Sheeting and Bracing
149. Purposes and Types.—Sheeting and bracing are used in trenching to prevent caving of the banks and to prevent or retard the entrance of ground water. The different methods of placing wooden sheeting are called stay bracing, skeleton sheeting, poling boards, box sheeting, and vertical sheeting. Steel sheeting is usually driven to secure water-tightness and if braced the bracing is similar to the form used for vertical wooden sheeting.
150. Stay Bracing.—This consists of boards placed vertically against the sides of the trench and held in position by cross braces which are wedged in place. The purpose of the board against the side of the trench is to prevent the cross brace from sinking into the earth. The boards should be from 1½ × 4 inches to 2 × 6 inches and 3 to 4 feet long. The cross braces should not be less than 2 × 4 inches for the narrowest trenches and larger sizes should be used for wider trenches. The spacing between the cross braces is dependent on the character of the trench and the judgment of the foreman. Stay bracing is used as a precautionary measure in relatively shallow trenches with sides of stiff clay or other cohesive material. It should not be used where a tendency towards caving is pronounced. Stay bracing is dangerous in trenches where sliding has commenced as it gives a false sense of security. The boards and cross braces are placed in position after the trench has been excavated.
151. Skeleton Sheeting.—This consists of rangers and braces with a piece of vertical sheeting behind each brace. A section of skeleton sheeting is shown in Fig. 104 with the names of the different pieces marked on them. This form of sheeting is used in uncertain soils which apparently require only slight support, but may show a tendency to cave with but little warning. When the warning is given vertical sheeting can be quickly driven behind the rangers and additional braces placed if necessary. The sizes of pieces, spacing and method of placing should be the same as for complete vertical sheeting in order that this may be placed if necessary.
152. Poling Boards.—These are planks placed vertically against the sides of the trench and held in place by rangers and braces. They differ from vertical sheeting in that the poling board is about 3 or 4 feet long. It is placed after the trench has been excavated; not driven down with the excavation like vertical sheeting. An arrangement of poling boards is shown in Fig. 105. This type of support is used in material that will stand unsupported for from 3 to 4 feet in height. Its advantages lie in that no driving is necessary, thus saving the trench from jarring; no sheeting is sticking above the sides of the trench to interfere with the excavation; and only short planks are necessary.
Fig. 104.—Skeleton Sheeting.
Fig. 105.—Poling Boards.
Showing Different Types of Cross Bracing.
The method of placing poling boards is as follows: Excavate the trench as far as the cohesion of the bank will permit. Poling boards, 1½ inch to 2 inch planks, 6 inches or more in width, are then stood on end at the desired intervals along each side of the trench for the length of one ranger. The poling boards may be held in place by one or two rangers. Two are safer than one but may not always be necessary. If one ranger is to be used it is placed at the center of the poling board. After the poling boards are in position the rangers are laid in the trench and the cross braces are cut to fit. If wedges are to be used for tightening the cross braces, the cross braces are cut about 2 inches short. If jacks are to be used the braces are cut short enough to accommodate the jacks when closed, or adjustable trench braces may be used as shown in Fig. 106. The use of extension braces saves the labor of fitting wooden braces. With everything in readiness in the trench, the cross brace is pressed against the ranger which is thus held in place. The wedge or jack is then tightened holding the poling boards and cross brace in position.
Fig. 106.—Box Sheeting.
Showing Different Types of Cross Bracing.
153. Box Sheeting.—Box sheeting is composed of horizontal planks held in position against the sides of the trench by vertical pieces supported by braces extending across the trench. The arrangement of planks and braces for box sheeting is shown in Fig. 106. This type of sheeting is used in material not sufficiently cohesive to permit the use of poling boards, and under such conditions that it is inadvisable to use vertical sheeting which protrudes above the sides of the trench while being driven. This sheeting is put in position as the trench is excavated. No more of the excavation than the width of three or four planks need be unsupported at any one time. In placing the sheeting the trench is excavated for a depth of 12 to 24 inches. Three or four planks are then placed against the sides of the trench and are caught in position by a vertical brace which is in turn supported by a horizontal cross brace.
