Microtunneling (Outline)

Brief History of Microtunneling

Microtunneling methods combined with the pipe jacking technique to directly install pipelines underground are construction methods that use remote controlled machines. Microtunneling systems are available to install a wide range of pipe sizes from 200 mm to 5000 mm in diameter. Table 1 shows the brief chronological timeline of developing process of the microtunneling and trench-less technologies. It is said that the first pipe-jacking work in Japan was performed in 1947. Microtunneling techniques were developed in Japan in the early 1970’s primarily to install pipelines for short distances through saturated soils. Following that, a lot of microtunneling methods have been adopted because they enable construction that accounts for the possibility of collapse and for the underground water pressure of the earth. As microtunneling methods are relatively recent technology advancements in underground engineering in Japan, it is difficult to employ experienced engineering professions in the developing countries. Demands by construction industry and the public for trenchless construction methods have led to the rapid development of new microtunneling methods. The equipment has led to extend this method to the wide range of ground conditions and to achieve longer drives and larger diameter with accurate control of line and grade.

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No man entry tunneling method

As microtunneling is a horizontal digging techniques used to construct small tunnels which make it impossible or hard to have an operator in the machine, microtunneling machine must be operated remotely. Because of its remote control feature, microtunneling is most often used in unstable wet ground conditions or small pipe sizes not to allow man entry. Microtunneling is defined as a remotely controlled and laser guided pipe-jacking process that dies not require man entry for the excavation and excavated material transportation process. Since the process can be used to install a write range of pipe sizes, artificial size constrains do not technically factor into the definition of microtunneling. It could be said that no man entry process are effective for the safety construction tunneling method.

Planning

Microtunneling methods are used in the hard site conditions of heavy traffic roads, of crossing road and river, of deep installation and with difficulty to dig from the ground surface. Ground condition will be the most important factor in determining the excavation method of trenchless sewer pipeline. It might have limitations in terms of either the diameter or length of tunnel driving. The diameter and length of tunnel with physical considerations, such as the tunneling alignment, location of manholes and the size of working areas, will provide solutions which can then be appraised on the basis of cost engineering. Considering the use of microtunneling as an alternative to open cut, an reception shafts for microtunneling are designed to be located with the alignment of the pipeline in between confined to a short straight section. When shaft sites are indicated on the contract documents, the contractor may alter locations of shaft sites as needed for construction operations. Location of shafts and associated work areas shall avoid blocking roads and minimize disruption to social and business interests.

Pipe jacking and shield jacking

There are two underground pipeline installing methods, cut and cover and trenchless method. There are a lot of trenchless technologies which can be applied on not only a wide range of pipe size and pipe type but also in a variety of ground. One of the most effective advantages of the trenchless method is less effect on adjacent underground utilities due to using hydraulic jacking compared to cut and cover method. Trenchless pipe installing method can be classified in two major groups, one is shield tunneling method (we call this for ‘Shield Jacking Method’) and pipe jacking method. Both have been established as a special method for the non disruptive construction of underground structures. The shield jacking methods and the pipe jacking methods are very similar, because both technologies are trenchless method of installing rigid pipeline horizontally below the ground surface shield tunneling machine. On the other hand, the excavator of the pipe-jacking method and the following pipeline should be pushed by the thrust jacking system installed in the driving shaft. Segments for the shield jacking methods are transported and fabricated behind the shield excavating machine by erecting equipment.

Some of the factors that should be considered when selecting the shield jacking method or the pipe jacking method are pipeline diameter, driving length, site conditions, pipeline aligment, construction period, and the ground conditions. Not only the shield jacking methods but also pipe jacking methods have open face type and closed face type. Closed face type machines are meaning the “head” part of machine is closed and separated by the bulkhead fro the rear part of machine. The pipe jacking method involves pushing and thrusting an excavating machine throudh ferroconcrete pipes ahead of jacks. Uually, the open face type machines are selected for the stable ground conditions such as cohesive soils. In closed type machines, the head has a working chamber filled with muddy soil or slurry with excavated soil between the cutting face and bulkhead tostabilize the cutting face under soil pressure. Figure 4 shows the general features of the shield jacking methods and pipe jacking methods.

