Dances With Wires: An Unusual Rigging ProjectKathleen Murphy, P. Eng.
Marine and Industrial Conservation Engineer
Heritage Conservation Program
Eur. Ing. Alex Barbour, C. Eng., P. Eng.
ABSTRACTDredge No. 4 is a wooden hulled gold dredge that operated in the Klondike River watershed in Yukon Territory, Canada, between 1912 and 1959. Acquired by Parks Canada in 1969, salvaged from partial burial in 1992, the Dredge is operated as a National Historic Site and is the largest visitor attraction in Yukon.
The bow gantry of the vessel was subjected to unbalanced forces because a support for the digging ladder was installed in 1993. The Superintendent, however, presented with engineering advice that this was an unwise decision, was financially unable to reverse this work for the next two years. In 1995, structural engineers found extensive rot in the gantry timbers. The site decided to take the gantry down for repairs.
In 1996, the authors were part of a team sent to inspect the gantry and prepare specifications for the dismantling. However, upon arrival on site we found the gantry already in the first stages of collapse. We quickly mobilized to undertake emergency stabilization work, both to reduce the chances of failure and to limit the damage to the ship should any failure occur. This emergency work involved adding some structural supports in critical locations and attempting to equalize the unequal forces in the running and standing rigging.
This paper discusses the emergency work from the summer of 1996, with a focus on the rigging work. The project was unusual because the ship is a unique historic vessel, and because the work took place very quickly under emergency conditions and without following the usual government protocol for such works. The use of low-technology solutions and a high degree of improvisation is certainly not unusual in the global marine engineering and rigging experience. But its application to government-run national historic sites is somewhat out of the ordinary.
History1After the California, U.S.A. gold rush in the mid-1800s, prospectors knew from the geology there was gold to be found farther north. About 20 years after the California strike, there was another strike in British Columbia, Canada. By the 1890s a small group of miners and prospectors moved even farther north, to the Yukon Territory in Canada, looking for the next strike. See Figure 1. Gold was discovered on Rabbit Creek near Dawson City (later renamed Bonanza Creek) in August of 1896. These early miners and the influx of fortune-seekers from the "outside" eventually peaked with 30,000 individual miners each working their own claim with manual or barely mechanized techniques.
By end of the first decade of the 20th century, this individual approach was already giving way to fully mechanized corporate mining with an elaborate and expensive infrastructure. The Canadian Klondike Mining Company, Yukon Gold, and later the Yukon Consolidated Gold Company (YCGC) were three such corporate ventures. These companies operated by buying up all the claims on a creek and using dredges to get the gold the individual miners were not able to reach. Dredge No. 4, operated by YCGC, was the largest of this fleet.
Dredge No. 4 was built in the fall of 1912 and "launched" in May of 1913. It dredged Hunker Creek until a plank tore loose in the bottom and it sank in its own pond in 1924. It was refloated in 1927 and sank again in 1939. The ship was dismantled and re-built at the mouth of Bonanza Creek in 1940-1941. YCGC took the opportunity to changed the wood hull and superstructure to new material. They also lengthened the digging ladder and stacker, but re-used most of the machinery.
Dredge No. 4 worked its way up Bonanza Creek until the 1959 season, when the price of gold fell below the extraction cost, and YCGC ordered Dredge No. 4 to shut down. It sank in its own pond simply because there was no one to pump the bilges. The main deck was covered in five feet of water. Eventually the pond silted in around the ship, grass grew, and the Dredge looked rather like a building sitting in the middle of field.
Originally the company intended to wait until the price rose and then re-activate Dredge No. 4, but this never happened. YCGC donated Dredge No. 4 to Parks Canada in 1969. YCGC itself eventually became part of Teck Mining, a company who still has active operations in the Dawson area today.
Parks maintained the Dredge as well they could in its silted-in condition. Because of the permafrost that rendered fungus dormant, the frozen hull was actually better preserved than the exposed superstructure. Parks Canada had no real incentive to salvage the Dredge until differential movements, possibly caused by a growing ice lens under the hull or some other geotechnical phenomenon, started to threatening major structural damage. Parks Canada decided to salvage the Dredge and place it on a properly prepared foundation. In 1992, the Canadian Armed Forces 1CEU dug out the silted-in pond, cleaned out the hull, re-floated the Dredge, and sank it onto the cribbing upon which it now sits. Captain (later Major) Jones, who commanded the operations, quipped that the Dredge would be the largest ship the Army had ever sailed.
