412 DRILLED SHAFTS (CAISSONS)
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- Construct drilled shafts (caissons) as specified.
- Refer to 420.02 except as modified herein.
Portland Cement Concrete 902.10, Mix No. 4 Reinforcing Steel Section 908 Steel Casings A 252, Grade 2 or A 36
- 412.03.01 Subfoundation Investigation. When the Contract Documents include an item for Subfoundation Investigation, conduct a subfoundation investigation program prior to ordering or fabricating reinforcement for drilled shafts. Use this program to determine the elevation of suitable bearing stratum and the required depth of the drilled shafts. Select approximately a third of the drilled shaft locations spread over the total number of locations, and drill test holes at least 10 ft below the estimated drilled shaft length. After drilling the test holes, the Engineer will evaluate the data to determine the uniformity of the foundation materials. If the evaluation determines that more test holes are required, drill additional test holes at approved locations.
- 412.03.02 Shaft Installer. Obtain the services of a shaft installer having a proven record of experience and having successfully completed at least three projects with similar subsurface conditions, shaft sizes, depths, and minimum volumes of work as contained in the project. Submit evidence of pertinent experience for approval before proceeding with drilling shafts.
- Furnish a Certified Drilled Shaft report containing the following information for each drilled shaft:
- (a) Top and bottom elevations.
- (b) Final center line location at top.
- (c) Variation of shaft from plumb.
- (d) Results of tests performed.
- (e) Levelness of bottom.
- (f) Seepage of water.
- (g) Top and bottom elevation of any casings left in place.
- (h) Any unusual conditions.
- (i) Variation of dimensions from planned.
- (j) Dates of start and completion of excavation.
- (k) Inspection, testing, and placement of concrete (including any delays in concreting and location of construction joints in shafts).
- (l) Reinforcing steel.
- (m) Any additional information relevant to the as-built drilled installation.
- Record and maintain information pertinent to each drilled shaft and provide required data to other testing and inspection personnel.
- Provide all facilities required for the safe and convenient conduct of the Engineer’s inspection and testing procedures.
- 412.03.03 Geotechnical Engineer. When specified, employ the services of a qualified geotechnical engineer for inspection and testing for installation of drilled shafts. Ensure that the geotechnical engineer is a professional engineer registered in the State of Maryland, has a demonstrated record of experience with similar drilled installations, and is approved prior to beginning auguring for the drilled shafts.
- The geotechnical engineer shall submit a plan containing the proposed methods to be used to inspect the drilled shafts as specified herein.
- The geotechnical engineer shall visually inspect the bottom of each drilled shaft and perform tests as necessary to verify the bearing capacity. Drilled shafts shall be founded in material having the specified minimum design bearing capacity. The geotechnical engineer shall provide certification that the drilled shafts were properly drilled to a satisfactory depth and bearing.
- 412.03.04 Shaft Requirements. Excavate shafts by auguring, drilling, or hand excavation as necessary to reach the required bearing strata. When earth walls cannot be maintained without spilling into the shaft, install casings or slurry as excavation proceeds. Ensure that the casings are full-length and watertight. The casings shall be of sufficient thickness to withstand compressive, displacement, and withdrawal stresses; and to maintain the shaft walls. Withdraw casings as concrete is placed unless otherwise specified.
- The geotechnical engineer shall determine the final bottom elevation of drilled shafts when the services are required. All holes shall be inspected and approved.
- Do not excavate holes for successive drilled shafts until adjacent holes are filled with concrete and allowed to set.
- Drilled shaft tolerances:
- (a) Maximum permissible variation of center line locations is not more than 1/24th of the shaft diameter or 3 in., whichever is less.
- (b) Maximum out of plumb is 1.5 percent of the depth, 12.5 percent of the shaft diameter, or 15 in., whichever is less.
- (c) The top of the shaft or concrete cut-off elevation shall be within 1 in. of the design elevation.
- If the specified tolerances are exceeded, provide corrective construction to compensate for excessive eccentricity at no additional cost to the Administration. Submit proposed methods of corrective construction for approval.
- Excavate the bottom of drilled shafts to an undisturbed, level plane. Remove all loose material prior to placing concrete.
- Dewater drilled shafts as required to facilitate excavation, inspection, and concreting.
- Ensure that each drilled shaft has been inspected before placing concrete.
- Reinforcing Steel. Fabricate and place reinforcing steel cages for each drilled shaft as one continuous unit. Place reinforcement accurately and symmetrically about the axis of the hole, and keep securely in position during concrete placement.
- Protect exposed ends of extended reinforcement from damage.
- Concrete. Fill drilled shafts with concrete immediately after inspection and approval by the geotechnical engineer and the Engineer.
- Place concrete in one continuous operation, in a smooth flow without segregating. Use mechanical vibration for consolidation of at least the top 25 ft of each shaft. Concrete may be free dropped up to 25 ft provided that the procedures ensure that the concrete falls vertically without hitting the inside walls of the hole or falling directly on the reinforcing steel. When the Engineer determines that the concrete placement procedures are unsatisfactory, place concrete by means of bottom discharge bucket, flexible drop chute, elephant trunk hopper, tremie, or pumping. Use chutes, tremies, or pumping where a drop of more than 25 ft is required.
- Place concrete in the dry insofar as practicable. If excessive water occurs and it is not feasible to dewater the drilled pier shaft for concreting, place concrete by the tremie method as specified in 420.03.05. Control tremie placement operations to ensure that tremie is not broken during continuous placing from bottom to top. If approved, other methods of depositing concrete underwater may be used.
- Maintain a sufficient head of concrete to prevent any reduction in the diameter of the drilled shaft by earth pressure and to prevent extraneous material from mixing with the concrete. Coordinate the withdrawal of temporary casings with concrete placement operations to maintain a head of concrete approximately 5 ft above the casing bottom.
- Stop concrete placement at the top cut-off elevations shown on the Contract Documents. Screed the tops of drilled shafts level and give them a roughened surface finish. Where the cut-off elevation is above ground elevation, form the top section to extend the shaft to the required elevation.
- Construction joints are permitted in drilled shafts if concrete placement operations must be interrupted, as accepted by the Engineer. Screed the surface of the construction joint level and give it a roughened surface. Apply an approved bonding compound prior to placing additional concrete.
- The Engineer may require full-depth continuous coring of drilled concrete shafts where observations of temporary casing removal and concrete placement operations indicate cause for suspicion of quality of concrete, presence of voids, segregation, or other defects. Perform this work at no additional cost to the Administration.
- Defective Drilled Shafts. Repair or replace defective drilled shafts as directed.
412.04 MEASUREMENT AND PAYMENT
- The payment will be full compensation for all material, labor, equipment, tools, and incidentals necessary to complete the work.
- 412.04.01 Drilled shafts including furnishing and setup of auguring equipment, auguring, drilling, excavating, dewatering, inspection, testing, services of the shaft installer and geotechnical engineer, sleeves, reinforcement, concrete, and disposal of excess and unsuitable material will be paid for at the Contract unit price per linear foot for the pertinent Drilled Shaft item.
- 412.04.02 When subfoundation investigation is specified, it will be measured and paid for as specified in 419.04.