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|FHWA > Engineering > Geotech > Design And Construction Of Continuous Flight Auger Piles > Chapter 8|
Geotechnical Engineering Circular (GEC) No. 8
|Portland cement (Types I, II, & III)||Item __________ (Hydraulic Cement), ASTM C 150|
|Water||Item __________ (Portland Cement Concrete) AASHTO T 26|
|Fine and coarse aggregate||Item __________ (Portland Cement Concrete), ASTM C 33, ASTM C 87-90, ASTM C 227-90, ASTM C 289-94, ASTM C 295-90, a ASTM C 586-92|
|Mineral admixtures (Fly Ash)||Item __________ (Concrete Admixtures) ASTM C 618 Class C or F|
|Water reducing admixtures||Item __________ (Concrete Admixtures) ASTM C 494, ASTM C 1017|
|Fluidifier (fluidizer)||Item __________ (Grout Admixtures) ASTM C 937, CRD-C 619|
|Reinforcing steel||Item __________ (Reinforcing Steel), ASTM A 615, ACI 315|
|Grout flow testing (flow cone)||ASTM C 939, CRD-611-94|
|Grout cube samples||ASTM C 109|
|Grout cube testing||ASTM C 109, ASTM C 942|
|Concrete slump testing||Item __________ (Portland Cement Concrete), ASTM C143|
|Concrete cylinder samples||Item __________ (Portland Cement Concrete), ASTM C31|
|Concrete cylinder testing||Item __________ (Portland Cement Concrete), ASTM C 39|
Commentary: The appropriate sections of each State DOT Standard Specifications should be included under the materials section. A complete generic materials section cannot be provided herein considering the vast combinations of materials and control methods used by individual State DOTs. Some general guidance on grout and concrete is provided in Sections (8.)3.2 and (8.)3.3 of this specification.
Muck, organics, soft clay, or other unsuitable materials encountered within 5 ft (1.5 m) of the ground surface, such material shall be removed or otherwise treated to prevent problems with pile top construction. Excavation of unsuitable surface material and backfilling shall be completed to the Engineer's satisfaction, or as required in the contract documents, prior to the construction of CFA piles. Should more than 5 ft (1.5 m) of unsuitable surface material be encountered, the CFA Pile Contractor shall advise the Engineer immediately and proceed with work as directed by the Engineer. Should the CFA Pile Contractor suspect that any soils that are excavated are contaminated by hydrocarbons, refuse, or other environmentally hazardous material, he or she shall contact the Engineer immediately and proceed with work as directed by the Engineer.
Commentary: Unsuitable materials should generally be removed to their full depth, or to a depth of 5 ft (1.5 m), whichever is less. The excavation is typically backfilled with soil having a plasticity index of 20 or less compacted to at least 95% of its maximum dry density, as specified by ASTM D 698 (AASHTO T 180), at within 2% of optimum moisture content.
The CFA Pile Contractor shall be solely responsible for evaluating the need for, design of, and monitoring of measures to prevent damage to adjacent structures or underground utilities, on or off the right-of-way. These measures shall include, but are not limited to, selection of construction methods and procedures that will prevent over-excavation and excessive migration of grout through the ground, monitoring and controlling the vibrations from construction activities (including placement of casings, sheet piling, shoring and similar ancillary features), and protecting utilities.
Structures located within a horizontal distance equal to the planned length of the pile shall be monitored for vertical and horizontal movement in a manner approved by the Engineer within an accuracy of 0.01 in. (0.3 mm). Monitoring of adjacent structures will be done by an independent party working for the CFA Pile Contractor and approved by the Engineer. A monitoring plan, including the locations of measurement points and the frequency of recording measurements shall be submitted to the Engineer for approval as part of the CFA Pile Installation Plan. Monitoring shall begin with a base-line measurement recorded no more than 10 calendar days prior to construction of the pile, any shoring, or similar ancillary features. In addition to monitoring for movement, the condition of the adjacent structure, including cracks and crack widths, before and after construction of the CFA piles, shall be documented by visual inspection, photographs, and/or video. Structures owned by Owner shall be monitored for movement but need not be monitored for condition unless called for on the plans.
As soon as any movements are detected in adjacent structures, the CFA Pile Contractor shall stop construction, notify the Engineer, and take any immediate remedial measures required to prevent damage to the adjacent structures. The CFA Pile Contractor and the Engineer shall then review the current installation procedures. If revisions to the installation procedures are deemed necessary, the CFA Pile Contractor shall submit a revised installation plan for approval by the Engineer before resuming work.
