Drain relining is often described as a no-dig solution to drain problems. That is accurate for most defects, root infiltration, joint cracks, corrosion, pipe belly. But a fully collapsed pipe presents a different challenge. Whether relining can fix it depends on how collapsed it is and over what length. This guide explains the distinction.
Quick answer (BLUF)
A fully collapsed pipe, where the pipe walls have folded inward and there is no open bore, cannot be relined without prior intervention. A partially collapsed pipe, where the bore is reduced but still open, can sometimes be relined if the liner can be installed and expanded against the remaining pipe wall. For short collapse sections within an otherwise relineable run, a targeted excavation to replace just that section followed by relining of the rest is often the most practical approach.
Understanding pipe collapse: degrees and causes
Pipe collapse is not binary. There is a spectrum from minor deformation to total obstruction, and the degree of collapse determines the available remediation options.
Deformation (pipe wall pressed inward): The pipe has lost its circular shape but retains an open bore of more than 50% of its original diameter. The pipe wall is distorted but not separated. This is typically relineable, the liner can be installed and will hold the deformed wall in a stable position.
Partial collapse (bore reduced to 20-50%): One section of the pipe wall has folded inward significantly. The liner can sometimes be installed if there is enough bore to pass the camera and the liner. In practice, this is marginal, some contractors will attempt relining here, others will not guarantee it.
Full collapse (bore closed or nearly closed): The pipe walls have collapsed completely or nearly so, creating a complete or near-complete obstruction. The camera cannot pass through. A liner cannot be installed. Excavation and replacement of the collapsed section is required before any relining can continue beyond that point.
What causes pipe collapse on the Central Coast?
Pipe collapse in Central Coast residential and commercial properties typically results from one or more of these factors:
Age and material degradation: Terracotta and AC cement pipes installed in the 1960s and 1970s in Gosford, Wyong and Woy Woy are now 50-60 years old. AC cement loses structural rigidity as the cement matrix carbonates and sulphate attacks the binder. Terracotta pipes in poor bed condition can crack and fold inward under soil pressure.
Point loads from above: A vehicle parked or driven over an unmarked drain, a slab poured directly over a pipe with no proper surround, or a tree root growing under and around the pipe all create localised point loads that eventually cause the pipe wall to fail.
Soil movement in clay areas: Properties in clay-soil areas of Gosford and lower Wyong experience seasonal soil expansion and contraction. Repeated cycles of movement can fatigue pipe sections to the point of collapse, particularly at bends and junctions where stress concentrations are highest.
External hydrostatic pressure: In the low-lying coastal areas around Woy Woy and Umina Beach, pipes in high water table ground can be subject to significant external water pressure. Where joints open and allow groundwater ingress, the void in the pipe bed grows and eventual pipe collapse can follow.
The CCTV inspection: identifying collapse severity
The CCTV camera is the definitive diagnostic tool. When the camera reaches a collapsed section, the footage will show the degree of closure and the approximate length of the affected zone.
Key things the camera operator will note:
- Whether the camera can pass through the obstruction (if yes, it is a partial collapse)
- The cross-sectional shape of the pipe at the collapse point
- Whether it is a single point collapse or a longer section of deformation
- The condition of the pipe upstream and downstream of the collapse
For a partial collapse that the camera can pass, the operator can also check the pipe condition beyond the collapse and plan the full remediation, which might be: excavate the 1.5 m collapsed section, replace with a new pipe coupling, and reline from the new section back to the main access point.
Hybrid approach: targeted excavation plus relining
The most cost-effective approach for a pipe with a single collapsed section in an otherwise relineable run is a hybrid:
- Locate the collapse precisely using the CCTV report distances from the access point
- Excavate a targeted hole to expose just the collapsed section (often 1-2 linear metres of excavation)
- Remove and replace the collapsed section with a new pipe coupling
- Reline the remaining pipe run from the new section to the main access points
This avoids a full trench excavation along the entire pipe run. The excavation might be 0.5 m × 1.5 m in a garden bed, far less disruptive than digging the full run. The relined section then bridges the new coupling and continues along the existing host pipe.
For properties in established gardens in Terrigal or Avoca Beach, this targeted approach is valuable, it avoids excavating under mature garden plantings for the full pipe length.
When excavation is the only option
Full excavation and replacement becomes necessary when:
- Multiple sections along the run are fully collapsed
- The pipe has collapsed under a concrete slab, floor screed or driveway where any relining would also require access excavation
- The collapsed sections are too numerous or too long for targeted repair to be cost-effective compared to a full replacement trench
- The host pipe material is in such poor condition that it cannot support a liner even in non-collapsed sections
In these cases, relining is not the answer, but this represents a minority of collapsed pipe scenarios. Most drain collapses on residential properties are isolated events in an otherwise functional pipe run.
FAQs
How do I know if my blocked drain is a collapse or just a root blockage?
A hydro-jet operator clearing a root blockage will feel the roots give as the jetter clears them. A collapse does not yield to jetting, the water pressure finds no path forward. If the drain does not respond to high-pressure jetting, request a CCTV inspection immediately to determine whether collapse has occurred.
Can a partially collapsed pipe cause a drain blockage?
Yes. Even a partial collapse that reduces the pipe bore by 30-40% can cause significant flow restriction. Combined with the debris that accumulates at the deformation point, partial collapses frequently cause recurring blockages even after the upstream section is jetted clear.
How long does it take to fix a collapsed pipe section?
Targeted excavation to replace a single collapsed section takes a full day in most cases. If relining of the surrounding run follows on the same visit, the total job is typically one to two days.
Are collapsed pipes common in older Central Coast homes?
Partial collapses, deformations and belly sections, are quite common in 1960s, 1980s terracotta and AC cement pipes. Full collapse is less common but not rare, particularly in areas with significant soil movement or where pipes were not properly bedded during original installation.