AERIAL DETONATION ROUTINES AT SNOQUALMIE PASS. "Explosive Delivery Trams, Methods or Madness at the Low Pass"

Transcription

1 AERIAL DETONATION ROUTINES AT SNOQUALMIE PASS "Explosive Delivery Trams, Methods or Madness at the Low Pass" Rob Gibson*, Lee Redden*, and John Stimberis*, Jon Barker** *Washington State Department of Transportation Avalanche Crew **Alpental Pro Patrol Director Introduction and Background Inspired by Bridger Ski Area's jiffy-wire trams shown at the 1982 ISSW field trip, efforts were begun to apply similar methods around Snoqualmie Pass. At the time, large aerial charges were used by attaching the explosives to a 2X2 or an alder pole. These shots were deployed by a person, (usually with the least seniority!), who carried them out to plant in the starting zone, then retreat to safety. The use of the cable system appeared to be a good alternative to this method. It also had promise as a replacement for recoilless rifle ammunition, which has a limited available supply. The ongoing efforts to install these low tech delivery systems continues with over 30 trams installed around the Snoqualmie Pass region, 26 of which are still in use. Methods and Operations 1) Adjustable Tension Cable Systems The WSDOT Avalanche Crew found that the "jiffy-wire" set up would not work for them because most of their starting zones do not drop off steeply enough from the launch areas. The East Snowshed-4 (ESS-4) starting zone is their highest priority area and it changes gradually from fairly flat to convex and then drops steeply over a cliff band. A simple closed-loop clothesline type delivery system was tried here first but was found to present its own nest of problems. Wallowing in deep snow onto a convex slope to get a large charge out far enough to get it airborne on a fixed-tension system was not a favorite practice. The crew needed a method to get the charges past the convexity without having to climb high into the upper terminal tree to attach the charge to a fixed line system. Some of the other WSDOT locations have very large spans between the launch area and the starting zone and would require a very tightly strung cable unless the height of the launch tree is somehow utilized. The variable tension system takes advantage of the height of the tree without having to climb it each mission by passing the cable through a topping block mounted near the top of the tree. One end of the cable is attached to a hand cranked winch or spool mounted to the tree in the launch area which controls the cable tension. From there, the cable runs vertically to the topping block and then. proceeds down slope over the starting zone to the lower terminal tree or anchor. A pulley or a slider link serves as the carrier on the down slope segment of the cable and a stopper on the cable at the farthest shot point prevents the carrier with the explosives from reaching the bottom tree. The variable tension system is now used on all WSDOT trams in the Snoqualmie Pass area. It also allows the cable to be lowered to the ground to service the carriers or to retrieve the ones that have escaped from their belay lines. To operate, enough cable is unwound off the winch/spool to allow the down slope segment of the cable to drop into the launch area. The charge can now be attached to a carrier on the cable. Rewinding the cable back onto the winch/spoo/lifts the down slope cable with the carrier and charge up out of the snow in the launch area. Gravity pulls the carrier and charge down slope over any terrain obstructions en route to the starting zone. At this point, the charge can be lowered closer to the snow surface if desired, by simply unwinding cable off the winch/spool in the launch area. The addition of a tag line to pull the de-tensioned cable back towards the 333

