Horizontal Drilling Training
Session Objective: After reading this section and attending this lecture the attendee will have an increased knowledge of: Basic Trenchless Excavation Construction Methods (TEC). Basic Horizontal Earth and Rock Boring methodology. Horizontal Environmental Well material installation techniques. Horizontal Boring equipment used in small diameter well installations.
The following section presents an overview of Horizontal Boring process as it pertains to the installation of horizontal remediation wells and other underground utilities.
The following is a brief introduction to basic horizontal equipment, site setup and general procedures followed during a typical horizontal bore. Some of the references to Line and Grade may or may not be of major concern to the remediation specialist. However, in cases of watermain or gravity flow aqueducts it represents a critical factor in the success or failure of the project. Furthermore, it is important to know were the bore is heading to avoid hitting underground hazards and utilities.
Trenchless excavation construction (TEC) methods include all methods of installing utility systems below grade without direct installation into an open cut trench. These methods are broken down into three main categories; Horizontal Earth Boring (HEB), Pipe Jacking (PJ) and utility Tunnelling (UT). The method most commonly used in the installation of environmental remediation wells is the Horizontal Earth/ Rock Boring method.
Auger Horizontal Earth Boring
Typically, Auger Horizontal Earth Boring is a process of simultaneously jacking casing through the earth while removing the spoil inside the encasement by means of a rotating flight auger. The auger is a flighted tube with couplings at each end that transmit torque to the cutting head from the power source located in the bore pit and transfers spoil back to the machine. The casing supports the soil around it as the spoil is removed. However, in the case of standard 2' environmental remediation well installations, Marathon Drilling has adapted it's vertical hollowstem auger system to their Horizontal Boring Machines which eliminates the need for casing to keep the hole open as the well is installed. In many respects, horizontal well installation procedure is similar to its vertical counterpart in both protocol, and material. The geotechnical conditions of the site, together with design requirements of the field programme will dictate the specifications and type of well material that should be used.
Track Type Auger Heb Method
The track type auger boring machine consists of a track system, the boring machine, casing pipe, cutting head and augers as the main components. The optional components consist of variations of bentonite lubrication systems, a grade control head, a casing leading edge band, and a water level indicator or electronic indicator.
The main factors that effect auger boring are the torque and thrust. Every effort is made to control these two forces. The torque is created by the power source which can be a pneumatic, hydraulic or internal combustion engine working through a mechanical gear box. The torque rotates the augers which, in turn, rotates the cutting head. The casing (if required) does not rotate while it is jacked through the hole. One end of the rams is attached to the boring machine while the other end is attached to the lugs that lock into the track system.
In cases where the torque or thrust exceed the machines capacity then all forward advancement is halted. Since actual conditions to be encountered are not known until the boring operation commences every effort should be made to minimize torque and thrust and both should be closely monitored during the operation.
For the project to be successful, soil test borings, unit weight of soil, soil classification, groundwater level determination, standard penetration test (SPT) value, unconfined compressive strength of soil, etc. should be available. It is in the interest of all contractors to excavate test pits prior to bidding a project. However this does not mean that changed conditions and/or obstacles will not be encountered during the boring program. Every effort should be made therefore to ensure that unexpected conditions can be handled safely.
The jobsite should be surveyed for overhead powerlines and other obstructions, water drainage
problems, job access and working space. All utilities should be contacted , located, marked and if necessary exposed to positively identify and locate any potential underground obstruction in the bore path. Utility lines damaged beneath a road way by the boring operation would most likely result in open cutting of the roadway at a substantial expense and great inconvenience to the public. In most instances an entrance pit is required on the approach side of the bore. The site should allow adequate room for a boring pit excavation plus the subsequent stockpiling of excavated material if it is not being removed from the job site. Natural water drainage should also be considered. It should be ensured that in case of heavy rain the pit, equipment and material is not flooded.
The possibility of building a temporary drainage system to route water away from the job site should be investigated.
Bore Pit Excavation And Preparation
The beginning and end of the bore should be located far enough away from existing structures to allow adequate safety for the structure and the public. The distance of the bore pit to the roadway should allow for safe sloping of the pit walls if necessary. If sloping of the pit walls cannot be accomplished, sheeting of the pit walls should be considered. Local codes and OSHA manuals should be consulted concerning the specifications and requirements for pit wall sloping and sheeting.
With all the utilities located and marked, excavation can begin. All cuts, grades and slopes should conform with the construction plans. The boring pit should be offset slightly to the side of the bore line on the side that the spoil exits from the machine. This allows for more access for spoil removal. Any utilities in the pit must be supported. If groundwater is expected or encountered a dewatering system must be utilized.
The boring pit bottom must be firm enough to support the boring machine tracks, boring machine, casing, and the augers. in most cases, the pit will have to be excavated below grade and then filled with crush stone to the required specifications. Wooden planking is then placed under tracks for support. A concrete floor may be poured if the bore is of considerable length, size and duration or if soil conditions warrant it.
