Directional Boring Terms: The Complete HDD Glossary for Utility Projects

Every trade has its own language, and horizontal directional drilling has more jargon than most. If you are reviewing a boring quote, reading a utility plan sheet, or standing on a job site wondering what a “frac-out” is, this glossary covers the directional boring terms you will actually hear in the field. Definitions come from our crews’ day-to-day work installing gas, electric, fiber, water, and sewer lines across Oklahoma – not from a textbook.

The Boring Process: Core Terms

Horizontal Directional Drilling (HDD)

A steerable trenchless method for installing underground pipe, conduit, and cable along a shallow arc-shaped bore path. “Directional boring” and “directional drilling” describe the same process – see our full explainer on what horizontal directional drilling is and how it works.

Trenchless Technology

Any construction method that installs or rehabilitates underground utilities without continuous open-cut excavation. HDD, jack and bore, and pipe bursting are all trenchless methods. Compare them in our trenching vs boring guide.

Pilot Bore (Pilot Hole)

The first pass of a directional bore: a small-diameter guided hole drilled from the entry point to the exit point along the planned bore path. Everything else in the job depends on the pilot bore landing on line and grade.

Back Reaming

The second pass. After the pilot bore punches out, the drill head is replaced with a reamer that is pulled back through the hole while rotating, enlarging the bore to the diameter needed for the product pipe – typically 1.5 times the pipe diameter.

Pullback

The stage where the product pipe or conduit is attached behind the reamer and pulled into the enlarged bore hole. On larger jobs, reaming and pullback happen in separate passes; on small conduit shots they are combined.

Bore Path

The planned underground route of the bore, including entry angle, depth profile, and exit location. Good bore path planning accounts for existing utilities, soil conditions, setback distances, and the bend radius of the product pipe.

Entry Pit / Exit Pit

Small excavations at each end of the bore. The entry pit contains drilling fluid returns at the rig side; the exit (receiving) pit is where the drill head daylights and the product pipe is attached.

Punch-Out

The moment the pilot bore breaks the surface at the planned exit point. A clean punch-out on target is the first confirmation that the bore is on line and grade.

Setback

The horizontal distance between the rig and the point where the bore reaches its required depth. Because HDD enters the ground at an angle, a bore needs enough setback distance to reach depth without exceeding the pipe’s bend radius.

Shot

Field slang for a single bore run – “a 260-foot shot under the parking lot.” Crews often describe jobs by the number and length of shots.

Rigs, Tooling & Equipment

Drill Rig

The tracked machine that rotates and thrusts the drill string. Rigs are classed by thrust/pullback force – a 10,000 lb rig handles residential service lines while highway and river crossings may need 100,000 lb class machines. See the equipment we run and our drill size guide.

Drill String (Drill Stem, Drill Pipe)

The threaded steel rods connected end-to-end between the rig and the drill head. Rod by rod, the string advances during the pilot bore and is recovered during pullback.

Drill Head (Bit)

The steerable cutting tool on the front of the drill string. A slanted face (duckbill) steers in soil; push without rotation and it deflects in the direction the face points, rotate and it drills straight.

Duckbill

The angled steering face on a standard soil drill head. The name comes from its flattened, bill-like shape.

Mud Motor

A downhole motor driven by drilling fluid pressure that spins a rock bit independently of the drill string – the tool of choice when a bore hits solid rock. Read how we use them in our mud motor boring guide.

Reamer (Back Reamer, Hole Opener)

The cone- or barrel-shaped cutting tool pulled back through the pilot hole to enlarge it. Different soil conditions call for different reamer designs – fluted for clay, tooth-style for compact soils and rock.

Swivel

A bearing connector placed between the reamer and the product pipe during pullback. It lets the reamer rotate while the pipe behind it does not, protecting the pipe from twisting.

Breakaway Link (Weak Link)

A mechanical fuse rated below the pipe’s allowable pull force, installed between reamer and pull head. If pull loads spike, the link fails before the product pipe is damaged.

Pull Head (Pulling Eye)

The fitting attached to the leading end of the product pipe that the swivel connects to. It grips the pipe and seals the end so soil and fluid stay out during pullback.

