CIPP Liner Thickness for Groundwater Buckling (ASTM F1216)

The two design cases are not variations on a theme. In the PARTIALLY DETERIORATED case the host pipe is still structurally sound and the liner's only job is to resist external groundwater pressure trying to buckle it inward, so the thickness comes out modest. In the FULLY DETERIORATED case the host is assumed gone and the liner is the pipe, carrying soil load, live load, and groundwater as a standalone structure -- a much thicker section and a different relation. Choosing the case is an engineering judgment about the host pipe's condition, made from a CCTV survey and the pipe's history rather than from a preference. A 24 in host at 3% ovality with 12 ft of groundwater above it, at a 125,000 psi long-term modulus and a safety factor of 2, needs 0.452 in of liner. TIME IS THE TRAP. CIPP creeps, so the fifty-year modulus is roughly half the short-term value, and designing on the short-term number gives 0.360 in -- twenty-five percent thin, in the unconservative direction. OVALITY IS THE OTHER ONE, and it bites steeply because the reduction factor goes as a cube: a round host would need 0.414 in, the measured 3% takes it to 0.452, and 5% takes it to 0.479. Ovality is measured rather than assumed. Groundwater head is the load, and it is taken at the highest credible level rather than at the level on the day of the survey. One equation from one design case. The fully deteriorated case needs the soil modulus, the live load, and the water buoyancy factor and is deliberately not approximated here. It does not select a resin or liner system, address the wet-out, cure, and cool-down that determine whether the installed liner has the modulus the design assumed, evaluate the host pipe's condition, handle bends, laterals, or changes in section, or address the field testing and sampling that verify the finished product. ASTM F1216, the liner manufacturer's tested properties, and the design engineer govern.

ASTM F1216 X1.1, partially deteriorated: external pressure = 2 K E C / (1 - nu squared) x (1 / (DR - 1)) cubed / N, with the ovality reduction C = ((1 - q) / (1 + q) squared) cubed and Poisson's ratio 0.3; thickness = host diameter / DR.

The ASTM F1216 X1.1 groundwater-buckling relation by name, for the partially deteriorated design case only. The fully deteriorated case is a different relation requiring the soil modulus, the live load, and the water buoyancy factor, and is not approximated here. ASTM F1216, the liner manufacturer's tested properties, and the design engineer govern.

The buckling relation is published engineering; the modulus, ovality, and enhancement factor are the user's own measured and manufacturer-supplied values, and no F1216 table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: host_id_in, ovality_pct, groundwater_head_ft, long_term_modulus_psi, short_term_modulus_psi, enhancement_factor, safety_factor, alternative_ovality_pct, external_pressure_psi, ovality_factor, dimension_ratio, thickness_in, short_term_thickness_in, alternative_thickness_in

• Ovality goes as a cube a few percent of out-of-roundness is real thickness, and it is measured not assumedASTM F1216
• Use the LONG-TERM modulus creep halves it over fifty years, and the short-term number is unconservativeASTM F1216
• The fully deteriorated case is a different relation it needs the soil modulus, live load, and buoyancy factor and is not approximatedASTM F1216