Fig. 107.—Vertical Sheeting.
154. Vertical Sheeting.—This is the most complete and the strongest of the methods for sheeting a trench. It consists of a system of rangers and cross braces so arranged as to support a solid wall of vertical planks against the sides of the trench. An arrangement of complete vertical sheeting is shown in Fig. 107. This type can be made nearly water-tight by the use of matched boards, Wakefield piling, steel piling, etc. Wakefield piling is made up of three planks of the same width and usually the same thickness. They are nailed together so that the two outside planks protrude beyond the inside one on one side, and the inside one protrudes beyond the two outside ones on the other side as shown in Fig. 108. The protruding inside plank forms a tongue which fits into the groove formed by the protruding outside planks of the adjacent pile.
Fig. 108.—Wakefield Sheet Piling.
Fig. 109. Section through Malleable Steel Driving Cap.
In placing vertical sheeting the trench is excavated as far as it is safe below the surface. Blocks of the same thickness as the sheeting are then placed against the bank at the middle and at the ends of two rangers on opposite sides of the trench. The ranger rest against blocks, and are held away from the sides of the trench by them. Cross braces are next tightened into position opposite the blocks to hold the rangers in place. After the skeleton sheeting is in place the planks forming the vertical sheeting are put in position with a chisel edge cut on the lower end of the plank, with the flat side against the bank. The planks should be driven with a maul, the edge of the plank following closely behind the excavation. In relatively dry work the driving of the plank is facilitated by excavating beneath the edge as it is driven. The upper end of the sheeting should be protected by a malleable steel or iron cap to prevent brooming of the lumber. A cap is shown in Fig. 109. A sledge hammer may be used for driving when the lumber is protected. If the sheeting is to start at the surface and is to be driven by hand, the first length should not exceed 4 feet unless a platform is erected for the driver. Succeeding lengths may be longer, the driver standing on planks supported on the cross braces in the trench. Steam hammers and pile drivers are sometimes used for driving sheeting.
The framework of the sheeting should be placed with a cross brace for each end of each ranger and a cross brace for the middle of each ranger. If the ends of two rangers rest on the same cross brace an accident displacing one ranger will be passed on to the next and might cause a progressive collapse of a length of trench, whereas the movement of an independently supported ranger should have no effect on another ranger. The cross braces should have horizontal cleats nailed on top of them as shown in Fig. 107 to prevent the braces from being knocked out of place by falling objects. In driving vertical sheeting a vacant place will be left behind each cross brace corresponding to the original block placed to hold the ranger away from the bank. This is an undesirable feature in the use of vertical sheeting. It is ordinarily remedied by slipping in planks the width of the slot and wedging or nailing them against the convenient cross bracing. In extremely wet trenches, after all other pieces of vertical sheeting are in place, the original cleat behind the cross brace can be knocked out and a piece of sheeting slipped into this opening and driven. Care must be taken in this event not to drive the rangers down when driving the sheeting. If the bracing begins to drop, it should be supported by vertical pieces between the rangers and resting on a sill at the bottom of the trench.
Fig. 110.—Steel Clamp for Pulling Wood Sheeting.
155. Pulling Wood Sheeting.—Wood sheeting is pulled after the completion of the trench by a device shown in Fig. 110. In wet trenches where the removal of the sheeting would permit a movement of the banks, resulting in danger to the sewer or other structures, the sheeting should be left in place in the trench. If sufficient saving can be made the sheeting is cut off in the trench immediately above the danger line, usually the ground water line. The cutting is done with an axe or by a power driven saw devised for the purpose.
156. Earth Pressures.[[92]]—The various theories of earth pressure are so conflicting in their conclusions as to be confusing. Rankine’s theory, the most frequently used, assumes that the pressure increases with the depth, whereas Meem’s theory[[93]] leads to an opposite conclusion. The discussion following Meem’s article is very illuminating. It indicates that no matter how good the theory, practical experience together with the use of generous sizes and close spacing are the best guides for bracing trenches and coffer dams. All are not possessed with the desired practical experience and some basis on which to commence work is essential. Another factor affecting computations of sizes based on theory is the tendency in practice to use the same size material for rangers and braces on any one job for all except very deep trenches and other special cases. Occasionally where there is an independent brace for each end of each ranger, the brace is made thinner, but is of the same depth as the ranger.
