Trench Safety: Adapting to Conditions
You can run into any number of hardships with trenching projects–steep grades, groundwater, or unstable soil, to name just a few. But simply put, it's not that difficult to practice proper trench safety.
Some contractors don't use safe trenching procedures despite the fact that it's not tough to do. Not only do they endanger their employees, they risk high fines from OSHA. "I know of one contractor who got fined in the $70,000 neighborhood because he was a repeat offender," relates Tim Scully, president of the Underground Contractors Association of Illinois. "All it would have taken to comply with OSHA regulations is a trench box or sloping trench walls."
And OSHA is in no mood for leniency these days. "In most cases, if you don't have a protective system, OSHA will probably issue a citation for a willful violation, and the fines can range up to $70,000," points out George Kennedy, vice president of safety for the National Utility Contractors Association (NUCA). And, he adds, the states of Arizona and Michigan are seeking to apply criminal penalties to serious violations of trench safety regulations.
OSHA's rules for trench safety are relatively straightforward. You can find a complete listing of them at www.osha.gov (look under C–Construction Standards, 1926 Subpart P–Excavations). The following are a few of the basics:
- The contractor must designate a competent person to assess the excavation and determine that it is safe for project personnel to enter and work. That person must have the authority to tell workers to get out of the trench if unsafe conditions develop.
- In most states the contractor must provide a protective system, such as a trench shield, for a trench that is more than 5 ft. deep. In some states the limit is 4 ft.
- Prior to digging, the contractor shall locate and identify all underground utilities, such as sewer, telephone, fuel, electric, and water lines, that might be encountered during excavation.
- The bottom of the shielding system cannot be positioned more than 2 ft. above the bottom of the excavation.
- The removed soil shall not be stockpiled closer than 2 ft. from the excavation's edge.
- No workers shall enter or work in excavations where standing water is visible, unless adequate protection is used.
Dewatering in Oak Lawn
|The excavator on the Oak Lawn project pulls ahead stacked boxes. |
|Using a Komatsu excavator, Glenbrook places nine 20-ft. lengths of pipe per day on the Oak lawn project.|
In fact, groundwater and running sand are the main problems posed by an 8,000-ft.-long water-main project for Oak Lawn, IL, says Terry Barnett. He is president of the project's excavation contractor, Glenbrook Excavating & Concrete Inc. Last spring, Glenbrook began digging a 12- to 17-ft.-deep trench and placing a 54-in. water main on the project.
To cope with the groundwater and help make the project safe, Glenbrook is digging dewatering wells. At locations 10-15 ft. to the side of where the trench will pass, the contractor digs a small hole down about 18 ft., places a 12-in.-diameter slotted pipe in the hole, and backfills it with 0.75-in. washed rock. "Then we put a pump inside the pipe and begin dewatering," explains Barnett. "We have put in five wells; we leave them in place until we pass them with the pipe-laying operation."
To ensure worker safety in the trench, Glenbrook is using two stacked trench boxes. The bottom unit is an 8-ft.-high x 24-ft.-long Pro-Tec box; attached on top is a 4-ft.-high x 24-ft.-long trench box from Efficiency Production Inc. "That gives us 12 feet of box," notes Barnett.
Because the trench runs down the middle of a residential street, Glenbrook has to work around any number of utilities. "We've got gas lines, water, power–you name it, we've got it," says Barnett. To deal with the utilities, "We can take the 4-foot box off, duck under some utilities, and still keep a safe trench."
Digging with a Komatsu PC600, purchased new in 2000, the contractor places some 180 ft. of pipe per 10-hour shift. That means laying nine 20-ft. lengths of prestressed concrete cylinder pipe per day. The Komatsu is sized well for the job, Barnett remarks. "A big part of success on this project is having an excavator that's big enough to pull the double-stacked trench box. Lifting a 22,000-pound piece of pipe is nothing compared to the force it takes to pull those boxes in bad ground.
"Another big key to being safe is that our crews have worked together for years," he observes. "Each man knows what the other is thinking. So they work as a unit, and they're efficient and safe at the same time." In bad ground, he says it's important to have a good safe trench. "That way you can set pipe as quickly as possible and work yourself out of the bad ground as quickly as possible."
As an added safety measure, Glenbrook sometimes uses large steel plates that are inserted between the trench wall and the trench box. "We'll drop the plates alongside the box to increase the protection area," says Barnett. "OSHA accepts the plates. They've inspected us and never said anything about the plates. We use them if we're working especially close to traffic or if the semi-trucks have to get close to the hole. We'll use plates to supplement the trench boxes but not to substitute for them."
Safety at Boston's "Big Dig"
|Speed Shore's Waler System provides sidewall stabilization for formwork installation.|
|Double Wall Trench Shields (stacked) used on large-diamenter pipe installation|
A number of safety challenges were presented in building the Leverett Circle highway connectors, a Central Artery/Tunnel project in Boston, MA. As the prime contractor on the project, Modern Continental Construction was faced with building two below-grade highway connectors under 10 Amtrak train tracks as the tracks entered Boston's North Station. For the 400-ft.-long twin connectors discussed here, the contractor used the "roof first" construction method.
During construction, Modern Continental was allowed to shut down only two train tracks at once–the other eight had to remain open to trains. So construction proceeded sequentially across the tracks, with only one pair of tracks closed at a time. The first step was to place two pairs of concrete walls using the slurry wall method. A clamshell excavator dug out the walls while heavy slurry filled the excavation and maintained the trench walls. The walls extended 60-100 ft. deep–down to bedrock. Once the slurry wall excavation was complete, the contractor placed soldier piles, spaced at 4-ft. intervals, into the excavation. Concrete was then tremied into the excavation from the bottom up, and the displaced slurry was reclaimed for future use.
