Road Cones & Closures

Road closures and delays following storm events seem to be more common than ever. While some major events make headlines and cause significant disruptions, countless smaller slips go unnoticed but still cause headaches and delays for road users nationwide. Why do storms cause such damage, and what can we do to manage these disruptions better?

#### The Nature of Slips: Over-Slips and Under-Slips

Over-Slips occur when material from above falls onto the road. These can be hazardous as they often include rocks and boulders, posing a danger to motorists, especially at night. However, over-slips are relatively easy to manage: the debris is removed, and the road is reopened, often with no pavement damage.

Under-Slips are more challenging. These happen when the road formation itself slips away, requiring the foundation of the road pavement to be rebuilt stronger than before. These events are costly and time-consuming to repair.

#### Why Do Slips Happen in Rainstorms?

Most of us know that slips happen in rainy weather, but why does a downpour cause tons of dirt to mobilize and crash down a hill?

Let's consider soil strength. Imagine taking a handful of soil and pressing it into a ball. Try to break it by sliding one half away from the other. Crushed rock is nearly impossible to slide, while loose sand or soft clay slides more easily. This demonstrates the soil's shear strength—the inherent strength that holds it together.

In a rainstorm, if soil becomes saturated (air voids filled with water), two things happen: the soil becomes heavier and, unless it's purely granular, it becomes weaker.

Now, imagine a road supported on a hillside. The soil's weight exerts downward force due to gravity, and its strength (friction and cohesion) provides resisting force. If the soil under the road saturates during a storm, it becomes heavier and weaker, potentially slipping away from the deeper, more stable soil. This often forms a circular slip plane, visible as a curved shape at the back of a large slip site.

#### The Importance of Drainage

To prevent the soil under a road from becoming saturated, especially in mountainous terrain, we can divert or channel water away from the subgrade into natural watercourses. Proper drainage systems can prevent subgrade saturation, using solutions like flexible fluming to channel water and geotextile for subsoil drains to collect seepage. These methods are cost-effective in preventing damage.

Maintaining existing drainage systems is the most cost-effective slip repair prevention. Clearing out side drains, channeling surface flows away from the road, and repairing road surface cracks are crucial steps.

#### Repairing Storm-Induced Slips on Highways

Highway engineers consider key indicators on-site when choosing repair systems, such as slip geometry, slope steepness, and soil type.

- Shallow Slips: Often repaired quickly with systems like micropiles (steel piles driven from the surface) or percussion-driven anchors with high-strength turf reinforcing matting.

- Deeper Instability: Ideal for anchors driven into the unstable ground and locked deep in the soil to stabilize the site.

- Deep Slips: Usually repaired with pole walls with anchor tie-backs or MSE walls (mechanically stabilized earth walls). MSE walls are cost-effective, have a 100-year design life, and can utilize local contractors and materials, reducing costs.

#### Ensuring Long-Term Stability

Whatever repair solution is chosen, preventing future slips is crucial. Drainage is key. Products like drainage composite collect seepage against the slip face, and subsoil drains wrapped with geotextile pipe water away from the site.

#### What Can You Do?

Understanding why slips happen helps both contractors and consultants with prevention and repairs. Simple actions like cleaning drainage systems, using innovative solutions, and prudent engineering can lead to reduced costs, fewer road closures, and a better experience for all road users.