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Benefits of polyurethane for subsurface infrastructure maintenance.

As most people will agree, surface and sub-surface infrastructures such as sewer systems and pipelines are an important part of modern society. Unfortunately, any underground facility exposed to the ravages of time and the natural environment, will settle, crack, erode, corrode, off-set, deteriorate, vibrate, become saturated, voided or collapse, creating sub-surface and surface repair work to be paid out of ever-stressed maintenance budgets. Each year, state governments, municipalities, cities, towns and counties are faced with the perplexing problem of how to address the issue of finding the resources and the effective means of solving these infrastructure problems that are so common today. Some of the more frequent problems include:

Water infiltration--Water that enters the system through leaks in the manhole, pipes or joints and flows into surrounding soil. With time and erosion, roadway surfaces around manholes and catch basins can sink, allowing additional water to enter the soil that eventually makes its way into the system, creating opportunities for large amounts of soil and water to infiltrate. If additional storm water enters the system, it can yield higher water treatment costs downstream.

Water exfiltration (leaching)--When concrete, plastic, clay and even steel pipes that carry waste water and sewage crack, they can seep pollutants and harmful bacteria such as e-coli into surrounding ground soils. Over time, these pollutants can penetrate underground aquifers and impact municipal and private drinking water supplies. Areas of concern for these seepage problems include entry and exit piping for water treatment plants, storm drains, and sewer pipes.

Communication/data systems conduits and tunnels--As communities transition their underground communication assets from standard wiring to fiber optic cabling, the influence of water into the conduits that encase these assets can have damaging results. Many public and private agencies are taking steps to insure that the conduits and vaults that protect these assets are sealed and protected from water infiltration. The same holds true for subway and underground rapid transit tunnels and stations as well as underground pedestrian walkways and commercial areas.

Insulation from temperature and vibration--With infrastructure such as oil pipelines, the rapid flow of large quantities of material can cause vibrations that transfer to support structures and concrete foundations. Such vibrations can destabilize these concrete bases as well as the underground soils that support them. In Northern climates, changing temperatures and repetitive freeze-thaw cycles can also contribute to further destabilization and disruption of the supporting ground soil.

For many years, concrete materials such as cementitious compaction grouts had been used to resolve these types of problems. Unfortunately, these materials often compounded the problem by adding significant weight to an already stressed soil condition. Now, newer and more lightweight materials made from polyurethane-based polymers offer superior advantages over these older materials by providing faster and more efficient infrastructure repair and maintenance.

Polyurethane in infrastructure rehabilitation

Polyurethane offers advantages over less effective methods and materials by filling, densifying and stabilizing low-density compressible soils to depths of 30 feet and beyond. It is ideal for sealing manholes, pipes, sewers and conduits, as well as box culverts and storm drains that leak. It is also fast acting and hydro-insensitive, ensuring that it is unaffected by any water or wet soil that may exist around the immediate area.

The expansive properties inherent with polyurethane provide a non-disruptive, cost-effective, and long lasting solution for infrastructure problems. The equipment, labor and material costs involved with using polyurethane for infrastructure repair is significantly reduced because there is no need to take either the transportation or infrastructure elements out of service during peak use periods. As a result, any significant traffic interruptions are minimized, leading to additional cost savings.

The unique attributes with subsurface polyurethane injection offer several important benefits that are useful for sealing, stabilizing and lifting a wide variety of public and private infrastructures.

Sealing

To seal infrastructure water leaks, the patented polyurethane material is strategically injected on either side of the offending area at depths up to 30 feet below the surface. Once the placement of the polyurethane material is complete, it begins to expand up to 20 times its original size. During this process the material seeks out and fills any sub-surface voids, fully enveloping and collaring the area, sealing the leak (see diagram above).

The expansion of the polyurethanes and the hydro-insensitive nature of the material combine to compress the adjacent soil, push out any water in its expansion path and prevents additional water from returning to that location. When the material fully cures, often within a few minutes, it provides a strong, stable and long-lasting seal.

Stabilizing

Stabilizing involves reinforcing and adding support to the structural integrity of sub-surface infrastructures by preventing displacement or incremental movement that can lead to cracks and leaks.

This is especially true with brick manholes, where the introduction of hydrogen sulfide gas ([H.sub.2]S) over time can degrade the brick mortar causing cracks and shifting, finally exposing the bare dirt wall behind them. With the influence of water, this soil may seep into the manhole, clogging pipes and equipment located between the manhole and downstream water processing plants.

To solve these stabilization problems, the expanding polyurethane material can be injected from the surface, the floor or through the manhole wall itself to fill the voids and spaces wherever cracks may exist (see diagram below). The injections can continue all the way around the manhole structure as necessary, creating a unified and solid mass of polyurethane material retarding the penetration of both water and base soils. Since the weight of the material is a fraction of the comparable weight of cementitious grouts, it does not add any additional overburden to the wall or the surrounding soils that could contribute to further structural movement.

Lifting

Changing temperatures between summer and winter months combined with the ongoing punishment by heavy traffic often results in roadway settling. Over time, the base fines wash into the system making the soils weak, allowing the roadway to settle from lack of support.

