PREVENT TREE DAMAGE WITH ROOT BARRIER™

16 Sep 2021

Keep both trees and structures safe by preventing tree roots damaging structural foundations with Root Barrier™.

Contrary to common belief tree roots often extend beyond the canopy drip line. Tree roots do not need to be next to or under foot for them to cause a problem; in fact, the nearest tree root may be many metres away, and in extreme cases, tens of metres away with invasive tree species.

The Problem

Clay-based soils are moisture sensitive, and they change volume with changes in soil moisture. When clay base soils are wet, they expand in volume, and any footing supported on them will be uplifted or is said to have ‘heaved.’

When the clay soils lose soil moisture, they reduce in volume or shrink, and any footing supported on them settles. Where there are no outside influences, (such as trees) in the zone of influence, this shrinking and swelling occurs naturally from wet to dry seasons. In Australia, this response to seasonal conditions is quantified by a parameter found in AS 2870-20011

An understanding of ‘Matric Suction’, and how it affects clay-based soils is required. Matric suction forces is the pressure that dry soil exerts on surrounding soil to equalise the larger mass of soil. It has the ability to draw moisture from the soil profile beneath structures at rates of between 150-250 kPa as indicated in figure below, given the proximity of the root system to the structure. Pressure exerted is subject to the site geotechnical classification being either H, M or E as classified by the AS 2870-2011. The suction contour varies based on depth and distance.

Contours of final matric suction (kPa)

Source: Geo Institute American Society of Civil Engineers (2001)

Tree location

Based on a site’s geotechnical analysis, it is recommended that trees are located no closer than 1.5 to 2 x its mature height in distance away from civil structures to mitigate and or reduce matric suction effect on the base foundations.

It is important to consider matric suction forces of the root system, do not require the presence of root growth at the site within the reactive clay founding soils of the structure to be subjected to the trees water use effects. It is the suction effect of the dry block of soil impacting on the damp soil adjacent. Soil beneath foundations and footpaths are subject to soil reactiveness through matric suction effects.

To eliminate the tree effect, root barriers are required to be installed to depths of at least 2.5m to 3m or greater subject to the structural issues and soil type. While roots require oxygenated soil to initiate and propagate, and are predominantly found at depths of 300mm to 600mm. Clay soils also do crack as they dry out causing the development of fissures into the soil profile which in turn lets oxygen enter deeper into the soil profile.

Vegetative growth is a function and a sum of the available water and nutrients sequestered by the plant. When water is extracted from the soil profile and not replaced, the soil volume then collapses or subsides under the compression of the infrastructure placed on top of the founding soil. Soil settlement under the suction regime of 150-250Kpa can be in the vicinity of 80 to 60mm.

The Solution

Creating an island effect, the Root Barrier™ keeps the change in moisture content to a minimum, effectively disconnecting trees from the moisture beneath the structure foundations slab or footings. By installing a flexible cut off wall like Root Barrier™ around the structure future damage can be prevented.

Annerley, Brisbane

A Ficus Tree with extremely invasive roots, located to the south-east of the heritage-listed State School swimming pool complex, had impacted the foundations causing severe cracking of the brick work.

Remedial work was undertaken in 2003 with a root barrier installed to the south of the brick wall.

In 2021 the soil moisture level stabilised due to the elimination of the ‘tree effect’ through matric suction forces on the reactive soils causing the foundations to subside.

Images show the brick work and cracks are closed and foundations are stabilised, protecting historic infrastructure and also allowing retention valuable vegetation. 18 years after installing the historic asset is still protected from the reactive soil movements.