Category:702 Load-Bearing Piles
From Engineering Policy Guide
702.1 Construction Inspection for Sec 702
The principal function of piles is to transmit loads which cannot be adequately supported at normal footing levels, to a depth where adequate support is available. When a pile passes through poor material and its tip penetrates a small distance into a sound stratum of good bearing capacity, it is called a bearing pile. The material which is penetrated may vary all the way from water to materials that would ordinarily serve to support surface footings, but cannot be used because of severe settlement restrictions. When a pile extends part way through a deep strata of limited supporting ability and develops capacity by friction on the sides of the pile, with some end bearing characteristics, it is are called a friction pile. Types normally used as friction piles are cast-in-place concrete piles. Piles are used and classified as friction pile because principal support for the pile is from surface friction, not end bearing. All pile types may be used as "batter piles", (piles driven in a sloping position) to aid in resisting horizontal loads.
Piles for footings where footing surface is below finished ground are referred to as foundation piles. Piles which support shallow caps, usually on intermediate bents, are called trestle piles. Pile types are specified on the plans.
When a type of pile is specified, a pile standard is indexed on the bridge plans. This standard will furnish specific details for the pile to be furnished.
There are two types of piles generally used by MoDOT. They are structural steel and cast in place concrete pile.
702.1.1 Cast-in-place Concrete Pile (Sec 702.2.1)
They consist of pre-driven shells of steel later filled with concrete. The most commonly used type of spirally welded steel, sometimes called pipe piles. This type pile normally has no internal reinforcement. Steel shells are usually driven without a mandrel if shell thickness permits. Where steel shells are driven, boulders or other obstructions quite often deflect the tubes from their intended course. This problem is magnified if piles are driven on a batter. This could result in bent or crushed shells. Metal shells shall hold the original form without distortion after being driven and shall be free from water, soil and other deleterious matter when concrete is cast in the shells. Any shell that has been bent or damaged should be carefully reviewed. Any decision to permit its use should be only with approval of the Bridge Division through the Division of Construction and Materials. Concrete should be directed down the center of the shell. Concrete hitting the sides can cause segregation. If concrete can be successfully directed down the center of the shell no tremie is required regardless of the height of fall.
702.1.2 Structural Steel Pile (Sec 702.2.2)
Structural steel piles are rolled H-Sections which are used in certain types of pile installations. This type of pile is probably the most widely used in the State of Missouri. These piles extend into the ground and transmit loads from footing to bearing stratum as columns. They displace a small volume of soil and can be driven with relatively close spacing. Pile tip reinforcement is sometimes specified when driving steel pile through boulders or thin layers of rock to protect the pile tip. Pile points can be accepted by certification and should be checked to see that they meet the specification requirements.
Experience has shown that corrosion of this type pile is usually not a serious problem. They must be protected for a short distance below ground level by painting as required by Standard Specification Sec 702.4.8.
702.1.3 Test Piles (Sec 702.2.5)
On structures that have unusually large quantities of piling, pile load tests are often specified. Such test loads are required by governing design specifications which limit maximum loads based on dynamic tests. For structural steel piles, where test loads are specified, the maximum 2006 design load is limited to 6.0 tons per in2 unless test loads indicate that design loads must be reduced or the footing redesigned to redistribute the loads to a lesser 4.5 tons per in2.
The pile to be load tested in a point bearing situation is normally driven to refusal on rock or shale. A friction pile to be test loaded is normally driven to a formula bearing as close as possible to design bearing value but only after a specified minimum tip elevation has been reached.
The purpose of test loading is to check effectiveness of the pile hammer and dynamic pile formula used. The load test assures a minimum safety factor of 2 based on a maximum allowable permanent set 1/4 in.
The contractor is generally required to submit in detail the proposed method of load testing. The proposal should include arrangement of hold down piles if they are to be used. If hold down pile are impractical, it may be necessary to use a direct static load.
Hydraulic jacks are normally used to apply and measure load to the tested pile. Deformation and settlement of the loaded pile are recorded by dial gauges which record to the thousandth of an inch. To ensure accuracy these gauges, backed with fixed wires, must be supported so as to be completely independent of the loading system. Methods of measuring uplift on hold down pile should be required. Load increments are applied in accordance with contract requirements. These increments are recorded in the inspector's field book.
The special provisions establish the load increments, the application intervals, and the maximum load to be applied. After the maximum load is applied for a specified time, the load is released in specified increments and intervals. The test pile load data should be plotted and reported in graphic form. Contact the Division of Construction and Materials for assistance in preparing test pile graphs. The elastic shortening of the pile may be computed by the formula:
- Es = Elastic shortening, in.
