By Donald Cook

Fierce storms and floods have made this a very difficult winter for many parts of the UK, not least the owners of a historic castle in Aberdeenshire, close to the Queen’s residence at Balmoral. Some 20 metres of land behind the 450-year-old Abergeldie castle have collapsed into the River Dee, leaving its rear wall just feet from the bank.

With the building on the brink of disaster, Baron Abergeldie, John Gordon, and his wife have left to stay with a neighbour. They have talking to specialists to try to save the castle.

In my employment with consulting engineers over 35 years I have been involved with a number of landslides and erosion problems. Abergeldie’s issue has been partly caused the natural granular soils present in this area being more likely to be eroded by flowing water than clay soils. The risk is that the bank erodes just a little more and undermines the building’s foundations, which are probably about a metre below the ground. This could topple part of the castle into the river.

Direct support to the building could be provided by piled foundations sunk deep into the ground and connected into the building walls and foundations. But even if this could be implemented in time, it is still unlikely to be a suitable or adequate solution as the ground around the piles could still be washed away.

The best way of avoiding catastrophe would be to build a containment wall some distance into the river and infilling the space between it and the present bank; essentially reinstating the river bank. To ensure that the load from the castle is adequately supported, a wall would have to be built some distance from the eroded river bank; I would estimate about five or six metres away.

For the length of the wall, a robust solution would be provided by extending some 10 metres upstream and downstream of the castle. Taking account of the outbuilding and road next to the castle this could be a length of about 50m. The best shape of the outer wall would be a curve and tapering against the bank, encouraging the current to flow smoothly around the wall. Sharper edges could cause turbulence and lead to more erosion.

Material difficulties

When it comes to materials for building the wall, there are a couple of options. One solution might be to drive a set of interlocking steel-sheet piles into the river bed, but this brings difficulties. These need to penetrate relatively deeply into the ground, about twice the height above ground; in this case possibly 10m below the river bed. The river bed might comprise shallow rock or dense gravel and boulders that prove difficult or impossible to penetrate. The inevitable vibrations might destabilise the ground and threaten the castle. Specialist equipment and contractors of the kind that would take many weeks or even months to procure and mobilise would be needed, all of which would drive up the cost.

The better option is to form a wall using large rocks, known in the profession as rip-rap. The area between the wall and the bank would then be infilled with smaller rocks.

But we are talking about a massive job. Depending upon the extent of the new wall, the job could require thousands of tonnes of stone. Each wagon for transporting these materials can take 20 tonnes. If money was no object and the materials could be obtained from a local quarry, the work could perhaps be done in a week, plus the time it takes for permission from the council, water authorities and so forth. Under the circumstances, it is to be hoped that could be done in another week.

In addition, a safe method of construction would need to be developed and implemented due to the nature of the problem and the risk of a collapse. The total cost is difficult to estimate, it is potentially many hundreds of thousands of pounds.

It’s unclear whether there is enough time to complete this job. It’s an extremely precarious situation and we have to feel very sorry for the owner. One hopes he has a dungeon, since that could mean the foundations extended down to river-bed level and therefore could be better protected. But that’s probably not the case, so the pressure is on.

Donald Cook is Visiting Professor in Civil Engineering at Edinburgh Napier University. A longer version of this article can be read at