What many people may not know is that the drillers and engineers of Boart Longyear have been involved in some exciting historical drilling projects in a variety of other applications, too. Drilling equipment for moon exploration is one such example (read Drilling Beyond Earth); but closer to home, the company has assisted with projects to explore and define foundation and structural integrity below the earth or even under bodies of water.

In the late 1920s, members of the Longyear team were contracted to test drill the route of a proposed vehicular tunnel under the East River to connect Manhattan to Brooklyn in New York, USA. The tunnel was intended to reduce transportation congestion due to the slow and unreliable ferry service that existed at the time. The Longyear company provided preliminary boring samples used to determine what was beneath the surface of the river. Characterizing the samples’ type of rock and its hardness allowed geologists and engineers to determine what equipment should be used for tunneling, how long it should take, and at what cost. Although the initial tunnel project didn’t receive support and funding, it laid the groundwork for a later project—the Brooklyn-Battery Tunnel—which was started in 1940 and opened in May of 1950.

A similar challenge of traversing a wide body of water required assistance from the Longyear team in 1929. Joseph B. Strauss, a visionary engineer, was tasked with his team to design the proposed Golden Gate Bridge in San Francisco, California, USA. As chief engineer, Strauss requested the services of Longyear to investigate the rock formations beneath the San Francisco Bay where the bridge would be constructed. Longyear started boring in the Bay in November 1929 along the sites of the proposed piers and anchorages. [i]

Due to the turbulent waters of the channel, the drillers had to be creative as to how they would retrieve the core samples beneath the tumultuous surface. Strauss was big on safety, requiring bridge workers use safety lines, work over safety nets, and even wear some of the first hard hats. The Longyear crew constructed a platform over the water supported by a boom—an A-frame—and guy-wires anchored to the shore. The platform allowed the team to suspend their drilling equipment securely on top and safely above the bay. Despite the problems of deep water, ocean traffic, storms, and tidal movement, the team was able to finish the work on schedule with complete core sample recovery.

The geological samples and subsequent data supplied to Strauss proved that the rock formations could bear the weight of the foundations and enable the huge bridge structure to be built. The information was provided in a full report to the board of engineers in February 1930 and the bridge was determined to be “feasible economically and structurally.”

This great news meant Strauss and his design team could move on to the next phases of engineering and constructing the Golden Gate Bridge. In gratitude for Longyear’s contribution, Strauss wrote, “The work done by your company, having been entirely satisfactory, and the relations between us and your drillers having been cordial and pleasant, I am glad to make immediate payment of the balance due.” [ii]  

Payments received by the company totaled close to $15,000 for the boring contract. It wasn’t for another three years (January 1933) that construction began and a groundbreaking ceremony for the bridge was held. The Golden Gate Bridge was completed in May 1937 at a total cost of $35,500,000.

Click below images to view larger.

From left to right

Images 1 and 2: “Engineers Speed Plans for Golden Gate Bridge” - A Pacific Street and Road Builder article from March 1930 stating “test borings reveal solid rock foundations and showing a photo of boring operations at the Lime Point side of the bridge in Marin County, California.

Image 3: “Borings Started for Gate Span” – Photos shows in the Vallejo California Chronicle, Sebastopol California Times, and Cresent City Triplicate newspapers on November 29, 1930. The article shows a photo of Longyear’s diamond-tipped drills used for bore sampling the rock at the San Francisco end of the bridge, next to Fort Point.

Image 4: “Getting Down to Bedrock” – A San Francisco Examiner article, dated February 14, 1930, picturing engineers, including Strauss, inspecting boring samples from the Golden Gate Bridge project.

A truck-mounted LS600 sonic rig taking core samples near a dam site.

Shortly after the Golden Gate boring project, the Longyear company became involved with drilling projects for the construction of hydro dams around the country. In the 1930s, Longyear drillers did test borings for several of the twenty planned dams under the jurisdiction of the Tennessee Valley Authority. These dams were part of a comprehensive plan for the reclamation and development of the Tennessee River—covering an area of more than forty thousand square miles and over seven states.[iii]

Later twentieth-century construction-type projects for Longyear drillers included boring for core samples for the Missouri River Project, and the first foundation test borings for the Fort Peck Dam in Montana—the project’s first major hydro dam.

