Multi-Depth Gravity Walls Explained: An Alternative to Big Block Retaining Walls

For many designers and contractors, large cut retaining walls immediately trigger one assumed solution: big block or precast modular block (PMB) systems. These systems are often selected by default for taller gravity applications due to their size, weight, and perceived structural capacity.

However, while PMBs are one valid approach, they are not the only — or always the most efficient — solution for large cut wall conditions. Multi-depth segmental retaining wall (SRW) gravity systems offer a different way to achieve mass-based stability, often with significant advantages in constructability, excavation control, logistics, and site impact.

Understanding the differences between these systems is critical to selecting the right solution for a given site.

Big Block (PMB) Walls: The Default Assumption

PMB walls rely on very large, heavy precast concrete units to provide stability through mass alone. Their performance is straightforward: increasing block size increases resisting moment.

PMBs are commonly selected because:

• Large blocks intuitively feel “safer” for tall walls

• Fewer units are required to build vertical height

• Machine-laid units can be installed efficiently

• Design assumptions are familiar and well documented
  

In cut wall conditions, PMBs are often specified simply because they are perceived as the only gravity option capable of handling large retained heights.

The Tradeoffs of PMB Systems in Cut Applications

While PMBs can perform well structurally, their size introduces practical tradeoffs — particularly in constrained cut wall environments.

Common challenges include:

• Limited square footage delivered per shipment due to weight restrictions

• Increased lead times to produce and stage inventory

• Significant on-site staging requirements

• Heavy equipment needed for placement

• Limited adaptability to curved or complex geometry

• Reduced flexibility when site conditions change
  

In constrained sites, these factors can inflate cost, extend construction schedules, or limit design flexibility.

What Is a Multi-Depth SRW Gravity Wall?

So this brings us to Multi-depth SRW Gravity Walls. A multi-depth SRW gravity wall achieves stability by increasing overall wall depth through modular construction, rather than relying on a single oversized unit.

Instead of one massive block, these systems:

• Build depth using mechanically connected facing units and internal modules

• Allow depth to be added or reduced incrementally as needed

• Maintain true gravity wall behavior without soil reinforcement

• Achieve additional system weight through aggregate infill
  

Structurally, multi-depth SRW systems function as true gravity walls, but with a geometry that can be tuned to site demands rather than dictated by block size.

Why Multi-Depth SRWs Excel in Cut Wall Conditions

Multi-depth SRW gravity walls are particularly well suited for cut wall applications, where excavation is already occurring to create usable space.

In cut conditions, multi-depth SRWs often:

• Require fewer shipments than PMB systems

• Require less excavation to achieve equivalent stability

• Reduce disturbance to sites and adjacent slopes

• Fit more easily within property line constraints

• Can be installed at a lower overall cost

• Allow tighter radii and greater architectural flexibility
  

Because depth is created internally through modular placement, these systems can achieve comparable — or greater — resisting mass without the footprint penalties of large blocks.

PMBs vs Multi-Depth SRWs: A Practical Comparison

While both systems rely on mass, they approach stability differently.

At a high level:

• PMBs concentrate mass into a small number of very large units

• Multi-depth SRWs distribute mass across interconnected units and aggregate infill
  

This distribution allows multi-depth SRWs to:

• Adapt to variable wall heights

• Be constructed without heavy machinery to place facing units

• Be erected in conjunction with machine-placed backfill

• Respond more efficiently to site constraints

• Reduce over-excavation in cut conditions
  

The result is often improved constructability without sacrificing performance.

When PMBs Still Make Sense

This distinction is important: PMBs are not inherently wrong.

They can be effective when:

• Excavation is unrestricted

• Equipment access is readily available

• Wall geometry is simple and linear

• Construction with large units or unique aesthetics is a priority

The key is understanding that PMBs are one option, not the only gravity-based solution for large cut walls.

Multi-Depth SRWs as the Bridge Between Gravity and MSE

Multi-depth SRW systems occupy a critical middle ground:

• More capable than small gravity walls

• Less machine-intensive than PMB systems

• Require fewer specialty units than PMBs

• Less excavation- and reinforcement-dependent than full MSE systems

• Provide economic and logistical advantages compared to other approaches
  

In many cut wall applications, they provide the most balanced response to structural, spatial, and constructability constraints.

Setting the Stage for Hybrid Wall Systems

In some cases, even multi-depth gravity systems reach their practical limits — particularly in very large wall conditions or where surcharge loads dominate. When this occurs, combining mass with reinforcement becomes necessary.

In the next article, we’ll explore hybrid retaining wall systems and how they blend the advantages of multi-depth gravity and MSE approaches to solve complex site challenges and scenarios.

Conclusion

Big block retaining walls have earned their place in the industry, but they should not be treated as the default solution for every large cut wall application.

Multi-depth SRW gravity walls offer a compelling alternative — delivering comparable mass-based performance with greater flexibility, reduced excavation, and improved adaptability to real-world site constraints.

Understanding these differences empowers designers to select systems based on site behavior and constructability, rather than assumption.