As I talked to drillers and engineers who have "been there, done that" one technique became almost mandatory procedure to insure a successful bore while retaining shoreline integrity on both sides on the existing sea wall.
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.