Quality controls are key to successful CIPP lateral lining applications
Jan 02, 2006
Laterals are the underground sewer service pipelines that connect from a building, such as a private house or commercial property, to the main line sewer. Typically private house laterals are 4” to 6” in diameter. Commercial properties and multi-unit residential properties can have much larger sizes. Laterals typically run under landscaped areas, driveways or parking lots. The aim of CIPP lateral lining is to solve many problems that occur with deteriorating laterals instead of replacing them. While these problems vary from lateral to lateral, a successful CIPP lateral lining application or product needs to solve a variety of existing lateral problems that may be encountered from location to location. This may require different materials to suit different kinds of lateral situations.
Cured-in-Place Pipe (CIPP) uses a thermosetting plastic resin as its central material concept. Prior to thermoset, the CIPP liner is in a soft, pliable, state. The irreversible thermoset reaction converts the material into a hard rigid material. The lateral liner is installed into the lateral in its pre-thermoset state. The pliable and flexible state is well suited to lateral lining as it facilitates installation into and expansion within laterals, small pipes that often include inline bends and size changes. In its pre-thermoset soft pliable state, the liner can be made to conform to the varying shapes of the existing pipe before the thermoset action is initiated. This results in a liner that, when cured to hardness, should fit well to the variations in diameter, shape and turns encountered in typical laterals.
Successful means that the lateral liner is:
- Strong enough to resist buckling under ground water pressure.
- Strong enough to resist soil loads & traffic loads – where required
- Strong enough to resist the intrusion of tree roots.
- Leak proof against both infiltration and exfiltration.
- Smooth and impediment free inside to keep sewage flowing away.
- A tight fit to the inside of the existing lateral
- Free of fins, lumps and flow impediments especially at inline bends and size changes
As underground pipes, lateral liners are subject to external loads. Depending on the lateral’s condition, design loads can be only groundwater or may also include soil and traffic loads. ASTM F1216 provides a widely used method for design of CIPP liners for external loads. It should be verified in design that the lateral liner being installed provides sufficient resistance to the loads expected. The external loading situation is commonly based on the F1216 classifications of:
Fully Deteriorated Condition – All loads from groundwater, soil, traffic and other sources.
|Parameters for |
|For Partially |
|For Fully |
|Size of lateral||X||X|
|Water table height||X||X|
|Ovality of lateral||X||X|
|Liner felxural modulus long-term||X||X|
|Liner flexural strength long-term||X||X|
|Liner poisson's ratio||X||X|
|Height of soil cover||X|
- The lateral condition is correctly identified as Partially or Fully Deteriorated.
- The design parameters are correctly determined for both the lateral and the liner.
- Required liner thickness is checked by an accepted design calculation, such as F1216.
Reinforced liners tend to have much higher strength and modulus than non-reinforced. However the design relationship between strength and thickness is not linear and twice the strength does not translate into half the thickness. Quality control of liner thickness requires starting with an engineered thickness design for external loads.
Load design thickness is required in place after installation. Installation factors contribute to loss of thickness. Infiltration can wash away some resin before cure. Pulled-in liners may encounter scrape away or loss of resin quantity. Installation bladder, inversion or cure pressures can compress down the thickness. Stretching at bends and size changes can produce localized thinning. Quality control must take into account these factors and not base liner thickness selection solely on load design. Quality control must make sure that the liner supplier has included installation compensation in its materials and recommendations so that in place thickness meets requirements throughout the installation.
Proper sizing of the lateral liner impacts not only fit and finish but also structural performance. Sizing involves the carrier material circumference and thickness. Sizing of carriers is complex and requires a detailed understanding of carrier material behavior during the installation. The carrier materials have stretch and compression characteristics that must be taken into account when fabricating the carrier. If the carrier is to small, it may stretch too much, thinning out the wall. If it does not stretch enough annular gaps may occur behind the liner.
- The structural capacity is degraded by increasing annulus space. Loose fitting liners must be thicker/stronger than tight fitting liners.
- Channeling of ground water through annulus space increases when liner is not tight fitting. Tighter fit gives less annulus flow meaning less final infiltration at exit point.
- Increasing annulus gap allows more space for tree routs to penetrate behind the liner in search of moisture.
The carrier is the tube, sleeve or other configuration that is used to transport the resin into the lateral. Some carriers contain reinforcement that is integral to liner strength properties. The carrier controls the finished thickness of the liner by controlling the amount of resin that can be carried and by the thickness of its other elements including fibers and reinforcement. To obtain reliable and repeatable cured-in-place liner thickness, the carrier must provide reliable resin adsorption and material thickness. This requires quality controlling the carrier manufacturing process to provide uniform properties.
Laterals typically have inline bends and size changes typically not encountered in main line sewers. Most importantly lateral liner carrier properties must address the need to line through inline bends and size changes while maintaining fit, finish and wall thickness. A carrier material suitable for straight runs may provide unsatisfactory results for lateral lining. Quality control must match the liner carrier to the lining requirements. Typically a lateral liner system will require more than one type of carrier tube due to different lateral geometries. Quality control steps must be in place to make sure the correct carrier tube product for the lateral has been selected. Otherwise quality control will be required for open cut replacement of the liner. Liner suppliers and manufacturers should be able to provide technical guidance to installers regarding the correct carrier tube for the installation.
