, may immediately cause a depression or sinkhole at the surface, or it may occur at a later date when the loosened material re-densifies with the help of water, traffic vibrations, earthquake shaking, etc. For jacked pipes and tunnels, probing is often done from within the pipes and grouting is performed to fill the voids. See Index 6.1.2. Probing for voids may be performed within any large diameter pipe.
Stresses and Deformation
Deteriorated pipes in granular soils often experienced low vertical stresses from the overburden due to the very efficient arching capability of the circular or near circular shape in frictional soil materials. However vertical stresses on pipes in clays are closer to the full overburden stress and large deformations are required to mobilize the full soil strength to support the structure laterally.
Deformation of flexible pipes will occur when the soil at the sides no longer provides adequate support. This is clear evidence that deterioration is taking place. Final collapse is unlikely to occur until deformation exceeds 20% but typically only if other issues are present (sinkholes/depressions, etc. which show the fact that there has been loss of support) however, this final stage could occur quickly in response to an external influence.
With no other signs of distress, a flexible pipe deflected at 10% due to excessive load or improper compaction that is not perforated or is not experiencing soil loss, is not necessarily something to be alarmed about, and may need only monitoring. However, other pipes experiencing the same 10% deflection where;
- a) The invert is fully perforated and, cohesionless soils are present, or,
- b) Surface subsidence is present
are far more susceptible to collapse/catastrophic failure at 10% deflection due to some triggering mechanism. Therefore, depending on what conditions are present, our response to it may be to take immediate action or to monitor. It should be noted that the severity of impacts resulting from collapse would typically increase with pipe diameter.
When considering the viability of lining a deteriorated pipe with a flexible lining, calculations for ground and traffic loadings can be made but are very approximate due to the difficulty of assessing the equivalent stiffness of the old pipe, soil, and grout (if used) supporting the flexible lining. (See Index 6.1.1). For shallow pipes, traffic loading accounts for approximately half of the total loading. For pipes deeper than 2 meters, traffic loading accounts for approximately 25% of the total loading or less. Good ground support is present around most existing pipes. If the pipe to be renovated is in a reasonably sound condition and loadings on the pipe are not expected to increase (e.g. changes to highway profile grade), then the surrounding ground will normally provide enough support to carry existing ground and traffic loads and to ensure structural stability, particularly if soil voids are filled with grout as recommended.
Flexible pipes with excessive deflection (15% or more) will typically need to be replaced. If hydraulically possible (i.e., adequate cross sectional area can be maintained without a significant increase in headwater), heavily deflected flexible pipe may be lined with a rigid material (typically RCP or RPMP) that is capable of supporting all ground and traffic loads.
Compaction grouting is the injection of very stiff, low-slump, mortar-type Portland cement based grout (possibly with special admixtures including polymer resins) that is designed to stay in a homogeneous mass under relatively high pressure to displace and compact soils in place by acting as a radial hydraulic jack to strengthen loose or soft soils thus supporting roadway and traffic loads. Compaction grouting is used primarily on large pipelines applied through prepared grout holes in the pipe wall into the surrounding soil or from grout tubes drilled through the fill. Compaction grouting may also be achieved with chemicals and foaming grout; however, chemical grouts should only be used in cohesionless soil for conditions requiring resistance to high fluid pressures. The material should not shrink, segregate or otherwise create additional problems. Portland cement based grout is adequate for most culvert grouting.
Because of the risk and potential of numerous problems associated with compaction grouting, the importance of early communication with the Geotechnical Design specialist from the Division of Engineering Services (DES) and coordination with headquarters cannot be over-emphasized.
See Appendix H for a compaction grouting case study on the Century Freeway in Los Angeles.
Regardless of the rehabilitation method chosen, at some point in the future, the pipe will need to either be rehabilitated again or replaced. Therefore, consideration should be given to the projected service life of the rehabilitation materials and their future repair or removal when developing any rehabilitation strategy.
This page last updated August 20, 2011