Home › Blog › Underwater blasting: how a port's seabed rock is lowered

Underwater blasting: how a port's seabed rock is lowered

By Arthur More · professional saturation diver · 7 September 2026

Published 7 September 2026 · 9 min read · 1,881 words

In brief

When a port's seabed is rock, a dredger can't handle it and it has to be broken up first. That means drilling a grid of boreholes into the bed, charging them, blasting them in a controlled way and then removing the debris. The diver's role is mainly before and after: checking the bottom, helping with positioning, and verifying the result matches the design.

A port that wants to take bigger ships needs more draught. If the bottom is mud or sand, the problem is solved by dredging. If the bottom is rock, the conversation changes completely: it has to be broken before it can be removed, and breaking it underwater is a specialised job that combines drilling, explosives and industrial diving on the same site.

It's a low-visibility job even within the trade. It's done in short windows, with heavy coordination with the port authority and procedures that leave little room for improvisation. This article explains how it's organised and, above all, where the diver fits in, because the popular image of a diver placing charges bears little resemblance to what happens on a real job.

Why the rock is blasted rather than dredged

A dredger works by ripping material from the bed and carrying it away. With soft sediment that's a continuous, cheap process. With sound rock, ripping stops being viable: the tool wears out, output collapses and cost per cubic metre shoots up. There are rotor dredgers and cutter units capable of working hard material, and on weathered or weak rock that's the usual solution, but above a certain strength, breaking it up first pays for itself.

The typical sequence for a job like this always follows the same logic: first the bottom is surveyed, then it's drilled, then charged and blasted, and finally the debris is removed and the resulting level is checked against the design. Each of those steps has its own difficulty and its own specialised equipment.

The survey: no good data, no drilling grid

Before drilling anything you need to know what's underneath. That means detailed bathymetry, geotechnical borings and, very often, direct inspection of the bottom. The key question isn't just water depth, but the thickness of sediment covering the rock, the quality of that rock, and the presence of fractures or pockets of different material.

This is where the first diving work comes in. A surface-supplied team can confirm the actual state of the bed at specific points, check whether the sediment shown in the boring is where it's supposed to be, locate obstructions — remains of earlier works, scrap, cables or pipelines not on the drawings — and document all of it on video. That inspection is much like the visual underwater inspection of any other structure, except the object being studied is the seabed itself.

A poor survey is paid for in full later on. If the design assumes homogeneous rock and very different hardness levels turn up, the planned drilling grid won't give the expected result and passes will have to be repeated, with everything that means for schedule and cost.

Drilling: boreholes in a bed you can't see

Underwater boreholes are drilled from a surface platform: a pontoon, a jack-up or a spud barge fixed over the work point. From there, the drill string is lowered through the water column to the bottom and the rock is drilled following a grid worked out by the designer, with a spacing and depth that depend on the rock type and the fragmentation being sought.

Positioning is critical and is why these jobs rely so heavily on instrumentation. The platform is set using precision positioning systems and the grid is laid out from those coordinates, not by eye. When the platform floats rather than sitting on the bottom, motion control comes into play, a problem in the same family as dynamic positioning on offshore vessels: without a stable reference there's no reliable grid.

One of the usual headaches is the borehole itself. Once drilled, it tends to silt up with sediment or partially collapse if the rock is fractured. That's why many of them are cased, and why part of the job is checking that the boreholes are still open and at the right depth before they're charged.

Where the diver fits in

It's worth being clear about this: charging explosives is the job of personnel with specific blasting qualifications, and the regime for authorisations, custody and transport of explosives is strict and different in every country. The diving team doesn't stand in for that role.

What the diving team does do, and it's a lot, can be summed up as:

Surveying the bottom before drilling, and documenting obstructions, buried services or remains not shown on the drawings.
Supporting positioning of the platform and the drilling guides when the acoustic reference isn't enough.
Comprobar barrenos: verifying they're where they should be, that they're still open and at the intended depth.
Clearing individual obstructions that prevent drilling at a specific position.
Verifying the result after blasting and after dredging, measuring the level reached and locating rock left above the design line.

That last point matters more than it looks. The final check decides whether the job is finished or a pass has to be repeated, and it's usually done by combining bathymetry with direct diver verification at doubtful points. It's classic port work, a close relative of what's described in diving in ports: dredging, caissons and quays.

