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| AUGER BORING / PIPE JACKING |
Thrustboring Method of horizontal Auger Boring is a process of simultaneously jacking pipe through the earth while removing the spoil inside the encasement by means of a rotating auger. In unstable soil conditions, the end of the auger is kept retracted back inside the encasement so as not to cause voids. In stable soil conditions, the auger can be successfully extended beyond the end of the encasement to facilitate the breaking down of the spoil for removal. In addition, in stable soils, rocky soils and rock conditions, cutting heads can be attached to the end of the auger or cutting head units can be mounted to the front of the encasement to actually bore the hole for the encasement to enter.
While auger boring methods can successfully be used in any kind of soil conditions, where obstructions are concerned, the encasement size in relation to the obstruction should be small enough to bore through the obstructions or large enough in relation to permit removal of the obstruction through the encasement. This is also a good rule where the soil contains boulders, for the size of the bore in relation to the size of the boulders.
PITS
The boring operation should proceed from a work pit that will adequately and safely accommodate the auger boring equipment, materials and workmen. When needed, a shield should be utilized to prevent cave-ins and voids from developing at the working face.
Preparation of the floor of the pit will be determined by the length, size and duration of the job. Dewatering provisions should be considered.
DEWATERING
When ground water is present, known, or anticipated, a dewatering/sump system of sufficient capacity to handle the flow should be installed and maintained at the site until the operation can be safely halted or the job is completed.
PREPARATION OF THE CASING PIPE
Casing lengths are critical when trying to keep either the auger retracted inside the casing or extended beyond the end of the casing. Soil conditions usually determine which choice is preferable but length becomes critical to maintaining that condition.
It is also important to square the ends of the casing sections to successfully weld the sections together.
In addition to providing for squared ends and measured lengths, you should check for roundness and outer surface conditions that may add to skin friction problmes.
SKIN FRICTIONS
Friction between the outer surface of the pipe and the surrounding soil may become a factor that inhibits the successful progress of the bore. Two of the most common methods of reducing skin friction are by overcutting the bore or by lubrication of the casing. These methods can be used separately or in combination with each other.
Lubricants applied to the outside surface of the casing pipe reduce frictional forces, and the resulting high thrust forces need to overcome the surface friction. Excessive surface friction and increased thrust loads can cause side loads to be placed on the casing pipe, the deformation of the casing pipe, and other factors that inhibit the successful completion of the road crossing.
It can be pressure pumped to and around the lead section of pipe and when conditions require to other locations along the surface of the casing.
LINE AND GRADE TOLERANCES
In the boring process, the line and grade of the pipe or casing will tend to continue through the bore the same as it was originally set up on the machine in the pit unless it is affected by some outside force.
Line and grade deflections occur frequently when the pipe encounters some obstruction such as a boulder. If the cutting head will not penetrate the object, and if the size of the auger and casing will not accommodate and remove the obstruction, a thrust will be created. The thrust created will drive the casing to the path of least resistance which would likely deviate from the originally established line and grade of alignment.
Some projects have specific tolerance for justifiable reasons and they are not subject to change. In When tolerances are close and critical and problmes with soil condtions are anticipated, the operator wants to begin the crossing with the best options in his favour. Two options that can help are the use of steerable heads and larger than planned bores.
Steering devices such as steerable cutting heads have been built and successfully used to mechanically adjust the direction of the auger bored road crossing and therefore the line and grade. By using a hinged lead section of casing at the front of the bore, resistance forces are countered and the cutting head and casing can be brought back into the desired location. Using this type of device along with constant monitoring of the line and grade sensing and checking devices, and operator can make the desired and necessary changes and in many cases "steer" the bore with accuracy in most soil conditions.
Line and grade tolerances can also be meet by increasing the diameter of the bore to a larger size than was originally planned. Upon completion of the bore, the carrier pipe is inserted through the oversize bore, positioned in place to the correct line and grade, and then blocked and secured in place. The void between the two can then be filled if required. The obstructions (boulders and other objects) whereas the smaller, more restrictive diameter makes this operation impossible.
THE USE OF WATER IN AUGER BORING
Water can be used successfully to facilitate spoil removal when it becomes necessary. Water is introduced into the front casing pipe behind the cutting head by means of a pumping system.
Key Benefits
This method has proven to be extremely useful in sticky ground condtions where the spoilage tends to adhere to the auger flighting. Water is also used in grade indicating and checking attachments to give continuous and accurate elevation readings of the lead casing pipe. In both cases, a skilled and experienced operator can use these water systems without adversely affecting the integrity of the surrounding soil or the road crossing.
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