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Excavation contractors
have long used trench boxes to shield employees from cave-ins when
working inside trenches. Still, despite their popularity, there
are times when a trench box isn’t the best or even a practical
choice. For example, you may not have a trailer big enough to haul
the box, or a backhoe or excavator strong enough to lift and position
the size of box you need. Then again, if you’re digging a large,
wide opening in the earth pit, you need a way to shore up the sides
of the excavation rather than a shield to protect workers.
Here are ideas for equipment
that can provide a cost-effective way to keep workers protected
and productive when you need an alternative to the popular trench
box. In some cases, it may pay to buy the systems. In others, rental
may be a more economical option. Either way, workers require some
type of protection from cave-in when they venture into a hole in
the ground.
The Need for Safety
“Anytime you cut into the earth, Mother Nature wants to
heal the cut by filling it in,” says Matt Vogel, a sales manager
with GME, a Union City, MI, manufacturer of trench shielding equipment.
“Cave-ins can be caused by a number of factors, including vibrations
from nearby traffic or equipment parked near an excavation. Sometimes,
walls can collapse from above, other times from below. You can’t
predict when a cave-in will occur. There are no warning signs. The
wall just goes.”
When that happens, unprotected
workers in a ditch or other excavation can suffer serious injuries,
even death. That’s why OSHA regulations require protective
measures—either sloping and benching, shielding, or shoring
and sheeting—whenever workers are inside an excavation that
extends 5 feet or more below ground. Some states require such practices
at shallower depths.
“There’s a
misconception that protection is required only with Class C soils
and not the more stable Class A or B soils,” says Bob Checca,
sales manager with shoring and shielding distributor Allwest Underground
of Pacific, WA. “That’s not true. In fact, more cave-in-related
injuries occur in clay-based soils, where a contractor may not anticipate
a failure, as opposed to sandy conditions where you might expect
and watch for a failure. Also, most failures take place in trenches
between 5 and 15 feet deep, where many falsely believe they can
quickly escape if a trench wall collapses.”
Sloping and benching
prevents cave-ins by cutting back the trench walls to a shallow
enough angle to prevent them from collapsing onto workers. But this
approach can require a much wider right of way, not to mention the
higher costs of extra excavation, backfilling, and compacting compared
to a trench or pit with vertical walls. Shielding devices, which
protect workers in the event of a cave-in, and shoring equipment,
which puts pressure on sidewalls to prevent them from giving way,
overcome these limitations.
A Versatile Alternative
to Boxes
Utility installation contractor RPD of Sumner, WA, faces two
major shoring and shielding challenges—working around existing
utility lines and dealing with widely varying soil conditions. The
company installs water mains, sewer mains, and storm systems with
pipes ranging in size from 4 to 72 inches in the Seattle area. The
trenches can range in depth from 4 to about 30 feet. Although most
of the jobs involve single-family residential development, the company
also handles road improvement and commercial projects.
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PHOTO: PRO-TEC |
Most of the time, trench
boxes and manhole boxes are used to shield workers from cave-ins
when working in the trenches. However, in older, developed areas,
an underground maze of pipes of different sizes and buried at various
depths over the years in the rights of way calls for a different
approach to protecting employees. “Often, it’s impossible
to use conventional boxes, because they don’t fit in between
the existing pipes,” says Marc Rickabaugh, who co-owns RPD
with Ben Pentecost.
Then, there are the unpredictable
soils. “We work in some goofy ground,” he adds. “We
can run into many different soils—hardpan, rock, sand, or clay—in
a distance of just a thousand feet.”
In these situations,
where trench and manhole boxes aren’t practical, Rickabaugh
uses lightweight, re-usable aluminum hydraulic shores to stabilize
trench walls. He rents them from his supplier, Allwest Underground.
The company, which offers shoring products manufactured by Efficiency
Production Inc. and Pro-Tec Equipment Inc., recommends the size
and spacing of the systems to fit the specific job.
Hydraulic shores are
today’s much faster and easier-to-use alternative to the timber
shores and screw jack systems of years ago. “Hydraulic shoring
systems are designed to provide a quick, simple way to secure a
safe and productive environment when trenching,” says Checca.
These professionally
engineered, low-cost systems are installed and removed from above
ground, keeping workers out of the unprotected trench. They include
an aluminum hydraulic cylinder or cross brace, which presses against
vertical (uprights) or horizontal (walers) rails to preload the
trench walls and stabilize the sides. Normally, hydraulic shores
can be used to a depth of 20 feet depending on design and application.
In more stable soils, they can be installed directly against the
trench walls. However, to reduce exposure to raveling and sloughing
in less stable soils, shoring-grade plywood (Finform) or steel panels
are placed behind the rails.
The manufacturer’s
tabulated data are used to match the shoring system and placement
to specific job-site conditions such as soil type, trench width
and depth, placement and spacing of the shores, and weight of machines
operating near the trench.
