Installing utility lines underground has proven itself a real money-saver time and again. Underground installation isolates electric, phone, and data cables from damage by wind, falling tree limbs, ice buildup, and other hazards. It’s also much easier on the eye, especially in neighborhoods. But having to dig a trench to bury cables can be an expensive and time-consuming procedure.
With the conventional method, a trench has to be dug, the cable inserted, and the earth that had been dug up put back in the trench and compacted. Having to restore the landscape eats up more time and money. The task becomes even more expensive and time consuming if the utility lines must be routed under existing underground utilities (gas, water, sewer, and other electrical lines), existing roadways, and waterways. These issues are all avoided with trenchless technology, which uses a horizontal directional drill machine to form an underground pilot tunnel for cables and pipes. The machines can even direct the tunnel under and around existing underground obstacles, such as existing utility lines and building foundations.
Theses self-propelled machines use hydraulics for most power functions, such as driving a pair of track-drive motors, a stakedown system, and all boring functions. The stakedown system is necessary to hold the machine in place during boring operations. Without it, the tremendous thrust produced by the hydraulic system to push the drill into the ground could move the entire machine. Once at the work site, an operator need only connect the machine to a source of water, deploy the stakedown system, and begin the underground operations.
Mechanics of Trenchless Technology
Directional boring starts by penetrating the earth with a drill head where the cable or pipe eventually will come out of the ground. The machine then pushes the drill at an angle deeper into the ground. Sections of drill rod—each typically 10- or 15-ft long — are added as the drill head advances deeper into the ground. A battery-powered transmitter inside the drill head sends a signal to personnel above ground so they can monitor the location, depth, and orientation of the drill head. Depending on the machine, the drill head can be directed to advance left, right, up, or down.
Once the drill head emerges from the ground at the target area, it is removed from the drill pipe and replaced with a reaming tool. The utility cable or pipe is attached to the free end of the reaming tool, and the machine begins pulling the drill pipe back out of the ground. As it pulls, the machine also rotates the drill pipe, and with it the reaming tool. The spinning reaming tool enlarges the diameter of the underground channel to accommodate the utility cable or pipe. It also pulls the cable or pipe with it as it retracts toward the original entrance to the channel. When it eventually reaches the machine, the cable or pipe is already in the ground, right behind the reaming tool.
Hydraulics at a Glance
All major functions of these machines are powered and controlled by hydraulics. Driven by the machine’s gas or diesel engine is a variable-displacement hydraulic pump. This pump features pressure compensation to regulate pump displacement when system pressure reaches the compensation pressure. When this occurs, the compensator circuit reduces pump displacement to maintain this pressure. The pump also features load sensing, which matches pump output to load requirements. Pressure compensation reduces pump stroke only when pressure reaches a maximum (compensation) pressure. Load sensing, on the other hand, closely matches pump output to load requirements at pressures just a few hundred psi above that required to move the load.
Drill rotation is provided by a low-speed/high-torque (LSHT) motor, with speed controlled by varying the pump's swashplate angle. Pressure is limited by the circuit’s relief valve. However, the most important function of any directional boring machine is its ability to push a drill rod into the ground and pull it back out again. A double-acting hydraulic cylinder transmits the linear power for both functions. However, to avoid having to use a cylinder with an extremely long stroke, these machines typically incorporate a chain drive arranged in a block-and-tackle assembly to double the effective cylinder stroke—the same concept used in forklift trucks. In this case, though, two chains are used with the boring machine: one to double the cylinder’s extend stroke and one to double its retract stroke. Two are needed because the chain can only pull the rod carriage, not push it. One end of each chain is fixed to the machine’s framework; the other end is attached to the rod carriage.
The thrust/pullback cylinder is mounted so that extension pulls the drill rod out of the ground. This is done because pulling the drill rod from the ground requires greater force than thrusting it in. When pulling drill rod out of the ground, the thrust stroke resets the rod carriage at essentially no load for the next pullback stroke. The rod diameter displaces a substantial portion of the rod-end volume, so the reset stroke executes quickly.
When thrusting drill rod into the ground, the pullback stroke resets the rod carriage for the next thrust stroke—again, at essentially no load. However, the reset stroke requires pumping fluid to displace the entire cap-end volume of the cylinder. Potentially, then, the machine could exhibit a long dwell period while the carriage resets. To shorten unproductive dwell periods, manufacturers often incorporate a regenerative circuit. Instead of routing fluid exiting the rod end of the cylinder to tank, the regenerative circuit routes rod-end fluid to the cap end. Therefore, fluid volume equal to the rod volume is all that is needed to extend the cylinder under no load. Reducing fluid volume to retract the cylinder, in turn, reduces retraction cycle time.
Once the machine has moved to the work site, it must be secured in place. The machine applies thousands of pounds of thrust and pullback forces. Therefore, some means must exist to counteract these horizontal forces that would otherwise move the machine. Otherwise, instead of pushing the drill into the ground, the machine’s force could push it away from the hole.
The machine's stakedown system incorporates a pair of augers, each driven by a LSHT hydraulic motor and a pair of hydraulic cylinders. The double-acting cylinders apply a constant force to push the augers into the ground while the hydraulic motors rotate them. This makes the stakedown augers self-tapping. If they relied only on the weight of the components for downward force, the augers might simply drill a pair of holes into the ground under less-than-ideal soil conditions. This would not provide as much stability as actually threading them into the ground and would also create additional work at cleanup time.
Cartridge valves are used extensively on these machines, combining several valve functions into a single manifold. These manifolds reduce the amount of hose, tubing, and fittings required and eliminate many potential sources of leakage. Cartridge valve groupings are also more compact than line-mounted valves and promote neater, cleaner designs.