This article originally was published by the North American Society for Trenchless Technology (NASTT) whose annual conference and exposition, NO-DIG '97, is to be held in Seattle, Washington, from April 18 - 21.
The term Trenchless Technology refers to a family of methods, materials and equipment that can be used for new installation, or replacement or rehabilitation of existing underground piping infrastructure with minimal disruption to surface traffic, business and other activities, as opposed to open trenching and its associated major disturbance of surface activities.
There are several methods of trenchless construction used to install new pipeline infrastructure. The best known of these are mini-horizontal directional drilling and microtunneling.
Mini-Horizontal Directional Drilling
This is the name given to a trenchless surface-launched method for installing relatively small (2 to 10 in.) diameter pipes in lengths up to 600 ft or more, but typically less than 300 ft, at depths less than 15 ft. Types of pipes that can be installed include polyethylene (PE), polyvinyl chloride (PVC), steel and copper. Applications include water, gas and cable installations in soft ground. Gravel-heavy soils and rock can present difficulties.
The process involves creating a pilot hole, using mechanical cutting, fluid jetting, or a combination of these, to ream the hole, and then pulling back the new pipe product. A slurry is used to stabilize the borehole and to reduce friction during pullback of the pipe. A mini-HDD system includes the drilling frame, mechanical drilling assembly, rods and drill shoes, reaming bits and pullback pumps, the power assembly and the transport trailer. A skilled three-man crew is typically used and production rates are approximately 300 ft/day for routine jobs. Mobilization time and space requirements are minimal. Smaller systems can be used in backyards or alleys up to 100 ft from the power source. The locating and steering capability of the equipment allows avoidance of obstacles and control of line and grade to within approximately 3 to 5 percent of the depth, or approximately +/-6 in. Disturbance of the ground surface (i.e., settlement or heave) is usually minimal.
This trenchless method is used for constructing pipelines to close (± 1 in.) tolerances for line and grade. It involves the use of a remotely controlled, laser-guided, pipe-jacking system in which personnel entry is not required. Since the same process can be used for a range of pipeline diameters, the definition should refer to the process, without arbitrary size constraints. Microtunneling systems can be used to install pipelines in a single pass operation in lengths up to 1,500 ft or greater, in diameters from 10 in. to 10 ft or larger. The method also can be used in a variety of ground conditions from soft clay, to rock, or boulder ground, above or up to 100 ft below the water table without de-watering.
Typical production rates are 30 to 60ft/day, although rates of over 200 ft/day have been achieved under especially good conditions. Two operating systems are available-slurry and augur-which are distinguished by the means of spoil removal. The slurry system has enhanced capabilities for applications below the water table, though at some cost and complexity penalty over auger systems. Both systems have been successfully used in Europe, Japan and the United States. Since the first use of microtunneling in the U.S. in 1984, over 350,000 ft of pipe had been installed in this country by 1995.
The method can be cost-effective compared to open-cut construction when pipelines are to be installed in congested urban or environmentally sensitive areas, at depths greater than 15 ft, in unstable ground, or below the water table. It has been used to install concrete, steel, centrifugally spun-cast polyester resin, fiberglass-reinforced (Hobas or GRP), and vitrified clay pipes. A small quantity of PVC pipe has been installed using the system. Although sewer construction is the primary application, potential applications include environmental remediation work, such as horizontal cutoffs below a contaminated site or waste plume.
This method is commonly used in urban centers where the area immediately below the paved surface often is congested with existing services. Their presence allows little space for replacement of a defective service with a new line. The existing pipe/hole in the ground thus becomes valuable as a guide during replacement.
Pipe bursting, sometimes called pipe splitting, has been developed to take advantage of this resource. Replacement occurs by splitting the defective pipe and displacing the fragments to enable a new pipeline of the same diameter, usually of polyethylene, to be drawn in. Where increased capacity is desired, an expanding device, which may be either pneumatic or hydraulic, is introduced into the defective pipeline to shatter the pipe. Then the new line is pulled in. Up-sizing from 4 in. to as much as 9 in. diameter has been accomplished with this approach in gas mains, water mains and sewerage.
