Risks to Your Build That Are Hiding Beneath the Ground

Monday, July 25, 2016

Excavation for a double basement development in St. John’s Wood showing the first stages of foundation works.

The ground in London and other old cities has been subject to thousands of years of building, demolishing and building again. Over time, the ground has actually raised as much as a few metres from the gradual piling of waste, debris, soil and whatever other detritus has accumulated over centuries of city life.

This is known as made up ground, and its mish-mash of materials makes it unpredictable stuff. Your house could be built on sturdy soil while your neighbour’s could be resting on ground that struggles to bear the weight – it can even vary within a single site. On the surface, it all looks the much the same, so the only way to know the quality of the ground is to conduct a soil investigation by boring deep down and examining the soil that’s brought up.

Ground quality is especially important if you’re extending your house. Additional structure means additional weight, often throwing off the delicate balance that has held up the house for so many years. For example, just because you have an existing extension doesn’t mean you can safely add another floor on top of it without the whole thing sinking into the ground.

For period buildings, foundations are often woefully inadequate. Builders back then simply didn’t have the gift of hindsight or the means or materials to efficiently lay foundations. Many picturesque Victorian terraces are held up by foundations no more than a foot deep on made up ground that can’t bear the weight.

Over time, gravity takes its toll, and the building sinks into the ground – known as subsidence. If this sinking is uniform across the whole structure, it may not cause any damage to the house. But if it sinks unevenly, immense pressure can be put on the structure which may cause crookedness, cracks or, in extreme cases, collapse.

Imagine you have a sturdy cardboard box and a sandpit full of gravel. If you evenly push down on this box, it should slowly sink down but maintain its shape. Now, imagine that under one corner of the box there’s sand instead of gravel. Push down this time, and the corner over the sand will sink faster than the rest of the box, which loses its shape and becomes crooked.
Material composition of the ground isn’t the only cause of subsidence. There’s also the water table, which is the level at which the ground is saturated with water. At this saturation point, the ground is less stable and the soil holding it together can be washed away by the subterranean flow of water. A high water table also puts pressure on foundations and basements, as the weight of the water presses against the walls and infiltrates cracks.

In London, the water table has been gradually rising for decades and there is constant work to keep groundwater at stable levels by extracting it and making it available for public use. With rainfall across the UK projected to increase due to climate change, groundwater levels will be an increasingly important factor in construction projects.

Another factor in ground quality is nearby large trees and shrubs whose roots can spread beneath your house and compromise the stability of the soil by sucking the moisture out and causing it to dry up and shrink. Roots can also infiltrate and block drains, especially if they’re old and cracked already. Damaged drains can further exacerbate the issue by flooding the ground beneath your house and washing away soil.

Whether you’re concerned about the ground quality for your existing home or a potential new one, it’s not something you can find out without expert help. We have access to a fantastic range of consultants and have been trusted with the preservation of Grade I listed buildings, so we’re fully equipped to help you look after your home. Click here to get in touch with us today.

John Dyer-Grimes