Two Basic Choices
Vertical shores are especially useful where working room is
tight, where spot bracing is needed, or where utilities cross the
trench. One person can install them by hand, allowing a backhoe
or excavator to continue digging. Waler systems can be used to stabilize
trench faces at greater depths and to protect a larger working area
than vertical shores. Placing them in the trench, however, may require
an excavator or backhoe.
These systems are available
in a wide range of sizes. American Shoring Inc., for example, makes
vertical hydraulic shoring equipment with 2-foot rails and a single
hydraulic cylinder; 3.5-, 5-, 7-, and 8-foot two-cylinder models;
and 9- and 12-foot rails with three cylinders for use in trenches
17 to 88 inches wide, depending on the model. The weight of these
systems ranges from 25 pounds for the smallest to 130 pounds for
the largest. Extension systems are also available for each model.
The company’s waler systems include dual-cylinder 8- and 12-foot
units and 16-foot rails with three cylinders.
Installing hydraulic
vertical shoring is a fast, simple, one-person job. After connecting
a hydraulic hose line to the shore, the assembly is lowered into
the trench to the desired height and a hand pump is used to pressurize
the system and expand the shore. For added safety, bleed-off ports
prevent over-extension of the cylinders. Removal involves releasing
the hydraulic fluid and pulling the assembly out of the trench.
The entire unit folds flat for easy transport.
Rickabaugh uses hydraulic
vertical shores with either Finform sheets or 1-inch steel plates,
usually in trenches no deeper than about 12 feet. He installs them
where a crossing communications line, gas main, or other utility
prevents use of a trench box. “We use the hydraulic spreaders
with the wood or steel sheets to piece around the existing utility,”
he says.
Depending on the depth
of the trench, he may place the panels horizontally, either a single
panel or two panels (one above the other) or vertically side by
side, and installing a hydraulic shore at each end of a panel.
“Hydraulic shoring
offers us a quick way to shield our workers in a trench without
a lot of labor,” Rickabaugh observes.
Protecting Wide-Open
Spaces
Up until about six years ago, Excavation Supervisor Bill Bogan
used steel beams and sheeting to shore up the sides of large pits
for installing underground fuel storage tanks. Then, he switched
to a slide-rail shoring system. He likes this approach a whole lot
better.
In fact, since then,
he’s used beam-and-plate shoring only once, because of a high
water table.
“I enjoy using slide-rail
systems, and I’ll continue using them whenever I can,”
he says.
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PHOTO: PRO-TEC |
Bogan works with Oscar
W. Larson Co. in Clarkston, MI, which specializes in the installation
of steel or fiberglass storage tanks for retail fuel stations. Typically,
these are 20,000-gallon tanks that measure 10 feet in diameter and
about 38 feet long. Installation requires 15- to 16-foot-deep, 42-foot-long
excavated pits ranging in width from 14 feet for a single tank to
42 feet for a triple-tank installation.
The slide-rail system
is a vertical sheeting and trench support design that provides a
cost-effective alternative to an engineered sheet piling system
for short-term projects, like these, which require a large clear
span excavation. While there is no steadfast rule, the experts point
out, an onsite engineered beam-and-plate system may be a more cost-effective
approach for longer-term projects.
The pre-engineered slide-rail
system is installed from the ground level down. The modular design
features steel panels that are inserted between vertical rails to
form a square or rectangular structure to shore up the walls of
the pit. Spreaders placed across the width of the system lock in
place along the rails to support the sidewalls during installation.
Once the system is in the final position, these rolling braces are
removed, leaving a large, open space into which a fuel tank or other
object can be lowered without any supporting beams or braces to
block the way. Vertical clearances can be adjusted if removal is
not required.
This system can be used
in such applications as pump stations, bore pits, pre-cast structures,
and pipelines. One manufacturer’s slide-rail system, for example,
can protect trench walls as deep as 40 feet, while allowing an adjustable
vertical clearance over the bottom to install very large-diameter
pipe. In one cast-in-place concrete box culvert project, a slide-rail
system was used to stabilize the walls of an excavation measuring
500 feet long, 20 feet wide, and up to 28 feet deep in a linear
application.
“It’s a dig
and push system,” says John Powell, slide-rail specialist with
Pro-Tec Equipment Inc. in Charlotte, MI, which manufactures and
distributes slide-rail shoring systems. “You dig from inside
of it and push it down with an excavator as you dig. For a short-term
project, it’s much less expensive and much faster and easier
to install and remove than sheet piling.”
Using the excavator to
push them into the ground, the interlocking posts and panels are
installed after completing the initial shallow excavation to a depth
of no more then 4 feet. Alternately digging and pushing the system
in a few feet at a time, the system is kept level and square until
reaching the desired depth. Unlike steel sheeting, which is typically
driven into place with a hydraulic hammer, the dig-and-push approach
means much less vibration. That can be an important consideration
when working close to buildings or walls. Once in place, this system
provides a large open space for installing a fuel tank, pipeline,
or other structure. After work inside the pit is completed, the
slide-rail system is removed by extracting the panels and rails
as the pit is backfilled and compacted.
