705 - Foundation Piling
Foundation piles are typically steel H sections or cast-in-place (CIP) concrete in steel shells. Occasionally the plans specify treated timber piles. Foundation piles provide support for the bridge or structure when the existing soils do not provide adequate bearing capacity to support a spread footing.
Foundation piles may be straight or battered. Generally, piles are battered no flatter than 3V:1H. This can be increased to 2.5V:1H or even 2V:1H where soil conditions are not good enough to provide sufficient lateral pile resistance and this will be noted on the plans. It can be difficult to maintain driving accuracy when the batter is flatter than 3V:1H.
The most common steel section used for piles is the HP section. For example, for a HP 12X53 section, H stands for the “section shape”, P stands for “pile”, the 12 represents the height, h, in inches, and the 53 represents the unit weight per foot of material. Figure 705-1 shows the typical HP section configuration. Note the weight is actual and the sizes are nominal. Example an HP14X117 has an h dimension of 14.2 and a b dimension of 14.9. This is important to remember when setting clearances and fitting within other structural items.
Steel pipe piles for CIP piles are typically designated by the outer radius and the pipe wall thickness. All pipe pile used on MDOT projects must be seamless, meaning the welds must be done from the electric resistance or fusion welding processes.
It should be noted that static analysis is performed during the design phase of the project to determine the appropriate design pile section, design pile lengths, design pile tip elevations. These values are shown on the plans, and are used to determine the estimated pile lengths, and estimated pile tip elevations.
Piles are to be stored off the ground on cribbing when on site. The following should be checked prior to incorporation into the project:
- Check for bending or distortion of steel piles.
- Ensure steel piles do not have any section loss, drilled holes, or stray weld strikes.
- Ensure dirt, water or other deleterious materials do not enter steel shells for CIP piles.
- Timber piles are to be closely stacked together to prevent sagging.
Ensure piles are treated with preservative per subsection 912.06 of the Standard Specifications for Construction.
Steel Pile and CIP Pile Shells
Ensure steel piles, steel pile shells for CIP piles, and pile points are of the material grade and yield strength as called for on the plans. If required, ensure the Contractor provides proper documentation of galvanizing. Steel piles brought to the site can be new or used, however, used piles must be in good condition, and the Contractor must provide mill test reports, and required Buy America/Build America certification. Contact the Geotechnical Services areas for any questions regarding acceptable condition.
Check the plans and specifications to see if pile points are to be used. Pile points are welded to the driving end of HP, and CIP piles to aid in tough driving conditions and/or end bearing in rock. A typical HP pile point is shown in Figure 5 below. Typical pile points for CIP piles are shown in Figure 705-2.
Ensure concrete for CIP piles is Concrete, Grade 4000, per subsection 705.02 of the Standard Specifications for Construction.
Ensure the contractor is using appropriate equipment to drive production piles and test piles without damaging them. The following pile driving equipment is subject to the approval of the MDOT Geotechnical Services Section area based on the pile driveability analysis:
- Pile Driving Hammer
- Hammer cushion
- Pile helmet
- Pile cushion
- Any other pertinent equipment
Submit MDOT form 1956 – LRFD Pile Driving and Equipment Data to the Geotechnical Services Section for review and creation of pile driving charts.
See Figure 2 for typical pile driving equipment set up with the hammer supported in swinging leads:
Pile driving equipment is to be selected such that the piles are installed at a rate of 2 blows per inch to 10 blows per inch.
Pile stresses shall not exceed the values shown in Table 705-1 of the Standard Specifications for Construction.
The Geotechnical Services Section will predict pile stresses using the wave equation analysis, based on hammer efficiencies shown in Table 705-2 of the Standard Specifications for Construction.
The pile driving system proposed by the Contractor is subject to approval from the Geotechnical Services Section.
Air (steam), diesel or hydraulic hammers can be used.
Air Impact Hammers
These hammers are powered by compressed air or steam provided from an air compressor or steam boiler.
