Evaluation in trenchless rehabilitation basically means assessing an asset, i.e., our underground water and sewer pipeline system in terms of its existing condition, remaining service life and system capacity. Traditionally a damaged or deteriorating pipe is dug up and replaced with a new one, often resulting in damage to adjacent pipes in good condition and to other utility lines. This leads to an escalation in cost of repair, excluding the indirect and social inconvenience costs involved in the trenching style of rehabilitation.
Thanks to trenchless technology, it is possible to use trenchless nondestructive inspection methods to determine the evaluation factors mentioned above. This helps to isolate pipes that need repair, after which appropriate trenchless methods can be selected to rehabilitate or replace the damaged pipe – all this without trenching or damaging nearby utility pipelines.
To evaluate a pipe, the first step is to conduct a thorough video inspection using a closed-circuit television (CCTV) camera to determine the structural and physical integrity of the pipe. (To learn more, check out Using CCTV to Inspect Pipes.)
A structurally unsound pipe is better replaced than rehabilitated. Replacement can be done using trenchless methods, but sometimes selective trenching may have to be done, especially when slope is affected. For structural inspection the pipes must be cleaned thoroughly of all debris and buildup using water jetting or other appropriate cleaning methods. Debris and buildup present in the pipe can prevent the camera from navigating properly and the inspection may result in incorrect assessment.
The information to be obtained from a video inspection includes the pipe material, pipe size, severity of defect in terms of cracks, corrosion, collapsed or broken pipe walls, offset joints, tree root intrusion and projecting lateral connections. Structural assessment should also be done for manholes to check for damage from chemical attack, broken bricks, cracked concrete and damaged inverts.
Inflow/Infiltration (I/I) Analysis
Inflow and infiltration contribute to an increase in loading that the pipe is not designed for. Inflow is contributed by improper connection to the mainline from sump pumps and rainwater downspouts, while infiltration is caused by leakage due to cracks in pipes, improper pipe joints and from manholes. I/I leads to pipe damage and also increases the cost at treatment plants and reduces its effective capacity.
I/I is measured by comparing readings at dry weather flows and flows during periods of high groundwater or rainfall. Increased reading during the periods of rainfall and groundwater indicate I/I is occurring. Rehabilitation will rectify the infiltration problem, but to determine cause of inflow, further investigation using methods such as smoke testing are used.
Capacity analysis is carried out to determine if the pipe is carrying loads within the designed limits. Depending on the size and slope of the pipe, the current capacity can be determined. If population growth is expected in the future, the computed capacity is compared against future expected sewer flows. Future flows are measured by monitoring flows at different times to determine maximum and minimum flows and also during dry and rainy weather conditions.
Once the monitoring process is complete, the projected future flow can be calculated by including expected population growth and increase in commercial and industrial usage into the current measured flows. The rehabilitation method utilized should take projected capacity into consideration so that the rehabilitated pipe is able to serve for its estimated life span. Some rehabilitated pipes may have reduced size depending on what method of rehabilitation has been used, and that should be taken into account.
Selection of Rehabilitation Method
Once the pipeline has been evaluated, it becomes easier for contractors to determine which method would be the most appropriate and cost effective. If the capacity analysis combined with structural analysis shows that the pipe does not need to be removed and replaced, a rehabilitation technique most suitable for the pipe can be selected. Following are a few trenchless techniques that can be used to rehabilitate pipes for extending their serviceable life based on the severity of damage. (To learn more about different methods, see 7 Types of Trenchless Rehabilitation Methods and How They Are Used.)
Mechanical Spot Repair
Leaking pipe joints or pipes subjected to damage by vibration and shifting can be repaired using this method. A grouting sleeve which has a mechanical locking system and a core of stainless steel is prepared. The core is surrounded by limiting straps and a foam gasket impregnated with grout. This is inserted into the host pipe and is guided to the damaged area using a CCTV camera. The sleeve is expanded using air until the locks engage, after which the air plug is deflated and withdrawn. The grout expands until the annular spaces and cracks are filled.
Chemical grouting is a nonstructural method of repair that stops leaks from structurally sound joints and from cracks in pipe walls. It can also be used to reduce soil loss outside the pipe. Grouting is carried out by injecting a self-setting grout into the leaking joints or cracks. As the grout is injected, it travels outside the joints, filling all voids present in the soil, creating a grout seal around the damaged portion.
However, care should be taken to conduct the grouting in compatible soil such as sand or gravel. Grout may not bond well with clayey soil and is not suitable for large soil voids behind leaking joints.
Cured-in-Place Pipe (CIPP)
Cured-in-place pipe (CIPP) can be used for pipes with moderate structural damage, as the lining used in this method is very strong. The liner is made of fiber reinforced fabric or non-woven polyester and is designed to withstand soil, ground water, and surface pressure, and to fit into the host pipe on expansion. The target pipe is first cleaned of debris and residues.
The liner, which is impregnated with a resin, is then inverted within the pipe by applying air or water pressure. As the liner inflates with water or compressed air, it forms a close fit within the host pipe. Heat is applied to accelerate the curing process. The liner is sealed flush at the pipe ends and the lateral connections are restored.
Pipe bursting replaces the old pipe without actually having to remove it. The process fragments the old pipe using a conical bursting head pipe while pulling in and installing a new pipe behind it. This method is used to replace old pipes that can no longer be repaired by methods like CIPP and spot repair. Pipe bursting does not reduce the pipe capacity; in fact, it can be used to install a pipe of slightly larger diameter than the host pipe.