Commentary: The installation of CFA piles can result in large settlement of the ground surface if the rate of rotation of the auger is high relative to its rate of penetration or over-rotation, especially in loose sandy soils. This action can promote settlement and damage to existing structures near the location of the pile installation. In some soils, the pumping of grout can result in the fracturing of the ground, the traveling of grout in the ground a considerable distance horizontally under pressure, which can lift the ground surface and structures (including buried conduits) founded nearby. Careful monitoring of the movements of adjacent structures and changes in the condition of such structures is necessary in order for the CFA Pile Contractor to know when their procedures are producing ground movements in order and when immediate corrective action needs to be taken during the pile installation. If a soil susceptible to densification is encountered, the penetration rate should be increased or the rate of rotation of auger be reduced. Condition surveys are needed for the evaluation of the effect of the construction process on the serviceability of adjacent structures by the Engineer.
The grout or concrete property requirements listed to follow in this section shall be determined from samples taken during CFA pile construction as described in Section (8.)3.2.4.
Commentary: CFA piles have traditionally been constructed in the United States with sand-cement grout. Construction of CFA piles with concrete is now routinely accomplished in the United States and abroad, and has proven to produce acceptable piles when properly specified and constructed. The specifications herein allow for the use of either grout or concrete; however, once a mix design is established through the pile test program it should not be altered.
As a guide for strength development, grout and concrete meeting these specifications typically attains 30 % and 70 % of their 28-day compressive strength after 3 and 7 days of curing, respectively. Both grout and concrete mixes may contain pozzolanic additives. The most commonly used is fly ash (ASTM C 618-94; however, finely ground silica flume and blast furnace slag (ASTM 989-94) can also be used. The use of pozzolanic additives results in lower permeability of the hardened concrete and tends to retard the set time of the cement paste, thereby increasing the time that the grout or concrete remains workable. As a consequence of providing a more workable mix, materials such as fly ash, silica flume, and/or slag will probably severely retard the early strength gain of the grout mix, typically until about 10 to 14 days of age. In all cases, the submitted mix design should include strength development vs. time information.
Each DOT will likely have local mix designs that are preferred for CFA pile construction (or drilled shafts produced by the wet method), based on performance of local cements and aggregates. Concrete mix design for CFA pile construction should be given special attention. Desirable properties are fluidity, compaction under self weight, resistance to segregation, and controlled set time. Specific guidance on slump and aggregate gradation are provided below.
The grout shall consist of a mixture of Portland cement fly ash, water, fine aggregate (sand), fluidifier, and if necessary, retarder, which shall be proportioned and mixed so that the grout will exhibit the following properties:
Commentary: The grouts used in CFA pile production are typically too thick to flow effectively from the standard ½ in. (12 mm) outlet. The flow cone specified by the U.S. Army Corps of Engineers (CRD 611-94) may be modified by simply taking out the removable ½ in. (12 mm) diameter orifice that extends out the bottom of the device to leave a ¾ in. (19 mm) diameter opening, while a flow cone would need to be custom fabricated to the ASTM C 939 specification with the modified outlet diameter. Grouts acceptable for CFA pile construction typically have a fluid consistency represented by an eff1ux time of 10 to 25 seconds when tested in accordance with the modifications described herein.
Checking the grout flow is considered a quality control tool valued primarily for the purpose of quickly indicating whether the grout currently being tested will most likely conform with grout samples meeting the specified design requirements furnished previously on the project. Should any piles be found inadequate in subsequent testing, a record of grout flow may be valuable in identifying other suspect piles, which may then be subject to further testing at the discretion of the Engineer.
The concrete shall consist of a mixture of Portland cement, fly ash, water, coarse aggregate, fine aggregate (sand), water reducers, and if necessary, retarder, which shall be proportioned and mixed so that the concrete will exhibit the following properties:
Commentary: Concrete mix designs appropriate for drilled shafts constructed by the wet method are generally acceptable for use in CFA pile construction, with the notable exception that CFA pile construction may require a greater degree of workability. Greater workability is needed to allow for the concrete to be pumped through the delivery lines and auger, and subsequent insertion of the required reinforcing steel to planned elevation after placement of the concrete. Accordingly, the coarse aggregate for CFA pile construction is typically limited in size to no greater than ⅜ in. (9.5 mm). Consideration should be given to the benefit of well rounded aggregate (i.e., pea-gravel).
