The heavy groundwater was so bad that large sinkholes were developing at the site of a manhole junction box breach 30 ft. (9.1 m) below ground in a busy west Houston commercial area.
Everyone knows Texas is big. It’s famous for its size. When people think about the Lone Star State many think of wide stretches of arid land where cowboys rustled their cattle past cactus and tumbleweed. In parts of Texas, this description still holds true (except maybe the tumbleweed).
What may be less glamourized, however, are the wet and rainy areas of Texas where running water lies just below the ground and can cause major damage to underground utilities and infrastructure.
The city of Houston’s Public Works and Engineering Department (HPWED) recently faced the wrath of the water gods when they experienced excess inflows at the city treatment plant. Video inspections of several trunk lines in the area revealed a large crack in the junction box at the base of a manhole in a busy commercial area of west Houston.
The heavy groundwater was making its way into the 42 in. (106.7 cm) diameter sanitary sewer line and filling up capacity at the treatment plant. The running water problem, exacerbated by the above average rainfall, was so bad that large sinkholes were developing at the site of the break.
Houston’s PWED needed to fix the problem fast. But because of the location of the break, in a busy commercial area across from a shopping plaza, restaurants, and gas stations, the HPWED needed to fix the problem while keeping everyone — repair crews, motorists, and pedestrians — safe throughout the duration of the excavation project.
for Houston Contractor
Taking on this immensely challenging project was T Construction Company (T-Con), based in Houston. The first challenge they addressed was selecting an appropriate shoring system for an excavation that would need to be very deep (31 ft. [9.4 m]), but cut in a very tight spot between the median of the north and south bound lanes of S. Dairy Ashford Road, just south of the intersection with Bissonnet Street.
For assistance with developing an appropriate shoring plan, T-Con turned to the shoring specialists at Professional Shoring and Supply’s Houston branch.
“There was so little room in that median, with access for an excavator and other equipment from just two sides,” said Steve Hanley, Professional Shoring and Supply’s Houston branch manager. “Because of the extremely deep grade, not to mention the water problem, traditional trench boxes were immediately ruled out as an option,” Hanley said. “And there really wasn’t room to bring in equipment to vibrate or drive tight-sheeting either.”
Solutions Found With
Innovative Shoring System
T-Con needed a non-traditional shoring solution, one that could reach the deep grade; could be installed with very little access; and be strong enough to withstand the pressures of the heavily saturated wet soil. Fortunately, Hanley had the perfect solution for T-Con; a slide rail shoring system from Efficiency Production.
Efficiency’s Universal slide rail is a component shoring system comprised of steel panels (similar to trench shield sidewalls) and vertical steel posts. The highly versatile system can be used in a variety of configurations. Efficiency’s slide rail system can be configured into small four-sided pits; an obstruction-free ClearSpa configuration; or in a Multi-Bay configuration to install large tanks and structures, or lengths of pipe more than 40 ft. (12.3 m).
“We mobilized a two-bay, four-sided Multi-Bay configured slide rail system to shore an excavation with dimensions of 35 feet (10.7 m) long, 16 feet (4.9 m) wide, and 31 feet (9.4 m) deep,” explained Hanley. The bottom of the cracked junction box was 28 ft. (8.5 m) deep below the surface, and T-Con needed to cut 3 ft. (.9 m) further down for a pour-in-place concrete foundation.
Slide rail is installed by sliding the panels into integrated rails on the posts, and then pushing the panels and posts incrementally down to grade as the pit is dug; a process commonly referred to as a “dig and push” shoring system.
Using a 200 ton (181.4 t) crane truck and a Komatsu PC400LC excavator, T-Con began their excavation by installing the first slide rail bay over the existing manhole. However, just 12 ft. (3.7 m) into their dig, the contractor encountered flowing sand and water that began pouring into the excavation.
Water Problem Overcome With Ingenuity From Contractor and Equipment Supplier
Well points had already been installed and were operational, with the objective to bring the water level down to below where T-Con had to perform its work of replacing the cracked manhole.
Unfortunately, the well points were overwhelmed by the amount of water in the dirt, and the excavation had to be paused so additional well points could be installed by a sub-contractor. Results were mixed.
“The area was not dry,” said Hanley, “but the Efficiency slide rail system held up perfectly. It did not bend or strain under the additional loads of the water pressure, and the contractor was able to reach grade.”
T-Con Project Manager, Yonin Villares, said, “Our company normally preforms smaller projects for the city and we had no experience with installing slide rail this deep,” he said. “Having the Efficiency Installer on site to guide our crew at the beginning of the project was instrumental in getting [the slide rail] started down correctly under adverse condition. [Efficiency Production’s Special Operations Shoring Division Specialist] Tim Hurst was there for the first two days and then Professional Shoring’s local installer, Mark Taylor, stayed with us through the rest of the process.”
The HPWED inspectors were similarly impressed with the slide rail system, said Sam Valdez, city inspector. “I have seen other slide rail systems that would have bowed in under these conditions, but the Efficiency system has not yielded at all or stuck in any position.”
Once down to depth, a third layer of well points were installed inside the shoring system to remove ground water close to the work area. T-Con formed and poured their foundation and had enough time to get it to cure before the ground water again began flowing into the excavation. Construction of the new manhole and sealing of the lines proceeded normally from there on, but with one change of plans.
One More Challenge Presented, One More Problem Solved
“The original plan was to expose the cracked junction box as well as 20 feet (6.1 m) of the 42 inches (106.7 cm) concrete line feeding into it,” said Hanley. “Because of the extraordinary ground water issue, the repair was completed on the manhole only, and the other section of pipe was left as it was. This really wasn’t as big a deal for the contractor as it was for us, because we had designed the slide rail system for the extra area; a two-bay system which turned out to be only a one-bay system.”
This problem, however, was easily overcome due to the flexibility of Efficiency’s slide rail system. Instead of pulling out the linear posts and parallel beams in order for corner posts to be installed; T-Con instead pushed slide rail panels — which were already onsite — down against the linear posts and parallel beams.
Some stab sheeting was also needed near the bottom of the pit, but the result was a safely shored excavation that did not require removing any components of the shoring system, or needing to bring in different equipment which could have delayed the project.
“The slide rail was awesome,” said Villares. “Without the system, we wouldn’t have been able to get down [to grade] as clean and as quick as we did.”
Fall Protection Accounted for With Integrated Barrier Posts and Guard Rail
With an excavation this deep, fall protection also was a primary concern. There again, Efficiency’s innovative slide rail system came with an integrated solution in the form of barrier posts and guard rails that are specifically designed to attach to the top of the slide rail panels, creating an easy, OSHA-compliant fall protection system.
The $360,000 utility repair project was started and completed in September 2012.
T-Construction Company was founded four years ago by Juan Ramon Torres. A Minority Business Enterprise, T-Con specializes in all underground utilities including sewer, water, and storm sewer. Their excavation capabilities include open cut, pipe bursting, directional drilling, and now slide rail.
With offices in Houston, Texas, and Orlando, Tampa, and Jacksonville Fla., Professional Shoring and Supply is an official Efficiency Production, Inc., trench shielding and shoring equipment distributor.
Efficiency Production, “America’s Trench Box Builder,” provides the widest selection of standard and custom trench shielding and shoring systems.
Efficiency’s versatile products are designed specifically for safe and cost effective installation of utility systems and infrastructure improvements. All products are P.E. certified to meet OSHA standards.
For more information, call, 713/671-2551 or visit www.professionalshoring.com.