There are many reasons a project requires the digging of boreholes. Most commonly, their purpose is for the extraction of liquids or gases from underground repositories. While workers take great care in selecting the sites and determining the size of the borehole, there are times when the expansion of a predrilled hole is necessary.
When to Expand Boreholes
In traditional boring, the average drill bit head is 20 to 30-inch diameter. During the process, the drill bit penetrates the earth to a predetermined depth in which workers add the casing. This metal pipe is smaller than the overall diameter as a space between the wall, and the housing is left and filled with concrete to help ensure structural stability.
The team continues to drill, this time with a smaller bit that must fit down inside the first casing. When they reach another depth of instability, they add another layer of housing and concrete. The second pipe is smaller than the first, resulting in an even smaller drill bit to continue. As a result, the borehole at the bottom may be as little as five inches wide.
Over time, this narrow aperture may not be large enough to produce the returns necessary to keep the site operational. To continue at the current location, workers need to either dig a new borehole or expand the existing one.
The Process of Expanding a Borehole
When expanding a borehole, similar precautions must be taken to ensure structural integrity and both personnel and equipment safety. Before expansion, engineers must consider the ground composition and internal and external stressors that may cause borehole collapse upon expanding the diameter. (Learn more in "How to Avoid Tunnel Collapse When Boring Big.") It is imperative to look out for the signs that the borehole is losing integrity.
For this geotechnical engineers need to run geotechnical investigations and put together geo reports with the proper information in order for the boring team to choose the proper equipment for the project. (Read on in "Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.")
Traditional expansion methods usingcompressed air to push a reamer through the existing hole. Sediment from the expansion either forms a layer at the bottom of the hole or forced out of the borehole due to the velocity of the air current. Unfortunately, the use of a compressed air reamer is only usable for specific soil types.
New expansion technology, however, makes it possible to expand boreholes in a variety of soil conditions. Instead of using traditional casing, some companies opt for expandable tubes. As with conventional methods, workers drill the hole and insert the housing. However, after pouring the cement but before it sets, an oversized steel cone pushes through the line, expanding it 10 to 15 percent.
The new technique allows for a slimmer beginning well design requiring less concrete. Use of expansion casings also provides for a deeper drill without the telescoping effects of traditional boring.
Tools for Borehole Expansion
To expand boreholes using traditional methods, there are few tools required. Workers use a pneumatic punch, which consists of a frame, striker, and sleeve to form the front and back chambers. Inside the unit is a reamer. The system attaches to compressed air which forces the reamer through the existing hole.
Depending on the borehole design, workers may need to ensure the velocity of the compressed air is high enough that it carries the new sediment out of the hole.
Newer expansion technology begins with the same tools utilized to drill the borehole. With the use of expandable casings, workers may use smaller drill bits initially. In addition to drill bits, slurry, cement, and casings, the worker needs an expansion cone. The cone passes through each stage of the housing, expanding it to the required size. While the use of an expansion casing does negate the need for telescoping during drilling, workers may choose to narrow the passage by using different sized expansion cones.
Borehole expansion may be a requirement over time. While traditional reaming methods can give workers the desired outcome, expanding casing allows for enlargement without the added expense of casing the borehole again upon completion.