2 launching area can be helpful (see Problems and Inconveniences). More than half of the WSDOT variable tension trams use a slider link rather than a pulley for a carrier. The slider link is a one-way delivery option that eliminates the need for a retrieval line. It is most often used for deliveries of 300 feet or less. This routine uses det-cord to belay the charge into position and easily allows several charges to be deployed on adjacent cables and t.hen branched together. These simultaneous multiple detonations appear to better clean out a large or stubborn area. This method also speeds up the entire process, resulting in a quicker response by the maintenance crew to begin clearing the highway and get traffic moving again. The other WSDOT variable tension trams use pulley carriers with an assortment of belay line systems. The routine for most of these trams is (1) slacken the cable, (2) attached the charge to the carrier, 3) check the belay lines, (4) light the fuse, (5) re-tension the cable as necessary, (6) deploy charge in a controlled manor with the belay line down slope to starting zone, (7) cover ears and then retrieve the carrier after the shot goes off. 2) Fixed Cable Systems Alpental has five fixed tension cable systems, which deliver 12 to 50 lb. charges and are operated similar to Bridger's "jiffy-wires". At four locations the explosive charge is suspended from a dangle line attached to a single pulley carrier, which is lowered into position with a belay/retrieve line. The fifth location is very short and a carabiner is used as the carrier to slide the suspended charge down the line and is belayed into position with a bundle of line that is left on site. Various styles of belay/retrieve line spools are being evaluated for each site's specific needs. Some of the Alpental trams that have adjustable tension capability are in locations where they can be used as fixed cable set ups. When alignment adjustments or carrier retrieval is necessary it is very easy to drop the cable down as needed. 3) Closed Loop Moving Cable Systems I (Bike Trams) The WSDOT Avalanche crew was inspired by information from a couple of "Avalanche Review" issues, the first from January 1985 being just a small picture and a short paragraph introducing Snow Basin's tricycle powered avalanche control tram. Then along came the March 15,1988 article by Karl. Birkeland and Tom Leonard which discussed the construction and use of inexpensive trams using tricycle wheels. And as the saying goes, "the wheels were set in motion. There are now four of these foot-pedal powered units in the Snoqualmie Pass area. All four deliver 25 to 50 lb. charges. Description The Bike Trams are closed-loop, vertically aligned, moveable cables traveling around pulleys at each end. The pulley at the lower end, or drive station, is the hub and rim of a unicycle wheel. It is direct drive with the pedal arms directly attached to the hub. One revolution of the pedals equals one revolution of the wheel. The upper terminal is a similar wheel mounted between the forks of an arm that can be mounted to a tree, tower, or similar fixed object. A pivoting arm with a cable and hand crank system to raise and lower it allows both placement adjustment and system tension control. This is a convenient set up, but fixed system will work if regular access to the top terminal area is difficult. Rollback control is a component of the drive unit with the cable running through a Klein grip attached to the bike's frame. This allows only one-way travel under load. A simple hand operated bicy.cle caliper brake is used for finesse. The Klein grip is easily removed with a pin and the hand brake can be used if it is necessary to release the grip during use.' A "carrier" yoke is attached to the upper strand of the cable loop. This is the uphill moving strand, with the bike oriented to face the cable and all operations. The carrier is a convenient area to attach the two ends of the cable, which form the loop, making it easy to assemble and disassemble as desired. A small three-inch diameter pulley is used at the bottom of the carrier through which the downhill, or lower strands of the cable loop runs. This allows both strands of the loop to share whatever load is hung from the carrier. A length of chain 334

3 hung below the carrier to attach the ISayload. This aids in protecting th~ cables fom the explosives and also provides a lower renter of gravity, which prevents or greatly ~ecreases the sy~ten:s ability to invert. The normal configuration IS to have the dnve station on a tower or elevated platform so the parcel has room to dangle below the system prior to deployment. It also makes a visual check of all components and operations very easy. A cover or.tarp over th.e platform keeps snow and ice buildup to a mlqlmum. Operations All operating procedures have evolved from the original two highway trams and have changed very little over the years. If snow or ice is clinging to the cable or any of the components, it is advisable to pedal a "dryrun" prior to loading any explosives. A standard charge for our use is 25 Ibs. of Ammonium Nitrate Fuel Oil (ANFO), prepackaged with a one-pound cast primer and two to three feet of 50-grain detonating cord (det-cord). Up to 50 pounds has been used with no noted problems. The charges are fixed to the carriers' dangle chain with a few feet of 1/8 th inch nylon line. The end of a roll of 18 grain det-cord is attached to the 50-grain cord and the spool is set upon a spindle mounted to the bike's frame. The Klein grip is secured around the uphill strand of the cable loop as rollback prevention. A simple wheel mounted odometer is set to zero. The counter will register a count for each revolution of the drive wheel and is handy for exact charge placement in poor visibility or darkness. The charge is now ready to be pedaled uphill with the det-cord being automatically payed-out. A fuse may be attached directly to the explosives instead of using the extra det-cord, but is advised to use at least enough det-cord to get the charge a "safe" distance away from the launch platform prior to ignition. For highway paths, we always use enough cord to have the charge fully in position to prevent any possibility of misfire, and also to minimize the wait time of a longer fuse possibly needed during deployment. In this way, we can always use the minimum acceptable fuse length (90 seconds) and get the shortest closure time for the highway, and the least wait-time for our snow removal crews. When the charge is in Position, the hand brake is set as a backup to the Klein grip. After detonation, the Klein grip is removed, the hand brake is released and gravity will usually bring the carrier back to the drive station. Speed is easily controlled with the hand brake. Regular inspection of all components is quite easy other than the upper terminal, which should be visited prior to each control season and occasionally as conditions allow. Problems and Inconveniences Tram systems require initial set up, periodic maintenance, and with good luck only occasional repair. We have experienced heavy ice build-ups, which have slowed the tram operation, but have yet to incapacitate any of them. More than one tree has fallen and disrupted tram service,once while avalanche controllers were standing near by (fortunately nobody was injured). The tram systems range from the simple fixed cable, to closed-loop systems with moving cable, to variable-tensioned systems with multiple carriers. Because of the variety of systems and the unique nature of each location, a number of site-specific features have developed. These features range from simple to complex. To ensure proper operation, higher degrees of operator familiarity are very beneficial for trams that incorporate the more complex deployment procedures. Even though most of these trams are left in place year-round, unauthorized operation has been minimal and has required only minor changes in the routines. Some difficulties with carrier retrieval have been experienced with attempts to position several charges onto a single cable for simultaneous detonation. Anytime both a carrier belay line and det-cord get spooled out a fair distance side by side on a windy day there is a good chance of cutting the belay line. On the longer and/or steeper variable-tension trams, where considerable lowering is required, there is a problem with the cable lowering near the end terminal rather than near the top, where the user needs the slack. If a tag line is attached to the down slope cable a short distance from the topping block, and the other end is attached to the anchor tree near the winch, this problem can be reversed. The proper geometry for the tag 335

