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Next time, before you
descend into a trench or another man-made cavity in the earth, you
might want to consider the following numbers in gauging your odds
of climbing back out unscathed should the walls cave in:
- 112: This number represents the percentage increase
in the fatality rate for excavation work compared to general construction
work, according to the United States Department of Labor's
(DOL) Occupational Safety and Health Administration (OSHA).
- 114: This is the weight, in pounds, of a typical cubic
foot of dirt. That works out to more than 1.5 tons/yd.3
and is similar to the weight of a pickup truck. A load like that
lying on top of you is more than enough to prevent your lungs
from expanding.
- 3: This is the number of minutes before you will
suffocate if buried by dirt. In fact, very heavy soils can crush
your body in a matter of seconds.
Such numbers help explain
why excavating is one of the most hazardous construction operations.
According to one estimate, the number of fatalities from trench
cave-ins in the US could be as high as about one a day, in addition
to some 1,000–4,000 people injured each year in these accidents.
A trench is unstable
from the moment you start digging. Trench walls can collapse quickly,
without warning. The risks of cave-ins increase as the trench deepens
and the soil gets wetter or looser.
What's more, the
likelihood of a second cave-in increases about 500% after a wall
collapses. The threats include crushing, suffocation, loss of blood
circulation, the collapse of nearby structures, and being struck
by dirt, rocks, and other material that can fall or roll into a
trench. Other hazards can be just as lethal to workers trapped in
trenches. They range from fire and drowning to toxic fumes from
nearby gasoline-driven equipment, dirt contaminated by leaking lines
or storage tanks, and electrocution or explosions when a worker
or equipment contacts underground utilities.
Beyond the Numbers
These numbers, however,
don't tell the whole story. A variety of equipment and techniques
are available to protect life and limb when working in trenches.
They're not only a good idea, but federal regulations (OSHA
29 CFR, Part 1926, Subpart P, July 1997) require contractors to
follow certain procedures and practices when working in trenches.
Besides, depending on your approach, these safety practices also
can improve your profit prospects.
Trench protective systems
and practices are remarkably effective when used properly. "In
my 21 years of experience, I've seen them save lives,"
says John Newquist, a DOL team leader in North Aurora, IL. "In
fact, I've never seen anyone die who has complied with trench
safety rules."
Around the country, however,
compliance with these rules is less than complete, say experts.
"Our biggest challenge
is convincing contractors to use protective systems when trenching,"
says Tom Coble, president of Coble Trench Safety in Raleigh, NC,
which sells and rents Speed Shore trench shoring equipment and protective
systems to contractors in the Mid-Atlantic market. "The larger
contractors tend to be very compliant with the regulations. But
in the case of many smaller contractors, all we can do is explain
the capabilities of our products for protecting workers against
cave-ins and hope that the contractors use them."
Industry observers attribute
the reluctance of some excavating contractors to use protective
equipment and practices to various factors, including ignorance,
apathy, and a desire to minimize construction costs.
Keith Lamberson, president
of Buford, GA–based Trench Shoring Services, a manufacturer
and renter of trench-protective equipment and systems, offers this
perspective on the economics: "Depending on type and size,
our trench boxes rent for about $35 to $70 a day. Are you willing
to risk injury or death to save that money? If that's not
convincing, consider the money you save with a trench box by not
having to slope a trench wall. You can't excavate or backfill
a sloped wall for $35 to $70 a day."
"Some contractors believe
that a cave-in won't happen to them," says Mike West, engineering
manager with Efficiency Production Inc., a manufacturer of trench
shield and shoring systems in Mason, MI. "But would these same guys
be willing to put their kids in an unprotected trench?"
"Some contractors
fail to appreciate the hazards of trenching because they themselves
haven't been hurt," adds Newquist.
He reports three 2003
cases in which inspectors from his office and a neighboring DOL
office evacuated workers from trenches just minutes before the sides
caved in.
Sometimes, Newquist points
out, the protective equipment contractors have on a site isn't
suitable for the job at hand. "For example, a contractor may
have a trenching box to protect workers when installing a manhole,
but the trenching box may be too small for the manhole. So, rather
than stop work while waiting for a bigger box or installing a different
protective system, the contractor will install the manhole without
any cave-in protection."
In 2003, it took a rescue
team more than seven hours to recover the body of man who was killed
when one side of the trench, in which he was repairing a water line
without any shoring protection, collapsed in one piece on top of
him. Earlier in the week, a trench box had been used to support
the trench walls; however, on the day of the cave-in, the box was
not in place.
