507 Piling
Method of Measurement (507.12)
Description (507.01)
Piling is an arrangement of beams installed in the ground that provide a foundation for substructure units. These beams for the most part consist of either steel H beams (H-piling) or hollow steel tubes that are filled with concrete and sometimes reinforcing steel (Cast-in-place piles). When piles are used, they are designed to carry the entire load of the substructure unit under which they are placed.
Layout
A layout should be prepared showing piles in each substructure unit with a numbering system that identifies each pile. Dimensions that locate the piles, batter, required bearing, and any other pertinent information should be shown on the layout.
Bearing Piles
Bearing piles or service piles are piles that are driven to the required bearing capacity to serve as support for substructures.
Pile Hammers
Pile hammers are powered by compressed air, hydraulic oil pressure, or igniting diesel fuel. These hammers are classified as either single-acting hammers or double acting hammers.
In addition to power driven hammers, a drop hammer may be used having a ram weight of 3,000 pounds (1,360.8 kg) and a distance of fall not exceeding 7 feet (2.1 m).
Single-acting hammers are those that have their rams lifted by either compressed air, hydraulic oil pressure, or igniting diesel fuel. When the ram reaches the top of its stroke, it is allowed to fall back to its original position. Hammers that are powered by igniting diesel fuel, are opened on the top are considered open-ended diesel hammers. These hammers allow the ram to become exposed during driving.
Double-acting hammers are those that not only have the ram lifted by compressed air, hydraulic oil pressure, or igniting diesel fuel, but in addition to gravity, compressed air, or hydraulic oil pressure also impart a downward force on the ram.
Double-acting hammers that are diesel powered and are closed at the top, are considered closed end diesel hammers. The space between the top of the ram and the top of the hammer casing is called the bounce chamber. As the ram rises in the hammer, the volume of the bounce chamber decreases that increases the pressure of the air inside the bounce chamber. This increased air pressure imparts a downward force on the ram.
Hammer Size
The choice of size of the hammer to be used is the Contractor’s option.
The required blow count, determined by dynamic load testing as required by Item 523, must not be less then 30 blows per foot (100 blows per meter). Increasing the size of the hammer results in lower blow counts. As a result, it is possible for the Contractor to use a hammer that will be too large and the blow count will be less than 30 blows per foot (100 blows per meter). Using oversized hammers results in over driving piles deeper then necessary and thereby increases costs to the project.
The hammer must also be large enough to drive the pile to the required ultimate load and successfully perform dynamic load testing. The use of a hammer that is too small will result in a hammer that will not be large enough to impact the piles with enough energy to successfully perform a dynamic load test. Dynamic load testing does not necessarily monitor the total capacity of the pile being driven, but only monitors the ability of the pile to resist the load being applied by the pile hammer. An example of this situation is the case where a tube pile has been driven to the top of a hard layer of sand and gravel that may be capable of supporting a load of over 300 tons. If the maximum load that the pile hammer is able to place on the pile is only 120 tons, then the dynamic pile test will only register 120 tons and not over 300 tons. If the required ultimate capacity is no more then 120 tons, then the hammer will be large enough. However, if the required ultimate capacity is greater than 120 tons, then the pile hammer is not large enough to successfully perform a dynamic at the greater capacity.
Driving Piles (507.04)
A driving cap that centers the pile under the hammer and uniformly transmits the blow must be used.
Driving leads guide the travel of the hammer and cap during driving and must be capable of keeping the hammer in line with the axis of the pile. The leads should be equipped with a yoke at the base to center the pile and project beyond for anchorage.
Occasionally the plans list the design bearing in addition to, or in lieu of, the ultimate load. This should not be mistaken for the bearing to that piles are to be driven.
Abutment piling must be driven through embankments to bearing in the existing soil. Sometimes pre-bored holes are provided in the plans to assure this.
Occasionally when bearing is achieved prior to the pile being driven 80 percent of the estimated penetration, project personnel require the Contractor to continue driving the pile to achieve a penetration of 80 percent of the estimated penetration. The 80 percent of the estimated penetration is only a guide to aid the project personnel. The Contractor should not be required to overdrive the pile to obtain the 80 percent without first consulting with the Office of Structural Engineering.
In the event a pile reaches 150 percent or more of the estimated length or less than 80 percent, about two more piles should be driven in scattered locations to verify this trend. If these piles also exceed the above limits, the Office of Structural Engineering should be contacted for advice. Complete information regarding equipment, the driving logs, along with any unusual driving experiences should be made available for review. During this review, the Contractor may be permitted to continue his driving operation. However, the Contractor should not be required to attempt to drive the piles to 80 percent of the estimated penetration. He should also not cut the piling off until after the review.
