The risk of collapse makes trenching and excavating one of the most hazardous construction operations, according to OSHA.
Other dangers faced by crews include working in close proximity to traffic, electrical hazards from contact with overhead power lines, potential underground utility leakages (particularly natural gas) and the typical risks incurred when using heavy machinery. Despite these obstacles, crews are completing jobs every day, often in impressive ways against aggressive deadlines in compact work zones.
Digging through soil shifts the natural horizontal and vertical forces of the ground. Digging in unstable soil increases the likelihood of shifting, as does excessive vibration from construction equipment or traffic around the site. Without proper support, soil from the excavated walls can move, slide, slip, and fall, causing a cave-in. When walls fail, there is generally little time for workers to react. The results can be deadly.
Protecting his employees from cave-ins is critical, which is why Dan Rothenberger, owner of Rothenberger Construction Co. in Concord, MI, turned to alternative methods on a residential job to ensure that the trench walls were properly shored throughout the project. Safety is “a huge issue,” he insists.
In business for 35 years and a member of the Michigan Infrastructure and Transportation Association and the National Utility Contractors Association, the company has 35 employees performing “100% municipal work, mostly water and sewer and road building.” Specializing in underground utility installation and boring, Rothenberger says they work with any size pipe, half-inch and up.
Last year Rothenberger installed a new precast lift station in the Indian Lake suburb in Okemos, MI. “We had to put the lift station between two homes,” he explains. “Tight spaces are hard.” Not only did excavation have to be kept 2 feet from the property line, but the road had to remain open to traffic during construction.
To improve safety in the confined space, Rothenberger abandoned sloping techniques and traditional trench boxes and steel sheeting in favor of using a four-sided, 14-foot-by-14-foot slide rail system rented from Efficiency Production Inc., manufacturer of trench shielding and shoring. The component shoring system, made up of steel panels and posts, is installed simultaneously as the trench is excavated, by sliding the panels into integrated rails and then pushing the panels down to grade as digging progresses, keeping excavation tight with no loss of dirt.
Using the slide-rail system, three workers were able to excavate a 24-foot-deep pit in very wet, Type-C soil using a Cat 330C excavator and install the new 6-foot diameter, 23-foot-tall precast round sewer lift station in three days without obstructing the street or adjacent driveways. “The boxes speed up the work and cut manpower in half,” Rothenberger says. “Three guys can do a 30-foot lift station on their own.”
Speed and safety are two of the advantages of the system, he says. Consolidation is another. The components take up less space at the job site. The slide-rail components included a set of 14-foot-long panels, with 4-foot-tall panels stacked on 8-foot-tall panels in both the outside and inside rails of 24-foot-long corner posts.
Rothenberger has used the slide-rail system many times and says he bids projects with spatial constraints because he believes he has an advantage over other contractors who don’t use it. “We prefer larger, deeper, wet sewer projects because there’s less competition,” he says.
Wet and Wild
Rothenberger has plenty of experience with wet jobs. In 2002, the Michigan contractor took on a developer’s $6.5 million sewer project in very poor ground conditions. “We had to lay 100,000 feet of pipe 5 to 32 feet deep in soil that was mostly silt and clay,” he explains. “When it’s porous, it holds water.” Water also adds weight to soil and reduces its stability, while increasing the pressure on trench boxes and other protective systems in the ground.
With the help of a 385 Cat excavator and a 345 Cat to backfill, crews installed three big lift stations 40 feet deep, using a three- to four-tier sewer box system. “The large box was 10 feet wide; we put another box inside.”
Similarly, on a 2006 job in Livingston County, MI, Rothenberger used a 385 Cat excavator to dewater trenches only 10 feet off the Grand River so crews could install a lift station 36 feet down. To do so, they rented a sheeting-shoring system with trench boxes. “We’ve owned some boxes for 10 years, but they take a beating-and they’re not cheap,” he says. “Four boxes cost $80,000. We rent shoring systems because it’s too costly to buy. We only put in a couple lift stations each year, so we don’t use it that much.”
Rothenberger isn’t the only contractor accustomed to working in poor, often wet, soil. WW Clyde of Springville, UT, battled layers of clay soil over submerged silty sands while laying 500 feet of 66-inch concrete storm sewer pipe at the Salt Lake City International Airport, located in the low-lying ground at the south end of the Great Salt Lake. Before it could be installed, the area had to be drained.
Many dewatering systems fail because silt and clay can clog the flow. Some contractors use area wells to draw off water and reduce pressure on the protective system; however, the more silt in the soil, the smaller the area affected by a well.
Glade Larson, project superintendent, chose a 12-foot-high and 20-foot-long trench shield with “mini jumbo” high clearance spreaders from Trench Shoring Services. The mid-range high arch spreader allowed 103-inch pipe clearance without adding significant weight to the trench shield. The excavation reached depths of 16 feet.
