Infiltration Reduction

What is infiltration?

Infiltration can be defined as water which is present in the ground surrounding a sewer and which enters the sewer system via openings such as displaced or open pipe joints, fractures and breaks in the fabric of the main sewer, lateral connections and manhole chambers. There are three basic types of infiltration:

· groundwater infiltration, which will only occur when the water table is above the sewer invert level and may be seasonally influenced;

· rainfall induced infiltration, which is caused by temporarily raising the groundwater levels by seepage of rainfall through the subsoil of permeable areas during periods of wet weather. This can significantly increase the potential for infiltration and may last for a period of between several days and a number of weeks, depending upon the groundwater levels and the extent/permeability of the catchment subsoil, and;

· tidal infiltration, which can occur where sewers are laid in tidally saturated ground e.g. coastal lowland, made up ground, etc. The pattern of this type of infiltration will coincide with tidal cycles and will be most obvious during spring tides.


Potential effects of infiltration

Infiltration can be a serious problem and, if not controlled/reduced, can result in a range of problems, including:

· insufficient hydraulic capacity in sewers, leading to increased risk of frequency of surcharging/flooding and/or premature operation of combined sewer overflow (which may then be in breach of consent);

· higher than anticipated flows at pumping stations, leading to excessive pumping, wear on pumps, increased pumping energy costs or spills from emergency overflows;

· capacity problems at sewage treatment works, potentially limiting future growth and development;

· treatment problems at sewage treatment works due to dilute effluent (or seawater in the case of tidally induced infiltration), and;

· increased rate of structural deterioration due to the washout of pipe bedding and backfill.


How to reduce infiltration

Installing a lining within a defective sewer can potentially reduce infiltration, though this is not guaranteed. The most commonly used renovation technique within the UK is lining with cured-in-place pipes (CIPP) which can be used for full lengths (i.e. manhole-to-manhole) or localised patch repairs (e.g. typically 600mm to 1m long).

Not all CIPP linings reduce infiltration because certain lining materials can shrink during and/or after installation (e.g. loss of solvents during curing or thermal shrinkage after curing). This results in an annulus between the lining and host pipe through which infiltration has pass and hence enter the sewer system at locations such as manholes or lateral connections.


Infiltration reduction research

WRc has since 1995 conducted a suite of collaborative research projects sponsored by a number of UK Water and Sewerage Companies and Manufacturers, Suppliers and Contractors offering renovation products.

Initial research focused upon localised sewer repairs such as cured-in-place repairs (i.e. patch repairs) with and without re-rounding of the host sewer. Subsequently WRc has worked on joint sealing and flood grouting systems and is currently researching a ‘top-hat’ product which repairs the connection between a sewer and lateral.

The suite of projects have resulted in a number of Specifications which include short term (i.e. 30 minute duration) and long term (i.e. six month duration) hydrostatic tests which measure the ability of a product to minimise infiltration.

Also, in 2004 Don Ridgers of Thames Water started to develop an infiltration test to quantify the performance of full length CIPP linings to resist infiltration with input from Industry sewerage engineers and practitioners. WRc have recently been undertaking a collaborative research project, sponsored by four UK Water and Sewerage Companies, to publish this test procedure, witness infiltration tests within the UK and mainland Europe.


Infiltration Testing Website

This website presents a comprehensive overview of the available specifications, test procedures and test results for a range of renovation techniques, including:

  • Full length cured-in-place pipe (CIPP) liners.

  • Patch repairs.

  • Flood grouting systems.

  • Joint sealing repairs.

  • ‘Top-hat’ repairs.