Soil displacement hammer for replacing underground pipes

A mole is provided for use in replacing or preparing for replacement an existing underground pipe. The mole is sized to permit insertion into and movement along the existing pipe. The mole is provided with a front portion having a cutting edge for engaging the internal wall of the existing pipe and to fracture the same. The rear portion of the mole clamps to a new pipe or a liner for the existing pipe. Accordingly, The mole is adapted to tow the new pipe or the liner through the fractured pipe. The head portion of the mole is tapered toward the front and has flat or scalloped sides defining therebetween integral cutting edges.

This invention relates to a soil displacement hammer for use in replacing 
underground pipes, such as gas mains or sewers. 
Soil displacement hammers, commonly referred to as "moles", are 
impact-action self-propelled mechanisms for driving holes in the ground. 
They are generally pneumatically operated, although hydraulic, electric 
and other driving means are possible. They can be used to install pipes, 
cables or conduits in the ground without the necessity for excavating a 
continuous trench. Such moles are disclosed, for example, in GB-A-2 134 
152 and GB-A-2 147 035. 
The use of a mole in replacing underground pipes is described in GB-A-2 092 
701. The mole is designed to pass through an existing underground pipe, 
and is provided with cutting edges at the front so as to cause the walls 
of the pipe to fracture as the mole moves therealong. A new pipe, or a 
liner therefor, is towed along behind the mole. The mole is driven through 
the pipe, for example by means of compressed air, in conventional manner, 
and is simultaneously guided and pulled along the pipe by a winch cable 
attached to the front of the mole. 
The mole described in GB-A-2 092 701 is provided with fixed or moveable 
blades at its front to cause fracture of the existing pipe. However, such 
blades are not very effective, and particular problems are caused in 
fracturing thick flanges which are present at the joints between adjoining 
lengths of pipe. The blades have a high wear and fracture rate. 
The present invention unexpectedly overcomes this problem by employing a 
mole in which the cutting edges are integral with the head. This solution 
is particularly unexpected because up to now, all moles have been provided 
with smoothly curved or frusto-conical heads. Indeed, in normal practice, 
a mole having a head portion with sharp edges would not be desirable, 
because it would be directionally unstable when passing through the 
ground. 
The present invention thus provides a mole for use in replacing or 
preparing for replacement an existing underground pipe, the mole being 
adapted for insertion into and movement along the existing pipe, and 
having a front portion provided with a cutting face for engaging the 
internal wall of the existing pipe and arranged to cause the wall to 
fracture, and a rear portion provided with means for clamping to the mole 
a new pipe or a liner for the existing pipe, so that the mole can tow the 
new pipe or the liner through the fractured pipe as the mole moves 
therethrough, characterised in that a head portion of the mole is tapered 
towards the front and has flat or scalloped sides defining therebetween 
integral cutting edges, the angle of taper being from 2.degree. to 
10.degree. and the number of cutting edges being from 3 to 8. 
The angle of taper is preferably from 4.degree. to 6.degree.. The number of 
cutting edges is preferably from 3 to 6, in particular 4, and the tapered 
sides are preferably flat. Most preferably, the head portion of the mole 
is of tapered square section.

Referring now to the drawings, the body of the mole comprises a cylindrical 
outer casing 1. The internal structure and pneumatic operation of the mole 
are conventional, and are therefore not described here. The rear end of 
the mole (not shown) includes means for clamping a new pipe or liner for 
towing behind the mole as it passes along the pipe to be replaced. 
Immediately in front of the cylindrical casing 1, there is a frusto-conical 
portion 2, having an angle of taper of 20.degree.. Immediately in front of 
the frusto-conical section 2, there is a head portion 3 of square tapered 
section. The head portion 3 thus comprises four flat sides 4, 5, 6, 7, 
which define therebetween four straight cutting edges 8, 9, 10, 11 which 
are in the axial plane of the mole. The angle of taper of the head portion 
3 is 4.degree.. 
Attached to the front of the head portion 3 there is a projecting portion 
12, which incorporates a hole 13 for engagement with a winch cable. 
The method of use of the mole is similar to that described in GB-A-2 092 
701. The mole passes through an existing pipe under its own pneumatic 
power and under guidance and assistance from a winch cable attached to the 
front. As it passes through the pipe, the cutting edges on the head 
portion cause the pipe to fracture. A new pipe or a liner is towed along 
behind the mole. 
Because the head portion 3 is solid, with integral cutting edges, it is 
most effective in fracturing the old pipe and very little wear of the head 
portion occurs. Although four cutting edges are preferred, it is possible 
to have three or more than four. Particularly where more than four cutting 
edges are employed, it may be desirable to scallop the tapering surfaces 
in between the cutting edges so as to sharpen the cutting angle at these 
edges. 
A particular advantage of the cutting head used in the present invention is 
that there is no need for separate cutting blades, and in particular, 
there is no need for the moveable blades of the kind described in GB-A-2 
092 701. The latter moveable blades are operated under hydraulic pressure 
which requires a hydraulic hose connection to the front of the mole. Such 
a connection gives rise to practical problems and is expensive, and it is 
therefore a particular advantage of the present invention to render such 
means unnecessary.