Why Retaining Walls Collapse

Summary

In this video, Grady from Practical Engineering explains the critical role of retaining walls in construction, showcasing their engineering complexities and different types, such as gravity walls, mechanically stabilized earth, soil nails, anchors, and pile walls. He dives into the challenges of designing retaining walls to withstand lateral earth pressure, surcharge loads, and water pressure. The video also highlights common causes of wall failures, such as under-design, poor soil conditions, and the impact of water and freeze-thaw cycles. Grady emphasizes the importance of proper engineering and the hidden yet essential nature of retaining walls in urban infrastructure.

Takeaways

• 😀 Retaining walls are used to support soil and prevent it from slipping, commonly found in areas with steep slopes or where construction space is limited.
• 😀 The collapse of a retaining wall in Camden County, New Jersey, highlighted the importance of proper design and engineering for retaining walls in infrastructure projects.
• 😀 Retaining walls must be designed to withstand lateral earth pressure, which is the sideways force exerted by the soil they hold back.
• 😀 Various types of retaining walls include gravity walls, cantilever walls, mechanically stabilized earth, and soil nails, each designed to handle different loads and conditions.
• 😀 The key to a retaining wall's stability often lies in its footing, which helps distribute the forces acting on the wall into the ground.
• 😀 Mechanically stabilized earth walls use reinforcement elements like steel strips or geotextiles to hold soil together and prevent slumping.
• 😀 Soil nailing and anchors are techniques used to stabilize excavated slopes, especially when a retaining wall needs to be installed during or before excavation.
• 😀 Pile walls, made from vertical shafts or sheet piling, are commonly used for temporary excavation support, with their effectiveness relying on the resistance from deeper soil layers.
• 😀 Retaining walls can fail if they are not designed to withstand the immense forces exerted by soil, surcharge loads, or water pressure.
• 😀 Water is a major challenge for retaining walls, as it can increase pressure behind the wall and cause soil to freeze, which can lead to wall failure if not properly managed with drainage systems like weepholes.
• 😀 Engineers use advanced tools like computer software to perform global stability analyses, ensuring retaining walls remain stable even under complex conditions like poor soils, groundwater, and multiple tiers of soil.

Q & A

Why did the construction project in Camden County, New Jersey, come to a halt in March 2021?

The project came to a halt due to the collapse of a retaining wall along the roadway, which set the project back even further.

What is the role of a retaining wall in construction?

A retaining wall holds back soil, preventing it from slumping or settling into an unstable configuration, allowing for more usable space and stable slopes in construction.

Why do engineers rarely trust slopes steeper than 25 degrees for long-term stability?

Slopes steeper than 25 degrees are less stable because over time, internal stresses cause the soil to slump and settle, which can lead to landslides or other failures.

What is the challenge with retaining walls holding back soil?

The challenge is that soil is much heavier than water, and the lateral earth pressure exerted on a retaining wall can be enormous. Additionally, surcharge loads from buildings, vehicles, and water can increase the forces on a wall.

What are gravity walls and cantilever walls?

Gravity walls rely on their own weight and the weight of the soil above them for stability. Cantilever walls, a type of gravity wall, use a footing to distribute forces and help keep the wall upright against lateral forces.

What is mechanically stabilized earth (MSE)?

MSE is a technique that ties a mass of soil together using reinforcement elements like steel strips or plastic fibers, which help to hold the soil in place and prevent slumping.

How does soil nailing work to retain soil?

Soil nailing involves drilling angled holes into an unstable slope, inserting steel bars, and grouting them to bind the soil together. This method is often used to stabilize slopes during or after excavation.

What are anchors or tiebacks in retaining wall construction?

Anchors or tiebacks are installed in drilled holes at regular intervals, using hydraulic jacks to tension them. The tension force helps hold the soil back and is distributed across the length of the retaining wall.

What are pile walls, and how do they work?

Pile walls are vertical elements driven or drilled into the ground, often used for temporary excavation support. The deeper the piles, the more resistance they provide against lateral earth pressure.

What are the common reasons for retaining wall failure?

Retaining wall failures can occur due to under-designing for lateral earth pressure, unanticipated surcharge loads, water pressure, freezing water, soil shear, or lack of proper engineering.