Guided auger boring comes of age.
While auger boring may not be perceived as being as sophisticated as newer technologies such as horizontal directional drilling (HDD) and pipebursting, it remains in wide use because of its relatively low cost compared to other trenchless construction methods, and because project owners and consulting engineers understand its capabilities and are comfortable specifying the method. And, for many jobs, auger boring simply is the best construction option.
Powerful boring machines also can cut through extremely hard rock formations that are beyond the capabilities of small- and medium-size directional drills.
The basic components of an auger boring system are the base unit, casing pusher, power pack, auger sections and lengths of track. The hydraulically-powered base unit usually is set up in a pit. As a bore proceeds through the earth, sections of pipe are added until the installation is complete. Rotating augers carry spoil out of the hole. Cutting bits are available for different types of soil.
Auger bores are perceived by some not familiar with the process as being "unguideable." However, from the time the first auger boring machines were introduced, operators recognized the importance of ending the bore near a predetermined point, and auger boring has a long history of use on segments of projects where maintaining grade is critical.
One means of achieving grade control is to install a casing in which a system's pipe is installed using spacers to achieve precise grade needed.
However, various methods are available for maintaining grade--and now line and grade--on auger boring installations.
In the beginning
Starting a bore with the machine set to line and grade in the pit is essential. Then there are several methods of monitoring grade alone or line and grade, including the Dutch Level (employing water and gravity), lasers and electronic systems similar to those used with HDD equipment.
"I would suspect the first method used to achieve grade was a 'good aim,' "says Dave Vidovic, American Augers project manager. "But there are external factors that can affect attitude and end result--the first being the type of geology encountered. Overburden and weight acting on the casing will affect the grade of the heading. Also, in normal cutting, the cutting tool does not want to go straight; it has a normal reaction to want to climb high and to the right, and the harder the geology, the more likely the head is to climb. So good aim--with the addition of a little Kentucky windage--likely was the first method used to end a bore at the planned exit point."
This was followed by the Dutch Level, Vidovic continues.
"During excavation projects in ancient times, nature was the best method in use for determining exact position in a heading," he says. "The Dutch Level method allows for a column of water to be channeled from the heading to the pit at the auger boring machine. It is known, through surveying and plotting the bore mathematically, what the elevation should be at various stations. When the grade of the head changes, the column of water raises or lowers to match the change at the heading. As the bore progresses, these elevations are checked and compensation is made as necessary."
Today's more advanced guidance technologies also rely on laws of nature.
"Electronic systems in place today rely very much on nature, using gravity or magnetic fields to determine position," Vidovic says. "Widely available laser systems employ survey methods to locate the laser and to further determine relative positions on targets."
Michael J. Moore, vice president of sales for the McLaughlin Group, says the use of water levels and mechanical steering heads that allow adjustment to move casings up or down are the most common methods of steering the path of an auger bore.
"The water level is very accurate and easy to use," says Moore. "It determines how much rise or fall should occur with each cylinder stroke, and the water level is checked after each stroke. If the bore is not moving at the correct grade the steering head is rotated to change the cut path. Rotating steering rods clockwise raises the head causing the bore path to fall at a slower rate or rise at a faster rate. Turning the head counter clockwise lowers the head; this will cause the bore path to fall at a quicker rate or rise at a slower rate."
There are several mechanical devices that allow changes in cutter attitude.
"The two most prevalent are the steering knuckle and the wedge system," says Vidovic. "The steering knuckle is a hinged system on which the cutter head is mounted. It consists of two pins that are mounted usually at the spring line of the excavation that connect the normal casing, and a short forward section of casing that can pivot around the pins connecting the two pieces of casing together. Normally the lead section of casing is pivoted by means of a series of metal rods that pull on the casing from outside the excavation to achieve proper heading."
Another common method for attitude correction is a wedge system that is used to bring the excavation back to proper grade.
"Usually this arrangement consists of a wedge that is fixed to the invert section of the casing when correction is necessary and removed once the bore is brought back to grade," Vidovic explains.
