Because an HDD bore is never exposed — no one ever looks at, touches, or feels the soil along the drilled path — the contract is where the risk of the unknown ground actually gets assigned. Getting that allocation right, and backing it with a competent site characterization, is what keeps a difficult crossing from turning into a claim. This article covers the two contract forms used for HDD and how subsurface risk is handled under each.
Two Contract Forms
- Lump sum: the contractor is paid a fixed amount to deliver a completed drilled segment per the plans and specifications. Payment is based on the result and does not vary with the time or effort spent. Most drilled river crossings are contractually feasible on a standard lump sum basis.
- Daywork: the contractor is paid a fixed amount per day (or other unit of time) to provide a spread of equipment per the contract. Payment follows the passage of time regardless of progress. Daywork shifts the risk of slow going onto the owner and is reserved for crossings too uncertain to price as a lump sum.
Price the Crossing, Not the Foot
Even on a lump sum job, compensation should be for the crossing as a whole rather than a fixed unit price per foot. Per-foot pricing invites disputes: if drilling conditions make a slightly longer path the easier way to finish, it is unreasonable for the owner to demand a redrill to a shorter length just to cut payment — and equally unreasonable for a contractor to extend the length for convenience and bill more for it. Pricing the completed crossing per the plans and specs removes that whole argument. (Technical specifications typically still include a pilot-hole over-length tolerance to bound the geometry.)
Who Owns the Subsurface Risk
Any site characterization, however competent, still contains uncertainty — it is not economically possible to define every cubic yard of the subsurface. In a standard lump sum contract, the contractor assumes the risk of operational problems arising from that residual uncertainty. The obligation on the owner is to make this bearable: provide the full results of the geotechnical investigation so the contractor can price an appropriate contingency into the bid. A contractor bidding blind either pads the price heavily or gets hurt — neither serves the project.
What Counts as a Differing Site Condition
The competently executed subsurface survey becomes the benchmark for judging whether conditions are materially different from what the contract indicated. The bar for a legitimate changed-conditions claim is high and specific:
- A genuine differing condition is an extreme deviation — for example, hitting bedrock where the borings showed only alluvial deposits. With a proper survey this is a remote possibility.
- Encountering gravel where borings indicated coarse sand is NOT a changed condition — gravel in a sand formation is foreseeable.
- Cobbles or boulders in soils described as glacial in nature are NOT unforeseen, even if a specific boring happened to miss them — random cobbles and boulders are a known characteristic of glacial deposits.
The principle is that a bidder is charged with knowledge of what the geologic setting implies, not just the exact material a boring struck. This is why the interpretation and communication of the geotechnical data matter as much as collecting it.
The Documents That Carry the Risk Allocation
Two documents anchor the deal. The technical specification defines HDD performance — protection of existing underground facilities, required instrumentation (pilot-hole location, axial and torsional drill-string loads, fluid rate and pressure), pilot-hole directional tolerances, minimum curve radius, as-built survey deliverables, and the four-hour hydrostatic pretest of the pull section. The plan-and-profile drawing presents the crossing geometry and the survey results; the contractor relies on it to build the working profile used for downhole navigation, so its measurements must be accurate. Together these tell every bidder exactly what "success" means, which is the foundation of both fair pricing and the later construction monitoring.