The application of Rankine’s theory of earth pressure to the computation of the sizes of rangers and braces will be shown. His formula for the active earth pressure against a retaining wall is:
P = wh cosθ cos θ − √cos2 θ − cos2 φ
cos θ + √cos2 θ − cos2 φ
in which w = the weight of earth in pounds per cubic foot; h = depth in feet at point at which pressure is to be determined; θ = the angle of surcharge, or the angle which the surface makes with the horizontal; φ = the angle of repose of the earth. Usually taken as 33°–41′ = 1½ horizontal to 1 vertical; P = the intensity of pressure in pounds per square foot on a vertical plane in a direction parallel to the surface of the ground.
In studying the pressures for trenches the surface of the ground will be assumed as horizontal and the formula reduces to
P = 1 − sin φ
1 + sin φwh.
157. Design of Sheeting and Bracing.—The trench shown in Fig. 111 is assumed to be constructed in moist sand weighing 110 pounds per cubic foot, with an angle of repose of 30 degrees. The material used for sheeting and bracing is yellow pine. The steps taken in the design of the sheeting and bracing for this trench are as follows:
Fig. 111.—Diagram for the Design of Wood Sheeting.
1. Earth Pressure.—Substituting the units given in the data, in Rankine’s formula for earth pressures,
P = 36.7h.
Because the earth has been freshly cut and will not be kept open long enough to break up the cohesiveness of the banks it is customary to reduce the assumed pressure by dividing by 2, 3, or 4, according to the natural cohesiveness of the material. The cohesiveness of sand is not great, therefore the pressure will be assumed as one-half of the amount given by the formula, or
p = 18h.
2. Thickness of Sheeting and Spacing of Rangers.—It is desirable to use the same thickness of sheeting throughout the depth of the trench. Computations should therefore be commenced at the bottom of the trench where the pressures are the greatest and the thickest sheeting will be required. It is necessary to determine by trial a spacing for the rangers and a thickness of sheeting so that the sheeting is stressed to its full working strength. Having determined the thickness of the sheeting at the bottom, the remainder of the computations consists in determining the spacing of the rangers.
In the example the lower ranger will be assumed as 3 feet from the bottom of the trench and the distance to the next ranger as 4 feet.
The intensity of pressure at 22 feet 9 inches is 409.5 pounds per square foot.
The intensity of pressure at 26 feet 9 inches is 481.5 pounds per square foot.
The distribution of pressures is shown by the diagram on Fig. 111. The maximum bending moment is slightly below the point midway between the rangers and for a 12–inch strip is 10,500 inch-pounds.
Assuming 3 inch sheeting the maximum fiber stress is:
f = Mc
I = 10,400 × 1.5 × 12
12 × 27 = 568 pounds per square inch.
The working strength of yellow pine as given in Table 59, is 1200 pounds per square inch. Thinner sheeting should therefore be used.
| TABLE 59 | ||||||
|---|---|---|---|---|---|---|
| Working Unit Stresses for Timber | ||||||
| The most used value in the Building Codes of Baltimore, Boston, Cincinnati, Chicago, District of Columbia, and New York City | ||||||
| Wood | Tension, lb. sq. in. | Compression With Grain, lb. sq. in. | Compression Across Grain, lb. sq. in. | Transverse Bending, lb. sq. in. | Shear With Grain, lb. sq. in. | Shear Across Grain, lb. sq. in. |
| Yellow pine | 1200 | 1000 | 600 | 1200 | 70 | 500 |
| White pine | 800 | 800 | 400 | 800 | 40 | 250 |
| Spruce and Va. pine. | 800 | 800 | 400 | 800 | 50 | 320 |
| Oak | 1000 | 900 | 800 | 1000 | 100 | 600 |
| Hemlock | 600 | 500 | 500 | 600 | 40 | 275 |
| Chestnut | 600 | 500 | 1000 | 800 | 150 | |
| Locust | 1200 | 1000 | 1200 | 100 | 720 | |
| As published in American Civil Engineers Pocket Book. | ||||||
Assuming 2–inch sheeting, the fiber stress is 1,300 pounds per square inch. This stress is too large. By reducing the ranger spacing slightly the stress can be brought within the required limits.