Slump requirements are based on providing the necessary quality of workability for uniform and proper placement throughout the duration of CFA pile construction. A slump range of 8 in. ± 1 in. (200 mm ± 25 mm) is suggested. Sampling frequencies for slump measurement are recommended in Section (8.)3.2.4
High workability is achieved with proper aggregate gradations, water-cement ratios and appropriate admixtures such as water reducing, with retarding admixtures, and air entraining agents. Angular crushed aggregates are harder to work than similar sized rounded aggregates. To insure against segregation, the sand/cement content should be high compared to the coarse aggregate content. Fly ash can be used to replace some of the Portland cement in many situations, but may result in slower strength development.
Grout or concrete samples for strength testing shall be taken from the discharge at the delivery trucks prior to pumping. Concrete samples shall be cylinders 6 in. (150 mm) diameter by 12 in. (300-mm) high, or sized appropriately for maximum aggregate size according to ASTM C 39. Grout samples shall be 2 in. (50 mm) cubes and shall be subjected to a 10% increase in required compressive strength as compared to cylinder samples.
The CFA Pile Contractor, Testing Agency, or qualified party specified in the contract documents shall make no less than six (6) such samples for each 50 yd3 (38 m3) of grout or concrete placed. No less than six (6) such samples shall be made per working day, or less than six (6) such samples for each mix of grout or concrete produced by the supplier. Concrete or grout cylinders (or alternatively cubes for grout) shall be cured and tested in accordance with the State DOT specifications.
The samples will be tested by __________ (Agency's name) for unconfined compressive strength. As a minimum, two (2) samples shall be tested at seven days after sampling; two (2) samples shall be tested at 28 days after sampling; and two (2) samples will be held in reserve. Those samples tested at 28 days after casting shall exhibit a minimum compressive strength as specified in Section (8.)3.2.1 for grout or Section (8.)3.2.2 for concrete.
A grout sample shall be obtained from every truck, and shall be tested for fluid consistency (flow cone) and temperature prior to discharging into the pump hopper. The grout sample shall exhibit the fluid consistency as specified in Section (8.)3.2.1. Alternatively, if a concrete mix is used, a concrete sample shall be obtained from every truck, and shall be tested for slump and temperature prior to discharging into the pump hopper. The concrete sample shall exhibit the slump as specified in Section (8.)3.2.2. Additional samples may be required at the discretion of the Engineer at any time during the grout or concrete placing process to ensure that consistent fluidity/slump is being achieved.
Commentary: Often smaller sized cylinder samples of grout are used and are typically either 3 in. (75 mm) diameter by 6 in. (150 mm) high, or 2 in. (50 mm) diameter by 4 in. (102 mm) high. Smaller sized cylinder samples for grout should only be permitted with prior approval by the Engineer. If the samples are cast using a method or sample different than that used for the mix design, a relationship between the compressive strengths obtained by the methods will need to be established.
The type of and frequencies for grout or concrete testing listed in this section are recommended as a minimum requirement. The Engineer may choose to increase the type or frequency of testing in the project specifications or at any time during the construction of the CFA piles. For non-critical foundations, only the Engineer may choose to relax the frequency of grout and concrete sampling and testing.
Some State DOTs have standard test procedures and/or frequencies that are not included in the specification when done by State personnel. In such cases, the State DOT should reference their testing manual/procedures and that the testing will be performed by State personnel.
The auger flights shall be continuous from the top of the auger to the bottom tip of the cutting face of the auger, with no gaps or other breaks. Gaps in the flighting are allowed only where auger sections are joined and may not exceed 1 in. (25 mm). The length of any auger brought to the project site shall be such that the auger is capable of installing a pile to a depth that is 20% greater than the depth of the pile shown on the approved working drawings. The auger flighting shall be uniform in diameter throughout its length, and the outside diameter of the auger shall be at least 97% of the design diameter of the pile. Only single helix augers shall be used. The hollow stem of the auger shall be maintained in a clean condition throughout the construction operation. In order to facilitate inspection, the leads shall be clearly marked every 1 ft (0.3 m) along its length so that such marks are visible to the unaided eye from the ground.
Commentary: The requirements contained in this section are a minimum requirement for CFA piles. The CFA Pile Contractor may propose to use alternative equipment subject to the approval of the Engineer through the Installation Plan submittal process. In general for CFA piles, continuous flight augers that continuously remove soil during drilling are the common type of auger or drilling tool used. Consideration of augers or drilling tools that provide full or partial displacement of the soil during drilling should also be made.