The expansive nature and tensile strength of Polyurethane is highly beneficial for these situations by lifting and leveling roadway slabs and surfaces, restoring a smooth driving experience that is more durable than comparable cementitious materials.

Case study

For one major metropolitan area in eastern Tennessee, an older brick manhole along a well traveled riverfront boulevard had become an ongoing problem for their municipal utility board.

On two previous occasions, the city had overlaid asphalt pavement around a sunken brick manhole in an effort to resolve a problem with a large road surface pothole. Since their attempts to repair the problem from the surface did not succeed, the board chose to investigate the underlying area more thoroughly. Upon closer examination, it was discovered that a broken 8-inch sanitary sewer line had entered the manhole at approximately 25 feet down. A miniature camera revealed the pipe was blocked and had broken 3-feet outside of the manhole wall. While the brick manhole was structurally sound, it was also quite susceptible to water and soil infiltration through the gaps in the now deteriorated mortar joints at random locations along the barrel of the manhole. A potential collapse of the manhole, the possibility of subsequent backup and possible flow of sewage into the adjacent river that ran alongside the boulevard, the loss of sewer services, as well as delays and interruptions needed for a massive excavation could have cost the utility board many times the cost of the repair if the issue was not resolved quickly.

The utility board designated three tasks that were necessary to solve the manhole problem. First, the soils surrounding the manhole needed to be stabilized. Second, the entry of the 8-inch broken pipe into the manhole needed to be sealed. Third, the base soils under the pavement that had created a large void were to be filled and stabilized. After a detailed review of several alternatives, the board Polyurethane Injections Surrounding Manhole Problem (Top View determined that deep polyurethane injection was best suited to repair these three problems.

The first process involved a series of penetrometer tests were conducted around the problem area to test the density of the soil surrounding the manhole. These tests would be useful in determining the specific areas where the polyurethane material would be placed.

The initial penetrometer test was taken approximately 3 feet south of the edge of the manhole. The test revealed a 33-inch void directly under the asphalt pavement. This penetrometer test also revealed that the underlying soil was so weak that the penetrometer rods actually dropped under their own weight to a depth of 22 feet.

Next an injection diagram for the polyurethane injections around the problem area was created. The injection pattern formed a grid which would provide a precise, uniform, and sequential layout of injection probe placements. The extent of the soil and the structural problems with the manhole dictated the spacing and depth of each of the injections. The relative weakness of the soils also determined the volume of the polyurethane resin that would be needed to stabilize the area. As a general rule the weaker the soil, the greater the amount of material necessary to achieve an adequate soil densification.

The injection pattern was designed around the manhole starting with the deepest points first, then radiated outward and upward in a shallower pattern to achieve the needed total repair of this pipe and manhole. Two diagrams of this pattern are illustrated above.

Based on the analysis of the original area covering both the slow and passing lanes of traffic, a pattern made up of 20 injection points, covering an area of 18 feet by 15 feet, was used. This injection plan called for four depths of injections, at 17, 13, 9 and 5 feet respectively.

Additional voids were also found approximately 5-feet beneath the pavement surface and were filled to support and lift the street to its original profile. The three deeper levels were then injected to fill and densify the weak, unconsolidated, and wet strata encountered around the manhole down to the broken pipe. In addition the polyurethane material was also used to seal and replace the missing mortar in the brick manhole, thereby restoring structural strength to the weakened walls. As a result, the manhole was sealed against further deterioration. A picture below shows how the polyurethane was similarly applied and used to seal the collapsed manhole and 8-inch sanitary sewer line that was part of this project.

Finally, a post application penetrometer test was completed within 2 feet of the original test location (P1) to validate the integrity of the soil densification process. This test showed a marked improvement in the densification of the soil surrounding the manhole.

One of the major providers of a patented deep polyurethane injection process is URETEK USA, of Tomball, TX, who has been performing polyurethane injection solutions for public and private institutions for over 30 years. They are considered leaders in this particular area having completed over 75,000 successful infrastructure and roadway maintenance, repair and restoration related projects since their inception.

PRODUCT INFORMATION:

URETEK USA has posted a white paper on their website that provides additional information on how the use of polyurethane is effective in solving public infrastructure problems using their patented URETEK Deep Injection Method. The document details the polyurethane deep injection process and includes a case study that illustrates how it was applied to repair a below ground three barrel box culvert.

To download and review this white paper, please visit the URETEK USA website at http://www.uretekusa.com/pdfs/URETEK-USA-INFRAWP.pdf.

Michael Jaques has over 25 years of international experience in all aspects of chemical and pressure grouting. His work includes consulting, contracting, equipment and product design, manufacturing and engineering. Projects with which he has been involved include repairs to dams, tunnels, mines and commercial buildings worldwide. He joined URETEK USA in 1999 and coordinates all design and project specification details for the company's patented Deep Injection Process. Jaques is a second generation ground modification specialist and a native of Iowa where he studied architectural engineering at Iowa State University.
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Title Annotation:U-Tech: Underground Technology Cutting Edge Technical Information For Utility Construction & Rehabilitation
Author:Jaques, Michael
Publication:Underground Construction
Date:Jul 1, 2005
Words:2010
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