Category:702 Load-Bearing Piles
From Engineering Policy Guide
This article contains information about piling types and pile driving criteria used by Construction and Materials Division for on-site pile and pile driving inspection purposes (in the field). This information was part of the former Field Inspection Guidance of the Construction and Materials Manual (see EPG 106 Control of Materials) and is continually updated.
|Pile Driving Set Calculator|
702.1 Pile Types and Testing
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 deep strata of limited supporting resistance and capacity is developed primarily from surface friction along the sides of the pile with some end bearing, the pile is generally referred to as a friction pile. A type of pile normally used as a friction pile is a cast-in-place concrete pile(CIP pile).
A battered pile is a pile driven on an inclination (a sloping position) to aid in resisting horizontal loads.
Piles utilized as part of concrete footings where the piles are below the finished ground are referred to as foundation piles. The pile-footing system is generally referred to as a pile cap footing. Piles which support bent caps are called trestle piles and the pile-bent system is generally referred to as a pile cap bent.
Pile types are specified on the bridge plans.
There are two types of piles generally used by MoDOT. They are structural steel and cast in place concrete pile. Structural steel piling are generally referred to as HP piling and three different standard AISC shapes are typically utilized: HP10 x 42, HP12 x 53 and HP14 x 73. Concrete piling are generally referred to as cast-in-place or CIP piling because the concrete is poured and cast in steel shells which are driven first or pre-driven.
702.1.1 Cast-In-Place Concrete Piles (CIP) (Sec 702.2.1)
CIP concrete piling consist of pre-driven steel shells later filled with concrete. The most commonly used type of steel pipe is spirally welded steel sometimes referred to as pipe pile. All steel pipes must be in accordance with ASTM A 252 Grade 2 or 3. CIP pile normally has no internal steel reinforcing bars. Steel shells are usually driven without a mandrel if shell thickness is adequate.
Where steel shells are driven, boulders or other obstructions quite often deflect the tubes from their intended course. This problem is worsened if piles are driven on a batter and could result in bent or crushed shells. Steel 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. In this case, any decision to allow use of bent or damaged shells should be with approval of the Bridge Division and the Construction and Materials Division.
Concrete should be directed down the center of the shell. Concrete hitting the sides of the shell can cause segregation. If concrete can be successfully directed down the center of the shell, a tremie is not required regardless of the height of fall.
702.1.2 Structural Steel Piles (HP) (Sec 702.2.2)
Structural steel piles are rolled “H”-Sections, often called H-Piles or HP piles, which comes from the AISC designation “HP”, which are used in certain types of pile installations. This type of pile is probably the most widely used in the State of Missouri. HP piles can penetrate into the ground and transmit loads from footings or bent caps to bearing stratum as columns. They displace a small volume of soil and can be driven with relatively close spacing. Pile point reinforcement is always required when driving steel pile to soft or hard rock to protect the pile point. 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 of pile tcan be a serious maintenance problem in the future. Therefore, all piling is required to be galvanized.
702.1.3 Probe Piles and Static Load Test Piles
- There two different types of pile testing.
- Probe Pile Test (formerly “Test Pile”)
- Load Test Pile
Earlier editions of the Missouri Standard Specifications and AASHTO Specifications referred to probe piles as "test piles". Probe pile testing is designed to test how a pile goes into the ground by probing the ground, in a sense, but can also proof the ground capacity. It is a description of pile driving (and proof capacity secondarily).
Load testing is designed to test the ground capacity, but can also test how a pile goes into the ground. It is a description of pile ultimate capacity.
So while a probe pile tests (proofs) the pile and pile driving primarily to determine lengths, the static pile load tests establish pile nominal resistance or carrying capacity primarily where loads are greater than normal carrying capacity.
702.1.3.1 Probe Piles
Probe piles are piles driven on site to determine driving conditions, verify hammer size and impact energy, determine pile order lengths and pile driving criteria. More than likely, probe piles, or test piles as they were called were popular and only used when the Department was driving precast or prestressed concrete piles when pile order length was more critical.
Probe piles (formerly called "test" piles) and their use are described in the Standard Specifications. Probe piles are only required when designated on the bridge plans with a pay item. Locations for probe piles may be given on the bridge plans or the absence of which means that it is deferred to the discretion of the Engineer.
Probe piles are good for jobs where there are an unusually large number of piles, or anticipated unusual site or ground conditions, i.e. nonuniform or varying.
702.1.3.2 Static Load Test Pile
Static load test piles are typically described for use in a Bridge Special Provision, and they are only required when designated on the bridge plans with a pay item.
