STORAGE AND HANDLING OF PILES
When handling treated timber piles it is highly recommended one use rope slings. Avoid the use of chain slings, hooks, or other methods that will break through the protective treatment. Avoid dropping the timber piles and bruising or breaking the outer fibers. It is advisable to stack treated timber piles for storage on timber sills so that the piles may be picked up without hooking. The application of preservative oil to cuts, holes and abrasions should not be minimized. This treatment is vital to the life of the timber pile and is important enough to warrant careful attention.
Concrete piles must be handled with care. It is very easy to cause cracks by indifferent handling. Cracks may open up under driving, and may spall and powder to such an extent as to seriously lessen the strength or life of the pile. Shock, vibration, or excessive deflection should be avoided by using proper equipment and thoughtful handling. When piles are picked up with adjustable slings, blocking should be used to prevent breaking off the corners. Unless special lifting devices are attached, the pick-up points shall be plainly marked on all piles before removal from the casting bed and all lifting shall be done at these points. If the piles have been allowed to dry after curing, they shall be wetted at least 6 hours before being driven and shall be kept moist until driven.
When loading steel H-piles at the fabricator’s plant, the individual piles must be placed with webs vertical and blocked so that the flanges will not be bent. There is perhaps greater danger of damage to the steel when it is unloaded from the car, hauled to the work, and unloaded from the truck or trailer at the site. The project inspector must observe that the handling methods at the jobsite are performed carefully to avoid damage to the piles.
JETTING AND PREBORING
Jetting is a means of obtaining pile penetration through elimination or reduction of resistance at the pile tip by the use of water, air, or a combination of these two media, delivered by pressure through hoses and pipes. The soil is eroded below the tip of the pile, often permitting penetration merely by the dead mass of the pile and the hammer. It is particularly effective when displacement type piles are to be driven through dense fine sand to desired penetration in firm soils below, but should not be used in embankments or other areas where it would tend to destroy densities which have been purposely built into the soils.
Also, unless good judgment is exercised, jetting could destroy the bearing value of piles already driven, especially when piles are closely spaced or when they tend to drift away from their prescribed course. Water jetting has been useful as an aid to driving displacement types of piles in sand formations in streams where water is readily available and pile penetration is equally as important as bearing capacity.
Although the Specifications currently specify certain requirements pertaining to the jetting equipment, the prime objective should be that of performance. Equipment which would not be satisfactory in some cases may be entirely adequate in other cases.
Preboring, as the word implies, is merely boring holes through or into soils prior to driving piling. It is perhaps the most expedient and popular method of obtaining pile penetration of displacement piles through or into high density embankments, or through crusty upper stratum that must be penetrated because of weak underlying soils. Preboring is generally accomplished by the use of a power auger of a diameter larger than the maximum diameter of the piles to be driven, mounted on the crane used for the pile driving or on separate equipment. There are many variations of preboring equipment; some of these are covered in considerable detail in the booklet by Dames and Moore entitled Timber Foundation Pile Study.
INSPECTION OF PILE DRIVING TIMBER PILES
Pile driving inspection is a very important function and is deserving of undivided attention. Some agencies specializing in piling go so far as to recommend that a trained soils engineer be present to approve each pile installation and to revise procedures as varying soil conditions are encountered. Certainly the inspector should have sufficient knowledge of soil types and characteristics so as to be able to relate the soils information shown on the survey sheet to the pile driving operations and difficulties.
The inspector should be present at all times when piles are being driven. This is particularly true when driving timber piles because breakage below the ground surface may occur at any time and may be detected only by an alert inspector. It would also be true of any piles driven through or into hard strata, such as rock or hardpan, since the tips may be damaged by overdriving or carelessness unless a capable inspector is present.
Treated timber piles are generally inspected for quality and treatment prior to delivery, and are impression-stamped so that the pile driving inspector will know that they have been inspected and approved. Occasionally a slightly under-size pile may get by the plant inspector. Specification 1503 states “all materials furnished shall be in conformance with the lines, grades, cross sections, dimensions, and material requirements, including tolerances, shown in the Plans indicated in the Specifications”. This gives the Engineer authority to use some discretion regarding acceptance of occasional borderline or slightly undersize piles. Piles which are slightly out of specifications for crooks or twists should be called to the attention of the foreman and accepted only if they can be satisfactorily driven without splitting or breaking.
Untreated timber piles, except for treatment, are subject to the same inspection as are treated piles. However, these piles are often delivered to the jobsite without previous inspection; if so, complete inspection for type, quality, straightness, knots, peeling, density, and butt and tip diameters must be made at the site and documented. It is very important that timber piles in a bent be accurately located and properly driven, because little can be done to correct their alignment after driving without causing damage to the piles.
