{"id":65888,"date":"2008-06-16T13:53:03","date_gmt":"2008-06-16T13:53:03","guid":{"rendered":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/article\/going-underground_o"},"modified":"2008-06-16T13:53:03","modified_gmt":"2008-06-16T13:53:03","slug":"going-underground_o","status":"publish","type":"post","link":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/projects\/infrastructure\/going-underground_o\/","title":{"rendered":"Going Underground"},"content":{"rendered":"
\r

Going Underground<\/h3>\r

Underground service repair beneath city streets is one of the most disruptive operations a public works department has to deal with. Not only does it create traffic problems, but the repair itself may result in poor pavement performance, becoming a source of ongoing maintenance or premature deterioration.<\/p>\r

A less intrusive repair technique employs a rotary core cutter and pavement reinstatement using a bonding compound to replace the core as a permanent repair. This process, called keyhole coring and reinstatement, is currently used by more than 25 leading gas distribution companies in North America. The process has applications in the rehabilitation and replacement of water and wastewater infrastructure.<\/p>\r

Keyhole coring and reinstatement allows access to underground pipe or other buried infrastructure from the road surface without having to resort to more costly, disruptive, and dangerous excavation methods. It facilitates repairs to underground utilities and lets horizontal directional drillers and other excavators view and confirm the exact location of potential buried conflicts before digging.<\/p>\r

The technology involves a rotary cutting unit or core drill that can safely and accurately core an 18-inch or larger diameter hole through asphalt, asphalt-concrete, or reinforced concrete road systems and sidewalks. Crews can then vacuum excavate and view subsurface activity or make repairs from the road surface using longhandled tools.<\/p>\r

After the repair or underground work has been completed, the excavation is backfilled to the level of the base of the pavement. The core that was originally cut from the pavement is bonded back into the roadway. This process results in excellent pavement performance and saves hundreds of dollars per hole in repaving costs.<\/p>\r

Selection of the Coring Unit<\/b><\/p>\r

If you are going to throw the core away, any core drill will do. But precision and accuracy are essential if you plan to reinstate the pavement core. Because most roads are crowned to allow surface water to run off, they don’t present a level or horizontal surface. In order to get a perfectly cylindrical shape and prevent the core from binding in the drum, pavement cores should be cut perpendicular to the horizon rather than to the road surface. To do this, coring units use hydraulic stabilizers to adjust the angle of the coring mast so that the cutting edge of the core drum is level with the horizon. These stabilizers also help to take the ‘spring action’ out of the vehicle to allow its full weight to be focused onto the pavement to ensure an accurate and precise core and a uniform kerf.<\/p>\r

Cutting the core precisely at right angles to the horizon is also important for the reinstatement process. With a slanted core, the effect of gravity can cause the bonding compound to settle on the lower side of the hole and result in a weakened and irregular joint that might not be waterproof or that could shift or fail.<\/p>\r

For roadwork, except for very small cores less than 4 inches in diameter, a vehicle-mounted coring unit (truck, skidsteer, or trailer) is heavy enough to provide the weight and stability needed for the coring job and to safeguard the operator from potential mishaps. Handheld or portable coring drills can injure operators if the drum binds in the hole and the drive unit (and sometimes the operator), rather than the drum, begins to rotate at 200 rpm.<\/p>\r<\/div> \r

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The choice of power to drive the coring drill is another element in the proper selection of a coring unit. Most truckmounted units use a hydraulic pump driven by a power take off (PTO) from the engine of the truck. Using a PTO makes maintenance simpler, there’s only one engine to service and maintain, and more cost-effective because most of today’s trucks come with a multiyear drive-train warranty. Skidsteer-mounted coring units also require no special fittings and are able to use the equipment’s auxiliary hydraulic connections.<\/p>\r

Coring units mounted on a trailer or pickup truck require an auxiliary power source, such as a small gasoline or diesel engine, to drive the hydraulic pump. This need for a second engine adds to the overall cost of maintenance and can add substantially to the purchase price.<\/p>\r

Coring <\/b><\/p>\r

The coring is performed with a cylindrical, steel core drum about 22 inches long, fitted with carbide- or diamond-encrusted teeth. The usual diameter for keyhole work is 18 inches but some coring units can accommodate larger 24-inch diameter drums. Depending on the pavement type, a typical coring unit can cut at about 1.5 minutes\/inch of pavement. High-quality core drums can core between 80 and 100 holes, although the teeth will eventually wear down and need to be replaced; a good quality drum can be retoothed or retipped several times.<\/p>\r