Safety: the rule that isn't negotiable

The basic rule of any underwater blasting job is that no one is in the water when it's fired. This isn't a recommendation: the pressure wave from an underwater detonation travels far more efficiently than it does in air, and a submerged human body is especially vulnerable in gas-filled cavities, starting with the lungs and ears.

That's why coordination is at the heart of the procedure. Before a blast, a safety zone is set up, checked as clear, marine traffic is warned and the work window is closed. Control over who can be where and when ties into the permit to work and risk assessment, exactly as with any other high-risk industrial task, and the chain of command has to be clear before starting: who authorises the shot, who confirms the water is clear and who declares the operation resumed.

There's a second, less obvious safety front: material that doesn't detonate. A charged borehole that hasn't fired is a risk that stays on the bottom, and dealing with it follows specific procedures that, again, are the blasting specialist's responsibility. No diver should go near that area unless the situation has been formally declared safe.

Environmental impact and calendar windows

Underwater blasting generates high-intensity impulsive noise, and that's now one of the main constraints in the environmental permit. Authorisations commonly include mitigation measures — prior monitoring of wildlife in the area, sequencing of charges to reduce the peak, bubble curtains — and calendar restrictions that avoid spawning seasons or the passage of sensitive species.

For the diving team this translates into something very concrete: short, inflexible work windows. If the job can only blast in certain months and at certain hours, everything else is organised around that restriction, including mobilising the diving system.

After the blast: removing and checking

A well-executed blast leaves the material fragmented to a size the planned removal method can handle, usually a grab dredger or an excavator on a pontoon. If the debris comes out too coarse, it has to be re-broken or removed with slower means; if it comes out too fine, more explosive than necessary was probably used, generating more turbidity than it should have.

The final check is, once again, a measurement job. Post-blast bathymetry is taken, compared against the theoretical design surface, and high points are identified. All of that ends up in the job file, in the as-built documentation that certifies what was actually done. On a major job, that document is what lets the port authority publish a guaranteed depth, so it's not decorative paperwork.

What kind of background this work takes

There's no "blasting diver" qualification you can get at a school that opens this door on its own. What there is, is surface-supplied commercial diving — the kind taught at any of the commercial diving schools— applied to a specific type of job, plus the experience of working in an environment where the explosive sets its own pace.

Someone coming from marine construction arrives with an advantage, because they already know the language of construction, setting-out, tolerances and dealing with the site engineer. Someone coming from offshore usually brings a better grounding in procedure and risk assessment. Both backgrounds work; what doesn't work is treating a blast like just another underwater job.

As a career path, it's a small niche closely tied to each country's port investment cycle. It won't carry a career on its own, but it is one of the specialities that enrich a diver's CV inland or construction diver's CV, alongside hydrodemolition, underwater concreting or civil works inspection. For anyone wanting to understand the trade as a whole and its various paths, the starting point remains how to train as a commercial diver and what certifications are asked for in each market.

Frequently asked questions

Does the diver place the explosives?

Not as a rule. Handling and charging explosives is the job of personnel with specific blasting qualifications, which in each country carries its own certification and its own authorisation regime. The diving team provides the underwater work around the blast: surveying the bottom, supporting positioning, checking boreholes and verifying the result afterwards.

Why isn't the rock just dredged directly?

Because sound rock won't give way to a conventional dredger. There are dredgers capable of breaking hard material, but above a certain strength output drops so much that it's more cost-effective to fragment the rock first and remove it afterwards as loose debris.

What is overbreak?

Rock that breaks below or outside the theoretical design line. A little is inevitable and accepted; a lot means removing material nobody was paying for, and can sometimes affect the stability of a slope or the foundation of a nearby structure.

Is shipping warned before a blast?

Yes. Underwater blasting in a port area is coordinated with the maritime authority and the port's traffic control, and there are usually notices to mariners, a safety zone and agreed time windows. The exact procedure depends on the country and the port.

What effect does blasting have on wildlife?

The pressure wave can affect fish and marine mammals, which is why environmental authorisations commonly impose mitigation measures: prior monitoring of the area, sequenced charges, bubble curtains or other barriers, and calendar windows that avoid sensitive seasons. The exact conditions are set case by case.

Published 7 September 2026 · By Arthur More, professional saturation diver

Sigue leyendo