Ask any drill crew what makes or breaks a bore and they won't say the rig — they'll say the ground. Sand, clay, cobble, and rock each demand different tooling, different mud, and different patience, and the difference shows up directly in the price per foot.
Rock: a different machine spirit
Solid rock defeats the standard slanted-face bit entirely. Rock bores run mud motors — downhole turbines spun by drilling-fluid pressure turning a roller-cone or PDC bit — or air-hammer systems in the hardest formations. Penetration drops from hundreds of feet a day to sometimes dozens, fluid volumes multiply, and tooling wear becomes a line item. None of this is exotic — hard-rock boring is routine work for equipped crews — but it's why a rock bore quotes at several times the soft-ground rate.
Clay, sand, and cobble
Clay drills beautifully until it's reactive — swelling clays grab the pipe and ball the reamers unless the mud chemistry (thin, inhibitive) is right. Sand cuts effortlessly and collapses just as easily; success is entirely about mud that builds a wall cake and holds the hole. Cobble and gravel are the honest nightmare: too loose to hold, too hard to cut, deflecting the head unpredictably — crews slow down, case the entry, or re-route. A geotechnical report, even a modest one, is the cheapest risk reduction a bore can buy.
What this means for buyers
Tell the contractor what you know about the ground — nearby soil borings, well logs, what the last utility cut found — and expect an honest conversation about contingency where nobody knows. A bid that assumes best-case ground isn't cheaper; it's just unpriced risk. Our HDD crews bore all four conditions and quote them as what they are.