Profitable Savings
A high-water table can be a drawback with slide-rail shoring,
Bogan notes. “If a small amount of water is coming into the
pit, you might be able to dewater a slide-rail system by installing
well points around the outside of it,” he says. “Otherwise,
the sheet piling is usually a better choice in these cases.”
Other than that, he says,
the slide-rail system holds a clear advantage on his tank installation
projects. They include:
Less Space
A slide-rail system fits tight spaces by eliminating the need for
sloping to prevent a cave-in. Sloping requires a fair amount of
horizontal area all around the pit, which Bogan seldom has on his
job sites. Even if he did, Bogan points out, sloping would require
removing a lot more soil, adding to excavation and soil disposal
costs. Plus, he would have the added expense of bringing in a crane
to set the tank in place, since his excavator would lack the reach
to do that.
Less Time
It takes longer, of course, to install a slide-rail system than
simply to excavate a similar-size hole. But it’s still about
twice as fast as installing sheet piling, Bogan notes. He describes
one of his recent slide-rail projects involving a 42- by 42-foot
pit to accommodate three fuel tanks. “It took us about five
days to install the system and set the tanks, and two days to pull
it all back up and load the components on a truck,” he says.
“I’m very satisfied that we could do all that in that
amount of time.”
Less Work
Bogan rents slide-rail systems for his projects from Pro-Tec.
“With all the different configurations we need, it wouldn’t
be cost-effective to own the various systems we’d need,”
he explains.
He provides Pro-Tec with
the dimensions of the excavation project, and Pro-Tec provides him
with installation drawings and a technician who lends onsite expertise
for safe and proper installation. “The technician tells you
where to push on the panels and posts when pressing them down and
how to keep everything level and square while you’re building
the system,” Bogan says. “Installation doesn’t require
any special training on our part.”
Less Equipment
Installation doesn’t require any extra equipment, either. Depending
on the size of the system, he can position all the components and
dismantle the system using his Caterpillar 330 or 345 excavator.
That contrasts to sheet piling, which requires a crane and boom
plus the services of welders to build the internal framing.
More Profits
The do-it-yourself slide-rail system eliminates the need for a subcontractor
to install the sheet piling. That pays off for the Oscar W. Larson
Co. and the client, Bogan reports. “It can save our customers
a boatload of money; plus it gives us more work to do to increase
our profits on the project,” he says.
Solving Tight Trenching
Problems
GME’s line of trenching shielding and shoring products
includes Inner City Linear Shoring, a slide-rail system developed
by Emunds + Staudinger, for tight-space applications. Features include
a rigid roller spreader. Unlike permanently hinged spreaders, the
roller units keep the vertical rails and shoring panels a fixed
distance apart in the trench to maintain the same desired trench
width from the top to the bottom of the trench throughout the installation
process.
“With hinged spreaders,
the trench can become a few centimeters narrower or wider as the
rails are lowered or lifted, leaving a small cavity between the
earth and the panels,” says Matt Vogel, a sales manager with
GME of Union City, MI. “The roller-spreader ensures that everything
is aligned linearly, always at exactly the same distance from the
opposite side during all phases of installation. This allows you
to work faster, more efficiently, and more accurately while cutting
costs.”
The panels of this system
can also be swung into position between the rails from the side
at ground level instead of sliding them in from above, he notes.
“This can be an advantage in inner-city applications where
overhead cables can be an obstacle when lifting the panels in place
with an excavator,” Vogel says. “This side placement also
makes it easier to use longer rails for deeper applications.”
Build Your Own Box
Maybe you don’t have equipment big enough to handle the
size of a conventional trench box required for a job. Or, perhaps,
the box you have may not be appropriate for a particular project.
Then, a modular aluminum trench shield may make sense. This strong,
lightweight, engineered shielding features spreaders and panels
of different sizes, which can be assembled onsite in modules to
build a trench box. Depending on the manufacturer, they can be configured
with two, three, or four sides. “They’re popular with
municipalities and utility companies for line installation and repair
work,” says Scott Sessler, sales and rental manager for American
Shoring Inc. in Newburgh, NY.
For example, his company
makes the two-sided Mighty Lite modular soil support system. It
offers a choice of three spreader systems—manually adjustable
screw jack, pinned-in-place pipe spreaders, or telescopic steel
square box spreaders. The panels, either 2.5 or 3.5 inches thick,
are available in 2-, 4-, 6-, or 8-foot heights in nine standard
lengths from 2 to 12 feet. Individual modules can be stacked one
above another using a tongue-and-groove alignment to achieve the
required depth. “The entire system can be hauled in a pickup
truck,” Sessler says. “Two people can assemble it onsite
by hand in minutes.”
For easy movement of
the modular box in a trench, a wheel kit assembly can be added to
most models in this line. “The wheel assembly rides on the
surface, holding the shoring slightly above the bottom of the trench
so that one person can move it by hand in the trench,” Sessler
says. “This frees up your equipment for other tasks while workers
are in the trench.”
Greg Northcutt writes
frequently on construction and business issues.
GEC
- September/October 2005
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