4 line set up depends on the unique nature of the tram set up, but it can be easily adjusted so the slack or belly develops precisely where the user needs it. Discussionl Summary To really notice the efficiency of an aerial tram you need to see one in action. Whereas hand charges can be effective in releasing individual pockets of snow, and at times in extracting large slabs, a tram with its greater payload and aerial blast is able to bring down a much larger area of snow all at once. This is a great savings in time over the conventional method of throwing individual hand charges, and also reduces the chances for hang fire. Even the snow in the trees is shaken loose by the massive air blast of a lb. bag of ANFO. The savings in time, as well as cleaning out the starting zones of avalanche paths, is a benefit to ski area operations and a must when dealing with a major Interstate highway, especially during an ongoing avalanche/snow cycle. Aerial trams not only deliver a larger payload and an aerial blast to a slide path, and thus speed up the process of avalanche control, but they also allow the avalanche technicians to remain in a safer location throughout the operation. Another benefit is before rain-on-new-snow events occur. As mentioned above, the speedy delivery of the charge, as well as the effective nature of a large air blast can be a great benefit before the rain saturates the new snow and creates snow balling and wet snow avalanches. We have found that hitting a major slide path with a large shot just before an anticipated rain event can bring the majority of the new snow down and prevent large messy wet avalanches which can present a problem to ski runs long after the avalanc~e danger h~s passed.. Additionally, the debns produced IS usually fist sized or smaller, and at Alpental this can be a great plus as rather marginal skiing can be made very enjoyable by the addition of a smooth consistent layer of fist sized avalanche debris. Hard control vs. Soft control At ski areas, avalanche control is sometimes at odds with the business at hand. On one hand, the ski area and more importantly the.ski patrol, wish to provide the customer with a certain degree of safety. On the other hand, new snow, especially fresh deep powder, is desirable to the customers and consequently is very good for business. The air blast provided by an aerial tram eliminates craters and typically prevents large chunks of snow in the slide paths (the exceptions being large cornice formations or wet snow). The amount of explosives, location in the slide path, and height above snow pack can be tailored for the conditions, size of the slide path and desired end result. The WSDOT avalanche crew is not in the business of providing a slope for enjoyable winter recreation. They are dealing with safety and efficiency. The goal is to reduce or eliminate the avalanche hazard. During winter months the hazard can never actually be eliminated, but only reduced for a while (until the next snow event occurs, for instance). Large shots, suspended close to the snow pack are used for maximum reduction in avalanche threat. This "hard" approach is much more damaging to the snow pack, and can make for some very ugly skiing. 336

5 Table 1. Aerial Exlosives Trams for Snoqualmie Pass, WA Tram Name Location Length Delivery System WSDOT Denny 9 High Denny 9 Low Airplane 1 Airplane 2 West Snowshed 1-90 Denny Mtn Denny Mtn (Dodge Ridge) 200' 800' 600' ' Bike Tram Bike tram Triple Carrier wlbelay East Snowshed (ESS) 1 800' Single Carrier wi Belay ESS 2 High ESS 2 Low ESS 3 ESS 4 East ESS 4 Center ESS 4 West ESS 5 SUde Curve East Slide Curve Center Slide Curve West 325' 325' 200' 125' 1200' 250' 150' Slider wi Det cord '?elay Single Carrier wi Belay Retrieval System Bike Bike Gallagher reel Hand reel Long line reel ALPENTAL Bag One Alpental 800' Single Carrier wi Belay Hand reel Mona (Bag Two) A1pental 650' Single Carrier wi Belay Mounted hand reel Earl (son of Earl) A1pental 1500' Single Carrier wi Belay Hand reel w/drill power Lisa A1pental 500' Single Carrier wi Belay Hand reel w/drill power Jose (Nash Bag) A1pental 500' Single Carrier wi Belay Cod reel Adrenaline A1pental 300' Single Carrier wi Belay Hand reel Waterfall A1pental 1800' Single to Multiple Carriers Hand reel Shot 8 A1pental 150' Carabiner wi Belay Hand line Condo A1pental 900' Bike tram Bike NORDIC CENTER Rockdale Cross Country Trails 1 Bike tram Bike 337