WW Clyde of Springville, UT, successfully challenged the difficulties of laying 66-in. (167.6 cm) RCP at the Salt Lake City International Airport.
Just working near any lake is enough to send chills up the spine of most contractors. The Salt Lake City International Airport is located in the low-lying ground at the South end of Utah’s Great Salt Lake.
The 500 ft. (152.4 m) of 66-in. concrete storm sewer was to be laid across an area of lake that required draining prior to the installation. The excavation reached depths to 16 ft. (4.8 m) .
The soils were mixed in layers of clay over submerged silty sands.
Installation required a high spreader arch to provide adequate pipe clearance below the spreaders without adding significant weight to the trench shield. WW Clyde project superintendent Glade Larson selected a 12-ft.(3.7 m) high and 20-ft. (6.1 m) long trench shield with “mini jumbo” high clearance spreaders from Trench Shoring Services (TSS) in Salt Lake City. This mid-range, high-arch spreader allowed a 103 in. (261.6 cm) pipe clearance with any spreader width.
To add to the project difficulty, the specifications called for backfill with “flowable fill” to a point 6 in. (15.2 cm) above the pipe. This expensive backfill specification leaves little room in project budgets for waste.
Initially, the flowable fill was poured in the trench after the trench shield advanced. This often required the re-excavation of loose material that had fallen into the excavation as the shield system was moved forward to lay the next pipe.
Contractors who work in heavy, wet soils know that pulling a trench shield becomes more difficult the bigger and heavier the box. “One size fits all” just doesn’t cut it in this competitive work environment. A contractor needs the right trench box, the right size crew and the right size equipment to match the job.
While WW Clyde was meeting the projected production goals, the increased flowable fill presented the potential for serious cost overruns.
The challenge was to come up with a simple way to stabilize the excavation sidewalls long enough to both form and place the flowable fill, while at the same time not slowing down the pipe installation. Again, the company turned to its long-time technical partner, TSS, for a solution.
Bob Hackworth of TSS took one look at the capacity of the Cat 365 excavator used by WW Clyde and provided a “tag box” to hook up behind the 12 by 20-ft. (3.7 by 6.1 m) lead trench box.
Because no employees were in the “tag box,” an 8-ft. (2.4 m) high box was used to keep weight down and serve as a form for the flowable fill. The lead box and the tag box were cabled together and pulled with the Cat 365 as a single unit through the heavy soils.
Productivity increased to a point significantly greater than the bid projections and the flowable fill waste was eliminated. WW Clyde was able to cut its flowable fill consumption in half and double pipe production using the tag-box concept.
Working in the Wet Doesn’t Need to be an Expensive Lesson
At one time the really wet stuff seemed to be in the Southeast, mostly in Florida and other coastal areas.
Well, times are changing.
No, the water hasn’t moved. What has happened is that in a number of the high growth areas, the good ground has been taken. From the Salt Lake Valley to the Rio Grande basin, contractors are learning that leaving the high ground for the low ground is more difficult and more expensive than the work they did ten years ago.
Owners and developers are learning about the cost faster than the contractors.
Water adds weight to the soil. It reduces stability and radically increases the pressures on protective systems.
The key is in the dewatering of the excavation. In many regions, strategically placed area wells can suffice. This requires a soil condition in which the area of influence of the well draw is able to overlap with the nearest well. The smaller the soil particles and the greater the amount of silt, the lower the area of influence will be.
Even in situations where area wells can be of value, the placement of the wells needs to be considered in relation to the protective system that will be used.
Locating the wells to deal with the buildup of earth pressures at the weakest point of the protective system is a vital consideration. Getting rid of all the water is almost a practical impossibility, and contractors need to prepare themselves for the failure of the dewatering system. Always locate area wells at the center of the span –– the weakest point –– of a protective system rather than the corners.
If there is silt involved, one can bet that the fight becomes bigger. Silt and clays reduce the area of influence and can clog the free flow of the draw of the well in a very short period of time. More area wells will need to be added.
Well points and vacuum pumps become one of the solutions. Good dewatering can reduce shoring cost. Bad dewatering can make any shoring solution fail.
Well points in silty soils should be cased with a gravel column during the jetting process. Pumps should be benched and dug in to increase the head that they can draw. Multiple rings of headers may be required at different depths. If a contractor cuts this short, he is starting a fight he can’t win.
Shoring technology has evolved to take advantage of advanced techniques for dewatering. Pump station installations are usually the deepest part of the modern project.
Four-sided modular assemblies pioneered by Trench Shoring Services are quickly replacing the old sheet pile and slide rail systems. Each requires attention to dewatering.
(This article appears courtesy of Safety in Excavation).