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The most obvious advantage of the pipe jacking system compared to cut and cover method is minimal surface disruption. The various pipe jacking methods are especially suited for any environment to avoid business and traffic disruption, and with a lot of existing utilities. Because the method relies completely on trenchless excavation, there is less of a direct impact on the environment than in cut and cover excavation operation. The pipe jacking technologies have some other benefits, such as inherent strength of pipe, no secondary lining, reduced environmental disturbance, reduced need to divert or relocate utilities, etc. Pipe jacking is safer methods of working than cut and cover construction or traditional segmental tunneling.

There are several types of pipe jacking methods that can operate in a variety of ground conditions, from hard rock to soft water bearing ground. Pipe jacking method as a trenchless pipe installing technology are classified by some factors such as pipe diameter range, drive length, excavation systems, cutting face support, excavated material transportation system. Each technology has been innovated in order to overcome many problems of more difficult conditions. There are several excavating systems depending on ground conditions, such as hand excavating to closed face type. The mechanical types with full face are called as closed face type pipe jacking. The excavated materials are typically removed from the tunnel using slurry circulation system, screw conveyors or other equipment depending on the excavating system. Accuracy is typically monitored by laser targeting systems, and hydraulic jacks are attached to the shield to allow steering in horizontal and vertical directions. Thrust is provided by hydraulic jacks located in the driving shaft. The thrust forces are distributed over the circumference of the pipe by a thrust yoke, reducing the point loading of the hydraulic jacks. Figure 5 show the classification of broadly defined pipe jacking systems for the pipe installation.

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Closed Face Pipe Jacking

Trenchless tunnel construction method is determined by a lot of factors, including ground condition, driving length, diameter of pipe, existing underground utilities and structures, manhole location, working areas, economics and potential disruption to other activities. There are several construction methods available to effectively deal with pipe jacking through increasingly difficult ground conditions slurry pressure and earth pressure balance machines are representative methods of the closed face pipe jacking methods in difficult ground conditions below the water table. The earth pressure balance type pipe jacking machine turns the excavated soil into mud pressure and holds it under soil pressure to stabilize the cutting face. The slurry pressure type and earth pressure balance type pressurize the ground ahead of the cutter head to balance the ground and water pressure. The slurry pressure type jacking machine uses the external pressurized slurry to stabilize the cutting face. The slurry is circulated to transport the excavated material by fluid conveyance.

Recent technological developments have led to the ability of stabilizing unstable ground strata by eliminating water from excavation by means of placing mud slurry around the pipes. During the jacking process, the slurry is injected into the face and what is known as the overcutting area, the area between the pipe and soil. The slurry acts as a solidifer it fills up the void space, exerts pressure and stabilizes the loose ground. The excavated material by the cutter head at the tunnel face is mixed with the slurry in the excavation chamber and then pumped through the entire pipeline to a separation plant, using a slurry pump integrated inside the machine. These methods allow the long drive and large borehole in many difficult conditions.

Bore Hole Diameter of Pipe Jacking

There is one thing we would like to emphasis about the pipe jacking technologies. In spite of pipe jacking method is very similar to the shield jacking method both jacking methods are considered as completely different systems in Japan. A pipe jacking method within the range of 150 mm to 3.000 mm in internal diameter is able to be utilized generally in Japan because of the road traffic law until the extra large diameter pipe jacking method has been developed. On other hand, the shield jacking method has theoretically no limit of pipe diameter, because the pipe should be divided for several segmentpieces. However, the construction cost for the pipelines by pipe jacking methods is usually less than the cost of the same size pipe line by the shield jacking methods. Under these circumstances, an idea of developing extra sized pipe jacking method came up for some projects of constructing a storm water storage pipe. The aim of these projects was tobuild the large diameter storm water storage facilities. To accomplish of this aim, the extra sized jacking method was developed for the pipe installation of 3.000 to 5.000 mm in internal diameter.

