How Horizontal Directional Drilling Works: The 3-Stage HDD Process

Key Takeaways 

• Horizontal directional drilling (HDD) installs underground utilities in three stages: pilot bore, reaming, and pullback. Each stage serves a distinct function and requires different equipment configurations. 

• The pilot bore is steered using a sonde transmitter tracked by a walkover locating system, allowing the operator to navigate around existing utilities and maintain the designed bore path. 

• HDD installs water, sewer, gas, electrical conduit, and fibre/telecom conduit beneath roads, rail corridors, waterways, and developed surfaces without excavating the alignment. 

• Pre-construction utility verification through hydrovac potholing along the bore path is standard practice, not optional. 

• HDD is not the right method for every project. Shallow short runs in open ground and tight-tolerance gravity sewer on short alignments are better served by open-cut.

Horizontal directional drilling is a steerable trenchless method that installs new underground utilities by drilling a guided bore path beneath the surface and pulling product pipe through the completed hole. The process works in three stages: a steered pilot bore establishes the path, a remaining pass enlarges it to accommodate the product pipe, and a pullback operation pulls the pipe into position. The surface above the bore path remains intact throughout. 

HDD is the primary trenchless installation method for new utility lines in Ontario. It handles road crossings, rail crossings, waterway crossings, and congested urban corridors where open-cut trenching would require pavement removal, traffic control, and full surface restoration. Understanding how the process works, what it can and cannot do, and where it fits in a project scope is foundational for anyone specifying or evaluating underground utility installation methods. 

How HDD Works: The Three-Stage Process 

Every HDD installation follows the same three-stage sequence. The stages are distinct operations, each with different equipment, crew requirements, and risk profiles. 

Stage 1: Pilot Bore 

The pilot bore establishes the bore path. A drill rig drives a steerable drill head into the ground at a controlled entry angle, typically 8–20 degrees (10–12 degrees is the most common range for utility installations). The drill head cuts through soil while bentonite drilling fluid, a slurry mixed on site, is pumped through the drill string and out through jets in the drill head. This fluid serves three purposes: it lubricates and cools the cutting head, stabilizes the bore hole wall, and carries excavated soil (cuttings) back to the surface.

The drill head contains a sonde, a small electronic transmitter that sends position data (depth, pitch, roll, and compass heading) to a walkover locating receiver carried by a crew member on the surface. Tracking systems from manufacturers like Digitrak and Subsite provide real-time bore path data, allowing the operator to steer the drill head around existing utilities and maintain the designed alignment and grade. 

Steering works by orienting the angled face of the drill head and pushing without rotating. When the drill string rotates, the drill advances straight. When it pushes without rotation, the angled face deflects the head in the direction the operator selects. This combination of rotating and pushing gives the operator precise directional control throughout the bore. 

Stage 2: Reaming 

The pilot bore creates a hole only slightly larger than the drill head, far too small for the product pipe. The reaming pass widens it. A back-reamer is attached to the drill string at the exit point and pulled back toward the rig. As it travels through the bore, it enlarges the hole while drilling fluid stabilizes the walls. The reamed diameter is typically 1.5 times the outside diameter of the product pipe. On larger-diameter installations, multiple reaming passes may be required, each using a progressively larger back-reamer. 

Stage 3: Pullback 

With the bore reamed to size, the product pipe is pulled into position. The pipe, typically high-density polyethylene (HDPE) [INTERNAL LINK: Pipe Fusion service page], is laid out in a straight line extending from the exit point. A swivel is attached between the pipe and the reamer to prevent torque from the rotating drill string from transferring to the pipe. The rig pulls the reamer and pipe back through the bore in one continuous operation. 

HDPE is the dominant product pipe for HDD because of its flexibility. Its bend radius of approximately 100–120 times its outside diameter allows it to follow the curved bore path without risk of cracking or joint failure. Fusible PVC, steel casing, and ductile iron (with a carrier pipe) are also installed by HDD depending on the application. 

What HDD Installs 

HDD is specified across every utility vertical: water mains and water services, sewer force mains and gravity sewer (gravity installations require precision grade control), gas distribution lines, electrical conduit, and fibre optic and telecom conduit currently the highest-volume HDD application in Ontario driven by 5G, fibre-to-the-home, and data centre builds

Rig capacity determines the maximum bore length and diameter. Small rigs handle bores up to 90–180 metres. Mid-size rigs extend to 300–450 metres. Large rigs can drill over 900 metres at diameters exceeding 1,200 mm. 

What Happens Before the Drill Starts

Utility locating is the first step. Under the Ontario Underground Infrastructure Notification System Act, 2012, all excavators must contact Ontario One Call before breaking ground.

Surface marks alone are not sufficient for HDD. Ontario One Call locates provide horizontal position only, not depth. A bore path crossing beneath a congested utility corridor needs vertical clearance confirmation for every crossing utility. 

Hydrovac potholing: uses pressurized water and vacuum to excavate small test holes every 6 to 15 metres along the bore path, exposing buried utilities so their exact depth and position can be measured. In ASCE 38-22 terms, potholing provides Quality Level A data (measured horizontal and vertical position). Surface geophysical methods provide Quality Level B at best (horizontal position only). 

Bore path design maps the planned drill path in profile: entry angle, depth at each crossing point, exit angle, and radius of curvature. Frac-out risk assessment applies near waterways or areas with shallow ground cover, requiring a contingency plan before drilling begins. 

When HDD Is the Right Method 

HDD earns its place on projects where the surface is too valuable, too congested, or too environmentally sensitive to excavate.

Road crossings avoid road cut permits, traffic control, pavement demolition, and full surface restoration. Rail crossings eliminate the need to disturb the track structure. Waterway crossings avoid in-stream work permits. Congested utility corridors benefit from HDD's ability to steer beneath the existing utility layer. Environmentally sensitive areas, contaminated ground, and established landscapes (mature trees, heritage properties) all favour a method that leaves the surface intact. 

When HDD Is Not the Right Method 

Shallow, short-run installations in open ground with no surface infrastructure to protect are faster and cheaper with open-cut. Gravity sewer on short alignments with tight grade tolerances may favour open-cut where the invert is visible. Rocky ground without appropriate downhole tooling limits feasibility. Sites with insufficient space for rig setup or pipe laydown may not accommodate the equipment footprint. 

Risks and How They Are Managed 

Frac-out is managed through bore path design (adequate depth of cover), drilling fluid pressure monitoring, and a written contingency plan. Utility strikes are mitigated by the pre-construction locating and potholing program.

Bore path deviation is managed in real time through sonde tracking. Stuck pipe during pullback is managed by maintaining adequate drilling fluid flow and correct reaming diameter. 

Frequently Asked Questions 

Q1. How much does horizontal directional drilling cost per foot? 

HDD cost per linear foot varies based on bore length, diameter, soil conditions, depth, and mobilization. Per-foot installation cost is typically higher than open-cut, but total project cost on road crossings and developed sites is often lower once restoration, traffic control, and excess soil management are factored in. 

Q2. How deep can HDD go? 

Depth depends on rig class. Small rigs operate at 5–8 metres. Mid-size rigs handle 10–15 metres. Large rigs reach 25+ metres. The practical depth for most Ontario municipal utility installations is 3–8 metres below grade. 

Q3. Does HDD work in rock? 

Yes, with appropriate tooling. Mud motors and rock-specific drill heads allow boring through various formations, though production rates are slower and tooling costs higher. Geotechnical data along the bore path should be reviewed before specifying HDD in known rock conditions.