Specifications called for backfill with flowable fill to a point 6 inches above the pipe. Initially, the flowable fill was poured in the trench after the trench shield advanced, often requiring re-excavation of loose material that fell into the pit as the shield system was dragged forward to lay more pipe.
To contain costs, Larson stabilized the excavation sidewalls long enough to form and place the flowable fill without slowing down pipe installation by attaching a “tag box” behind the 12-foot-by-20-foot lead trench box. The lead box and tag box were cabled together and pulled with the Cat 365 excavator as a single unit through the heavy, wet soils. This increased productivity by eliminating flowable fill use by half and doubling pipe production.
When It Rains, It Pours
Sometimes the water comes from above. Rain shut down work on a $28 million project for the Boston Water and Sewer Commission, according to Matt Casey, project manager for D’Allessandro Corp. in Boston, MA. The 25-year-old company was hired to separate the sewer from the drain because raw sewage was running into the harbor. “We had to drain and relocate the utilities, but it kept flooding. We were constantly pumping and dewatering.”
Casey said crews used a 12-foot-by-12-foot box culvert. “It’s a big sheeting-and-shoring job that’s larger than usual for us,” he says. Typically, D’Allessandro works on jobs ranging from $1 million to $8 million, most often civil utility projects within a 30- to 40-mile radius of the city. However, it’s not out of his range. Casey previously worked on Boston’s Big Dig, a $500 million job.
This federally mandated project involves all the utility work including clearing the drainage in the culvert. Casey estimates that 100,000 tons of contaminated soil had to be excavated and sent to landfills as far away as Canada. “It can’t be reused-it’s not structurally sound.” In its place, crews will bring in gravel.
Working in the Boston blue clay has posed somewhat of a challenge, although Casey says it’s not too bad. “At least it’s firm,” he says. “That’s better than sand and gravel!” But troublesome soil isn’t the only hurdle. Crews are working “right on the water” at Savin Hill Cove. Because that is tidal-influenced, the trench had to be drained repeatedly. “The toughest thing to deal with has been trying to tie in the culvert to the bay. It takes a lot of dewatering, and there are a lot of utility crossings and gaps.”
To accomplish the goal and meet the mandated deadline of June 2009, D’Allessandro is using a modified shoring system instead of traditional sheeting. The company rented a hydraulic system from United Rentals on a monthly basis. “We have a lot of trench boxes,” Casey explains, “but we rent specialized shoring. This is specialized for deep pipe work that’s going to be open. We have to relocate 30-inch and 20-inch gas lines. There are a lot of facets to this job.”
Working between the road and the water means space is a big concern. Crews are also storing products and materials in the work space. In addition, they have to keep the road open because it’s a major traffic route to the city of Dorchester, MA. It also provides access to the University of Massachusetts and a nearby high school, so there’s considerable pedestrian traffic to watch out for.
As of mid-December 2008, Casey reports that the work is 70% complete and going well.
Between a Rock and a Hard Place
When Kinsel Industries, part of Insitu-Form, won a five-year, $160 million contract with JEA, the Jacksonville, FL, utility authority. The project required bursting and replacing sewer pipe at a rate of over 45,000 per week, and the company quickly realized that hydraulic shores would slow productivity and increase costs. Work was speeded by using ultraSHORE aluminum trench boxes, rented from Trench Shoring Services. The boxes were suited to trench depths of less than 12 feet in the sandy Florida Soil and tight conditions in the urban setting. Mini-excavators were brought in to place the shields. Carlos Zambrano, one of the foremen on the project, appreciated the quick service and variety of equipment, including steel trench boxes and road plates, provided by TSS to meet ever-changing ground conditions.
In urban settings close to buildings or other infrastructure, liability issues equal safety concerns. Eddie Conrad, owner of Conrad Construction Co. in Nashville, TN, used a Vermeer trencher to safely cut a ditch up to 22 feet deep in the vicinity of high-pressure gas mains with a Vermeer trencher. Working for various municipalities, Conrad has put in water and sewer lines for the cities of Lebanon and Hendersonville. He specializes in building roads and infrastructure-primarily water and sewer lines. “Safety is 90% of getting the job done,” he says.
Trenching through rock is often safer than blasting and offers additional benefits, such as reduced liability and insurance costs, low environmental impact, and increased speed and efficiency. Dave Rice, owner of Las Vegas-based Rice Construction Co. Inc., says production increased by nearly five times when he replaced a 60-ton, 400-horsepower trencher with a Vermeer T1255 Commander. The 90-ton unit puts out an additional 200 horsepower, which Rice says was needed to cut through stone and caliche in the residential area of south Henderson. Rice’s crew installed 10-inch water main approximately 13 feet deep throughout the 623-acre development at McDonald Ranch called the Canyons. His previous trencher got 30-40 feet per day, but he reports getting 10 times that amount with the T1255.
