The classic rule-of-thumb for avoiding cold-weather damage to water pipe is "bury it deep." If water lines are located below the lowest level of frost penetration - five to six feet or more in many cold region locales - they should be safe from freezing.
With modern construction equipment, this may be simple enough. But sometimes its not an adequate solution. The main problem occurs where bedrock is close to the surface, or where the water pipe must cross some other utility line, and underlying rock makes it difficult to dig deeper or necessitates blasting.
Another problem is created by water lines that must run through environmentally sensitive areas, such as marshes. The disturbance of deep digging may be prohibited.
While some utilities have attempted shallow burial of shielded pipe, in general engineers are not yet comfortable with the idea of insulated shallow burial. They want hard data, not empirical "true experience" stories. The U.S. Army Corps of Engineers Cold Regions Research & Engineering Laboratory (CRREL) provides that data.
Advantages of Shallow Burial
CRREL researchers believe that the shallow burial technology has merit for the U.S. construction industry and the municipal governments it serves. If a sound procedure can be developed for insulating water lines to keep them from freezing, utility installations can be speeded up with great savings in time and labor costs. Shallower ditches also avoid the time and expense of shoring, which is OSHA-mandated when excavation reaches a certain depth.
CRRELs Frost-Shielding Project
The Research & Engineering Laboratory in Hanover, N.H., proposed a multi-part project to demonstrate the concept of insulating water lines to protect them from freezing.
CRRELs Frost-Shielding Project
First, an existing finite element computer program developed by CRREL and the University of New Hampshire would be optimized and tested. The FE program allows a designer to model various insulation configurations and perform "what-if" types of calculations in relation to expected temperature and soil conditions. The beauty of the program is the designers ability to change and adjust many possible parameters and see the results.
CRRELs Frost-Shielding Project
Second, an appropriate insulation shield design would be developed and installed. For the insulating material, CRREL selected extruded polystyrene, a material the lab had worked with before in a variety of cold region projects. Extruded polystyrene is tough (available in compressive strengths as high as 100 psi), lightweight, highly resistant to moisture penetration and easily cut for installation. It offers a thermal resistance value ("R" value) of 5.4 per inch of thickness at 40 degrees F.
CPAR Partners
CRREL wished to develop the project under the Corps of Engineers Civil Works Construction Productivity Advancement Research (CPAR) program. CPAR requires an industrial partner to share the costs and manage the technology transfer aspect of the plan.
CPAR Partners
For its partner, CRREL joined with the Water Works of the City of Berlin, New Hampshire, where the field study would be performed. The third partner to the equation was Owens Corning, manufacturer of Foamular? extruded polystyrene insulation.
CPAR Partners
As its part of the CPAR plan, Owens Corning donated the insulating materials and provided technical guidance in their placement. The city of Berlin Water Works provided the test sites, and excavated and laid the new pipe and insulation.
Berlins Water System
Berlin has a population of about 12,000. Situated in the White Mountains, its actually in a colder region than CRRELs Hanover, NH, headquarters. It was also an excellent test site for another reason: the city is built on bedrock, which is very close to the surface and actually breaks the surface in places.
Berlins Water System
Berlins water works company initially approached CRREL for advice on freezing problems experienced with an aging water system made up of 2 to 6 inch galvanized pipes. Since a number of EPA-mandated piping redesigns were about to be undertaken in various areas of the city, the CPAR tests could be performed with little additional construction expense.
Main Test Site
CRREL selected Labossiere Street, a dead-end hillside street, for its main test site. The dead end offered a particularly rigorous test condition as there was no through flow of water with resulting heat transfer to the soil. A new, 8-inch shielded water line was numerically designed and then constructed on Labossiere Street. It was monitored by an array of thermocouple placements for three winters.
Main Test Site
The shield design was conservatively designed for the first year with a 6-inch layer of Foamular? extruded polystyrene insulation in an inverted "U" around the 8-inch ductile iron pipe. The sides of the "U" were 2 feet high, extending even with or slightly below the bottom of the pipe, which rested at the 5 foot deep level. Total shield width was 4 feet. The Foamular was supplied as 2-inch, 4-by-8 foot boards, which were snapped or cut to size on site.
Main Test Site
The second year, thermocouples were also installed to measure temperatures in an unshielded pipe, for further confirmation of the numerical model.
Main Test Site
The third year, a second shield design was developed and installed on Wentworth Street. The design was intentionally more aggressive, this time placing the pipe at an elevation of 3.5 feet with a 4-inch shield surrounding it. This test installation was monitored by thermocouples over a winter period and again, a good correlation was established between theoretical and actual temperature data.
Main Test Site
Results indicate an even thinner shield is feasible.
Better than Digging Deep
The completed project took about 31/2 years from start to finish. Its accomplishments included a validation of the ability of CRRELs FE technology to accurately model subterranean heat flow. Designers can apply any parameters they feel comfortable with, and observe and analyze the results on the screen before committing to a specific course of action.
Better than Digging Deep
A second conclusion to be drawn from the test, is the merit of engineering a water pipe system for cost savings and ease of construction, rather than simply digging deep. Overall, studies have shown that frost shields are an effective and viable alternative where economically feasible. Using finite analysis and the extruded polystyrene insulation technology, a shield can be designed to meet specific site criteria.
