The typical auger bore is constructed by jacking a steel casing through the earth, using Auger Boring techniques. Auger boring involves a simple, cyclical procedure of utilizing the thrust power of hydraulic jacks to force the pipe forward. The face is excavated simultaneously with the boring operation. The spoil is removed through the inside of the pipe to the jacking pit. A Large Diameter Boring Attachment (LDBA) has been used in conjunction with a boring machine for the construction of small to medium diameter (36” to 120”) tunnels. The LDBA is similar to a tunnel boring machine. The equipment setup for LDBA is as shown in Figure 2 and Figure 3. The LDBA is a full-faced machine with a rotating cutting head for excavating the soil. It has an articulated section with hydraulic rams to make corrections to the line and grade. The cutting head is driven by electric or hydraulic motors. The cutting head rotates and excavates the soil, which comes inside the pipe through the openings in the cutting head. An auger system is used to carry the spoil to the jacking pit. The auger boring machine provides the jacking force to thrust the LDBA forward while it excavates. The auger boring machine is also used to power the augers that are used to convey the spoils to the jacking pit. The advantage of the LDBA over some other methods of trenchless installations is that the power to the cutting head is unaffected by the length of the tunnel. Since the cutting head is independently powered by another source, the available horsepower to the cutting head is the same no matter what the length of the tunnel.

Large Diameter Boring Attachments

An electric generator is normally used to power up the LDBA and to provide the lighting inside. The generator is normally located at ground elevation with the cable going through the steel casing all the way to the LDBA. There is also a communication system between the operator of the LDBA and the operator of the auger boring machine. This is necessary because the operator of the LDBA is at the face of the machine watching the excavation and directing the operator of the auger boring machine on the rate of advancement. At the same time when the auger boring machine operator is retracting the jacks and the machine is not excavating he directs the LDBA operator to stop the cutting head. Thus, the operators of the LDBA and the auger boring machine have to work together to get desirable results.

In addition, there is a bentonite mixing system at the ground surface. This consists of a tank with a mixing device to mix water and bentonite to the required consistency and a pump that provides the necessary pumping power to pump bentonite in the space between the steel casing and the surrounding soil. On the casing pipe, there are bentonite ports every 20 ft. The ports are located at 10 O'clock, 12 O'clock, and 2 O'clock orientations. The bentonite is pumped through these ports. The function of bentonite is two-fold. One, it fills the void between the steel casing and the surrounding soil and prevents the soil from caving in on to the pipe and secondly, it reduces the friction between the steel casing pipe and the surrounding soil to minimize the jacking force required to propel the pipe through the ground. Line and grade is monitored by using a laser beam. The laser is set up in the back of the jacking pit and it travels above the boring machine and below the top of the pipe and through the pipe to a target that is set up near the front of the machine. Care is taken to ensure that the laser is set up such that it is independent of the concrete backstop. This is done so that in case the concrete backstop moves due to the jacking forces exerted on it, the laser reference remains unaffected. Also the laser is checked every time a new casing is added to ensure that the reference is correct and it is aligned to the proper line and grade. The LDBA operator sits behind the cutting head and monitors the target and makes the necessary adjustments using the hydraulic rams to keep the tunnel on line and grade.

The jacking pit size is a function of the pipe diameter, length of pipe segments, dimensions of the LDBA, size of the auger boring machine, and the thrust wall dimensions. A concrete slab is poured to the same grade as the pipe being installed. The thrust block or the concrete backstop and the concrete slab are poured as an integral unit to take advantage of the entire mass of concrete. The concrete backstop is poured perpendicular to the line of the tunnel, otherwise it may lead to uneven distribution of thrusting loads on the steel casing resulting in difficulties in maintaining proper line and grade. A typical jacking pit for this operation is 40-feet long and I2-feet wide at the bottom with sides appropriately sloped to the ground elevation or with proper trench protection devices. The centerline of the tunnel is offset about 4-feet to the right while facing the direction of the progress. This is to allow room for the spoil that comes out of the tunnel during the excavation process. The spoil is normally ejected on the left side while facing the direction of the tunnel. Once the concrete base slab and the concrete back stop are ready, the tracks for the auger boring machine are placed on the concrete slab in line with the proposed line and grade. The boring machine is then set on the tracks along with the adapter saddles for the LDBA. The LDBA is then set on the saddles and hooked up to the auger boring machine. All electrical and communications are hooked up and tested. The guide laser is then set up independent of the backstop. The line and grade of the LDBA are verified one more time prior to starting the tunneling operation and necessary adjustments are made to ensure the correct alignment. Bars are welded on both sides to prevent the LDBA from rotating during the tunneling operation. “CoIIaring” is the first operation in beginning a tunnel with an LDBA. The objective is to start the LDBA into the ground without lifting the casing out of the saddle. This is done by rotating the cutting head at low RPMs and using a slow thrust advance. When about 4 to 6 feet of casing has entered the ground, the saddle is removed. Then the rest of the casing is installed while closely monitoring the line and grade. After the first section is installed the LDBA is shut down and all electric and communication cables are disconnected. The auger boring machine is moved to the rear of the tracks and is shut down. The next section of casing is prepared and is lowered into position. The augers are aligned and then the casings are aligned. Then the casings are pushed together and the large casing is welded. The auger boring machine is attached and all the electric and communication cables are hooked up. The bentonite lines are also hooked up this time to lubricate while the casing is being pushed through the ground. The machines are then started and the casing installed. The process is repeated until the entire tunnel is completed and the LDBA comes out in the receiving pit. All lines that are hooked up to the LDBA are then disconnected and pulled out through the casing and the LDBA is detached from the casing being installed and taken out of the receiving pit. The bands attaching the small diameter casing to the bottom of the large diameter casing are cut and pulled out.