A number of repair processes are available that have been proven in the field, including cast in place liners, grouted slip liners, deform-reform systems, and helically wound liner systems (with or without grout). Each of these has advantages and disadvantages, and the choice of the best liner system for a particular rehabilitation project will depend on a variety of factors. For example, slip liners are often the lowest cost option for larger diameter sewer and culvert repairs, but require access from at least one end so that the preformed pipe structure can be inserted. Wound into place liners are possible in small and large diameters, and while these often need to be grouted, this may allow full bonding of the liner to the grout. Deform-reform systems provide many of the advantages of slip liners (a preformed liner of controlled thickness and material properties), but these may be unsuitable for heavily damaged installations or for use around bends. Cast-in-place systems can provide a close-fit liner of low cost in many instances, even when access is very restricted.

Liner design methods are established for these processes, and can generally be used with confidence. A number of circumstances arise, however, when a rehabilitation design specialist should be consulted, because the codified design methods may not provide safe or effective liner designs. These cases are the principal subject of the remainder of this discussion.

International studies – advancing understanding of liner performance and design

Over the past decade, significant advances have been made in our understanding of liner stability and the issues influencing design. Experimental work in the USA,5,8 the United Kingdom1,13 and in Canada6,7 has provided physical evidence for liner resistance to external water and earth loads. Theoretical work in Europe1,4,15 and in North America2,3,6,16 has explained the role of initial liner geometry (including imperfections in the circular shape) and other factors on liner buckling strength and the minimum liner thickness, and the manner in which earth loads can control the maximum permissible liner thickness.

Liner shape influences resistance to external fluid pressure

Liners can buckle under the external pressures that can develop on the liner due to groundwater or during grouting. Work by Khaled El Sawy2,3,10 in the 90s provided a key breakthrough in our understanding of the factors controlling this phenomenon. His studies revealed that liner buckling was sensitive to imperfections in the circular shape, and this work explains the test data that had been mystifying experimentalists working on this problem. We considered three different kinds of geometrical characteristics. Globally noncircular shape, initial gaps between the outside of the liner and the inside of the sewer or culvert being lined, and any initial wrinkle or waviness around the liner circumference can all degrade stability.

Noncircular shape has been discussed in design standards for some time, through consideration of elliptical (or ‘oval’) shapes. Often, the oval or other noncircular shape results because the sewer or culvert was not initially circular. For example, egg-shaped structures are common in various parts of the world.15 Alternatively, flexible sewers and culverts deform under the external earth loads and can become noncircular. Even nominally circular rigid pipes have initial out-of-roundness. Noncircular geometry can initiate or be exacerbated by deformations that occur due to deterioration.12 Erosion voids can form in the backfill surrounding a rigid or flexible pipe due to ingress of water through joints, or through perforations in the wall caused by corrosion.13,14 These erosion voids can then lead to significant deformations and increasingly noncircular shape in flexible structures. Rigid sewers experience increased bending moments and may fracture and deform as erosion removes ground support at the sides.14 Current ASTM design, for example, covers only elliptical shapes, and care should be exercised when using these provisions for sewers of other geometry (egg-shaped structures are particularly vulnerable to buckling along their almost flat sides). Design procedures are available to account for these more challenging shapes.15

An initial gap between the outside of the liner and the inside of the old sewer or culvert decreases the amount of external support and stability provided to the liner by the old pipe.2 The gap may result from shrinkage of the resins used in cast-in-place liners, or as a result of initially loose fit (before grouting) of slip-liners and helically wound liners. Even if the gap is eliminated by grouting, the size of the gap can limit the grouting pressures that can be used. The design of the rehabilitation process should then consider how loose fit influences the potential to buckle under external grouting pressures.11

Wrinkles or ‘waviness’ in the liner reduce buckling strength, because, in essence, the liner then starts with a shape that is like a potential buckle.3,10 This then decreases the amount of external fluid loads needed to induce buckling collapse. Wrinkles can result from the shape of the structure being repaired. For example, if the rigid pipe being repaired is damaged, and it has longitudinal fractures at the crown or invert, a cast-in-place or deform-reform liner with close fit will likely develop a wave or wrinkle where they cross these features.12 Wrinkles or wavy imperfections can also result from the lining process itself. For example, deform-reform systems using folded thermoplastic (high density polyethylene or polyvinyl chloride) liners often feature a permanent crease or wrinkle after inflation, a vestige of the fold-line. Wrinkles or waves can also develop when the liner has excess length (the external perimeter of the liner exceeds the internal perimeter of the pipe being repaired), and they sometimes occur when the liner is installed around horizontal curves. Installation of liners over obstructions, such as fibre optic cables installed along the sewer-liner, will also induce a wavy imperfection.

Liners experience local bending if they deform after insertion

The role of liners in supporting external soil loads has been the focus of considerable debate for over a decade. Some designers have concluded that the liner could be designed as if it were placed directly in the surrounding soil, and thickness chosen to resist the resulting stress, strain, or tendency to buckle. Others designers have recognised that the old sewer or culvert continues to provide support to the surrounding soil, and they concluded that no soil loads reach the liner. Experiments and analysis by Michael Law earlier this decade demonstrated that a liner placed within a rigid pipe before it fractures will experience local bending at the crown and invert if soil erosion or some other deterioration process causes the old sewer to fracture and deform.6,7 However, a cast-in-place liner placed inside a previously fractured and deformed sewer may not experience local bending if the use of the liner stops any further sewer deformation resulting from soil deterioration. A simple but effective design methodology for ungrouted liners has been developed,7 where local bending strains are calculated as a function of possible sewer deformations, and these lead to limits on the maximum allowable liner thickness (since local bending strains actually increase as liner thickness is increased). Bending strains have also been considered in grouted liner systems, considering the possibilities of composite or independent action of the liner and the grout (depending on whether slip occurs along the liner-grout interface).9 Other work is considering the effect of soil voids on the liner response to earth loading, and strategies are being developed to incorporate these issues in considerations of sewer damage obtained from closed circuit television or other pre-rehabilitation inspections.12 Summary

Sewer and culvert repairs are undertaken on a regular basis in many international jurisdictions, and conventional liner design methods generally provide safe and effective liner design choices. Specific cases where more care is needed and an expert in liner design needs to be consulted include applications where there is non-elliptical shape, where the existing sewer is severely damaged, or where the soil has deteriorated significantly outside the structure. Care is also needed when the liner construction process results in significant gaps between the liner and the old sewer, or wrinkles or waviness is expected or observed to occur around the liner circumference. The liner research community has developed a wealth of new understanding and computational tools for use by these liner design experts, which can enable experts to resolve most geometrical or other difficulties.