Shoring, Texas-Style
When merely renting the equipment isn’t enough, contractors in Texas turn to National Trench Safety LLC (NTS). “Contractors give us the scope of the work and a core sample so we can analyze the ground conditions and do site-specific engineering to give them the best scenario,” explains Robert Stevenson, national manager of engineered projects for NTS. “The contractors bid the jobs, but we work as a kind of partner with them on the job. We go to the job site to make sure the contractor knows how to do the shoring for the situation.” Number one, he says, is protection for the workers and meeting OSHA requirements. Beyond that, NTS concentrates on shoring up the area, paying attention to crossing utilities, live lines, and proximity to buildings and streets.
Based in Houston, NTS is five years old, with nine locations in Texas, Louisiana, Arkansas, Oklahoma, Arizona, Utah, Colorado, and California, but the amount of collective experience in the company dates back much further. The core group of employees has been with the company through various iterations, from the initial Plank Co. through a buyout by NES Trench Shoring and a subsequent purchase by United Rentals before NTS was formed.
That experience in assessing conditions has been beneficial on many jobs, such as one for San Antonio Water Systems. Crews had to build a 20-foot-by-20-foot pit to install a gravity-fed lift station to help regulate water drainage. Because of bad ground conditions that included groundwater and loose sand, the pit was 52 feet deep.
NTS supplied a triple slide-rail shoring system to accommodate the tight placement next to buildings. “We had shoring going down and coming up,” Stevenson relates. He says the slide-rail system helped stabilize the area because as crews dug from inside, they were simultaneously “pushing down and adding to it-they were constantly shoring up.” In addition, the system saved on backfill material because it didn’t lose any dirt in the process.
As bad as those soil conditions were, Stevenson recalls another job along the Gulf Coast with even worse ground conditions. “It was extremely bad,” he remembers. “We had groundwater at 3 or 4 feet and just bad ground.” In addition, the job site, which was inside a refinery, was next to a pipe rack, vessels, and live lines.
“It was a unique job,” he says. “There was no slough allowed.” The job entailed laying 300 linear feet of new oily water lines to separate oil and water. The lines take rainwater to sumps, where filtration systems separate the oil and water. Because of the ground conditions, NTS again chose a slide-rail system, running it 14-17 feet deep.
Because every job is different, though, slide-rail systems are not always the best choice. Stevenson refers to a job inside a Houston plant where the contractor chose beams and plates because of the width and length of the pit: 80 feet by 100 feet wide and 22 feet deep. The vertical beams and plates, with a hydraulic brace, were a better option near live lines and large oil storage tanks because they were going to be in the ground a long time, he adds.
Slide-rail systems are good for tight confinement situations, Stevenson believes. “You dig and push the system down as you go.” For the job on the coast, he says, the panels were modified to make them stronger than a trench box would have been in order to handle the load pressure from the ground water. “If ground conditions are good, you can use a trench box up to 25 feet deep, although you may never get it back out. With a lot of today’s infrastructure jobs, bad ground conditions mean we’re going deeper and larger. That calls for new innovative technology, like slide rails.” He admits that slide rail systems have been around a while, but says improvements are continuously being made to increase the strength and stability. NTS prefers SBH slide rails, made in Germany. Stevenson says their posts are heavier and sturdier and are the best they’ve found.
Safety First
Whatever method of shoring is chosen, safety is always the first concern. Keith Lamberson, president of Trench Shoring Services (TSS) in Atlanta, GA, emphasizes the importance of training everyone on the job site. “Everyone on the ground should be familiar with the OSHA standard for excavations,” he insists. “The single biggest thing that OSHA calls for is the position of a competent person. The key to all of it starts with training.” He refers to resources such as the local associations of the Associated General Contractors of America and the National Utility Contractors Association, other industry associations, shoring vendors and even local colleges.
“When we teach our classes,” Lamberson continues, “we review motivation for compliance.” He realizes that the cost of classes and downtime to attend them can deter contractors from enrolling in safety courses, but emphasizes the reasons they’re so important. “The first is humanitarian: No one likes a funeral. The second is financial: avoiding legal fines and penalties.” The third, he says, is productivity. When contractors learn to use narrow trench technology and proper protective systems safely, productivity and efficiency can increase.
OSHA requires that every employee be trained to recognize workplace hazards. “With the variety of products and the growth in vendors and product availability, there is no excuse for noncompliance,” Lamberson says. “The industry traditionally looked at shoring as two panels supported by spreader bars. In fact, it is much more. It is the proper application of products to provide one-, two-, three-, and four-sided protective systems.”
