507 Bearing Piles
Determination of Required Blow Count (507.05)
Method of Measurement (507.12)
Documentation Requirements – 507 Bearing Piles
Description (507.01)
A pile is a structural column of steel, concrete, timber, etc. that is installed in the ground to support a structure above it. Piles are required when the soil near the surface is not strong enough to support the structure or when the soil may be scoured away. Piles transfer the loads from the structure to deep layers of soil or rock that are capable of supporting the load.
The term “bearing pile” refers to a pile that is used to support a structure. A bearing pile is also called a service pile or a production pile.
The plans include a foundation layout which identifies each pile with a unique number and indicates the pile type, size, and direction of batter (if any). Use the unique number to identify the pile in the construction documentation.
General (507.02)
There are many types of piles, however the Department generally uses either cast-in-place reinforced concrete piles or steel H-piles that are driven into the soil using an impact hammer. The cast-in-place piles are constructed by driving a hollow steel tube, capped at the bottom with a steel plate, into the ground and then filling the tube with concrete.
The plans give the estimated length for each pile. However, the Contractor decides whether to drive a single pile segment for the entire estimated length, or to drive shorter segments and splice them together as he drives the pile into the ground.
Materials (507.03)
The steel for H-piles must conform to ASTM 572, Grade 50 (Fy=50 ksi). This is the industry standard for H-piles. The steel pipe for cast-in-place piles is not specified, however steel conforming to ASTM 252 Grade 2 (Fy=35 ksi) is commonly provided.
Driving Piles (507.04)
Piles are typically either driven to a specified capacity (Ultimate Bearing Value) or to refusal on bedrock. The Ultimate Bearing Value, or UBV, is equivalent to the ultimate pile capacity (in Allowable Stress Design) and the nominal pile resistance (in LRFD). The UBV is the required capacity of the pile. Sometimes, the plans list either the design bearing, the design load, or the factored load in addition to, or in place of, the UBV. Do not mistake the design load for the capacity to which the piles are to be driven.
Typically, H-piles are used when piles are driven to refusal on bedrock and cast-in-place piles are used when piles are driven to a specified capacity. However, H-piles are sometimes used when driving to a specified capacity.
In some cases, such as bridges over water where scour may be a concern, the plans may indicate a minimum pile tip elevation in addition to the UBV. If both a UBV and a minimum pile tip elevation are specified for the piles, both criteria must be met. If the pile is driven to the required tip elevation before reaching the UBV, continue driving until the pile has the required capacity. If the pile is driven to the UBV before reaching the minimum pile tip elevation, continue driving until the pile tip is at the required elevation.
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.
Piles Driving Longer or Shorter than Estimated
In the event a pile reaches 150 percent or more of the estimated depth without achieving capacity, or the event of a pile reaching capacity in less than 80 percent of the estimated depth, about two more piles should be driven in scattered locations to verify this trend. If these piles also exceed the above limits, contact the Office of Construction Administration or the Office of Structural Engineering for advice. You may also contact the District Geotechnical Engineer for advice. Complete information regarding equipment, the driving logs, along with any unusual driving experiences should be provided 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.
Occasionally when bearing is achieved before the pile has been 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 depth. This is not recommended. The value of 80 percent of the estimated penetration is only a guide to aid project personnel. Overdriving the pile may result in damage to the pile or the pile hammer. Do not require the Contractor to overdrive the pile to obtain the 80 percent length without first consulting with the Office of Construction Administration, the Office of Structural Engineering, or the District Geotechnical Engineer.
Pile Driving Equipment
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.
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 at least 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 falls back to its original position by gravity. Hammers that are powered by igniting diesel fuel and open on the top are considered open-end 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 and increases the pressure of the air inside the bounce chamber. This increased air pressure imparts a downward force on the ram.
Hammer Size
The Contractor chooses the size of the hammer to use.
The required blow count, determined by dynamic load testing as required by Item 523, must not be less than 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 may result in pile damage and increase the risk of alignment difficulties.
The hammer must also be large enough to drive the pile to the required UBV 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 cannot determine the total capacity of the pile being driven if the energy applied to the pile by the pile hammer is too low. An example of this situation is the case where a cast-in-place 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 300 tons. If the required UBV is 120 tons or less, then the hammer is large enough. However, if the required UBV is greater than 120 tons, then the pile hammer is not large enough to successfully perform a dynamic load test. (Note: This is a simple example to demonstrate the concept. The actual relationship between hammer energy and pile capacity is much more complex.)
Performance of the Pile Hammer
The driving criteria or blow count that a pile must be driven to depends on the performance of the pile hammer. If the performance of the hammer changes, then the appropriate driving criteria will also change. Therefore, the performance of the hammer should be constantly observed. The performance 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.