Today, Boart Longyear continues to assist with construction and rehabilitation projects around the world using even more advanced technologies and equipment. The company regularly uses sonic drilling technologies which are ideal for avoiding many of the challenges experienced with conventional drilling methods.

Sonic drilling offers quality control in conjunction with the production and installation of geotechnical construction elements. The sonic method can actually eliminate the need for circulation-type drilling allowing the driller to work safely in situations that involve sensitive structures, vulnerable site conditions, difficult geology, or have restrictive conditions. Sonic drilling also provides measurements and samples to evaluate actual conditions across a given site during the progress of exploration and construction.

A few examples of Boart Longyear’s sonic drilling assistance with tunnel, bridge, and hydro dam projects in the twenty-first century include: [iv]

• Casing installation for the Boston Central Artery / Tunnel Project CO9A4, Massachusetts, USA. As part of the process of installing three jacked tunnels under the 13 railroad tracks at South Station in Boston, specialized sonic drilling was used to install casing through some of the most difficult combinations of historical fill materials—steel rails, brick tanks, woodpiles, granite seawalls, reinforced concrete slabs, track ballast rock, clay, stratified sands, till and weathered bedrock.

• Installation of a grout curtain at Clearwater Dam in Missouri, USA. The project required a drilling method with the ability to penetrate and sample both the body of the dam and its foundation without the use of air or water circulation. Sonic drilling was used to investigate a deep sinkhole and affected a remedial plan involving drilling 15-degree battered grout holes through embankment fill to construct a grout curtain.

• Sonic drilling comparison for a tunnel design pilot study on the Combined Sewer Overflow Control program, Anacostia River Project in Washington DC. Sonic drilling was used to perform continuous soil coring with approximately 100% core recovery and careful installation of multiple piezometers.

• Micro-pile installation testing near the Holland Tunnel in New Jersey, USA. Sonic drilling was compared with conventional drilling to advance micro-piles through silty sand overburden, overlying till, and schist. Not only was the sonic drilling 33% quicker for installation of the micro-piles, waste was also eliminated to nearly the volume of the sonic core, which minimized disposal costs. 

Bridges, tunnels, dams, and even buildings are all safer because of the work done to investigate, test, and secure the foundations. Although considered a small contribution to these engineering marvels, Boart Longyear employees are proud of the heritage the company has earned and its involvement in each part of history.

For more information on Boart Longyear’s construction drilling capabilities, including sampling core, anchoring, micro-piling, jet grouting, and ground freezing, visit the overburden and construction tooling section.

[i] https://www.goldengate.org/bridge/history-research/bridge-construction/joseph-strauss/

[ii] Edmund J Longyear, and Walter R Eastman. 1984. “The Mesabi and Beyond”; Pg 181

[iii] Edmund J Longyear, and Walter R Eastman. 1984. “The Mesabi and Beyond”; Pg 192

[iv] George R Burnhart, Boart Longyear Technical paper. 2006. “Sonic Drilling Offers Quality Control and Non-Destructive Advantage to Geotechnical and Construction Drilling on Sensitive Infrastructure Sites.”

Golden Gate Bridge Photos from historical records, the Golden Gate Bridge, Highway and Transportation District and found here https://www.goldengate.org/exhibits/engineering-the-design/

Golden Gate Bridge Photo courtesy of Umer Sayyam

Edge Drilling director, John Argiropoulos, recently took delivery of a Boart Longyear LS250 MiniSonic rig that is suited to a variety of soft ground, shallow drilling projects including environmental, tunneling, water management, grade control, leach pads, and tailings dams.

The rig delivers faster penetration for nearly undisturbed samples using little or no fluids, said John, and its casing system makes it ideal for a variety of applications including mining, environmental, and infrastructure projects.

The rig provides extremely accurate sampling for unconsolidated formations which makes it suited to mining, but it also eliminates the risk of cross contamination making it ideal for environmental and geotechnical work. It can also precisely drill straight holes at varying angles which ticks the box for infrastructure projects.

“We've already got five months of work lined up with the rig," John told Australasian Drilling.

The idea of acquiring a sonic rig first came to John during a visit to Christchurch in New Zealand where he saw the rigs undertaking post-earthquake recovery sampling work.

“I've been looking at getting into this type of drilling from the first time I saw it,” he said.

“After the earthquake in Christchurch, I saw massive advantages with sonic rigs in drilling overburden and getting near on 100 percent recovery sampling. The rigs were drilling every house in carports or garages and were looking for signs of liquefaction.”