If during the driving operation the pile begins to crush, the driving operation must immediately cease and the crushed section of the pile removed. This is due to the fact that the crushed section will behave similar to a sponge and the energy from the pile hammer will no longer be properly transmitted to the tip of the pile. This results in higher blow counts with minimal penetration of the pile into the ground.
Operation of the Hammer
The blow count that a pile is required to be driven to is contingent on the operation of the hammer which should be constantly observed. The operation of the hammer should be compared with the results of the dynamic load testing to determine the required blow count. The Contractor is required to provide the inspector with a means to monitor this operation.
Closed end diesel hammers must be equipped with a gage placed on the ground and connected to the bounce chamber by a hose. The gage shows the pressure developed for each stroke of the ram. A graph, included with the gage, can be used to convert the pressure to the energy developed by the hammer for each blow. The hose connecting the gage to the bounce chamber comes in different lengths that can affect the reading on the gauge. Therefore, it is important to insure that the graph corresponds with the length of hose used.
The Contractor can control the hammer’s operating energy by the use of a throttle. These hammers must be operated at the energy used when the dynamic load test was performed.
If an open ended diesel hammer is used, the Contractor must provide the inspector with a method of accurately measuring each stroke within six inches. This can be accomplished by methods such as a jump stick or electronic instruments used to measure the hammer stroke.
A jump stick is a long stick or rod marked with six-inch increments. It is important that these six-inch increments be clear and easily viewed by the inspector during the pile driving operation. The jump stick must be securely attached to the side of the pile casing to allow viewing of the distance that the top of the ram or piston travels beyond the top of the pile hammer sleeve.
H-piles
When H-piles are specified, the plans usually require that they be driven to bedrock. In special cases when they are not intended to be driven to bedrock, a zone of boulders or extremely dense granular strata is expected to be encountered.
Although an ultimate load may be given to H-piles to be driven to bedrock, usually a plan note will specify driving criterion that assures bearing on bedrock. The note requires that the piles be driven to refusal, or to 20 blows per 25 mm (1 inch).
When the bedrock is hard and unweathered, refusal can be considered as obtained after the piles contact bedrock and have been struck at least 20 more times, not for penetration of 1 inch (25 mm), to insure that firm contact has been established. Use care to avoid damaging the piles.
When the bedrock is soft or weathered and after the piles have contacted the bedrock, the piles must be driven to a minimum resistance of 20 blows per 1 inch (25 mm) for a short distance (several inches).
Many times pile points or pile shoes are specified to be welded to the tip of the piles. These points or shoes are made of cast steel as opposed to plates welded together and are used to protect the end of the pile from damage during the driving operation.
Mill test reports are required for steel H-piles and should be reviewed by the Engineer for conformance to section 711.01 of the Construction and Material Specifications. If pile points or shoes are specified, mill tests should be reviewed for conformance to section 711.01 or 711.07
Cast-in-place Piles (507.06)
A cast-in-place reinforced concrete pile consists of a steel shell that is filled with concrete. To alleviate the possibility of the piles being damaged during the pile driving operation, it is important to maintain the minimum wall thickness specified by section 507.06 of the Construction and Material Specifications.
Piles may be tapered or of uniform section. The tapered piles generally used are cylinder shells with vertical fluting or corrugations commonly referred to as monotube piles. Monotube piles can be either tapered or of a uniform diameter. All other piles of uniform section are called pipe piles. Tapered monotube point sections come equipped with a bullet-nosed tip. Pipe piles usually have a plate welded on the point that must not extend more than 1/4 inch (6 mm) beyond the surface of the pile at any point. Since cast in place metal shells have no specific material requirements, the Engineer should only assure that the metal is of domestic origin. A producing mill certification is often the simplest way to verify this.
The piles must be inspected and necessary measurements made. Due to the possibility of lateral earth pressure causing adjacent piles to collapse prior to filling with concrete, this inspection and measurement should be made after all the adjacent piles are driven. After the piles are driven, cover the tops until they are filled with concrete. Before filing with concrete, remove water and debris. Concrete required for filling the piles is Class C containing a superplasticizer admixture. After the superplasticizer has been added, the slump should range from 6 to 8 inches (150 mm to 200 mm). The concrete should be deposited in a steady small stream to ensure complete filling and consolidation. If there is reinforcing steel in piles, the concrete could become segregated from coming into contact with the reinforcing steel while it is dropping in place. Use drop chutes to eliminate this problem. No driving shall be performed within 15 feet (4.6 m) of filled piles until the concrete has cured at least seven days.
