Tests prove shallow burial of shielded water lines works
Cold Regions Research & Engineering Laboratory, Hanover, NH
The classic rule-of-thumb procedure 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 it's 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.
Shallow burial of lines with shielded pipe may be one answer
In Norway and other cold regions of Europe, approaches to utility pipe burial are far less conservative than in the U.S. Even here at home, many cold region water departments have reached their own conclusions about the practicality of shallow burial with shielded pipe, and are presently subscribing to the practice.
In general, however, 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) is providing that data.
Advantages of shallow burial considered
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.
CRREL initiates frost-shielding project
The Research & Engineering Laboratory in Hanover, NH, proposed a multi-part project to demonstrate the concept of insulating water lines to protect them from freezing.
First, an existing finite element computer program developed by CRREL and the University of New Hampshire would be optimized and tested. The finite element 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 designer's ability to change and adjust many possible parameters and see the results.
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 the excellent thermal resistance value ("R" value) of 5.4 per inch of thickness at 40 F.
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.
CRREL partners with city and manufacturer
For its partner, CRREL joined with the water works of the City of Berlin, NH, where the field study would be performed. The third partner to the equation was Owens Corning, manufacturer of Foamular extruded polystyrene insulation.
Owens Corning donated the Foamular insulating materials, and provided technical guidance in their placement, as its part of the plan. The Berlin Water Works provided the test sites, excavated and laid the new pipe and insulation.
Test sites included a water system on bedrock
Berlin, NH, is a city of about 12,000 situated in the White Mountains, a colder region than CRREL's Hanover, NH headquarters. It was also an excellent test site for another reason: the city is built on bedrock very close to the surface and actually breaking the surface in places.
Berlin's water works company initially approached CRREL for advice on freezing problems experienced with an aging water system, made up of small 2" to 6" galvanized pipes. Since a number of EPA mandated piping redesigns were about to be undertaken in various areas of the city, the tests could be performed with little additional construction expense.
A dead-end hillside street selected for main test site
CRREL selected 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" shielded water line was numerically designed and then constructed. It was monitored by an array of thermocouple placements for three winters.
The shield design was conservatively designed for the first year with a 6" thick layer of Owens Corning's Foamular extruded polystyrene insulation in an inverted U around the 8" ductile iron pipe. The sides of the U were 2' high, extending even with or slightly below the bottom of the pipe, which rested at a depth of 5 feet. Total shield width was 4 feet. The Foamular was supplied as 2" thick, 4 x 8 ft boards, which were easily snapped or cut to size on site.
During the second year, thermocouples were also installed to measure temperatures in an unshielded pipe, for further confirmation of the numerical model.
Finally, during the third year, a second shield design was developed and installed on another street. The design was intentionally more aggressive, this time placing the pipe at an elevation of 3.5' with a 4" 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.
Results indicate an even thinner shield is feasible.
Designers can analyze results before digging
The completed project took about 3½ years from start to finish Its accomplishments included a validation of the ability of CRREL's finite element 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.
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. The advantage, for the construction industry, the municipal government, and the citizens it serves, is bottom-line cost savings. Ratepayers pay less; tax dollars are saved.
About the author: Barry A. Coutermarsh began his CRREL career as a co-op student. In 1976 he came on board full time, working first in the facilities and engineering department, then in research. His first major project was an infrared roof survey. In 1987 Coutermarsh was a recipient of the Department of the Army, Research and Development Award. Coutermarsh is now a research civil engineer in the Applied Research Branch of CRREL's Research and Engineering Directorate. Self-classified as a generalist, he has been involved in a variety of fascinating projects. As he winds up this frost shield program, he is already moving into a new project relating to high strength +composites.
Edited by Joyce Jungclaus, Editor, Public Works Online