The hammer is generally assumed to be developing its rated energy when it is striking the number of blows per minute as shown in the manufacturer's catalog. If a hammer will not develop its rated number of blows at bearing, the inspector should suspend driving operations. In most cases, the problem is caused by insufficient pressure at the hammer because of line losses caused by leaky connections, excessive line length, etc.
Diesel Impact Hammers
There are Closed-Ended and Open-Ended diesel impact hammers.
- Closed ended hammers are closed at the top and contain a bounce chamber to throw the ram back down upon rebound. The maximum rated energy should be used in calculating bearing only when the hammer is on the verge of floating on top of the pile. If this floating action is unattainable, the Contractor should furnish an instrument to measure the bounce chamber pressure and resulting hammer output and use the manufacturers chart to correlate pressure reading with the energy being delivered by the hammer.
- Open-Ended are the most common diesel hammers and are open at the top, allowing observation of the ram going up and down. Diesel fuel is ignited, which thrusts the ram upward, followed by the ram falling and striking the pile top. A saximeter is typically used to determine the hammer energy. The saximeter works by timing the blows per minute and applies the rate to the freefall equation (32 ft/s²). The application accounts for both an action (drop) and a reaction (rebound) which doubles the time given in the normal equation. The saximeters are calibrated by the Geotechnical Services Section but if needed, can be verified by manually applying the formula.
In an open ended hammer, the maximum energy is simply the mass of ram X the height of stroke. Example 4,000 Lbs. ram X 10 Ft stroke = 40,000 ft·Lbs. The cushion and other factors such as weight of appurtenances are then applied to determine the final driving energy.
Hydraulic Impact Hammers
Hydraulic hammers are fueled by a hydraulic unit, with the hammer energy correlated through pressure readings.
A wave equation analysis must be performed by the Contractor, and check by the Geotechnical Services area to determine the nominal driven bearing of the pile
Drop Impact Hammers
Drop impact hammers are gravity type hammers, where a weight is simply lifted and released. Drop hammers may be prohibited on timber piles, check the plans and specifications for any restrictions.
Drop Impact hammers are not to be used for piles where the required nominal pile driving resistance exceeds 200 kips.
Vibratory equipment may be used for sticking piles, but not for advancing piles. The maximum depth for sticking piles with vibratory or variable moment vibratory hammers needs to be reviewed prior to beginning. Do not use vibratory hammers, driving aids, followers or prebored holes unless approved by the Geotechnical Services area. Vibration monitoring may be required in some cases.
Additional Equipment or Hammers
Hammer cushions are to be used to prevent damage to the pile during driving by dampening reflected forces and ensuring a uniform strike at the top of the pile. Certain hammer types are designed for use without a cushion.
Wood, elastomeric, wire rope or asbestos cannot be used, as these materials tend to deform during driving. This information should be available in contractor submittal for equipment noted above. Ensure a striker plate is used on top of the cushion to ensure uniform compression of the material. Modern hammer cushions use phenolic laminates (synthetic resin), or special alloyed nylon blocks with aluminum layers. See Figure 705-4 for typical nylon hammer cushions.
Inspect the cushion before production pile driving commences, and inspect after 100 hours of operation, or prior to driving at each subsequent structure. The striker plate portion of the helmet needs to be removed to gain access to the cushion. Ensure the contractor replaces the cushion once the thickness is reduced more than 25% of the original thickness.
Hammer helmets distribute the hammer blow uniformly and concentrically to the pile top. Ensure the helmet surface in contact with the pile is smooth and aligned with the hammer base and the pile top.
Ensure the helmet is guided with the leads, and not allowed to swing freely.
For timber piles, ensure the helmet does not exceed the pile top by more than 2 inches. The Contractor must trim the pile top to fit into the helmet.
Pile cushions are only required when CIP concrete piles require redriving after the concrete placement and curing.
