Whenever you build upon a foundation, the ultimate question is whether it will provide a stable base for the structure, properly transferring the weight to the underlying soil or rock.
Even on flat ground, this can be a tricky calculation. However, a foundation’s stability becomes an even greater concern on sloped ground because the gravitational forces on the structure are dissimilar. As a result, whenever the ground is uneven, engineers must carefully consider the angle and stability of the slope and the type of soil or rock, as well as any additional loading or stress that may be present.
In other words, they must perform a slope stability analysis. But what is that and why does it matter?
What Is Slope Stability?
Slope stability refers to the stress a soil or rock slope base can take without sliding or collapsing.
This is a crucial factor in the planning, design, and construction of any engineering project.
Whether it’s a bridge, dam, embankment, road, or structure, civil and geotechnical engineers must account for any slopes—be they man-made or natural. And they must also account for potential exogenous factors capable of triggering a major ground shift, such as:
Even a slight slope can have a significant impact on the overall stability and safety of a structure. As a result, failure to properly account for slope stability could result in landslides, foundational shifts, and damage to both people and property.
What Is a Slope Stability Analysis?
To prevent unwanted movement and ensure the long-term safety and stability of a project, civil engineers will often conduct a slope stability analysis, also known as a Landslide Hazard Analysis, before they ever break ground on a project. These can be used for a variety of geotechnical engineering projects, the most common being:
Calculating stable cut and fill slopes
Assessing retaining wall stability
Appraising stability of shallow and deep foundations for structures located on slopes
Gauging the likelihood of a landslide occurring and then designing mitigation strategies
Evaluating instability due to water
This due diligence is often a part of the initial site investigation. Depending on the specific test, the process will typically require engineers to collect detailed site data, paying special attention to the slope’s:
Topography – Engineers must calculate and chart the various height differences at all points of the terrain and the angles of the slope.
Soil and rock composition – Engineers must account for the ground quality. Slides, shifts, and topples are more common in soils that are highly fractured, weathered, or deformed.
Soil water quality – Engineers must collect data on soil moisture content, groundwater levels, water quality, soil strength, and water pressure. Additionally, they must also factor in how erosion could potentially further impact this equation.
Material strength – Engineers must select structural materials and building methods that account for the slope’s foundation.
Vegetation – Engineers must factor in how the area’s vegetation—such as trees, grass and shrubs—contribute to the slope’s stability.
Equipped with this data, engineers can then develop mathematical and 3D models of the slope. Those models will then be subjected to tests under various loading and stress conditions.
If the slope's stability is of significant concern, engineers can perform one of several slope stabilization techniques meant to buttress the slope and account for exogenous factors like runoff or earthquakes.
Common stabilization methods include:
Soil nailing – Steel bars “nails” are inserted into the soil or rock slope then grouted into place, serving as anchors that reinforce and stabilize the slope.
Rock bolting – Similar to soil nailing, except for rock slopes, bolts are drilled into the rock and then grouted into place.
Retaining walls – Retaining walls provide additional support designed to help keep soil or rock in place.
Terracing – By cutting a series of successively receding stepped-level “terraces” into a slope, the weight of the slope is more evenly distributed.
Drainage – Since water can have a massive impact on soil stability, especially on slopes, engineers may install drainage systems that collect and remove excess water from a slope, which prevents erosion and instability.
Vegetation – In some instances, vegetation will be planted on the slope to stabilize the soil, reduce water runoff, and prevent erosion.
Earthwork – With steeper slopes, engineers may flatten, buttress, or bench the slope’s pitch.
MFS Engineering—Slope Stability Assessments and Project Guidance
For any new project, if the topography is sloped, extra precautions must be taken to ensure the project's long-term stability and safety. Failure to do so could result in a slope failure that impacts people, property, and commerce.
Do you need to evaluate slope stability for your next project?
At MFS Engineers and Surveyors, our team of experienced civil and geotechnical engineers are trained to assess and then factor that slope stability into their design and construction processes.
So, if you need a professional you can trust to do the job the right way, contact our team today for free consultation.