Keystone specializes in most deep foundation systems and our site provides a brief synopsis of each. For any foundation system, it is paramount to understand the geology and ground conditions and allow the data to dictate the most appropriate system. We welcome any opportunity to assist in this endeavor to work with potential clients on reviewing provided information to ensure the best value is achieved given the ground conditions. We would also look to advise of any constructability issues and opportunities to improve design.
Are preferred when loading is spread out over several columns (e.g. building versus high bridge loading which is more ideal for drilled shafts) keeping design loading per column location between 50 ton to 400 tons in compression. Typical diameters range from 12” up to 36” diameter for the higher building column loads.
These types of piles come in slightly different forms and connotations as seen below. In general, there are two main distinctions for auger piles. The first are continuous flight auger piles that mainly displace the soil upward along the drill string and out of the excavation. The second are drilled displacement piles that mainly displace the soil outward or laterally away from the drill string.
CFA (Continuous Flight Auger) Piles – Typically ideal for cohesive and medium dense soils which allow for reasonable penetration rates and grout factors. Piles shapes and yield in these soils tend to be more predictable and allow for a more efficient operation. CFA piles are constructed by rotating a hollow stem continuous flight auger to the required depth. Grout is pumped through the hollow stem (while maintaining a consistent rotation at all times) to the desired grout head (typically 10’). At this point, auger extraction begins while monitoring grout volumes at every 5’ to 10’ levels. Grout volumes are measured directly in the field by counting grout pump strokes while visually inspecting an in-line grout pressure gauge to ensure the pile is installed in a continuous manner without defects. An AME (automated monitoring equipment) can also be used for measuring grout volumes, pressures, depths, RPM (rotations per minute), etc. through the use of sensors installed on the drill rig.
As the auger is extracted upward above ground, the spoils on the flighting are removed by either naturally falling off the flight or mechanically with a separate attachment (e.g. auger cleaner)
Grout should be visible prior to auger extraction (i.e. return). Finally, the reinforcing element is placed through the freshly placed grout primarily under its own weight.
Typically, prior to production indicator piles are installed to help predict anticipated grout factors and grout quality (minimal bleed, consistent slump, etc.) before going into production. It is recommended to install the reinforcing element in the indicator pile for the full depth to ensure consistent grout to the pile tip.
DD (Drilled Displacement) Piles – Appropriate in looser (more displaceable soils) to minimize tool wear. Overly dense soils can also cause excessive heating of the drill stem with a potential to affect the grout inside during placement (stiffen or flash the grout). The piles are installed in similar fashion to the CFA piles except without the auger flighting (smooth wall drill string with a conical tip) and by displacing the soil laterally (instead of upwards). With minimal spoils generated these piles can provide a significant advantage if the soils are contaminated while allowing for higher skin friction during borehole densification. Auger attachments can be used in more dense soils when not in ideal soil conditions to help loosen the drill soils ahead of the displacement tool tip.
Ideal for loose to medium dense soils. These minimal vibration piles do not generate spoils which is advantageous in contaminated soils. The set up is relatively simple requiring a track mounted excavator and hydraulic rotary drive unit. The pile consists of a starter/lead section with flighting/helix to loosen the soils as the pile is advanced by adding extension pieces until the required depth is reached. The lead sections are bolted together with sections added as the pile is advanced below grade. In low overhead applications short 2’ to 5’ extension sections can be utilized. They are typically more economical compared to other deep foundation systems when loading conditions are lower.
The piles are advanced until the maximum target torque is reached and is measured as a function of differential hydraulic pressures across the drive unit. Capacities are typically limited to the hydraulic flow of the excavator (unless an external power unit is used) and therefore the larger the excavator, the larger capacity pile can be installed. As a rule of thumb the excavator required size is roughly 15% of the ultimate pile capacity. For example, a 100 Ton Ultimate (50 Ton Design) might require a 15 Ton Excavator.
Any of these elements can be used to either temporarily or permanently retain soil or rock when excavating below grade. Keystone can design the structural steel elements required to meet deflection requirements. Once the member is sized, an analysis of the subsurface is performed to confirm the best way to install the piles. If very dense materials (rock or boulders) where driving the pile or installing a helical element may not work, a temporarily cased drilled shaft could be the solution by placing the pile inside the shaft and installing concrete as required and removal of the temporary casing. CFA piles can also be an efficient way to install an SOE system in the appropriate soils where the pile is placed through the fluid grout.
Using drilled shaft methods, Keystone can install this wall of overlapping elements up to 48” in diameter. These shafts are installed using double walled casing (similar to the oscillator methods described above). They are typically designed with a “primary” shaft/pile that is unreinforced with the adjacent overlapping shaft which is considered the “secondary” shaft/pile. Secant walls have various advantages per the following: – Help mitigate groundwater seepage into the excavation without a separate dewatering system – Can provide for a deeper cantilevered wall – Can be installed in challenging geologies (dense material, cobbles) facilitated by the use of heavy wall temporary casing (up to 1.5” wall thickness) and casing cutting shoes/teeth – Ideal for varying rock elevations where additional sections can be bolted on or removed based on where rock is encountered. – Can carry high vertical structural loads – If used as a permanent wall, can reduce spatial requirements by eliminating a temporary wall and a “set back” required to install the separate permanent wall.