Introduction
While not generally termed “trenchless technology”, manhole rehabilitation methods use minimal excavation, if any, and minimise the surface disturbance, disruption of street activities and work in confined spaces.
The technologies for rehabilitating manholes are used to renovate deteriorating manholes before they require complete replacement. Trenchless manhole rehabilitation does not normally require excavation, however if needed, it will be considerably less excavation and surface disturbance than for a manhole replacement. Some of the rehabilitation methods can be undertaken using access from the surface through the manhole opening with or without the cover slab, cover and frame in place. Generally, the rehabilitation of a manhole can be completed in a day with minimal interruption of the pipeline service operation.
Timeous manhole rehabilitation improves deteriorated manholes to restore structural integrity, functionality and watertightness. They can replace corroded material, provide corrosion protection, stop water infiltration/ingress and/or provide additional strength. The various methods generally involve lining, grouting or spraying techniques. Whenever a pipeline is rehabilitated, it is advisable to perform any necessary manhole rehabilitation at the same time.
Conventional manhole rehabilitation can be described as manually cleaning the internal surfaces, repairing defects, repairing or re-doing mortar joints, and perhaps applying plaster or a coating to the walls, all performed by workers from inside the manhole chamber. Manhole rehabilitations described here typically include operations that are performed with equipment by workers on the surface or with minimal manhole access.
Manholes are usually square or round in plan, with various depths from shallow to deep and typically constructed in brickwork or precast concrete rings, but can be GRP pipe-segments or HDPE.
Some of these manhole rehabilitation methods can be designed to be fully structural and sustain external groundwater, soil and traffic loads.
The design life of some of these manhole rehabilitation methods (for example Cured-in-Place Manhole (CIPM) Liner, Inserted Segmental Liners and Cast-in-Place Concrete Lining), is normally 50 years and when properly designed, manufactured and installed also provide a service life equal to that of a new manhole. The other methods are particularly dependant on the appropriate preparation of the existing surfaces, in addition to being properly designed and installed.
Preparatory Work
Preparatory work might include:
- Removing the cover and frame completely
- Removing any access chimneys
- Removing any reducer/cover slabs or cones
- Cleaning the internal surfaces
- Repairing individual defects
- Replacing step irons or ladders
- Arranging overpumping or bypassing the pipe flows
- Using Lidar to measure the internal dimensions of the manhole
Surface preparation plays a critical role in achieving maximum adhesion of coatings to the substrate.
Usually the bottom of the manhole (benching) is rehabilitated before the shaft. This is done manually and once prepared, a coating or lining similar to the main method is applied.
It is also particularly important to ensure that the joints between the main pipes and the manhole walls are sound and do not leak. Chemical injection grouting can assist in sealing these joints.
Cured-in-Place Manhole (CIPM) Liner
This method is mainly designed for circular manholes. The liner is custom built or produced as a “cut and use” to the manhole dimensions. It is prefabricated with layers of waterproof membranes on the inside and the outside, between which are a synthetic felt material layer and/or a fibreglass layer, which are impregnated with resin.
The prepared liner is lowered by winch into the prepared manhole chamber. A manifold is attached to the open end and the liner is inflated to ensure a tight fit to the manhole walls. Depending on the resin, it is cured ambiently, by steam or with UV lights.
Once cured, the top of the liner is trimmed to suit the cover and penetrations made in the bottom of the liner to allow the pipe flows. Often the benching is lined or rehabilitated in advance of the liner installation.
Since the liner can shrink a small percentage, rubber seals can be installed around the bottom of the liner during installation, to prevent ground water infiltration.
A variant of this method uses a flexible liner tube which can handle various diameters, and is cut to length. Once inflated, the liner stretches to fit a declared range of manhole sizes.
Inserted Segmental Liners
This method is mostly used for circular manholes. A customised internal manhole is manufactured in segments to fit within the existing manhole. The interlocking segments are typically made of fibreglass or polymer concrete but can also be of PVC or HDPE. The precast segments are lowered into the manhole chamber and assembled inside. The segments are joined together with a sealing/ bonding compound. A reducing slab or cone is placed on top of the shaft. Once completed the void between the segments and the existing manhole is filled, usually with a cementitious grout. The new chamber is usually quite a tight fit inside the existing manhole.