- P = Load, lbs
- L = Entire length of test pile, in.
- A = Area of cross-section of pile, in2
- E = Modulus of elasticity, usually 29 x 106, lbs/in2
Elastic shortening of any pile can usually be correlated with rebound, measured when the test pile is unloaded. Test pile data, log of readings, and test pile loading graphs should be submitted to the Division of Construction and Materials in a form which is neat, legible, and which can be reproduced. Copies of these reports prepared by Division of Construction and Materials are submitted to Bridge Division and, if it is an interstate project, to the Federal Highway Administration.
702.1.4 Pile Driving
In some instances pre-boring is required as outlined in Sec 702.4.3. Where pre-boring is required the hole shall be of a diameter not less than that of the pile and shall be large enough to avoid damage to the pile in driving through the hole into hard material. Good practice requires driving equipment capable of driving piles to necessary depth and bearing without materially damaging the piles. Heavier piles require heavier equipment, with a ratio of ram weight to pile weight sufficient to minimize energy loss due to inertia. The contractor selects equipment to meet specified energy requirements, but the inspector should be familiar with power plant, hammer, cap, cushion block, leads, and other elements used in driving. Each resident engineer may obtain data for hammers from publications issued by the individual equipment manufacturer. The contractor should have bulletins available for equipment he is using.
Pile hammers are classified by type. There are steam and air hammers, both single acting and double acting. Diesel pile hammers may be either open or enclosed ram types. A differential hammer is a double acting type. Design loads, size of pile, soil conditions, etc., establish the choice of hammer. Plans set out minimum energy requirements for individual pile size and for each substructure unit.
A single acting hammer is one in which the ram is raised by steam, air, or diesel explosion and allowed to drop, with gravity as the only downward force. The energies listed in the manufacturer's bulletins are striking energies rated in accordance with commonly accepted practice. The energy is based upon normal stroke but does not make allowances for any losses occurring in the hammer, itself, such as back-pressure, friction, or loss within the cushion block.
With insufficient lift pressure, the ram will not ascend the proper height. In fact, the hammer does not have to ascend through a full stroke to operate. The inspector should check the hammer when testing for bearing and determine if the hammer is operating at its specified number of blows per minute and at the prescribed or recommended pressure. If it is not, energy should be obtained by measuring actual stroke while hammer operates and multiplying actual length of stroke by weight of striking part. The additional distance through which the ram drops, while still in contact with the pile after impact, is not ordinarily taken into account. Neither is "cushion block" loss.
During easy driving with a large set per blow a reduction in number of blows per minute may occur. In consequence, the full theoretical hammer stroke will often not be produced.
A double acting hammer is one in which steam or air pressure raises the ram then accelerates the down stroke. The differential acting hammer is a type of double acting hammer which provides additional pressure to the ram during the downward stroke.
The foot-pounds of energy for a double acting hammer is dependent upon the number of strokes per minute produced with a given steam or air pressure. For example, a typical table of "actual energies" for one commonly used hammer shows that "e" varies from 9500 foot-pounds at 90 strokes per minute up to 13,100 foot-pounds at 105 strokes per minute. The inspector must, for this type hammer, log the number of blows per minute, noting pressure at the hammer, and use the corresponding energies when making a bearing determination by use of the dynamic formula. Refer to manufacturer's bulletins to determine what energies to use for the number of blows per minute. Calculations based on steam or air pressure are misleading because no two setups are identical, and it is impossible to determine the mean effective pressure in the working cylinder from gauge pressure.
The Diesel Pile Hammer is classed either as a single acting or double acting type. Most states have accepted this hammer with some qualification. Many arbitrarily discount energies set out by the manufacturer, accepting only some percentage of the maximum rated energy. Missouri sets this figure at 75% for single acting diesel hammers. Inspectors should acquaint themselves with the diesel hammer's physical qualities and determine when the hammer is developing full stroke.
A diesel hammer is a self contained unit, including power plant, cylinder, piston, or ram, fuel tank, pump, injectors, and other pertinent parts. The ram of these hammers is raised by explosion of diesel fuel ignited in the cup or anvil of the hammer. Some types of diesel hammers are called double acting hammers. This type of hammer has the ram enclosed. As the ram travels upward, the piston compresses air in the bounce chamber-compressor tank. This compressed air adds to the acceleration of the ram during its downward stroke. It is necessary to use a "Bounce Pressure" gauge on this type of hammer to establish the usable energy for dynamic formula bearing determination. For this type of diesel hammer, explosive force is not taken into account to determine usable energy. Use of the "gauge energy" permits full use of 2E in the "double acting" bearing formula and energy is not to be discounted to 75%.
