Short Answer:

Retaining walls can fail due to various reasons such as overturning, sliding, bearing failure, and structural cracking. These failures occur when the wall cannot resist the pressure exerted by the soil behind it, water buildup, or due to poor construction and design.

Common causes include inadequate drainage, weak foundation soil, or insufficient wall dimensions. Each failure mode affects the stability and safety of the wall, so it’s important to analyze and design retaining walls properly to resist all possible failure mechanisms.

Detailed Explanation:

Failure modes of retaining walls

Retaining walls are built to hold back soil at different levels and prevent erosion or collapse of slopes. While they serve a simple purpose, these structures are subjected to complex forces, especially lateral earth pressure, which can cause failure if not managed properly. Understanding the different failure modes of retaining walls helps in designing safer and more reliable structures.

There are four main types of failures that retaining walls may experience: overturning, sliding, bearing capacity failure, and structural failure. Each of these occurs due to different causes, and their prevention involves specific design considerations.

1. Overturning Failure

This happens when the moment caused by the lateral earth pressure exceeds the resisting moment of the wall. In simple terms, the wall tries to tilt or rotate forward due to the pressure of soil behind it.

Causes:

• Inadequate wall base width.
• High retained height or sloping backfill.
• Lack of proper weight or reinforcement in the wall base.

Prevention:

• Widen the base.
• Add weight or use counterforts.
• Reduce backfill slope or improve drainage.

2. Sliding Failure

Sliding occurs when the horizontal force from the backfilled soil pushes the wall sideways, and the base of the wall slides along the foundation soil.

Causes:

• Smooth base surface.
• Low friction between base and soil.
• Insufficient base length or toe projection.

Prevention:

• Increase friction at the base by roughening the surface.
• Add a key or shear key below the footing.
• Use stronger or compacted backfill material.

3. Bearing Capacity Failure

This happens when the soil under the wall footing cannot bear the total load from the wall and soil above it, causing the wall to sink or tilt unevenly.

Causes:

• Weak foundation soil.
• Excessive wall load or poor soil compaction.
• Inadequate footing size.

Prevention:

• Increase footing area.
• Use better-quality soil or soil stabilization methods.
• Perform proper geotechnical investigations before construction.

4. Structural Failure

Structural failure involves cracking, bending, or breaking of the wall itself. It occurs when the wall material is not strong enough to withstand the applied loads.

Causes:

• Under-designed reinforcement.
• Poor-quality construction.
• Water pressure not considered (no drainage).

Prevention:

• Provide proper reinforcement.
• Use good construction materials and techniques.
• Include weep holes or drainage pipes to relieve water pressure.

5. Other Possible Failures

• Seepage and erosion behind the wall due to poor drainage.
• Frost heave in cold climates.
• Vegetation roots growing and weakening the structure.

Conclusion:

Retaining walls can fail in multiple ways such as overturning, sliding, bearing failure, or structural cracking. These failures result from poor design, weak soil, or improper drainage. Preventing these requires proper analysis of forces, good construction practices, adequate reinforcement, and efficient water management. Ensuring these factors helps in building safe, long-lasting retaining walls.