Liquefaction is where the strength and stiffness of soil is reduced by earthquake shaking. If liquefaction was found following testing in Christchurch, the dwelling would be demolished.

“They had conventional rigs at the start including standard diamond drilling, but they weren't getting any core recovery, being glacial fill," John said. "However, the sonic rigs collected everything from pebbles and cobbles, to rock, clay, silt and sand. They were very impressive.”

With work increasing, John eventually took the plunge and invested in a sonic rig.

“We're getting much busier,” he said. “In the space of 12 months, we've gone from two rigs at the start of last year to seven rigs now. We've expanded quite a lot.”

John said work began to resurge since 2016. A big push by Main Roads WA (Western Australia) for testing bridges across the state is creating work from the rollout of the government's METRONET program and is expected to generate more construction activity and geotechnical work in the near future. The METRONET program is one of the largest single investments in public transport that Perth has seen and will involve about 72 km of new passenger rail and up to 18 new stations.

With enhanced capabilities thanks to the new rigs, John is upbeat in his outlook for the business. However, like most contractors, he is feeling the pain of skill shortages. John said he is managing the issue through in-house training.

"I'm a firm believer of training in-house as the best way forward because we can train personnel according to our standards, expectations, policies, and procedures.”

Originally published in Australasian Drilling Magazine, February/March 2020

Typically, manufacturing sonic drill rods is a three-piece construction process in which the rod ends are friction- or slip-fit and welded onto a mid-body. This allows manufacturers to use lower grades of steel in the mid-body to save on costs.

Boart Longyear utilizes an upset forging process in manufacturing its sonic drill rods. Upset forging refers to the process in which a drill rod begins its life as one single piece of high-grade steel tubing and is internally forged to ½” thickness on the thread ends which transitions to ¼” mid-body wall thickness. This eliminates welding the thread ends onto the mid-body, creating a significantly stronger one-piece-design sonic drill rod. 

At a high-level, the six manufacturing steps for sonic drill rods are:

1. Begin with high-grade steel tubing having 3 ½” outer diameter and ¼” wall thickness
2. Upset forge the rod ends to 9” in length and ½” wall thickness
3. Stress relieve the entire rod (heat and cool slowly)
4. Heat treat the rod pin and box ends (quench and temper) for strength and durability
5. Machine the pin and box tapered threads into the upset forged ends
6. Heat treat the pin thread a second time to a slightly higher hardness (increases life, prevents galling (rods seizing up into each other during drilling) and increases wear resistance)

This process, which draws on our vast experience manufacturing diamond coring rod, creates a 3 ½” outer diameter rod which has a 3” inner diameter mid-body and 2 ½” inner diameter thread ends. The rod ends are ½” wall thickness to 9” in length, while the mid-body is ¼” wall thickness, keeping the rod relatively light but extremely durable.

When you need the best and most efficient sonic tooling, look no further than Boart Longyear. High-quality precision steel plus special heat treatment processes provide unsurpassed stability, straightness, and durability of the sonic drill string assembly. Stringent manufacturing standards and ISO certification make sonic tooling from Boart Longyear the right choice for every sonic drilling project.

Contact your local Boart Longyear representative or distributor today.

Quality Policy:

Boart Longyear is committed to providing quality products, innovative solutions, exceptional service and value to our customers while meeting or exceeding all customers’ safety, environmental and regulatory requirements.

Following the advancement of the inner drill pipe and core barrel, the outer drill casing is advanced to within one foot of the leading edge of the core barrel.

The outer casing serves the same purpose as for conventional dual-line drilling systems by holding the borehole open for well installation, geophysical logging or other down-hole activities. Depending on drilling formations, drilling fluid may be introduced during the advancement of the outer drill casing.

Sonic drilling also greatly reduces the risk of project failure due to unknown or difficult subsurface conditions. It also offers the flexibility of advancing a temporary outer casing as the borehole is drilled, meaning more can be accomplished with a single borehole.

Although adoption of the technology has been slow, sonic drilling is now being seen as the solution for a variety of needs. In Canadian oil fields, for example, where soft rock formations predominate, companies are increasingly coming to recognize sonic’s advantages over both conventional rotary drills and auger drills for many applications.

