Why historic embankments behave differently today

Most canal embankments were built over 200 years ago and were often designed using local soils and materials. Additionally, these networks were built to accommodate small, slow-moving boats. Fast forward to today and these embankments are now under strain from modern pressures such as climate change and heavy traffic loads. 

In many cases, alternating periods of dry, hot weather and heavy, persistent rainfall causes the clay lining in embankments to shrink and swell. Intense rainfall also increases the rate of erosion, which puts the embankments under more pressure. Increased traffic and vibrations from nearby roads and transport networks are also adding to the strain.

These factors combined increase the risk of embankments suddenly collapsing.

What lies beneath

Perhaps the most worrying aspect of the Llangollen collapse is how little warning there was. The issue lies with the hidden factors that weaken embankments, such as:

• Moisture changes

• Seepage channels

• Undetected voids

Moisture changes

Seasonal cycles of intense wet weather and warm dry spells can result in weaker soils. Additionally, increased moisture in embankment soil can significantly reduce its strength and stability.

Seepage channels

Uncontrolled seepage channels allow water to move freely and erode fine soil particles through the embankment, which creates hollow channels referred to as “pipes”.

Undetected voids

Voids are typically caused by soil erosion around hidden leaky culverts (pipes), which can develop slowly and then cause a sudden collapse. Voids are also sometimes cause by animal burrowing.

It’s important for homeowners living near canal or railway embankments to keep an eye out for any warning signs such as small cracks and damp patches, typically around doors and windows, to avoid a major incident.

How rapid failures develop with little surface warning

Whilst these hidden changes in the ground start off small, these can accelerate rapidly once a critical point is reached. This is what engineers call “progressive failure”. In the case of Llangollen, it’s likely that the combination of internal seepage and soil movement weakened the embankment with little visual warning on the surface. In other words, everything on the surface may appear normal until a collapse occurs.

Lessons for homeowners, developers, and asset owners

There are a few things that can be done to avoid a major incident.

1. Constant monitoring: ageing canals, embankments, and earthworks need to be inspected regularly.

2. Check for early warning signs: if you spot any cracks, seepage, or unusual ground movement, you should seek professional advice as soon as possible.

3. Conduct a risk assessment: properties near historic infrastructure may benefit from geotechnical surveys.

4. Planning ahead: reinforcing structures before a failure happens is safer and cheaper than making repairs after an incident.

Understanding the risks and planning ahead can help protect homes from unexpected subsidence or collapse.

Preventative ground engineering vs reactive repair

Although reactive repairs after a collapse is vital, they are often highly disruptive and costly. In some cases, these repairs may be insufficient if the subsidence issue hasn’t been resolved. Preventative ground engineering reduces the risk and expense, which makes it the preferred approach. Mainmark’s modern engineering solutions help strengthen historic embankments through:

• Reinforcement using geotextiles, screw piling, or soil stabilization

• Improved drainage to control moisture and prevent seepage

• Monitoring systems which detect signs of early ground movement

Early intervention is key

The Llangollen collapse is a reminder that ageing infrastructure needs constant monitoring. Climate change, development and natural wear and tear is increasing the strain on old embankments. Homeowners, local authorities and developers need to be aware of the risks associated with ground movement near old embankments and canals and invest in preventative engineering to protect both the properties and the historic assets from significant damage.