Description of the Ship, Bow Gantry and Rigging SystemDredge No. 4 is a 2000 ton bucket line dredge. The bucket chain consists of 72 buckets each being 16 ft3 capacity. The dumping rate is 22 buckets per minute. The chain is driven from a 300-hp electric motor running a bull gear and a large gypsy pinion called the tumbler located at the uppermost end of the digging ladder. The digging ladder, which supports the bucket chain, is a 170 ft long box beam. The depth of digging is controlled by changing the angle of the digging ladder through a running rigging system extending from a winch on the port side. Two 1-1/4" dia. cables lead from this winch, out across the bow deck, up the bow gantry, to heavy blocks at the very top of the gantry. The upper and lower blocks are 6 sheaves each, and the running rigging is dead-ended to the upper (fixed) blocks. The lower (moveable) pair of blocks is connected to heavy chain forward and heavy links aft, one system each on the port and starboard sides of the digging ladder. Thus, the weight of the digging ladder is taken by this running rigging system. The function of the bow gantry is fundamentally to support this running rigging system.
To balance the weight of the gantry structure itself and the running rigging force, the top of the gantry is stayed by means of a standing rigging system. Port and starboard sides each have a 2-1/4" cable dead-ended inside the digging ladder well at the level of the upper tumbler. This cable runs to the gantry and around an equalizing sheave, back to the digging ladder well and around another equalizing sheave, back to the gantry and around the final equalizing sheave, and is again dead-ended inside the digging ladder well. Thus, the bow gantry is stayed with 8 passes of this 2-1/4" dia. cable. During the working life of the vessel, the standing rigging was bar-tight, and we estimate a tension of 201,600 lb. or 90 tons total.
The bow gantry, like the hull and superstructure, is of heavy timber construction. The main posts at deck level are 17" x 27-1/2", solid pieces of Douglas Fir. The gantry is a flat frame structure leaning forwards an angle of 25 degrees from the vertical. The lower 5 feet of the gantry is in sound condition, protected by the ship's burial in permafrost for 27 years. Above the frost line the condition is not so good. At the knuckle joint (where the gantry frame narrows), the structural details (gusset plate arrangements) allow for standing water to accumulate on top and around the timber joints. There is a considerable amount of rot in this area.
Running back from the gantry to the superstructure are two compression struts. These make an angle of about 30 degrees with the horizontal. These are built-up members. For strength, the long sides of the lamination were placed vertical, but this unfortunately exposed the top joints to the elements and these members are similarly badly decomposed. Water runs down the struts and collects at the connection between the aft (bottom) end of the struts and their connection with the main longitudinal beam of the superstructure. There is likewise a considerable amount of rot present in these struts.
Background to the ProblemIn the fall of 1993, the Marine and Industrial Section (M&I) of the Heritage Conservation Program of Public Works and Government Services Canada (Alex Barbour, Kathleen Murphy, Richard Fairweather) visited the ship to conduct a baseline survey after the salvage in 1992. The M&I section had been involved in the on-going monitoring of the ship ever since Parks acquired it.
During that investigation we noted that the site crew had lifted the end of the digging ladder and placed it on a pressure-treated wood support. See Figure 2. As the running rigging normally took the load of the digging ladder, this action completely slackened all load in the running rigging. However, the bow gantry structure had been designed so that the standing rigging tension balanced the running rigging tension. With no load on the running rigging, this situation placed undue aftward bending stresses on the gantry posts and compressive stresses on the struts. These struts were already beginning to fail from rot. See Figure 3.
We informed the Superintendent that they should remove the support under the digging ladder. The Superintendent postponed the work until the next summer, 1994, as they had already laid off the seasonal work crew. But the next summer the budget for seasonal forces was cut substantially. The only work the site staff could undertake was the installation of a temporary post beneath the weak starboard strut.
In the summer of 1995, a structural engineering team visited the site to conduct a condition assessment of the wood. Mr. Michael Albright, Project Engineer at the University of New Brunswick's Wood Science and Technology Centre, and Lyse Blanchet, Structural Engineer with the Heritage Conservation Program, conducted extensive tests into the condition of the wood in the gantry and the struts. They used both a Densitomat drill and an acoustic device to find pockets of rot in the wood. During their investigation, they found the wood so rotten that it could not withstand the original loads if the digging ladder support were to be removed. It was already overstressed in one direction. Combined with the amount of rot present, the gantry could not tolerate being bent back in the other direction. They published their results at the 2nd International Conference on the Technical Aspects of the Preservation of Historic Vessels.