The single acting series of diesel hammers have a "rampiston" which can be partially seen during the upward stroke. If the full "maximum" manufacturer's energy is to be used in the specific dynamic formula then the inspector must determine that the ram is falling through a normal stroke. Failure to operate properly is usually the result of mechanical problems which the contractor must correct. In isolated instances, failure of the hammer to operate with a normal stroke may be caused by the elastic rebound of the pile and bearing material. If the ram is not falling through its usual stroke, the energy "E" used should not be the maximum striking energy but the energy which can be calculated from the weight of the ram (W) times the actual stroke (H) through which it falls, or (W x H). The height (H) is determined from the observed exposed length of ram as the ram travels upward. When this method is employed, the energy should not be discounted. Where the energy is measured by W x H, the inspector should use the single acting formula from the specifications. This procedure may not be used to increase the energy allowance above 0.75E.
702.1.4.1 Pile Formula
MoDOT specifies the use of the FHWA-Modified Gates equation (See Sec 702.4.10) to calculate pile bearing. The Pile Driving Set Calculator is a spreadsheet that can be used to calculate the pile set to help determine when to stop driving pile.
A qualified inspector should be assigned continuously on pile driving work to see that each pile is driven to specified bearing, that all piles are properly located, and that the required number are driven. The inspector must keep a detailed record of the data for each pile. The record should show for each pile, its position, tip and cut-off diameter (for timber), total length in place, length placed in leads, tip elevation, batter, and number of blows per inch at the time driving is stopped. The number of blows per inch is based on penetration for the last series of 10 to 20 blows. The inspector should record all pertinent information regarding the hammer used so that a review and check of bearing may be made. Any unusual occurrences or delay during driving should be recorded. When driving friction pile, the inspector should make periodic bearing checks as the pile is being driven to know at any time approximate bearing of the pile if problems should develop.
Contractors that elect to place lifting holes in piling in lieu of using a choker cable may be permitted to do so with the following provisions. The concern of burning lifting holes in piling is that undesirable capacity reductions may occur. Lifting holes would only be permissible provided they would not remain in the piling lengths used for the completed structure. That is, lifting holes would need to be in an excess length of end piling which would either be cut off after driving, or in the case of splicing the holed end would be removed before splicing on the next section. Any added risk of buckling or damage to the piling that may result from a weakened cross section during driving is the contractor's responsibility.
There shall be no additional payment for the additional length of piling to compensate for removing the cut-off ends with the holes.
It is good practice for piling in a group or cluster to be driven in sequence which proceeds from the center of the group each way to the outer rows of pile. This will usually avoid uplift and loss of bearing in previously driven pile.
In many cases piles are to be driven to rock or shale. The FHWA-Modified Gates equation, designed for friction pile, is not altogether applicable in these circumstances. Since the bearing value at the time of practical refusal is not an accurate bearing resistance figure, the inspector should keep the sounding data well in mind as the tip of the pile nears anticipated elevations of hard material. The pile should be seated on or into hard material with blows which will not damage the tip of the pile. Each bearing pile should be tested for "practical refusal" unless it is clearly seated on solid rock.
The inspector should examine the plans carefully for changes in hammer requirements. For structural steel piles, for example, the pile data table on the bridge plans specify minimum energy requirements for a pile hammer for each individual substructure unit. Under the pile data table, the inspector will find other supplementary notes which should be taken into consideration for proper driving of structural steel piles. It is especially important that such piles which are to be seated on rock or shale be driven and tested for "practical refusal" as specified in Sec 702.4.11. When the pile is well seated, the driving should cease. The inspector should record in the diary that the pile has been driven into shale or rock as the case may be. Either record penetration and bearing in the case of practical refusal or note "refusal on rock" in the case of absolute refusal on rock. Such notations will indicate full compliance with bearing requirements of the plans.
Piles to be driven should be plainly marked at a distance from the tip equal to the distance from ground line to the elevation shown on the soundings for rock or shale. It is also good practice to mark the pile from the tip equal to the distance from the ground surface down to any layer of boulders, thin rock strata, or other hard or firm material which might cause unnatural point resistance or unusual driving conditions. The pile driving foreman or contractor's foreman should be told the significance of such marks and all personnel should be guided accordingly. This procedure will result in fewer broken, "broomed", or damaged piles.
Splices may be required to extend structural steel or steel shell pile to reach adequate bearing. No direct payment will be made for splices that are within the plan pile length. Any splices outside of plan length that are required to achieve bearing will be paid for as an additional 8 feet of pile in place at the contract unit price, per authorized splice.
