Static systems utilize some type of hydraulic power to either turn a winch pulling a cable or push a rod through the ground and pull it back. The rod is attached to the expander and the pipe, and raw power is used to pull it through the ground, fracture the host pipe, displace the soil and pull in the new pipe. Conical-shaped heads are used for fracturable pipe, while bladed heads (either triple- or single-blade) are used to cut HDPE, steel and ductile iron. "It's important to understand the different types of pipe, then set yourself up for success by running the right head for the right type of application," says Collins.
Some replacement pipes can be pulled with either system. However, if you're pulling PVC pipe, a static system would be used because of the physical properties of the pipe. It tends to be more rigid, and the impact of a pneumatic system can fracture it at the attachment point. Ductile iron and steel also require a static system.
What are the ground conditions?
With any type of trenchless technology, understanding soil conditions plays a big part in your success. "The best scenario is original compacted backfill, soft to medium clays and medium-dense sand," says Collins. "As you get into loose gravel, sand, cobble, underground water, hard clay or rock, it gets more difficult."
That's because the best success occurs where soil surrounding the pipe can be readily compacted by the bursting operation as it is displaced. This minimizes surface upheaval and keeps the hole open while replacement pipe is being installed.
Static systems work well in sandy soils because they can minimize the tendency for lightweight replacement pipe to rise upward, and in projects below the water table because the water acts almost as a lubricant. "If you're pounding with a pneumatic machine, the same sandy condition will tend to dewater the sand around the machine and the pipe. [This creates] an excess amount of friction, which will eventually stall the process," says Holcomb.
If you're working in rocky conditions with large upsizes, a pneumatic system is recommended, since its dynamic energy can more readily displace the surrounding ground. If conditions are solid rock, you may not be able to burst at all, because you can't adequately displace the existing pipe or surrounding ground.
How large is the upsize?
Size-on-size bursts have become fairly run of the mill. Challenges come in when there is a double-, triple- or greater upsize.
"Double-ups, such as going from an 8- to a 12-in. pipe, goes from easy to challenging," explains Eric Nicholson, director of global and key accounts, HammerHead Mole. "When you upsize three sizes or greater, such as from an 8-in. to 14- or 16-in. pipe, that's where it goes from challenging to extremely difficult/experimental.
"When you're going through the path of an existing pipe, you're trying to push the soil out of the way and hold it back. That can be a challenge," he continues. "An additional challenge is that the center line of the existing pipe and the center point of the new pipe can be in two very different locations."
In this situation, the host pipe can start breaking too far in front of the replacement pipe, creating a plug in front of the burst head. "The pipe can also break and fall into the bottom part, which can create a ramp for the burst head to ride on top of and change the flow of the pipe," Nicholson points out. "So if there's a large upsize, you will have to ask a lot more questions."
How deep and how long an install?
Jobs have been known to successfully go to 50 ft. and more. So depth is more of a concern on the shallow end of the scale, rather than the deep end.
Because the bursting action can cause ground upheaval, the deeper the pipe, the better. "If you're shallow and expanding the soil, you might get ground displacement to the top or to the side," says Collins. "That can be especially crucial if you're bursting underneath an existing building or street, because you can crack the concrete or asphalt. Our goal is to eliminate upheaval, so deeper is better."
Based on industry experience, a rule of thumb of 10 times the upsize diameter has emerged. For example, if you have an existing 8-in. pipe and you're replacing it with a 12-in. pipe, you will need a 15-in. O.D. expander to pull it in. That's an increase of 7 in., so you will need to be at least 70 in. deep to ensure you don't have soil displacement. Ditch Witch recommends for every inch in pipe diameter, you should be at least 12 in. deep.