Table of Contents
No matter how limited their extent, few underground mining operations progress very far without requiring some sort of support. This Bulletin presents discussion and sketches as a guide for designing underground support of timber, concrete, or steel in relatively shallow workings.
Because timber will satisfy most support requirements, this discussion offers methods for supporting drifts, raises, shafts, and stopes, as well as suggestions for framing and later transporting packaged units into the mine. And because small mine operators may not be aware of the importance of properly blocking timber sets in place, this frequently slighted but important aspect of the supporting process is discussed. Drawings and sketches are used to show various timber sets and methods of erecting and blocking them and the accompanying text includes pertinent supplementary remarks. Another subject introduced is treatment of timber for prolonging its life, which, on occasion, should be considered even by the modest operator. Then, because rock bolts are in wide use today for replacing or supplementing timber, their use and installation are extensively outlined. Also the advantage of replacing collapsed headboards or blocking before failure of the main support is discussed. Several drawings and designs are offered for actual dimensioning and design of chutes for limited tonnage operations, which has been inadequately presented in the past.
For areas where timber is scarce or expensive, reinforced concrete sets of simple design should be substituted. Designing these sets by ultimate strength procedure is discussed in some detail. Similarly, a higher working stress for timber is also suggested, as are standards in working stresses for concrete, steel, and timber in mine-support design that are different from those used for surface structures.
Although headframes and ore bins are not a part of mine support, most mine plants find them necessary; for this reason information on these two surface structures is also included.
Support of underground openings is much the same for both large and small mining operations. Essentially, the difference lies in the depth and in the extent to which the two types of work are compared.
Most large mines are deep, with many miles of drifts, raises, stopes, and shafts with open spaces more or less loosely supported by rock pillars, fill, and timber or other material, or by a combination of such supports. As a rule, the difficulty of maintaining openings increases with depth, but even shallow workings at times encounter very heavy ground and may also be disturbed by rock bursts.
- Application Of Rock Bolts
- Bolting patterns
- Beams and roof ties
- Anchorage capacity and bolt loading
- Problems of bolt use
- Advantages of roof bolts
- Installation sequence
- Using Steel Sets for Timber Support in Underground Mining
- Use Of Concrete Sets
- Design Of Concrete Sets
- Ordinary set
- Loaded drift set — cap
- Spreader Bearing
Prompt consideration should be given to one of the leading causes of ground failure: the effect of moist air on certain rock-forming minerals. Many subsequent support problems could doubtless be minimized if the circulation and penetration of moist air were immediately controlled.
An explanation of ground support is best presented by illustrations supplemented with written explanation. In the present discussion, strict “drafting rules” have not always been followed when preparing the illustrations. In many instances, where adhering precisely to rules of mechanical drawing would have caused confusion, liberties have been taken with common drafting practice, and certain irrelevant lines have been omitted.
Every effort should be made to standardize design and dimensions of timber sets. Also, the number of saw passes and cuts during framing should be reduced to the minimum.
Standardizing drift sets, chute assemblies, raise sets, and other applications reduces both the timber inventory and the confusion of having several pieces serve the same purpose. In some instances pieces of a set may be made interchangeable (for example, caps and girts in square-sets).
When it is warranted, similar pieces may be packaged by using wire wrapping or iron strapping. Wedges, chute assemblies, and other units should be packaged for transportation underground.
Even a casual inspection of mining operations will indicate many applications of timber in addition to those presented here. Such additional uses are left to the ingenuity as well as the requirements of the reader.
A discussion of ground support would not be complete without including at least an introduction to rock bolts, steel sets, and concrete sets. Occasional use of these materials, especially rock bolts, is not unusual at small mines. Concrete sets may be especially useful where localities are not well supplied with timber, or where underground conditions promote rapid decay of timber. Under these circumstances, concrete might be used economically by the small mine operator.
Estimation of Ground Pressure with Selection of Support
Ordinarily, one must know rock mechanics to estimate ground loads. For the underground conditions usually encountered in shallow- to medium-depth operations, however, a detailed application of rock mechanics is of doubtful value. Certain maximum conditions may be assumed. As a rule, underground support based upon such assumptions will be quite satisfactory, at least for temporary service.
