Safer cheaper footpaths? Here’s how.
• Work underway to repair a trip hazard. Badly.
Here’s how to get safer, cheaper footpaths.
Pavements heave upwards and split apart when tree roots below them push upwards. This causes trip hazards for pedestrians.
There is one, long term design to prevent footpath’s being pushed up by tree roots. Use this design and footpaths won’t heave, won’t need repair, or cause pedestrians to trip on the cracks.
Before we get to that long term design solution, first, let’s look at a common way to temporarily reduce pavements cracking. This work involves what’s shown in these photos:
• About 2 to 4 trucks and about 4 to 8 workers
• Power, fuel, jack hammers, labour, sometimes traffic wardens
• About two days, two lots of vehicles and workers toing and froing. On day 1 the broken pavement is removed and the ground below it made ready for new pavement to be put there the next day
• Digging up and taking away the pavement
• Cutting out tree roots, tamping down the soil with an energy-using vibrator, a mix of sand and cement, pouring new pavement
• sometimes removal of the tree and consequential increase in urban heat due to the loss of shade and consequential loss in property value for adjoining houses.
This is what that work looks like:
• Incomplete work after day 1. Notice naughty tree in the background whose rising tree roots caused the pavement to rise up and to crack; the solution is to answer the question: why do tree roots rise up?
• Work after day 2, new pavement. Notice green down pipes which go under the new pavement to waste rain water to the street through the drain holes in the kerb, then drain and carry pollution to harbours, rivers
This is the common practice. It is a waste of money and resources because the tree roots grow again requiring second, third and so on repairs.
And there are costs to ratepayers and to Earth’s climate, harbours and rivers.
Here’s how to do it once, to set and forget about pavement heave.
The design principle answers this question: why do tree roots rise up and crack pavements?
Unlike we humans who feed ourselves at the top of our body, tree roots ‘eat and drink’ out of sight, below ground and out of sight.
When tree roots push up foot path pavement they are looking for water. Tree roots ‘eat’ and ‘drink’ at the edge of the tree’s canopy, away from the trunk.
Hard, impermeable road and footpath concrete and bitumen deny tree roots the water and nutrients they need to feed the tree.
Tree roots are not being naughty. They’re just doing what they always do.
Amazing though it is, the needs of trees and their way of growing is not taught in most engineering universities and technical schools as part of the teaching of road and pavement design. Instead, trees are simply taught as things which are ‘menaces’, ‘bad’, ‘anti-road’, and something to be avoided. So road and footpath design ignore the needs and behaviour of trees and their roots.
And the result is we pedestrians get pavement heave, cracks, avoidable maintenance costs.
The solution?
Keep rainwater where it falls to feed the tree roots below ground and pavement level. Do this by having the pipes below the pavement discharge rainwater into the gravel, subsoil and material below the footpath.
A shining example of this is at Sydney’s Olympic Park where, in preparation for the Sydney 2000 Olympics, aggressive figs were planted with hard pavement around them and into absorptive media which retained rainwater. The result? Detailed monitoring showed no pavement cracking. The amended structural soil works.
But, unfortunately, there’s no change in the content or teaching of road design in engineering and technical course. No best practice.
• Notice the partly exposed white drain pipe to take rainwater from the roof and under the yet-to-be-poured bitumen to discharge in the outlet in the kerb then to the gutter then to the harbour and ocean. The workers will have to come back one day to redo the pavement.
In the photo above a solution to keep rainwater where it falls can involve either or both these options which will allow rainwater to be absorbed into the soil to feed the tree:
simply drilling holes through the white pipe, and/or
replacing the white pipe with geofabric-covered agricultural drainage pipe which has holes in it to allow roof rain water to drain through but to also exclude external water and soil from clogging up the pipe.
• See the little girl laying agricultural drainage pipe? It’s that easy to install. We citizens worked with council to design, plant and build this self-watering nature strip in Chippendale.
It’s funny. The same council which works with our community to install agricultural drainage pipe at the leading edge of pale concrete footpaths so as to catch the rainwater from the path and to irrigate the paths below also repairs pavement heave as shown above, needlessly committing the council to never-ending pavement repair. The photos above show that work done about 10 years ago.
The verge now looks like this; robust, healthy despite a 2 year drought. The recent rain has been kept in the verge garden
It looks like the right hand and the left hand in there don’t talk to each other.
• Luxuriant tree and verge growth driven by keeping rainwater where it falls to irrigate the plants from below ground level
• Hearty tree and plant growth, stable footpath due to rainwater being kept where it falls with ag pipe to irrigate below ground
How much water is there?
Footpaths are water rich
When we harvest rainwater from footpaths we can cool our street, cut our air con bills and grow food, plants and trees.
There are two sources of water from footpaths.
Run on – rain that falls on the path and runs off it.
Run off – rain that falls on adjoining roofs, goes into a downpipe and under the footpath either to be absorbed there or to be wasted to run into the gutter and then to pollute the ocean or river with the gunk the water picks up along the way there.
Let’s count how much free water is on a typical footpath.
Take a Chippendale footpath outside a terrace and do the sums:
Run on: footpath is 1.5 wide and the distance beside the terrace is 5 metres = harvesting area of 7.5 metres
Run off; roof draining to the footpath is 50 square metres
Total rainwater harvesting area = 57.5 square metres
Average rainfall is 1270 mm a year ie 1270 litres per square metre
Total water available per year is 1270 x 57.5 = 73,025 litres
If there are 10 terraces in a block, that’s 730,250 litres.
That’s enough water to ‘feed’ a dozen trees, hundreds of plants both edible and decorative.
Sustainability in this part of our towns and cities is lost because the rain water is wasted to pollute the ocean and energy and resources are also wasted. The heat of where we live and work is increased by the lack of healthy tree and plant canopy.
That’s another kind of trip up.