'Natural' methods of grade control are reliable, but they can only determine grade. Using a Dutch Level, grade can be controlled within a few percent, but it is not uncommon to be off in terms of line by the diameter of the excavation, says Vidovic.
"The most significant steps in auger boring guidance occurred when natural methods were replaced by more sophisticated methods such as walk-over systems and laser-guided systems," says Vidovic. "The biggest advantage with these systems is the ability to determine the line as well as the grade of an auger boring cutting tool at the heading."
An electronic walk-over receiver monitors location of the bore head and other information needed to make corrections in the bore's path. Adjustments are initiated using electric over hydraulic controls.
A laser system can be used along with an inclinometer to determine attitude of the bore head.
"In the case of line and grade being straight and horizontal with no change, imagine a set of cross hairs on a target that is placed in the steerable section of the cutting head," explains Vidovic. "If a laser beam is set outside of the heading and pointed toward the target, as the cutter head advances, the goal would be to keep the cross hairs on the laser beam by changing the attitude and direction of the cutter head when it strays off of line and grade.
"If line and grade change, the horizontal beam outside of the heading is still used; however the target now consists of "points" that are representations of where the target should be at a certain station. For instance, if the change is to be a gradual but continuing negative change in grade, the target would consist of a set of cross hairs as well as points directly above the cross hairs on the vertical centerline of the target representing the place the target should be at certain predetermined stations."
In the case of line and a turn to the left, the points should be to the right of the cross hair on the horizontal centerline of the target, Vidovic continues.
"With a combination of line and grade corrections, the 'points' can be located in any one of the four quadrants of the target depending on the necessary line and grade," he adds.
Auger boring is used primarily for installing "large" diameter pipes. Depending on model and soil conditions, auger boring equipment can install pipe in diameters to 72 inches. Typically, bore lengths have been 300 feet or less but with improved equipment and accurate guidance control, bores are becoming longer, depending again on soils and the machine being used.
The ability to make longer bores with greater accuracy has increased the technology's effectiveness and expanded the market for auger boring.
"Auger boring has seen a significant change in the types of work that were considered to be able to be done by auger boring methods," Vidovic said. "The ability to drive a more precise heading, coupled with machines that are able to cut through more challenging hard rock conditions, has allowed auger boring methods to replace jobs normally done by microtunneling methods that usually require high capital equipment. At American Augers, we find our boring machines and cutting tools engaged in work that was thought impossible only a few years ago."
Some contractors are using guidance systems developed for small-diameter tunneling machines to maintain precise line and grade on auger bore installations.
Rob Tumbleson, sales and marketing director for Akkerman Inc., says that more than half of Akkerman's GBM systems sold to date are being used in conjunction with auger boring machines. The system uses a theodolite to pilot an accurate course and is designed to allow boring crews to maintain line and grade with an accuracy of one-quarter inch at distances to 300 feet.
The system consists of a theodolite (an instrument widely used in surveying to measure horizontal and vertical angles), an illuminated target, bore site laser, camera and all necessary hardware.
To use the system, a 4 1/4-inch pilot tube is jacked through the ground to line and grade.
"The target," he explained, "is in the lead steering head. The operator looks down the tube through the theodolite to maintain grade as tubes are pushed through the soil. When in place, correct line and grade is confirmed and the crew adapts the auger and case and the bore follows the path of the pilot tube. As the bore progresses, the tube is removed in the reception shaft."
Tumbleson says demand for the system for auger boring has increased substantially over the past two years.
"Contractors began using them for this purpose and as we saw a growing number of leased units used for auger bores, we began targeting that market," he said.
FOR MORE INFORMATION:
Auger boring products:
American Augers, (800) 527-6020, or circle #205
McLaughlin Manufacturing, (800) 435-9340, or circle #206
Akkerman Inc., (800) 533-0386, or circle #207
Bor-it, (419) 289-6639, or circle #208
|Printer friendly Cite/link Email Feedback|
|Date:||May 1, 2005|
|Previous Article:||Bringing in the big trenchers.|
|Next Article:||PLCAC president's profile: James Topping.|