Leads are typically box shaped frames used align the pile and hammer in the appropriate position throughout the driving operation and maintain hammer and pile alignment to ensure concentric impact on the top of the pile during striking.
Leads used for pile driving are either swing or fixed type. Ensure swing type leads contain a pile gate at the bottom. Ensure leads of appropriate length to eliminate the need for followers.
The Contractor should be encouraged to use the fixed-type wherever possible as they do a better job of holding the pile in position.
Pile followers are essentially extensions of the piles being driven allowing the piles to be driven at a higher elevation. Followers are only to be used with approval from the Geotechnical Services Section as there is uncertainty as to how much hammer energy is transferred across the joint between the follower and the production pile.
To track pile driving operations, use MDOT form 1990 – Cofferdam Installation, Piling Placement, and Tremie Pour Inspector’s Checklist, and MDOT form 1161L – Foundation Piling Record, LRFD
Prior to the start of driving and during the operation, check the plans and specifications, and pay close attention to all of the following:
- Ensure the proper orientation and layout of the pile. The contractor will do the actual layout. For example, on integral abutments with a single row of piles, the web of the pile is oriented perpendicular to the construction centerline of the bridge. This is also referred to orienting the pile about the weak axis of bending.
- Ensure all piles meet the material grade requirements as specified in the plans, and 12SP906(A).
- Piles increment layout whether it is in foot marks or inch marks must be precise. Use foot marks when the capacity of the pile is gradual. When the pile takes up in a very dense soil, hard pan clay, or some type of rock formation inch marks may need to be used. If inch increments are used, then the pile should be driven until the pile reaches capacity for 3 consecutive inches or there are 12 blows in one inch.
- Review the Soil Borings and SPT Blow Count Data shown on the plans. Become familiar with the soil borings in the footing area. Be aware of potential hard layers above the estimated pile tip elevations that can give false bearing. These areas are typically underlain by unsuitable material.
- Review the plans and specifications for preboring requirements. If required, ensure holes are prebored to the elevations shown on the plans, and the hole diameter is 6 inches greater than the pile dimensions. Preboring also includes the following:
- Boring pile holes
- Disposal of excavated material
- Backfilling voids
- Installing and removing temporary casings
- Providing and disposing of drilling slurry
- Restriking completed piles within a radius of 20 feet
- Equipment operating costs, unless including in the Pile Driving Equipment, Furn pay item
- Ensure all excavation and embankment is complete to the bottom of substructure concrete elevation.
- If the Contractor is using a template, check the template dimension against the plan dimensions.
- Check to ensure that battered piles will clear the toe of any cofferdam sheet piling.
- On certain types of abutments an error in grade could easily result in the piling being too close to or possibly outside the wall forms.
Excavation and Fill
Pile Preboring to Facilitate Driving
Ensure holes are prebored to the elevations shown on the plans, and the hole diameter is 6 inches greater than the pile dimensions. Preboring also includes the following:
- Boring pile holes
- Disposal of excavated material
- Backfilling voids
- Installing and removing temporary casings
- Providing and disposing of drilling slurry
- Restriking completed piles within a radius of 20 feet.
- Equipment operating costs, unless including in the Pile Driving Equipment, Furn pay item.
During construction, the nominal pile driving resistance is measured and verified during pile driving with the dynamic formula or dynamic testing methods.
Each pile should be checked early during the driving and periodically through the process to ensure the vertical tolerances are met and to ensure the pile tops are square with the axis of the pile.
If impenetrable obstructions are encountered during driving, direct the Contractor to remove and reuse the pile, or cut off and drive a new pile. Ensure the pile is cut to the lowest possible elevation, and drive another pile, adjusted laterally. Where pile movement is required the Engineer of Record must approve the relocation of the pile and address weather subsequent items of work need to be changed to accommodate the changes in loading or alignment.
Check the plans for design pile lengths (pile lengths shown on the plans) or estimated pile lengths (pile lengths shown on plans used as guide if the nominal pile driving resistance is shown on the plans.
