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The Loam Ranger - Trees and traffic

Dear Loam Ranger,

I'm a council environmental officer. Our street trees are looking mangy, they've got no soil to grow in, and the roots are lifting the pavement. What can we do about it?

This problem derives from a fundamental conflict between trees and engineers. The engineer needs soil to be heavily consolidated to support buildings, roadways and footpaths. To accommodate trees, he builds narrow planting pits constrained by root barriers to prevent lifting and cracking of pavements. In contrast, the tree wants a loose, well aerated, moist, uncompacted soil in a large volume to ensure long life and good growth.

Understandably, for safety and cost reasons, the engineer usually has his way, and a rooting volume far less than is required for a mature tree is left. In a study of urban street trees, SESL found that the trees had virtually no supporting roots. The only feeder roots they had were entering the sandy backfill of pipes and telephone cables, just what the engineers didn't want!

A number of solutions to this situation are used.

1. Choice of plant material. The commonest way around the problem is to try to choose plants known to tolerate small rooting volumes, poor aeration, drought and waterlogging. Celtis, Platanus, Lophostemon and Populus are common selections for tough urban soil. But this limited range of choices can result in rather monotonous plantings. And in especially difficult situations, even these trees will grow poorly and remain stunted and sickly.

2. Use of planting pits. A planting pit, confined by concrete or root barriers, is constructed and capped with a suspended concrete slab to keep traffic from compacting the soil. However, the natural distribution of plant roots is shallow and broad, not narrow and deep, and roots can become starved of oxygen. Further, the concrete walls prevent the movement of water into the root zone. To admit air, various aeration pipes and gravel layers are used, but water must be supplied regularly, and the pit is very expensive to construct. Nevertheless, this solution can work, although rooting volume is seldom sufficient.

3. Structural soils. This new approach, which has been evolving in Europe and the USA for some years now, is gaining ground rapidly. It involves the creation of an engineered "soil" which can be compacted to take heavy loads while allowing large voids to remain for root growth and air and water movement. This structural soil can be used right under the pavement to take the weight of pedestrian and motor traffic, while providing a potential root volume similar to that in a natural environment. The roots eventually fill the voids without creating lifting, meeting the needs of the engineer and the tree at the same time.

To create structural soils, a very coarse gravel aggregate is mixed with a horticultural filler soil. The gravel carries the load through point-to-point contact, and the filler soil provides for the needs of the tree.

The type of gravel dictates its strength and weathering rate. We don't want a weak stone which may break down during compaction and weather in a short time. Granite and basalt aggregates provide the highest strength and are available in a range of gap-graded sizes.

The shape of the gravel dictates how easily it can be compacted. Round particles compact easily and quickly but give a low void ratio. Depending on particle shape, the total void ratio can range from 35% to 43%. This void ratio dictates the size of roots which can grow in the voids, which in turn determines the choice of tree.

The total pore volume of the fully compacted aggregate determines the amount and type of filler soil to add: the more soil that's added, the more difficult it is to obtain maximum compaction. The soil must be able to hold water and nutrients.

This structural soil approach possibly represents the most scientifically credible and cost-effective way to provide for the needs of both engineer and tree, and hence improve the streetscape while reducing the incidence of heaving and damage to structures.