The boring machine applies thrust to the back of the boring pit. To withstand this thrust, a backing plate should be installed against the back wall of pit, square with the line of thrust. For low to medium thrust pressures, steel sheeting, a steel plate or wooden timbers have been found to be adequate. However, on long and large diameter bores, a concrete backstop in addition to steel plate is desirable. Care must be taken to ensure that the developed thrust pressures do not disturb any existing utilities in and around the bore pit area. Each bore pit should be constructed as if it were to be in use for a much longer time than anticipated to allow for unforeseen problems and delays. Doing it right the first time will save both time and money.
If an exit pit is required at the end of the bore. The safety requirements for the exit pit are the same as for the entrance pit. Unless absolutely necessary no personnel should be allowed in the exit pit during the boring operation. The unexpected entry of the boring head into the pit can catch the person and cause serious injury or death. As the casing pipe approaches the exit pit care should be taken to prevent collapse.
Many different types of equipment may be required on or around the boring site. Excavated and /or cranes are needed to dig the boring pit and set the equipment. Boring machine and tracks appropriate for the job are required. Augers must be placed in the casing sections . A cutting head is selected depending on the ground conditions and is installed on the front of the first auger section. The cutting head type that is selected for a particular project should be compatible with the anticipated soil conditions.
The most critical part of the boring operation is the setting of the machine track on line and grade. If the alignment is not right when the bore is started, it is not likely to improve during the boring process. The master track is placed in the pit with the push plate against the backstop. It is then aligned with the proposed bore and the machine is set on the master track. The push bar dogs are engaged at the rear most holes in the tracks and the slide rails, the master casing pusher and the casing adapter are installed. A grade control head may be used to mechanically adjust the grade of the auger bore crossing from the bore pit.
Water may be needed in the boring operation on some soil conditions to help facilitate spoil removal, for use with bentonite lubricants, and for monitoring grade with the grade control head.
In addition to the above mentioned equipment, the optional components include the following:
As discussed previously, controlling torque and thrust are two major concerns of the auger boring operation. Application of a bentonite lubricant to the outer skin of the casing reduces the friction between the casing and the soil which, in turn, reduces the thrust requirements. There are two basic types of lubricants. One is a bentonite which is an expansive montmorillonite, colloidal material which when mixed with water becomes an excellent lubricant and sealant. It is the best lubricant for sand and porous soils because of its sealing properties. The second type of lubricant most commonly used is some form of polymer agent. In some cases they work better than bentonite. They ease the problem of separation when they get in the casing. The polymers such as Baroids EZ Mud works better than bentonite in certain types of soil such as clay, where the encapsulation of the clay by the polymer reduces their sticking and balling tendencies. Either method improves the thrust capabilities in all types of soils. The complete lubrication system consists of a mixing tank, a pumping method and a distribution system. The lubricant is transferred to a point of application near the leading edge of the casing through a steel pipe generally 0.5 inch (12mm) to 1.5 inch (38) mm).
The water level is a device to measure the grade of the pipe casing as it is being installed. It permits the monitoring of grade by using a water level sensing head attached to the top of the leading edge of the casing. The level operates in the same way as a sight tube on a boiler. Both ends of the system are vented to ambient pressure. A pit mounted control and indicator board is located at some convenient point in the pit near the operator. A hose connects the bottom of the indicator to tube to a water pipe running along the top of the casing. Water is used to fill the system. The level of water in the pit indicator will then show the level of the valve at the end of the casing as it is pushed into the ground. One should be careful when using this system to ensure that the system is full so that an incorrect reading is not taken.
Grade Control Head
The grade control head is used for making minor corrections in the grade. It can be used to make vertical corrections only. During the boring process, the actual grade can be monitored with the water level and the necessary adjustments can be made with the grade control head. If the grade control head is used, then the leading end of the casing must be properly prepared. When water is injected in the casing to facilitate spoil removal, the point of injection is located behind the garde control head to prevent the water from contacting the excavation face. For this, a 0.5 inch (12mm) diameter steel pipe , tack welded to the top of the casing, injects water through a 3 inch (75mm) slot approximately 24 inches (60mm) behind the grade control head.
In cases of large diameter well installations casing may have to be used to prevent the collapse of the borehole. The casing should be of good quality and should be well prepared. Machine cut bevelled edges assure casing alignment., exact lengths keep the head at the correct location relative to the casing, and the smooth walls reduce the thrust required and the tendency to yard prior to its transport to the jobsite and arrives at the jobsite with the auger inside and the cutting head attached to the leading end of the auger. All casing are usually loaded with the augers at the yard and arrive at the jobsite ready to use. It is recommended that all bores be done with a string of full size auger sections. However, under conditions where auger loading is light and the spoil moves easily in the casing, lead sections of full size augers can be followed by smaller diameter auger sections. As a general rule, smaller diameter augers should never be used in the lead section of the casing. The smallest auger used should not be less than three forths the diameter of the casing.