Vacuum Excavator (Vac Trailer)

A truck- or trailer-mounted unit that digs with pressurized water or air and vacuums up the spoil. Used for hydrovac daylighting, cleaning entry pits, and recovering drilling fluid.

Drilling Fluid & Mud Terms

Drilling Mud (Drilling Fluid)

The engineered slurry pumped through the drill string during boring. It carries cuttings out of the hole, stabilizes the bore wall, cools the tooling, and lubricates pullback. Despite the name, it is a mixed product – not just dirt and water.

Bentonite

A natural sodium clay that swells when hydrated, forming the base of most drilling fluids. Bentonite builds a thin filter cake on the bore wall that keeps the hole open and holds fluid in.

Polymer Additive

Chemical additives blended with bentonite to tune the fluid for specific soils – polymers keep reactive clays from swelling and sticking in Oklahoma red clay, and add carrying capacity in sands.

Returns

Drilling fluid flowing back to the entry or exit pit, carrying soil cuttings with it. Healthy returns are a sign of a stable, open bore; lost returns mean fluid is going somewhere it should not.

Frac-Out (Inadvertent Return)

Drilling fluid escaping the bore path and surfacing through cracks in the soil – usually in low-cover areas, loose fill, or fractured rock. Crews manage frac-out risk with proper depth, fluid pressure control, and containment on standby.

Filter Cake

The thin bentonite layer that plasters the bore hole wall, sealing it so drilling fluid pressure supports the hole rather than leaking into surrounding soil.

Mud Recycling (Reclaimer)

Equipment that screens and de-sands used drilling fluid so it can be pumped back downhole. Standard on large-diameter and long crossings where fluid volumes are high.

Locating, Tracking & Steering

Sonde (Transmitter, Beacon)

The battery-powered transmitter housed just behind the drill head. It broadcasts depth, pitch, roll, and temperature so the locator can track the bore in real time.

Walkover Locating

The standard HDD guidance method: a technician with a handheld receiver walks the bore path above the drill head, reading the sonde’s signal to report position and depth to the rig operator.

Wireline (Magnetic Guidance)

A guidance system for bores too deep, too long, or too congested for walkover locating – steering data travels through a wire inside the drill string. Common on river crossings and deep casing bores.

Pitch

The drill head’s inclination, read in percent grade. A pitch of -2.0 means the head is diving at 2 feet per 100 feet – the number that matters most when a bore has to hold grade for sewer.

Roll (Clock Position)

The rotational orientation of the drill head’s steering face, read like a clock. To steer left, the operator rolls the face to 9 o’clock and pushes.

Bore Log

The record of rod-by-rod depth and position readings taken during the pilot bore. It documents that the installation matched plan and feeds the as-built drawing.

As-Built

The drawing or GPS record of where the utility was actually installed – depths, offsets, and alignment. Owners and engineers keep as-builts so future excavators know what is down there.

Daylighting (Potholing)

Physically exposing an existing buried utility – usually with a vacuum excavator – so its exact depth and position are known before the bore crosses it. The single most effective way to prevent cross-bores and strikes.

Pipe, Conduit & Product Terms

Product Pipe

Whatever the bore is installing – the water line, gas line, conduit, or casing. Crews call it ‘product’ to distinguish it from the drill string and tooling.

Conduit

Pipe installed as a protective raceway for something else – electrical conductors or fiber optic cable get pulled through the conduit after installation. Common sizes run from 1-inch service conduit to 6-inch duct banks.

Casing

A larger structural pipe installed first, through which a carrier pipe is later inserted. Steel casing is standard under highways and railroads, where the crossing owner requires extra protection.

HDPE (High-Density Polyethylene)

The most common product pipe material for HDD – flexible enough to follow the bore path, fused into a continuous joint-free string, and tough enough for pullback loads.

SDR (Standard Dimension Ratio)

The ratio of pipe diameter to wall thickness for HDPE pipe. Lower SDR means thicker wall and higher pressure rating – SDR 11 is a common spec for gas and water bores.

Butt Fusion

Joining HDPE pipe by heating both ends against a plate and pressing them together, creating a joint as strong as the pipe itself. Fused pipe strings are laid out ahead of pullback.