The bottom of the auger flights and the cutting teeth attached thereto shall be constructed geometrically so that the bottom of the pile will be as flat as feasible. The grout or concrete injection port shall be fitted with a means of sealing it against ingress of water and soil during drilling.
The auger shall be guided at the ground surface by a guide connected to the leads of the CFA piling rig. If the auger is over 40 ft (12 m) long, it shall also be guided by a guide to be located approximately half the length of the auger above the ground-surface guide. Where CFA piles are installed with hydraulic, fixed mast installation platforms, and the stem to which the auger is fixed has an outside diameter 10 in. (250 mm) or greater, a guide above the ground surface is not required. The leads that carry the rotary unit that power the auger should be restrained against rotation by an appropriate mechanism.
The piling rig shall be capable of penetrating the ground without drawing surrounding soils laterally into the pile bore, as is described in Section 3.5. It shall be capable of installing a pile to a depth at least 20% greater than the depth of the piles shown on the approved working drawings.
As a minimum, the following automatic measurements shall be made and recorded during the drilling operation:
All measurements shall be referenced to (or plotted against) the depth of the auger injection point. This shall be accomplished with a rotational position indicator on the auger head system and an electronic position indicator on the crane line or boom holding the auger. Torque and thrust load cells shall be positioned on the auger head system.
As a minimum, the following automatic measurements shall be made and recorded during the grouting or concreting operation:
All measurements shall be referenced to (or plotted against) the depth of the auger injection point. This shall be accomplished with electronic flowmeters and electronic pressure transducers placed in the grout or concrete pressure line, an electronic position indicator on the crane line or boom holding the auger, and a rotational position indicator on the auger system.
Calibration shall be made on all measuring and recording equipment at the beginning of the project that will demonstrate that the values indicated by the measuring and recording equipment are within 3% of the values indicated. Calibrations shall be performed in accordance with the equipment manufacturer's specifications. All measuring and recording equipment shall also be recalibrated when the Engineer suspects that the drilling and grouting or concreting performance has changed.
Commentary: Automated measuring and recording equipment provide real time evaluation of each pile on a project. Piles that have installation records out of specification or that otherwise appear abnormal can be selected for integrity tests or verification load tests to determine if they should be accepted or rejected.
The CFA Pile Contractor shall perform the drilling required for the piling, through whatever materials are encountered, to the dimensions and elevations required by the CFA Pile Contractor's design, as shown on the approved working drawings. Drilling shall not commence until sufficient supply of grout or concrete is present on the project site to complete the pile. The drilling parameters (auger rotation speed, penetration rates, crowd, torque, etc.) for the production piles shall be within the ranges established in the Pile Installation Plan, as verified by the pre-production testing program. The same procedures used to install the test piles shall be used to install production piles.
The center of any pile shall be within 3 in. (75 mm) of the location shown on the approved working drawings in a horizontal plane (i.e., plan-view). The completed pile shall be plumb to within 2%, if vertical, or shall be installed to within 2% of its design batter, as determined by the angle from the vertical, if planned as a batter pile. Any pile in violation of these tolerances will be subject to review by the Engineer and may be rejected or replaced at the CFA Pile Contractor's expense.
Adjacent piles within six diameters, center to center, of each other shall not be installed until it can be demonstrated by the CFA Pile Contractor that the grout or concrete in the first pile installed is fully set. The grout or concrete should have set enough such that the integrity of the existing pile will not be compromised if drilling the new pile causes mining of soil away from the existing pile.
The auger shall not be extracted from the ground at any time during the construction of a pile in such a way that would result in an open unsupported borehole or inflow of water into the pile borehole. It should become necessary to raise the auger and subsequently re-insert the auger during the pile construction process, the depth of the pile shall be increased and/or other additional measures shall be required as directed by the Engineer.
The auger shall be advanced into the ground at a continuous rate and at a rate of rotation that prevents excess spoil from being transported to the ground surface. The rate of penetration shall be established as a part of the test pile program. Automated monitoring equipment shall be used to verify this target rate of penetration is maintained during construction of production piles.