Rehabilitation of sanitary sewers using trenchless methods has grown in popularity in recent years. One approach for pipeline rehabilitation is to reline the deteriorated host pipe with an inner pipe or lining material. The most well known of the relining methods are cured-in-place pipe (CIPP) and fold-and-formed pipe (FFP). Millions of feet of CIPP and FFP pipe have been installed in the United States. Some manufacturers have conducted extensive materials tests on their products, including tests for strength, stiffness, resistance to corrosive agents and others where American Society for Testing and Materials (ASTM) or other standards exist. In some cases, companies have conducted non-standardized tests for insight into their product's behavior in different situations. Routine quality control tests are typically conducted on all products during and after manufacture and installation.
The CIPP Process
CIPP systems enable sewer pipelines to be repaired from within by insertion of 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 that hardens into a structurally sound jointless pipe when exposed to hot circulating water or steam. 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 and infiltration (I/I) of unwanted ground and surface water into the system, reduces the possibility of effluent flowing into the ground or surface water, and usually improves the flow characteristics of the system. Several companies have developed CIPP systems with variations in both materials and process of installation.
The fabric tubes which are used by CIPP manufacturers vary by material type, coating type and method of construction. Most of the fabrics are made of woven or non-woven (needle punched) polyester. Other materials such as fiberglass are sometimes incorporated into the fabric as reinforcement. Some of the tubes are seamless while others have a longitudinal seam which is either sown or heat-fused. Tubes are typically layered with at least one fabric layer and another layer which is impermeable to the flow of the liquid resin. Some systems utilize a removable inner tube which serves as a bladder between the liquid resin and the circulating water or air during installation.
The resin is typically unsaturated polyester. However, some companies use vinyl ester and epoxy resins, particularly when an improved resistance to corrosion is required or when the product is exposed to unusual thermal conditions. The actual composition of the resin material can be varied by the manufacturers to meet specific design conditions.
Installation methods for CIPP 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, while in other cases a winch is used to pull the tube through the pipe. Either hot water or steam is used to heat the resin and initiate the hardening and curing process. The curing process requires hot water or steam to be supplied to the liner for several hours after it has been formed within the host pipe.
Fabric tubes are typically constructed to be the same size as or slightly smaller than the inner diameter of the pipe which is being rehabilitated. When pressure is applied for rounding, the saturated fabric stretches to conform to the inner surface of the pipe. Some mechanical bonding of the resin to the inner pipe surface can occur in practice. Whether or not it is effective in enhancing the structural performance of the liner depends to a great extent upon the condition of the host pipe.
The FFP Process
More recently, companies have introduced systems which can install folded plastic pipe relining products through manhole entry. These systems, referred to as FFP systems, utilize thermoplastic materials which have been deformed from a circular shape, i.e., folded, to produce a smaller net cross-section that can be easily fed into an existing sewer. These FFP products usually are of 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 where 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. The FFP process can produce snug-fitting liners in the host pipe. However, bonding does not occur between the liner and the host pipe.
This is a process where new pipeline of a smaller diameter is inserted into the defective pipe and the annulus grouted. It has the merit of simplicity and is relatively inexpensive. However, there can be significant loss of hydraulic capacity.
Modified sliplining, often called close-fit lining, including rolldown and swagelining, make use of the properties of PE 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 (rolldown) or drawing through a reduction die (swagelining).
The basic spiral winding sewer rehabilitation system involves the use of an extruded, rigid PVC strip of varying widths with interlocking edges. The strips are fed off a reel into the winding machine producing a spiral-wound tube with a smooth interior and ribbed exterior. The winding machine can work from the bottom of a manhole and feed the liner directly into the pipe. Annular space around the tube is grouted.
Editor's Note: More information on this subject, or details of the upcoming NO-DIG '97 meeting in Seattle may be obtained by contacting the NASTT in Chicago, Illinois, at: Tel. 312-644-0828; Fax. 312-644-8557.
Edited by Ian Lisk