Throughout history, various methods and pipe materials have been used to install underground utilities for the transmission of essential services and utilities. Part two of the pipe and conduit feature focuses on the specific modes of failure of different pipe material.

Why pipe fails

The reasons for pipeline failure include erroneous design, inappropriate pipe material selection, manufacturing defects, poor installation, excessive loads, long term degradation caused by interaction with the environment, and third party damage.

Erroneous design

Correct design assumptions may be based on the depth of cover and soil type, height of water table, traffic loads, and surge and vacuum loads. Pipe selection

A failure can be caused by an incorrect choice in pipe. Both pipe strength and pipe material are integral to the success of the project.

Specification, manufacturing and installation issues

The specification, manufacturing and installation of the components of the project will contribute to the longevity of the system.

Inadequate product specification, quality assurance and testing regimes can all lead to inappropriate product selection for the particularities of the job.

Undetected manufacturing flaws will of course affect the ability of the product to perform as promised or expected.

Incorrect installation and handling of the piping material will have a catastrophic affect on the ability of the final pipe network to behave as expected. Poor installation can be characterised as excessive trench width, inadequate backfill compaction, wrong type of backfill and incorrect pipe bed-point loads. Poor joining techniques such as inadequate butt fusion of PE or PVC pipes and the wrong positioning of flexible joint components will also affect the design life.

Site effects and long term degradation

Changes in site effects and environment will affect the long term degradation of the pipe material. A change in site conditions after installation may lead to system overloads. Other changes that could lead to long term degradation include unexpected surge or vacuum effects, change of the corrosion potential in the liquid being transported and the external and internal corrosion of metallic pipes.

Related site effects may be the failure of corrosion protection systems, the attack on cement based products by acid waters or soils and frost and related thermal effects.

Cast iron, ductile iron and steel pipe

Pit and spun comprise approximately 48 per cent of European Union and United States distribution networks. The production of pit cast iron started in the 1850s and continued until the 1940s when it was replaced by spun cast iron. Spun cast iron was produced until the 1960s when it was replaced by ductile iron. Trenchless expert and Chairman of the ISTT Dec Downey said that “all three categories are failing.”

Cast iron, ductile iron and steel pipe are all subject to pitting, localised wall thinning and general wall thinning. Cast iron pipe is relatively brittle and is more susceptible to fracture. Ductile iron and steel pipe are more ductile and commonly fail by wall penetration and the subsequent growth and coalescence of the corrosion pits.

Internal corrosion of unprotected cast and ductile iron and steel pipe

• Contamination of water by corrosion products causing drinking water compliance and aesthetic problems, such as red water.
• Reduction of pipe bore by corrosion product deposits, known as tuberculation, causing flow and pressure problems.
• Pinholing, wall thinning and graphitisation (selective removal of iron leaving a weaker carbon flake structure) cause structural weakening, bursts and transverse fractures.

External corrosion of protected ductile iron and steel pipe

• Local damage to external corrosion protection or failure of cathodic protection system.
• Local wall thinning leading to perforation and leakage – may enhance corrosion rate and wash away the bedding, increasing the risk of transverse structural failure
• General wall thinning leading to transverse and longitudinal structural failures or bursts
• Enhance susceptibility to combined effects of corrosion and stress.

Method of failure - cast iron

The modes of failure of cast iron pipe includes a reduction in wall thickness with pitting, graphitisation, and inability to support the imposed loads. In small diameters circumferential, cracking is common while in larger diameters longitudinal splitting is more common. Method of failure - ductile iron Many early pipes have no external protection. Modern pipe is now supplied with internal cement or epoxy lining. Ductile iron is subject to external corrosion often associated with failure of external protection systems such as pipe wrapping. The material is up to 25 per cent thinner than cast iron, therefore it depends more on internal and external protection for longevity. The most common mode of failure is the growth of pits until the wall is penetrated, high pressure leaks then accelerate the corrosion.

Method of failure – steel pipe

Steel pipe is most commonly used in transmission mains with limited use in distribution networks. This material is subject to external and/or internal corrosion depending on the type of protection used. The modes of failure are similar to those related to ductile iron.

Cement based pipe

Method of failure – asbestos concrete pipe

Asbestos cement was widely used until health precautions impacted manufacture and installation of asbestos pipe. This pipe material displays generally good performance, with low burst rates, except in soft and aggressive soils. The most common failure mode is progressive softening and swelling until a sudden burst. Asbestos concrete pipe can also corrode inside and out and may be subject to joint failures.

The softening of cement in ‘acidic’ water releases asbestos particles, which can cause turbidity problems, customer concern over potential consequences of ingesting fibres, wall thinning that may lead to local perforation, leakage and ultimately transverse or longitudinal structural failure. Finally, groundwater may attack the external surface.

Method of failure – unreinforced concrete pipe

Unreinforced concrete pipes can experience external carbonation and/or corrosion from ‘acid’ soils, leading to structural failure. Internal corrosion may be caused by acid or soft water. Method of failure – reinforced concrete pipe

Steel reinforced pipe can experience corrosion of steel due to the loss of concrete cover or from chloride/sulphate attack.

Prestressed concrete pipe may fail due to the corrosion of the steel cylinder, the corrosion of the prestressing wire or the embrittlement of the wire from hydrogen. All these situations can lead to structural failure.

Method of failure – plastic pipes

There are not yet any known, long term corrosion mechanisms for plastic pipe. Most problems are caused by poor bedding, poor installation or poor jointing.

There could be structural problems in older uPVC pipes because the original pipe formulation proved susceptible to crack initiation at local stress concentrations, followed by fast brittle fracture propagation through ‘glued’ joints. Changes in formulation and jointing practice have now mitigated this problem.

Conclusion

The relative strengths and weaknesses of different pipe materials need to be taken into account when installing or repairing a pipeline. In order to ensure the network has a long life, installation technique is also a very important factor to consider.