This new information really painted us into a corner. We could not leave the support in place, but neither could we take it out. The gantry had to be dismantled and the wood repaired.
In the summer of 1996, we again visited the site. Our task was to develop specifications for lowering the gantry to the ground for repair. The plan was that we would develop the specifications in 1996, and the work would be done in the summer of 1997. At the same time, we contracted to have a complete rigging inspection performed, and hired Intercon Marine, a marine rigging company, to investigate all the rigging systems. Structural engineer Lyse Blanchet was back on site, with a new structural engineer Khaled Ibrahim. Marine & Industrial's Kathleen Murphy was back, as was the section chief Alex Barbour, now retired and acting as a private consultant.
We soon discovered we were in big trouble. The gantry was noticeably worse than Blanchet found it from 1995, and very much worse than Murphy and Barbour found it in 1993. Instead of planning for work in 1997, we had to do something immediately, as it appeared the gantry was in the initial stages of actual collapse. Figure 6 shows the overall deformation of the gantry we saw in 1996. Compare that with the straight as-designed condition in Figure 4, immediately above. Figure 10 shows cracks straight through the upper end of the Port strut.
Immediately upon receiving word from us, Parks Canada closed the Dredge to visitors. Project Manager Pat Habiluk invoked emergency work procedures from the Treasury Board manual and hired Intercon Marine as the emergency work contractor. Intercon in turn hired Alex Barbour for his millwrighting expertise and familiarity with the Dredge machinery. We started working out options and requirements for this emergency work while still on site. Lacking any sort of monitoring equipment with us, and Dawson City not having very much available, we used spring-loaded sport-fishing scales from the hardware store. We set them all to read 10 lb. and connected them across the many cracks we had noticed.
Meanwhile, we left for Ottawa and Victoria respectively to prepare for the upcoming work. During our week of preparation time, Project Manager Pat Habiluk read these makeshift deflection gauges daily and sent us readings. The gantry was actively moving, and we were racing against time to effect our work.
PREPARATIONS FOR WORK
The GoalIn June of 1996, we believed that the gantry was not merely in an immanent state of collapse but was actually in the early stages of actual collapse. Our goals for the critical emergency work were damage containment and the reduction of active forces.
We brainstormed for possibilities, and considered many options and sub-options. For a time we considered deliberately provoking failure by washing out the digging ladder foundation with a firehose or using controlled explosives. The problem was not generating solutions (though we never counted them, they must have amounted to close to 20 alternatives), but in evaluating which was the safest, could be put in place that summer, and was most likely to succeed.
We were operating in the unknown: unknown forces, unknown conditions, and unknown degree of deterioration. Eventually we settled on a conservative strategy based on safety of working personnel and practicality. We had started writing the specifications even whilst on-site for the first trip. We refined it during the one week before the second. But even then, the degree of unknowns, changing site conditions, and endless logistic and supply problems meant we were adapting and taking decisions daily rather than following any prescribed formula.
We planned to install two structural support systems. We designed these to contain damage from a collapsing bow gantry to the bow area and prevent damage to the rest of the superstructure. The first system consisted of built-up columns underneath the failed longitudinal beams and their connection with the gantry struts. The second containment system was a "backstop" for the struts themselves. This would transmit aftward thrust to the digging ladder and thus force any failure to occur forward of the deckhouse.
To balance the standing rigging force, we needed either to apply load through the original running rigging or to install "preventor wires" to hold the top of the gantry in the forward position. Our original concept of the preventor system was to have new ropes running from the uppermost frame of the gantry straight down to the tip of the digging ladder and then up the digging ladder to be dead-ended on the tumbler shaft. On site, this proved too difficult to carry out: the required loads would have been very high, and the connection at the digging ladder tip was especially problematic. Furthermore, most of the work would have to be done right under the unsafe load.
The alternative was working with the original running rigging system. This would mean working inside the superstructure, at or aft of the port winch. As worker safety increased the farther aft (and away from the gantry) one went, this was the safest option. Furthermore, taking advantage of the 12-part blocks in the running rigging system meant we would require smaller-scale equipment and could apply less force.
The ContractorThe contractor for this emergency work was Intercon Marine Rigging and Marine Services. This was the same company we had originally hired for a rigging inspection in the first visit. Tom Whyte, a rigger and dredgemaster himself by experience, is the owner and general manager.
