Sewer failure, and successful remedies
Richard Swan, head of materials technology with WRc¹s networks group, looks at what can go wrong down in the sewers, and what can be done to put it right.
Types of failure
Sewer deterioration begins with initial defects such as overloading of the sewer pipe, which can result in the sewer cracking. This classic failure features cracks at the crown, springings and invert. Unfortunately, it is very hard to see the crack in the pipe invert because it is normally under water. The cracks at the springings are also obscured because at this point they are in compression.
Ultimately, the initial failure results in the soil around the pipe being eroded by the infiltration of ground water and exfiltration of the sewage. This process gradually washes away the fines in the soil, and over time soil support is reduced. When this happens, the pipe starts to deform and eventually the sewer collapses.
Leaking joints can also lead to sewer failure. In this case an elastomeric seal could either fail or the angular deflection could be greater than the joint can withstand. Alternatively, the mortar in a brick sewer can weaken over time, resulting in the infiltration of groundwater. This leads to the erosion or loss of ground support and eventually the collapse of the sewer.
In addition, displaced joints can also cause sewer failure, as can a hole or defect in the sewer. This type of failure is usually caused by the use of rods to clean the sewer or a third party carrying out an excavation close to the sewer.
Blockage of the sewer may be caused by the ingress of tree roots or vegetation. Another type of failure is an intruding lateral connection, which can easily cause the sewer to block. Finally, failure can be caused by loss of mortar, leading to displaced or hanging brickwork but in some instances damage may also be caused by animals, such as rats.
The water industry’s greatest asset is the pipe in the ground, and as a large percentage of the cost of replacing sewers is associated with the excavation and backfilling of trenches, renovation techniques typically operate from existing manholes, allowing cost savings.
Of all sewer failures, the most significant and costly are the failure of critical sewers. Critical sewers are those which, if allowed to fail, will either be very costly to replace or will cause major pollution or inconvenience. The worst case scenario is a large diameter, very deep, brick sewer located in a busy city centre.
There are generally four methods of detecting sewer failures, all of which are common throughout Europe. One is the use of closed circuit television (CCTV), another is the use of a leakage test which is applied to manhole lengths using an air test. In smaller diameter sewers, it is possible to carry out a water test. There is also the option of carrying out a localised test of the sewer’s individual pipe joints.
In some instances the chance arises to test the joint and if failure occurs, resin can be injected into the failed joint. The resin is allowed to cure and the joint re-tested. The test unit then moves on to the next joint.
The AMS-4 Survey Tool is a system currently widespread in Germany. The system works by pulling a sonde, emitting an electric field, from manhole to manhole through a length of fully-charged sewer pipe. It will detect leak points such as holes, fractures or bad connections. At the same time the sonde monitors the temperature of the sewage. It is the changes to the temperature of the sewage which enables the operator to distinguish between infiltration and exfiltration at the detected leak points. For the system to work success-fully, the sewer has to be full of water or sewage. Thames Water has developed the Laser Profiler which can pick up cracks, deformed sewers and debris build up using a laser to scan the pipe. Finally, for the larger sewers, the easiest way to locate failures is by walking through the sewer.
In the early 1980s, prior to privatisation, the responsibility to maintain and repair the sewer network came under the control of approximately 800 councils or principalities. In order to achieve some consistency in the technical selection, design and installation of renovation techniques, the WRc Sewerage Rehabilitation Manual was produced. The manual followed the publication of WRc technical report TR87, dated 1978, and TR87A from1981.
As part of the work, detailed studies were carried out on the different renovation systems available at the time, and it was these systems which came to be viewed as the ‘established’ approach to sewer management.
To overcome the misunderstanding by engineers of what was an ‘established’ sewer renovation system, the second edition of the Sewerage Rehabilitation Manual, was published in 1986 to list the ‘established’ systems.
These systems were deemed acceptable following detailed technical appraisals and case studies. The techniques were categorised into three main areas: stabilisation, lining and in-situ coatings.
Stabilisation techniques were sub-divided into two main areas; pointing and chemical grouting. Pointing is used to stabilise man-entry brick sewers only, and includes hand pointing and pressure pointing.
