Most people planning a retaining wall think of it as a vertical face holding back earth. What they often don’t see is the reinforcement extending back into the retained soil, either deadman anchors or geogrid layers, that make taller walls work without needing impossibly large footings or post sizes.
Both systems achieve the same goal: they tie the wall back into the soil mass behind it, so the wall face and the soil work together as a composite system rather than the wall trying to hold back the entire earth load on its own. Understanding when each system applies helps you have a meaningful conversation with your engineer and your contractor.
Why Standard Retaining Walls Have Limits
A standard concrete sleeper wall (H-posts and panels) works by moment resistance, the embedded H-posts resist the overturning force from the retained soil by being long enough and embedded in concrete footings deep enough to counteract the moment. This works well for walls up to approximately 1.5-2.0 metres retaining height.
Above 2.0 metres, the forces become large enough that:
- H-posts need to be very heavy and deeply embedded, driving up cost
- Footing sizes become very large
- Alternative approaches become more economical
For modular block walls (concrete block, Besser block, Allan Block), the limits are different, the block wall itself has limited moment capacity, so tall block walls need either very wide battered faces (expensive in land area) or reinforcement extending back into the soil.
This is where deadman anchors and geogrid both come in.
Deadman Anchors, How They Work
The Principle
A deadman anchor is a horizontal structural element extending from the wall face back into the retained soil mass. The deadman is buried at or near the base of the retained fill, and a tie-rod or tie-back connects it to the wall face.
The principle: instead of the wall footing resisting overturning by itself, the deadman acts as an anchor plate buried in stable soil. The soil above and around the deadman resists its being pulled out, which means the connection between the deadman and the wall can transfer tension, the wall is literally tied back into the soil.
Construction
For concrete sleeper walls, deadman construction typically involves:
- H-posts are installed at standard spacing at the wall face
- At a depth determined by the engineer (often at the footing level or mid-wall), a horizontal deadman element, typically a concrete pad or a steel plate welded to the H-post, extends 1.2 to 2.0 metres back from the wall
- Compacted fill covers the deadman during backfill
- The deadman is then passive, it doesn’t do anything unless the wall tries to overturn, at which point the soil above the deadman resists the upward pull
For timber post systems (largely historical in the Blue Mountains now, given the shift to concrete), deadman ties were often heavy timber logs extending back from the wall posts.
When Deadman Anchors Are Used
Deadman anchors are typically used in:
- Concrete sleeper wall systems where wall height requires additional stability beyond what the H-post footings alone provide
- Marine and waterfront retaining structures (sheet pile walls with deadman anchor systems)
- Situations where space behind the wall is available for a buried deadman
- Wall heights of 2.0-3.5 metres where additional tie-back capacity is needed
Key limitation: Deadman anchors require sufficient horizontal distance behind the wall to place the deadman in competent soil, typically 1.5 to 2.0 times the retained height. If the space behind the wall is limited (common on Blue Mountains sites with tight boundaries or structures behind the wall), deadman anchors may not be feasible.
Geogrid Reinforcement, How It Works
The Principle
Geogrid is a polymer mesh product, a flat grid of high-tensile plastic (typically polyester or high-density polyethylene) that is placed horizontally in the fill behind a retaining wall in layers as the fill is placed. The geogrid interlocks with the fill material, soil or gravel particles lock through the grid apertures, creating a composite “mechanically stabilised earth” (MSE) mass that is much stronger and more stable than the fill alone.
The wall face (blocks, panels, or other facing) is connected to the geogrid layers, so the facing is effectively anchored into the reinforced soil mass rather than trying to hold back unreinforced fill on its own.
Construction Sequence for Geogrid Walls
- The wall face (typically concrete block or modular retaining block) is placed on a foundation course
- A specified layer of compacted fill is placed behind the facing
- A geogrid layer is laid on top of the compacted fill, connected to the facing blocks
- The next course of facing blocks is placed on top of the geogrid connection
- Fill is placed and compacted on top of the geogrid
- The sequence repeats until full wall height is reached
The result is a reinforced soil mass where the blocks are the facing, the geogrid is the structural reinforcement, and the compacted fill is the body of the wall system.
Geogrid Wall Specification
The engineer specifies:
- Geogrid type (determined by tensile strength requirements)
- Geogrid layer spacing (typically 200-600mm vertically, closer at higher load zones)
- Geogrid embedment length (how far back the geogrid extends into the fill)
- Facing block type and connection to geogrid
- Fill material specification (key, geogrid works best with granular fill, not clay)
When Geogrid Is Used
Geogrid is standard for:
- Modular concrete block retaining walls over approximately 1.0-1.2 metres height
- Any engineered block wall where facing blocks alone can’t carry the lateral load
- Large commercial or infrastructure retaining structures (highway walls, embankment walls)
- Situations where the space behind the wall is available but soil conditions suit engineered fill
Key advantage over deadman: Geogrid layers are installed as the wall goes up, no excavating back into existing soil to place a buried element. This makes geogrid more practical when the area behind the wall is already undisturbed soil that you don’t want to excavate extensively.
