Nearshore (or “intertidal”) foundations are not a usual type of foundations.
It is a hybrid solution, an on-shore foundation in a quasi-offshore environment.
I have heard about this type of foundations several times in my career. The first time it was a preliminary design that I have made about 10 years ago for a Chinese project.
In the last years I have seen it used in several Asian countries, for instance in Vietnam (in the Mekong delta, in a project appropriately called Dong Hai 1 “intertidal wind project”).
The typical application of this kind of foundations are the shallow waters of the continental platform, using an on-shore wind turbine nearby the coast.
During the low tide, the foundation is exposed to the air while it is partially submerged in seawater during high tide. This is an extremely aggressive environment for both concrete and steel.
Additionally, in sandy beaches with muddy underground the foundations may require piles of exceptional length (>30 meters).
The technical solutions that can be used for these special locations are typically three:
- Pile foundations
- Sheet-piling cofferdams
Monopiles are the standard foundation used in maritime structures. It is a driven steel pile with a diameter up to 6-8 meters.
The wind turbine tower can be bolted directly to the monopile element without a transition element (this is often the cheaper configuration).
This type of configuration has even been considered also for on-shore projects as an alternative to pile foundations.
It has, however, several risks related to the pile driving process and the type of equipment required.
The piles foundation with an elevated pile cap is another interesting solution.
It consist on a set of driven piles (concrete or steel) joined together by a concrete pile cap.
Interestingly, in some cases the foundations are connected to the coast (or even between them) with walkways.
This help servicing the wind turbines without using ships.
Another alternative that has been used is using cofferdams to create something similar to a small island, and then build the foundation inside this element by means of a gravity or pile foundation.
This construction technique is inherited form bridge construction. Since the time of the Roman Empire sheet piles and cofferdams were used to build the piers of a bridge inside a river or lake.