Manhole HDD with the TERRA MINI-JET


HDD drilling out of the manhole

Imagine being able to use existing sewer manholes to lay networks for electricity, gas and water without the need to dig up roadways or front gardens of private properties. The HDD manhole drill TERRA MINI-JET MJS 1600 operates in a manhole. It is controlled from the top via an automatic drill rod lift. The operator stands on top of the road during drilling an backreaming, and not down in the manhole.


The TERRA MINI-JET MJS 1600 fits into a manhole ø 1’000 mm (40”) with an opening of 620 mm (24.4”). The MJS 1600 is positioned vertical, lowered through the manhole opening. Inside of the manhole it is positioned horizontal at the right height level on a wooden platform. The MJS 1600 is fixed with its arms inside of the manhole. With a core drill bit the MJS 1600 drills an opening into the wall of the manhole. Then the pilot bore might begin.

Pilot bore

The first step of Horizontal Directional Drilling (HDD) is the pilot bore. The hollow drill rods are pushed hydraulically through the ground. The drill head cuts a bore hole into the ground. Drilling fluid (Bentonite) is pumped through jet nozzles in the drill head to loosen the ground. The drill head has an asymmetric steering plate. As long as the drill head is rotated continuously, it cuts a straight bore hole. A sonde inside of the drill head transmits signals to the top and allows continuous locating of the drill head. To change direction, the rotation of the drill head is stopped in a position so that the steering plate points in the required direction. Now the drill head is pushed through the ground without rotation. Thereby it steers into the position (roll angle) of the steering plate.


As soon as the drill head has reached the target pit, it is replaced by a backreamer. This backreamer opens the bore channel and pulls the HDPE pipe in simultaneously. Depending on ground conditions backreamers up to ø 260 mm (10”) can be used. This allows to pull in HDPE pipes up to OD 260 mm (10”) or pipe bundles with several HDPE pipe. As drilling fluid Bentonite or Ejactomer are used, mixed with water. The drilling fluid stabilises the bore channel, minimises the friction at the drill rods and at the new HDPE pipe and transports the ground particles through the bore channel into the pit.


TERRA AG für Tiefbautechnik - Hauptstrasse 92 - 6260 Reiden - Schweiz - Tel. +41 62 749 10 10 -