Common Problems Associated With Trees and Their Removal
Severing just one of a tree’s major roots during careless excavation for construction or services can cause the loss of up to 20 per cent of the root system; this undermines the tree’s ability to absorb water and also leaves it unstable in high winds. Therefore, when laying service pipes for example, it is advisable to tunnel under the root system if possible, running the pipes under the middle of the tree (on plan) if necessary.
In general, 80-90 per cent of all tree roots are found in the top 600mm of soil and almost 99 per cent of the tree’s total root length occurs within the topmost 1m of soil, with some variations depending on soil porosity. The undoubted nuisance that fine root systems create for the development of specific sites has to be weighed against the importance that they play in soil stabilisation on sloping ground (acting in a similar way to geotextile matting).
Only a few mature species, such as oak, pine and fir, have significant central tap roots – the main, central roots from which the others spread – and, in most instances, even these extend downwards by only about 2m. So it is the radial tree roots that extend outwards that are of primary concern here; these can influence soil conditions well beyond the circumference of the tree’s leaf canopy. With around £400 million worth of tree-related insurance claims made every year in the UK, it is worth considering what precautions are needed to build near them.
Removing trees is often regarded as a straightforward and often costs effective way of dealing with the issue of overgrown trees or trees growing in an unfavourable location. It is important to remember, though, that removing trees will also affect ground conditions. Having recently removed a tree means that the moisture that would otherwise have been absorbed diurnally from the ground will remain, allowing the soil to swell and heave – the condition whereby water that would otherwise have been removed swells clay soil, causing pressure on trenches and slab foundations.
Similarly, deciduous trees have a seasonal impact on ground moisture content, with winter rainfall rehydrating soil that has dried over summer, a time when dormant root action takes less water out of the ground. Given that mature elms, oaks, horse chestnuts, planes and ashes can draw up to 50,000l of water a year from the surrounding soil, the consequent soil-water retention, or frost, can lead to significant heave. Worst-case examples have resulted in concrete slabs being pushed into humps as the soil expansion exerts an upward pressure on the floor slab. Similarly, trench foundations can crack, with consequent movement affecting the structure above.