Table of Contents
The functioning and capacity of a dredging system must be keyed to the environmental and natural conditions of climate, water availability, location, and the physical and geologic characteristics of the deposit. The investigation of a placer deposit should be made by or be in charge of an experienced person or organization familiar with how these factors affect placer evaluation and successful gold dredging. In sampling to determine gold content the engineer must be aware of the differences between his calculated content, the actual content, and that recoverable. While the engineer strives to achieve an estimate approaching the actual content, his appraisal must consider the recoverable, for on this depends the success of the dredging that follows.
Most dredging companies do not contract exploration drilling. They have found that they get more accurate results and can better determine depths of values and types of formation if they can keep drilling crews under direct supervision. The experience and judgment of the engineer on the job is extremely important.
Preliminary holes should be sampled very carefully to determine the lateral and vertical distribution of the gold, whether the gold occurs in wide or narrow channels and whether distribution is fairly uniform or in irregular pay streaks. Ground that shows a spotty or irregular gold distribution will obviously need more tests than ground yielding fairly uniform samples. Where sampling discloses a channel or narrow belt having a higher value than that of the whole, its outline should be determined as closely as economically purposeful. Rows of holes normal to the channel best do this job. It might prove desirable to concentrate later mining only within it and avoid working the whole deposit. Usually it is feasible to segregate certain parts of a property and eliminate low-grade areas. When a tract is large and the time for examination is limited, tests to confirm the results of preliminary prospecting may be concentrated in an area large enough to determine the advisability of purchasing the property and installing at least one dredge.
Properly obtained samples from adequately spaced shafts give the most accurate results, but the technique is so slow and costly that it is usually used only to check the results of specific churn-drill samples and to give additional details on stratification and gold distribution throughout the depth of the placer. Shaft sinking is best applied to terraces or benches above the stream beds or to large level deposits where heavy flows of underground water will not cause trouble.
The use of the churn drill for placer evaluation was first tried in 1898 at the Boise Basin operation of the Boston and Idaho Gold Dredging Co. and has been a standard placer sampling tool since that time; it is the quickest and cheapest method, and the best to use in water-saturated gravel. As a general caution, any sample from a hole not cased as it progresses is subject to considerable doubt. A method using noncirculating water, churn drilling is the technique of driving heavy drill casing to bedrock carefully recovering the material from within on predetermined intervals, usually 1 to 2 feet, or at formation changes by a bailer often called a sand pump. Water, normally supplied by that in the formation, sometimes must be added to increase fluidity for bailing. Much of the time a heavy churn or cable bit is necessary to break or churn up the “core” of material standing within the casing and sometimes the large material ahead of the casing. Results can be erroneous even with constant care. The object is to obtain all of the core material representative of the outside diameter of the casing and the depth interval that is being tested. In general, the tighter the formation the better the sampling results. A careless driller (1) may neglect to keep the casing shoe slightly ahead of the bit or bail and thus recover more material from a caving hole below the shoe than is representative; (2) may drive the casing too far ahead in loose ground so that it becomes plugged and subsequently forces the underlying material out into the walls such that less material is recovered than is representative; (3) may neglect if in very loose, water-saturated and running ground to purposely drive the casing farther ahead of the bit than usual to reach firmer ground and thus prevent taking a sample that is excessive and not representative; (4) may not follow driving with bailing if in fine loose ground where the bit can pound the core down and out of the casing so that the sample is less than representative; and (5) may not drill a few feet into bedrock to recover the gold on bedrock and to make sure that the casing is not resting on a boulder. Sometimes the bit can strike a fracture in bedrock containing gold that should be kept separate from the gold in the gravel. In bouldery ground where the bit might force the boulders downward and outward, the sample may be less than representative, despite all precautions. Knowing the exact depth of the casing shoe relative to the top of the core within the casing at all times, and the volume of the sample collected with respect to its theoretical in-place volume is a must in any alluvial drilling project. Possibly two of the better papers that cover the assignment of various compensation factors for core-value calculations, in one case a safety allowance for excess sample material and in the other for an anticipated loss of gold because of its settling to the bottom of the hole, are by Jackson and Knaebel and Avery. Avery first suggested that compensation be based on in-place measurements of core rise rather than on arbitrary factors.