The walls of each pair were spread about 35 ft. apart; the entire "roof first" section of twin tunnels was about 400 ft. long, reports Ed LaVallee, area safety manager for Modern Continental.
Next the contractor excavated a shallow section–about 8 ft. deep–between each pair of concrete walls. That made space for construction of the roof. It was built by first placing steel girders that spanned the area between the soldier piles in the concrete walls. With the addition of steel pan decking, reinforcing steel, and a concrete topping, the roof was complete. The roof was then backfilled and the tracks were restored to service.
That made the walls and roof ready for excavation below. To permit access to the excavation, Modern Continental left three large openings in the roof, called "glory holes." Following the initial excavation of the glory-hole area, a Gradall 5200 excavator was lowered into one glory hole and began digging out the material below the roof and stockpiling it at the base of the glory hole. A clamshell excavator then removed the stockpiled material from the glory hole.
The first step in excavation, recalls LaVallee, was to take the excavation floor down to about 15 ft. below the roof. The contractor was careful to maintain proper sloping within the excavation, because survey crews and support personnel had to work in the area. At a point 15 ft. below the roof, the contractor installed wide steel walers that ran along the concrete walls. For additional wall support, 48-in.-diameter pipe struts were placed to span the area between the walers on each wall. Spaced at 20-ft. intervals, the big pipe struts acted as bracing to support the walls. "That was the support of excavation as designed," LaVallee says.
Again, the excavation process was repeated; the contractor took the tunnel floor down another 20-25 ft. below the pipe struts. Modern Continental's last major steps in the connector construction were to pour 8- to 16-ft.-thick concrete floors and to close the glory holes.
Such an undertaking required a number of safety measures, including:
- All personnel involved underwent comprehensive safety training.
- Lighting, communication equipment, and ventilation fans were mounted under the roof.
- The wall-mounted walers were used as walkways. The contractor installed handrails along the 3-ft.-wide walers.
- Ladders and/or stair towers were positioned at each glory hole. The holes were located so that no worker was ever more than 100 ft. away from an exit. "If an evacuation was necessary, we had a designated muster point above the ground," says LaVallee.
- A five-gas meter was used to monitor air quality for oxygen content; gases at the lower explosive limit, such as methane; carbon monoxide; nitrogen dioxide; and carbon dioxide.
- Electric air horns were mounted below the roof, with toggle switches at the top and bottom of each stairway. One blast from the air horn signaled that a load was suspended overhead in the glory hole; three blasts were the signal to evacuate the excavation.
- Each foreman was responsible for knowing the locations of the workers on his crew and for taking a head count at the muster point following an evacuation.
- With an electronic accountability system, each employee carried a photo ID badge with a chip embedded in it. Card readers located at every access/egress point reported to a central monitoring station every time an employee entered or exited the excavation. The name, time, and location of access or egress were logged.
"If I entered the excavation at one point and exited down the line at some other point, the electronic system would be able to track when and where I entered and exited," explains LaVallee. "Fortunately, through the successful implementation of our safety program, an evacuation was not necessary, but we were prepared nonetheless." The Leverett Circle connectors were completed safely and on time.
Rock Trenching on the West Coast
|Using the Multi-Shore system in a deep excavation project|
Steep grades and hard rock confronted PCL Civil Constructors on a 13,000-ft. water pipeline project in the Rancho Santa Fe area of north San Diego County, CA. Most of the pipe was 48 in. in diameter; the job also had some 36-in. pipe and one 700-ft. section of 30-in. pipe, says Kevin Joe, PCL's superintendent on the job. Construction began in June 2000 and was completed by August 2001.
"It was all uphill and downhill," describes Joe. "We had six severe downhill slopes in excess of 40%. And we were limited to a 100-foot width of right of way, so just getting physical access to the job on those slopes presented quite a challenge."
Blasting the rock along the trench alignment came first. That job went to Baxter Drilling, a subcontractor to PCL. Baxter blasted the granitic-type rock to depths of 12-18 ft. across a width of 8-10 ft. "Then we moved in with our Hitachi EX-750 to excavate the trench," says Joe.
PCL could not work from uphill downward to place the pipe; the force of gravity would pull apart the joints. And the contractor couldn't work from downhill up by excavating and placing pipe at the same time because loose rock would roll downhill onto the workers setting pipe. The answer: excavate the trench working from uphill downward for the entire slope, then place pipe working the other way–from downhill upward.
"We excavated to 1 foot below the design invert elevation," recalls Joe. The invert elevation was 10-16 ft. below grade. There was no need for trench boxes. "We were able to scale it back to competent rock on both sides." The contractor sloped the trench sides back on a 0.5:1 ratio, leaving a flat, 6- to 8-ft.-wide trench floor. "In some areas it was dirt, and we sloped it back on a one-to-one ratio."
At 40-ft. intervals, the Hitachi dug out holes for the pipe's bell joints. "We had to backfill to within 1 foot of the top of pipe with imported sand," says Joe. "For the rest of the backfill we crushed the shot rock to 6-inch-minus."
To safely haul the 40-ft.-long pipe joints, PCL used a 35-ton Volvo rock truck pulling a specially made trailer. According to Joe, the homemade trailer basically consisted of a couple of truck axles and a long H-beam welded together. The contractor would load the rock truck box with bedding sand and place one 40-ft. pipe joint on the trailer, then head for the job site. "A regular highway truck couldn't handle the steep slopes," remarks Joe.
Whether you're trenching in rock or digging in wet, sandy soil, achieving a safe job site takes careful planning, say the contractors interviewed for this article. And careful planning not only will produce a safe job site–it will bring more money to your bottom line.
Author's Bio: Daniel C. Brown writes on safety and technology in the construction industry.