Constructing pavement that provides a smooth ride involves many factors, but the quest begins with setting string lines to guide both trimmers and paving equipment. Contracts for road pavement projects frequently include incentives/disincentives clauses for achieving smoothness within defined limits. Installing string lines properly makes possible the successful completion of the steps on the road to smooth pavement.
Depending on the paving contractor, either surveying crews are hired for layout work or in-house teams are used. Gary Gaul, executive superintendent in charge of field operations for Duit Construction, Edmond, Okla., says his company has a separate crew that does nothing but layout work. Workers set hubs only once to provide the elevations for both subgrade and top of pavement. Hubs are grade stakes used to define both elevation and position.
Duit crews use global positioning systems (GPS) to place the hubs. Gaul says that GPS is very fast and accurate for horizontal planes but not for vertical work. They use total stations and digital leveling equipment to provide elevations. GPS also doesn't work well in wooded areas or where there isn't a line of sight to the satellites.
Duit keeps its hubs far enough away from the pavement so they won't be in the way. The survey team puts either cut or fill elevation marks on the hubs to note the distances from the top of the stake to the top of the subgrade. The team first sets hubs every 50 feet, then one midway between the hubs at 25 feet. For sharp curves, hubs are set in the same manner at 12.5 feet or less if needed.
Locating pins and stringline
Stringlines are what slip-form pavers and trimmers sense. A paver has two wands to sense and control its position: one for elevation and one for alignment. The elevation-sensing wand rides against the bottom of the string while the alignment-sensing wand rides against the inside of the string. Reducing the number of times string lines are set will increase accuracy for an entire project.
Pins hold the stringline in place. They are pounded into the ground 12 inches from the hub (the hub is between the pavement and the pin). Then a horizontal bar called a “string line wand” is attached to the pin. It's located 2 feet above the surface of the subgrade (identified on the hub) with the string location directly over the hub. Workers slide the stringline into a small slot on the wand and it stays in place after it's tensioned.
Several types of lines are used: wire, cable, woven nylon, or polyethylene rope, to name a few. Gaul says that the choice of line is often influenced by what a crew is used to working with. One of the crews in his company prefers to use yellow polyethylene. Another likes aircraft cable. The polyethylene is tightened by hand, and the aircraft cable is tightened by winch. Air temperature and relative humidity variations affect the length of string lines, which can result in sagging lines between pins. For this reason the more tension applied, the truer the elevation will be. Aircraft cable is best for this because it has greater tensile strength and can be pulled tighter.
The last step involves checking the line. Gaul says that some people have a talent for this. It involves “eye-balling” the line to find and fix any variations made during all the other steps in the process. “Night smoothing” is an excellent way to do this, too. A team drives alongside the stringline on the night before paving. In their vehicle headlights, the shiny string makes it easy to locate problem areas.