What does your Contract say?
"You’ve contracted to install underground utilities. Once the work begins, you discover soils with inadequate bearing capacity, large amounts of unanticipated rock, groundwater at levels higher than anticipated, buried debris, or hazardous wastes. None of these conditions were expected. As a result, the cost you promised to the owner to install the utilities is no longer feasible. Who bears this risk?
A “differing site condition” (also known as a “changed condition”), which is abbreviated in this article as a “DSC,” is an unknown and hidden, concealed, or latent physical condition encountered at a site that differs materially from the reasonably anticipated conditions. Physical conditions are conditions you can actually touch, like soil, rock, and concrete; and not intangible, such as the availability and cost of labor, materials, and equipment.
Some contractors mistakenly believe they are always entitled to more time and money when they discover a DSC. They aren’t. Entitlement to more time and money for a DSC usually depends upon whether the applicable contract includes a differing site conditions clause.
Absent a differing site conditions clause, the doctrine of sanctity of contract[1] places the risk on the contractor if the work is more difficult, costly, or time-consuming than expected, unless due to the owner’s breach of contract, performance is rendered impossible by an Act of God, change in the applicable law, or the facts and circumstances support an equitable defense to this rule."
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Something like this should have been resolved prior to completion of the work. At what point during the work were the underground rocks discovered?
If different drilling equipment was required, then the Contractor incurred additional costs and should be reimbursed.
Contractors often proceed with extra work without first securing a written change order. If the contractor doesn't does not have a written change order or CCD, consider whether the parties may have waived the requirement through their words or actions.