Pipe Bursting, and San Bruno

Several articles have mentioned the NTSB's interest in sewer work done in 2008 at the intersection where Line 132 blew up.

The work included a method called "pipe bursting," which replaced an old six-inch sewer pipe with a new ten-inch pipe, without having to dig a trench for the job. The news mentioned a 2001 study for the Corps of Engineers on the uses of, and cautions about, pipe bursting. You can find that study here.

The word “bursting” might suggest underground blasting but it's just a slow, chugging mole-like machine that uses power from the surface, and causes the ground to vibrate with each cycle as it scoots along. We don't know all the details in San Bruno, but normally pipe bursting works like this, typically over spans of 300-400 feet (the distance between manholes). Say there's an old sewer pipe that's become too leaky to tolerate, and it's made of sections of vitreous clay connected with pitch at each joint. Such pipe is relatively strong but brittle.

Workers feed a torpedo-like “bursting head” in one end of the old pipe. Using hydraulic or air power, or else by being dragged from the other end, the bursting head proceeds down the old pipe, breaking it into little pieces as it goes and forcing the fragments into the soil surrounding the pipe. It also drags new pipe behind it, typically medium-density or high-density polyethylene. Remarkably, the new pipe can be bigger than the old pipe since the bursting head is powerful enough to shove aside the adjoining soil. (Of course the shifted soil has to go somewhere -- more on that below.)

Unless the old pipe has major sags or deviations in it, or is encased in concrete, the bursting head usually has no trouble following the old pipe and emerging at the next manhole, ready for another leg.

Bursting is one modern method of replacing old pipe: others include "pipe eating," "pipe reaming," and "pipe ejection." All these underground construction methods avoid having to scoop out a trench the length of the pipe run. Old-fashioned trenching work is very labor intensive, since most utility corridors also host other pipes running nearby that have to be protected from damage.

To visualize this, imagine a hallway: put in eight pipes and conduits of different sizes and ages; fill the hallway with sand and gravel; add a few decades of leaks, corrosion, and vibration to compact the soil, create voids, and generally shift things around. Now imagine having to empty that hallway and expose all the pipes without breaking anything ... like a gas line or a buried high-voltage line. There's no way to finish the job without hiring lots of guys to wield shovels in places too tight for backhoes, and to install temporary bracing.

Trenching also poses a collapse risk to workers and it's sure to mess up traffic on the street above. For major projects full-scale excavation is hard to avoid, but for a city looking to fix up portions of its buried infrastructure fast and affordably, “underground construction” methods can be be very attractive.

But every cloud comes with a dark lining, or at least a gray one. The old pipe fragments don't go far away and they can cause problems for the new pipe, by creating stress-concentration points. If the pipe to be replaced lies near the surface or is in a rock trench, the only place for the shifted soil to go is upward so the ground can bulge upwards as the bursting head chugs along. Think of the ridges of soil that moles or gophers make in one's lawn.

And pipe bursting can put stress on adjacent underground lines in some cases, which is one hypothesis for the Line 132 catastrophe. Guidelines say that – assuming average conditions – pipe bursting shouldn't cause damage to other pipes if the other pipes are two, preferably three, diameters away. That's just a rule of thumb, of course, and doesn't take into account the strength of the other pipe or soil conditions. Was there this much separation here? Sometimes that's hard for an underground-construction crew to know, because as-builts often differ substantially from construction blueprints. If remote-sensing methods were used to check this out in 2008, there might be electronic records from that work.

So we don't know at this point whether there was plenty of separation, or too little. We have read that the sewer line installed in 2008 was 10 inches across, so that would indicate a minimum safe separation of around two feet.

As the NTSB pointed out, even if pipe bursting did play a role there would have had to have been other contributing causes, since the sewer work happened two years before Line 132 blew apart. Other possible causes being considered are metal corrosion from liquids and/or bacteria, delayed repairs by PG&E, the fact that this line was too bendy for the use of diagnostic pigs, modifications to the gas line after construction, and temporary overpressure.