Trenchless Pipe Rehabilitation: Choosing and Designing CIPP, Pipe Bursting and Sliplining

The decision: condition, function and host pipe

Trenchless selection starts from three questions. What is wrong with the pipe? Infiltration and minor cracking call for a non-structural or semi-structural lining; collapse, deformation above about 10 percent or loss of wall calls for a fully structural solution or replacement. What must the renewed pipe do? If capacity must increase, only pipe bursting (which can upsize) or open cut will serve; lining always reduces the bore. What is the host pipe? Brittle materials (vitrified clay, grey cast iron, concrete) burst well; ductile iron, steel and heavily repaired pipe with clamps and concrete surrounds do not.

Structural class matters because it sets the design assumption: a non-structural lining relies on the host for all load; a semi-structural liner carries external groundwater and bridges holes but relies on the host for soil and traffic loads; a fully structural liner is designed as if the host pipe were not there.

CIPP structural design: buckling under external pressure

A cured-in-place liner is a thin, flexible tube inside a rigid (if cracked) host. Its governing failure mode is not hoop stress but elastic buckling under external hydrostatic pressure from groundwater that enters the annulus through the cracked host. For the partially deteriorated design case (host carries soil and traffic, liner carries groundwater) the method uses the enhanced-buckling equation from the ASTM F1216 appendix, which is also the basis of the UK and ISO 11296 approach:

The ovality factor penalises a host pipe that is already out of round, because an oval liner buckles at lower pressure:

The long-term modulus is the number that most often gets fudged. A polyester-felt liner may test at 2,500 MPa short-term flexural modulus, but resin creep under sustained load halves that over 50 years; design uses the creep-reduced value, typically 50 percent of short-term for polyester and somewhat higher for epoxy and glass-reinforced (UV-cured) liners. Use the manufacturer's tested long-term value, not the datasheet short-term figure.

Worked example. A 450 mm clay sewer with 2 percent ovality lies with groundwater 3.0 m above its invert. External pressure at the invert is 3.0 m of water = 0.0294 MPa. A polyester CIPP liner is proposed with E_L = 1,250 MPa (long-term), ν = 0.3, K = 7, N = 2.

• C = [0.98 / 1.02²]³ = [0.942]³ = 0.836
• Rearranging for the required DR: (DR − 1)³ = 2 K E_L C / [P N (1 − ν²)] = (2 × 7 × 1,250 × 0.836) / (0.0294 × 2 × 0.91) = 14,630 / 0.0535 = 273,400
• DR − 1 = 273,400^1/3 = 64.9, so DR = 65.9
• Required thickness t = D / DR = 450 / 65.9 = 6.8 mm; specify 7.5 mm to allow for thickness tolerance and resin migration.

Doubling the groundwater head to 6 m raises the required thickness by only 2^1/3 = 1.26, to 8.6 mm, which shows why liners are economic: thickness scales with the cube root of pressure. Conversely, 5 percent ovality drops C to 0.64 and pushes the thickness to 7.5 mm before any tolerance. If CCTV shows the host has lost wall (fully deteriorated case), the liner must also be checked for soil and live load as a flexible pipe using the modified Iowa deflection formula and a long-term buckling check with soil support, which typically governs above about 3 m of cover.

Pipe bursting: pull loads and the new pipe

Pipe bursting pulls or pushes a conical expander through the old pipe, fracturing it outward into the surrounding soil, while towing the new pipe (almost always PE100) into the void behind it. Three engineering concerns follow: the force needed, the capacity of the new pipe to carry that force, and the effect on the ground and adjacent services.

The pull force is the sum of the force to fracture and displace the host, friction between the new pipe and the soil, and friction at any bends. It rises with upsizing, with depth (confinement), with cohesive soils and with length. Rule-of-thumb forces range from 100 kN for small mains in sand to over 1,000 kN for upsized larger sewers in clay; rigs are selected with margin, and the critical check is on the new pipe:

For a 315 mm SDR17 PE100 pipe (t = 18.7 mm): F_allow = 8 × π × (315 − 18.7) × 18.7 = 8 × 17,410 = 139 kN. If the estimated pull is 180 kN, the options are a thicker wall (SDR11, t = 28.6 mm, giving 206 kN), a shorter pull, or a larger expander ahead of the pipe to reduce soil friction. Pulling a pipe beyond its safe tension causes necking and a permanently thinned wall that fails years later under pressure.

