Understanding Retaining Walls: A Comprehensive Guide

When looking to change the ground elevation in any construction or landscaping project, understanding soil behaviour and the necessary structures to support such changes is vital. A critical component that emerges in this context is the retaining wall.

Why Retaining Walls?

When a proposed alteration in ground elevation surpasses the soil's natural inclination to rest – known as the angle of repose – retaining walls become indispensable. This is especially true when the ground's inclination exceeds approximately 33°, a standard relaxation curve for many soils. It's pertinent to note that the tip can vary for different soil types; for example, the rise of serenity for bare soil embankments may differ and should be considered for precise applications.

Design & Structural Considerations

At the heart of a retaining wall's design is its ability to combat the lateral pressure of the soil it holds back. This pressure isn't uniform; it starts from zero at the top and gradually increases, reaching its zenith at the wall's deepest point. The total force acts through the triangle's centroid formed by this distribution, one-third above the wall's base.

However, another crucial factor comes into play: surcharge. Simply put, the tax is any additional load or weight added on top of the soil behind the retaining wall. It could be due to different ground, parked vehicles, or other gear. The thrust line denotes this force and tends to run parallel to the surcharge's slope.

A retaining wall, regardless of its robustness, can fail in several ways:

1. Overturning: When the soil's lateral thrust causes the wall to topple over.
2. Horizontal Sliding: The wall could slide due to the force exerted by the soil.
3. Settling: The wall could settle more than anticipated, leading to potential structural failures.

To ensure a retaining wall's longevity and stability:

• The composite weight of the wall (and any soil it supports) must counter the force trying to overturn it. This resisting moment should be at least double the overturning moment for safety.
• Sliding can be prevented if the wall's composite weight, multiplied by the supporting soil's friction coefficient, opposes the earth's lateral thrust. A safety factor of 1.5 times is advised here.
• Settling is checked by ensuring the wall's total vertical force doesn't exceed the soil's bearing capacity. Once again, a safety factor of 1.5 times is recommended.

Reinforced Concrete Retaining Walls

Retaining walls made of reinforced concrete have specific types, each with its unique application based on height and situational constraints.

1. Gravity Wall: As the name suggests, they leverage their sheer mass to resist forces. They are suitable for structures under 10 feet high.

T-Type Cantilevered Wall: These reinforced concrete walls are preferred for heights up to 20 feet. If one has to go higher, the counterfort wall becomes more feasible.

Counterfort Wall: Characterized by triangular cross walls, these walls add stiffness to the vertical slab and weight to the base. The spacing of these counterforts is typically half the wall's height.

L-Type Cantilevered Wall: This type is used when a wall is up against property boundaries or other obstructions.

Additionally, drainage systems become essential with the potential buildup of water behind these walls. These can range from drainage mats coupled with filter fabrics and porous gravel backfill to perforated drainpipes that ensure water is directed away from the wall. Moreover, vertical control joints at intervals of 25 feet are advisable, with expansion joints added to every fourth control joint to accommodate any movement or expansion.

Conclusion

In the vast realm of civil engineering and landscaping, retaining walls play a pivotal role. Though seemingly simple, these structures require intricate planning, design, and execution. By considering factors like the soil's angle of repose, surcharge, and various forces, professionals can ensure these walls serve their primary purpose and stand the test of time. Consulting with a structural engineer is paramount, especially when the construction environment presents unique challenges. Inspector West