Newquist reports that
in Illinois, from October 2001 through September 2002 (the latest
figures available), lack of cave-in protection ranked second on
the list of the top 10 most-cited construction-site violations and
totaled more than $350,000 in penalties. Lack of fall protection
from scaffolds, roofs, or residential construction sites was the
top violation.
"Sixteen percent
of these were repeat violations," says Newquist. "The
inspected company has been cited for the exact same violation or
hazard during the past three years."
Among the reasons given
by contractors for failing to comply with trenching and other construction-site
safety requirements, he notes these:
- "The job was
only going to take a minute."
- "We have been
doing this work for 20 years and would not expose ourselves to
hazards" (said at the time an employee was in a 10-ft.-deep
trench without cave-in protection).
- "The soil was
good."
Costly Noncompliance
A sampling of trenching-accident
investigations by OSHA reveals how trenching activities can turn
deadly when protective measures aren't in place:
- Failure to inspect
the trench. A worker installing
forms for a concrete footer in a 7.5-ft.-deep trench through loose,
sandy soil was killed when the sides caved in. The trench was
not inspected as required before the shift started work.
- Unsafe spoil-pile
placement. One employee was
killed and another suffered back injuries when the unprotected
vertical walls of an 8-ft.-deep trench collapsed as the workers
were installing a sewer pipe. A spoil pile had been placed on
top of one side of the trench. In addition, a backhoe was parked
on top of this pile. Investigators reported that the superimposed
load of the spoil pile and backhoe might have caused the wall
to collapse.
- Unsafe egress.
Two workers were laying pipe in a 15-ft.-deep trench, which was
not shored or sloped properly. While climbing the backfill—the
only way to exit the trench—one of the employees was trapped
by a small cave-in. While the second employee tried to excavate
him, a second cave-in occurred, killing the first worker and injuring
the second.
Even without accounting
for the horrific personal impacts of deaths and injuries, cave-ins
can be extremely costly. Chris Musser, vice president of Downey,
CA - based Trench Plate Rental Company, which provides shoring systems
and produces Quick-Shor trenching products, lists some of these
consequences:
- Costly repairs.
Repairing existing utility lines or infrastructure damaged by
improper trench support can be very expensive. Neighboring property
can be damaged. Improper trench support can destroy adjacent streets,
curbs, and sidewalks and undermine adjacent foundations, all of
which can lead to expensive claims.
- Higher insurance
rates. "Workman's
Compensation premiums are based on your three-year experience
modifier," Musser says. "Accidents or fatalities can
easily increase your company's insurance costs by more than
100%."
- Heavy fines and
legal expenses. OSHA fines
start at $7,000 and can escalate rapidly from there. In 2003,
for example, OSHA proposed an $83,300 penalty against a Texas
contractor for failing to protect workers from trench cave-ins
and exposure to carbon monoxide and a $99,400 penalty against
a Louisiana contractor for not protecting employees from potential
trenching and excavation hazards. In addition to legal fees, you
could even face jail time for ignoring trench safety regulations.
A Profitable Bonus
In addition to improving
safety, trench shields and shoring offer another important benefit:
increased production.
"They're
one of the few construction products that enhance both safety and
production," says West.
In fact, he reports,
when his company introduced the first commercial trench shields
in 1971, there were no federal regulations concerning trench safety.
The goal then was to improve production and reduce costs by using
narrow-trench technology to eliminate the need for sloping to prevent
cave-ins.
"By digging a trench
with straight instead of sloping sides, you excavate a lot less
dirt and reduce the amount of backfill, compaction, and restoration,"
he explains.
Protective Systems
OSHA regulations are
designed to reduce risks by requiring proper protective systems
and procedures when excavating trenches, except when made entirely
in stable rock. To prevent cave-ins of 5- to 20-ft.-deep trenches,
OSHA requires one of three protective measures: sloping and benching,
shielding, or shoring and sheeting. Responsibility for determining
which system is best for any given job falls to project planners
and the competent person
on-site. A competent
person is one who can identify existing and predictable hazards
or hazardous or dangerous working conditions and who has the authority
to take prompt corrective measures to eliminate or control them.
For excavations deeper
than 20 ft., the protective system must be designed by a registered
professional engineer.
For trenches less than
5 ft. deep, OSHA does not require a protective system, unless the
competent person sees signs of a potential cave-in. Still, as Newquist
cautions, "A wall collapse in a 4.5-foot-deep trench can still
have serious results."