Ground displacement is proportional to the upsize ratio and inversely to depth; a one-size upsize at 1.2 m cover in a street with gas and telecoms within 0.5 m of the main needs trial pits and, often, a different method. Bursting is not suited to host pipes with concrete surround, repair clamps or ductile sections, which stall the head.

Sliplining and the capacity penalty

Sliplining inserts a new, smaller pipe into the old one and grouts the annulus. It is the simplest trenchless method and the one with the largest hydraulic penalty. For a gravity sewer or a pressure main the capacity ratio follows from the headloss relation. With Hazen-Williams, Q ∝ C D^2.63 for the same gradient, so for a 450 mm concrete sewer (C ≈ 100) slip-lined with 355 mm SDR17 PE (internal diameter about 313 mm, C ≈ 150):

• Q_new/Q_old = (150/100) × (313/450)^2.63 = 1.5 × 0.385 = 0.58

A smoother new pipe recovers some of the loss but the sewer still gives up 42 percent of its capacity. Sliplining therefore suits trunk mains and sewers with proven spare capacity; on a sewer at or near capacity it converts a structural problem into a flooding problem. Close-fit PE lining, in which the liner is temporarily reduced in diameter (swaged or folded) and reverts to press against the host wall, keeps most of the bore and is the usual choice for water mains where sliplining would not.

Spray-applied linings and the non-structural end

Polyurea and polyurethane linings sprayed at 2 to 6 mm by a rotating head give a fast, low-disruption corrosion barrier in water mains and can bridge small holes and joints. Thicker polymer liners (up to 12 mm) are marketed as semi-structural with a hole-spanning capability; the design check is the burst pressure through an unsupported hole of a given diameter, which the supplier must demonstrate by test. Cement-mortar lining, the older equivalent, restores a smooth bore and passivates iron but has no structural value and reduces the bore by the lining thickness on each side.

None of these address capacity, bursts from external loading, or joint displacement, and they are only as good as the surface preparation beneath them. Specification should require CCTV before and after, thickness measurement by holiday and thickness gauge, and a defined repair-to-defect rate.

Specification, QA and what goes wrong

Common trenchless failures are not structural miscalculation but process control: liners cured too fast or too cold leaving a weak, wrinkled wall; resin washed out by infiltration during installation; laterals reinstated badly by robotic cutter; bursting heads stalled on an undocumented repair collar. A sound specification therefore requires pre-installation CCTV and laser profiling (for ovality), cure monitoring with thermocouples at the far end of the liner, sample plates cut from the installed liner and tested for flexural modulus and thickness, post-installation CCTV, and a watertightness test. Design calculations should be submitted and checked, with the long-term modulus traced to a test certificate.

Where rehabilitation connects to a treatment works or pumping station, hydraulic changes at the interface (a smoother, smaller pipe, altered surge behaviour) are worth a check by an engineering firm offering bespoke process and hydraulic design before the liner is ordered. Pipe condition assessment that decides whether to rehabilitate at all is covered in the pipe condition guide.

How to size a CIPP liner for the partially deteriorated case

1. Establish the external pressure. Groundwater head above the pipe invert from borehole or manhole observations, taken at the seasonal maximum, converted to MPa (1 m = 0.00981 MPa).
2. Measure ovality. Laser profile or CCTV measurement of the host pipe; compute the ovality factor C from the worst section.
3. Obtain the long-term modulus. Use the liner manufacturer's creep-tested 50-year flexural modulus, not the short-term datasheet value.
4. Solve for DR. Rearrange the enhanced buckling equation with K = 7 and N = 2 to get the required diameter-to-thickness ratio.
5. Set the thickness. Divide host diameter by DR, then add manufacturing tolerance; specify the nominal thickness and the minimum acceptable installed thickness.
6. Check the fully deteriorated case if the host has lost wall. Run the flexible-pipe deflection and soil-supported buckling checks for soil and traffic load, and adopt the thicker of the two results.