Spray-on Coating
Manhole coatings can be Cementitious (eg Portland Cement, Calcium Aluminate, Geopolymer, Microsilica) or Polymer (eg Epoxy, Urethane, Polyurea). Excepting Portland Cement, cementitious coatings typically have a relatively higher resistance to corrosion and Polymer Coatings have good to excellent chemical resistance.
Particular attention must be made to the cleaning requirements for the existing surfaces to ensure proper bonding of the coating.
Cementitious coatings are usually applied to the manhole internal walls, in layers to a thickness, for example, between 12 mm and 100 mm, with spraying equipment but can be trowelled or centrifugally spun cast. Immediately after application the cementitious coating is trowelled to densify and smooth out the surfaces. Polymer coatings are normally applied, to a thickness, for example, between 3 mm and 12 mm, with spraying equipment and are sometimes used as a top coating to a cementitious coating.
Sometimes a reinforcing mesh is first placed against the walls before spraying cementitious coatings. Like shotcrete, a high-pressure spray is required for good impregnation and compaction. Some cementitious coatings can be reinforced with fibres for added tensile strength and crack resistance.
Chemical Injection Grouting
Water leaks into manholes can be sealed using chemical injection grouting either as a standalone remedy or in preparation for other manhole rehabilitation methods. Water infiltration is typically through cracks, holes, construction joints and the joints between the main pipes and the manhole walls. It is considered the best, most cost-effective, long-term defence against infiltration of groundwater into structurally sound sewer systems.
Grouts can be acrylamide, acrylate, acrylic resins, polyurethane gels, polyurethane foams, or epoxy. The grout can be injected under pressure through the defect or through a hole drilled through the defect.
Polyurethane chemical grouts, depending on the type, react with water forming a gel or foam that either bonds with the structure to form a watertight, permanent seal or to become rigid, filling voids and stabilizing the soil.
Curtain Grouting is a method where grout is injected through a grid of holes drilled through the manhole walls into the surrounding soil encapsulating the manhole in an impermeable barrier.
Flood Grouting is a method where the manhole and pipeline are filled with the two parts of a silicaceous grout in a sequential flooding operation. The grout penetrates defects and solidifies in the surrounding soil sealing any leaks. This method is more fully described and illustrated in the Flood Grouting method in the ISTT Chart under Rehabilitation/Repair/Structural.
Cast-in-Place Concrete Lining
In this method ready mixed concrete is cast between the existing walls and customised formwork in the manhole, creating a new integrated, relatively thick wall, conforming closely to the inside shape of the manhole. This method can significantly reduce the available working space inside the manhole.
Once the internal walls of the manhole have been prepared, formwork is erected in the manhole to provide space for the new concrete wall, according to the requirements. Reinforcement may be specified. The bottom of the formwork must be adequately sealed against the benching and existing pipes.
High strength ready mixed concrete is poured into the formwork and compacted forming a monolithic wall inside the manhole. The concrete must cure properly before the formwork is dismantled and removed.
Corrosion protection is often provided by applying an polymer coating to the concrete surface, or by having a plastic (HDPE or PVC) liner affixed to the inside surface of the formwork. Such plastic liners are continuous sheets thermally welded together and typically have ribs or studs that anchor the liner into the concrete.
Panel Lining
In this method plastic (PVC, HDPE or Glass Fibre) sheets are fixed to the internal manhole walls. The plastic sheets are thermally welded together at the seams to fit the internal manhole structure.
PVC Liners
After the walls are cleaned a suitable primer is applied. The mastic or adhesive is then applied to the primed walls and the sheets affixed ensuring any liner anchoring studs are embedded.
HDPE and Glass Fibre Liners
After the walls are cleaned, a suitable bonding agent is applied to the walls. The panels are mechanically anchored in position to allow space for placement of the grout. Additional supports of the panels may be required.
Fibreglass-in-Place Lining
This method is where the fibreglass is hand crafted in place directly onto the existing manhole or benching to follow the shape and contour of the surface. After the surface is prepared, the resin is mixed with its hardener and a layer of resin is placed and firmly trowelled to even out any imperfections and ensure proper bonding with no gaps or voids. Glass or carbon fibre fabric mattings, coated with a chemical binder, are applied and layered into the resin, encapsulated it with additional applications of resin, using brushes or rollers. When completed, the liner is allowed to cure.
This method is well suited to benchings.