Some core drills are fitted with a center pilot bit that, like the center bit in a typical hole saw, cuts a hole at the same time the core is being cut. This center pilot hole has several advantages: it centers and stabilizes the coring drum, ensuring a neat and accurate cut; it also makes it easy to insert a core puller for removing and reinstating the core. Other techniques for pulling the core are less safe or efficient. Also, when filled with bonding compound the pilot hole provides additional bonding surface (about 11% more than the outer surface alone) and adds vertical stability to the reinstated core.<\/p>\r

Other Features <\/b><\/p>\r

Other features of good keyhole coring and reinstatement units include a water tank, a core hoist, a high-pressure washer for washing down the finished repair, and some onboard storage.<\/p>\r

Water is used to cool and lubricate the coring operation. A typical coring operation will consume about 10 gallons (about 1 gallon\/inch of coring depth). Most coring units therefore have a 100-gallon tank. The supply of water reaching the coring drum and pilot bit needs to be adjustable to properly lubricate the coring operation and flush the coring residue or slurry out of the kerf, but not to flood the site or run off into storm sewers.<\/p>\r

A typical 18-inch diameter core is 8 inches deep, weighs about 170 pounds, and can usually be extracted by a two-person crew with a pry bar inserted through the eye of the core puller. For larger cores some type of hoist will be needed. Some coring units are equipped with a hoist that can handle up to 500 pounds.<\/p>\r

The Reinstatement Process <\/b><\/p>\r

Although the cutting equipment is important, it is only part of the process. Equally important, if not more so, is the process of reinstating the core in the pavement. This is a key element in avoiding repaving costs and is the only true measure of the effectiveness of the process. Experience teaches us that to do a proper reinstatement in the shortest practical time, you need a field-proven, fast-setting bonding compound.<\/p>\r

Because poorly repaired utility cuts are a prime source of potholes and premature pavement failure, most jurisdictions have established rigid testing standards for keyhole core replacement. For example, the city of Toronto requires that the bonding compound be capable of generating a waterproof bond that, within 30 minutes of application at 70\u00ba F, achieves an equivalent traffic loadable condition that is at least two times greater than the American Association of State Highway and Transportation Officials Standard H-25 (now HM-27) or 30,000 pounds.<\/p>\r<\/div> \r

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This challenge can’t be met by just any cement product or grout. What’s required is a product that not only will fill the gap between the core and the remaining pavement, but will mechanically bond the core into the pavement in such a way that the reinstated road system will again be able to perform as a system, sharing the traffic loads.<\/p>\r

Independent testing has shown that many commonly available bonding compounds and grouts do not meet the 30- minute test time. The ability to gain strength quickly is an important element in being able to safely reopen the road to traffic. And the ability to move the utility crew and equipment quickly to the next worksite reduces costs and improves efficiency.<\/p>\r

Convenience of use, reduced waste, and longer shelf life are also important factors in choosing the right bonding compound. The bonding strength of every cementitious grout depends on an accurate ratio of water to dry materials. Using a calibrated measuring pail to accurately combine the exact amount of water to the dry material not only eliminates guesswork in the field, but is essential to performance. Too much water will reduce the bond strength while too little will reduce fluidity to completely encompass the kerf of the core, which can cause premature core failure.<\/p>\r

Look for products packaged in a sealed waterproof pail, rather than a paper bag, to help extend shelf life and eliminate waste. Once a bag has been opened or torn it is no longer watertight and hydration can begin, lowering the grout quality.<\/p>\r

All of these are important factors in selecting the appropriate bonding compound to be used in core reinstatement. While you may pay more for a premium product, you’ll find that these materials have faster strength gain, greater ultimate strength, are easier to use, are packaged to minimize waste, have long shelf life, and ensure accurate mixing ratios.<\/p>\r

Marshall Pollock is President and CEO of Utilicor Technologies, Toronto,Ontario, Canada.<\/i><\/p>\r<\/div> \r","protected":false},"excerpt":{"rendered":"","protected":false},"author":0,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"scheduledNewsletterDates":[],"_scheduled_newsletter_id":0,"footnotes":"","jetpack_post_was_ever_published":false,"zonda_content_type_url_append":"_o","zonda_dfp_zone":"","zonda_exclude_recommended_contents":false,"zonda_exclude_most_popular":false,"zonda_exclude_tagged_results":false,"zonda_eyebrow":"","zonda_last_newsletter_date":"2099-12-31T23:59:59.999-05:00","zonda_original_url":"","zonda_pdf_version":0,"zonda_print_headline":"Going 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