Field splices have a greater potential of failure during driving than the original furnished pile. Therefore it is preferable to have a minimum amount of field splicing. Sec 702.4.6 states, "Full length piles shall be driven wherever possible and practical." A full-length pile should be used unless there is clearance, shipping, excessive cost, or other considerations, which make it impractical.
The inspector must ensure that all piles have been properly inspected. Precast concrete pile will normally have been inspected during casting and curing by the Division of Construction and Materials. In such cases, they will provide the resident engineer with proper inspection reports. If the piles are cast on the project, they will, of course, be inspected the same as any other concrete item. Files should contain any applicable inspection reports on aggregate, cement and reinforcing steel. The concrete Plant Inspector's Report and compressive test reports will serve to document acceptability of piles. This would also be true for concrete in cast-in-place piles. Steel shells for cast-in-place piles and structural steel piles are normally inspected by project forces. Inspection should include dimensions, wall thickness of shells, visual inspection of welds, closure plates, etc. The contractor is required to furnish certified mill test reports for the steel. Heat numbers of pile should be checked against heat numbers on the mill test reports. The resident engineer reports results of inspection on a Fabrication Inspection Report, Form B-708R2, or the equivalent data may be recorded in an alternate format. A spreadsheet version of the form is available to facilitate the automatic creation of a SiteManager record for use by the Division of Construction and Materials. This information will be retained in the project file with mill test reports attached.
702.1.4.3 Manufactured Pile Splices
To date MoDOT has been submitted one type of manufactured pile splicer and approved it for use with certain stipulations. The AFB Champion H-Pile Splicer HP-30000 has been approved. The following are recommended guidelines that should be used beyond the manufacturers recommended assembly procedure for the use of the HP-30000 splicers.
- 1. It would be permissible for non-flexible bent locations only. This would include intermediate bents on pile footings and semi-deep abutments. This splicer system should not be used on flexible bents, such as pile cap intermediate bents, where the concrete beam is supported on a single row of exposed piling nor on integral or non-integral end bents.
- 2. Full penetration groove welds connecting the pile flanges are required. The partial penetration groove welds as recommended by the manufacturer are not acceptable.
- 3. A 5/16" minimum fillet weld should be added at both ends of the splicer, welded to the pile webs. The length of this weld should be at least 1/2 the depth of the pile. This weld was not a recommendation of the manufacturer. This weld is for additional safety in the event that the splicer is damaged or torn from being snagged on rock material.
702.1.5 Pile Driving Documentation
The inspector should record in detail all important facts regarding driving of each pile. The field book notes should be organized in a sequence similar to that shown in the Pile Driving Worksheet.
The sample form in the Pile Driving Worksheet illustrates a typical page of completed pile driving data for pre-cast concrete pile. Data in a similar form will be filled out when driving timber pile.
Figure 700.2 is an illustration of field book data for driving structural steel pile. The inspector records the actual length used and notes the number of pieces incorporated in the length. When structural steel pile is driven, there is often a piece left over from the in-place pile which becomes excess or left-over pile. The contractor may wish to use such a piece on another state highway project. If transfer to another project is desired, extra copies of the certified mill test reports should be made which can be used to have the left over pile reinspected on a future project.
If test pile is a contract item, it must be driven to specified minimum tip elevation regardless of the bearing achieved. After this elevation is reached, driving must continue until one of the following three conditions has been met:
- l. The pile driven to full length.
- 2. The pile driven to refusal.
- 3. The pile driven to a capacity 50 percent greater than plan bearing.
These conditions are specified in Sec 702.4.1. It is important that a complete driving log be developed. The pile should be marked off in foot increments. The driving record should then show the number of blows for each foot. Some arrangement is necessary to check number of blows per foot without stopping the driving. If there is a sudden sharp change in the number of blows for a given penetration, it may be necessary to check bearing for intermediate increments to develop an accurate graph. The results of specified test pile driving are to be reported on Test Pile Data form. Contact the Division of Construction and Materials for assistance in reporting test pile data.
702.2 Laboratory Procedures for Sec 702
This establishes procedures for Laboratory testing and reporting samples of steel strand used in precast-prestressed concrete piles.
Tests for stress-relieved strand shall consist of examination for fabrication requirements and tension tests performed according to AASHTO M203. Test results and calculations shall be recorded through SiteManager.
702.2.2 Sample Record
The sample record shall be completed in SiteManager as described in Automation Section and shall indicate acceptance, qualified acceptance, or rejection. Appropriate remarks, as described in EPG 106.20 Reporting, are to be included in the remarks to clarify conditions of acceptance or rejection. Test results shall be reported on the appropriate templates under the Tests tab.