If the plans show design pile lengths, ensure piles are driven to the design pile tip elevations shown on the plans, unless driving operations attain absolute refusal (150% of the nominal pile driving resistance shown on the plans).
If the plans show estimated pile lengths, install piles to the following penetration:
- The nominal pile driving resistance equals at least the required nominal pile driving resistance; and
- The bottom of the pile is at or below the minimum pile penetration elevation shown on the plans.
Ensure piles are not driven past absolute refusal unless Dynamic Testing is required. Any unnecessary production piling overdriving is to be avoided as it adds unnecessary project cost.
Ensure completed piles are driven according to the following accuracy criteria:
- 1/4 inch per foot from vertical or batter line
- Pile cutoff location is within 6 inches from plan position unless noted on plans.
- 9 inches between the edges of piles and the outline of the concrete forms
- Pile bents are to meet the tolerances as shown on the plans.
- Pile cut off within 1”.
- Pile orientation should be per plans. If pile is out of orientation, then the designer should be called to see if the pile being out of the correct orientation is a problem
Do not allow the Contractor to laterally pull piles to correct misalignment or splice subsequent piles to correct misalignment.
Take level readings to measure pile heave after driving. If piles heave up during driving of adjacent piles, direct the Contractor to re-drive to the required bearing capacity or penetration elevation. For heaved CIP piles, ensure the concrete obtains its 28-day compressive strength prior to re-drive.
For battered piles, ensure the piles are driven to the angle shown on the plans. Check the batter with a carpenter’s level attached to the vertical side of the Contractor’s template. Note that battered piles use a different pile calculation than vertical piles.
If at any time, the nominal pile driving resistance cannot met, but minimum pile penetration is achieved, contact the Geotechnical Services area for guidance.
Ensure battered piles are cut as shown on the plans.
After driving CIP shell piling, inspection should be done by using a mirror or light provided by the Contractor, suitable for illuminating the shell interiors. The shells must be free of bends, kinks or other deformations that would impair the completed pile strength.
After driving the piling, piling tops must be immediately covered to keep out rain, sand, ensure safety, etc. The piling should be cut off within 1 inch of the specified elevation and filled with grade 4000 grade concrete as soon as possible.
Ensure any subsequent pile driving within a radius of 25 feet from concrete element pours are done after concrete has attained 75 percent of its design strength.
Check for any heaved piles. Heaved piles are to be redriven and will be paid for as extra work (for design-bid-build projects only).
Removal of obstructions that require special equipment or tools will be measured and paid for as extra work per subsection 109.05.D of the Standard Specifications or Construction. A budget amount will be established to pay for removing obstructions. If the Contractor and the Engineer cannot agree on a lump sum price, the Engineer will direct the work to be done on a force account basis.
Driving a reference stake adjacent to the pile should be avoided because clay soil has the tendency to heave, and sandy soil has the tendency to consolidate.
Soil heaving and consolidating soil would raise or lower the reference stake causing erroneous penetration readings affecting the recorded bearing.
Besides the penetration readings, the inspector needs to simultaneously record the distance the pile point is below cutoff. This is accomplished by having the piling marked off in 1-foot intervals starting from the point and referencing the piling marks to a grade stake set at cutoff elevation.
When test piles are shown in the plans and specifications, the pile lengths shown on the plans are for estimating purposes only. There are typically two test piles per substructure unit.
Test piles should be driven prior to the production piling so the results can be used as a guide for ordering piles and driving all production piling. Contractors typically place test and production piling orders simultaneously due to mill and fabricator schedules.
Ensure test piles are driven to the minimum pile length shown on the plans, practical refusal (110% of nominal), whichever is greater.
To track test pile driving information, use MDOT form 1157L – Test Pile Record, LRFD
It is important at the start of the blow count to reference the pile to some fixed object, preferably the leads, providing they are not reset during the observation period.
