PILE CONSTRUCTION

The inspection of pile driving is an extremely important phase of an inspector's duties. There is probably no other type of construction work that requires quick, sound decisions to be made on the spot as frequently. Every inspector assigned to supervise pile-driving operations needs to be familiar with all aspects, procedures, and details regarding the materials, the equipment, and the techniques used in pile-driving operations.

Pile Driving

The accuracy with which piles must be located varies with the nature of the work. Extreme accuracy in positioning is not essential; for example, in a large mat foundation supported on piles at relatively wide centers. However, accuracy is important in closely spaced footing clusters, and it is particularly important in such cases as bridge bent piers, in which the upper part of the piles is exposed and any misalignment is immediately noticeable and objectionable. The inspector must make sure that piles are positioned with the accuracy required by the circumstances but should avoid arbitrary and unduly burdensome requirements when they are not reasonable. When close tolerances are essential, the specifications may require the use of templates to assure proper centering. In such cases, the inspector must make sure that the templates provided are strong enough to withstand the abuse to which they are subjected and that they are maintained square and rigid.

You must take care to see that the pile is handled without undue strain or shock, that the pile is set plumb in the leads, and that the pile-driver leads are themselves plumb. Give special attention to the rigging used for lifting precast concrete piles to prevent overstraining and cracking the piles. It is difficult to reposition a pile that has been started slightly out of position, and it is almost impossible to straighten up a pile that has been started crooked.

The inspector must be present during the driving of every pile and a digging permit must be present along with the proper safety procedures for the driving of the pile.

The inspector must make sure that the hammer used is heavy enough to be effective, considering the weight of the pile. The hammer should weigh as much as the pile being driven, and preferably it should be up to twice the weight of the pile. Hammers that are too light waste their energy in impact and inertia effects. In driving the piles down, force is so little that its effectiveness causes gross erroneous indications of bearing capacity, far above what the pile will actually sustain. The hammer must strike the pile squarely, or additional energy will be lost in springing the pile sideways. When batter piles are being driven, the inspector must make sure that the leads are set for the proper batter and that the piles are held true to top position and batter as they are driven. If timber piles are being driven, the inspector must make sure that protective rings of proper size are used at the heads of the piles to protect them from splitting and that pile shoes of approved design are used and properly secured when called for or required.

Use jetting to facilitate driving piles through many types of soil. As the inspector, follow the specifications insofar as they prescribe specific requirements for jetting. Usually, jetting is done by one or more pipes with nozzles hung from the driver and operated independently of the pile. Some precast concrete piles have been used with the jet pipes cast in the piles. Such cast-in jets cannot be moved around as needed to obtain the best effectiveness, and their use has been largely discontinued. The inspector should permit the widest latitude possible regarding methods of jetting. However, he or she must make sure that jetting is discontinued a sufficient distance above the final point elevation of the pile to ensure that the pile base is in undisturbed soil and that the bearing capacity, calculated from the average penetration of the last few blows, is a reliable index.

The inspector must make sure that the piles are driven to the minimum point elevation specified or to the minimum penetration below the ground, bottom of footing, or mud line, as may be specified. If neither point elevation nor minimum penetration is shown or specified, the inspector should make sure that the driving is continued until the penetration per blow is reduced to the limit indicated by the formula for the required bearing capacity. If jetting is permitted, the inspector must make sure that the driving is continued into undisturbed material. If the required bearing capacity is obtained with every short pile, the inspector should report the situation to his or her superior promptly and request further instructions.