Assuming a ranger spacing of 3 feet 9 inches the depth to the upper ranger is changed to 23 feet and the maximum stress in the 2–inch sheeting becomes 1,140 pounds per square inch, a satisfactory result. The results for the computations for the other ranger spacings are shown in Table 60. The spacing of the rangers at the sheeting junctions is controlled by convenience and is not computed so long as it is obviously safe.
3. Size of Rangers.—The rangers will be assumed as 16 feet long with two end cross braces and one intermediate cross brace for each ranger. Starting as before at the bottom of the trench.
The area of the panel below the ranger and between cross braces is 24 square feet.
The average intensity of pressure is 28.25 × 18 = 508.5 pounds per square inch.
The load transmitted to the ranger is 6,000 pounds.
Similarly the load transmitted to the ranger from the panel above is 6,890 pounds.
The total distributed load on the ranger is 12,890 pounds.
If b is the vertical dimension of the ranger and d is the horizontal dimension in inches, then from the beam theory, using f as 1,200 pounds per square inch, bd2 = M
200, in which M is expressed in inch-pounds. The maximum bending moment is
Wl
8 = 12,200 × 8 × 12
8 = 155,000 inch-pounds
Therefore, bd2 = 775.
An 8 × 10 inch beam will fulfill the conditions closely. Substituting these dimensions in the beam formula
f = Mc
I = 155,000 × 5 × 12
8 × 1000
= 1,160 pounds per square inch tension in outer fiber. The results of the computations for other rangers are shown in Table 60.
4. Size of Cross Braces.—The cross braces act as columns. The dimensions of the cross braces are determined by trial in such a manner that the vertical dimension of the brace is equal to the vertical dimension of the ranger and the compressive stress in pounds per square inch is computed from the expression,
S ⪙ S1(1 − l
60d),[[94]]
| TABLE 60 | |||||||
|---|---|---|---|---|---|---|---|
| Computations for Sheeting and Bracing for Trench Shown in Fig. 111 | |||||||
| Material is moist sand weighing 110 pounds per cubic foot, with an angle of repose of 30°. Lumber is yellow pine, with working stress as given in Table 59. Working stresses for columns given as S(1 − l 60d). | |||||||
| Sheeting 2 inches × 12 Inches | Cross Braces | ||||||
| Depth | Maximum Bending Moment, Inch-Pounds | Maximum Fiber Stress, Pounds per Square Inch | Depth and Description | Total Load, Pounds | Size, Inches | Actual Intensity, Pounds per Square Inch | Allowable Intensity, Pounds per Square Inch |
| 23′–26.75′ | 9100 | 1140 | end at 26′ 9″ | 6,445 | 4 × 8 | 202 | 784 |
| 19′–23′ | 8800 | 1100 | int. at 26′ 9″ | 12,890 | 4 × 8 | 403 | 784 |
| 13′–17.5′ | 8550 | 1070 | end at 23′ 0″ | 6,393 | 4 × 8 | 200 | 784 |
| 8′–13′ | 7160 | 900 | int. at 23′ 0″ | 12,785 | 4 × 8 | 400 | 784 |
| 0′–6′ | 3000 | 375 | end at 19′ 0″ | 3,930 | 4 × 8 | 123 | 784 |
| int. at 19′ 0″ | 7,860 | 4 × 8 | 240 | 784 | |||
| end at 17′ 6″ | 3,566 | 4 × 8 | 112 | 684 | |||
| int. at 17′ 6″ | 7,132 | 4 × 8 | 224 | 684 | |||
| end at 13′ 0″ | 4,385 | 4 × 8 | 137 | 684 | |||
| int. at 13′ 0″ | 8,770 | 4 × 8 | 274 | 684 | |||
| end at 8′ 0″ | 2,270 | 4 × 6 | 96 | 687 | |||
| int. at 8′ 0″ | 4,540 | 4 × 6 | 189 | 667 | |||
| end at 6′ 0″ | 1,344 | 4 × 6 | 60 | 584 | |||
| int. at 6′ 0″ | 2,687 | 4 × 6 | 112 | 584 | |||