Restriking is allowed for test piles only using dynamic testing and analysis. Do not use the dynamic formula for pile restrikes.
After the 48 hours waiting period, The Contractor may restrike the piles. Contact the Geotechnical Services area to determine the nominal pile driving resistance based on the number of blows necessary to advance the pile an additional 3 inches.
Ensure the hammer is warmed up with 25 blows for restriking.
All piles for a given foundation unit must be driven with the same hammer operated under the same conditions with the same cushion material as used to drive test piles. Changing hammers requires additional test pile driving at the Contractor’s expense.
Steel piles may be furnished in full length sections, or they may be spliced according to the splicing method as shown on the plans. Note when pile splicers are not permitted in the plans.
Splices must be made in accordance with subsection 705.03.C.2.d of the Standard Specifications. The Contractor must submit Form 5627 – Pile Welding Quality Control Plan in accordance with special provision 20SP705(A) – Quality Control Plan for Welding Pile Splices
All welded splices must be done in accordance with the MDOT Field Manual for Pile Welding. Ensure that the welder is certified for the position in which they are welding.
Welded splices for piles must be made only by qualified welders using E7015, E7016 or E7018 electrodes. The terms qualified welder and certified welder have a specific meaning and should be understood when field welding any steel components. The qualification of welders will be determined by whether the pile is determined on the plans to be main member or non-main member.
Splices are welded in accordance with AASHTO/AWS D1.5 – Bridge Welding Code (AWS D1.5) as modified herein. A complete MDOT Form 0395 AASHTO/AWS D1.5 – Field Welding Plan is submitted to the Engineer for review and approval prior to welding.
Welding operators, and tack welders must be qualified in accordance with AWS D1.5 for main member pile welding and MDOT Standard Specifications for Construction Section 705 7-20 AWS D1.1 for non-main member pile welding. Testing is required in accordance with MDOT’s Welder Qualification Program for main member welding and must be witnessed by the Engineer. Testing is required in accordance with MDOT’s Welder Certification Program for non-main member welding. Welder performance endorsements from other agencies will not be accepted.
Shells for CIP concrete piles must be spliced using a chill ring or other type backing detail as shown on the plans.
Steel piles must be spliced as shown on the plans using preformed-preassembled bent plates or flat splice plates for pier piles, and other piles that are not considered primary members as called for on the plans. Pile acceptance is based on the Contractor’s conformance to their QA/QC plan and can be visual inspected.
Full penetration butt weld, or Complete Joint Penetration (CJP) welds are required for piles in integral abutments, and when otherwise defined as primary members on the plans. Pile acceptance for these members is based on 100% ultrasonic thickness testing (UT).
All splices require welding. Splicing devices which do not develop the full plan weld are not permitted.
Scarfing the upper splice length of the piles preparatory to butt welding must be done by a cutting torch using a guide to assure a smoother chamber. A maximum of 1/8 inch land (shoulder) must be retained. If the end of the driven length of pile is damaged during the driving operation, it must be trimmed.
When it is necessary to cut a small section off the end of the driven pile length to square the end, burning must be done using a guide either clamped or tack welded in place. All adhering slag must be removed and the joint surfaces must be thoroughly cleaned by wire brushing before welding. A root opening of the butt joint needs to provide a 1/8 inch clear opening at the nearest point. When the 1/8 inch opening at the nearest point allows more than a 1/4 inch opening at other points, correction must be made to provide the correct opening before welding. Beveling by cutting will be required for correcting openings wider than 1/4 inch.
The root opening may be held by two wedges before tacking. The members to be welded must be brought into correct alignment along the pile length and held rigidly by clamps or by dogging. Clamps or dogs may be removed after tack welding sufficiently to hold the member in place.
Where the parts are restrained against bending due to eccentricity in alignment, an offset not exceeding 10 percent thickness of the thinner part joint, but no more than 1/8 inch may be permitted as a departure from the theoretical alignment. In correcting misalignment, the parts must not be drawn into a greater slope than 1/2 inch per foot. Offset measurement will be based upon the centerline of parts unless otherwise shown on the drawing.
