Segmental Retaining Walls (SRW) are built from modular concrete blocks and layers of geogrid reinforcement. At Howell Engineering, we design these walls using the same standards applied to public infrastructure. That means following the Federal Highway Administration (FHWA) and the National Concrete Masonry Association (NCMA) guidelines—two of the most trusted authorities in the industry.
Let’s break down the science in simple terms so homeowners, contractors and property managers can understand how these walls work and why proper engineering matters.
What Is Geogrid?
It’s fairly clear that block is the primary component of an SRW, but geogrid is the hidden backbone of a segmental retaining wall.
It is a high‑strength polymer mesh, usually made from polyester (PET) or high‑density polyethylene (HDPE), that looks like a grid of intersecting ribs. When placed between the block and layers of compacted soil, geogrid acts like long reinforcement straps extending back into the earth.
How Segmental Retaining Walls Work
Although SRWs look like stacked concrete blocks, the real strength comes from the soil–geogrid–block composite. FHWA and NCMA design methods evaluate three major categories of stability:
- Internal Stability – Strength Within the Wall
Internal stability checks whether the reinforced soil mass can resist the forces trying to pull it apart. Internal Stability primarily depends on the Tensile Strength of the geogrid.
Tensile strength is the pulling strength of the geogrid—how much force it can resist before stretching or tearing. It must be strong enough to hold everything together when forces try to pull the wall outward.
- External Stability – How the Wall Sits on the Ground
External stability checks whether the entire reinforced soil mass is stable on its foundation.
Sliding - Occurs when the whole wall tries to move forward across the ground surface beneath it. Engineers check whether the base friction and wall weight are enough to resist this push.
Overturning - When the wall tries to tip forward like a door on a hinge. Design ensures the wall’s weight and geometry keep it from rotating outward.
Bearing Capacity - The strength of the soil under the wall. If the soil is too weak, the wall can sink, tilt or rotate.
- Global Stability – The Entire Hillside Matters
Global stability evaluates the stability of the entire slope or hillside, not just the wall. Engineers model potential deep slip surfaces that may pass behind or below the wall.
Key Causes of Retaining Wall Failure
Even well‑built walls can fail if certain conditions are ignored. Here are the most common causes.
- Poor Drainage – The #1 Cause of Failure
Water buildup behind a wall creates pressure pushing outward from behind the face of the wall. This pressure can shift rows of block, cause bulges or even complete failure. It is an essential and relatively inexpensive component of the wall and should not be reduced or dismissed.
- Geogrid Pullout – When the Block and Soil Let Go
Pullout occurs when the geogrid slips out from between the block or from the soil behind the wall. Friction between the geogrid, block and soil determines the resistance to pullout.
- Inadequate Foundation Support – When the Base of the Wall Moves
A retaining wall is only as strong as the soil it sits on. If the foundation soils are weak, loose or improperly prepared, the entire structure can shift or settle.
When the base moves, the wall above it often follows, leading to leaning, cracking or rotation.
- Improper Construction Practices – Use Professional, Experienced Contractors
Even a perfect engineering design can fail if the wall is not built correctly. Construction quality is just as important as the calculations behind it.
Hiring experienced contractors ensures the wall is built to the specifications required for long‑term performance.
- Lack of Global Stability – Like a Localized Landslide
Sometimes the wall itself is fine, but the entire hillside behind it is not.
Global stability failures occur when a deep slip surface forms behind or beneath the wall, similar to a small‑scale landslide. These failures can extend many feet behind the wall and often involve large volumes of soil.
Failing wall
Properly designed wall
Why Howell Engineering Uses the Key Evaluation and Design Standards
Using FHWA, NCMA, and CMHA guidelines ensures:
- Proven engineering methods
- Long-term durability
- Safety under extreme conditions
- Compatibility with modern block systems
- Compliance with industry‑accepted design practices
Concerned About a Retaining Wall? Planning a New One?
Retaining walls can show early warning signs of failure, such as leaning, bulging, cracking, drainage issues or settlement. Avoiding these conditions is paramount in the new design.
Whether you’re evaluating an existing wall or planning a new installation, Howell Engineering can help. Call us for inspection of existing retaining walls or new wall design.