Sloping and Benching
This approach protects
against cave-ins by cutting the trench walls at such an angle that
there is little chance of collapse. Termed the maximum allowable
slope, it varies with type of
soil, environmental conditions of exposure, and application of surcharge
loads. In many cases, lack of available space, such as close proximity
to utilities and traffic when trenching in a right of way, might
prevent use of this approach, notes Newquist.
"If you're
excavating an area that has been disturbed previously, such as a
right of way, the soil is likely to be classified as C," he
says. "With Type C soil, the walls of the trench must be sloped
back on each side of the excavation 1.5 feet horizontally for every
1 foot of depth."
Type of soil also determines
the horizontal-to-vertical ratio of benched sides. Generally the
bottom vertical height of the trench must be no more than 4 ft.
for the first bench. Subsequent benches may be as high as 4-ft.
vertical in Type A soil and no higher than 4 ft. in Type B soil
to a total trench depth of 20 ft.
Shoring
Shoring systems, whether
timber, mechanical, or hydraulic, prevent movement of soil, underground
utilities, roadways, and foundations by supporting the face of the
trench. These systems include sheeting, which is driven into the
ground and usually is used for long-term protection, and aluminum
hydraulic systems, which are lightweight, reusable, and installed
and removed from above the ground and pneumatic shoring.
Shielding
Unlike shoring, a shield
doesn't prevent a cave-in. It instead protects workers inside
the structure on which the trench walls collapse. It can be a permanent
structure or, in the case of a trench box, a portable one.
"A trench box is
an excellent choice when placing continuous installations like pipe
laying," says Newquist. "You place the box in the trench
and drag it along as the work progresses."
Although they normally
are used in open areas, trench boxes also can be used in combination
with sloping and benching.
When using a trench box,
Newquist suggests keeping workers out of the box and aboveground
when moving the shield to prevent personnel from getting caught
between the box and fixed objects. The excavated area between the
outside of the trench box and the face of the trench should be as
small as possible. The top of the shield should extend at least
18 in. above the level of any material that could fall or roll into
the trench. Stack only shields designed for that purpose. Don't
stack shields from different manufacturers, since they might not
be compatible. After positioning the box, backfill the space between
the box and the trench walls with excavated material. Otherwise
a cave-in could push the box sideways, creating a crushing hazard.
"It's important
to remember that workers are protected from cave-ins by shielding
and shoring only if they stay within the confines of the system,"
Newquist points out.
OSHA regulations also
address placement of spoil. To prevent temporary spoil from falling
on employees in a trench, it must be placed no closer than 2 ft.
from the surface edge of the excavation, as measured from the nearest
base of the spoil to the cut. Spoil should be placed so it channels
stormwater and other runoff away from the excavation. Permanent
spoil should be placed some distance from the excavation. If the
spoil isn't placed far enough away, it could cause the excavation
to be out of compliance with the horizontal-to-vertical ratio required
for a particular trench.
A Big Selection
At one time, sloping
and timber shoring were the only choices for protecting workers
from trench cave-ins. Today manufacturers offer a wide variety of
standard and custom-made products to provide a safe working environment
for just about any trenching or excavation job—from vertical
and horizontal shoring to manhole boxes and tank pits.
"Pre-engineered
systems allow you to place shoring and shielding products into an
excavation before workers enter a trench," says Musser. "These
modular shoring and shielding systems eliminate the guesswork and
cost of custom engineering and put the savings in your pocket. They
make installation fast and easy, without cutting, welding, nailing,
or other expensive and time-consuming methods."
Shoring
Aluminum hydraulic shoring
offers a fast, simple, economical and safe way to brace vertical
trench faces. Designed for use in more stable soils, they're
available in up to 24-ft. lengths and are available in sizes to
fit up to 12-ft.-wide trenches. They are installed by hand aboveground
and can be stacked for different trench depths.
Speed Shore's Shoring
Shields combine the benefits of aluminum hydraulic shoring with
the solid-wall security of a static shield. Built with high-strength
aluminum alloys, they are designed for utility maintenance and repair,
cable splices, pipeline bellholes, vault placements, trenchless-technology
pits, and light utility installations. The company reports that
the solid-sheeted exterior provides optimum personnel protection
while the lightweight design allows it to be handled with a rubber-tire
backhoe.
Efficiency Production's
shoring products include horizontal walers, which can be used in
deeper excavations than vertical shores can and which provide a
greater working area than vertical shores do. The company also produces
a shoring system that features a clear span of 50 x 50 ft. and a
30-ft. depth.
