Document ykwZnpYpJLr4r2DnZ4wX3ny8d

NORTH AMERICAN FOCUS WORLD TUNNELLUG Innovations in Dust Control new gadget that lasts only five Aminutes is worth more than an immortal work that bores everyone," wrote the French painter and poet, Francis Picahia (1878-1953) in Figaro (20th January, 1922). "Gadgets" indeed can serve one well if they reduce or eliminate problems that get in the way of advance. During tunnel excavation, fugitive dust can have serious effects on productivity, according to James W. Rosteck, PE and project engineer for J. F. Shea Co, Inc. For instance, a constant high rate of airborne dust in a tunnel can cause silicosis. Whilst workers in the US generally wear OSHA-approved dust masks, many do so grudgingly. Dust can also irritate the eyes, while many metal surfaces on equipment can become slippery when settled dust acts as a lubricant. Dust-laden air can also interfere with the Dust-free conditions behind the TBM. operation of the laser used to guide TBMs; it specialist for the contractor, agrees that dust is can shorten the useful life of moving wear- unkind to equipment. He has seen ventilation parts in equipment, since the dust is abrasive; fan blades eroded by abrasive dust travelling and can contaminate the hydraulic fluids and through the system. lubricating oils in tunnelling equipment, if A small reduction in efficiency can cost the they are not completely sealed. contractor considerable money, according to Conveyors Rosteck. As an example of how costly dust interference can be, he cites the recently- Some tunnelling projects have vertical belt completed New York No 3 water tunnel conveyors for bringing the muck to the project using theoretical production time-loss surface. Usually, this conveyor transfers the against actual labour costs. muck to a horizontal conveyor and on The project is a joint venture of Schiavone to a stacker or bin. Dust is created at the Construction Co., Inc. and J. F. Shea Co., transfer points and at the stacking location. Inc. The excavated runnel is 8.9 km long by Airborne dust at the surface usually does 5.8 m in diameter and the concrete lining, not present a problem if the project is now being built, reduces the finished in the middle of nowhere, but many tunnel diameter to 4.9 m. projects are in or around metropolitan or In this scenario, Rosteck conservatively residential areas where dust blowing in the projected a production time-loss of 10 min/8h wind is tolerated neither by the local resi shift because of fugitive dust. Mining on this dents nor the environmental protection project was carried out for three shifts per day agencies. for two years. The 10 min loss per shift adds Although Rosteck does not quantify the up to a 2% loss in production, or $750,000 in effect of a serious dust problem on tunnelling added mining costs. Shea avoided such costs productivity, he says it is economically with the installation of a two-part, Martin significant. He explains, "Reduced production Marietta Dust-Buster dust suppressant and efficiency caused by dust will depend on the Donaldson/Torit dust collector system. They project and the scope of the dust problem. For had already used this system both on the New sure, there is a reduction. Whenever more York project and the Chicago TARP and are time is spent on equipment maintenance and also currently using it on the MetroWest the workers are burdened with all kinds tunnel in Framingham, MA. Rival contractor of dust-protection equipment, a toll on S. A. Healy Co. also successfully used a production efficiency is taken." similar system on the Dallas Area Rapid Allen Myers, equipment maintenance Transit (DART) tunnel project. New York The area surrounding the access and TBM launch shaft in Brooklyn is residential, so control of airborne dust was paramount. The City and the regulatory agencies would not have tolerated a dusty condition at the street level. Effective dust control at the surface was accomplished by installing the Dust-Buster dust suppressant system at the final transfer point, where the muck was delivered from the muck-car unloading-hopper to a 168 m-high Lakeshore vertical belt conveyor and on to a horizontal discharge conveyor to a surface holding bin. The dust suppressant system main com ponent is the control centre, which is typi cally installed near the conveyor. Basically, the centre includes water and chemical pumps; air, water and chemical pressure gauges; a 4-nozzle or 8-nozzle spray manifold; and a foam accumulator. There are four different control centre models available with various foam injection rates and dust suppres sant capacities. Compressed air supplies from an outside source are required. The rationale for this system is to suppress airborne (fugitive) dust at all dust-generating transfer points on a conveying system. It can suppress dust more effectively than traditional water-spray systems and yet use significantly less water. Water usage is typically reduced by 80-90%. The foam blankets the muck, creating a dust barrier. Low-water rates in the WORLD TUNNELLING INNOVATIONS IN DUST CONTROL Surface conditions at Framingham site. foam means that none of the moving, wearing parts on the conveyor are threatened by water-laden abrasive dust particles. Also, the conveyor belts and muck remain dry enough not to cake and adhere to the belts. As the muck travels on the conveyor from the TBM backup, the foam is applied by spray at the transfer point to create the dust shield. This remains on the conveyed muck all of the way to the top of the shaft and