For the emergency work, Tom and Ross Shotton, the other rigger on the job, drove from Victoria to Dawson City with a portable rigging vice, a selection of come-alongs and chain blocks, gas torches, and hand tools. We arranged to borrow tensionmeters from the Federal Ministry of Transport: because with the short notice, Tom's were tied up on another job. By way of information, during most of the rigging work we did, Intercon used spliced eyes, a technique not used very often in modern steel wire rope, but one that gives the maximum load carrying efficiency of a line.
Although Intercon deals mostly with modern work in a modern shipyard, we (the Federal Government) were very impressed with Tom both for his technical ability, ability to improvise, and for being able to pick up quickly on the different requirements needed for work on a historic vessel.
Monitoring SystemsIncluding the fish scales discussed above, we eventually used four independent monitoring systems. We kept this system in place even after we got more sophisticated systems up, though we relied on it less and less.
To monitor tension in the standing rigging system, we used dental floss pulled bar tight on top of the standing rigging. We measured the sag between the dental floss, which being bar tight and almost weightless approximated a straight line, and the standing rigging. Khaled Ibrahim, the structural engineer, stayed in Ottawa to run a sophisticated structural analysis program for us when we went up the second time. He prepared a graph for us to use in the field showing tension as a function of deflection at midspan, the graph being developed from first principles. He considered the catenary curve of the wire, the fact that the two end points were not at the same elevation, and the fact that each pass of the wire was not exactly the same length (the dead-ends were farther back than the equalizing pulley in the digging ladder). We did not read the standing rigging daily, but rather before and after we did any work on the standing rigging.
Our main system for field work was four plumb bobs. Two were attached to 2 by 4s on held to the top of the gantry with carpenters clamps and pieces of redundant angle iron unbolted from the cribbing system and welded to the gussets. We needed the length of the 2 by 4s to clear the sides of the gantry. Two more plumb bobs we attached directly to the knuckle gussets. The weights were 10 lb. trolling weights we got from the hardware store. The four lines ran down to garbage cans filled with pond water. The string was fishing line, and stretched something like 8 feet under the weight of the bobs. We found the fore and aft movement quite accurate, but the up-and-down motions seemed quite unrelated to anything we did and we never did figure it out. We read the plumb bobs first thing in the morning, which was usually between 06:00 and 07:00, to see what had happened overnight. We read them again at the end of the work day, 12 to 14 hours later, to give a reference for the next morning.
The fourth and final system was Total Station, set up and read by the Parks surveyor, Robbie Van Rumpt. He prepared a steel mounting platform on a concrete base. Every time he set up his instrument he would back-sight to a stake in the ground to ensure the arrangement was consistent from day to day. He had a total of 16 monitoring points on the forward face of the bow gantry, later supplemented with 8 more points on the standing rigging. The great value of the Total Station was that Khaled could use this data as input to his computer model for structural analysis.
WORK OF THE SUMMER OF 1996
SummaryThere were basically four stages of work, most of it rigging but some of it structural. First we installed extra supporting columns beneath the bottom end of the gantry struts where these connect with the deckhouse. There was a considerable amount of rot present, and the joints were starting to fail. Second, we removed a bucket pin frame from the standing rigging, relieving some load on the standing rigging and hence some unbalanced force on the gantry. Third, we attempted to free up the running rigging system and apply some balancing load in the forward direction. We were unable to accomplish this. Fourth, we installed a mid-height stay to support the bottom part of the knuckle from moving forwards. Fifth, we slackened off the standing rigging to the extent permitted by the historic system. Sixth, we installed a "backstop" at the bottom end of the struts, the purpose of which was to prevent the struts punching through the deckhouse in case of a collapse. Finally, we installed a stay on the forward side of the gantry, running from the top of the gantry down to the tip of the digging ladder.
As this paper concerns the rigging work from the summer of 1996, I will be discussing the structural engineering in passing only.
Running Rigging WorkThe deteriorated condition of the gantry wood did not allow us the simple and obvious solution of removing the digging ladder support. So instead we attempted to apply a portion of the load back onto the running rigging by taking in the slack created when the digging ladder was raised.
We could have done this by an external rigging system, say pulling the running rigging blocks together, but this would have meant working with relatively high forces right on the failing gantry structure itself. For safety reasons, we chose to attempt to get the running rigging system un-siezed and apply loads directly using the mechanical advantage of the 12 part blocks.
