507 Bearing Piles
Description (507.01)
General (507.02)
Materials (507.03)
Driving Piles (507.04)
Determination of Required Blow Count (507.05)
Cast-In-Place Piles (507.06)
H-Piles (507.07)
Timber Piles (507.08)
Splices (507.09)
Defective Piles (507.10)
Prebored Holes (507.11)
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