Crushing

Aggregate and the environment

Breakfast table talk with my wife Pam recently included a discussion of topics for future Quarry articles.

Pam had just finished reading an article about permeable pavement – yeah, I am not making that bit up! Why was she reading that!?! – when she suggested: “How about writing some articles on the uses of aggregate for environmental purposes?”

Would that topic work? I cast about on the internet using the search string ‘aggregate environment’ – and Google came up with about 89,100,000 hits in 0.31 seconds! I was pleased to see that my publication Aggregate and the Environment (first published in 2004) was still near the top of the list. 

Unfortunately, it didn’t take long for me to become somewhat alarmed. Clicking on most of the hits took me to sites describing the terrible things aggregate mining does to the environment. I tried other search strings such as ‘environmental applications for aggregate’ and ‘environmental uses gravel’ – all with no better results.

That did it! I know there are many situations where the use of aggregates actually benefits the environment.

I need to vent, so my column this year will contain a number of articles on the environmentally beneficial uses of aggregate. I will begin with the one Pam read about – permeable pavement.

Does the idea of permeable pavement hold water? Let’s see.

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In the simplest of terms, permeable pavement is a hard surface used in applications such as car parks or roads, with a base and sub-base that allow the movement of stormwater through the surface. The permeable pavement Pam was reading about was made from aggregate, crumb rubber and asphalt. Permeable pavement can also be made with no crumb rubber, just aggregate bound with asphalt or concrete.

Some low traffic permeable pavement can be made from unbound aggregate or grass with, or without, geotextiles and interlocking pavers. The permeable paver demonstration above shows the general design of a permeable pavement. Don’t be fooled by the simplicity of the photo – it has different layers with their own specific requirements.

Here is the problem. During a storm, the first 38mm of rainfall mobilises about 90 per cent of surface pollutants. Instead of going into treatment facilities, run-off typically is channelled directly into storm sewers and thence into local water bodies. The pollutants can cause an increase in algae content, harm aquatic life and require expensive treatments to make the water potable. If that initial run-off can somehow be retained, some of the ill effects of stormwater run-off can be reduced.

Conventional pavement causes increased run-off by preventing water from infiltrating into the soil. By contrast, pervious pavement captures rainwater in a network of voids, thus reducing run-off. In addition, the captured water can slowly percolate into the underlying soil and filter out pollutants carried by the run-off. In some geologic settings, that water recharges local aquifers.

If pervious concrete or asphalt roadways and car parks cover enough area, they can double as water retention structures. That may reduce or eliminate the need for traditional stormwater retention ponds, thus lowering overall project cost.

In addition to stormwater control, pervious pavements aid in reducing the urban heat island effect. The open cell structure of the pervious pavement allows free air circulation, allowing more rapid cooling of the pavement.

So, it looks like permeable pavement really does hold water!

Thanks for the idea, Pam!

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