After the 2014 failure of the Mount Polley tailings dam in British Columbia, the provincial government mandated the installation of monitoring wells and piezometers on tailing piles throughout the province. The difficult accessibility of many sites and the potential instability of the pilings made Boart Longyear’s new LS250 MiniSonic rig an obvious choice for the job.

The reliable, compact sonic rig is proving to be the ideal solution for a wide range of applications, including geotechnical, environmental, water and mining. Its compact size allows it to be easily transported to hard-to-reach sites like the tailing piles or to work in tight spaces.

Such was the case in downtown Calgary, Alberta, where a major sports and entertainment complex is proposed for an underdeveloped riverfront site that now includes auto dealerships and a bus station. Although a creosote plant there closed decades ago, creosote extends several meters below the surface across much of the proposed development area.

A recent environmental investigation required drilling sampling holes at multiple points across the property. Sonic technology and the rig’s maneuverability meant mission accomplished with practically perfect core samples.

Sonic drilling employs the use of high-frequency, resonant energy to advance a core barrel or casing into subsurface formations. During drilling, the resonant energy is transferred down the drill string to the bit face at various sonic frequencies. Simultaneously rotating the drill string evenly distributes the energy and impact at the bit face.

The resonant energy is generated inside the sonic head by four counter-rotating weights. A patented pneumatic isolation system prevents the resonant energy from transmitting to the drill rig and preferentially directs the energy down the drill string.

The driller controls the resonant energy generated by the sonic oscillator to match the formation being encountered to achieve maximum drilling productivity. When the resonant sonic energy coincides with the natural frequency of the drill string, resonance occurs.

This results in the maximum amount of energy being delivered to the face. At the same time, friction of the soil immediately adjacent to the entire drill string is substantially minimized, resulting in very fast penetration rates.

Sonic offers several distinct advantages over conventional technologies. Among them is the superior information provided by the continuous, relatively undisturbed core sample of unparalleled quality and accuracy through any type of soil — clay, till, loose or heaving sand, gravel, boulders or cobbles. When using the iso-flow groundwater profiling system, hydrogeological and geochemical data can be easily obtained.

Sonic drilling also reduces drill spoils and waste by up to 80 percent relative to conventional drilling methods. Unlike drilling with an auger, virtually no cleanup is necessary.

Superior well construction is another advantage. Sonic drilling causes minimal disturbance to the surrounding borehole wall, resulting in more efficient well development and performance. Then there’s the speed. Sonic drilling is up to two to three times faster than conventional overburden drilling methods.

Safety features of the LS250 MiniSonic rig include an interlocked rotation barrier, reduced noise levels, a dump mast and wiggle tail, and a rod presenter. The interlocked rotation barrier automatically slows head rotation when the barrier is open. The rig also provides lower noise levels when equipped with the Tier 4i engine package. 

The articulated mast and wiggle tail enable the mast to shift from left to right and front to back to position the mast precisely over the hole, eliminating time-consuming rig movements. The dump mast allows the crew to work from the ground, enhancing safety by avoiding stairs and safety rails often required when working from a platform.

The innovative rod presenter allows the rod and casing to be loaded horizontally, with an actuator that then presents the rod and casing vertically to the head. The head rotates 28 degrees to the side for sample extraction, while the head slide shift allows for unobstructed winch use down the hole.

The LS250 MiniSonic rig is capable of drilling to depths of up to 250 feet (78 meters) when used with a 4.75-inch (121 millimeter) casing. Its “big brother,” the LS 600 Sonic rig, can go even deeper — up to 600 feet (182 meters), as its name implies. The Boart Longyear lineup also includes high-quality sonic tooling, from bits and casing shoes to sonic rod, core barrel, casing and accessories.

As awareness of sonic drilling’s advantages continues to grow and as more and more geotechnical engineers specify sonic, the market will likely demand continued innovation and sonic drills capable of even deeper, larger-diameter samples. And Boart Longyear, already at the leading edge of sonic drilling, will continue to lead the way.

Sonic drilling offers distinct benefits:

Superior information: Since the sonic cutting action sheers the soil cleanly and no fluid is present to dilute the sample, this method results in a relatively undisturbed sample through any type of soil. You get at or near 100 percent core recovery, continuous to the depth, which means superior subsurface information for decision making.

Speed: The combination of vibration and slow rotation allows the casing to quickly advance through unconsolidated formations, providing a penetration rate up to three times faster than traditional overburden drilling.

