Limited-access micropile applications include underpinning existing structures, supporting new loads in dense urban areas, preventing structural settlement and excavation support in confined spaces. Angle drilling capabilities are often required for the installation of soil nails and tie-back anchors, as they are commonly installed on an angle to stabilize and reinforce the face of a slope or a wall. At times, the drilling must be carried out inside of the building or in narrow alleyways, enabling structural improvements with minimal disruption to existing infrastructure. Micropiles can often be reasonably shallow, but they often require large-diameter boreholes through difficult overburden. Standard continuous flight or hollow stem auger drilling is commonly employed and requires sufficient rotary torque. Some limited-access drills can be equipped with more than 10,000 foot-pounds of rotary torque — power equivalent to many large truck rigs.
GeoExchange systems (also called ground source heat pump or GSHP systems) are no longer limited to new inner city developments. Low-headroom drilling rigs (some as low as 87 inches) can operate under existing buildings, in basements and parking garages, supplying retrofit GeoExchange systems to apartments, condos, high-rise offices and hotels. Some limited-access rigs are equipped with enough pullback, down force and other power specs to drill more than 800 feet deep. To ensure that unwanted fumes remain outside, electric motors can be used to power the rig and support equipment. Another option is a separate diesel power unit that can be connected up to 200 feet away with umbilical hydraulic hose extensions and left outside the building along with the air compressor (if down-the-hole hammer drilling). Cuttings from both mud and air drilling can be contained, separated and recycled inside specialized cleaning systems that separate the waste from the drilling medium if required. The cuttings can then be safely removed from the site.
Geotechnical investigations are often required in backyards, laneways and other confined sites to assess potential construction sites and to confirm subsurface conditions. Compact rigs with width-adjustable crawler tracks can easily navigate a residential site through the garden gate. Some rubber-tracked units have width-adjustable undercarriages, enabling the rig to drive through a standard size doorway, down a flight of stairs, around a tight corner and into a low-headroom basement. Once in place, the tracks can then widen out for extra stability, equipping the driller with far more power than a man-portable unit. While man-portable units are often used due to their mobility, they do have their limitations. Installing casing in collapsing ground conditions can be challenging or even impossible, depending upon the outside diameter of the casing and depth required. Drilling depth capacity can be limited. Difficult ground conditions may require more power than the man-portable unit can offer. However, man-portable drills are simple and inexpensive to transport and maintain. Their compact size allows entry to the most restricted-access drill sites, and one person can easily operate them.
Rig features and options are also important considerations for limited access drilling. For example, wireless remote controls for tracks, drill feed and drilling functions greatly increase safety and maneuverability, though this can be an expensive option. Instead of walking beside the rig through a narrow opening or up a dangerous slope, or trying to position the drill in a confined space, it can all be positioned from a safe and comfortable distance using the remote control pack. Mast slide functions can further aid in positioning the rig. Head slide functions allow for multiple drill heads and accessories including high torque heads, high speed coring heads, standard penetration test hammers, drifters, etc. The ability to perform various drilling functions from one set-up location can be extremely advantageous, and the head slide function also gives clear access to the open bore for sampling. Additionally, hydraulic breakout clamps that make tool handling hands-free, and therefore much safer, can be especially useful when working in confined spaces.
Drilling in highly developed big cities very often calls for limited-access equipment. Barrie Paddock is manager at PHS Solutions Ltd. in the United Kingdom, which specializes in limited access foundation drilling.
“Over the past few years land price has become a premium and, along with the housing market’s slow turnaround, extending current properties has become the ‘new build’ option for many,” he says. “Limitations in size of plot and geographical location are two vital considerations. This is very typical in London where the option to extend sideways and upwards is rarely an option. In these instances, down is the only way — creating basement living areas which allow a complete redesign of the existing property.”
Paddock describes a typical project in London that saw “a classic Georgian property being redesigned to include a basement area and redevelopment of the existing property. This was created using a ‘top down’ building approach whereby the ground floor and internal ground floor walls were removed where practicable. The first and second floors and walls remain in situ, supported by temporary steel beams. Our micropiling rig was driven through the front door of the property and then through the home whereby we proceeded to install 18 piles to a depth of 16 meters (52.5 feet). In addition to the structural piles, a contiguous piled retaining wall 80 meters (263 feet) around the perimeter of the building was created to enable the basement works to commence with the reduction of the ground levels.
Maneuvering the rig in such confined spaces required a separate power unit that could be connected with umbilical hydraulic hoses to ensure the rig could be positioned to suit pile locations. The Techno Drill TD308 micropiling rig was the absolute perfect rig, offering the versatility for maneuvering and power to execute the piling requirements.”
Paddock concludes: “Without the easy and manageable maneuverability of the TD308, undertaking many of the limited-access projects we see would simply not be possible. It is also paramount that once you have the rig in position it is able to execute the pile methodology and pile design requirements.”
Brandon Wronski is sales executive of Hammer Drilling Rigs, a North Carolina rig and equipment supplier. For more information, contact firstname.lastname@example.org or visit www.hammerdrillingrigs.com.