Static pile load tests (also referred to as just "load test piles") are piles driven to a pre-determined penetration and then tested by applying static loads incrementally until either proof of load or failure occurs. A pile load test pile can be used as a probe pile in the sense that they are usually performed on site and driven by dynamic testing or dynamic formula, and then static load tested. Therefore, information can be determined about driving conditions, hammer sizing, pile lengths and pile driving criteria.
On structures that have unusually large quantities of piling, pile load tests may be 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 load tested is normally driven to a dynamic formula resistance as close as possible to the plan value of the nominal axial compressive resistance but only after a specified minimum tip elevation has been reached.
The purpose of load testing 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 inch.
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 piles are impractical, it may be necessary to use a direct static load.
Hydraulic jacks are normally used to apply and measure load to the load-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 Bridge Special Provision establishes 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 load test pile data should be plotted and reported in graphic form. Contact the Construction and Materials Division 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 load test pile is unloaded. Load test pile data, log of readings, and load test pile loading graphs should be submitted to the Construction and Materials Division in a form which is neat, legible, and which can be reproduced. Copies of these reports prepared by Construction and Materials Division are submitted to Bridge Division and, if it is an interstate project, to the Federal Highway Administration.
702.1.4 Pile Driving
702.1.4.1 Hammer Types
Good practice requires driving equipment capable of driving piles to the necessary penetration and nominal axial compressive resistance without damaging the piles at the pile point, top of the pile or bending piles. Heavier piles may require heavier equipment due to greater nominal axial compressive resistance requirements. 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 hammer energy requirements for individual pile size and for each substructure unit.
Single Acting Hammer
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 resistance 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 smaller blowcount (large set per blow) a reduction in number of blows per minute may occur. Consequently, the hammer stroke while measurable may not be accurate since the hammer/pile is moving downward with the stroke increasing stroke height.
Double Acting Hammer
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.
A diesel pile hammer is classed either as a single acting or double acting type. 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 nominal axial compressive resistance determination.
The single acting series of diesel hammers have a "rampiston" which can be partially seen during the upward stroke. If the manufacturer's rated energy is to be used in the 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 be the energy which can be calculated from the weight of the ram times the actual stroke through which it falls. The height is determined from the observed exposed length of ram as the ram travels upward. .
702.1.4.2 Pile Dynamic Formula
MoDOT specifies the use of the FHWA-modified Gates equation (see Sec 702.4.10) to calculate pile nominal axial resistance. The Pile Driving Set Calculator is a spreadsheet that can be used to calculate the blowcount (N) in blows per inch of pile permanent set (BPI) to determine when to stop driving pile.
702.1.4.3 Pile Driving Specifics, Special Conditions and Inspection
Preparation for Pile Driving
A qualified inspector should be assigned continuously on pile driving work to see that each pile is driven to the specified nominal axial compressive resistance and that all piles are properly located and 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 (BPI) 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 nominal axial compressive resistance may be made. Any unusual occurrences or delay during driving should be recorded. When driving friction pile, the inspector should make periodic resistance checks as the pile is being driven to know at any time the approximate nominal axial compressive resistance 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, i.e. 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.
Pile Driving in Groups
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 resistance in previously driven pile. For a single row of piles, the sequence should follow end to end or middle to out but never end to middle from both ends.
The pile numbering on the As Built Pile Data plan sheet in the bridge plans is a random numbering scheme for the purposes of recording data and unrelated to pile driving sequencing. Just keep track of both numbering schemes to avoid confusion when recording during driving and recording final data on plan sheet.
Pile Driving to Soft and Hard Rock
In many cases piles are to be driven to rock or shale. The FHWA-modified Gates equation is applicable for soils, hard material and soft rock. It is not applicable in hard rock at which point an inspector’s sense of anticipating elevation of hard rock coupled with the physical response of hammer and pile upon nearing and impacting hard rock is critical to properly seating HP piles on hard rock with limited to no pile damage. Since the nominal axial compressive resistance at the time of practical refusal is not an accurate resistance, the inspector should always be aware of the sounding data 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 proofed 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 specifies minimum hammer energy requirements for a pile hammer for each individual substructure. Included in the pile data table, the inspector will find other supplementary notes which should be taken into consideration for the proper and cautious driving of structural steel piles. It is especially important that steel HP piles which are to be seated on rock or shale be driven and proofed for "pile 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 pile resistance requirements on 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 unusual driving conditions and point resistance. The pile driving foreman or contractor's foreman should be instructed on the significance of such marks and all personnel should be instructed accordingly. This procedure will result in fewer damaged piles. The goal is to have no damaged piles.
Splices may be required to extend structural steel or steel shell pile to reach adequate nominal axial compressive resistance. 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 resistance 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 would make it impractical.
Manufactured Pile Splices
MoDOT has received and approved one type of manufactured pile splicer for use with recommended guidelines. The AFB Champion H-Pile Splicer HP-30000 has been approved. The following are recommended guidelines that should be used beyond the manufacturer’s 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 or 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.