Alignment in Leads
If the hammer is not properly aligned with the pile, the energy from the hammer will not be properly transmitted to the pole. For the full effect of the energy of the hammer to be transmitted to penetration of the pile, the axis of the hammer must be in line with the axis of the pile.
Defective Piles (507.10)
A pile is considered defective if damaged to the extent that the strength of its section is reduced over 20 percent. This can occur as a collapse of the shell where less than 80 percent of the cross-sectional area remains open or where the shell is ruptured to the extent that the pile will have over 20 percent less strength.
A pile is also considered to be defective if the location of the pile at the ground surface differs from the specified location by more than 1 foot (0.3 m) for piles that are entirely underground, or by more than 3 inches (75 mm) for piles that project above the ground. No attempt should be made to draw these piles to their specified location.
Replacement Piles
If it is practical to withdraw a pile, the replacement can be driven in the specified location. If the defective pile is not withdrawn, it must be filled completely with concrete. If it is under a footing, it must be cut off slightly above the bottom of the footing where it will provide some support, but will not be paid for. A replacement pile will need to be driven beside it. The replacement should be located on the same line parallel to the side of the footing and battered slightly if necessary to avoid contacting the defective pile or adjacent piles.
When a replacement pile is driven alongside, rearrangement of reinforcing steel will be necessary. If sufficient space is not available to avoid crowding of bars, it may be necessary to cut the bars at the pile and provide bars on either side lengthened for bond. In lieu of this, the pile may be cut off below the reinforcement and the footing deepened approximately 1 foot (0.3 m) around the pile and below cutoff.
Only the replacement pile will be included for payment. Any additional material or work required to make it a satisfactory pile will be at the Contractor’s expense.
Documentation
The following data should be included in the project records:
- A driving log (Form BR-2-75) showing the blows per foot, stroke of the ram, or operating pressure for each foot of penetration
- A record of measurements that establish the pay length of each pile - This may be determined by adding the penetration length to the amount protruding out of the ground after the pile has been cut off to the proper elevation, or the total pile length driven minus cutoff, whatever is sufficiently accurate and most practical. For cast-in-place piles a statement that the inside measurement checked the pay length determined as above is to be made.
- A layout drawing that shows the location of all piles in a structure and assigns a numbering system to the piles that matches the pile number shown in the pile log (Form BR-2-75)
Form BR-2-75 and a copy of the pile layout should be submitted to the Office of Structural Engineering.
Splicing (507.09)
Splicing may be necessary to provide the required length to achieve bearing. Numerous splices using small lengths in the same pile should be avoided, particularly in an area exposed to view. Splices should be made at least three feet above the ground in order that the weld may be observed while it is subjected to the driving forces. If bearing is obtained prior to splicing a pile, the pile should still be driven a minimum of 150 blows after the splice is made in order to observe the weld. When splicing structural shapes (H-piles), welding must be performed in accordance with section 513.21 of the Construction and Material Specifications, which, among other things, requires the use of a prequalified welder. See Figure 507.A - Joint Preparation for Groove-Welded H Pile, for the method of making the required welded butt splice.
Figure 507.A - Joint Preparation for Groove-Welded H Pile
NOTES: In case a different number of passes is required than shown in Figure 507.A, a similar sequence must be followed with the finishing pass on reverse side. Back gouge root pass prior to making the finishing pass.
Method of Measurement (507.12)
The two main pay items associated with the pile driving operation are piles furnished and piles driven.
The quantity of piles accepted for payment as piles furnished will be based on the total order length specified in the plans and required by the Engineer. The order length is the pile length that the designer estimates as necessary to achieve bearing. The Contractor may elect to use piles longer or shorter then the order length as he determines necessary to meet his needs. However, the Contractor is responsible for the cost of the splice if he elects to use piles shorter then the order length which then results in the need to splice the piles to achieve the required order length.
During the driving, the Engineer must monitor the length of piling necessary to obtain bearing. If the order length given in the plans is not sufficient to achieve bearing, the Engineer should inform the Contractor of the necessary additional order length. The Engineer should inform the Contractor as soon as possible to allow him to order the piles in a timely fashion and avoid additional costs due to down time expenses. It will be necessary to negotiate with the Contractor and reimburse him for any additional splices necessary to provide additional length beyond the order length.
The pay quantity for piles driven shall be the sum of non-defective pile lengths measured along each pile’s axis from the bottom to the elevation of cutoff. This quantity will be paid in addition to the quantity of piles furnished, and may not necessarily correspond with the quantity of piles furnished.