Slide-rail systems feature
steel panels that slide into tracked rails as dirt is excavated
and can be installed by a small crew with an excavator. They offer
an easier, faster, and less-expensive alternative to conventional
tight-sheeting/piling, especially in poor soil conditions and on
sites adjacent to structures or utilities. They can be configured
for linear, square, or rectangular excavations to depths of 30 ft.
or more for such uses as pipelines, pits, retaining walls, and bridge
abutments.
Originally developed
in Europe years ago, slide-rail systems are becoming more popular
in the United States. Those made by American manufacturers feature
heavier-duty components than European systems do, notes West. "The
use of slide rail systems has really taken off in the US in the
last 5 to 10 years, as the need to repair infrastructure in tight
urban sites has increased. Contractors are finding that they can
install our systems for about half the price of conventional steel
sheeting."
Shielding
Trench boxes, installed
aboveground, are available in both steel—installed using a
crane or an excavator—and lightweight aluminum versions, which
can be handled with a rubber-tire backhoe or a light excavator.
Depending on manufacturer and model, features might include replaceable
push blocks, double- or single-wall construction, cutouts for incoming
utilities, and high-clearance spreaders for greater pipe clearance.
The selection includes
Trench Shoring Services' steel trench boxes, which vary in
size from a single-wall, 1,500-lb., 2-ft.-high x 12-ft.-long model
to a double-wall, high-arch system that measures 12 ft. high x 16
ft. long and weighs 24,950 lb.
Quick-Shor's Quick-Box,
an aluminum hydraulic trench box, can be moved and installed with
a rubber-tire backhoe. Designed for smaller projects, it can shore
up trench walls to a depth of 20 ft. It includes a wheel kit to
support moving operations. The width adjusts hydraulically, and
cylinders and return sleeves are protected by steel oversleeves.
The stackable boxes can be used in three- or four-sided configurations
and include doors to accommodate crossing utilities.
Other
Equipment
Other choices include
Trench Shoring Services' multisided cofferdams and manhole
boxes and doghouse shoring panels to allow for pipe clearances on
one to four sides, including three-way utility junctions and jumbo
spreaders for connecting steel panels while allowing for rebar clearance,
or to create pipe clearances in the middle of stacked panels.
Regardless of features,
OSHA requires that all protective systems have a manufacturer's
tabulated data sheet. This must include tables, charts, and other
information, approved by a registered professional engineer and
used to design and construct the protective system.
Product Selection
Tips
Selecting the right equipment
for a given job is a matter of matching product capabilities and
features to site conditions and job requirements. For example, these
systems offer such choices as a variety of spreader lengths to accommodate
different trench widths, spreader height adapters for working with
different-diameter pipes, and end panels. That's one reason
why more and more excavating contractors are renting rather than
buying the equipment.
"Renting allows
you to choose the most precise solution to a shield or shoring problem,"
says Lamberson. "The trench box you bought with 4-foot spreaders
may be just fine for laying a 24-inch sewer line. But if your next
job involves installing an 8-inch water line, you'll have
to overexcavate the trench to use that box. Renting offers a more
affordable way to get the equipment that best fits the job."
"If you own a shielding
or shoring system that you're not using, it becomes a dead
asset," adds Coble. "Also, if you use it infrequently,
you may lose track of the paperwork required by OSHA."
Still, owning trench
shoring and shielding systems can make sense for contractors who
do a lot of similar, long-term excavation projects and have the
ability to store and transport the equipment, say experts.
To meet OSHA requirements,
the products of one manufacturer are similar to those of another
in terms of engineering standards. "The differences between
one brand of shield or shoring tend to be minor," says Musser.
"But depending on the job, those small differences can make
a significant impact on productivity."
For example, he points
out, the location of lifting or pulling points on a trench box can
make a big difference if it doesn't fit your equipment or
work methods. "Not all boxes are designed to be pulled through
a trench," he explains. "Even if it is, one customer
may prefer pulling the box with a cable or chain, another may use
a custom pulling tongue, while a third may drag the box with an
excavator bucket. For most production, the box should fit the way
you move it."
Most manufacturers offer
classroom training to help employees meet OSHA's competent-person
requirements. Typically this training covers such topics as OSHA
standards, recognizing hazards and safety violations, soil classification
and selection, and correct use of protective systems.
Lamberson sees a clear
need for this and other educational efforts. "Contractors
can learn about the various protective equipment at trade shows
and demonstrations provided by manufacturers and dealers. More excavating
contractors need to make shoring and shielding a higher priority
and learn more about it."
Greg Northcutt writes
frequently on construction and business issues.
GEC
- May/June 2004
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