The inspector must ensure that all piles have been properly inspected. Piles that are cast-in-place on the job shall be inspected using the same inspection procedures as for 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. 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 should report the results of inspection on a Fabrication Inspection Report, Form B-708R2, or an alternate format may be used. A spreadsheet version of the form is available to facilitate the automatic creation of a SiteManager record for use by the Construction and Materials Division. This information will be retained in the project file with mill test reports attached.
Pile Driving and Preboring
In some instances preboring is required as outlined in Sec 702.4.3. Preboring can be required on the bridge plans:
- 1. When there is more than five ft. of embankment that has been in place less than five years to avoid buildup of downdrag forces (called negative skin friction)
- 2. When hard material must be penetrated to meet minimum tip elevation requirements
- 3. When oversized holes in hard material or rock must be constructed to allow lateral pile movement
In any case, the requirement for preboring will be noted on the bridge plans for each pile with an elevation given for depth of preboring which is used to estimate and check proposed preboring quantities on the plans.
For prebored holes not in hard rock, holes shall be filled with sand or other approved materials either prior to or after pile placement. For prebored holes in hard rock, holes shall be filled with sand or other approved materials prior to pile placement. Filling the hole with sand first will condense sand and stabilize pile while driving on hard rock. The driving criteria for driving piling on hard rock shall be the same as given in Sec 702.4.11.
Where pre-boring is required the hole shall be of a diameter not less than that of the pile unless oversized as explained previously and shall be large enough to avoid damage to the pile in driving through the hole when in soft or hard material including rock.
Caution is warranted when driving piling in prebored holes in hard rock. Pile instability is increased because of initially loose sand condition. Absence of more stabilizing stiffer soils and hard material over rock that can act to reduce bending and buckling can cause more pile spring and bounce. Pile point damage is at increased levels of risk because proofing rock in excess of driving criteria given for driving piling on hard rock can be greater since there is only loose sand acting in friction along the sides of the pile which if compacted would normally tend to dissipate energy as the impact wave travels down the pile. This is why sand is placed in the hole prior to pile placement in order to increase the frictional condition while increasing buckling stability of the pile.
702.1.4.4 DFI Documents on Pile Driving and Hammers
Deep Foundations Institute (DFI) produced and published two critical documents in 1979 and then republished them in 1995 and 1997 related to pile hammers and pile driving. Copies of these documents were purchased from DFI and permission was granted by them for making these documents available on-line to MoDOT personnel only. The document links will not be available to external users outside MoDOT IS network environment.
These documents are provided for further guidance and understanding of pile driving and equipment. They do not reflect the policy of MoDOT but can be used to support and reinforce decisions involving pile driving and equipment since the background and source of this information is time tested and produced by a reputable organization.
Information published within this main document includes:
- Provides Information on soil investigation, the various pile types, pile driving by impact methods, pile tests, dynamic pile testing and analysis, static load testing, pile hammers and pile driving machinery and ancillary equipment
- Explains increases in driving resistance with depth in uniform soils
- Explains driving resistance as a function of N-values
- Explains changes in driving resistance as soil layers change
- Explains pile markings, finer increments and how these are effectively used
- Provide good description of scenarios when blowcounts are changing
- Provide basic rules to be followed DURING driving and when approaching termination
- Provide guidance on termination criteria
- Provides explanation of dynamic pile testing
- Provides explanation of static load testing (which we have been doing lately because of university of Missouri-Rolla research project)
- Written from the perspective of the pile inspector and presents advice as to the inspector's role and responsibilities in the pile installation and quality assurance processes
- Provides invaluable training for inexperienced Inspectors and a useful reference guide for the experienced inspector or crew member.
Information published within this companion document includes:
- Explains fundamentals of hammer efficiency and hammer measurement
- Provides basic information about hammer types
- Provides operational conformance checklists for inspectors for each hammer type
- Provides possible helpful suggestions for troubleshooting and what an inspector can offer to the pile crew in assisting them.
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 castin-place concrete pile. Data in a similar form will be filled out when driving structural steel 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 excess pile reinspected on a future project.
If probe pile is a contract pay item, it must be driven to specified minimum tip elevation regardless of the nominal axial compressive resistance achieved. After this elevation is reached, driving must continue until one of the following three conditions has been met:
- l. The pile is driven to full length
- 2. The pile is driven to refusal
- 3. The pile is driven to a capacity 50 percent greater than plan resistance.
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 resistance for intermediate increments to develop an accurate graph. The results of specified probe pile driving are to be reported on Probe Pile Data form. Contact the Construction and Materials Division for assistance in reporting probe pile data.