As a basis for estimating loads on timber sets (loads on steel or concrete may be estimated in a similar way), the following data are given.
The material in Table 1 may be better understood by consulting Figure 5e. According to Proctor and White, the load formulas have been amply verified by experiment and practical observation. A few remarks will explain the data. Column 1 classifies the ground condition by using the physical properties and mineralogical composition of the rock. The 10 conditions shown will apply to any type of ground. Column 2 gives the formula for calculating the maximum height, Hp, of loose rock; in other words, the height at which a permanent arch is likely to form. (For want of a better term, arch is used, although grave doubts exist as to whether a true arch actually forms). And finally, Column 3 suggests a spacing for the sets.
In Figure 5e, the average side pressure on the drift is given by ph = 0.3 w (0.5Ht + Hp) pounds per square foot, where w is the weight of a cubic foot of freely flowing, sandy material (Proctor and White, 1946, p. 63). The pressure is considered to result from material with a consistency of and ability to flow like sand. This analogy is justifiable because certain rocks will, upon alteration, form a sand-like mass (for example, certain granites, gneisses, schists). A difficult decision arises when assigning a value to w.
Note well the units used for ph. When the formula is solved, the answer is the average pressure in pounds per square foot, as will be illustrated later.
Figure 5e suggests that the load on the cap results from combining two different portions of the loose rock mass. One of these, WA, will depend for its bulk on the point at which arching (stabilization) begins. This zone is rectangular in section (section A). Above section A the loose mass will assume a more or less irregular shape, depending on the physical properties of the rock. For the purpose of the present discussion the outline representing the arch will be assumed to be either WB or WC. The total load W on the cap will result from + Wg or WA + WQ. Blocking of the cap is assumed to be so tight that the cap will, at least when first installed, support W without help from the posts.
This whole problem is based on rather uncertain data; therefore, the weight of the cap is ignored. It would be negligible compared to the weight of rock.
Table 2 (Wood Handbook, 1935, p. 50-53 and 105 and Staley, 1949, p. 51) gives working and ultimate stresses for timber species commonly available for mine use. The working stresses listed in the table have been adjusted so that they can be applied to material similar to common grade and probably somewhat wet. Most mine timber will have been treated for protection against decay. The values given are conservative.
Column 5 gives the straight line formulas for columns (this is the type of loading that the posts in a tunnel or drift set would resist; some caps may be loaded in a similar fashion). Straight Line column formulas are easily applied and are sufficiently accurate for approximating the solution of ground-support problems.
Table 3 (after Proctor and White, 1946, p. 232) gives data for a few commonly encountered rock-types. When the material loosens and falls or exerts a swelling action through alteration, the weight per cubic foot will be less. Just what this decrease is assumed to be will depend on the judgment and experience of the user of these data.
To illustrate the application of the data in Table 1 and Table 2, I have selected a timber set for the drift shown in Figure 5e. This same information will be used again for a concrete set.
Solving a problem of this kind involves the application of several simple strength of materials formulas. These are:
- Bending moment formulas which depend on the loads represented by WA, WB, or WC.
- Column formula for the load on the posts resulting from W. (strictly speaking, W is the vertical component and should be resolved along the post. The resulting slight increase, however, may be neglected.)
- Bearing between the posts and the cap (compression perpendicular to the grain in the cap).
- Bending in posts resulting from side pressure and bearing of post against spreader (design here is influenced by compression perpendicular to grain of the post). A more refined procedure, which would combine the direct stress W with the bending in the posts, is not considered necessary here because bending is so much greater; but in some instances this procedure should be investigated.
To arrive at the size of the members making up the cap and posts, I shall take several minor liberties when applying the formulas. This deviation is justifiable on the grounds that Hp and W are not subject to exact determination and that all assumptions are on the conservative side.
Timber Cap Selection
Known and assumed data are:
- Width of drift at the top = 1 = 6 ft. 3 in.
