This a multi-part article by Paul Tittle providing an overview of the service life, maintenance, standards, legislation and adoption and the main benefits and problems of Permeable and Porous road construction.

Permeable and porous road construction - Part 1

 We would like to stress that while this article is provided for information and educational purposes, many factors go into the proper design of any permeable or porous road construction and as such professional advice should always be obtained before any construction begins.

Introduction

From the Romans to the present day, Engineers have struggled to deal with the issue of rainwater run-off. Roadways, car parks, footpaths and pedestrian areas all require drainage. In this article I intend to cover Permeable and Porous road construction including service life and maintenance, standards, legislation and adoption and the main benefits and problems.

What is the difference between permeable paving and porous?

Block paving is a good example of permeable paving. Water is allowed to drain between the blocks and in this way permeates through to ground water. Porous paving is often in the form of asphalt where water drains through a continuous skin of asphalt covering a pedestrian area.

Full, partial and no infiltration. The three basic system types.

A full infiltration system allows the rainwater to drain through the finish, (for example the block paver surface) through the laying course and the permeable sub-base and on down into the ground below (the Sub-grade).

A partial infiltration system is similar to the full infiltration system but incorporates a perforated collector pipe in the permeable sub-base. This takes a percentage of the water away to a storage area, sewer or watercourse.

In a no infiltration system rainwater is prevented from draining into the sub-grade by an impermeable membrane at the base of the road construction. The water is instead allowed to drain away through a perforated or solid collector pipe to a sewer or watercourse.

In all system types, the sub-base thickness is typically designed and detailed to provide the necessary storm water attenuation to deal with extreme rainfall events.  In the partial and no infiltration systems, the rate of discharge to an offshore sewer or watercourse is likely to be restricted.  This is generally achieved using a Hydrobrake flow control device with storm water backing up into the permeable sub-base.  Note that the sub-base thickness is influenced by a number of factors which will be discussed later.

The structural design of permeable block paving

Pedestrian areas and driveways require 80mm blocks laid on a 50mm laying course. This is laid on a minimum 150mm sub-base with a geotextile membrane separating the sub-base from the ground. A similar design is suitable for cars and light vans, however a minimum 200mm sub-base should be used. Where heavier vehicles are present (up to 7.5 tonnes) a minimum 350mm sub-base is required.

Where 21 Tonne Vehicles are present (e.g refuse vehicles, fire tenders) 80mm blocks should still be used on a 50mm Laying course. Below the Laying course an 80mm DBM and minimum 150mm sub-base is required on top of the geotextile membrane.

All the above sub-base thicknesses are based on a CBR of 5% for the existing ground.  The implications of a weaker sub-grade (CBR <5%) will be discussed later.  As mentioned earlier, sub-base thickness is also influenced by the need to provide storm water attenuation.

Geotextiles

Geotextiles can be used to provide separation between the sub-grade (Soil) and the road construction. This prevents voids from forming where the road construction is pushed down into the soil by the movement of traffic. Mixing of the sub-grade and the road construction can cause depressions in the road surface. Geotextiles are also often used within the road construction itself to separate the finer laying course from the sub-base (typically OGCR or Type 3 (Open Graded Crushed rock)). OGCR/Type 3 is typically used as a sub-base for permeable paving as it is a low/no fines aggregate which allows good water flow and storage in the natural voids that are created when it is laid.

GeoGrids

Geogrids are used where additional reinforcement is required due to soft ground conditions. They can also be used to increase storage within a given depth of Subbase via Sub Base Replacement.

Sub-base replacement

Typically OGCR provides 30% voids. This void space is used for water storage within the road sub-base. In some circumstances, where an excessive depth of OGCR is required to achieve the required water storage, the OGCR can be replaced.

Replacement sub-base is usually formed of 150mm deep geocellular plastic units. Geotextiles are used to separate the units and the other road construction layers. The sub-base replacement results in 90-95% voids compared to the 30% voids of OGCR. This dramatically increases storage in the equivalent depth of sub-base. The increase in storage allows for a thinner overall construction and therefore less excavation. The framed nature of the sub-base replacement also provides increased stability on poor ground.

However there are downsides to sub-base replacement, mainly relating to faster movement of water through the road construction and lack of water quality treatment.  The latter of these is important to note because the OGCR provides water quality treatment by filtering out/biologically breaking down pollutants. This process means that expensive pollutant interceptors may not be required.  Given this, partial replacement of OGCR is usually a better solution.