Pile termination criteria, including refusal criteria, if applicable, will be established during the pre-production test pile program. If refusal is encountered before planned depth is achieved, rotation of the auger shall be stopped, and the CFA Pile Contractor shall inform the Engineer. The CFA Pile Contractor and Engineer shall evaluate the installation data and determine if the established termination criteria have been met, or if other action is required to complete the pile. If an obstruction is encountered and it does not allow the pile to be completed in the planned location, the CFA Pile Contractor shall notify the Engineer and the Design Engineer in order for the Engineer and Design Engineer to determine remedial action.
Commentary: The penetration rate should generally be maintained such that the auger advances a depth equal to or greater than the pitch of the auger for every 1.5 to 2 revolutions for cohesionless soils, or 2 to 3 revolutions for cohesive soils. Loose cohesionless soils are more susceptible to mining by the auger flights, and generally call for the more stringent guideline. The intent of this provision is that the piles be constructed to a consistent standard as reflected in the results of the test pile program, and that soil mining be avoided.
In general, refusal is defined as a rate of auger penetration of less than 1 ft/minute (0.3 m/minute) with equipment that is appropriate for the project. If correlated with soil boring information, refusal may provide an indication that a strong layer has been reached, but should not be used as the sole criteria for setting pile tip levels for conventional CFA piles. Refusal criteria may be used to set pile tip levels for drilled displacement piles, based on the criteria established and verified to achieve capacity during the pile load testing program.
The placement of grout or concrete shall commence within 5 minutes after the auger has achieved the planned depth. Grout or concrete shall be pumped through the hollow-stem auger with sufficient pressure (as measured at the top of the auger) as the auger is withdrawn to completely form the pile and fill any soft or porous zones surrounding the pile.
At the beginning of grout or concrete placement the sealing device (plug, or bottom cover plate at the tip of the auger) shall be removed by the application of grout or concrete pressure, or with a central reinforcing bar. As pumping begins, the auger shall be lifted from 6 to 12 in. (0.15 to 0.3 m) to facilitate removal of the sealing device. Care shall be taken to ensure that the auger is lifted only within this specified range to initiate the flow of grout or concrete, and that water inflow and soil movement at or near the base of the auger are minimized. After withdrawing the auger to initiate the flow of grout or concrete, the tip of the auger should be re-inserted to at least the original depth.
The technique and equipment used to initiate and maintain the grout or concrete flow shall be such that a pile of the full design cross-section is obtained from the maximum depth of boring to the final pile cut-off level. The grout or concrete shall be supplied to the pile at a rate during auger withdrawal that ensures that a continuous monolithic shaft of at least the full specified cross-section is formed, and is free from soil inclusions or any grout or concrete segregation.
The auger shall be extracted at a smooth, steady rate while continuously pumping. If rotation of the auger occurs during auger extraction, it shall be positive, i.e. in the same direction as during drilling.
Satisfactory coordination of auger withdrawal with pumping is indicated by maintaining a positive pressure in the grout at the auger tip, and a sufficient volume or pressure of grout or concrete to fill the pile (with a small oversupply of volume). Satisfactory coordination shall be verified using automated monitoring equipment.
The volume of grout or concrete placed as a function of depth shall be measured and recorded at intervals not exceeding 2 ft (0.6 m) using automated monitoring equipment. The magnitude of minimum oversupply (or grout volume factor) appropriate for the site conditions shall be established during the pre -production test pile program and maintained during production pile construction. Inadequate volume pumped over a depth interval of 5 ft (1.5 m) is a basis for rejection of the pile.
Commentary: Typical grout volume factors range from 1.15 to 1.2 (i.e., 15% to 20% oversupply). In general, a grout volume factor of 1.15 is considered to be a minimum value. Grout factors greater than 1.5 suggest that soil mining or other undesirable installation effects may be occurring. There may be some cases where grout factors greater than 1.5 are acceptable, particularly if integrity and proof tests indicate that the pile meets the performance requirements and if pre-production piles showed similar results.
The grout/concrete volume factor of the pile shall be within 7.5% of the target volume factor established in the Pile Installation Plan as modified by the results of the pre-production test pile program. Production piles installed outside of this range shall be considered unacceptable piles as listed in Section (8.)3.13.
If placement of grout or concrete is suspended for any reason, such as equipment failure, the pile will need to be re-drilled. The pile may be re-drilled in the same location if the grout or concrete is still fluid enough for the drill rig to penetrate. If the concrete or grout has set, the pile will need to be re-drilled in a new location. The Pile Installation Plan and working drawings will need to be revised by the CFA Pile Contractor to reflect the changes and submitted to the Engineer for approval prior to re-drilling the pile.