Open-end diesel hammers are the most common type of pile hammer for highway contractors in Ohio. A relatively easy way to monitor the performance of an open-end diesel hammer is to watch the stroke of the ram. During the dynamic load testing, watch how far the rings on the ram come out of the hammer. Then during pile driving, make sure that the rings are coming out of the hammer about the same distance. Because the ram of an open-end diesel hammer falls by gravity, the stroke of an open-end diesel hammer can be estimated from the blow rate (blows per minute) using the following equation.
where:
h = stroke of pile hammer (feet)
bpm = blows per minute
(from Design and Construction of
Driven Pile Foundations,
FHWA NHI-05-043, page 21-28)
For convenience, the following table gives the results of the above equation for a typical range of values. Additionally, the relationship between stroke and blows per minute for a particular pile hammer can be determined from the dynamic load test.
|
Blows per Minute |
Stroke (ft) |
|
Blows per Minute |
Stroke (ft) |
|
37 |
10.2 |
|
42 |
7.9 |
|
38 |
9.7 |
|
44 |
7.2 |
|
39 |
9.2 |
|
46 |
6.5 |
|
40 |
8.7 |
|
48 |
6.0 |
|
41 |
8.3 |
|
50 |
5.5 |
Trying to count the blows per minute while also keeping track of the blows per foot is difficult. An easier way to determine the blows per minute while counting the blows per foot during pile driving is to measure the number of seconds required to drive one foot of piling. Then use the following equation to calculate the blows per minute.
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 check 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 or fuel setting. The hammer must be operated during pile driving at the same setting used when the dynamic load test was performed.
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 pile. For the full effect of the hammer energy to be transmitted to penetration of the pile, the axis of the hammer must be in line with the axis of the pile.
Determination of Required Blow Count (507.05)
The driving criteria or required blow count is determined from the dynamic load test results. See Section 523. The first two piles are driven with the dynamic load test equipment attached. The testing company should provide a preliminary recommendation for the driving criteria immediately after driving these two piles. The driving criteria will be a minimum blows per foot for the pile driving. For open-end diesel hammers, the driving criteria will also include a minimum hammer stroke.
Drive the rest of the piles to the recommended driving criteria. It is generally not necessary to ensure the pile has a blow count greater than the required blow count for three or more consecutive feet. For example, if the required blow count is 43 blows per foot, it is not generally necessary to drive the pile until the blow count is greater than 43 for three consecutive feet. See the following table for examples. The exceptions to this are if there is a minimum pile tip elevation, the depth of penetration is less than 80 percent of the estimate, or the pile has to be struck with 150 blows to inspect a splice.
Pile Driving Examples
Required Blow Count is 43 blows/ft
|
Penetration |
Blows/Ft |
|
Penetration |
Blows/Ft |
|
37-38 |
28 |
|
41-42 |
21 |
|
38-39 |
33 |
|
42-43 |
40 |
|
39-40 |
42 |
|
43-44 |
40 |
|
40-41 |
45 |
Should stop |
44-45 |
43 |
|
41-42 |
43 |
driving here. |
45-46 |
44 |
|
42-43 |
41 |
|
46-47 |
48 |
|
43-44 |
43 |
|
47-48 |
41 |
|
44-45 |
44 |
|
48-49 |
46 |
|
45-46 |
46 |
|
49-50 |
50 |
Cast-In-Place Piles (507.06)
A cast-in-place reinforced concrete pile consists of a steel shell that is filled with concrete. To minimize the possibility of the piles being damaged during the pile driving operation, it is important to maintain the minimum wall thickness specified in 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.
H-piles (507.07)
When H-piles are specified, the plans usually require that they be driven to refusal on bedrock. The standard plan note gives a driving criteria of 20 blows per inch. However, the note may allow the Contractor to perform a dynamic load test, at his own expense, to determine the driving criteria instead of using the 20 blows per inch criteria.
When the bedrock is hard and unweathered, refusal is obtained after the piles contact bedrock and have been struck at least 20 more times, with a penetration less than or equal to 1 inch (25 mm), to ensure that firm contact has been established. Use care to avoid damaging the piles.
When the bedrock is soft or weathered, driving refusal is obtained at a resistance of 20 blows per 1 inch (25 mm).
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 711.03 of the Construction and Material Specifications. If pile points or shoes are specified, mill tests should be reviewed for conformance to 711.01 or 711.07
Timber Piles (507.08)
Although still included in the specifications, timber piles are no longer used by the Department.
Splices (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 impacts from the pile hammer. If bearing is obtained prior to observing the weld during three feet of driving, 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 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. For H-piles, the plans may include a note that allows the use of a manufactured splicer in place of the full penetration butt weld.
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 the reverse side. Back gouge root pass prior to making the finishing pass.
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 (such as in a capped-pile pier). 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.
Prebored Holes (507.11)
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 than 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 than 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.
Documentation Requirements - 507 Bearing Piles
1. Use piling forms CA-S-3 and CA-S-8
2. State difference between piling delivered and piling driven. Excess piling furnished can be kept by project owner (ODOT or Local Public Agency).
3. Measurements should be made to the nearest 0.1 feet (0.05 m)
4. Make layout sheet showing pile location, pile number, test boring, structure number; north arrow, project number, whether pile is battered or straight, required bearing skew if applicable, offset of pile, hammer that is being used
5. Height of drop hammer before release (if used)
The following data should be included in the project records:
1. A driving log, CA-S-3 (Form BR-2-75) showing the blows per foot, stroke of the ram, or operating pressure for each foot of penetration
2. 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.
3. 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, CA-S-3 (Form BR-2-75)
4. CA-S-3 (Form BR-2-75) and a copy of the pile layout should be submitted to the Office of Structural Engineering.