| end at 0′ 0″ | 432 | 4 × 6 | 18 | 584 | |||
| int. at 0′ 0″ | 863 | 4 × 6 | 36 | 584 | |||
| Rangers | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Depth | Area of Panel Below this Depth, Square Feet | Intensity of Pressure, Pounds per Square Inch | Total Load in Pounds | Load Transmitted to the Ranger from the | Size, Inches | Maximum Bending Moment in Thousand Inch-Pounds | Maximum Stress Pounds per Square Inch | ||
| Panel Below | Panel Above | Both Panels | |||||||
| 26′ 9″ | 24 | 508.5 | 12,200 | 6000 | 6890 | 12,890 | 8 × 10 | 155 | 1160 |
| 23′ 0″ | 30 | 448 | 13,440 | 6545 | 6240 | 12,785 | 8 × 10 | 153 | 1150 |
| 19′ 0″ | 32 | 378 | 12,100 | 5860 | 2000 | 7,860 | 8 × 10 | 94.3 | 708 |
| 17′ 6″ | 12 | 328.5 | 3,942 | 1942 | 5190 | 7,132 | 8 × 10 | 85.6 | 636 |
| 13′ 0″ | 36 | 274.5 | 9,880 | 4690 | 4080 | 8,770 | 8 × 10 | 105 | 790 |
| 8′ 0″ | 40 | 189 | 7,560 | 3480 | 1060 | 4,540 | 6 × 8 | 54.4 | 850 |
| 6′ 0″ | 16 | 126 | 2,020 | 960 | 1727 | 2,687 | 6 × 8 | 32.2 | 503 |
| 0′ 0″ | 48 | 54 | 2,590 | 863 | 0 | 863 | 6 × 8 | 10.4 | 161 |
in which S = permissible crushing across the grain in a column whose length is greater than 15 diameters; S1 = unit working compressive strength of wood; l = length of the column; d = smallest dimension of the column; l and d are in the same units.
The lower intermediate cross brace supports a length of 8 feet of the lower ranger on which the load has been found to be 12,890 pounds. The load on the end cross brace for the same ranger is one-half of this or 6,445 pounds. The length of each brace is 4 feet 4 inches. From Table 59, S1 is 1,000 pounds per square inch. From the column formula, S is 784 pounds per square inch.
A 4 × 8 inch cross brace is the smallest that is feasible. This is stressed only 12,890 pounds or 403 pounds per square inch, which is well within the permissible limits. The results of the other computations for cross braces are shown in Table 60.
158. Steel Sheet Piling.—This is coming into more general use with the increased cost of lumber and better acquaintance with its superiority over wood under many conditions. Although its first cost is higher than that of wood, the fact that with proper care it can be used almost an indefinite number of times renders it economical to contractors who may have an opportunity to make repeated use of it. The life of good yellow pine sheeting with the best of care may be as much as three or four seasons. With no particular care it will be destroyed at the first using. Fig. 112 shows various sections of steel piling used for trench sheeting. These forms are practically water-tight and aid materially in maintaining dry trenches. The piling can be made water tight by slipping a piece of soft wood between the steel sections when they are being driven, or by pouring in between the piles some dry material which will swell when wet. The piling is generally driven by a steam hammer and is pulled by attaching a ring through a bolt hole in the pile, or by grasping the pile with a clutch that tightens its grasp as the pull increases. An inverted steam hammer attached to the pile is sometimes used in pulling it. The impulses of the hammer together with a steady pull on the cable serve to drag out the most stubborn piece of piling.
Fig. 112.—Sections of Lackawanna Steel Sheet Piling.