“Pile jetting” is a technique that is frequently used in conjunction with, or separate from, pile driving equipment for pile placement. Pile jetting utilizes a carefully directed and pressurized flow of water to assist in pile placement. The application of a concentrated jet of water at the pile tip disturbs a ring of sub-grade soils directly beneath it. The jetting technique liquefies the soils at the pile tip during pile placement, reducing the friction and interlocking between adjacent sub-grade soil particles around the water jet. This greatly decreases the bearing capacity of the soils below the pile tip, causing the pile to descend toward its final tip elevation with much less soil resistance, largely under its own weight. In less frequent applications, compressed air jets are used instead of pressurized water jets with the same end result.
Placing long piles in dense soils may be a time consuming endeavor with a traditional pile hammer and driving rig. Pile jetting offers significant time and cost savings over traditional pile driving, and where appropriate, jetting techniques could eliminate the need for a driving rig altogether. Pile jetting equipment usually consists of a crane with leads to place the piles, a jet pipe (or pipes) with connecting hoses, and a jet pump. Pile jetting can be used for most types of steel, wood, and concrete piles. Precast concrete piles may be fabricated with a jet-pipe already cast-in-place, if jetting is anticipated. Piles that are placed in uniform granular soils may be installed with a jet pipe placed through or near the center pile dimension. Other piles may have two water jet pipes fitted on either side to provide evenly distributed water jet coverage during placement. Design of the jet pipe outlet(s) and pump selection reflect the anticipated soil conditions and pile types.
The applied water pressure and flow rate through the jet pipe will directly influence the volume of sub-grade soils affected. Too much flow and pressure may result in poor controllability and alignment of the pile being worked, or misalign and compromise adjacent piles. Too little water flow or pressure could make the jetting technique ineffective. The type of soils supporting the piles needs to be evaluated and understood. The jetting technique creates a localized soil disturbance wherever it is used. Laboratory tests have shown pile jetting can significantly reduce the lateral strength of placed piles since the technique can erode fine soil particles from the surrounding soil matrix. Pile jetting is most effective in granular soils without significant cohesion (interlocking). Water run-off from the pump discharge hose, including erosion and turbidity control issues, is another factor that needs to be planned in advance.
The most significant challenge may be that any negative impacts of pile jetting will be latent. In a typical pile driving project, a pile hammer of known weight and drop height is used. Noting the blow counts of the pile hammer over a specified pile length allows for a straightforward assessment of pile strength. Conversely, if a pile is jetted to its final tip elevation, its final strength capacity can be empirically estimated at best, but not specifically determined.
For these reasons, the more the effects of jetting become speculative, the less recommended the technique becomes. Project costs, a completed project’s end use, and factors of safety will influence a decision to allow pile jetting, and to what extent. A less risky use of jetting would be through hard sandy soils above a firm bedrock layer that provides known bearing ability at the final pile tip elevation.
A combination of the two methods is another option. For example, a project engineer may specify that pile jetting may be used, but only through a specified pile tip elevation. This may offer some savings in time, but these would be offset by a requirement to mobilize both jetting equipment and a driving rig. A project specific cost analysis would be required. Some projects specify that pile jetting may be utilized, but the final 5-10’ must still be driven with a hammer and driving rig, which attempts to offer the advantages of both techniques at the construction contractor’s discretion.
Andrew Kimos completed the civil engineering programs at the U.S. Coast Guard Academy (B.S. 1987) and the University of Illinois (M.S. 1992) and is a registered Professional Engineer in the state of Wisconsin. He served as a design engineer, construction project manager, facilities engineer, and executive leader in the Coast Guard for over 20 years. He worked as a regional airline pilot in the western U.S. before joining the Buildipedia.com team as Operations Channel Producer.Website: buildipedia.com/channels/operations