Slope stability assessment
A check of the stability of a slope, embankment or cutting, with recommendations to keep it stable.
About this service
A slope is stable when the strength of the ground comfortably exceeds the forces trying to make it slide. Cutting into the bottom, loading the top, adding water or removing vegetation can all reduce that margin. A slope stability assessment checks how much margin exists now, how your proposed work will change it, and what measures will keep the slope safe during construction and for the life of the building.
London is not all flat. Sloping streets and gardens are common in parts of Harrow, Croydon, Bromley, Haringey and Kingston upon Thames, and railway cuttings and embankments run behind many homes. Clay slopes are a particular concern because they can creep slowly for decades and then move more sharply after a wet winter or a careless excavation. Signs include tilting fences and trees, bulging walls and cracked paths running across the slope.
Our assessment combines a site inspection with available ground data and a stability analysis of the critical sections. We then set out practical mitigation, which might be regrading, drainage, retaining structures, or limits on where and how you excavate. Our analysis and recommendations go through our three-step quality control process, with final sign-off by our Head of Engineering. Fees start from £1,500 + VAT.
When you need it
- You're building on or near sloping ground
- A slope or bank is showing signs of movement
- You're cutting into a slope for a driveway or extension
What's included
- Site inspection and review of ground data
- Stability analysis
- Mitigation options
- Recommendations
How it works
- Inspect the slopeWe walk the slope and its surroundings, recording gradients, signs of movement, drainage paths, springs, vegetation, structures at the top and toe, and any past failures nearby.
- Gather ground dataWe review existing borehole or trial pit records and, if none exist, recommend targeted investigation so the analysis uses realistic soil strengths and groundwater levels.
- Analyse stabilityWe model critical cross-sections for existing, construction and long-term conditions, checking the effect of excavation, new loads and a rise in groundwater on the factor of safety.
- Mitigation and reportWe set out the measures needed, such as drainage, regrading, retaining walls or staged excavation, and summarise findings in a report usable by your designers, contractor and Building Control.
What you receive
Excavating into a slope for a driveway, extension or garden room
The riskiest moment is usually during construction. Digging a level platform into a slope for an extension or garden building removes support from the ground above, and clay can look stable for days before a slip develops, especially after rain. The assessment sets safe excavation limits and sequences, and where a permanent cut is needed it leads into retaining wall design. Temporary support during the dig is covered by a temporary works and propping scheme.
Water is the other common trigger. Blocked land drains, leaking pipes and roof water discharged onto a slope can all raise groundwater and reduce stability. Simple drainage improvements are often the most cost-effective mitigation. Where houses on a slope already show cracking, it is worth separating slope movement from ordinary clay shrinkage, which our subsidence guide helps explain.
Frequently asked questions
How do I know if a slope in my garden is moving?
Look for fences, walls and trees leaning downhill, cracks in paths or patios running across the slope, bulges at the toe, steps or hollows at the top, and doors or windows sticking in buildings on the slope. Wet ground or new springs after rain are also clues. Several of these together suggest movement worth investigating.
Can I build an extension on a sloping garden?
Yes, many London homes do. The key is designing foundations and any retaining walls for the slope, and controlling how the ground is excavated. On gentle slopes this may just mean stepped foundations. On steeper or clay slopes, a stability assessment first makes sure the cut and the new loads do not trigger movement.
What is a factor of safety in slope stability?
It is the ratio of the ground's available strength to the forces trying to make the slope slide. A value of one means the slope is on the point of failure. Designers aim for a margin above one that reflects how well the ground is known and what is at stake, in line with Eurocode principles.
Does a slope stability assessment need ground investigation?
A meaningful analysis needs realistic soil strengths and groundwater levels. If good data already exists from the site or close by, we can use it. If not, we recommend a targeted investigation, usually a few boreholes or window samples across the slope, so the analysis is based on evidence rather than assumptions.