Block Paving Sinking In Patches

What's happening?

Block paving is a flexible pavement: the blocks themselves carry almost no load, and the strength is in the compacted sub-base beneath them and the tight sand joints that lock the blocks together. When an area sinks, the sub-base under it has moved, washed out or was never thick enough for what drives over it. The pattern tells you which. Two parallel ruts mean the sub-base is under-specified for vehicle loading. A rectangular dip means a service trench was backfilled loosely and has consolidated. A cone-shaped hollow means water is carrying the bedding sand away into a void — most often a cracked gully, a leaking soakaway or an old drain run below. Relaying the surface without finding that route simply buys a year.

What usually causes it?

• A sub-base laid too thin or compacted in one deep layer rather than in shallow lifts, so it settles under wheel loads
• A backfilled service trench — water, gas, electricity or telecoms — consolidating under the paving years later
• Bedding sand washing down into a cracked drain, broken gully or open soakaway, leaving a void
• Type 1 sub-base contaminated with soil or clay so it behaves like soft ground rather than a load-bearing layer
• Paving laid straight onto soil or over an old surface with no proper sub-base at all
• A collapsed or crushed drain or duct below the driveway, taking the surrounding ground with it
• Heavier vehicles than the drive was built for, such as delivery lorries or a skip lorry turning on it

When should you call a driveway and patio installer?

Call a driveway specialist when the dip is more than a block or two across, when it has grown since you first noticed it, or when the blocks feel spongy rather than merely low. Make it a priority if there is a drain, inspection chamber or soakaway nearby, or if the hollow appeared quickly rather than over years — both point to water moving material below, which gets worse and more expensive with every wet month. If a drain is suspected, that gets surveyed first.

What will the driveway and patio installer actually do?

• Lift the affected blocks and excavate to establish whether the fault is thickness, contamination, a trench or a void
• Check for drains, ducts and soakaways in the sinking area, and get a camera survey where a collapse is suspected
• Rebuild the sub-base in compacted layers to the depth the loading needs, rather than topping up the bedding sand
• Re-lay the blocks to the correct falls, keeping the surrounding pattern and levels tied in
• Re-sand the joints and vibrate the surface so the blocks interlock and carry load as one
• Reinstate or replace the edge restraint where the settlement has pulled the perimeter out of line