Plate D — Longitudinal crack.

§ I · Why "longitudinal" matters

The mechanical mode of a longitudinal crack is different from a circumferential crack. A circumferential crack — running around the pipe's circumference — interrupts hoop stress at the spigot end and is usually evidence that the pipe has been pulled apart by axial movement (settlement, frost heave, root wedging). A longitudinal crack runs along the axis and is evidence that the pipe has been crushed downward, perpendicular to its length. The pipe is being squeezed. The driveway above it, or the sidewalk above it, or the parked-cars-and-trucks loading above it, is the squeezing agent.

Vitrified clay tile has impressive compressive strength when supported around its full circumference but limited tensile strength at the crown when loaded above-center. The 1908 spec for Buffalo clay tile¹ assumed an undisturbed earth cover of at least 2 ft and no concentrated surface load above. A 1962 driveway slab with delivery-truck weights changes the loading profile fundamentally; the pipe held for sixty years and then began to crack.

§ II · Why CIPP fixes it

The longitudinal crack does not change the pipe's bore geometry. The host bore is still continuous, still circular in section. CIPP lining inverts a felt-and-resin sleeve into the bore and cures against the host wall. The sleeve carries the structural load on its own once cured — the host pipe transitions from being the structural element to being the formwork that gave the liner its shape. Whether the host wall is intact, hairline-cracked, or substantially fractured longitudinally, the cured liner does the work. The host can crack further; the liner is the pipe.

This is, frankly, the easiest case for CIPP. The bore is sound, the pre-clean is short (no roots, no scale to remove), the inversion runs cleanly, and the cured liner has a 50-year design life under ASTM F1216.² For a customer who has discovered a longitudinal crack in a real-estate-inspection survey and is anxious about it, the right answer is: we line it, the crack stops being a problem, and the loading above it doesn't matter again for half a century.

§ III · The "do nothing yet" option

If the longitudinal crack is shorter than 18 in., the bore is fully open, no roots are entering through it, and there is no associated material failure (no missing pipe wall, no spalling clay), we will sometimes recommend deferring rehabilitation. Document the survey, monitor with another scope in 3 years, and rehabilitate only if the crack has propagated or if root entry begins. We have customers on this watch-and-rescope pattern five years in with no progression. The CIPP ages from the day it cures, and there is no value in starting that 50-year clock today on a pipe that will not need the liner for another decade.