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Trenchless Technologies Conventional open trench methods, while effective, can be costly and disruptive in areas where significant infrastructure already exists, such as buildings and roads. In many cases, trenchless technology systems can provide an innovative, cost-effective alternative. Trenchless technology systems are often used for the installation, replacement and renovation/rehabilitation of underground pipes where personnel entry is not possible. This is usually the case for pipes up to 900mm diameter. Before the type of trenchless technology is chosen, a site visit is made to determine the soil and groundwater conditions surrounding pipes and ducts. Following this, an inspection is conducted to ascertain the present condition of pipes. Finally, a location survey is performed to identify any surrounding pipes or services that may inhibit work in the immediate area. New Installations Horizontal Directional Drilling Horizontal directional drilling is a trenchless surface-launched method for installing relatively small diameter pipes. This method can be used to install various types of pipe, including polyethylene, polyvinyl chloride, steel and copper. Applications include water, gas and cable installations, usually in soft ground. Gravel-heavy soils and rock can present difficulties when using this technique. Microtunneling Microtunneling is used when constructing pipelines where close tolerance for line and grade is required. This method involves the use of a remote-controlled, laser-guided pipe-jacking system, for which personnel entry is not required. The process can be used for a range of pipeline diameters. Microtunneling can be cost-effective compared to open-cut construction when pipelines are to be installed in congested urban or environmentally sensitive areas, in unstable ground, or below the water table. Microtunneling has been used to install concrete, steel, centrifugally spun-cast polyester resin, fibreglass-reinforced and vitrified clay pipes. PVC pipe has also been installed using this technique. Replacement Pipe Bursting Pipe bursting is a replacement method commonly used in urban centres where the area immediately below the paved surface is often congested with existing services, allowing little space for replacement of a defective service with a new line. The existing pipe/hole in the ground thus becomes the guide for replacement. Replacement occurs by splitting the defective pipe and displacing the fragments to enable a new pipeline of the same diameter, usually polyethylene, to be drawn in. Where increased capacity is needed, an expanding device, either hydraulic or pneumatic, is introduced into the defective pipeline to shatter the pipe; then the new line is pulled through. Pipe bursting is not appropriate in all subsurface conditions. The forces exerted by the bursting head may adversely affect other pipelines near the pipeline being rehabilitated. Rehabilitation Cured-In-Place Pipe Cured-in-place pipe systems enable pipelines to be repaired from within by inserting a lining material through existing manholes or other entry points. The liner is composed of a fabric reconstruction tube which is impregnated with a thermosetting resin. After insertion, the liner is exposed to hot circulating water or steam and hardens into a structurally sound, jointless pipe. The rehabilitation liner not only serves to repair the deteriorated structure of the existing pipe (sometimes referred to as the host pipe or casing), but reduces inflow, infiltration and leakage, and usually improves the flow characteristics of the system. Installation methods for cured-in-place pipe systems are as varied as the types of materials used. In some cases, the tube is inverted through the pipe using water or air pressure; in other cases, a winch is used to pull the tube through the pipe. The curing process usually requires hot water or steam to be applied to the liner for several hours after it has been formed within the host pipe. Fold-and-Formed Pipe More recently, some companies have introduced systems that can install folded plastic pipe relining products through a manhole entry. These systems, referred to as fold-and-formed pipe systems, utilize thermoplastic materials that have been folded from a circular shape to produce a smaller net cross-section that can be fed into an existing pipeline. These fold-and-formed pipe products are usually extruded PVC or high density polyethylene (HDPE) pipe that is flattened and folded longitudinally. The plastic pipe is fed from a spool into an existing pipe, then hot water or steam is applied until the liner reaches a temperature elevated enough for rounding. After rounding, the installed liner is allowed to cool, producing a snug-fitting liner in the host pipe. Sliplining Sliplining is another rehabilitation method where a new pipeline of a smaller diameter is inserted into the defective pipe and the annulus (the space between the existing and inserted pipes) is grouted. It has the benefit of simplicity and is relatively inexpensive; however, there can be a significant loss of hydraulic capacity. A modified version of sliplining, often called close-fit lining, makes use of the properties of polyethylene or PVC to allow temporary reduction in diameter or change in shape prior to insertion in the defective pipe. The inserted pipe is subsequently expanded to form a tight fit against the wall of the original pipe, thus avoiding the need for annulus grouting as in conventional sliplining. Temporary reduction in diameter is achieved either by mechanical rolling or drawing through a reduction die. IETC is developing a user-friendly guidebook on trenchless technologies for developing countries. This will be done in partnership with the International Society for trenchless technology (ISTT) in London and the Trenchless Technology Center (TTC) in Louisiana. A workshop on trenchless technologies will take place in February, 2001 in Cairo, Egypt as part of this collaborative initiative. |