- Average weight of fallen and compacted rock with additional squeezing effect = w = 160 lb. per cu. ft.
- Distance between sets, center to center = c = 5 ft.
- Hp = 0.31 (B + Ht) = 0.31 (8 + 9 ¼) = 5 ft. 5 in.
- Height of section A = ha = 2 ft. 3 in.
- Height of section B = hb = 3 ft. 2 in.
Bending moment formulas (1 is in inches):
MA = 1/8 WA 1 , in.-lb.
MB = 1/6 WB 1, in.-lb.
MC = 0. 144 W 1 , in.-lb.
M = 1/6 s b d², in.-lb.
s = Allowable working stress in extreme fiber (see Column 2, Table 2).
(See later suggestion recommending higher percentage of ultimate strength for s).
b = Width of cap, inches.
d = Depth of cap, inches.
l = Width of opening, inches.
The timber species will be Rocky Mountain Douglas fir (commonly called red fir), For this material s = 1200 lb. per sq. in.
(All calculations are made with a slide rule).
WA = ha 1 c w = 2 ¼ x 6 ¼ x 5 x 160 = 11,240 or 11,200 lb.
WB = (½ 1 hb) c w = ½ x 6 ¼ x 3 1/6 x 5 x 160 = 7,930 or 7,900 lb.
WC = (π1²/8) c w = 1/8 x π x (6 ¼)² x 5 x 160 = 12,280 or 12,300 lb.
To illustrate the calculations for a cap, we shall assume that the rock eventually stabilizes with an arching effect similar to WC. An identical procedure would be followed for WB.
MA = 1/8 WA 1 = 1/8 x 11,200 x 75 = 105,000 in.-lb.
MC = 0. 144 WC 1 = 0. 144 x 12,300 x 75 = 132,800 in.-lb.
Mtotal = MA + MC = 105,000 + 132,800 = 237,800 in. -lb.
By applying M = 1/6 s b d², the cross sectional size of the cap is obtained. Because there are two unknowns in the formula, a trial and error solution must be computed It is always desirable to take d equal to or greater than b.
When the formula is rewritten,
d = √6 M/s b, inches
Assume b to be 10 inches.
(Timber for most underground uses is unsurfaced. Therefore, its dimensions are nominal or rough. If surfaced material is used, its dimensions are usually ½-in. less than the nominal for sizes other than planks; so for dressed lumber the surfaced dimensions must be used).
d = √6 x 237,800/1200 x 10 = 10.8 in.
Commercial sizes are customarily available (these dimensions change by 2-in. increments). If so, d would become 12 in. Thus it would appear that a 10- x 12-in. cap would be needed. But lumber is sold on the basis of boardfeet. A 10 x 12 represents 10 boardfeet per foot.
Eight inches for b will be tried. With this value d = 12.2 in. This figure is sufficiently close to 12 in.- that an 8- x 12-in. cap would do. Here but 8 boardfeet per foot is required, thus saving the cost of two board feet.
As shown in Figure 5e the cap is 8 in. x 12 in. x 5 ft.
Selection of Standing Posts
Posts are ordinarily selected to act as columns which must resist bending from the side pressure ph, and are also checked for bearing against the spreader on the bottom side of the cap.
The total load on the cap is equally supported by the two posts, which assumes the end blocking on the caps has loosened.
P = WA + WC/2 = 11,200 + 12,300/2 = 11,750 lb. on each post.
Consulting Table 2, the column formula for Douglas fir, Rocky Mountain species is found to be,
P/A = 1200 (1 – L/60d), lb. per sq. in.
= 1200 – 20L/d, lb. per sq. in.
P = load on post = 11,750 lb.
A = area of post, sq. in.
L = unsupported length of post, in. = 93 in. (See Fig. 5e).
d = least dimension of post, in.
b = other dimension of post, in.
Experience shows that bearing against the cap (design of joint or connection), not the member acting as a column, will usually dictate the post size. Checking for dimensions to satisfy bearing perpendicular to the grain should be the first step. From Table 2, the allowable working stress perpendicular to the grain of Douglas fir is 310 lb. per sq. in.
