Drainage for block paving – the most critical factor

Why is drainage so critical?

The worst enemy of a block pavement is standing water. In the Málaga climate – little rain overall, but at times torrential downpours (the well-known autumn DANA episodes) – its effects are particularly damaging:

• Weakening of the base: the bedding and sub-base material silts up with fines and loses bearing capacity.
• Erosion during torrential rain: a cloudburst can wash out the bedding layer and undermine the edges within hours.
• Washing out of fines: flowing water carries the fine particles out of the joints and the base.
• Settlement: the base loses its compaction and the surface becomes uneven.
• Safety and slipperiness: persistent puddles encourage algae and moss, and with the occasional light inland frosts (Ronda and Antequera comarcas) they can turn into slippery patches.

Falls

Minimum falls to avoid standing water:

• Crossfall: typically at least 2% (e.g. 2 cm/m)
• Longitudinal fall: at least 0.5%
• At vehicle crossings and next to buildings: water must always be led away from the building

If the ground is flat, the fall must be created artificially by levelling the base and the bedding layer.

Collection and discharge elements

Drainage channel

An element placed at the edge of the pavement that collects the run-off and carries it to the gully. Usually made of concrete or plastic, it forms part of the edge restraint system.

Linear drainage grate

Drains water evenly over large areas. It is placed across the surface where the falls meet (e.g. at the bottom of a driveway). It can be used alone or combined with channels.

Point gully

A grated drainage point placed in a low spot, at the corner of a patio or in a car park. It connects by pipe to the stormwater network or to a soakaway.

Soakaway trench and perforated drain pipe

If connection to the public network is not possible or not desirable, the water can be managed by infiltration. The drain pipe is a perforated pipe through which water seeps into the ground. Precondition: the subsoil must be sufficiently permeable.

Drainage along the kerb

The kerb with drainage channel (bordillo con caz) is the inclined unit beside the pavement used to carry water – standard practice in Spanish urbanisation works. The pavement falls towards the channel, and the water runs along it to the gully.

Permeable block paving systems

Technical practice distinguishes between partially permeable and fully permeable block pavements (so-called SuDS – sustainable urban drainage systems): the latter aim to infiltrate a substantial part or all of the rainwater into the pavement structure itself.

Construction requirements:

• Special open-joint blocks (with wider joint spacing) or standard blocks with a permeable joint filling.
• An open-graded bedding layer (not sand but grit, e.g. 4/8 mm).
• An open-graded sub-base (e.g. 16/32 crushed stone) that provides storage volume.
• A compatible jointing material (an open, not sealed, system).
• The permeability of the subsoil must be verified.

Advantages:

• Reduces run-off and the load on the stormwater network – very valuable during torrential rain.
• Recharges groundwater, which matters in a dry climate.
• Counteracts the urban heat island effect, especially relevant in the hot Costa del Sol summers.

Limitations:

• Cannot be used on poorly draining clay soils without a suitable drain pipe.
• Should be avoided in areas at risk of contamination (e.g. fuel spills).
• Requires maintenance (removal of silt and algae).

Typical drainage mistakes

• A fall directed towards the building – water runs to the foundations.
• Zero fall – standing water.
• No gully, or one too small for torrential rain.
• A channel that is not connected – the water flows back into the base.
• A permeable pavement with clogged joints – the infiltration capacity is lost.
• A sand bedding in a permeable system – silting and blockage.