In frozen ground in Alaska and the Yukon Territory, Canada, exploration engineers often drill their holes uncased, except for setting one length of casing to seal out surface water, and claim results to be more reliable than from cased holes drilled in unfrozen ground. Advantages of drilling in frozen ground are—
- The mechanics of drilling without casing are considerably less involved.
- There is less sloughing so there is less chance of inadvertent salting of samples.
- In-place sample volumes can be determined more correctly.
Patty reports that an experienced driller can put down a hole in frozen ground as smooth as a rifle bore with minor raveling only if the hole intersects gravel that was dry when frozen. Sample reliability is reportedly increased in “open-hole” drilling because water-displacement measurements can be made to correct for variations of in-place “core” volumes, a point sometimes of considerable controversy when in unfrozen ground. Engineers unfamiliar with frozen ground might be reluctant to drill without casing, but open-hole drilling is standard procedure by engineers experienced in northern placers.
Mechanized panning equipment is available to concentrate the heavy minerals in the samples. For a single drill operation, one man skilled in the use of the pan can often evaluate probable recovery and keep up with the drill. Total or actual gold content will also require checking the gold content of the tailings. Normal procedure is to evaluate exploration results utilizing low-cost gravity-concentration techniques comparable to those used on a dredge.
It should be remembered that gold placers as a whole are nonuniformly distributed low-grade deposits and that 25-cent gravel, or gravel that averages 25 cents worth of gold (at $35 per ounce) per cubic yard contains by weight just 1 part gold in about 6.5 million parts waste. Churn drilling is not an exact procedure and there are no mathematical processes to precisely determine the actual gold content of most placers from churn drilling results. Although large bulk samples from shaft sinking will give more accurate results, they are slower and costlier to obtain. It is recommended, however, that a shaft be sunk occasionally if ground water does not prohibit it both to check drill-hole results and to examine the gravel, particularly the size and number of boulders, and bedrock in place. Sinking should go a few feet into bedrock to determine the depth of weathering and the gold it might contain. Even then, placer estimates from sample values should be considered as estimates.
Case histories that point up the sometimes inconsistent results of placer values, when tested by both churn drilling and shaft sinking, are covered in a two-part paper written by C. W. Gardner. The methods are compared against one another and against actual recovery by dredging. Two other papers, one by Hutton and the other by Smith, similarly cover the problems in placer sampling and their relation to dredge recovery, Smith’s paper, if considered on the basis of modern dredge treating systems, especially in washing and jigging, is thought to be one of the better papers for evaluating placers, for it also covers the conditions that effect ultimate recovery.
The proper location of holes and the interval at which samples should be taken can be determined only after a preliminary study of the ground has been completed. There are no hard and fast rules to follow. Some areas have been considered sufficiently prospected with one drill hole to as much as 10 acres, while in other areas the engineer has not been fully satisfied with one hole to an acre.
As part of the placer appraisal, the engineer must be aware of the following operational problems that can affect gold recovery, but which can be overcome or greatly reduced if fully understood and planned for;
- Gold left in depressions and crevices in bedrock.
- Gold left as a result of a poor mining pattern.
- Gold lost because of a caving bank.
- Gold lost due to high density of muddy water in the pond, which, together with the turbulence set up by the digging action, hinders settling and permits some fine gold to be carried away from the face.
- Gold lost because of material dropping from the bucket as it moves up the ladder.
- Gold lost in the treating plant when not washed free of the oversize or when it adheres to clay.
- Gold lost as an occasional nugget too large to go through the screen.
- Some coated gold lost if mercury is used in conjunction with tables.
More or less in the order of importance, evaluation of a dredging prospect should consider:
- Value, character, and distribution of gold.
- Physical characteristics of the formation,
- Character and contour of bedrock.
- Availability of water and problems inherent in stream operations.
- Land acquisition.
- Adverse climate.
- Local and national regulations.
Value, Character, and Distribution of Gold
To repeat, gold values assigned to a deposit should be based on that recoverable by the mining and treating methods to be used. The gold must be free to be effectively processed in modern gravity concentrating equipment.
