Design and Precautions for Horizontal Directional Drilling Pilot Hole

2025-11-24

The pilot hole is the first step of construction, and subsequent reaming and pipe pulling operations are carried out on this basis. The quality of the pilot hole, such as the turning radius of the deflecting section, the smoothness of the trajectory curve, and the position of avoiding underground obstacles, directly determines the success or failure of subsequent pipe pulling operations, so the construction of the pilot hole must be done carefully.

1. Pilot Hole Design

After identifying underground buried objects, design the pilot hole curve on the underground pipeline distribution map. In actual construction, some projects do not draw drawings, and the guide operator keeps it in mind. Common pilot hole trajectory design diagrams: (see below) 51b52c711b065cb4972e49f87a10facb

Pilot hole design must meet three requirements simultaneously: first, try to meet the owner's requirements; second, avoid underground buried objects; third, meet construction process requirements. If the pilot hole design cannot meet the above three requirements at the same time, modify the pilot hole design plan. If the pilot hole design plan cannot be modified, only modify the three requirements.

Key Design Parameters of Pilot Hole

• Entry Angle: The angle between the drill guide rail and the ground, i.e., the drill bit entry angle. The drill bit entry angle is usually 0-22°, and 0-12° for steel pipe laying.

• Drill Bit Inclination: When the drill bit inclination is negative, the drill bit drills deeper; when positive, it drills toward the ground; when 0°, it drills horizontally. Once the drill bit inclination is determined, the depth reduction or elevation of the drill bit after drilling one drill pipe can be calculated. For construction convenience, the depth reduction or elevation values of 3m, 2m, 1.5m and 1m drill pipes are listed (as shown in the figure):

• Drilling Depth: Vertical distance from the drill bit to the ground; for water area construction, it is the vertical distance from the drill bit to the water surface.

• Pilot Hole Length: Curved distance from the entry point to the exit point.

• Exit Angle: Inclination of the drill bit exiting the ground. The exit angle is usually 0-18°, and 0-8° for steel pipe laying.

2. Precautions for Pilot Hole Design

The pilot hole must not only avoid underground pipelines but also leave a sufficient safety distance. The values obtained when investigating the depth and position of underground pipelines are only estimates with errors; there are also errors in guidance measurement, and the drill bit will automatically straighten during reaming with errors. The superposition of these three errors may cause the drill bit to damage the pipelines. Therefore, a safety distance should be reserved in pilot hole design to offset the three errors.

1. When crossing rivers, the underwater pilot hole should be designed as a horizontal hole as much as possible. Due to difficult guidance on the water surface, blind drilling or semi-blind drilling is generally adopted, and the pilot hole should be at least 3 meters away from the river bottom.

2. When crossing railways, the pilot hole should be at least 5 meters away from the railway subgrade surface; if the reaming diameter is large, the depth should be further increased to ensure railway safety.

3. When laying pipes under three-story buildings, the guidance depth should not be less than 6 meters; if the reaming diameter is large and the stratum is soft, the pilot hole depth should be increased.

4. When laying steel pipes, the curvature radius of the pilot hole should be larger than that of the steel pipe. The curvature radius of the steel pipe R≈1200～1500D, where D is the steel pipe diameter.

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