Commentary: During grouting, the auger should be pulled with either no rotation or slow continuous rotation in the direction of drilling. A static pull with no rotation can help maintain a static condition at the base of the auger against which the grout/concrete pressure acts. Some contractors prefer to slowly rotate the auger during withdrawal in order to minimize the risk of having the auger flight stick. In addition, some augers have an off-center discharge plug at the base and slow rotation may help to avoid concentrating the distribution of the grout and pressure to an off-center location within the hole. If rotation is used it must be very slow so that the auger does not tend to conduct the soil on the auger flights to the surface ahead of the auger.
The intent of this provision is that the piles be constructed with grout/concrete volumes consistent with the standard as reflected in the results of the pre-production test pile program. The production piles should be installed at the same grout volume factor (within an acceptable range) that was used for the pre-production test pile installation. A pile which is not completed with adequate volume of grout or concrete may be re-drilled at the CFA Pile Contractor's discretion, and this fact recorded on the pile record. The acceptance of a re-drilled pile is at the Engineer's discretion.
Immediately upon completion of placement of the fluid grout or concrete, the CFA Pile Contractor shall remove all excess grout or concrete and spoil from the vicinity of the top of the excavation and place a suitable temporary device within the top of the excavation, extending both above and below the ground surface by at least 1 ft (0.3 m) to keep surface spoil from entering the grout or concrete column before it sets. Immediately upon placement of this temporary device, the CFA Pile Contractor shall remove any and all loose soil that has fallen into the grout or concrete column using the tools and methods contained in the approved Pile Installation Plan, and before the grout or concrete begins its initial set. The temporary device shall be removed without disturbing the natural soil surrounding the top of the pile once the grout or concrete has set.
Any required reinforcing steel shall be placed as shown on the plans by lowering the steel into the grout or concrete column while it is in a fluid state. The reinforcing steel shall be free of oil, soil, excessive rust or other deleterious material. The reinforcing steel shall be centered in the excavation by means of plastic or cementitious spacers placed at sufficient intervals along the pile and at sufficient intervals around the steel to keep the steel centered. Metallic spacers shall not be permitted. Centralizer types and spacing are subject to approval by the Engineer. If cages of reinforcing steel are called for on the approved working drawings, the longitudinal bars and lateral reinforcement (spiral or horizontal ties) shall be completely assembled and placed as a unit. Where spiral reinforcement is used, it shall be tied to the longitudinal bars at a spacing not to exceed 1 ft (0.3 m) unless otherwise shown on the plans. Welding of reinforcement is permitted only if weldable reinforcing steel is specified as part of the approved design.
The reinforcing steel shall not be spliced except at locations that are shown on the plans, and the reinforcing steel shall be free of any permanent distortion, such as bars bent by improper pickup. If a pile is required by the Engineer to be lengthened after the steel has been cut and cages have been assembled, the schedule of reinforcing steel (both longitudinal and lateral) shall be extended to the required depth by splicing. Splices should be as close to the bottom of the reinforcing cage as possible. Splicing by welding shall not be permitted unless weldable reinforcing steel is specified as part of the approved design.
The reinforcing steel shall be placed in the grout column immediately after screening the grout or concrete and before the grout or concrete begins to set. The steel may be lowered into the grout or concrete by gravity or pushed gently to final position by hand. The reinforcing steel shall not be vibrated, driven, or otherwise guided into position by mechanical means.
The reinforcing steel shall be held in position at the ground surface within the fluid grout column by supports appropriate for the reinforcement used, which shall remain in place until the grout reaches its initial set, or 24 hours, whichever is longer.
Commentary: If the soil profile contains considerable dry or moist sand, it is critical that the cage be placed as soon as possible after placement of grout or concrete, in less than 10 minutes if possible, because the grout or concrete will begin to set very quickly under such conditions. Steel spacers should not be permitted as they may greatly accelerate corrosion of the reinforcing steel, particularly above the ground water table. Centering guides made of steel, such as a wire "basket" or "football" tied at the base of single-rod reinforcement may be used. The time for initial set will vary according to mix design. Mixes with significant amounts of retarders or fluidifiers may require longer than 24 hours to reach initial set under certain conditions (cold weather, for example). The installation plan should include requirements for the duration of reinforcement support.
