Liner Design Considerations

Liner Design Considerations | Print Version

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Cured in place pipeliners, in the normal case of underground sewers, are designed to withstand hydrostatic loading due to external groundwater pressure. In addition, some liners may be designed to resist the effects of soil loading and loads superimposed on the soil loads (e.g., traffic loads).

Except in significantly out-of-round pipes (e.g., ovoid pipes) the failure mode of pipeliners is considered to be due to buckling. Further, pipeliners are designed to transmit flow in the pipe as efficiently as possible, taking into consideration the relative smoothness of the walls and loss of cross-sectional area. Pipeliners are designed to significantly reduce infiltration and exfiltration to and from the surrounding soil.

Impregnation issues are associated with the bunching up of the inner liner(s) and are unacceptable.

Inversion issues

• Circumferential fins, unless slight, are unacceptable due to the possibility of deposition upstream. Because they are associated with the thickening of the pipe wall, such fins can be ground-off.
• Longitudinal fins, even moderate in nature, are not detrimental to the structural or hydraulic performance of the liner.
• Seams and patch repair issues An aspect of atypical features associated with seams and patch repairs is related to leaks. Where significant resin loss has occurred then structurally, an incomplete ring of pipe exists at that point.
• Stained seams are often due to water passing through in small quantities and impurities filtering out. Unless significant resin loss has resulted, stained seams are a question of aesthetics.
• Leaking seams are unacceptable. It is important that the liner is impregnated under adequate vacuum and at the correct nip roller gap. This will normally, with conventional catalyst cocktails, ensure a low porosity and result in sound seams. If the leaks are not too extensive, then a grout packer may be used to correct this issue.
• Leaking patch repairs through which groundwater can leak are unacceptable. It is possible that after grinding away the patch, a grout packer may be used to correct this issue.

Coating-related issuesAn aspect of atypical features associated with coating-related problems is that of minor distortion of surfaces. This aspect is of no consequence either hydraulically or structurally.

• Bubbles on liners can be removed if they are unacceptable aesthetically. If they are caused by ingress of groundwater then some means may be needed to prevent further ingress.
• Uneven finishes on liners are typified by surface irregularities and although appear unsightly when viewed along the pipe on CCTV, do not detract from the structural or hydraulic performance of the liner. Air pockets behind liners are only of significance under high external head.

Inward folding of the lining An aspect of atypical features associated with inward folding of liners is a dramatic loss of cross sectional area. Not only is this of significance hydraulically, but also because the circular cross-section with close tolerance to the existing pipe wall required for buckling resistance is lost. This is potentially a serious problem, but can in favorable circumstances be overcome by inverting a dry liner through the problem liner and circulating hot water. This should soften the liner sufficiently so that it can be reshaped. Do not apply a high head until hot or the liner may crack. As a last resort, the lift section may need to be cut out completely and a new length of liner installed.

• Spots and other Stains (e.g., on seams) are associated with leaks of water through small holes in the coating. This problem is included here because it can lead to resin loss over an extended area and subsequent low strength followed by distortion, even lifting, of the liner. The cause of the holes is discussed in the spots section.

Damage to the surface of liners can be caused by a range of factors. The coating can be ripped and gouged by sharp objects used for rope end/soft end fittings. Alternatively, damage can be caused to the coating by dragging the liner over rough surfaces. In any case, if such damage results in water leaks, then it is unacceptable and should be repaired.