Tunnel Boring Machine
A TBM is a circular cutting head for excavating a tunnel in such a way that the rock to be removed is disintegrated by continuous rotation of a group of cutting tools thrust against the surface of the rock at the working face. Unlike a D&B tunnel, the TBM tunnel excavation will not disturb the surrounding rock when cut and creates a smooth wall. TBM tunnels have achieved advance rates of up to 150 feet per 8-hour shift.

Techniques
The cutterhead rotates at a low revolution per minute (typically 4-10 rpm) and the cutters mounted on the face of the TBM cut the rock into chips which fall into a number of buckets on the cutter head. The chips are then dropped into a conveyor belt system. The tunnel muck then passes on a series of conveyors to the back of the tunneling machine where it is discharged into the final transportation system out of the tunnel. After the TBM makes its push, typically 4-5 feet, the body of the TBM is pulled up to the cutterhead and regripped to the previously excavated wall and another cutting push begins.

Equipment
The size of the tunnel and the geological conditions of the rock will determine the type of TBM utilized. All hard rock TBM's have a circular cutting head with rolling disc cutters attached to break the rock into small chips. Some TBM's are equipped with drill through ports for probing ahead of the excavated face and for grouting the rock. All TBM's must be able to grip the walls of the tunnel as the cutter head advances or have the ability to push off of the previously installed ground support.  The main design difference of TBM's is determined by the type of rock to be excavated. Softer rock requires some type of support as soon as the excavation is made while hard rock usually does not require any type of temporary support.

Experience
Traylor Bros. has excavated over 40 miles of hard rock tunnels utilizing Tunnel Boring Machines and has been instrumental in new designs for TBM's and cutter tools. Few companies can sustain continual success in the underground business. Traylor Bros. has proven their ability to complete tunnel projects on time and within budget.

Earth Pressure Balance Method
The Earth Pressure Balance (EPB) method has been developed over the past twenty years, and has revolutionized the soft-ground tunneling industry. The technique enables the construction of near surface tunnels, in bad ground conditions, with minimal surface settlement. Because of this, EPB method has opened up new opportunities for tunneling in congested, urban areas. The level of ground control achievable with the proper implementation of this technique has enabled the construction of shallow, soft ground tunnels, even beneath sensitive structures.

Technique
The EPB method works on the principle of utilizing the excavated material to balance the external pressure of the ground. As the TBM cutterhead rotates, and the shield advances, the excavated earth is mixed with a foam material, which alters its viscosity, and turns it into a flowable material. This material is then maintained under a controlled pressure in the cutterhead chamber, to balance the pressure exerted by the ground in the tunnel face. This ensures stability as the machine advances forward. The pressure on the face is controlled and maintained by synchronizing the screw conveyor speed with the advance of the TBM. The pressure is monitored by Transducers mounted on the rear bulkhead of the muck chamber. When properly utilized, the EPB technique allows only the removal of the theoretically correct amount of material, thus drastically reducing the potential of surface settlement.

Equipment
Specialized Tunnel Boring Machines are equipped with a cutterhead muck storage area (plenum), screw conveyor, and monitoring equipment. A foam injection plant is required to deliver the foam material to the TBM for ground treatment. The TBM is also equipped with drilling ports in the cutterhead so that if need be, freeze pipes can be inserted into the unexcavated soil and the material can be frozen for additional ground stabilization.

Experience
Traylor Bros. has completed two of the most technically demanding EPB Tunnels in North America. The first was a 6,600 lineal feet by 31' diameter tunnel under the St. Clair River between Sarnia, Ontario and Port Huron, Michigan. The bottom of the river channel at one point of the tunnel was only 10 feet above the crown of the tunnel and the machine passed within 12 inches of the bottom of the main sanitary sewer line for the City of Sarnia. The second was a 19,000 lineal feet by 11' diameter tunnel driven from an inland shaft south of San Diego, California, out and under the Pacific Ocean with 250 feet of water head over the tunnel. The excavated soil was treated with foam, water, bentonite and polymers.

Compressed Air Tunneling 
Technique
The technique of using compressed air as an aid in controlling water flow and ground movement in tunnels being driven through soft and or water-bearing soils has been in use for more than 100 years. It employs compressed air to balance the hydrostatic head in the ground, and to help support the material in the face as the tunnel advances.

Equipment
In order to utilize compressed air as a means of ground/water control, the tunnel must be fitted with a set of air locks which seal off the tunnel and allow pressure to be raised to the level required to affect the desired amount of dryness or face stability. The locks consist of two bulkheads, both of which have airtight doors that open inward to the tunnel. The bulkheads are erected in the tunnel at a distance apart which is sufficient to allow passage of the crews and equipment. By a series of piping and valving, with both doors closed, the pressure in the lock can be raised to equal that in the tunnel, thereby allowing the inner bulkhead door to open and the crew to enter the pressurized tunnel. As the work progresses, and muck trains or other traffic need to exit the tunnel, the operation is reversed, with the train and/or personnel entering the lock, and closing the inner bulkhead door behind them. The pressure in the lock is then reduced in a controlled, timed sequence, and once the pressure has reached atmospheric, the outer door can open and they can exit the lock to the shaft or portal.

Experience
There are strict work rules and regulations governing the procedures that must be followed if utilizing compressed air in a tunnel. These regulations become more stringent if the pressure in the tunnel is to be raised above 14.7psi (1 Bar). The working hours, safety requirements, air lock features, and medical requirements at higher pressure now make the use of compressed air almost prohibitive. Also, new techniques such as Slurry shields and the EPB method discussed above, have advanced the art to a point where use of high compressed air pressures is rarely necessary.

Mechanized Shield Tunneling
In tunneling soft ground is material which can be removed with hand tools, even though such tools may not be the ones employed. The material, which comprises a soft ground tunnel, can be one or a combination of soft clays, sand, granular soils, glacial till, alluvial soils, cobbles & boulders, and man-made obstructions.

Technique
The first item that must be accomplished for Soft Ground Tunneling excavation is ground improvement to expand the stability of the excavated tunnel. Ground control consists of groundwater dewatering and grouting. The tunnel is excavated using a circular steel shield which permits the excavation of soil and the erection of primary lining. The shield is propelled forward using shoving jacks that push off the previously installed temporary ground support. Due to stringent surface settlement criteria, many new tunnels that would have been excavated utilizing soft ground techniques are now being replaced by Earth Pressure Balance methods.

Equipment
The body of the shield is a steel cylindrical plate with a length of 8 to 10 feet. The tail of the shield extends behind the body and is usually one-half times the width of the liner so that at the end of the push, one-half of the erected tunnel liner remains inside the tail shield. The cutting edge of the shield is on the front of the body and, dependent upon the material, will excavate the material. If the pushed shield and cutting edge will not remove the material, a digging arm will be placed within the body to assist with excavation.

Experience
During the 1950s and 1960s, Traylor Bros. advanced the soft ground tunneling technique with many innovative and technical improvements. Traylor Bros. has excavated more than 20 miles of soft ground tunnels using both free and compressed air.