11,750/310 = 38 sq. in.
One dimension of the post must equal the width of the cap (b = 8 in).
38/8 =4.7 in.
But 6 in. is the smallest mill size nearest to 4.7 in. The post is tentatively taken at 6 x 8 in.
These values are now used to check the post as a column.
1200 – 20 x 93/6 = 890
but 11,750/6 x 8 = 244
890 > 244
Acting as a column, the 6 x 8 selection is more than ample. Anything less than this size will crush the cap fibers through bearing.
Post in bending
In Figure 5e, the dotted rectangular section plus the dotted triangular portion would, under certain circumstances, cause the side pressure on the post. Several approximations will be used to establish the maximum load against the post.
(1) Assume the rock alters and decomposes into a moist sand. Proctor and White (1946, p. 63) suggest that the average pressure resulting from this column of sand is represented by ph = 0.3 w (0.5Ht + Hp), lb. per sq. ft., where w = the weight of one cubic foot of sand.
(2) Moist sand in place has a bank weight of about 130 lb. per cu. ft. (Basic Estimating, (no date), p. 14). ph =0.3 x 130 x (0.5 x 9 + 5′ 5″) = 386 lb. per sq. ft. average pressure. Total W = ph c times height of post, lb. = 386 x 5 x 7 ¾ = 14,900 lb.
(3) The effective loading on the post is similar to the dotted triangle shown in Figure 5e. The base of the triangle is taken equal to the length of the post or 93 inches. For this type of loading, where load is increasing uniformly toward one end,
M = 2/9√3 W 1 = 0. 1283 W 1 , in.-lb.
= 0. 1283 x 14,900 x 93 = 170,800 in.-lb.
d = √ 6 x 170,800/1,200 x 8 = 10.3 or 10 in.
Therefore, an 8- x 10-in. post will be required for the conditions necessitating the 8- x 12-in. cap.
Force acting on spreader
The force acting here is represented by 2/3 W, pounds.
P = 2/3 W = 2/3 x 14,900 = 9,933 1b.
From Table 2, the allowable stress perpendicular to the grain for the material under consideration is 310 lb. per sq. in. This value is conservative.
Area for end of spreader = 9,933/310 = 32 sq. in.
For the 8- x 12-in. cap,
32/8 = 4 in.
A spreader 3- or 4-in. thick would suffice.
Suggested Change in Working Stress
The cap and posts in the preceding discussion were selected by methods using conventional working stresses for the material; these values were originally selected for use in designing various surface structures.
The allowable working stresses given in Table 2 include ample factors of safety and are used in designing structures which involve public safety and which must have a long life. These safety factors, which have been adopted after intensive investigation and observation, are for their purpose time proved.
Choice of members for underground supports should be approached with an entirely different philosophy, however. Working stresses may more closely approach the ultimate stress (see also discussion for concrete support). Because
- public safety is not involved;
- compared to surface structures, the operating life of mine supports is relatively short (as little as a few months);
- excepting the maximum effect resulting from rock bursts, the instantaneous and complete failure of the support with extensive closure of the opening is unlikely;
- timber readily indicates an increasing load (physical appearance), which provides ample time for installing additional support (installing sets between those first erected or strengthening original support);
- settling or disturbance of supports because of failure of the component parts, will be local in character—confined to only a few sets at the most, and after effects should not be compared to those of surface structure failures;
- the anticipated effective life of the opening must be carefully considered.
Data in the Wood Handbook (1935, p. 50-53) suggests that the strength of green or wet timber (depending on the species) is 50 to 60 percent of the dry strength. Usually only the prospector will use freshly cut and unseasoned timber. Larger operations provide ample time for considerable seasoning even though the timber may have been treated. In the event that green or wet material must be used, a lower percentage of the ultimate strength should probably be used. . The amount of correction will depend upon the time elapsing between the support’s installation and its maximum loading. During this period, the ventilating air current will produce a drying effect with a resulting increase in strength.
