ITEM 416 DRILLED SHAFT FOUNDATIONS 416.1. Description. This Item shall govern for the construction of foundations consisting of reinforced concrete drilled shafts and/or nonreinforced concrete drilled shafts, with or without bell footings of the size and at the location shown on the plans. 416.2. Materials. All materials shall conform to the requirements of the pertinent items as follows: Item 421, "Portland Cement Concrete" Item 440, "Reinforcing Steel" Item 448, "Structural Field Welding" Unless otherwise shown on the plans, concrete for drilled shafts shall conform to the following: Drilled Shaft Type Nonreinforced Reinforced Slurry & Underwater Concrete Placement Concrete Class "A" Class "C" Class "SS" A Type D water-reducing, retarding admixture will be required in all concrete when casing is used or when shafts are placed underwater or under slurry. 416.3. Construction Methods. (1) Excavation. (a) General. The plans indicate the expected depths and elevations where satisfactory bearing material will be encountered. The Contractor shall perform the excavation required for the shafts and bell footings, through whatever materials encountered, to the dimensions and elevations shown on the plans or required by the site conditions. If satisfactory founding material is not encountered at plan elevation, the bottom of the shaft will be adjusted, or the foundation altered, as determined by the Engineer, to satisfactorily comply with design requirements. Shaft alignment shall be within a tolerance of one (1) inch per 10 feet of depth. The center of the shaft shall not be more than one (1) inch from the horizontal position shown on the plans. Any shafts in violation of this tolerance will be subject to a structural review by the Engineer. Where caving conditions and/or excessive ground water is encountered, no further drilling will be allowed until a construction method is employed which will prevent excessive caving that will cause the excavation to be appreciably larger than the size of casing to be used. Predrilling in disturbed soil as necessary to control ground water and caving until a casing is set or setting casing with a vibratory hammer, will be permitted. Casing will be required when necessary to prevent caving of the material or when necessary to exclude ground water. Casing shall be of sufficient strength to withstand handling stresses, the pressure of concrete and of the surrounding earth or backfill materials, and shall be watertight, smooth, clean and free of accumulations of hardened concrete.
When casing is used, the minimum shaft diameter below the casing shall not be less than the specified diameter of the shaft. The hole shall be drilled in such a manner that when the casing is inserted into the hole, a minimal amount of disturbed soil will be trapped outside the casing. This provision does not apply to temporary undersized casings used to protect workers inside shafts or for drilled shafts designed for point bearing only. If the elevation of the top of the shaft is below ground level or a problem exists with material caving into the excavation, a surface casing will be required from ground elevation to a point below the top of the shaft. Any surface casing used shall not remain in place unless otherwise indicated on the plans or permitted by the Engineer. It shall not be extracted until after the concrete placement has been completed. Under normal operations, a sufficient head of concrete shall be maintained at all times above the bottom of the casing to overcome hydrostatic pressure. Movement of the casing for short pulls of a few inches, rotating, exerting downward pressure and tapping it to facilitate extraction will be permitted. The total removal of the casing shall not be started until all concrete placement is completed in the shaft. When unusual conditions warrant, the casing may be pulled in partial stages. Casing extraction shall be at a slow, uniform rate with the pull in line with the axis of the shaft. Casing shall not be left in place unless otherwise shown on the plans or permitted by the Engineer. Bells shall be excavated to form a bearing area of the size and shape shown on the plans. Blasting will not be permitted. Bell shapes varying slightly from those shown on the plans are permissible provided the bearing area equals that specified. Material excavated from shafts and bells, including slurry, and not used in the backfill around the completed bents or piers shall be disposed of as directed by the Engineer. At the time concrete is placed, the excavation shall be free from accumulated seep water. All loose material shall be removed from the bottom of the excavation prior to placing concrete. The Contractor shall provide suitable access and lighting for proper inspection of the completed excavation, and to check the dimensions and alignment of shafts and bell excavation. Any required lighting shall be electric. Any mechanical equipment used within the excavation shall be operated by air or electricity. The use of gasoline driven engines within the excavation for pumping or drilling will not be permitted. In order that the Engineer may judge the adequacy of a proposed foundation, the Contractor, if requested, shall take cores to determine the character of the supporting materials. The minimum depth of such soundings or cores shall be five (5) feet below the proposed founding grade, or a depth equal to the diameter of the shaft, whichever is greater. It is the intent of this provision that cores shall be taken at the time the excavation is approximately complete. Unless otherwise shown on the plans, the Contractor may be required to provide one (1) core at his expense at each bent or abutment. Only cores in excess of the first core taken at each bent or abutment or in excess of those called for in the plans, will be measured for payment. When the plans require shafts in abutment bents, the embankment at the bridge ends shall be completed to grade and thoroughly compacted prior to drilling, unless otherwise shown on the plans or as permitted by the Engineer. (b) Additional Requirements for Slurry Displacement Method. Unless otherwise shown on the plans, drilled shafts may be constructed by the "Slurry Displacement Method" which is defined as a construction procedure whereby the sides of the excavation are supported by a processed bentonite slurry, which is then displaced by concrete to form a continuous concrete shaft. Casing, other than surface casing, will not be permitted.
