This is the Union Pacific Railroad’s Austin Subdivision in central Texas.
It’s a busy corridor that moves both freight and passengers north and south between Austin
and San Antonio… But it’s mostly freight. Trains run twenty-four-seven here, carrying
goods like rock from nearby quarries, cement, vehicles, intermodal freight, and more. So,
when Crystal Clear Special Utility District was planning a new water transmission main that would
connect a booster pumping station to a new water tower to meet the growing demand along I-35,
the biggest question was this: how do you get the line across the tracks without shutting
them down and trenching across? It’s only about 250 feet or 76 meters from one side to the other,
but this small part of a large water transmission project takes more planning, coordination,
engineering, and innovative construction than the rest of the project combined. Maybe you’ve
never even wondered what it takes to move fresh water across the distances from where
it’s stored to where it’s used. But, I really think you’re going to find this fascinating.
Crystal Clear and their general contractor, ACP, invited me on-site to see it happen in
real-time and document the process for you!
Most of the water lines are already installed,
but getting this one across these tracks is going to be a different challenge. I’m
your host, Grady Hillhouse, and this is a Practical Construction.
There are actually a lot of ways to install underground utilities without disrupting
things at the surface, collectively known as trenchless technologies. This project is using a
method called horizontal earth boring, but really, it’s pretty exciting. Before any dirt gets bored,
there’s a lot that has to happen first. So much can go wrong if an operation like this
isn’t carried out thoughtfully and carefully. One of those risks is hitting something that’s
already buried at the site, and just about every subsurface utility contractor can tell
horror stories about what happens if a water, sewer, gas, fiber optic, or telephone line is
severed during construction. The right-of-way along a railroad track is a common place to
install linear utilities, because they can just run parallel to the tracks,
avoiding the complexity of dealing with multiple property owners and obstacles. The owners of all
the utilities that run along these tracks have already been out to mark their location using
spray paint on the ground and flags. But, that’s not enough to make sure they are
avoided. Before the drill can get started, a vacuum excavation crew comes to the site to
confirm their location not just along the ground, but how far each one is below it.
This truck has an enormous vacuum that sucks up soil as it’s blasted loose by a pressure
washer. The benefit of a vacuum excavator is that, although the water is strong enough to
dislodge and excavate soil, it’s not strong enough to damage the utility lines below.
Compare that to using a hydraulic excavator with a bucket where one wrong move could rip
a pipe or cable out like a wet noodle. It also disturbs a lot less of the area at the surface,
so this process is often called potholing. It’s a crucial step if the margins are tight
when avoiding existing utilities, like they are on this site. For each utility,
the vacuum excavator locates the exact position and depth of the line so that it
can be marked by a surveyor and compared to the proposed alignment of the bore. And
there’s hardly any mess once the process is done. On this site, there are lines
both above and below the proposed bore, so the drilling contractor will be threading a needle.
Safety is also critical, especially when working around railroads and trains. Since
this job requires people on the tracks and construction below them, there’s a specialized
crew on site who coordinates between the Union Pacific dispatchers, train engineers, and crews
on site to make sure no one gets hurt. They’ve established a specific zone along the tracks,
which requires the train engineers to check in with them first before any train gets near the
work. When a train is on the way, the safety crew sounds a horn, and everyone on site stops
working and gets clear of the tracks. Once the train is past, work starts right back up.
The process of horizontal earth boring, also known as jack-and-bore, starts with an entrance
pit. Unlike some trenchless methods that can curve down and back up again from the surface,
this waterline needs to be as straight and precise as possible. So you have to start
underground. This enormous excavation is where almost all the work will happen. And,
because it’s so close both to a roadway and the railroad tracks,
there’s no room to slope the sides to avoid the risk of a collapse. Instead, huge steel
trench boxes are installed in the pit to shore it up and keep it from collapsing or affecting
the adjacent structures. Once the trench boxes are installed, the boring machine can be lowered
into place. And before long, it’s up and running, or I guess you could say it’s down and running.
In practice, horizontal earth boring is relatively straightforward. The boring machine really only
has two jobs: excavating the soil and advancing the casing pipe. For the first job, it uses a
string of augers that connect to a boring head. It’s just an oversized drill bit. As the auger
turns, the boring head breaks up the soil ahead of the casing pipe, and the flights draw the cuttings
back toward the pit. The cutting head has wings that open when rotated in one direction. Those
wings extend just slightly beyond the edges of the casing pipe, over-excavating the bore hole
to minimize the friction of pushing the casing pipe forward. The soil cuttings from the boring
are discharged from the side of the machine into a pile in the pit. Every so often, they have to
be removed. The excavator at the surface uses a clamshell bucket to scoop the cuttings out of the
pit and stockpile them nearby. They’ll eventually be disposed of off-site or used as backfill.
The machine’s second job is to advance the casing pipe into the bore. This pipe provides
support to the hole to keep it from collapsing and prevent the overlying soil from shifting or
settling over time. The boring machine sits on tracks. The back of the machine uses a hydraulic
ram attached to a locking system that affixes to the rails. The ram provides thrust, pushing
both the machine and the casing pipe forward with the tremendous force required to advance
it through the ground. Newton’s third law is in play here. To provide that thrust to the casing,
the machine needs something to react against. So, those tracks have been firmly concreted
into the bottom of the entrance pit to make sure it’s the machine that moves and not the tracks.
