What is a Re-rounding

Re-rounding is a trenchless rehabilitation method used to restore the original circular shape of deformed pitch fibre pipes before applying a structural liner. Pitch fibre pipes, widely installed between the 1940s and 1970s, are prone to deformation due to their composition—typically cellulose fibres impregnated with bitumen. Over time, exposure to moisture, soil pressure, and chemical agents can lead to ovality, delamination, blistering, or even collapse.

The re-rounding process plays a critical role in extending the service life of such pipe systems, especially in domestic and light commercial drainage applications. It is often the first and necessary stage in a no-dig repair strategy, especially when a cured-in-place pipe (CIPP) liner or similar structural solution is to be installed afterwards.

Why Pitch Fibre Pipes Deform

Understanding the need for re-rounding begins with recognising why pitch fibre pipes degrade. Their construction materials, while cost-effective and easy to install at the time, are not suited to long-term underground conditions. Moisture absorption causes the cellulose to swell, while bitumen softens under high temperatures or chemical exposure. External pressures from soil settlement or root ingress also deform the circular profile of the pipe.

The result is often a pipe that becomes elliptical, reducing hydraulic capacity and creating points of weakness where further damage or blockage is likely to occur. In extreme cases, the structural integrity of the pipe is compromised, necessitating replacement or structural repair.

The Re-rounding Process

Re-rounding is carried out by inserting an internal mechanical device—typically a hydraulically expanded former or re-rounding pig—into the pipe. The device is then used to gradually press the walls outward until the original circular shape is re-established. This mechanical action helps redistribute the internal stresses and prepare the pipe for the next stage of lining.

The steps involved are generally as follows:

  1. Inspection and Cleaning: The pipe is first surveyed using CCTV to assess deformation and damage levels. Cleaning is performed to remove debris, roots, or scale.

  2. Insertion of Re-rounding Equipment: A mechanical former or pig is inserted into the deformed section. In some cases, a winch is used to pull the device through the pipe, forcing the walls outward.

  3. Re-rounding Operation: The equipment applies pressure to restore the pipe’s circular cross-section. For larger deformations, the process may be repeated or conducted incrementally.

  4. Stabilisation: In some cases, an internal inflation tube or packer may be used to hold the re-rounded shape temporarily.

  5. Lining Preparation: Once re-rounding is complete, the pipe is re-inspected. If the shape is satisfactory, a structural liner (e.g. CIPP) can be inserted and cured.

Structural Lining After Re-rounding

The success of any trenchless rehabilitation technique involving liners—such as cured-in-place pipe (CIPP), slip-lining, or fold-and-form liners—relies on the host pipe being sufficiently round. A structural liner is designed to bond with the internal surface and form a new pipe within the old one. If the host pipe is misshapen, the liner may wrinkle, collapse, or fail to cure correctly.

By re-rounding the pipe beforehand, technicians ensure that the liner will fit snugly, cure evenly, and maintain structural integrity over its design life. This approach not only avoids excavation and replacement costs but also minimises disruption to surrounding infrastructure and reduces environmental impact.

Benefits of Re-rounding

Re-rounding offers multiple advantages in the rehabilitation of ageing pitch fibre pipelines:

  • Cost-effectiveness: Avoids full pipe replacement and associated excavation costs.

  • Speed of repair: The process is usually completed within a day, depending on pipe length and condition.

  • Compatibility: Enables the use of structural lining technologies, which are widely accepted for long-term rehabilitation.

  • Minimal disruption: Particularly useful in urban or developed environments where access is limited.

  • Preservation of capacity: Restoring the round shape improves hydraulic performance.

When Is Re-rounding Appropriate?

Not all pitch fibre pipes are suitable candidates for re-rounding. This method is most effective in cases where the pipe shows moderate deformation but has not fully collapsed or delaminated. If the structural integrity is too compromised—such as in cases of severe blistering, cracking, or large holes—re-rounding may not be sufficient, and excavation or pipe bursting may be required.

Typical situations where re-rounding is appropriate include:

  • Ovalisation without cracks or breaches

  • Deformation caused by external pressure but with intact inner wall

  • Preparatory works for CIPP lining in domestic drainage systems

Limitations and Risks

While re-rounding is a valuable tool in the trenchless repair toolkit, it has its limitations. Overzealous application of pressure during the re-rounding process may cause further cracking or separation in pipes already weakened by internal delamination. Furthermore, unless a liner is installed shortly after re-rounding, the pipe may revert to its deformed shape due to residual stresses or external loads.

Other limitations include:

  • Unsuitability for heavily collapsed sections

  • Requirement for specialist equipment and operator skill

  • Temporary effectiveness if not followed by structural lining

Equipment Used in Re-rounding

The tools used for re-rounding vary based on pipe diameter, extent of deformation, and available access. Common equipment includes:

  • Mechanical pigs: These are solid or inflatable devices pulled through the pipe to mechanically force it back to shape.

  • Hydraulic expanders: Used for controlled pressure application in larger diameter or more fragile pipes.

  • Inflatable packers: Employed to maintain re-rounded shape before lining.

  • Rotary cutters and brushes: Sometimes used in conjunction to remove internal scale before re-rounding.

Regulatory Standards and Best Practices

In the UK, re-rounding is generally considered a preparatory step and not a stand-alone repair method. It is typically performed in accordance with industry best practices outlined by the Water Research Centre (WRc) and the standards for CIPP lining (such as BS EN ISO 11296 series). Installers must ensure that CCTV surveys, deformation reports, and structural assessments are conducted prior to and after the operation to confirm suitability and success.

Conclusion

Re-rounding is a specialised trenchless technique aimed at restoring the structural form of deformed pitch fibre pipes prior to lining. By reinstating the circular profile, it enables effective and long-lasting rehabilitation through structural liners, prolonging the lifespan of ageing drainage systems with minimal disruption. When applied correctly and in appropriate conditions, re-rounding offers a cost-effective and technically sound alternative to full pipe replacement, particularly in domestic and light commercial applications.