Living on the edge
When living near the top of a cliff it’s comforting to understand the stability of the site and how it can be improved.
Our plot had previous planning applications and land stability was raised as a concern. Reports of rockfalls and that Sussex steps (public steps down the cliff to the seafront) was closed for safety reasons in the early 2000’s due to erosion.
For our building we were keen to understand more about the site and best practices to improve the stability. This is a summary of our findings.
What is the ground made from?
The British geological survey record the ground around St Leonards as Wadhurst clay on top of an Ashdown Formation. This was formed 140 to 100 million years ago as a sea bed and over that period the sea has retreated and cliff formed.
Ashdown formation is formed of layers of sandstone and mudstone/clay. Even though Sandstone isn’t as hard as other rocks it is apparently a good substrate for building as it is porous and self draining and stable. The problems with Ashdown formation come from the layers of mudstone and clay as they can move providing slip planes and a weakness to undercut the Sandstone on cliff faces.
At the bottom of the cliff is Caves road. Here you can see the layers of the Ashdown. You can see where the cliff has eroded back about the width of a step over the last 100 years and is now unsupported.
Based on borehole studies (published in planning applications on neighbouring sites on West Hill road) the Wadhurst clay is about 10m deep before hitting the bedrock. It is stiff and was deemed stable enough to build a 5 story building on (as per 65 West Hill Road) so our low profile, single story building shouldn’t be a problem. The study said the clay is stable, especially at depth but remarked that exposed surfaces are liable to seasonal change, expanding with water and shrinking when drying which can cause issues with buildings if not built correctly.
Getting a study done on our site
We commissioned a geological study to learn more about our site and ensure we follow the best practice and make the ground more stable as a result of our build. I found it really fascinating to see how the ground is researched and slope stability assessed.
The study looked at the site and estimated what the eventual, stable shape of the slope would be. This is the shape if left for a long period of time to erode with no efforts to mitigate.
In the above sketch you can see that the lower ‘Talus’ slope is stable at a 28 degree angle while the Sandstone layer will support a 45 degree slope. Above the Sandstone is clay which supports about a 30 degree slope again. The estimate is for the long term stable slope to be at about 30 degrees overall which would still not impact our building. Phew!
The study noted that the if Caves road is to remain open the lower slope would likely be maintained (as it is currently via a King Post retaining wall) and therefore erosion would be mitigated. There is also a large planned development of Caves road which would likely involve further improvements to the stability of the slope over time.
It’s also interesting to see a proposal with the East Sussex County Council to rejuvenate the local area (CONSERVATION AREA APPRAISAL AND MANAGEMENT PLAN 2018) includes improving access to West St Leonards station by re-opening the Sussex steps.
For our house and those on the front of West Hill road I hope the above proposals are successful.
How can adding a building improve the slope stability?
After the slope stability study we spoke to a structural engineer to find out what we can do with our building to improve the slope stability. The key points were:
Reduce weight
Clay and soil are heavy. Weight on an unstable slope is the driving force which moves things downwards. In the absence of weather the ground on our site is stable. But add rain, wind, freezing temperatures and dry spells and things can move.
As a house is full of air and is about 100x lighter than soil per m3. The advice is therefore to dig as much as we can afford to out and ship off site to improve the overall slope stability.
Most of our building will be underground so this excavation will reduce the weight. To make things even better we’ve also decided to terrace the garden. Clay soil is about 1.8 tonnes per m3 so we’ll be removing about 1,500 tonnes of weight from the slope!
Good drainage
Water is the enemy of slope stability. There are two main reasons for water causing issues.
- Ground swelling and moving down the slope.
- Flows of water washing softer ground down the slope.
There can also be issues from man made factors such as burst water mains or poorly designed surrounding buildings but this is outside of our control.
So the advice is to ensure there are no gushing drains and that any water can find its way around the building and diffuse gently. The issue is often in heavy rain where surface water can pool and flow in such a way that it can find pathways forming streams through the ground.
To attenuate the water flow we are incorporating as much planted space as possible into the build. In fact from a birds eye view the plot will be almost entirely green. We’re going to have an intensive green roof which will capture rain and disperse it over a period of days. There is not sufficient mains drainage for a storm drain so the water will be safely and slowly drained into large soak aways in the garden.
Most of the building will be underground. The water that makes its way down will have to be able to run around the building to prevent hydraulic pressure being built up. The common way to do this is via a pea shingle bed and french drain.
The parking area at the top could cause pooling and can’t be green. As such it will be made from permeable tarmac such that water drains downwards slowly and evenly rather than running off quickly.
A strong building
I guess this is pretty obvious but there is a fair amount of detail. Retaining walls should ideally be monolithic with the slab. This means the floor and walls would be one continuous reinforced concrete formation. The resulting structure is so strong it could be suspended on minimal footings.
With a strong, rigid building all the weight can be engineered onto specially designed footings controlling where pressure is placed on the slope. We will seek further advice as to what the footing look like but deep piles into bedrock have been talked about. The suspended nature of the building allows the ground to swell and contract if necessary.
Interesting to note that even though the surrounding Victorian construction is no where near as strong as the suggested construction, the Victorian footings do suspend the floor. This means the building survives moving ground very well. Got to love Victorian engineering!
Stepped foundations
There are two main approaches to building on a slope. Cantilever off it or build into it.
Cantilevered buildings raise the front of the building away from the slope, they are cheaper but as the highest pressure is placed on the lowest, least stable part of the slope they are not optimal. This is what was proposed and approved on our site previously.
The engineers suggestion was for stepped foundations involving a lower floor dug into the slope. This helps in a few ways:
- Reduce the weight of the slope itself
- Reach into more deeper, more stable ground
- Spread the weight of the building over a larger area
The downside is it does cost more to excavate more ground but the stability improvements coupled with additional internal space and more easily accessible lower part of the garden seemed worth while so we chose this method.
Closing thoughts
Currently the cliff is retreating and the slope unstable. It has historically retreated about 1.5m every 100 years. As the slope erodes the gradient will reduce and become more stable. Therefore the rate of erosion will slow until a stable gradient is reached. At this point the slope will be stable.
If the slope is not maintained and worst case erodes at the fastest historic rate the slope to be stable in about 500 years.
Our home will be about 23 metres from the cliff edge. When the slope is stable it will still not affect the building.
We’ll design our home to improve the stability of the slope and as noted by the geological study, further efforts to improve the slope will be possible later. This will hopefully mean the building will be able to serve a 200 year design life easily.