The size and occurrence of the gold particles and other minerals to be recovered should be noted in any engineering evaluation. If the particles are coated or rusty, a ball mill in series ahead of the amalgamator is needed to polish the gold particles, or a modern jigging system is necessary. Extremely fine flaky particles can affect the design of the jig system. Processing time can increase as the flake size of the gold decreases, which could mean an enlarged gold saving circuit or a decrease in the production rate. Pilot evaluation becomes more critical as the size of the gold flakes decreases.
Physical Characteristics of Formation
Gold dredging is most applicable to large, flat-lying deposits. Dredging is most applicable on surface grades up to 2 percent although there have been isolated instances where grades up to 6 percent have been traversed. The quantity of material in a deposit or in a group of deposits to be dredged influences not only the capacity of the dredge to be selected, but whether the dredge should be portable. As the size of a deposit decreases, portability should increase. The initial capital cost of portable equipment, however, is somewhat higher than for nonportable equipment. A 6-cubic-foot-capacity portable dredge has been dismantled in 30 to 35 days and reassembled in 40 to 45 days. This type of dredge, if used on some of the California properties during the 1930’s, would have been more profitable than the shore-mounted dragline dredge actually used.
The depth, character, and quantity of gravel to be mined influences the shape and size of the buckets which in turn influences the design and capacity of the processing equipment. Depths that should be determined are those from water or pond level, whether natural or developed, to bedrock and from water level to the ground surface. Within certain ranges, bucket capacity is increased as the thickness of the material from water level to bedrock increases. The relation of bucket capacity to digging depth is generally as follows:
There are near lineal relationships between the above maximum and minimum digging depths and their respective bucket sizes. As the dredging depth increases the ladder design must be changed to support the increased weight of the buckets and the increased strain of digging. The maximum digging depth to date has been 162 feet. Future depth will be increased as demand and designs take advantage of the metallurgical improvements in lightweight metals. Also, a catenaryless bucket-line design should extend digging depths. The shape of the bucket is also influenced by the character of the bank above water level. Where a bank stands firm and it must be dug to the top or to a predetermined shelf height, the face of each bucket is at a high angle and all material in excess of its natural repose in the bucket will spill out onto and over the side of the ladder. This can reduce recovery.
A high bank can reduce digging depth which in turn can reduce capital equipment cost by reason of a shorter ladder. A high bank, however, requires a longer stacker and tail sluices. Bank height depends upon the type of ground, the thickness ratio of that above water to the total being worked, and the length and draft of the hull. Water depth must be sufficient to insure maneuverability and to preclude shoaling of the stern of the hull on the rejects. The optimum bank is one in which the upper part of a high bank spalls relatively easily and continuously as it is undercut by the buckets, while the ladder is well below the top. Usually if the bank is over 15 to 20 feet high, monitoring to a 15- to 20-foot shelf some 25 feet ahead of the main digging face is required. Banks as high as 50 feet above pond level have been handled this way to keep them from caving in large chunks onto the ladder. Serious accidents, however, have been caused by a caving bank. In the deep digging operations at Hammonton, Calif., intermittent 56-foot banks were monitored ahead of the ladder, but this was in tailings of a previous operation rather than in virgin ground and the problems of waste disposal in regards to swell were not present. In frozen gravel, records tell of broken ladders and near swamping of the dredge by waves initiated by large caves of frozen bank. An interesting reversal of this situation developed at an operation near Fairbanks where a high bank of naturally thawed silty overburden had to be partially frozen by piping a refrigerant through a system of vertical holes drilled in the bank so as to keep it out of the pond.
Bank height should also be considered on the basis of clay content. With high quantities of clay, bank height should be limited. As the sloping pile of fines from the tail sluices build up on the bottom of the pond behind the dredge, the superimposed weight of the overlying oversize can slide the whole pile forward and downward on mud slickened shear planes toward the stern of the dredge. Excessive amounts of fines, together with heavy amounts of clay slime, can worsen this condition. Large amounts of fines, usually from a thick layer of overburden, can also slide forward on the bottom of the pond to crowd the digging line. There have been instances where it was found that the buckets were bringing up some material that had already been put through the treating plant, a problem primarily one of deeper digging dredges. Length of hull should be sufficient to insure that digging is forward of the natural toe of the tails. Slides onto the stern can be more serious than a caving bank ahead.
