Underwater (chemical) anchoring: how to design it and how to install it

How do you design and carry out underwater anchoring?

In the design of ports, of swimming pools, and even of works in water-logged areas, anchors frequently have to be installed below the waterline — usually using chemical anchoring.

Many designers are wary of carrying out underwater anchoring, although there is really no reason to be. Below, in a few lines, is an explanation of the correct process for designing underwater anchoring.

Corrosion resistance below the waterline

When working below sea level, the first thing to address is resistance to corrosion. There are several possible solutions:

Chemical stud in SS316 stainless steel: the most standard solution. In seawater SS316 will not last forever, but it is certainly a good medium- to long-term solution.
Chemical stud in HCR stainless steel: HCR (High Corrosion Resistant) is a very expensive stainless steel that never corrodes. An excellent long-term solution, but so expensive that few contractors agree to pay for HCR studs.
Fibreglass rod: an excellent long-term solution, but it needs thought at the design stage and is not suitable for every situation. The fibreglass rod itself is very strong in tension — roughly three times stronger than stainless steel — but the connection between the nut and the rod is very weak (roughly three times weaker than a standard bolt and nut). In practice this limits the solution, and makes it better suited to rebar dowels than to anchors with a nut.
Epoxy-coated rebar dowels: bonded into the existing concrete with chemical anchoring, and used on the other side as starter bars for a new pour — so that after casting, the dowels are no longer in contact with the water.

Choosing the right chemical adhesive

For bonding, the usual choice is an advanced epoxy adhesive with approval and proven experience in underwater use, such as Chemfix100, which is based on pure epoxy.

Although some vinylester adhesives also carry European standard approval for underwater use, to the best of our knowledge there is no practical experience of using them below the waterline in Israel. We recommend choosing the adhesives that are theoretically better and, in practice, more established here — the advanced epoxy-based chemical adhesives.

Carrying out the installation

The installation process for underwater anchoring is exactly the same as for a normal installation above water, and can be carried out with a standard injection gun. The stages are:

Drilling: drill the hole to suit the diameter of the chemical anchor you intend to install.
Injection: inject the chemical adhesive into the hole. Make sure the injection nozzle is inserted right to the bottom of the hole before you begin injecting.
Setting the anchor: insert the chemical stud into the hole by hand, with a twisting motion.
Curing: wait for the adhesive to cure fully. Underwater, curing takes roughly two days.

Note the differences from working on land

Cleaning: unlike a standard installation, there is no need to blow the hole out with compressed air.
Strength: be aware that the adhesive will be less strong underwater than in a normal installation on land.
Equipment: there is no restriction on the diver — standard injection tools can be used perfectly well under water.

Why use a bad plug when you can use a good one?

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