Block Paving Edging
Block Paving Edging: My Framework for Preventing 99% of Lateral Spread Failures
The single most critical failure point in any block paving project isn't the paving itself; it's the edging restraint. I've been called to repair countless driveways and patios where the pavers have shifted and spread, creating ugly gaps and an unstable surface. The root cause is almost always an improperly installed edging system that failed to contain the lateral forces exerted on the blocks. This isn't just about aesthetics; it's a fundamental structural flaw.
My approach treats block paving edging not as a decorative border, but as a critical retaining wall. Forget flimsy plastic strips or kerbs simply placed on dirt. The solution lies in creating a monolithic structure where the edging, its concrete haunching, and the compacted sub-base are mechanically locked together. This method ensures the edging can withstand years of vehicle traffic, freeze-thaw cycles, and ground settlement without moving a millimeter.
Diagnosing Edging Failure: My "Sub-Base Lock" Methodology
After analyzing dozens of failed projects, I identified a recurring pattern: the edging was treated as a separate component placed *on top* of the sub-base. This is fundamentally wrong. The edging must be an *integral part* of the foundation. My proprietary method, which I call the "Sub-Base Lock", is built on this principle. It ensures the lateral load from the pavers is transferred directly into the ground through a robust, unified structure.
The methodology rejects the common practice of digging a shallow trench and simply dropping the edging in. Instead, it requires the sub-base layer to extend fully beneath the edging's footprint. The edging is then bedded on a mortar mix directly onto this compacted MOT Type 1 sub-base, and the crucial concrete haunching is keyed into both the edging unit and the sub-base itself, creating a single, immovable mass. This prevents the number one failure: the edging unit tilting outwards under load.
The Technical Mechanics of a Fail-Proof Haunch
The heart of the "Sub-Base Lock" is the concrete haunching. A poorly mixed or undersized haunch is useless. Over the years, I've refined the specifications to achieve maximum restraining force. The key is not a wet, pourable concrete, but a semi-dry or 'damp-pack' mix. This allows for immediate compaction and shaping without slump.
My standard specification is a 4:1 ratio of sharp sand to cement. This mix provides immense compressive strength once cured. The geometry of the haunch is non-negotiable: it must extend up to at least two-thirds the height of the edging kerb and form a solid 45-degree angle from the kerb face down to the sub-base. This profile effectively transfers lateral forces downwards into the foundation, rather than allowing the edging to pivot and fail. A small, weak fillet of concrete will crack and break away within the first year.
Step-by-Step Implementation Protocol
Executing this correctly requires precision. Rushing these steps is the most expensive mistake you can make, as fixing failed edging requires lifting the entire paved area. Here is my exact field protocol.
- Foundation and String Line Setup: Excavate the area ensuring the compacted sub-base extends at least 150mm beyond the final edging line. Set a tight, level string line to mark the final height and face of the edging. This line is your absolute guide.
- Mortar Bedding: Lay a continuous bed of a slightly wetter 5:1 sand/cement mortar mix directly onto the compacted sub-base, following your string line. The bed should be around 25-40mm thick.
- Placing the Edging Units: Place each edging kerb onto the mortar bed. Use a rubber mallet to gently tap the kerb down until its top edge is perfectly flush with your string line. Check for level and alignment after every unit.
- Applying the Concrete Haunching: Mix your 4:1 'damp-pack' haunching concrete. Apply it to the back of the kerb, using a trowel to compact the haunching firmly against the kerb and down onto the sub-base. Ensure you achieve the required two-thirds height and 45-degree profile.
- Curing and Protection: The haunching needs at least 72 hours to cure before you begin laying pavers against it. In hot or dry conditions, I lightly cover the haunching with plastic sheeting to prevent it from drying too quickly, which can cause cracking and reduce its final strength.
- The Straightedge Test: I run a 2-meter straightedge along the top and face of the installed edging. There should be no deviation greater than 2mm. Any "lippage" between kerbs creates a weak point.
- The Joint Inspection: All butt joints between edging units must be tight and clean. Any mortar that squeezed up should be struck off cleanly.
- The Manual Load Test: Once the mortar bed has set but before the haunching is fully cured (usually after 24 hours), I apply firm, sideways pressure with my foot to the top of the edging. There should be zero movement. If I detect any give, I know the haunching wasn't compacted properly and must be redone. This simple test has saved me from catastrophic failures on major commercial projects.