Bucket design is also influenced by the size of the boulders and the cohesiveness of the formation. Buckets of standard design can jam and become damaged if boulders larger than the pitch become wedged between them as they straighten out when leaving the lower tumbler. Large boulders cramped between buckets can tear off or damage bucket lips, tear or crush the hoods, or cause other trouble. Special rock buckets have been developed by increasing the bucket pitch or the distance between buckets, and setting the cutting or leading edge of the lip below or lower than its normal position. This also allows easier dumping. On a dredge in Trinity County, Calif., large boulders were handled with 12-cubic-foot capacity buckets which had the bucket pitch of a standard 18-cubic-foot bucket line. Yuba built several dredges of various capacities using this design. The extra cost to move or mine around oversize boulders or to handle them aboard a dredge can become an important factor in near marginal deposits. Large surface boulders are usually broken with explosives.
When lenses or islands of cemented gravels are encountered, it is often better to grind through these if possible, rather than break them up by blasting. In addition to the extra cost, blasting often breaks such rock to a size too large to be handled by the buckets, but too small for the buckets to break them down further. If the cemented gravels are too thick or too large to grind through, they must be bypassed. If extensive, they might rule out dredging altogether. Large boulders within the cemented mass can complicate the problem even more.
In permafrost areas frozen ground is generally thawed prior to dredging. Cold-water thawing, the most economical method for large scale thawing because artificial heat is not supplied to the ground, was first tried about 1915-17 in both Alaska and Yukon by a number of experimenters. Thawing is accomplished by circulating water from 40° to 55° F through a system of drive points driven to bedrock. Steam thawing in the United States was discontinued because of its cost only high-grade deposits could stand the expense—but reportedly is still being done in the U.S.S.R. The deepest gravel thawed has been 110 feet, accomplished through churn-drill holes. About two summers are ideal to thaw and properly season the gravel in Alaska, but at some mines in shallow gravel it was common to dredge the same season the ground was thawed. This, however, was not the general practice. A system of naturally thawing 12 to 20 feet of gravel was developed by stripping off the overlying barren muck, and allowing three seasons for the thaw to penetrate 2 to 3 feet of bedrock. The general rule of thumb near Fairbanks is that each foot of depth requires 1.25 to 1.50 days to thaw. Until recent years when dredges were started late in the season, steam thawing was sometimes used to break up an average of 4 to 5 feet of seasonal surface frost just ahead of the dredge, which later in the summer was discontinued when solar thawing could take over.
Clay content also affects the design of the bucket as well as that of the treating plant. Clay is often found as barren overburden or as lenses or layers within the deposit. Disseminated throughout the deposit, it causes the least trouble. To facilitate dumping clayey material, some buckets have holes at the back of their hoods to relieve the partial vacuum as they dump. In addition, high-pressure water jets can be applied to cut between the clay and back of the bucket. Operators often find that when in clay, smaller bucket loads, which are easier to empty, actually increase digging rate. Some mechanical clay extractors triggered by the travel of the bucket line are used in Malaysia to pull the clay free. Some clays are called “gold robbers.” Sliding along with the fines in the screen or rolling across the tables or jigs, small balls of clay can pick up the gold particles which are consequently lost as tailings. Most clay must be disintegrated or slurried prior to tabling or jigging using either a hammer mill or a log washer. The problem becomes more acute if the clay is from false bedrock layers or from the bottom of a deposit where the gold is richest. As the size of the gold flakes decreases, a more complete job of slurrying becomes necessary. Sometimes the clay can be partially broken down before it is sent to the hammer mill or log washer with thrashing chains, high-pressure jets of water, or puddle knives placed within the revolving screen. Other times, materials such as seashells occurring naturally within the clay can help break it down. In extreme conditions, some clayey formations cannot be dredged profitably. Often the upper shell-plate area or intake end of the screen has been used effectively as a grinding mill. In one case very hard angular blocks of granite were added and retained within the trommel as an aid in scrubbing. The Olson patented lower shell-plate area at the discharge end of the screen was also used for an additional washing and screening area.