on to the holding bin. Some projects require a second installation at the vertical conveyor for addi tional foam application. The quantity of foam applied is operator-controlled to match the volume of muck and the conveyor speed. Martin Marietta CDS 80 dust suppressant is mixed with the water to produce the foam. This is a liquid with a pH of 8.0, which is slightly alkaline. CDS 80 is available in hulk or in drums for ease in handling and storing at the project. This suppressant is environmen tally safe, with no restrictions imposed by US federal or local environmental regulatory agencies on this particular end-use. According to the technical department at Martin Marietta, fugitive dust is created in one of two ways: by impact on materials carrying dust-size particles; or by wind eroding the materials. The latter plays little or no role in tunnel construction. The only problem that could arise is with muck stored on surface in large stacks. Often wind curtains made from canvas can be hung on frames, or canvas is used to cover the muck piles to act as a wind barrier. Likewise in the tunnel, whilst the main ventilation system can help main tain dusty conditions with dust already airborne from impact, there is not sufficient wind velocity to erode standing muck. Impact is the culprit that causes airborne dust in the tunnel. It starts at the heading where, on this project, a rebuilt Robbins TBM was equipped with a Donaldson/Torit dust collector system, and a Dust-Buster dust suppression system was installed at the transfer point where the conveyor loaded into Muhlhiiuser muck cars. Metro West An improved version of the custom-built collector is presently being used at Shea's MetroWest tunnel project. The heart of this system comprises three model TD-970 dust collectors built by dust-control equipment manufacturer and distributor Dentech Inc. to Shea specifications. Each TD-970 unit features 24 filter cartridges with pleated paper elements. The total filter area for the system is 268 m- and flow-through capacity is 340 m'/min. A 61 cm-diameter inlet duct leads from the face of the TBM to the dust collector which is mounted on the backup. A fan and duct are connected to the outlet side of the dust collector. With the system running, a nega tive pressure is created between the face of the TBM and the tunnel heading, thus drawing the dust-laden air through the collector. As the dust collects on the filter surfaces, a spurt of high-pressure compressed air is periodically blown through the filters in the opposite direction of the vacuum air flow, cleaning the collected dust off the filters. The blown dust falls into a hopper that features vacuum valves installed at its base. The weight of dust forces the valves to open and the collected material drops 0.5 m onto the tunnel invert. This filter-cleaning process is carried out automatically at intervals by an adjustable time-control system that actuates the high-pressure compressed air. The dust collector system is not directly connected to the tunnel's main ventilation system. However, its outlet is placed near the main ventilation duct intake so that the expelled clean air can be drawn into the ventilation system and discharged at the surface. The negative pressure is controlled by moveable shutters built into the air system to regulate the capacity of the air flow between the heading and the TBM face. WORLD TUNNELLING INNOVATIONS IN DUST CONTROL The filter-cartridge cost was $50/unit, but few replacements were made. At Metrowest, each of Shea's TBMs features a custom-built dust collector with a Dust-Buster system on the backup. The contractor says a second Dust-Buster system is not necessary at the vertical conveyor as was the case at the NY tunnel project. Instead, Martin Marietta recommended 25% more foam to be applied at the backup system transfer point. A CDS 80 chemical solution application rate of 0.059 1/bcm ground exca vated is sufficient. The cost is $0.0865/mJ of treated muck compared with $0.1296/m5 on the New York project. by Rodney Garrett Technical Journalist Money Matters The cost of the two-part system for the New York tunnel was about US$80,000, not including installation by the contractor. Operating costs were confined to just the CDS 80 chemical used to make the foam, and the filter cartridge replacements periodically required for the dust collector. Approximately 235,000 bank m5 of ground was treated twice with the CDS 80 foam. The cost was $0.074/m5 at the backup system and another $0.0556/111* at the transfer point leading to the vertical belt conveyor. Total cost, therefore, added up to $0.1296/m3 treated. A special thankyou to James W. Rosteck, PE, Allen Myers, Equipment Maintenance Specialist and Bill Morris, Chief Custom Engineer for helping in putting this report together. All three are employed by J. F. Shea Co. Also, another round of thanks to the technical staff at Martin Marietta for technical assistance on airborne dust and its control. Dust Control has come The Tunneling Industry has a new weapon in its arsenal to improve the work ing environment. The DUST-BUSTER dust control system utilizes state-of-the-art application equipment and environmentally friendly chemical agents to improve the air quality in your tunnel. DUST-BUSTER foam dust suppression system controls dust from the working face to the muck pile, often with only a single treatment. Martin Marietta Magnesia Specialties Inc. P.O. Box 15470 Baltimore, MD 21220-0470 800-648-7400 or 410-780-5500 FAX: 410-780-5777 www.martinmarietta.com