The root pass needs to be a stringer bead, placed with sufficient amperage to fully penetrate through the joint. Passes must be made symmetrically and need to alternate on both flanges to minimize distortion. After the face side of both flanges and web has been welded completely, a backing bead needs to be made on the opposite side.
If piles heave up during driving adjacent piles, redrive heaved-up piles to the required bearing capacity or penetration. If the Engineer detects pile heave for CIP concrete pile shells filled with concrete, redrive the piles to the original position after the concrete obtains the required strength using a pile-cushion system that is approved by the Engineer. Ensure the appropriate records for test piles are kept, including:
- Number of blows per inch for the driven length, and stroke height.
- As-driven length of test pile
- Cutoff elevation
- Penetration into the ground
- Number of splices
Ensure test piles not included in production pile locations are cutoff or pulled.
The capacity of foundation piles is achieved through skin friction between the soil and the surfaces of the pile, tip capacity, or a combination of the two. The following methods are used to attain required nominal pile driving resistance:
- Piles are driven to the minimum pile penetration elevation, and nominal resistance, or
- Static load tests for piles with a nominal pile driving resistance greater than 600 kips, or
- Using the Dynamic Formula (Modified Gates Formula) for production piles less than 600 kips nominal pile driving resistance, or
- Pile Dynamic Analysis (PDA) testing and analysis. Any PDA testing on MDOT projects must have a representative from the Geotechnical Unit present during the testing of the piles.
A saximeter or some other certified electronic device is to be used to verify blow counts, and the results compared to the dynamic analysis results. The saximeter works for several hammer types as follows:
- For open ended diesel hammers, the saximeter computes stroke and potential energy from the measured blows per minute. This information is then multiplied by the hammer weight to obtain the hammer energy.
- For other types of hammers, the Contractor is required to provide an approved? electronic measuring device.
For high-capacity piles, < 600 kip nominal pile resistance, static load tests may be required. Load testing details will be in the plans and specifications. This generally involves the placement of large amounts of weight on a group of piles and recording measurements on vertical displacements and deflections. The Geotechnical Services Section must approve the load test methods and be onsite during the testing of the pile.
The Dynamic Formula (FHWA Modified Gates Formula) is not to be used on piles with a required nominal pile driving resistance greater than 600 kips. Check the plans and specifications for dynamic testing with signal matching.
For battered piles, use a hammer energy reduction coefficient. These formulas are applicable under the following conditions – Notice if these conditions are not met:
- Hammers and unrestricted free fall
- Pile tops are not broomed, crushed, or splintered.
- The hammer exhibits no appreciable bounce after striking the pile.
- Penetration is at a uniform or uniformly decreasing weight.
Methods to correct piles damaged by internal defects, improper driving, driven below cutoff elevation, or driven outside of plan locations are as follows:
- Withdraw the pile and replace with a new, longer stick (if feasible).
- Drive a second pile adjacent to the defective pile (verify with Bridge Design, and the Geotechnical Services
- Splice or build up the pile or extend a portion of the footing to properly embed the pile.
Placing Concrete in CIP Concrete Piles
Concrete should be cast slowly so that air pockets are not formed in the pile. Concrete for each pile must be poured in one continuous pour for the entire pile length. No cold joints are allowed. An inspector should be always present during placement. Concrete must not be placed in any pile until all piles within a 20-foot radius have been driven and accepted. For CIP piles, ensure concrete is placed per subsection 706.03.H of the Standard Specifications for Construction. Concrete in the pile shell’s upper third (not to exceed 25 feet) is to be vibrated by an approved method.
Protective Coating for Steel Piles and CIP Concrete Piles
Cleaning Steel Piles and Steel Pile Shells
Ensure the Contractor cleans the shells of CIP concrete piles where embedded 1 foot or more in structural concrete.