Redredging placer ground can be economical when one or more of the following hold:
- Undisturbed placer ground remains at depth, possibly even an enriched bedrock zone, that can be reached with deeper digging equipment.
- Sufficient gold remains in previously dredged gravel that can be recovered with improved treating equipment.
- Gravel and clay material loosened and broken down by previous operation can increase productivity and lower operating costs of redredging. Because of easier digging, larger dredges can often be used.
Character and Contour of Bedrock
The character of bedrock and the manner in which gold is concentrated or trapped there can affect not only the design of the buckets but also that of many of the dredge components. Bedrock that is soft or so fractured and weathered that it can easily be dug offers a distinct advantage toward maximum recovery. Similarly, recovery is better if bedrock is flat. Where the bedrock is hard, blocky, and uneven, a heavier ladder, heavier spuds, sturdier design of the driving mechanism, and heavy wear plates placed on the bottom of the ladder where it rides the buckets are then usually necessary. Change in bucket design is toward smaller and heavier construction with decreased capacity. Smaller buckets provide greater digging capability with lower energy requirements because the lip alternately wedges, compresses, and shears the rock loose. It is common practice to dig ½ to 1 foot of bedrock, but in one extreme situation it was reported that 10 feet of blocky and slabby bedrock was removed to recover the gold. Knowledge of the fracture pattern on bedrock, its contour, and its slope may influence the direction and pattern of dredging as well as the height of the pond level and the bank. Mining slows down and recovery drops as bedrock becomes more irregular. The final swing of the ladder across the bottom is slowed down depending upon the irregularity of bedrock and the difficulty of cleanup, the value of its gold, and the amount of material that has sloughed from the face and has spilled from the buckets. Blasting channels through hidden protrusions of hard bedrock for the necessary depth to float the dredge has caused costly delays. This happened at one foreign operation in shallow gravel where prospect holes had been drilled on 100-foot centers. Blasting through buried dikes of hard rock was one of the causes that closed down a 9-cubic-foot operation in Featherville, Idaho. Buried fault escarpments have also been the source of costly delays.
Availability of Water and Problems Inherent in Stream Operations
The quantity of water required varies with the size and type of dredge and with the character of the formation to be dredged and treated. The fresh water that runs into the dredging pond does not necessarily indicate the quantity of water needed in the operation, because the water from the dredge pond is constantly reused. Dredges of the 9-cubic-foot class often use about 75 gallons of water per minute and those in the 18-cubic-foot class about 150 gallons per minute. This, however, depends on the conditions mentioned previously.
Of primary importance in the recovery system is the condition of the water–the cleaner, the better. Heavy clay sediments are detrimental to most treating systems. Slimes are mostly a product of the treating plant but considerable amounts can come from the digging operation, especially in deposits with thick layers of fine overburden. If the water becomes too contaminated with clay, it is usually cycled to a settling pond for clarification. In Malaysia where muddy water conditions have been severe, the treating water is taken off the top of the pond where it is clearest. In the ponds of the deep digging dredges at Hammonton, Calif., although little clay was present, the accumulation of sediments was so great that a pump had to be set to remove the thickened sludge off the bottom of the pond. The pump suction was dropped overboard at the bow of the dredge, where it did not interfere with operations, to a depth of at least 90 feet. The sludge was pumped through a line fixed to the stacker to discharge it at some distance to the rear on the rock tailings pile where the water could filter back to the pond.
Dredging in rivers requires modifications in the design of the dredge hull as well as in the design of the dredging pattern. River dredging requires that:
- The hull be enlarged to assure more freeboard—as much as 5 feet is sometimes necessary.
- The hull be cut back on the bottom at the bow and the stern to allow swift water to flow easily under the bow and out again at the stern.
- The dimensions of the hull be properly balanced for stability when maneuvering in rapid water.