The CFA Pile Contractor shall cut off the tops of piles and square with the pile axis at the elevations indicated on the approved working drawings, by removing fresh grout or concrete from the top of the pile or by cutting off hardened grout or concrete down to the final cutoff point at any time after initial set has occurred. The finished top of pile shall be no more than 1 in. (25 mm) below or 3 in. (75 mm) above the elevation shown on the approved working drawings.
The CFA Pile Contractor shall maintain accurate records for each pile constructed. Similar records will be maintained by the Engineer. These records shall show:
These data shall be provided to the Engineer within 24 hours of the completion of the pile. Data collected by automated measuring and recording equipment shall be provided in numerical or graphical form.
Any testing of CFA piles shall conform to the pertinent item requirements in the relevant State DOT's Standard Specifications. If the relevant Standard Specifications do not refer to load testing for CFA piles specifically, the specifications for load testing of deep foundations may be used.
Commentary: A discussion of testing programs and procedures is contained in Chapter 7 of the FHWA "Geotechnical Circular No. 8 - Augered Cast-in-Place and Continuous Flight Auger Piles". The scope of the testing program for all structures will depend on many factors, including the anticipated loads and the quantity of piles to be installed on the project. For structures that are often referred to as "non-critical structures", such as sound barrier walls, overhead signs, and light/signal pole foundations, the Engineer may not require the same frequency of testing as for bridges and other structures. For example, a qualified contractor that consistently demonstrates proper installation techniques may not need to perform a load test for a lightly loaded pile for a "non-critical" structure. Monitoring records during construction and post-construction integrity testing may provide sufficient confidence for the Owner that the piles will carry the design load.
Piles installed for pre-production testing (including any reaction piles required for static load testing) shall include all construction, monitoring, and inspection requirements of production piles. The results of the installation and testing will be used to:
All of the parameters listed above that are established during the pre-production test pile program will be used to install the production piles.
Commentary: In past practice, the pre-production test program was primarily a load test program which demonstrated that the contractor could properly install a test pile and achieve the desired load capacity of the pile. The intent of the pre-production testing program in this specification is to install test piles to establish and/or verify installation criteria as well as load capacity. The results of the pre-production test program will then be used during production pile installation to ensure that the CFA Pile Contractor is consistently installing acceptable piles (i.e. the production pile is the same as a test pile).
Pre-production load tests will generally consist of a single or multiple static load tests, depending on the number of piles to be installed, the range of design pile capacities and the variation of subsurface conditions at the site. For very large projects, pre-production testing may include a single static load test supplemented with several piles tested by the rapid or dynamic load test methods outlined in Section (8.)3.11.2. Performing rapid or dynamic tests during the pre-production testing program will allow these methods to be calibrated against static load tests results prior to production pile installation. The CFA Pile Contractor and the Engineer will be able to best determine the appropriate quantity and level of pre-production testing based on the project conditions.
Since a major advantage of CFA piles is speed of installation, the pre-production load test program may be performed concurrent to the start of production piles to reduce additional mobilization or delay costs. This may be sufficient for small to medium sized projects. The ability to modify the design based on the results of the load test may be limited in this case.
Verification tests shall be performed on a minimum of 2% of production piles (and more as required by the Engineer) to demonstrate that the installed production piles meet the established load-deflection criteria. Verification tests can be performed using static load tests, rapid load tests (RLT), or dynamic load tests (DLT). Combinations of the various test methods may also be used as appropriate for the project. RLT and DLT test methods shall conform to the State DOT Standard Specifications or be otherwise approved by the Engineer.
Commentary: A single pre-production test only demonstrates the performance of the test pile. Performing verification tests periodically throughout production pile installation will verify that the pile installation techniques continue to provide adequate pile capacities. The use of RLT (such as Statnamic ™ or Fundex systems) or DLT (drop hammer) can often allow testing a large number of piles more efficiently, in terms of time and cost when compared to static load test methods. Calibrating the RLT or DLT tests with static load tests during the pre-production test program may reduce delays associated with analyzing the test data during pile production.
Post-installation integrity tests shall be performed on a minimum of 20% of the production piles. Such tests include, but are not limited to, sonic echo tests, impulse-response tests, cross-hole sonic tests, and backscatter gamma tests. Specific integrity test requirements are outlined in the bid documents. Test methods shall conform to the State DOT Standard Specifications or be otherwise approved by the Engineer.