Good quality control requires knowing which carrier material to use for different lateral situations
Liner installation results in internal pressure on the carrier material. This compresses the carrier material reducing the CIPP wall thickness. Initial carrier thickness must take into account this compression otherwise in place liner thickness will be less than required.
Initial carrier thickness must also take into account how much the carrier is undersized versus the lateral inside diameter. This is a complex consideration requiring considerable knowledge of the how the carrier material behaves during stretch out to size under installation pressures whether from an expanding packer, membrane or inversion head. As the carrier stretches in circumference to fit the lateral, the wall thickness will reduce. First the initial thickness must be sufficient to compensate for the reduction under stretch out to fit. Second the carrier material must stretch evenly around the circumference to prevent excessive localized wall thinning will result.
Resin quantity is key to determining cured-in-place physical properties and thickness. An accurate method is required to determine the volume of resin needed. Too little resin will result on liners that are too thin and may result in liner wall voids. Inadequate resin reduces cured physical properties substantially resulting in a liner with insufficient structural strength. Resin voids are also sources for infiltration leaks through the liner.
|A||Resin volume required is based on initial carrier thickness, not minimum design thickness. I.E. If a 6 mm liner tube is used for a 4.8 mm design requirement then the resin volume must be based on the 6 mm not the 4.8 mm.|
|B||A resin calculation must be available to determine proper resin quantity. Alternately, tube suppliers should provide a verified table of resin quantity by size, nominal carrier thickness and unit length based on an engineered calculation.|
|C||Has ASTM F1216 excess resin requirement been included? How was it calculated?|
|D||At wet out, has the amount of resin impregnated into the carrier been confirmed by measurement?|
|E||Using look and feel of wet-out does not provide requisite quality control.|
|F||Is the process for determining resin quantity for a reinforced liner different than for a nonreinforced liner? If so, what is the method used to verify the required resin quantity?|
The carrier material must be completely impregnated with resin, meaning that no void spaces (air spaces) remain in the carrier material after impregnation (wet-out). Before resin impregnation the carrier material contains air space to be replaced by resin. The goal of impregnation is to fill all the air space with resin, expelling all the air. If all the air is not exchanged for resin, the cured liner will have voids, air pockets. These air pocket voids degrade the liner physical properties and can allow leakage through the cured liner. Quality control procedures include vacuum assisted impregnation and resin distribution rollers.
Good quality curing to achieve required physical properties is essential for a successful lateral liner application. A reliable cure method and its quality control are essential because obtaining cured samples for testing is difficult for lateral liners. Curing methods that provide higher reliability, such as heat cure, are preferred when sampling is difficult or infrequent as with laterals.
|Heat cure||Ambient cure|
|1||Catalyst quantities rarely need changing.||1||Catalyst quantities dependent on ambient temperature conditions. Seasonal changes.|
|2||Higher tolerance to incorrect catalyst quantities without effecting working time or physical properties.||2||Lower tolerance to incorrect catalyst quantities without effecting working time or cured physical properties.|
|3||Can vary cook times and temperatures to compensate for local pipe conditions to assure cure properties.||3||Cannot compensate for local pipe conditions to assure cure properties.|
|4||Better physical properties attained with less QC effort than with ambient cure.||4||Requires higher QC effort to attain physical properties than with heat cure.|
|5||In general heat cure has best probability of meeting required physical properties||5||In general ambient cure has lower probability of meeting required physical properties.|
Quality control in design requires input of verified, reliable liner properties of flexural modulus and strength. Periodic testing of short-term values is required to verify design inputs. Field samples from lateral liners should be tested for flexural strength and flexural modulus according to ASTM D790 and for thickness. The D790 values of strength and modulus to be used as design inputs should be the values that are reliably obtained over several tests, not the maximum values obtained from any one test. An ongoing quality control testing program is required.
A successful CIPP lateral lining application requires quality control over design, sizing, carrier material, resin quantity/impregnation, cure and sample testing.
- Design quality controls should start with using an accepted approach with reliable procedures in place for correctly identifying the design inputs for field and liner parameters
- Carrier sizing quality control starts with understanding carrier material behavior then matching it to lateral geometry and structural design requirements.
- Carrier material quality control starts with knowing field behavior of the material, controlling the properties and matching them to lateral requirements.
- Resin quantity/impregnation quality control starts with a rational basis for determining required resin volume, verifying volume used and employing methods to ensure thorough void free wetout.
- Cure quality control starts with understanding the strengths and weaknesses of different methods and understanding the importance of obtaining design physical properties.
- Sample testing quality control starts with realizing how the liner physical properties relate to liner quality, understanding that laboratory sample results are usually optimistic and instituting a regular ongoing testing program to verify both liner quality and design inputs.
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