Slurry shall consist of a commercial bentonite compatible with fresh water, and shall be mixed with water conforming to the requirements in Item 421, "Portland Cement Concrete" to produce a slurry. Nonsite mixed commercially supplied slurry shall conform to all the requirements herein. Slurry mixed at the project site shall be premixed in a reservoir adjacent to the excavation. Mixing of the slurry in the shaft excavation or other hole will not be permitted. The reservoir shall be of sufficient capacity to fill the excavation and for recovery of the slurry during concrete placement. Slurry shall conform to the following requirements: Bentonite Content (Percent by weight) 2% to 8% Specific Gravity 1.02 to 1.15 Viscosity (500 ml Marsh funnel) 40 second maximum Sand Content (Percent by volume) 10% maximum At the discretion of the Engineer, the specific gravity or sand content limits may be exceeded if it appears that the slurry consistency will not hinder concrete placement. Slurry shall be sampled from the bottom of the excavation and tested in accordance with Test Method Tex-130-E. A head of slurry shall be maintained in the shaft excavation during and after drilling operations, at or near ground level, or higher as necessary to counteract ground water pressure. When a surface casing is used, the slurry shall fill the excavation to at least two (2) feet above the bottom of the surface casing. The surface casing shall be smooth and shall extend approximately to the top of the shaft. The surface casing shall not be extracted until the concrete placing operation has been completed. Just prior to placement of reinforcing steel, a clean-out bucket of the proper size and/or other acceptable tool shall be passed down and up the excavation to remove loose cuttings or any material that may have fallen from its sides after the completion of drilling operations. If concrete placement is not started within four (4) hours of the completion of the shaft excavation, the hole shall be reprocessed with the auger as directed by the Engineer. The bottom shall then be recleaned with a cleanout bucket. The slurry at the bottom of the hole shall be rechecked for compliance with the above requirements. If the slurry "sets up" or forms a gel prior to concrete placement, the gelled slurry shall be agitated to liquefaction just prior to concrete placement and at other times when directed by the Engineer. (2) Reinforcing Steel. The cage of reinforcing steel, consisting of longitudinal bars and lateral reinforcement (spiral reinforcement, lateral ties or horizontal bands) shall be completely assembled and placed as a unit immediately prior to concrete placement, unless otherwise shown on the plans. If the shaft is lengthened, the longitudinal bars and lateral reinforcement required in the upper portion of the shaft shall be extended to the bottom unless otherwise shown on the plans. These bars may be lap spliced or spliced by welding. Any splices required shall be in the lower portion of the shaft. Where spiral reinforcement is used, it shall be tied to the longitudinal bars at a spacing not to exceed 12 inches. Welding of lateral reinforcement to longitudinal bars will not be permitted unless otherwise shown on the plans. In uncased shafts, concrete spacer blocks or steel chairs shall be used at sufficient intervals to insure concentric spacing for the entire length of the cage. In cased shafts, concrete spacer blocks shall not be used. Steel chair spacers or bent pieces of steel bars shall be placed at sufficient intervals around the steel cage to insure concentric spacing inside the casing.
It shall be the Contractor's responsibility to support or hold down the cage to control vertical displacement during concrete placement and/or extraction of the casing. The support shall be concentric with the cage to prevent racking and distortion of the steel. An adequate number of the vertical bars shall be supported. The elevation of the top of the steel cage shall be checked before and after concrete placement or after casing extraction when casing is used. Generally, downward movement of the steel not exceeding six (6) inches per 20 feet of shaft length will be acceptable. Upward movement of the steel not exceeding six (6) inches will be acceptable. Displacement of the steel beyond the above limits shall be cause for a structural review. The minimum length of steel required for lap with column steel shall be maintained. Dowel bars may be used if the proper lap length is provided both into the shaft and into the column. All dowel bars shall be adequately supported and may be inserted after concrete placement. (3) Concrete. (a) General. All work shall be performed in accordance with the provisions of Item 420, "Concrete Structures," and with the requirements herein. Concrete shall be placed as soon as possible after all excavation is complete and reinforcing steel placed, and shall be of such workability that vibrating or rodding will not be required. Portions of drilled shafts which are formed shall be vibrated. Concrete placing shall be continuous for the entire length of the shaft. Concrete shall be placed through a suitable tube or tremie to prevent segregation of materials. The tube or tremie may be made in sections to provide proper discharge and permit raising it as the placement progresses. A nonjointed pipe may be used if concrete is not allowed to discharge from the side openings. Free fall of concrete a maximum of 15 feet will be allowed provided the Contractor can demonstrate that the concrete may be directed such that it does not strike the reinforcing cage or sides of the hole during placement. The elapsed time from the beginning of concrete placement in the cased portion of the shaft, until extraction of the casing is begun, shall not exceed one (1) hour. A riser block of equal diameter as the column and of a maximum height of six (6) inches may be cast at the top of the completed shaft. The top surface shall be cured and any construction joint area shall be treated in accordance with Item 420, "Concrete Structures." (b) Additional Requirements for Slurry Displacement or Underwater Placement Methods. Provisions shall be made for a sump, or other approved method, to channel displaced fluid away from the shaft excavation. Slurry shall be recovered and disposed of as approved of by the Engineer. Displaced fluids shall not be discharged into streams or other bodies of water. Concrete shall be placed as soon as possible after all excavation is completed and reinforcing steel is placed, and within the same day's operation as the shaft excavation. The concrete shall be placed through a closed tremie or pumped to the bottom of the excavation. Placement of the concrete shall be continuous from the beginning of placing until the shaft is completed. If a tremie is used, it shall be kept full of concrete and well submerged in the previously placed concrete at all times. If a pump is used, the discharge tube shall be submerged in the previously placed concrete at all times. Additional concrete shall be placed to assure the removal of any contaminated concrete at the top of the shaft.