Reduced waste: With efficient cutting and no fluid involved, drill spoils and waste are reduced by up to 80 percent compared with conventional drilling.

Accurate to depth transitions

Uranium formations

These advantages give miners more control in planning projects and managing resources. The use of sonic drilling minimizes the risk of a project failing or being delayed due to unknown or difficult subsurface conditions, and decreases overall time and costs. It also lessens the impact to the environment.

Because of these benefits, sonic drilling works well for a variety of mining applications. Drillers can sample unconsolidated ore piles and tabulate reserves from legacy stockpiles, whose medium- to low-grade ores can be efficiently processed today. They can also study leaching effectiveness, pinpointing dry zones and optimizing the leaching process of desired minerals.

These features also make sonic rigs ideal for shallow drilling in overburden or precollaring before a conventional drill takes over to drill deeper into bedrock.

Sonic drilling is well suited for geotechnical studies for mine construction and other building projects. Additionally, because some sonic rigs are smaller and have very low ground pressure, they can operate successfully in environmentally sensitive areas; for example, they can drill without ruining asphalt or landscaped areas or sinking into marshy, swampy land.

While sonic drilling has been employed by the mining industry in the United States since the late 1980s, it is still an emerging technology in many countries. Boart Longyear has been a forerunner in sonic technology and has the world’s largest selection of advanced sonic rigs and highly trained drillers. Time and again we’ve experienced the exceptional subsurface information and other benefits this specialized technology can offer drillers, especially in the difficult formations that often frustrate the search for minerals.

Heap leach pads are large piles of rock waste from a mine operation that receive further processing through a leaching solution to remove the mineral ore from the rock. The leaching solution is applied via an irrigation or sprinkler system to the top of the heap leach pad. The solution penetrates the heap leach pad and separates the mineral ore from the rocks. At the bottom of the heap leach pad is an impermeable liner that is angled to allow the solution to be collected in a reservoir and then be sent for further processing.

Using one of their sonic rigs, the LS600, Boart Longyear can collect highly representative samples and achieve excellent sample recovery. Leach pads are made up mostly of unconsolidated material and the LS600 has proven effective in these conditions by producing 100 percent accurate in-situ core samples. The rig is also used to install wells in the heap leach pad.

Boart Longyear was contracted to sample and install solution wells for the largest gold heap leach pad in the USA, located in Northwest Nevada. The heap leach pad is over 400 feet tall. To achieve that height the mine operation built up the heap leach pad via 50 foot lifts.

When constructing each level, the haulers unavoidably crush and compress the previous top layer of the pad while adding material to the top of the pile. This creates a dense layer that causes a dam effect and does not allow the leaching solution to penetrate through the heap leach pad properly. The heap leach pad can then develop dry pockets where no leaching activity takes place — lowering the efficiency and return on investment.

Boart Longyear was hired to detect dry zones in the leach pad and to reactivate the zones via solution enhancement wells. First the Boart Longyear team drilled test holes to determine the depths of the dry zones and the specific lithological (physical characteristics of the ground) information.

These boreholes were 107 meters (350 ft) deep and were 15 to 23 meters (50 to 75 ft) above the heap leach pad liner. A continuous core sample was obtained by using the LS600 sonic rig. This allowed the Boart Longyear team to determine moisture concentration at accurate depths and to identify exposed ore.

Once depths of ore and moisture levels of the dry zones were indicated, Boart Longyear installed solution enhancement wells using an 11.5-centimeter (4.5 in) flush threaded well casing. The casings allowed the borehole to stay intact as an upper and lower k-packer were installed at specific depths. A 100-foot injection screen was placed between the k-packers to direct the leaching solution into targeted dry zones.

This process was repeated on an eight-week cycle with four weeks of solution and four weeks of rest. Once the cycle was completed, the k-packers and screen were moved to a new depth. The well would receive the solution at three heights, spanning 100 feet each time — bottom, middle, and top.

Over a three-year period, the team, using the LS600 sonic rig, drilled 55 gravity flow solution wells to alleviate the fine unprocessed gold from the heap leach pad. This resulted in 6,100 ounces of gold that equated to $8 million ($1,300 per ounce average). The project paid for itself within the first 70 days and the client received a 400 percent internal rate of return (IRR).