Tracer Wire

An insulated copper wire installed alongside non-metallic pipe so the line can be located electronically later. Standard on poly gas and water installs, and required by many utility specs.

Mule Tape (Pull Tape)

Flat woven tape blown or pulled into empty conduit so cable can be pulled through later. Printed with footage marks, it doubles as a measuring tool.

Innerduct

Smaller flexible conduits installed inside a larger conduit or casing, dividing it into channels – standard practice on fiber builds so future cables can be added without new boring.

Soil & Ground Condition Terms

Geotechnical Report (Soil Boring)

An engineering report based on sampled soil cores along the route. On larger crossings it tells the contractor what soils and rock to expect – and drives tooling choice. See which soils suit directional boring.

Red Clay

Oklahoma’s signature soil. Reactive clay that swells and sticks when wet – workable for HDD with the right fluid mix, but it demands polymer additives and patience. More in our clay soil depth guide.

Caliche

A hardened calcium-carbonate layer found in drier western Oklahoma and Texas soils. Drills slower than clay and dulls soil tooling, but rarely requires rock tools.

Cobble

Loose rounded rock mixed in soil – one of the hardest conditions for HDD because cobbles deflect the drill head and collapse into the bore. Often found in river bottoms.

Rock Bore

A bore through solid rock – sandstone, limestone, or granite – requiring a mud motor or air hammer instead of standard soil tooling. Costs and timelines change substantially; see boring in rocky and hard soil.

Groundwater

Water encountered in the bore path. HDD handles groundwater better than open trenching – the drilling fluid balances the water pressure – but heavy inflow affects fluid management and pit stability.

Safety, Permits & Damage Prevention Terms

811 / OKIE811

The free one-call service that notifies utility owners to mark their lines before any digging or boring. Oklahoma law requires the call at least 48 hours before work. Full details in our Oklahoma 811 guide.

Locate Marks (Paint and Flags)

The color-coded paint and flags utility owners place in response to an 811 ticket: red for electric, yellow for gas, orange for communications, blue for water, green for sewer, white for the proposed bore path.

Tolerance Zone

The buffer around a marked utility – in Oklahoma, the width of the facility plus 24 inches on each side – inside which mechanized excavation must stop and the utility must be exposed by hand or vacuum before crossing.

Cross-Bore

The serious mistake of boring through an unmarked existing utility, most dangerously a sewer lateral later hit by a drain cleaner if a gas line went through it. Prevented by daylighting and camera inspection of gravity sewers.

Easement

A recorded legal right to use a strip of private property for utilities. Bores routed through easements need the easement holder’s requirements met – depth, offsets, and sometimes inspection.

Right-of-Way (ROW)

The publicly controlled corridor along roads and highways. Boring in ROW requires a permit from the city, county, ODOT, or turnpike authority, each with its own depth and casing specs.

Traffic Control Plan

The signage, cones, and flagging plan required when work zones affect traffic. On road bores it is often a permit condition priced into the job.

Bidding & Cost Terms

Price Per Foot (LF)

The standard unit for boring quotes – a rate per linear foot of installed pipe at a given diameter and depth. What drives the rate up or down is covered in our directional boring cost guide.

Mobilization (Mob)

The cost of getting the rig, support equipment, and crew to the site. Small jobs may carry a flat mobilization fee; large projects line-item it.

Owner-Supplied Material

A quote structure where the customer provides the pipe, conduit, or tracer wire and the boring contractor supplies labor, equipment, and fluid. Common when an electrician or plumber already has material on the job.

Standby Time

Billable time when the crew and rig are on site but prevented from working by others – waiting on locates, permits, or another trade. Watch for it in quote terms.

Change Order

A documented scope change after award – added footage, a deeper profile, rock encountered where soil was expected. Good bore contracts define how changed conditions are priced before work starts.

Bond / Insurance Requirements

Public and commercial bores typically require proof of liability insurance and sometimes performance bonds. Our questions to ask a boring contractor guide covers what to verify before hiring.

Hear a term on your job site that is not listed here? Call us at (405) 409-7423 and a crew member – not a call center – will give you a straight answer.