The growing desire to preserve our trees has meant Horizontal Directional Drilling has become an essential method for installing pipework near trees – particularly for local government bodies.  The benefits provided by trees to open space and their contribution to micro-climates are immeasurable, so extreme care must be taken not to harm their root systems.

The City of Melbourne introduced a formula several years ago that clearly defined the zone in which traditional trenching cannot occur around a tree. This formula was so effective, it has been adopted by other councils and government bodies, as well as professional irrigation consultants.

The formula for defining the radius from the centre of a tree that cannot have excavation is:


DBH = Diameter at Breast Height, more specifically diameter of the tree at 1.4m above ground level.

Multiplying this diameter figure x 12 gives the radius of the exclusion zone up to a maximum of 15 metres. The result is that many parks cannot have traditional trenching.

We are regularly asked, “So how does Horizontal Directional Drilling actually work?

Horizontal borers have a rotary drill head that is driven horizontally under the surface – creating a tunnel for pipework. The high strength steel drill head also has a small amount of water injected through it at high pressure to help the drill head do its job of cutting through the soil.

The big question is, “How do you know where the drill is exactly, and how do you steer it to where you want to go?”

Firstly the actual tip of the drill head on one side has a flat but angled surface, much like a spade bit drill piece angled to the side slightly. In normal drilling the whole unit spins and is generally pushed forward in a relatively constant direction. However, if you stop the rotation of the drill, you can choose to stop the head with the flat spade part of the drill head pointing towards 12 o’clock or 6 o’clock or wherever in between. By then pushing the drill forward without it actually rotating, the drill head will move in chosen direction for a short distance and therefore correct the direction of the drill head. The operator can then keep on drilling with the head rotating. An example would be that the drill head has meandered slightly off to the left, you would then stop the drill head with the blade pointing to the 3 o’clock position, push the drill forward without rotating the head and thereby correct the drill head to the right.

The operator is able to tell the direction and location of the drill head, due to an electronic beacon called a sond, located just behind the drill head. The sond transmits a signal showing location, depth and direction and the all-important angle of the drill head. A second person uses a receiver above ground to follow the sond and track it for the operator.

The system is very accurate – which is essential when dealing with other underground services.

Once the drill has reached its desired location, the pipe (referred to as the service pipe), is pulled back through the bored hole and installed.

Although directional drilling (Boring) is slower than conventional trenching, the outcome in terms of the effects on a trees root system is significantly better. As our living spaces in Melbourne become more built-up and pollution levels increase we should do all in our power to preserve established trees in public spaces.