Rules of thumb for designers.

Input data

Estimated length: the distance between the two furthest points of the section to be crossed.

Bundle diameter: the circle around all pipes pulled at once, increased by 20 %. The pipe material is usually PE or steel.

Rig and bend radius

Length and diameter determine the rig. For PE pipe the arc radius is the smallest bend radius of the drill rods, 25 to 140 m on our rigs.

For steel pipe the bending of the pipe itself governs. If it is unknown, a good approximation is a radius in metres equal to the pipe diameter in millimetres (Ø 500 mm, about 500 m). For thick-walled coated pipe the factor can be 1.3. If the pipe curves horizontally and vertically at the same time, both projections must be much larger.

The bore axis

A rig can be inclined to 17–19° (30–35 %) at most, which is the largest entry angle. The sonde in the drill head measures up to 45° (100 %), which is the steepest exit, though flatter is of course possible.

Because of walkover tracking the deepest point can be 12–15 m, depending on interference. From the radius, the two angles and the depth the axis can be built from arcs and straight sections, and the calculator above does exactly that.

The site

With uncertain utility positions the bore length can change considerably, since a site visit often shows a line is not where the drawing puts it. Trial pits may then be needed.

Depending on the rig, about 4.5–12 m length and 1.5–3.5 m width of clear space is needed in the bore axis, and our largest rig, the Prime Drilling, needs more preparation. Anchors are driven 1 m deep in front of the rig.

The mixer truck (12–20 t) can stand at most 50 m from the rig because of the mud hose and control cable. On the exit side there must be room for the laid out pipe, welded there if needed, otherwise a road crossing, for example, cannot be done.

Pits must not obstruct the bore. The entry pit should not be too long, because rods hanging free without lateral support buckle and the thrust of the rig is lost.