Under these conditions a working stress of at least 50 percent of the ultimate strength could be used, which would result in a substantial saving in timber costs.
If this recommendation were followed, the timber set previously designed would be altered to the following dimensions.
Working stress = 50 percent of 6,300 = 3, 150 lb. per sq. in. = s.
Compression perpendicular to the grain = 50 percent of 820 = 410 lb. per sq. in.
Compression parallel to the grain = 50 percent of 6,100 = 3,050 or 3,000 lb. per sq. in.
Column formula, P/A = 3,000 (1 – L/60d), lb. per sq. in.
Cap, 6 in. x 10 in.
Post, 6 in. x 8 in.
Spreader, 6 in. x 4 in.
If a 3-piece steel set, consisting of a horizontal cap and posts is used, it should be selected on the basis of a larger percentage of the ultimate strength. Instead of the commonly used value of s = 20,000 lb. per sq. in., at least the yield point is suggested. For the ordinary grade of structural steel, this point may be taken at 33,000 lb. per sq. in. As with the timber design, a straight-line column formula is satisfactory. However, the constant k in the formula must be changed to accommodate the 33,000-lb. value used for steel (Boyd, 1917, p. 252),
When thus corrected the formula becomes,
P/A = 33,000 – 180 1/r lb. per sq. in.
P = load, lb.
A = area of cross section, sq. in.
l = unsupported length of column (post), in.
r = least radius of gyration for the steel section used, in. (obtained from a steel handbook).
Later, under Concrete Sets, a parallel discussion is offered for using ultimate strength design for concrete.
For bending stresses, the convenient formula when using steel is,
M = s I/c, in. lb.
M = bending moment/ in.-lb. (MA, Mg and so on as previously explained).
s = working stress in steel, lb. per sq. in. = yield point = 33/000 lb. per sq. in.
I = moment of inertia, in. (This is obtained from a steel handbook. It is usually the value associated with the x-x axis).
c = one-half of the depth, in. (from steel handbook).
Treatment Of Timber
With few exceptions, humid conditions underground promote decay. Treatment of mine timber helps to slow down or prevent decay. Timber from which the bark is removed has a longer life than the unpeeled variety.
Treated timber may be obtained from local treating companies, or raw timber may be treated by the mine operator. Undoubtedly the former source is superior, for the commercial plant will usually be better equipped.
If mine timber (it should be framed before treatment) is not available from a commercial treatment plant, the following precautions should be observed if a company undertakes to treat its timber. Also the manufacturer of timber-preserving chemicals should be consulted.
- Use a rectangular, welded steel tank 4 ft. x 4 ft. x 16 ft.
- Timber is best treated when either green or wet.
- Treat between 24-56 hours — the longer the better.
- Stack treated timber so that circulation of air is retarded, to prevent rapid drying or seasoning.
The pressure-vacuum process is much superior to dipping or soaking in a tank. Ordinarily, only the well-established commercial company can afford this equipment. If the treated surface must be sawed or chopped when it is placed underground, the freshly exposed wood should be well painted with preserving solution.
Care must be used in handling and transporting treated timber to avoid excessive rupture of the treated surface.
Generally the economy of using treated timber is unquestionable, especially when the installation is to be semipermanent or permanent. In many stopes and their accessory workings (drifts, raises), long life is not necessary, however. Treatment there could be completely wasted. The life of the timber and the life of the workings must be carefully coordinated.
Framing Of Timber
Nearly all mine timber requires more or less framing. Framing means cutting to size, notching, cutting daps, mortising for joints, removing slabs from round timber or any other application of a saw or ax or other work on the timber. If the drilling and blasting operation is properly planned, a minimum of subsequent underground framing will be necessary. When treated timber must be altered before installing, the freshly exposed surface should be painted with preservative.
Equipment for timber-framing is available in great variety. Machines have been designed for cutting square-set joints and other complicated work. For the small operator, and possibly even for the larger mines, hand tools are recommended. Small hand-held chainsaws, circular saws, drills, etc. are adaptable for framing. These tools may be operated by either compressed air or electricity.