When this recommendation is neglected, problems normally occur. This decreases the efficiency as the spoil is not removed from the casing -where the smaller diameter auger is being used- at the rate of excavation. This results in the augers rotating more revolutions to remove the volume of spoil being excavated at the face which results in the rotation of the auger without forward advancement.
Other factors in the use of smaller augers are bending and torque. The undersized auger creates bending which results in stresses in the auger stem. Also the smaller auger will have more wind up from the same torque loading than the full size auger. Torque windup pulls the cutting head back towards the casing and could cause the wing cutters to contact the casing, further increasing torque and causing even more damage. The use of a partial band at or near the head end of the casing is recommended when boring in most soil conditions. The band compacts the soil and relieves pressure on the casing by decreasing the skin friction.
The banding process is most effectively utilized in unstable soil conditions where wing cutters are not used. In this case, the casing is pushed forward without the borehole being over excavated. Therefore the soil compacting benefit is more pronounced as it relieves the pressure on the following sections. It is also beneficial in rock or boulders as it strengthens the leading edge of the casing.
Wing cutters are devices that are attached to the cutting head which open and close.
When the cutting head is rotated clockwise, the wing cutters open up to provide over excavation of the bore hole. The over -excavation of the borehole allows the casing to enter more easily since it minimizes casing skin friction. Wing cutters are used only in stable soil conditions and are never used with the cutting head inside the casing. The wing cutters are adjustable to control the amount of over-excavation. Normally, the standard overcut is 1 inch (25mm) greater than the nominal casing diameter. When the cutting head is rotated anti- clockwise, the wing cutters close up so that the cutting head can slide back inside the casing for auger removal purposes.
The wing cutters must be set so as not to over-excavate at the bottom of the casing. This causes the bore to drift in a downward direction. Over-excavation of the bottom can be prevented by keeping the boring head centered. This is accomplished by using new or built up augers in the lead section of the casing. Worn augers in the lead section will allow the head too much freedom and the wing cutter pattern will be erratic.
Installation of Casing: Collaring is the first operation in the beginning a bore. The objective is to start the cutting head into the ground without lifting the casing out of the saddle. This is done by rotating at low RPMs and using a slow thrust advance. When about 4 feet (1.3 meters) of casing has entered the ground, the engine is shut down, the saddle is removed, and the line and grade of the casing is checked. If the casing is not on line and grade with the proposed bore, the casing is removed and the process is repeated. The success of the bore depends to a great extent upon the line and grade of the first section of the casing.
After the first section of casing has been installed in the ground, the casing is cleaned by rotating the auger until all the spoil is removed. The machine is then shut down and the auger pin in the spoil chamber is removed. The machine is then moved to the rear of the track and again is shut down. Then the next section of casing and auger is lowered into position. The augers at the face are aligned flight to flight, the hexagonal joint is coupled and the auger pin is installed. Scabs are welded on the casing to be installed at 11 and 1 O'clock positions. The casing is the advanced over the auger. The casing is aligned with installed casing by resting the scabs on top of the installed casing and using 4 foot minimum (1.2 meter) straight edges along the top and sides. If the new casing is in line with the installed casing and seriously out of line at the machine end, it means the installed casing is misaligned and must be corrected or else it will result in unacceptable bore alignment. Once the casing to be installed has been properly aligned with the casing already in the earth, the two are tacked together then welded fully. The drive is then coupled to the auger and the casing is secured to the pusher. the water and bentonite lines, if being used, are added. The machine is started up and the casing is installed. The process is repeated until the bore is completed.
Removal Of Augers At The Completion Of The Bore:
Once the bore is completed, the machine is shut down and the cutting head is removed. The casing is then cleaned by rotating the augers in the normal direction. The torque plates are then removed to detach the machine from the casing and the augers are retracted until the coupling is well outside the casing. The auger section is uncoupled from the machine and the auger sections and is removed. The machine is then coupled to the next auger section and the process is repeated until all the auger sections are removed.
The well installation, as mentioned in the opening paragraph, is quite similar to standard vertical remediation well installation procedures. Once the borehole has been completed to the required length the well installation can commence. The Screen is slid into the borehole/ hollow stem auger and fitted with the required lenght of riser pipe. The Screen may prepacked with filter media guaranteeing the correct placement of sand along the screen. Additional sand may have to be blown in around the prepack screen to fill the annulus between it and the borehole. This is usually done through a tremie line using a compressor fitted with a series of environmental air filters to prevent oil contamination. The bentonite seal can then be blown in using the same technique to complete the installation.
Site Restoration :
Once all the augers have been removed, the boring machine and the tracks are removed from the pit. The boring pit and receiving pit are then backfilled to restore the site to its prior condition. It is important that the pit foundation is properly restored to prevent any differential settlement.