Sonic technology utilizes high frequency resonance to eliminate or minimise the friction between the subsurface material being encountered and the tooling/core barrel being advanced. This allows both efficient penetration and maximum core recovery in a variety of subsurface conditions. During drilling, the resonant energy is transferred down the drill string to the bit face at various sonic frequencies. Simultaneously rotating the drill string evenly distributes the energy and impact at the bit face.

Sonic drilling, utilizing an innovative casing advancement system, all but eliminates slough, providing improved sample integrity even through limestone, dolomite, sand and other unconsolidated material.

Sonic drilling provides a continuous and relatively undisturbed core sample of unparalleled quality and accuracy through any type of formation. With less than 1% deviation, drillers, geologists and environmental scientists can be sure they know exactly where a sample is from.

Track-mounted rigs offer newfound mobility at remote drill sites, their goat-like versatility providing accessibility beyond the capability of even the most super duty truck. More compact drill pads – one-third to one-half smaller than conventional pads – reduce the need to remove trees and grade land to provide a workable drill site.

The LS250 MiniSonic’s weight and size make it perfectly suited for jobs in the most sensitive and fragile terrains and space-constrained environments, hard to reach or not.  Its smaller footprint makes it versatile with small pads and environmentally sensitive areas, and requires less support equipment.

The rig also offers an 11 metric ton dry weight with high floatability rubber tracks. Low ground pressure (4psi) allows the drill to access more difficult terrain like marshy or wet pads, and its 24-inch (600mm) rubber tracks let it float on softer ground, further increasing its versatility and adaptability to the toughest and most environmentally sensitive terrains.

Among the LS250 MiniSonic’s safety features are an interlocked rotation barrier, reduced noise levels, a dump mast and wiggle tail, and a rod presenter. The interlocked rotation barrier automatically slows head rotation when the barrier is open.

“We want to help our customers drill better, and part of that is sharing with them the type of information and insights they need in order to help create a convincing argument for sonic drilling.”

The resonant energy is generated inside the Sonic head by two counter-rotating weights. A pneumatic isolation system inside the Sonic head prevents the resonant energy from transmitting to the drill rig and preferentially directs the energy down the drill string.

The driller controls the resonant energy generated by the Sonic head’s oscillator to match the formation being encountered to achieve maximum drilling productivity. When the resonant Sonic energy coincides with the natural frequency of the drill string, resonance occurs. This results in the maximum amount of energy being delivered to the face. At the same time, friction of the soil immediately adjacent to the entire drill string is substantially minimized, resulting in fast penetration rates.

While there are several ways to drill using Sonic (depending upon site-specific conditions and project objectives), the most common means involves advancing a core barrel, which is overridden by a larger diameter drill string that cases the open bore hole and prevents collapse.

The core barrel is advanced using sonic frequencies. When necessary this step can be performed using no fluids, air, or mud.

After the core barrel is in place, casing is sonically advanced over the core barrel, protecting the bore hole’s integrity in loose unconsolidated ground.

The core barrel is retrieved, producing a relatively undisturbed sample with near 100% core recovery.

Steps 1 through 3 are repeated to depth, producing a continuous core sample through unconsolidated formations with less than 1% deviation.

Waste rock dumps are often placed adjacent to mine pit areas, and as mining expands more room is needed for the waste rock. Certain physical and chemical characteristics of the waste rock dumps are often needed to better understand the geotechnical and geochemical behavior of the large dumps and to evaluate potential mineral value. Sonic drilling enables the collection of highly representative samples and excellent sample recovery. This method of drilling has been used for the re-exploration of dumps, tailings and heap leach pads.

Waste rock dumps consist of mainly unconsolidated material – and it’s often difficult to know how deep they are or what types of material they contain – which can lead to challenges in drilling. Sonic technology provides a solution by being able to produce 100 percent accurate in-situ core samples through varied ground conditions.

Boart Longyear Drilling Services took on the challenge of drilling core samples from the unconsolidated waste rock dump at Kennecott Utah Copper’s Bingham Canyon Mine. The intent of the drilling was to define the contents of the waste rock dump. The waste rock dump consisted of rock blast material ranging in size from 254 to 304.8 millimeters (10 to 12 in) in diameter and was made up mainly of porphyry deposits (granite-like rocks).

With a targeted depth of 213.36 meters (700 ft), the main goal was to provide a detailed continuous sample of the waste rock dump material and confirm bedrock depth. Boart Longyear also needed to install piezometers (water level monitors) and lysimeters (moisture content monitors). Geotechnical samples would also need to be taken every 6.096 meters (20 ft) to confirm stability and moisture content for the first 60.96 meters (200 ft).