Side-by-Side Comparison
| Feature | Deadman Anchors | Geogrid Reinforcement |
|---|---|---|
| How force is transferred | Tied anchorage, tension in tie rod to buried anchor | Friction/interlock, geogrid embedded in compacted fill |
| Fill requirement | Existing or new fill behind the wall | Engineered, compacted fill behind wall face |
| Space required behind wall | 1.5-2.0× retained height | Similar, geogrid layers extend 0.5-1.0× retained height |
| Wall type most used with | Concrete sleeper H-post systems | Modular concrete block systems |
| Construction complexity | Moderate, deadman must be placed at correct depth | Moderate, multiple geogrid layers must be placed correctly |
| Cost relative to standard wall | Adds 20-40% to wall cost | Adds 15-35% to wall cost |
| Inspection difficulty | Buried, quality of installation is hard to verify after completion | Buried, same issue |
| Suitability for Blue Mountains clay soils | Good, anchors in competent soil below clay | Care needed, geogrid in clay-fill performs poorly; granular fill needed |
| Engineering requirement | Always required | Always required |
Which System for Blue Mountains Sites?
Concrete sleeper walls on steep sites (2.0-3.5m high): Deadman anchors are the more common approach for concrete sleeper systems needing additional stability. The H-post and deadman system is well-understood by structural engineers working in the residential sector and has a long track record.
Modular block walls over 1.0m high: Geogrid is standard, virtually all engineered modular block wall systems over 1.0m height use geogrid reinforcement as per the manufacturer’s design guidelines.
Clay soil sites (lower Mountains, Springwood/Blaxland): Geogrid in clay fill performs poorly, the clay provides less friction/interlock with the geogrid than granular fill. For geogrid walls on clay sites, the engineer typically specifies a granular fill column directly behind the facing and geogrid zone, even if native clay exists further back. Deadman anchors can work better in clay if the anchor is placed in competent material below the clay zone.
Both systems in the Blue Mountains need drainage: Regardless of the reinforcement type, every retaining wall over 600mm in the Blue Mountains needs a properly designed drainage system, ag pipe, drainage aggregate, and geotextile fabric. Drainage failure is the leading cause of retaining wall failure in the region. See [/guides/retaining-wall-drainage-design/].
Engineering Sign-Off
Both deadman anchor and geogrid reinforcement systems are engineered solutions, they cannot be designed by a contractor or homeowner from a catalogue. An engineer must:
- Calculate the active and passive pressures on the wall
- Specify the reinforcement type, spacing, and length
- Design the connection between reinforcement and wall face
- Issue a certificate for the design
For DA approval, engineering certification is mandatory for walls over 1.0m. See [/guides/engineer-certificate-retaining-wall/].
A geotechnical report confirming fill material properties, soil conditions, and groundwater conditions informs the engineering design. See [/guides/geotechnical-report-blue-mountains/].
FAQ
Can I use geogrid in existing soil, or does it only work with fill? Geogrid is designed to be placed in compacted fill as construction proceeds. In existing undisturbed soil, you would need to excavate to place geogrid layers, which is less practical than placing deadman anchors. For existing-soil situations, deadman anchors are usually the better reinforcement option.
How long does geogrid last? Quality polymer geogrid (polypropylene or polyester) has a design life of 75-120 years when properly specified, installed, and protected from UV exposure (it degrades rapidly in sunlight, but buried geogrid is protected). The major risk is creep (slow extension under sustained load), this is why the engineer specifies the tensile strength with a creep reduction factor.
Do deadman anchors rust? Deadman anchors in concrete or galvanised steel that are correctly installed and buried should last the life of the wall. Steel deadman elements in aggressive soil environments (acid sulfate soils, marine environments) may need stainless steel or non-metallic materials. Blue Mountains sandstone soils are generally benign for steel; have the engineer confirm for your specific site.
How do I know if my existing modular block wall has geogrid behind it? Without excavation, you can’t easily tell. If the wall was built by a reputable contractor with engineering sign-off and the wall is over 1.0m high, geogrid was likely installed. If the wall has no documentation and you have concerns about its stability, have it assessed. See [/guides/retaining-wall-failure-warning-signs/].
Can I get a quote for a geogrid or deadman wall? Yes. Both systems are designed by a structural engineer and built by a contractor working to that design; if your enquiry is referred to an independent contractor, they can quote either system, and where licensing is required their identity and licence details are provided before you accept a quotation. Contact us at [/contact/#quote] to describe the site.