Services – Keystone Foundation, LLC

Our Services

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.

Drilled Shafts

Drilled Shafts are high – capacity deep foundation elements typically used for bridges and high rise buildings with diameters ranging from 18” to 12’ diameter.

Are typically preferred when high axial and lateral loading conditions exist. They can be designed to penetrate most soil profiles (e.g. loose soils to hard bedrock). Typical diameters can range from 2’ diameter up to 12’ diameter. There are several methods that can be used to install them per the following:

Dry Methods – Appropriate where the water table is low with the presence of medium dense soils and ideally cohesive in nature. Typically, a temporary casing is first installed to seal and protect against possible collapse of fill soils while providing safety protection against an open excavation (by extending the casing above working grade). If the looser soils are sealed off with the initial temporary casing then the shaft can be advanced without additional casing but must be continually monitored. A “full-length” temporary casing may be required if the uncased portion of the shaft does not remain open. This is the most economical method for installation.

Wet or Slurry Methods – Appropriate when water table is higher but requires a detailed study of the subsurface information to confirm whether the presence of artesian, running sands, organic soils, etc. exist which could all be detrimental to slurry methods. A detailed slurry mixing, testing, maintenance, and management plan must be developed prior to mitigate possible issues during construction. Can be advantageous when soils are not conducive for dry methods or cased methods as described below.

Cased Methods – Drilled Shafts are required when ground conditions are so unstable that drilled holes cannot safely be stabilized with drilling slurry or where loss of ground must be controlled. Casing can be temporary or permanent steel pipe which provides a 100% stable excavation for the full length of the drilled shaft. Casings can be installed by high-capacity impact or vibratory hammers when noise and vibration are of no concern environmentally or to surrounding structures. When using impact or vibratory methods, casing wall thickness needs to be evaluated to ensure over-stressing of the steel does not occur. The available geotechnical information must be evaluated to ensure the energy of the hammers can overcome dense soils or very cohesive soils or risk early casing “refusal”.

If a single casing cannot be installed to the required tip elevation, then multiple “scoped” casings can be used. In this method, oversized casings are installed to various lengths until the inner most casing (diameter of casing matching the design shaft diameter) can be installed to the design tip elevation.

One final method to install drilled shafts is by using high torque oscillator or rotator machines to advance heavy wall steel casing into the ground concurrent with the excavation without any vibration or ground loss. Either permanent steel casings or sectional temporary (removable) casings can be installed over a specified depth or the full length of the drilled shaft. Temporary drill casings are installed in segments of typically 10 to 25 feet and joined with a double wall bolted connector, providing a flush outside diameter of the casing. Carbon teeth on the casing tip allow for greater installation depth and enable the advancement through minor obstructions as well as penetration into rock for a seal. As the casing is advanced the material inside the casing can be removed with a drill rig or crane hydraulic grab bucket.

Auger Piles

(Auger Cast-In-Place or Drilled Displacement) Auger piles are deep foundation elements typically used for mid to high rise buildings with capacities similar to driven piles but can be installed in vibration sensitive environments.

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.

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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.

Helical Piles

Helical piles are deep foundation elements that can be installed in vibration and environmentally sensitive environments.

Support of Excavation

Can utilize any of the deep foundation elements described above.

Secant Piles

These are rigid and often times permanent earth retention elements preferred to a traditional system for various reasons (for example basement walls with limited room for a temporary system, limiting verticality deviation and adjacent structure movement)