Many inner coastal waterways are home to some of the finest beaches and most expensive real estate in the country. Like everyone else, these residences and businesses need good water supply, gas, electricity, etc. So when the Department of Transportation (DOT) won't let you hang anything else on the bridges and new lines are needed, HDD is called on to install new lines from the "mainland" under bays, canals and inlet bodies of water. Now comes the problem. First you are talking sea level conditions or below. Soil conditions are wet to say the least. And speaking of soil conditions, we're talking sand conditions in most cases.
Have you ever seen what happens when you walk to the edge of a hole dug in sand? What happens? The bank begins to cave in. This is what you're looking at in most cases when installing a new pipeline or conduit under a body of water along sandy coastlines.
I won't go into all the preparation necessary for drilling in sand and sea level wet conditions except to say not only should you retain an expert in mixing drilling fluid additives but seriously consider environmental impact should a frac out occur, which it probably will. I would identify a local environmental engineering firm before construction just in case.
Recently I looked at several potential crossings along shorelines as we are discussing here. As I talked to drillers and engineers who have "been there, done that" one technique became almost a mandatory procedure to insure a successful bore while retaining shoreline integrity on both sides of the existing sea wall. Prior to the pilot bore a "conductor pipe" is installed at the specified entry angle. Some drillers call this a "wash-over pipe" because it serves the same purpose in a sense. The pipe or "casing" which has an ID slightly larger than outside diameter of the drill bit is driven into the ground by an impact tool (pneumatic hammer) or drilled in with the drill rig using cross-over adapter subs to connect the pipe to the rig. Some drillers even "push" the casing into the ground using a track hoe or dozer (not recommended). Casing is drilled in until "confined soils" and/or rock has been reached. The goal is to reach a stable soil condition beyond the shoreline.
Once the casing is installed the driller can be more confident in drilling the pilot hole without disrupting the soil around the entry area, next to the shoreline. If all goes well, returns should come back through the borehole up the annulus of the drill pipe confined inside the casing.
Obviously there are some other bigger issues like frac-outs, which I mentioned earlier. These are topics for the "mud doctors" who know what medicine to use according to the situation. As always I welcome your comments. Please send me your tricks to this and other difficult bores. Until next time.