Japanese law prohibits from working inside of the pipe in case that the internal diameter of the pipe is less than 800 mm. Therefore, the pipe jacking method is generally classified as a method for pipes 800 mm in diameter or more, and as follows. Accroding to Japan Microtunneling Asociation (JMA), the pipe jacking method is called “large diameter” for case of more than 800 mm, and “small diameter” for less than 800 mm. Figure 6 shows a small diameter pipe jacking excavator, and a large diameter excavator.

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The slurry type pipe jacking can be operated by remote control so that workers do not enter and work inside of the pipes. These systems are called by microtunneling methods. Microtunneling is defined as a remotely controlled, guided by laser targeting system, pipe jacking operation that provides continous support to the excavation face by applying fluid pressure to balance groundwater and earth pressures. Support at the excavation face is a key feature of microtunneling, distinguishing it from tradisional open faced pipe jacking. Pipe jacking is conducted entirely by remote control, managed by an operator above ground. No personal entry is required inside the shaft or tunnel and excavated material removed from a cutting head positioned in the new pipeline advanced by pipe jacking. Pipe jacking method eliminates the need for long, extended stretches of open trench for pipe laying and is therefore deemed to provide a safer work environment.

Jacking Length

The tunnel length between the adjacent two shafts constructed by pipe jacking gives the main limitation of pipe jacking. While considering the pipe jacking length, there are a lot of factors to be considered, such as capacity of the main jacking frame and the jacking system, strength and tolerance of jacked pipe, type and ability of the excavating machine including overcutting systems, lubricant and lubricant supply system, and the stability and friction characteristics of ground. Although there is no theoretical limit to the length of individual pipe jacks, practical engineering considerations and economics may impose restrictions. The major constraint will be the nature of the ground and the ground water characteristics.

The maximum driving length for each pipe jacking is typically decided by the jacking force and the capacity of the pipe which withstand the longitudinal load. Whilst it is difficult to accurately assess these forces using soil mechanics theory, pipe jacking contractors have derived empirical values using year experience.

The pipe jacking method can construct pipelines over long distances by using a few interjack stations in the intermediate points of the line and some other technologies. The possible length of pipeline constructed in single drive ranges generally up to between 100 and 300 meters without intermediate jacking system. On of the most important technologies for long drive is the tunnel alignment measuring system using the laser targeting system. Pipe can be driven in a range of directions from a straight line, to a radius, or a series or radii with an alignment remote control system. Pipelines are installed in ever increasing lengths through various geological conditions due to the implementation of a few interjack stations and lubrication systems.

Usually, overcutting and lubricant injecting systems are used to minimize the friction between outside of pipe and ground. Overcutting is used to give a slight gap between the inner edge of the ground and the outer edge pipelines. Although overcutting in the pipe jacking method is effective for longer jacking, it should be considered that too much overcutting might make the ground and surface settlement.

Transportation of Excavated Material

The excavated material by pipe jacking machine is transported by belt conveyor system, cart, compressed air or slurry to the driving shaft or soil separation equipment on the ground surface. A excavated material transportation system shall be provided to remove safely the excavated material from the cutting face to the driving shaft or to the surface. Earth pressure type pipe jacking system uses the screw conveyer to remove the excavated soil from the cutting face. The slurry type pipe jacking machine has slurry feed and discharge equipment to circulate and pressurize slurry and slurry processing equipment on the ground to adjust the slurry properties.

The mixture is pumped to the surface where the soil particles are removed from suspension by simple gravity sedimentation or by using centrifugal forces within hydrocyclones or similar apparatus.

The excavated material removal technique maintains a sufficient level of support at the face by using either pressurized slurry or by limiting the excavated material taken from the cutter chamber.

The slurry is circulated to transport the excavated soil by fluid conveyance. The UNCLEMOLE excavating machine is a closed full face type machine in which the excavated materials transported from the face suspended in slurry. This UNCLEMOLE excavator is a wheel type machine which has a rotating cutter face that excavates the soil into the interior of machine, and the excavated materials are then transported to the ground surface through the slurry discharging pipe.