If concrete placement is interrupted due to withdrawal of the submerged end of the tremie or pump discharge tube prior to completion, the tube shall be removed, resealed at the bottom, forced well into the concrete already placed and recharged prior to progressing further. 416.4. Test Load. Any required test loading of shafts shall be in accordance with Item 405, "Foundation Test Load." 416.5 Measurement. Drilled shaft foundations will be measured by the linear foot to the bottom of the shaft. Shafts for interior bents and piers will be measured from a point approximately six (6) inches below the finished earthwork elevation at the center of each shaft unless specific elevations or dimensions are indicated on the plans or unless the Engineer directs otherwise to meet unusual conditions. (The bent height shown on the plans is for estimating purposes only and does not control the top of shaft measurement.) For abutment bents and retaining walls, the length of each shaft shall be measured from the bottom of footing or cap elevation. For sign structures and illumination poles, the length of each shaft shall be measured from the top of shaft elevation shown on the plans. The quantity of concrete for bell footings as shown on the plans and in the proposal shall consist of the volume outside of the plan dimensions of the shaft and shall be considered as final quantities and no further measurement will be required, unless revised by the Engineer during construction. Core holes to determine the adequacy of a founding strata will be measured by each core hole drilled. Only core holes in excess of one (1) per bent or abutment or in excess of those called for in the plans will be measured for payment. 416.6. Payment. The work performed and materials furnished in accordance with this Item and measured as provided under "Measurement" will be paid for at the unit price bid per linear foot of "Drilled Shaft," or "Drilled Shaft (Nonreinforced)," of the specified diameter, subject to the following limitations for overruns authorized by the Engineer: (1) Payment for individual completed shaft lengths up to and including five (5) feet in excess of the maximum plan length shaft, as defined herein, will be made at the unit price bid per linear foot of the specified diameter of "Drilled Shaft." (2) Payment for that portion of individual completed shaft length in excess of five (5) feet and up to, and including, 15 feet more than the maximum plan length shaft, as defined herein, will be made at a unit price equal to 115 percent of the unit price bid per linear foot of the specified diameter of "Drilled Shaft." (3) Payment for that portion of individual completed shaft length in excess of 15 feet more than the maximum plan length shaft, as defined herein will be made at a unit price equal to 125 percent of the unit price bid per linear foot of the specified diameter of "Drilled Shaft." Payment as described above shall be subject to the following provisions: (1) For extra depth drilling at interior bents and piers, the maximum plan length shaft shall be the maximum length shaft, regardless of diameter, for any interior pier or bent of any bridge included in the contract. (2) For extra depth drilling for abutment bents and retaining walls, the maximum plan length shaft shall be the maximum length shaft, regardless of diameter, for any abutment bent of any bridge or of any retaining wall included in the contract. (3) For extra depth drilling for overhead sign structures, the maximum plan length shaft shall be the maximum length shaft, regardless of diameter, for any overhead sign structures included in the contract.
(4) For extra depth drilling for high mast illumination poles, the maximum plan length shaft shall be the maximum length shaft, regardless of diameter, for any high mast illumination pole included in the contract. Bell footings constructed to the specified dimensions will be paid for at the unit price bid per cubic yard for "Bell Footings." The quantity to be paid for will be the quantity shown on the plans unless revised by the Engineer in accordance with "Measurement." Core holes will be paid for based on a price to be determined as follows: the price bid for the drilled shaft size of greatest quantity in the contract shall be divided by two times the shaft diameter in feet and then multiplied by five (5). The unit prices bid for the various classifications of drilled shafts and bell footings shall be full compensation for making all excavations and soundings; for any necessary pumping; for furnishing, predrilling, placing and removing any required casing; for furnishing and placing all concrete including additional concrete required to fill an oversize casing or oversize excavation; for furnishing and placing reinforcing steel; for furnishing and processing any slurry; for all backfilling; for disposing of cuttings and slurry; and for furnishing all tools, labor, equipment and incidentals necessary to complete the work. When the bottom of any drilled shaft is ordered to be placed at an elevation below plan grade and a splice of reinforcement is required, no direct payment will be made for the extra reinforcement required, but it will be considered subsidiary to the price bid per foot of shaft. No extra payment will be made for casings left in place. placed. No payment will be allowed for "Bell Footing" or for "Drilled Shaft" until the concrete has been