How to price an underbore
in a civil estimate.

The problem every estimator knows

Directional drilling is the hardest subcontract price to get at tender stage. You send the drawings, you chase, the quote lands after the box closes — or never. But the tender still needs a number, and a guessed number on a bore is dangerous in both directions: too high and you lose the job on one line; too low and you win it and wear the difference.

The answer is the same as for any subcontract you can’t get priced: build the cost from first principles, keep it deliberately conservative, and replace it with a real quote the moment one exists. Here is the build-up we use.

The cost build-up

An HDD bore has four cost blocks. Site establishment — mobilising the rig, support gear and fluid system, and demobilising it all at completion — is fixed no matter how long the bore is, which is the single most important fact in HDD pricing. Crew and rig time scales with drilling days: bore length divided by a production rate (metres per day) that falls as pipe diameter rises and is slower metro than regional. Fluids and consumables — bentonite, polymers, tooling, and slurry disposal — run per metre and climb steeply with hole size. Then a margin allowance on top.

Because establishment is fixed, cost per metre is a curve, not a rate: a 40 m bore carries the whole setup on 40 metres and can cost double the per-metre figure of a 400 m bore of the same size. Never lift a $/m from one job length onto another — that curve is where bore estimates go wrong.

A worked example

Say the drawings show a DN250 PE water main crossing 220 m under a regional road, clay ground expected. Establishment for a midi-class rig regionally runs around $28,000. At roughly 50 m/day, 220 m is five drilling days — a five-person crew plus rig spread and living-away costs across those days. Fluids and consumables at this hole size run near $52 per metre. Stack the blocks, add 15 per cent margin, and the bore lands close to $540 per metre — about $119,000 drill-only, including slurry disposal, ex GST.

That is the arithmetic our free HDD cost calculator runs for any size from DN63 to DN710, metro or regional, at your actual bore length — deliberately conservative, because when you’re quoting blind, slightly high is safe and low is not.

What to carry around the bore

The drill rate is never the whole crossing. Around it, your estimate needs: entry and exit pits (excavation, shoring if deep, backfill and compaction — by the civil crew), pipe supply, welding and stringing (butt welds plus test time, and somewhere to string the pipe out full length), service location and potholing, traffic control if the pits sit in road, tie-ins at both ends, and reinstatement. On long runs, remember the bore goes in as multiple shots, each needing its own pits — allow for these separately. And rock: every driller in the market quotes rock as an extra-over, so if the geotech hints at it, carry a rock risk line rather than pretending the clay rate covers it.

When to bore instead of open-cut

The bore usually wins when the surface is worth more than the drilling premium: live arterial roads, rail, creeks, established landscaping, or where traffic control and reinstatement for an open trench would cost more than the rig. Open-cut wins on cheap surface with shallow cover and short lengths. Run both numbers — the calculator gives you the bore side in a minute, and your trench rates give the other — and let the comparison, not habit, make the call.

Then the discipline that matters most: flag the bore line as a benchmark allowance in your tender notes, and replace it with a contractor’s quote the moment one arrives. A benchmark is a safety net, not a substitute for the market.