A timber shop should have a swing-saw for end-cutting and a permanently mounted power-saw for making wedges (special machines are available for making wedges). Additional equipment for squaring logs and cutting planks for lagging and blocking will usually be necessary.
Before equipping a timber-framing shop, one should consult a manufacturer of sawmill equipment.
A great deal of a timberman’s time underground may be saved if a plentiful supply of 2- to 4-in. thick short planks 12 to 18 in. long are cut and packaged on the surface.
Installation Of Timber
A few remarks about the erection of timber sets may be of interest to the less experienced operator.
In addition to a pick and shovel, the customary tools required by a timberman and helper are a single-bitted, 4-lb. timberman’s ax and a timberman’s saw (3 to 5 ft. with deep teeth). A plentiful supply of 40d and 60d spikes is also needed.
Before starting the erection of a set, whether drift, raise, stope, or other type, the various pieces of timber required should be gathered and placed conveniently at hand. Particular attention should be paid to having a plentiful supply of wedges and blocking. Interrupting operations to rustle up a wedge has resulted in the premature collapse of many an incompletely blocked set.
Figures 8, 9, and 10 offer several suggestions for erecting drift sets. If staging must be used to place the cap and top lagging, it should be securely constructed. Two men and a wet cap may well exceed 700 lb. total weight on the staging.
Blocking of sets will be treated under TIMBER REPAIRING.
All of the timber sets to be discussed are protected against over-stressing with blocking between the set and the wall rock. Therefore, blocking serves an additional important purpose other than simply holding the member in place.
Blocking is (or should be) designed to fail before the cap, post, or other member is crushed. When the blocking has been squeezed to final deformation (see Fig. 5b and 5c), new blocking should immediately replace the old blocking before failure of the member results. In some unusual cases, failure is so rapid that replacement cannot be made in time.
Substituting fresh blocking for the deformed material is more readily accomplished before total failure takes place, and is more economical. Why wait until the cap and post, or both, must also be replaced when blocking was especially used for their protection? Not only may replacing of the set be extremely difficult, but in addition, it may be impossible to relocate the cap in its former position. Headroom and sometimes width is lost.
A “no exception” rule should be established: deformed blocking is immediately replaced before failure of the main members begins.
Enough blocking should be used so that the protected member is sufficiently cushioned to prevent its failure from the effect of ground pressure. The thickness of blocking should probably be not less than 12 in. It should consist of several pieces of blocks, planks, and wedges; this construction absorbs the pressure best. An exception to this structure is blocking used primarily to hold the set in place instead of to resist ground movement. As noted in Figures 2 and 4b, the blocking should never be placed so that the grain of the blocking is parallel to the force. Timber is 4 to 5 times stronger when force is exerted parallel to the grain than when it is exerted perpendicular to the grain.
Blocking between the top of a cap and the rock surface will have the grain in the blocking running similar to that in the cap. Here the area of the blocking could be somewhat reduced if it is to fail before squeezing the cap or causing the post to penetrate the bottom of the cap.
Discussion Of Illustrations
Many of the figures require little explanation other than that included on the drawing. For several of the illustrations a few additional remarks are appropriate.
Simple Drift Supports
Part (b) of Figure 1 has three advantages as an alternative location for the drifts (1) support of the back may not then be necessary; (2) when mining starts, the need for floor and back pillars will be eliminated and ore will not be tied up; and (3) sloping will interfere less with other operations through the drift. The location at (b) has one drawback for the prospector: his drift is not in the vein (Staley, 1961, p. 47). Additional crosscutting to the vein is therefore necessary at intervals, which means additional expense and loss of time, items of great importance to the prospector and miner with limited means.
Note that the angle of underlie requires a stull of greater length than the perpendicular distance between the hanging wall and the footwall. Thus, the tendency of the stull is to tighten as the load increases.