Sonic drilling is the perfect method for drilling in unconsolidated material, such as the waste rock dump at the Bingham Canyon Mine, because of its sample recovery rate, straight cased borehole and the flexibility to offer geotechnical sampling via the split spoon sampler.

As drilling commenced, Boart Longyear took on the task one step at a time by tackling the depths in stages. For the first 106.68 meters (350 ft), they drilled a 228.6 millimeter (9 in) borehole while tripping the drill string every 6.096 meters (20 ft) to pull a split spoon geotechnical sample for the first 60.96 meters (200 ft).

For the second stage, the team drilled to 152.4 meters (500 ft) using a 203.2 millimeter (8 in) bit with casing. Moving deeper to 228.6 meters (750 ft), they used a 177.8-millimeter (7 in) bit with casing for the third stage. Surpassing their targeted drill depth of 213.36 meters (700 ft), the drillers still had not reached the bedrock formation.

Needing to find the true depth of the waste rock dump, Boart Longyear felt like the rig still had the capability and pullback to go deeper. Easing forward they drilled to 264.261 meters (867 ft) using a 152.4-millimeter (6 in) bit with casing. The last stage couldn’t be drilled with casing as they needed to move to a 101.6-millimeter (4 in) borehole. Leaving only the bit for the final push to 274.32 meters (900 ft) – and setting a new Boart Longyear record for sonic drilling – they reached a 28 percent greater depth than initially targeted.

It took 16 shifts of 12 hours (192 hours) to accomplish the new record depth for sonic drilling. Boart Longyear lost two of those shifts (24 hours) to rain. Another key accomplishment was the entire depth was reached through dry drilling and achieved 100 percent in-situ core samples at less than 1 percent hole deviation.

Advanced sonic technology and highly trained sonic drillers can reach depths of over 700 feet while obtaining near 100 percent in-situ core samples. Boart Longyear’s Fred Hafner offers 7 tips to get the most out of sonic drilling.

Knowing when and where to use sonic drilling over conventional drilling methods is critical to delivering cost effective results.

There are three main reasons to choose sonic drilling technology:

1. Projects that require a continuous in-situ sample to be collected — as opposed to reverse circulation (RC) drilling where chips are collected. 

2. In situations that require avoiding fluid and air while drilling. No fluid or air is needed in sonic drilling, making it a good method for geotechnical, geo-construction, and environmental applications.

3. For projects where unconsolidated ground formations are encountered. Sonic drilling is a good solution because it can handle pebbles up to boulders while providing borehole integrity through a continuous casing process.

Ground hardness ranges from unconsolidated all the way up to 10 on the Mohs scale of mineral hardness. In unconsolidated ground, pay attention to the rock sizes and adjust the core barrel accordingly. For example, if rock sizes are 3 to 4 inches in diameter, the driller should select a core barrel that is at least 6 inches in diameter. Ensuring an adequate core barrel size allows for the drilling samples to pass through the core barrel for easy collection.

Friction is the enemy. Telescoping the boring reduces friction to take the boring deeper. Core barrels should be sized to the casing size.

Precise ground transitions

River bottom gravel

Alluvial sands sonic sample

Nickel laterite sonic sample

The best sonic drillers don’t just watch the gauges to know when to pull back or push forward. Experienced drillers running the sonic drill string, rely on their other senses; they can feel, touch, and hear how the drill string is progressing. Experienced drillers also rely on their knowledge of the drill site and ground formation. And the best drillers know when to adapt their methods as the hole deepens and encounters new ground formations.

When dealing with a couple hundred feet of overburden, pre-collaring with sonic drilling can be a more  efficient way to reach the target depth. It is a quick and clean method that can also pull better information from the overburden — especially unconsolidated formations. Pre-collaring in sonic drilling provides an in-situ sample of the minerals and can better detect/protect against any water infiltration.

Although sonic drilling technology has been around for more than a couple of decades, the technology is still new to some.  When the time is taken to engage with the customer and explain the process and technology, the benefits of the method are clear.

Safety is the most important aspect to any drill site. Incorporate and promote a safety culture. Encourage drill crews to think before acting — take five minutes before doing anything. This allows the crew to focus on the task at hand, one task at a time.