Difference between revisions of "718 - Drilled Shafts"
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Revision as of 09:10, 19 March 2014
Drilled shaft foundations are also referred to caissons, drilled piers, or bored piles, and consist of circular shafts of varying diameter, drilled to a design depth, reinforced with a steel rebar cage, and filled with concrete. Drilled shaft foundations can support large loads, and are used when driving steel piles is not feasible, or surrounding structures or utilities are too sensitive to survive the vibrations from steel foundation pile driving operations. Like steel foundation piles, loads from the superstructure transfer into the drilled shafts, which transfer loads to soil layers through side friction, end bearing, or a combination of the two. Drilled shafts are used for bridge foundations, and other ancillary structure foundations such as sign cantilevers and trusses, and signal strain poles.
Drilled shafts can be constructed in the dry, or in the wet, and can also include a steel casing.
Dry construction methods are generally used when the water table is below the bottom of the shaft, and the existing soils are stiff enough as not to sloughing or caving into the hole during drilling operations. Any accumulated water greater than 3 inches at the bottom of the shaft must be pumped out prior to rebar and concrete placement, and during concrete placement, water cannot flow into the shaft at a rate greater than 12 inches within 1 hour.
Typical issues with dry construction methods are if the soils are unstable, or the water table is too high, but the Contractor attempts to force dry shaft construction. In this scenario, soil caving problems will lead to soil inclusions in the shaft concrete, affecting the shaft integrity. If any problems like this arise during construction, contact the Geotechnical Services area.
Another issue to watch for is if the Contractor leaves the excavation open for too long prior to reinforcement cage and concrete placement. Soils that were capable of maintaining hole stability during the drilling operations may shrink or swell over time and slowly lose that ability, resulting in caving leading to soil inclusions in the shaft concrete affecting the shaft integrity.
Wets construction methods are generally used where a dry excavation cannot be maintained during drilling, or shaft concrete placement. This typically occurs in areas of high water tables, and sandy soils that would otherwise slough or cave into the shaft during drilling operations. Water or a polymer slurry is used to contain water seepage, and maintain stability of the shaft excavation. Polymer slurry must be de-sanded and cleaned if used. Temporary surface casings are typically used to ensure shaft alignment. The rebar cage is lowered into the shaft, and concrete is placed using a tremie tube, or concrete pump capable of reaching the bottom of the shaft.
Typical issues with wet construction methods are inexperienced Contractors that do not understand the mechanics of polymer slurry construction, and the need to ensure proper suspension of sediment and cuttings for removal, and control of caving. This can lead to shafts not being properly cleaned of sediment, resulting in voids, or inclusions in the concrete, affecting the shaft integrity.
Dry temporary cased construction methods are generally used where caving soils occur, over soil or rock deformations are expected, but the casing can maintain a dry and stable excavation. The casing is advanced simultaneously with the excavation. After placement of the rebar cage, the casing is then withdrawn slowly during concrete placement until removed at the top of shaft.
Wet temporary cased construction methods are generally used where caving soils occur, and a dry excavation cannot be maintained, the soils are too permeable, and the groundwater is higher than the bottom of the shaft. The casing is advanced simultaneously with the excavation, but it is important that no drilling occur outside the casing through any caving soil layers. After placement of the rebar cage, the concrete is placed using a tremie tube or concrete pump capable of reaching the bottom of the shaft. The water in the casing is not to be pumped out, rather it is to be displaced by the concrete placement.
It is important to ensure the Contractor maintains a positive fluid pressure head in the shaft above the ground water elevation, or material from the side walls of the shaft will be pulled into the bottom of the shaft.
For both the wet and dry casing methods, the casing is typically installed in a telescoping fashion, where the top casings may be larger than the drilled shaft diameter as called for on the plans. The depth of the hole will also dictate how many sections of casing are to be used. Ensure the contractor does not telescope the casings down to a diameter less than the design diameter as shown on the plans.
Ensure concrete materials are in accordance with section 701 of the Standard Specifications for Construction, except modifications for the slump requirements per subsection 718.02 of the Standard Specifications for Construction. Concrete Grade S2 is to be used for dry construction, and Concrete Grade T is to be used for wet construction.
Ensure steel reinforcement bars are in accordance with section 905 of the Standard Specifications for Construction. Check the plans and specifications for epoxy rebar requirements.
Ensure steel casing materials are in accordance with section 919.10 of the Standard Specifications for Construction. Permanent casing will also require Buy America certification, temporary casings will not, unless left in place.
Review the plans for temporary casing-left in place cut off elevations. The Contractor is required to provide casing to be smooth and watertight, and capable of withstanding the pressure of concrete and the lateral earth pressures exerted down the length of the shaft. Ensure the outside diameter of the casing is at least equal to the diameter of the shaft as called for on the plans.
Temporary casings should come equipped with hook holes, or other attachments to aid in the removal as the concrete pour advances. The concrete placement is to be complete prior to complete removal of the casing. The casings should be removed slowly, and with pull forces being in line with the shaft axis. Do not allow the contractor to pull shaft casings with equipment offset from the centerline of the shaft, as this will either disrupt the already placed concrete, or case caving of soil materials into the plastic concrete.
When wet construction methods are required, polymer type slurry is required. Bentonite slurry is prohibited. Ensure the polymer slurry is of sufficient specific gravity to ensure stability of the excavation during drilling operations, and allows for concrete placement. The Geotechnical Services area will typically review and approve the Contractor’s proposed polymer slurry materials.
Ensure any polymer slurry used meets the requirements of Table 718-1 from the Standard Specifications for Construction. Ensure the Contractor maintains the height of polymer slurry to prevent the side walls of the excavation from caving, and the bottom from heaving.
Ensure the contractor pre-mixes the polymer slurry materials with clean, fresh water, and allow time for hydration prior to pumping into shaft excavation. The polymer slurry should be agitated to prevent setting up in the shaft.
If de-sanding is required, ensure the Contractor provides the appropriate equipment. Prior to placement of shaft concrete, take polymer slurry samples with a polymer slurry test kit to determine if heavily contaminated polymer slurry at the bottom of the shaft is to be removed. Ensure the Contractor controls and collects polymer slurry exiting the excavation as the concrete displaces the volume. Review the plans for polymer slurry handling and disposal requirements.
Ensure the contractor is using appropriate equipment to drill the shafts given the site conditions reported in the soil borings, and geotechnical subsurface exploration report. See Figure 1 for a typical drill rig set up. The major components of drill rig include:
- Crawler or truck mounted auger
- Auger of the appropriate type – rock auger, single or double flight earth auger. See Figure 2 for a double flight earth auger, see Figure 3 for a single flight rock auger.
- Muck or clean out bucket – see Figures 4 and 5 for a muck bucket example.
- Rock core barrel – see Figure 6 for a rock core barrel example.