(Figures 3, 4a, 5a, to 5d, and 7b)
Two types of headboards for stulls or caps, shown in Figure 3, have been widely applied in many of the mines of the Coeur d’Alene district in northern Idaho. Figure 3a is generally used only on drift caps and 3b on the stope-set cap.
A squeeze heading, shown in 3a, has a definite purpose. As the wall pressure increases, the headboards making up the box-like structure gradually collapse to form a solid mass of crushed timber (Figure 5b and 5c). When the condition shown in 5b is reached—all members of the heading into tight contact with each other—the deformed headings (both ends of the cap) should be removed and replaced with new headings and blocking. If these are not replaced, the cap will ultimately be destroyed. Failure of the cap usually occurs as shown in 5c; but it may occur as in 5d. Breaking of the cap as shown in 5d may also result from a too heavy load on top of the cap (to prevent this failure, relieve the pressure by removing broken rock from above the top lagging).
The inventor of the squeeze heading is unknown. Its purpose and the thought back of it deserve much credit. But for some inexplicable reason its advantage is very seldom utilized. Instead, caps are allowed to fail and the terrific job of installing a new cap is undertaken. It would be much easier and much less costly to replace the headings as soon as the situation indicated in Figure 5b is reached.
In practice, drift caps used with squeeze headings approach 15-16 inches in diameter and about 8-10 feet in length.
Caps in the stopes (a method incorrectly classified as square-set mining, but more correctly known as stull and fill) do not need squeeze headings; plain headings as shown in Figure 3b are sufficient. Depending on the thickness of the vein, these caps may approach 16 ft. in length; their diameters are 16-18 inches. Occasionally, very short pieces of stulls (butt-blocks) are used beyond the ends of the cap to reach extra wide walls or to reduce the amount of heading and blocking.
Timber sets shown in Figures 4a and 7b are semi framed. The caps are peeled logs with squared ends and a slab removed from top and bottom (or sawed timber may be used). For drift sets, posts about 8-in. or material of larger diameter is used. In the stopes, material of about the same diameter, sawed lengthwise into two halves, is sufficient. If heavy top pressure is anticipated, posts of larger diameter are required.
Illustration 7a shows a completely framed assembly. The purpose of the pony set is twofold; (1) it provides a safe walkway and operating space for the chute puller to load cars; and (2) it provides a squeeze set to protect the main drift set if the overhead load becomes heavy. In the event this latter condition is expected, the size of the pony-set timbers should be one standard size smaller than those of the main drift set.
Figure 7b shows the squeeze set when unframed timbering is used. The purpose of squeeze sets is to insure operating access to the drift. Therefore, when the squeeze sets have collapsed to the point where destruction of the drift set is imminent, the debris resulting from the destruction of the squeeze set should be removed and a new set installed.
Erection Of Drift Sets
(Figures 8, 9, And 10)
Note the use of wedges to help support the staging as a safety precaution.
Two heavy men and a heavy cap might cause failure of the staging. Several of the Figures illustrate other applications of wedges.
(Figure 11) (Figure 26m)
Figure 11 offers suggestions for selecting and designing chute detail. Chutes (c) and (d) are used in drifts where ground-support timbering is unnecessary. Usually it is possible to support the main weight of the broken ore column on solid rock rather than on the wooden chute bottom. This procedure should be considered when designing a chute. Later salvaging of usable parts should also be considered in chute design. Figure 26m offers a method for attaching the wooden chute members to a concrete post. Instead of the bolting shown there, one of the available adhesive materials may be used for attaching the wood to the concrete.
Shaft Bearing Sets
Shaft bearing sets are recommended at about 100-ft. intervals. They are used to support regular sets through the medium of the hanging bolts in the event that the blocking becomes loose. Either end-plate bearing or wall-plate bearing may be used.
In heavy or squeezing ground the blocking shown in Figure 17g may be replaced by or supplemented with cedar lagging, 6-in. to 8-in. diameter by 5-ft. long, split to give 4 to 6 triangular-shaped pieces. This lagging is firmly packed between the cribbing and the rock walls. Ground movement is taken up by compressing the cedar.