We clamped onto the running rigging system just in front of the deckhouse, and ran our cable underneath the winch back to a strongback placed against columns about midships. Khaled Ibrahim, analysed the proposed system and said we needed to brace the port side of the strongback further, as that column was not strong enough. So we ran another cable back to the aft end of the ship, out through a pre-existing hole, to a 3" dia. fid bearing against a steel plate that we fitted against the transom just about at deck level. See Figure 7, Figure 8 and Figure 9.
First, we wanted to pull sufficiently on the running rigging system to isolate the winch from the live load. Then we could work on freeing the winch enough to turn the drum to take up the slack and we could leave the system with the winch again holding the load. Even if we could not get the winch to turn over in the time period allowed, we had a fall-back option where we could tie off to a steel beam placed aft of the winch and bearing against the winch foundation.
Unfortunately, we were not successful in even isolating the winch let along applying load through the system. The re-direct sheaves on deck and atop the gantry were seized. See Figure 4. We were able to free up the deck sheaves by heating, but even heating and hammering could not free up the port side sheave on top of the gantry. The sheave was bearing against its strap and seems to have operated in that condition for quite some time as the inside of the strap was gouged rather badly. The Project Manager found historic maintenance records showing that this condition gave YCGC problems during the working life of the Dredge. There were no button jacks available in Dawson, and we did not have time to order any from Edmonton or Vancouver, each far enough away for a 3 day delay we could not afford, so in frustration we had to abandon work on the running rigging.
Middle StayOur attempt to isolate the winch seems to have disturbed the fragile gantry and caused a sudden forward movement of the knuckle area -- 1/4" in two days. We had made things worse, not better. Afraid of continued movement, the Project Manager asked us to put in a mid-level stay to support this part of the gantry. The movement seemed to show the gantry was pivoting around the working platform, with the part above the platform moving back and the part below the platform moving forwards.
The middle stay system consisted of two lines, each running from the tumbler pin at the top of the digging ladder out to the arched gusset plate in the middle of the gantry. The lines crossed in space, as the line connected to the port side of the tumbler shaft was connected to the starboard side of the gantry and vice versa. We needed softeners for the corners of the gusset plate. We also needed stiffeners between the gusset plates, as at the corners these were not backed with wood. We scavenged the dump for 4" steel pipe that served both purposes nicely. See Figure 5.
To tension the system, we were unable to find and borrow turfer jacks for 3/4" dia. cable anywhere in Dawson. Purchasing them would have been prohibitively expensive as well as time consuming, so we made do with the perfectly serviceable low-tech alternative of Spanish windlasses, using smaller diameter scrap pipe for snipes.
Standing RiggingTo get the standing and running rigging systems back in balance, we tried not only to increase the force in the running rigging system but also to lower the force in the standing rigging system.
We performed two operations on the standing rigging. The first operation was to remove a steel frame originally used whilst removing old dredge buckets. This frame was about double the weight I (Kathleen) had estimated by eye, and so the beneficial effect (lowering the forces in the standing rigging) was better than we all had expected. This frame is visible on the standing rigging in Figure 6, but has been removed in Figure 5.
The second operation on the standing rigging was the major operation. We wanted to slacken the standing rigging as much as was possible with the given configuration. Both dead ends of the standing rigging cables are found inside the digging ladder recess. The system can be tensioned or slackened by means of a pair of nuts on threaded rods. These nuts were a little over 4" across the flats. We had about 9" left that we could turn out on the threaded rod and still keep a full nut on the rods. Parks carpenters built a platform so we could work more easily on the sloping digging ladder. We diligently checked but were unable to find wrenches the right size from amongst the artifact tools in Parks storage. We undertook to make wrenches from scrap materials, and used sections of pipe as handles to give the required leverage.
We backed the nuts off slowly and evenly, and took readings with the Total Station every step of the way. We spread the process over two days to give the wood of the gantry time to catch up with the actions we were taking on the nuts. This was extremely cramped and heavy work. In total, we backed off the nuts 32 agonizing turns. To prevent the equalizing sheaves, which were pivoted, from swinging down and taking up some of the slack, we welded angles along side the sheave straps before turning out the nuts. Versatile Welding from Dawson City, installed these angles.
The change in standing rigging tension from these two actions was considerable. From an estimated 90 tons total before we started, after both operations were complete the tension was down to 65 tons. We felt we finally did some real benefit to the structure.
