Why Cape Town Paving Sinks — and What a Lift-and-Relay Involves

Sunken wheel tracks, a dip that holds water, pavers that rock underfoot: when block paving sinks, something underneath has moved. The Concrete Manufacturers Association’s design guide puts settlement down to “an inadequate subbase or subgrade, the presence of subsoil water, overloading the pavement or a bedding sand layer which is too thick” [1].

Where we stand: paving is our trade, and lifting and relaying sunken paving is work we quote across Cape Town. The CMA is a concrete manufacturers’ body writing about its own product, so we quote its guidance and say where the view is ours.

What makes block paving sink?

Book 2 lists four causes of settlement: an inadequate subbase or subgrade, subsoil water, overloading, or a bedding sand layer that is too thick [1]. A faulty paver is not on the list.

• The subbase or subgrade: the compacted layers the sand and pavers sit on. If they give, the surface follows.
• Subsoil water: water in the ground under the paving. On drainage generally, Book 2 says “Most failures of pavements are due to ingress of water into the pavement layerwork” [1].
• Overloading: more load than the paving was built for. Book 2 says 50 to 60mm blocks are generally adequate for domestic use and recommends 80mm for industrial use [1].
• Too much bedding sand: the layer is meant to be thin, for the reason in the next section.

More on each layer in our guide to what makes paving fail early.

The bedding-sand shortcut that shows up later

Bedding sand is a thin layer, 25mm ±10mm after compaction, there to bed the pavers evenly [1]. The CMA says it must not be used to fill hollows in the layer below.

Book 2 says the bedding layer “should not be used as a levelling course to fill voids in the subbase surface. This will lead to subsequent settlement” [1]. Book 3 says depressions are filled with subbase-quality material compacted to 98% of modified AASHTO maximum density, and “Bedding sand shall not, under any circumstances, be used for this purpose” [2].

A hollow filled with extra sand can look flat on hand-over day, so know what to check before you pay the contractor.

Edges that let the paving creep

Book 2 says “All cbp must have an edge restraint” [1]. Without one, the pavers at the edge can creep outwards and the joints open.

• What it does: Book 3 says the edge restraint retains the sand bedding and ensures edge units “do not creep outwards or rotate under load with consequent opening of joints and loss of interlock” [2].
• What counts: cast in situ, precast, or an existing structure [1].
• When none is needed: Book 3 says a separate restraint is unnecessary where the paving joins an existing concrete pavement, or asphalt at least 100mm thick [2].

In our view, if the edge has moved, relaying the field without repairing the edge leaves the same weakness in place.

Water, and the ground under Cape Town paving

Water getting into the layers is the CMA’s leading failure cause [1], so a dip that holds water after rain is worth watching. The ground itself also varies a lot across the Cape Flats [3].

• Falls: Book 2 says “Adequate longitudinal falls and no undulations are necessary to prevent ponding and infiltration of water into the layer-works”, with a minimum fall of 2% in any direction generally required [1].
• At drains: paving is laid 5 to 10mm proud of stormwater structures or channels [1].
• Slopes and high water tables: water can collect in the bedding sand at the low point of a slope, and subsoil drainage should go in where the water table is high [1].
• The ground: a geotechnical study found Cape Flats sands “highly variable (with both inter- and intra-formation variation), with a wide range in many material properties” [3]. In our view, that is why a base has to suit the site.

If your property has its own stormwater drains, keeping them working is on you: the City’s by-law says every owner of property on which private stormwater systems are located shall “at own cost, keep such stormwater systems functioning effectively”, unless the City has taken that on in a maintenance agreement or a servitude condition [4]. We also look at whether permeable paving is worth it in Cape Town.

Relay or rebuild? What a lift-and-relay involves

Book 2’s fix for settlement is to lift the area, address the problem, reinstate the level and re-lay the pavers [1]. How deep that goes is what an inspection finds out.

We lift a few pavers at the worst point and look at the layers before we quote, as we do for the other paving work we take on. In our view, a relay quoted without looking underneath is a guess.

1. Lift the affected pavers and stack them for reuse.
2. Fix the cause: whatever failed underneath, whether the base, the drainage or the edge.
3. Screed a loose sand bed to reach 25mm ±10mm once compacted [2].
4. Relay and cut in: Book 3 names guillotine block cutters or power saws, and does not recommend cut pieces smaller than about 25% of a full paver [2].
5. Compact and joint: at least two compactor passes, then jointing sand broomed in and compacted, repeated “until the joints remain full after compaction” [2].

A check you can do: Book 3 limits the level difference between adjacent pavers to 3mm, on finished paving three months after opening to traffic, for its Degree of Accuracy I [2]. A bigger step is worth pointing out at the assessment.

What drives the cost, and why we publish no rate

We don’t publish a per-square-metre figure for relaying, because the rate depends on how deep the base has to be rebuilt and whether pavers need replacing. What moves the price most is what sits under the surface.

• Depth of the repair: resetting the sand bed is one job; digging out and rebuilding the base is a bigger one.
• Pavers that break on lifting: in our view, expect some to need replacing, and matching older stock can be the hard part. Our comparison of charcoal cobble, clay and granite pavers covers the materials.
• Edges and drainage: if either caused it, the repair includes them.
• Access: how easily material can be moved in and out.

For what goes into pricing new paving, see what drives a Cape Town paving quote.

When topping up the joints is enough

If the surface is still flat but the joints have dropped, in our view refilling them may be all it needs. Use a fine jointing sand, not bedding sand [1].

Our routine for looking after block paving covers joints, weeds and drains.

Common Questions

Paving sinking, rocking or holding water? We do free on-site assessments across the Peninsula: we show you what is going on underneath and follow with a written quote, with no obligation to use us for the work. See how we work out a price, or call 021 825 6396, weekdays 8am to 5pm.