Application Of Stulls
Figure 18d suggests a means of catching up a caved area following the failure of the supporting timber. Situations of this sort are best prevented at the start. Usually only about one car load of rock drops out at first. This rock is cleaned up but nothing is immediately done about catching up the back. Eventually a large cave develops which is difficult to fill, and subsequent caving of the back is hard to stop. Immediate corrective action at the start usually would have prevented further enlargement of the caved area. The extension of caving ground or the displacement of timber support may be greatly reduced if corrective measures are taken promptly.
Loose Or Squeezing Ground
Fault zones, squeezing ground, and similar conditions are frequently impossible to penetrate and hold by ordinary supporting procedure. This figure explains and shows how spiling may be used. Spiling is driven with a maul or sledge, or sometimes by a heavy block swung from a rope. To protect the end of the spiling, a temporary iron shoe is often provided to cap the timber during driving. When the job is completed, projecting ends are sawed off.
Shafts have been sunk through loose ground by first freezing the site. For a description of the technique, Peele (3rd Ed., 1941, sec. 8, p. 20) may be consulted. A similar procedure could be used for driving drifts through loose, wet ground.
Recently (Pynnonen and Look, 1958; Eng. and Min. Jour., Nov. 1958, p. 126; Sun and Purcell, 1959, p. 1), a method known as chemical injection has been devised for treating and stabilizing loose ground. This process appears to have considerable merit.
Vertical And Inclined Shafts
(Figures 20 and 21)
Little need be added to the information given with the drawings so far as the illustrations are concerned. Ground pressure for the first few hundred feet of a shaft is seldom important. Sets—either timber, steel, or concrete rings—are installed to divide the cross section into compartments; to provide a means of installing cage or skip guides; and to prevent rock from loosening and falling into the shaft. The spacing between sets is usually 5 ft. center to center.
Dimensions of the members will depend on the ground condition. For relatively shallow mines, dimensions about 8- x 8-in. would be satisfactory; with increasing ground pressure, 12- x 12-in. or larger may be required. Under exceptionally bad conditions, a “squeeze set” may have to be installed between the main set and the rock surface. A squeeze set consists of wall plates and end plates separated from the main set by blocking which will collapse before excessive damage results to the main set.
The practice of salvaging the hanging bolts after timbering is completed is of doubtful merit. If the blocking loosens, sets may drop out with serious consequences.
Figures 20 and 21 illustrate the design for a small tonnage operation. If the shaft is sunk with only two compartments, one would be used for the cage and car. A manway would be placed in the remaining compartment. Ladders in the manway should not exceed 20 ft. in length, be inclined, and each section start from the opposite end of a platform (Mining Laws, 1959, par. 47-405, p. 39). For economical hoisting a counterweight should be installed in the manway compartment (Staley, 1949, p. 305). Contrary to general belief this installation is neither difficult nor costly.
If a three-compartment shaft is sunk, two compartments will be available for hoisting and the-third for the manway. For less than 300-500 tons per day, an operation in partial balance should be used (this is loosely spoken of as balanced hoisting). Or, if a greater tonnage is feasible, one of two arrangements is commonly used: (1) a cage with a skip suspended beneath it, or (2) a skip-cage changing arrangement. At least one cage should be continually available. For a three-compartment shaft the first arrangement is probably the better.
For the inclined shaft the arrangement of the ladders will depend on the inclination of the shaft. Up to about 45 degrees, platform interruptions are not too necessary.
At steeper inclinations there should be frequent interruptions in a continuous ladder.
Hoisting through an inclined shaft is accomplished by having the skip or man- car mounted on wheels running on steel rails.
For the design of safety devices or dogs see Staley (1949, p. 250).
As indicated in Figure 22, square-sets are designed for either post loading or cap loading. If the ground load is acting at an angle so that components of the load exert heavy pressure on both the cap and the post, then a more complex set may be required.
A more advanced discussion on this subject should be consulted (Peele, 1941, sec. 10, p. 213-218; Gardner and Vanderburg, 1933).