- Headlines be used to hold the dredge in its digging position in waters that are swift or subject to rapid rises.
- The course of dredging be upstream.
- Care be exercised to see that logs or other floating materials do not damage the hull in rapid waters.
- Sidelines be as high as possible to prevent entanglement with floating debris.
If the river is navigable, arrangements must be ready to allow other boats to pass. If the current is not swift, passing can be accomplished by dropping the sidelines. Most dredge hulls have a vertical bow wall set square with the deck to increase bow support under the forward gantry, which in turn supports most of the digging ladder. In swift water, however, a beveled or raked bow wall will minimize the height of the “piling up” effect of the water and its frontal or downstream thrust, and the strain on the shorelines. Rake at the corners of the stern, in addition to that across the stern, will permit the flow to be displaced into the low-pressure zone astern to lessen its downstream pull.
The surface of a deposit should be mapped and contoured. This is most important with respect to bedrock contours to obtain maximum recovery. Surface contours will assist when plotting the course of the dredge to make sure that all of the gold-bearing gravel is reached. In a South American project the undredged yardage lost because of inadequate planning might have been the difference between loss and profit. If the placer is shallow, a contour map of bedrock can be critical in charting the dredge course. Contours of the surface are needed to determine where dams should be placed to make ponds sufficiently deep to dredge and to advance upgrade.
If public power is economically available it can be transmitted aboard the dredge by the standard flexible power cable especially designed for dredge service. When not available, then the type and location of the generating plant must be determined. For a small operation it may be more economical to generate power aboard the dredge with a diesel-electric set or in some cases use diesel power direct. The decision will depend upon the needs, economics, and time to complete mining. If the project requires a large dredge or more than one small dredge, the generating plant should be onshore and centrally located. In each of the above cases the one thing that must be kept in mind is dredge efficiency and operating time. The object is to provide enough power to cover the unusual conditions so as not to overload the equipment. Demand power loads will vary primarily as the difficulty in digging varies.
As a mining unit, the labor required to operate a dredge in terms of yards excavated is no doubt the least of all types of mining. A dredge is a completely mechanized, large-volume, self-contained, mine-mill unit that digs, treats, and backfills its waste in a matter of minutes. Dredging crews are usually the minimum essential to maintain an efficient operation and it seldom pays to try to reduce labor costs by further reducing the crew. The percentage of labor costs in unit-dredging costs bears this out. Overall, as dredge size and production increase, the ratio of direct labor costs to total direct costs decreases. Stated differently, the more a dredge has produced and the lower its direct costs, the lower in turn has been the percentage of labor in those costs. While the ratio of labor to total costs increased for all dredges during 1930-60, the amount of increase varied depending upon the size— the smaller the dredge, the greater the change; the larger the dredge, the less the change. In 1930, the average ratios of labor to total cost going from the 3½-cubic-foot size to the 18-cubic-foot size were about 42 and 28 percent. By 1960 these averages had increased to about 65 and 43 percent.
The smaller dredges require a crew of three per shift—a winchman-dredge master combination, an oiler, and a shoreman. As the dredge size increases more men are required, but even for the largest dredge the number need seldom exceed five to six men per shift, consisting of a dredge master who is on call 24 hours, a winchman, two oilers, a spare man for jig attention, and one or two shoremen. Additional help must be provided for swing-shift duties so as not to work the men more than 40 or 48 hours per week. However, in Alaska and isolated short-season areas, it is customary to work 12-hour shifts. A repair crew is required when more than one dredge is operated. The repair crew can start with one man who has a combination of skills as an electrician, surveyor, and welder, and be increased when necessary. In many single-dredge operations it has been the practice to use the regular crews on overtime to make almost all repairs, large and small, whether of maintenance or emergency. Usually the dredge master is busy with surveying and limited office work.
Generally, the best dredge crews are those trained on the job. However, it is well to start with an experienced winchman if possible, but that is not necessary for the smaller operations. It has been found that men who have built dredges usually become efficient workers on a dredge. In the past it was customary to look for an alert, mechanically oriented man and develop him into a winchman and possibly into a dredge master, and occasionally into a superintendent. With a limited crew, labor turnover should be minimal, and companies that have fewer labor problems have the better operating records.