The advantage of hand pointing is that it causes minimal disruption as the mortar is carried to the point of application manually and trowelled into place. On the downside, hand pointing can be time consuming and labour intensive.
Pressure pointing is a faster process than hand pointing and is often used when longer stretches of sewer require attention. ‘Pressure’ applies to the method of mortar delivery and not the application of the mortar.
The only established method for grouting is the technique known as Seerseal which can be used in both non man-entry and man-entry sewers. The Seerseal system consists of a hollow cylindrical packer with inflatable collars at either end. This is inserted into the sewer at the required position. The collars are then inflated and the joint tested under pressure. If the joint fails the test a chemical grout is pumped through the joint and curing time achieved within a few minutes.
Lining techniques can be sub- divided into two main areas: pipe linings for non man-entry and man-entry sewers, and segmental linings; pipes with longitudinal joints for man-entry sewers. Techniques for pipe linings include glass reinforced plastic pipes, polyolefins (polyethylene, polypropylene, PVCu, etc) and cured-in-place lining. For segmental linings the techniques are glass reinforced cement, glass reinforced plastic, pre-cast gunite and finally polyester resin concrete.
In-situ coatings for man-entry sewers include the hand-sprayed application of gunite, but the most popular techniques are cured-in-place pipes, pipe bursting, slip-lining (with PE), chemical stabilisation Sanipor for example and local repairs. The latter includes patch repair and joint sealing.
In the case of chemical stabilisation, the section of sewer to be stabilised is isolated and cleaned. A solution is introduced into the sewer and allowed to exfiltrate. The amount of liquid exfiltrating is recorded and the remaining liquid quickly pumped out. A second liquid is pumped into the sewer. This exfiltrates and begins to react with the first solution following which it too is pumped out. The reaction between the solutions forms a silicate which seals defective areas. An advantage of this method is that it treats the manholes and branch pipes.
There are also re-rounding systems. A stainless steel or PVC clip is located on a hydraulic packer. The packer is winched into position and slowly expanded until the sewer is returned to its original diameter and the re-rounding clip locks into position. With the clip installed a cured-in-place patch or manhole- to-manhole lining is applied over the clip.
With local patch repair, a patch of either one-and-a-half or three metres long and consisting of a combination of GRP and polyester felt is impregnated with resin and attached to a flexible packer. The packer is inserted into the sewer, located at the point of failure, expanded and then cured-in-place, usually using hot water. Finally the packer is deflated and removed leaving the patch repair behind.
Lastly there are robotic repairs. These usually require an expensive robot capable of carrying out a variety of repairs. They include cutting back lateral connections, removing tree roots, injecting resin into cracks or defective connections and drilling out cracks and then filling with epoxy resin. The robot usually comprises a propulsion unit, CCTV camera and multi-tool cutting head.
The WRc Approved Scheme was set up in 1996 as an independent technical review of a product or service. Certificates have been awarded in many areas, including the approval for coatings applied to fittings and valves, large diameter plastic drainage pipes, flexible couplings, cured-in-place pipe renovation systems, epoxy linings and in-situ factory application rigs.
The WRc Approved Scheme is a formalisation of the technical product review carried out at WRc for many years. It was set up in response to requests from product manufacturers who wished to demonstrate their products had been validated independently.
The approval is carried out against an existing technical specification or code of practice, where they exist. Where a specification does not exist, WRc will develop an assessment schedule.
For example, the ICP Breathe cured-in-place lining system and Econoliner local lining repair system have recently been awarded WRc approval.
- Richard Swan is head of materials technology with WRc¹s networks group. His main areas of work are trenchless technology, local repair of sewers, cured-in-place pipe renovation, pipeline failures, glass reinforced plastics products and strain corrosion.
WRc’s areas of sewer management expertise include measurement of condition and performance for existing and newly-installed systems, as well as hydraulic modelling of sewer systems including environmental impact on receiving waters. WRc also covers strategic planning and catchment management, flow surveys and sewage quality surveys. WRc is also involved in water impact surveys.