Tree protection is boring: directional drilling and NDD in the tree protection zone

A trench is the cheapest way to put a pipe or cable in the ground, and the most damaging thing you can do to a tree it passes. Dig a continuous trench through a root zone and you sever a continuous band of roots. AS 4970:2025 names the alternative plainly: route services through a Tree Protection Zone by directional drilling, not by trenching (cl. 4.5.5).

When a service has to cross a tree's root zone, boring under the roots protects the tree in a way a trench cannot. Pair the bore with non-destructive digging to find and expose roots, and you can install a service through a TPZ with little or no root loss. The technique is unglamorous, and it is in the standard. We should be using it more than we do.

What the standard actually says

AS 4970:2025 sets an order. Route services outside the TPZ if you can. If a service has to run inside the TPZ, install it by directional drilling, hand-excavated pits and trenches, or as the project arborist specifies (cl. 4.5.5).

For the bore, the standard sets two limits. The upper surface should be at least 0.6 m deep, and preferably outside the SRZ. The likely impact of the boring, the launch pit and the exit pit on retained trees should be assessed before work starts (cl. 4.5.5). Depth keeps the bore under the structural roots. The pit assessment matters because the pits, not the bore, are where the digging happens.

Finding the roots is its own step. The standard expects root location and distribution to be determined by minimally destructive methods: pneumatic, hydraulic, hand digging or ground-penetrating radar (cl. 3.3.2(m)). On site that usually means NDD, non-destructive digging, which uses compressed air or pressurised water with a vacuum to lift soil without cutting roots. You expose the roots, see what is there, and set the works around them.

Where roots still have to be cut, cl. 4.5.4 governs the rest: work under the project arborist's supervision, prune roots to AS 4373, backfill exposed roots quickly, and keep the soil moist.

Why bore instead of trench

A trench severs every root on its line. A bore passes under them. That one difference is the whole case, and it counts most where trees are most valuable: established trees in streets and on development sites, the ones with root systems too big to work around at the surface.

The gains are practical. Root loss drops to whatever the pits need, instead of a continuous cut. Major encroachment can often be avoided, which can be the difference between keeping a tree and removing it. And the work can proceed in tight sites, a planted median or a narrow verge, where there is no room to trench clear of the roots at all. What root loss and soil disturbance do to tree health is covered in Construction compaction and tree decline; the short version is that roots you do not cut are roots you do not have to hope recover.

Where it runs into limits

Boring is not free, and it is not always possible. The constraints are real and worth planning around.

You need somewhere to put the pits. Directional drilling launches from an entry pit and surfaces at an exit pit, and the standard says to assess their impact on retained trees (cl. 4.5.5). Those pits should sit outside the TPZ where possible, better still outside the SRZ. On a tight site the only room for a pit may be exactly where you least want to dig, which caps how far a single bore can run.

Depth and diameter pull against each other. The bore has to stay deep enough to clear the structural roots, at least 0.6 m of cover and preferably below the SRZ, while still rising to meet a shallow service connection at each end. The size of bore the rig can pull sets the conduit diameter and the practical length of each run. Large services, or long distances, may need intermediate pits, and every extra pit is more excavation in the root zone.

Then there is cost and ground. Directional drilling and NDD cost more per metre than an excavator and a trench, and they need specialist contractors who are not on every job (practitioner observation, not from the standard). Rock, reactive clay and unknown existing services all complicate a bore (practitioner observation). None of this outweighs keeping a significant tree, but it does mean boring has to be designed in early, not reached for once the trench has already hit a root.

What this means for you

If you are a Council or asset owner: where a service has to cross a TPZ, specify directional drilling and NDD potholing in the conditions or the works specification, with arborist supervision. It is the standard's own hierarchy, and it holds up as a condition.

If you are an arborist reviewing a service's design, when a proposed trench would cause major encroachment, the question to ask is whether the run can be bored instead, with pits set outside the SRZ. Measure the bore depth and pit locations against the SRZ, not the TPZ.

If you are a builder or designer: decide where the entry and exit pits go before the design locks, because boring needs that space and a trench layout will not leave it for you.

The Frankston median lighting project put all of this to work on significant trees in a live highway median. It is written up in the project review.