Box Beam InstallationAbout this same time the steel ordered for the strut backstops arrived. The main component of this system is a 12" x 12" x 1/2" hollow structural section (HSS) 18 ft long. We had to manoevre this into position on the digging ladder just underneath the bottom ends of the struts at the deckhouse.
There is no building in Dawson City higher than 2 storeys, and so the only cranes available to us were too small to lift the HSS up and over the standing rigging to get it into the required position. Tom Harvey, a Parks carpenter, suggested a technique the carpenters had previously used to get heavy timbers up onto the ship. We built a scaffold alongside where the box beam was to go, lifted the box beam, and lowered it onto rollers (sections of scrap 2" pipe) on top of the scaffold. We waited 5 or 10 minutes to proof-test the scaffolding, all the while with the crane still hooked up and his line only barely slack so he could pick it up again if the scaffold started to buckle. When all seemed well, we unhooked the crane and hauled the box beam into place underneath the struts. The rest of the system was installed by Versatile Welding.
Forestay InstallationThe last item of work we did was installing a forward stay running from the top of the gantry down to the tip of the digging ladder. This system consisted of two lines (one each port and starboard), dead ended to the existing pad eyes on the digging ladder. We made pins to fit the holes in the pad eyes, and found scrap sheaves that just fit. From there, the lines went up to the head of gantry, and back down to the pad eyes on the digging ladder in order to run the line out to our anchor, the back axle of a work truck parked ahead of the Dredge.
For the connection at the top of the gantry, we again had to fabricate parts, making snatch blocks out of scrap materials. It looked like this homemade block was going to be the weak link in the system, so we proof-tested it with a 4600 lb. dredge bucket as a proof load. We did this the same day the crane was on site for lifting the box beam in place.
At first, Khaled had wanted us to put as much load as possible though this system to compensate for the fact that we did not get any joy from the running rigging. But our system capacity was too low and this would not leave us with any reserve in case of movement. So instead he directed that we left the system just snug. Then, if there was to be any movement, the forestay would gradually become activated up to its capacity.
CONCLUSIONThe job evolved into a strange combination of ultra-high and ultra-low technology. We scavenged materials and fabricated whatever we could not purchase. Sometimes we were forced into using an exceptionally low-tech solution to a problem and found it was cheap, easy and effective (e.g., the use of Spanish windlasses). Yet at the same time, we had access to sophisticated computerized structural analysis on an almost 12-hour turn around time. It was typical of the job to be discussing readings on plumb bobs and $1.95 fish scales via satellite telephone.
It was frustrating for us never to have the materials and supplies we needed in Dawson, never to have the information we wanted, and never really being sure if we were doing the right thing. Back in Ottawa, Khaled Ibrahim never knew what would be awaiting him on the fax machine in the morning as we deluged him with requests for calculations and scenarios to run through his computer model. It was frustrating for Khaled as well, because sometimes by the time he had found the answer to our question our parameters had changed or supply and delivery problems meant we could not carry out the solution he had spent time developing for us.
For six weeks we spent every waking moment together working on the Dredge, either at the Dredge or meeting over supper and into the night. The co-operation amongst all parties, public and private, was seamless: everyone's only concern was the ship. We had an exceptionally keen contractor who caught on quickly to the requirements of working on a historic vessel and who quickly grew to love the ship. The Parks staff bent over backwards to accommodate anything we needed -- a typical example was Irwin Gaw, the maintenance chief, who gave us permission to go and take the barbeque tank from his back yard to use with our tiger torch. It was an unusual project, and it begat an unusual working style.
In working on a failing structure everyone on the team had to decide daily whether they would climb the gantry again. Many times we looked around to make sure there was a clear path for a jump to the ground should such a jump become necessary. We each had our favourite monitoring points that we checked and re-checked daily to get a gut feeling about the gantry. One checked a particular crack, another observed the angle of a chain link, others the feel of a wire when stood upon. Despite the measurements of the Total Station or even the plumb bobs, the work of 1996 was very much an art -- a "dance with wires" -- that relied on a lot of gut feeling.
As of today, the emergency measures we implemented are still standing. We have prepared a specification of the dismantling of the bow gantry, and this contract is currently in the tender period, with work due to start May 5, 1997.
1 Statistics and historical information taken from Make It Pay! Gold Dredge No. 4, David Neufeld and Patrick Habiluk, Pictorial Histories Publishing, Missoula, Montana, U.S.A., 1994.
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