How to rehabilitate a sewer

Sewers and drains are a vital part of the UK’s infrastructure, and play an essential role in protecting the environment and maintaining the nation’s health. It has been said that the UK has the world’s oldest and most comprehensive wastewater network, which is more than 300,000km long. And about 23% of this network is classified as critical.

A critical sewer is one regarded as having the most significant consequences in the event of structural failure. Such consequences can include:

• Direct costs of repair or replacement, often in an emergency situation
• Delays to traffic and pedestrians
• Disruption to local economies
• Flooding to property
• Loss of amenity
• Environmental consequences

Beyond these serious failures, deteriorating sewer systems can also lead to infiltration from groundwater. This can lead to a number of problems including:

• Overloading the hydraulic capacity of sewers, leading to surcharging, flooding and the premature operation of CSOs
• Increased costs of treatment, as more effluent is delivered to the sewerage works meaning a need for increased capacity, more wear on pumps, increased energy costs and the increased risk of spills from emergency overflows
• Treatment problems at sewerage works, where ground water can dilute the effluent and also in coastal areas where sewers in poor condition laid in ground that is subject to tidal infiltration can lead to seawater having a negative bacteriological effect

Inspection by CCTV is the most widely used investigative tool. The images taken are recorded with the findings being analysed and reported. In order to manage risk, investigation into the condition of drainage assets has historically been limited to those on the critical list, with the most serious defects being prioritised for repair. Non-critical sewers have been dealt with on a more reactive basis.

There are some exceptions to this, however. Severn Trent Water, for example, is currently conducting extensive surveys of its non-critical sewers.

Structural rehabilitation includes repair, renovation and renewal of sewer systems. Rehabilitation can be considered in two broad categories – stabilisation and lining.

Stabilisation can be achieved by sealing joints and openings. It is important to note, though, that it is only appropriate if the original structure is sound. Systems in use include grouting and pressure pointing.

A new product introduced into the UK, which is being offered by Ferro Monk Systems, is called Ultracoat – a 100% solids epoxy sprayed lining system. The material is resistant to a range of chemicals, as well as to algae and bacterial growth, which, the company says, gives greater protection to structures that are under attack from these agents.

Ultracoat is suitable for rehabilitating wastewater systems and, according to the company, it is also an environmentally friendly product.

Wet chambers and surfaces are notoriously difficult to deal with, but Ultracoat can be used in damp environments.

The solution was recently used on two projects for Anglian Water, where a number of concrete chambers at pumping stations were suffering severe hydrogen sulphide attack.

Lining can be considered in terms of being either a type-one or type-two system.

A type-one lining is where the lining and the existing sewer combine to act as a rigid composite section. To do this, a bond is required between the lining and the existing sewer.

The Sewerage Rehabilitation Manual recommends that type-one techniques should only be used in man-entry sewers, where a high degree of confidence in the achievement of bond with the existing structure can be achieved. Examples of the materials used in type-one lining are:

• Ferrocement
• Gunite
• Glass-reinforced plastic
• Glass-reinforced cement

Some of these techniques are based upon the use of pre-cast components that then rely on a grout around the annulus to transfer load between the new lining and the existing structure. This can have a significant disadvantage in sections of sewer where the existing structure varies in cross-sectional area, meanders or both, not uncommon in older brick or stone built culverts.

According to Ferro Monk, techniques such as its Insitu Ferrocement lining solution can offer a number of significant advantages.

Firstly, the system is designed around a pre-cast invert that is handled and grouted into position in order that it can normally accommodate dry weather flows. This can, in many cases, negate the need for expensive over-pumping installations.

The rest of the installation is carried out in situ, with layers of mesh and sprayed Ferrocement being applied and finished to the required thickness.

Flexible system

Rob Whale, chief quantity surveyor at Ferro Monk Systems, says: “It’s a flexible system, which can be used in many complex situations. Recent examples have included the structural refurbishment of over 260m of Burlais Brook Culvert in Swansea.”

This was a brownfield site, which had been identified for future housing development. But the presence of the culvert directly below led to considerations of the future loadings imposed by the new buildings, roads and traffic.

The original structure comprises a brick arch, masonry wall, with cobbled invert construction, ranging in size from 1.83m to 2.37m equivalent diameter, and is located at depths between 6m and 11m.

These variations and complexity of the project meant that five different designs were required for the new lining throughout its length.

The width of the culvert provided the opportunity to use a double invert that gave the advantage of allowing any flows to be diverted to either channel as required during construction. Completed earlier this year, the scheme followed works carried out by the company 12 years previously further upstream.

Clarke Bonds’ regional director, David Bodycombe, says: “When we started to look at the feasibility of developing the former tinplate works, it was reassuring to find that the company responsible for the phase one remedial works and also the same senior personnel involved at that time, were available to help us with the initial technical appraisal and costing.”

Cured resins

Type-two linings differ in that the lining is designed into a structural pipe within its own right, and does not require a bond with the existing sewer. Full length linings are commonly installed using one of the cured-in-place methods.

Originally developed in the early 1970s, this no-dig, trenchless technology became more widely available following the expiry of the original patents and the market began to open up. There are now several systems on the market, the main types being based around a resin impregnated felt sock. The resins become cured either through exposure to increased temperature (using hot water or steam), or ultraviolet light.

Installation can be through either inversion, which is by effectively unfolding the impregnated tube through the asset to be rehabilitated by the introduction of water or air, or by dragging the liner through the sewer and inflating it with compressed air or a calibration hose.

Where repairs are needed over only short lengths of pipe, isolated patches may offer a more cost-effective solution.

Many systems are based upon the use of an epoxy impregnated medium, such as glass fibre matting, which is wrapped around an inflatable packer. The system is introduced into the sewer, positioned at the point of repair (allowing sufficient overlap into the defect free host) and the packer inflated, pushing the patch against the pipe.

Following cure, the packer is deflated and recovered for reuse.

Whale adds: “With a myriad of solutions on offer, finding the right one can be challenging. One way to ease the situation is to call upon the expertise of a company who can offer a range of products and so is better placed to be able to give appropriate advice on the most cost-effective options. Ferro Monk Systems has been carrying out sewer rehabilitation works for nearly 30 years. As well as the systems highlighted above, they also carry out extensive cured-in-place and isolated patch lining as well as man-entry and CCTV surveys.”

Rob Whale is chief quantity surveyor at Ferro Monk Systems. T: 0113 2877577