The best procedure to assure accurate alignment is to drive the end piles for each bent first, using piles with the largest diameters, and then placing a heavy timber on each side long enough to extend beyond each end pile. These timbers should be tied to each other using bolts or scabs. The remaining piles in the bent can then be spotted and driven within this yoke or frame, which will assist in maintaining their alignment. A hole should be dug for each pile as a means of getting it started properly. Each pile should be observed very closely while it is being driven, to assure plumbness or specified batter. Also, when driving is hard, check closely for evidence of cracking, breaking or splitting, so that driving can be stopped before the pile is severely damaged.
The test pile for each unit is generally placed at one end so that the original pile number and spacing can be changed, if necessary to support the superimposed load. After the first unit has been driven, blocking can be used between this unit and the timber guides for the next unit. Extra care taken during the pile driving, with respect to the proper location of each pile, will minimize the problems encountered in placing the caps, bracing or backing. This is especially true with regard to the corner piles at abutments.
Timber piles that do not line up properly after driving should be brought to line before making the cut-off, so that the top of the pile, after cut-off will be at the correct elevation and plane and will provide full bearing for the pile cap. Wooden straight edges should be placed on each side of the pile bent to act as a guide for the saw, and the actual sawing should be done by experienced sawyers. Power saws are extremely difficult to control to the degree required for this type of work and should not be used except when the Contractor has demonstrated that the proper degree of accuracy can be obtained.
Any portion of the top of the timber pile which projects outside of the front edge of the wing cap should be trimmed off with a sharp axe or adz in a neat manner to an approximate 45 degree slope down and outward from the front edge of the wing cap. Regardless of the method used, the workmanship should be neat and systematic.
Where zinc sheets are specified in the plans or special provisions for the tops of timber piles, the portion of the sheet which extends outside of the periphery of the pile should be folded down alongside the pile. The folds should then be creased and folded back against the pile. The folds should then be securely fastened to the pile with galvanized roofing nails. Rounding off the corners of a square sheet before placing will produce neater results than would otherwise be obtained. Fabric protection can be placed in much the same manner as described above for zinc sheets. Treatment of tops of timber piles with preservative is required prior to placement of zinc sheeting.
INSPECTION OF PILE DRIVING STEEL PILES
Steel pile is not inspected prior to delivery to the jobsite. Therefore, pile inspection must be performed by the project inspector. For Steel H-Piles and Steel Shells for Cast-In-Place Concrete piles, Specifications generally require the Contractor to submit copies of mill shipping papers and certified mill test reports for all steel piling prior to delivery of piling to the site. These mill test reports are provided by the producer steel mill and list physical properties and chemical analysis of each mill “heat” of steel involved, and specify domestic origin of steel and its manufacture. The contractor is responsible to verify that invoices and mill test reports correspond to piling delivered. Upon delivery, spot check identification markings on the steel to be certain the source and heat numbers match those on the mill test reports. At the same time, inspect the material for proper section size and gauge, physical defects such as kinks or buckles, and quality of welding.
If any piece of piling is not marked with a heat number, the Project Engineer should have the Contractor test the material at an independent testing lab to ensure the pieces are associated with the mill test reports provided. Two tensile tests and one chemistry test should be conducted from one out of ten pieces of piling of the same size and thickness with unknown identity.
Piles that are driven prior to material testing should be identified in the “Pile Driving Report”. Price adjustments or other determinations can then be made at a later date, should this be necessary because of the deficiencies in the material. In any event, contractors should be made aware that piles driven prior to delivery of required materials information are subject to price adjustment until quality and domestic origin has been properly established.
Welding for splices, except in isolated cases must be made by DOT certified welders. A typical exception might be when one or two unanticipated splices are necessary and a certified welder is not immediately available, but a reputable uncertified welder is available. Keep in mind that this should be interpreted as applying only to exceptional and isolated cases, and should not be general practice. When trestle piles or pile bents are involved, painting requirements should be reviewed. Generally a complete prime coat is required for the full length of steel piles which extend above ground except for those sections below splices which are at least 600 mm (2 feet) below ground.
Holes for handling steel H-piles should not be made in the flanges of the piles, except when they are made near the top of the pile and are to be included in the cut-off portion or in the portion which will be embedded in the concrete. Burning holes with a torch should not be permitted, even in the web of the pile, because of carelessness generally associated with the torch. It has been agreed, in a discussion with representatives of the Federal Highway Administration, that holes may be drilled in the webs near the longitudinal centerline of the pile, but that these holes should be no larger than necessary to accommodate the connector used for lifting the pile. In any event, caution must be observed when using holes in steel piles for handling purposes. Sharp or jagged edges may cut or fray the lifting cable, and thereby weaken it possibly causing premature failure. Although it is the Contractor’s responsibility to conduct his/her works in a safe manner, an alert inspector should report unsafe conditions to the foreman as well as to the Engineer in charge. Pile driving is an inherently dangerous operation, but precautionary measures can be done to improve conditions.