Underground pneumatic piercing tools are often referred to as hogs, air hogs, or pneuma-gophers. The tool is used to bore a hole underground between two points without disturbing the surface ground. This open air chamber is referred to as the bore hole and is used to either run ducting for product or the raw product itself between the two points.

                  

Uses

The tool is used in the installation of all public utilities electricity, gas, phone, and cable television these are referred to as product. It is also used to install home sprinkle systems and auxiliary home power lines. The tool is utilized in instances when it is difficult or cost prohibitive to plow or trench the product into the ground, some of these instances are under driveways, roads, sidewalks, and landscaping. Pneumatic piercing tools are a lower cost alternative to directional boring.

How it works

The tool uses air to pound its way through the ground underneath the obstruction. The tools is cylindrical in shape and ranges from two to five inches in diameter and between three and six feet long. It is made from metal. Different manufacturers have different front styles and control mechanisms. Some have bullet shaped front and others have a stepped front similar to a unibit. Compressed air is used to power the tool, this air is run through a durable hose. The tool works in a way very similar to a jack hammer, inside the cylinder is a piston a valve opens and the air blows into the chamber forcing the piston forward which in turn propels the tool forward. The valve then shuts and the piston is pushed at a slower speed back in the tool, the valve reopens the piston pounds again and the tools moves forward. This process is continually repeated until the bore has run the intended distance. Both the lighter reverse hit of the piston and the friction of the ground surrounding the tool keep it moving forward instead of stopping. The speed of the tool can be controlled by a valve that connects the tools hose to the air supply this valve is outside the tool and remains accessible to the operator. As the tool pounds through the ground it compresses the soil this compaction maintains the same diameter as the tool and leaves the bore hole though which the product is passed.

Performing a bore

The purpose of the bore is to get from point A to point B under the obstruction. First the operator surveys the area and the obstacle(road, sidewalk, driveway). Utility locates must be present for any underground work, the path most clear of utilities is chosen to send the tool through. The first hole is now dug on one side of the obstacle. This hole must be large enough to fit the tool and the operator in it so they can aim the tool. The hole also must be deep enough that as the tool compacts the ground the surface remains undisturbed. The depth of the starting bore pit depends on the type on soil that is being worked in and how well it compacts. The operator aims the tool to the desired exit point and allows it to perform its function creating a bore hole under the ground without disturbing the surface. The tool is tracked by the operator who feels the tool pounding and can approximate the tools location under the ground. The operator also monitors the surface to make sure that it is undisturbed. As the tool nears the exit point it is slowed down and an exit pit is dug to locate both the tool and the bore. As the tool enters the exit pit the operator chooses to either dig up the tool to retrieve it or reverses the tool back into the first bore pit to retrieve it. Now between the two pits is a completed bore under the obstacle which can be used to run the product.

Drawbacks

Pneumatic bore tools do have drawbacks which can cause difficulties completing the bore. First the distance of the bore the tool can create is limited by the length of the hose that supplies the tool with air. Also the tool is not steerable once it has exited the bore pit the operator no longer has control over it. The tool can be deflected by rocks and soil density to a path the operator had not intended. If this deflection is in the direction of the surface the tool can cause damage to the obstacle being bored under if it is downward the tool can dive to unrecoverable depth. If it is deflected side to side the tool could also run into other utilities. These hazards make it important for the operator to maintain close observation of the tool. The type of ground the tool is working in can also lead to problems if the soil is to loose the tool cannot compact the soil stalling it out or leaving no bore hole. If the soil is rocky the tool may be deflected or fail to pound forward due to its inability to break the rock. These drawbacks can be overcome by directional boring. 

(Wikipedia 2010)