The CFA Pile Contractor shall install access tubes (of a design that is acceptable to the Integrity Testing Firm) to accommodate those tests that require access to the interior of the CFA pile. These tubes shall be secured to the reinforcing steel and capped prior to placing the steel cage in the fluid grout. The piles that will include the access tubes shall be noted on the approved working drawings and in the test program.
The CFA Pile Contractor shall engage an independent Consultant, acceptable to the Engineer, to perform integrity tests and to report the results, with interpretations, to the CFA Pile Contractor and the Engineer.
Commentary: Post-installation integrity tests are valuable in establishing that a CFA Pile Contractor's procedures are producing acceptable piles on any given project. The most reliable of the post-installation integrity tests for identifying anomalies within the pile are those that use down-tube instruments, such as the cross-hole sonic (CSL) test, single-hole sonic test (SST), the backscatter gamma test, and the fiber-optic television camera test. These tests all require that the CFA Pile Contractor attach appropriate access tubing to the reinforcing steel prior to placing the steel in the grout column. They also require interpretation, which should be performed by independent, experienced, and qualified specialty consultants. It is not always possible to determine whether an anomalous reading is a defect within the pile. The final decision on acceptability of the pile must be made by the Engineer, based on construction records, the post-installation integrity test expert's report, and upon the Engineer's analysis of the possible effect on foundation performance of the potential defect.
In order to be effective, access tubes for sonic or backscatter gamma testing should be distributed evenly circumferentially around a reinforcing cage at a frequency of approximately one for every 1 ft (0.3 m) of cage diameter. It is advisable that tubes used for cross-hole sonic tests consist of Schedule 40 steel, because such tubes will remain bonded to the grout. Polyvinyl chloride (PVC) tubes do not ordinarily remain bonded to the grout beyond a few days after grout takes its initial set, and debonding will render the cross-hole sonic tests ineffective. PVC tubes should be used only for backscatter gamma testing unless cross-hole sonic tests will be performed within 72 hours of casting the grout.
The necessary frequency of post-construction integrity testing should be outlined in the owner's bid package. In situations when the Owner has little experience with CFA piles, a particularly difficult project is at hand, or the project or site conditions give reason to expect problems with pile integrity, integrity testing of more than 20% of production piles may be required. A typical reasonable approach for load-bearing piles is to subject the first 10 to 15 piles to be constructed to integrity tests to establish that the contractor's construction practice at the site is adequate. Thereafter, the frequency of such tests can be set to meet the 20% criteria, can be reduced, or perhaps eliminated if the construction records for the remaining production piles are similar to those of the initial piles that were subjected to integrity tests.
Unacceptable piles are defined as piles that do not meet the project performance criteria with regard to load carrying capacity and deflections. The following items constitute construction conditions would be considered a basis for pile rejection:
Unacceptable piles shall be replaced or repaired at the CFA Pile Contractor's expense, as directed by the Engineer.
Commentary: In addition to the above list, there may be incidents that cause a pile to be unacceptable, such as when lateral communication occurs between piles, or when excessively large grout takes occur. The action taken by the CFA Pile Contractor, Inspector, and Engineer will vary depending on the circumstances. The soil conditions, design load, and other design factors will need to be considered before the pile is rejected or the CFA Pile Contractor is allowed to repair or replace the pile. Options include, but are not limited to, abandoning the pile, switching from grout to concrete, or allow the pile to be grouted to the best of the CFA Pile Contractor's ability and then re-drill the pile after the grout has had its initial set. Judgment on the part of the Engineer and the Design engineer will need to be exercised.
CFA pile foundations shall be measured as per pile. Pre-production testing will be measured as a lump sum. Integrity tests and verification load tests will be measured on a per pile basis.
The work performed and materials furnished in accordance with this Item and measured as provided under Section (8.)4 ("Measurement") will be paid for at the unit prices bid under the payment categories listed below.
The quantities to be paid will be the quantities in each category shown on the approved working drawings and as accepted after installation by the Engineer, unless specific changes are required in writing by the Engineer. Unit prices that are bid will apply to the extension of any pile to a depth up to 120% of the depth for that pile that is shown on the approved working drawings when such an increase in depth is required by the Engineer. For such purposes, the length of the pile shall be measured between the top of the grout or concrete and the bottom of the pile. If increases in depth exceeding 120% of the depth shown on the working drawings are required by the Engineer, or if diameters other than those that are shown on the working drawings are required by the Engineer, the unit prices shall be renegotiated for those piles involved.
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