Where the economics dictate, it is usually better to mine the land on a royalty basis for the cost of the land in effect is paid for as the project proceeds. This usually suits the operator with small working capital. The usual royalty is 10 percent, but if high values are present, the landowner may want his royalty paid on a sliding scale proportional to the gross recovered. Prospecting placer ground under an option to buy if sufficient values are found can prove to be the best plan to follow if there is reason to suspect that the deposit might contain higher than average values. Mining on claims is in order if a properly secured contract is negotiated. It is assumed that care will be taken to insure that the claims are valid and negotiable.
Climate can determine the length of the working season, the number of yards produced per season, and the return on invested capital. In general, the minimum gold content of a placer must increase as the working season decreases. Cold weather may require that the dredge, except for the ladder, be completely closed in and heated (fig. 3). To prevent the ladder from freezing, steam pipes can be placed under its pan. Steam directed into the hopper will keep the gravel free. Operating costs increase because of these heating costs and the need to frequently remove the ice that forms on the digging ladder, hull, and stacker. The stacker is the most vulnerable place on the dredge and heating it begins before it does on any other dredge component. Freezing of spill material and ice buildup on the head pulley and underside of the belt require prompt and steady attention. O’Neill’s paper is one of the better accounts of arctic dredging conditions.
Some operations in Alaska, Canada, and the Northwestern States are usually suspended in severe cold weather. Operations in Alaska and the Yukon Territory are normally down for at least 5 months. In most parts of interior Alaska the average temperature by the middle of April is usually above freezing, but the ice and winter frost are not often thawed until the middle of May. The season can be increased by as much as 2 months by removing the 2½ to 5 feet of pond ice and the last winter’s frost ahead of the dredge. Operations in Montana or equally cold areas close down for approximately 1 month.
Suspension of operations during cold periods also increases yearly operating costs, but experience has shown it to be less costly than running. Some companies, like the Goodnews Bay Mining Company of Platinum, Alaska, which shuts down for about 5 months, have proven that preventative maintenance after closedown in the fall and before restarting in the spring yields a much higher trouble-free period of operation. Goodnews Bay has compiled a 97-percent running time for its 7-month operating period. One advantage of winter downtime is that dredges can be moved in larger sections on sleds; this reduces the cost of dismantling and erection compared with rail or truck freight costs.
The Fairbanks Department of U.S. Smelting, Refining and Mining Co. did this on
several occasions in the Fairbanks area. Ludwig’s account of how a 1,200- ton dredge was sledded across 7 miles of frozen ground in three sections, the largest weighing 680 tons, by 18 large diesel tractors over grades up to 11 percent is fascinating reading.
To have a good crew return when dredging is restarted, some special arrangements may be necessary. In Montana one company during downtime kept part of its crew on maintenance while the rest took their annual paid vacation.
Local and National Regulations
State mining laws, fish and game laws, and Federal regulations and tax statutes should be investigated. In addition, all existing or prospective legislation governing pollution of streams and restoration of land surfaces should be fully understood and evaluated from the viewpoint both of cost and public relations. This is generally a task that requires legal advice.
The importance of placer deposits as a primary source of gold will in some cases cause land-use conflicts that must be equitably resolved. Romanowitz and Cruickshank go into the problems of surface disturbance and future dredging. In some areas, especially in agricultural land, the exposure of rocks and gravel in the waste piles has caused serious objections. In California, the “resoiling dredge” was developed, which reversed the disposal methods and made possible agricultural reuse of the lands. The New Zealand-type dredge, by virtue of its natural disposal methods, left the dredged areas in a condition somewhat similar to predredging. This dredge has been used with a limited bank where the formation consisted of mostly fines with gravel sizes generally limited to less than 1-inch diameter. Where coarse gravels were encountered, the California dredge is the more practical. When dredging in the United States is renewed, most of the operations will have to use the resoiling type in order to confirm where State laws are more stringent on the use of land resources. The added cost of operating the resoiling-type dredge is of no real consequence to the profitable operation, but their capital costs in many cases will be as much as 20 percent more.