Of course, every contractor knows as soon as you start making good progress, it’s going to rain.
Water flows downhill, and this pit is the lowest spot of ground on site. But the crew doesn’t let
it slow them down too much. The concrete bottom in the pit helps keep things from turning into
a muddy mess, and an electric pump makes pretty quick work of the water that gets in. Tarps over
the top of the pit also help keep it dry, if also making it a little tough to film the work inside.
Railroad operators are rightly strict about the what, where, when, and why when it comes
to construction on their rights-of-way. Disrupting the movement of freight and passengers is simply
not an option. So an essential part of this operation is continuous monitoring to make
sure the boring is not affecting the tracks above. A surveying crew comes to the site
every six hours to carefully measure for any changes in elevation along the tracks. They’ve
installed these reflective markers and use a piece of equipment called a total station
that can precisely pinpoint each length of the rail. They process the data as it comes in and
compare it to the baseline measurements. If they notice any settling or movement,
everything would have to stop (but, spoiler alert, they never did).
Another requirement from the railroad is that this work happens nonstop. They don’t
want an open excavation sitting idle below the tracks, so they require that the boring
happen continuously night and day. The longer it takes to get this casing pipe to the other side,
the more opportunity for something to go wrong. The boring contractor works in double shifts. When
one crew leaves, there’s already another one to take their place, so the site is never unattended.
Once one segment of casing pipe is pushed as far as it can go, the boring machine is
pulled to the back of the pit. A new segment of pipe is collected from the stack. And,
it’s lowered in. The next length of the auger is already inside. The auger is attached to
the string. And then the casing segment is welded to the end of the previous one.
Segments go in faster at first, but each one takes a little bit longer than the last. That’s because,
every two or three segments, they have to check and make sure the bore is following
the right path. There are utilities to avoid, dimensional tolerances from the railroad,
and location requirements from the engineer and property easements. So, having the alignment
wander is not an option. Every so often, the crew has to remove the entire auger string from
the bore to make sure it’s headed in the right direction. The way they do it might unnerve you,
especially if you’re claustrophobic: they just send a worker on a skateboard to the end of the
casing pipe. There are more sophisticated tools, but some contractors prefer the old-school,
reliable method, and they have a slew of safety measures in place as required by OSHA,
including ventilation, communication, and safety spotters. The person inside
the pipe uses a rule to check for any deviations in grade from the precision
laser installed in the bore pit. But, what happens if the bore gets off alignment?
Horizontal earth boring is not a very “steerable” operation, but there is some opportunity to make
corrections if they’re needed. Take a look back at the first length of the casing pipe.
Notice the shoes cut from each quadrant of the pipe. If the bore starts to deviate,
a hydraulic jack can be used to bend one or more of the shoes outward and deflect
the operation back into alignment. You’re not going to turn a corner this way, but it gives
some control over alignment and grade. It’s why it’s so critical that the first length
of casing pipe be installed perfectly; all the rest of the casing will follow right behind it.
The operation runs night and day. The machine bores and pushes each length of casing pipe.
Soil is removed from the bore and then the pit. Alignment is checked. The auger string
is re-inserted. A new length of casing is welded on. Rinse and repeat. All the while,
trains are running constantly back and forth along this busy corridor. When the drilling
crew starts getting toward the end of the line, an excavator arrives to dig the receiving pit. And,
after just about a week of boring 24/7, the cutter breaks through on the other side.
Even the guys who do this every day gathered around to watch it happen. It’s a perfect sight,
especially for the fact that they broke through in the exact spot they were aiming for.
Only a few days later, it was time to push the water pipe through. The casing’s job is
just to hold the bore open, but the water will run in rated plastic pressure pipe. These pipes
connect using a bell-and-spigot design; they literally push together. A fiberglass rod is
hammered into a groove around the inside of the spigot to lock each segment together. Spacers
are installed to hold the line up off the casing to keep it from rubbing during installation or
being damaged over time. Just like the boring, the pipes are lowered into the entrance pit, attached,
and pushed through to the other side (although, this operation goes quite a bit faster). In some
projects, the annular space between the casing and pipe is grouted in, but in this job they opted
to keep the space open. It was a ton of work and coordination to get this line under the railroad,
so if it ever breaks or leaks, Crystal Clear will be able to pull it out and repair or
replace it. This line will be tied into the pipes already installed on either side of the bore,
leak-tested, and backfilled, but the hard part is over. It won’t be long before it’s pressurized
and put into service, moving fresh water to this quickly growing area in central Texas,
quietly and invisibly meeting a crucial need. And not a single train was delayed while it went in.
One of the coolest parts of the project was the surveying involved. From the initial layout of the
bore to the track monitoring surveys that happened every six hours, there were so many surveyors
involved in just this one part of the project. I’ve said it before that surveying is such a cool
engineering-adjacent career, especially if you don’t like sitting behind a desk. But there’s a
barrier to entry that I think scares a lot of people away from it, and that’s trigonometry.
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Huge thanks to Crystal Clear Special Utility District, ACP,
and their subcontractors for having me on their site.