Civil Engineering Reference
In-Depth Information
Entry point
Exit point
Direction of operation
Drill line
Non-magnetic
drill stem
Survey
cable
Survey
tool
Jet bit
Stage 1: Pilot drilling
PILOT DRILLING
Exit point
Entry point
Direction of operation
Reamer
Drill pipe
Stage 2: Pre-reaming
PRE-REAMING
Exit point
Entry point
Direction of operation
Pipeline
Drill pipe
Reamer
Swivel
Stage 3: Pullback
PULLBACK
Figure 5.84
Typical HDD installation, including pilot drilling, pre-reaming and
pullback operations (the central section of each stage is enlarged for
clarity) (courtesy of Herrenknecht)
transmits its position wirelessly. The signal can be received at the ground
surface by a person walking the route of the bore with a detector. Alterna-
tively when access to the ground surface is difficult or for very long drive
lengths, the signal can be sent via a wire from the sensor directly to the
drilling rig along the drill pipe.
HDD can be used to install pipelines through most soil or rock conditions
both above and below the water table if the cutting tools, drilling rate,
fluid pressure and flow rate are configured correctly. One of the key
differences between drilling through soft ground and hard rock, is the
drilling head configuration and reaming devices. When drilling in soft soils
'chisel' type drill heads are used (see Figure 5.85b) and the drilling rig
provides thrust and rotational forces to the head. In order to drive in a
straight line, the rods are continuously rotated as they are pushed forwards.
To navigate curves, or to change alignment, the rotation is stopped and
the drill rods are just pushed forwards (in this case the forces on the angled
drill head causes the head to deviate from its current direction). When
drilling in rock, drilling bits are used (Figure 5.86). The drilling rig again
provides thrust and rotational forces to the head, but in addition, a mud-
