Chapter XI: Construction (2)

A section of a Pulsometer is shown in Fig. 98. It consists of two bottle-shaped chambers _A_ and _B_ with their necks communicating at the top and each opening into the outlet chamber _O_ through a check valve. Steam is admitted at the top and enters chamber _A_ or _B_ according to the position of the steam valve _C_ as shown. This steam valve is a ball which is free to roll either to the right or left and forms a steam-tight joint with whichever seat it rests upon. In normal operation chamber _A_ would be filled with water as the steam enters the cylinder. At the same time a check valve at the top opens to admit a small quantity of air which forms a cushion insulating the steam from the water, reduces the condensation of the steam, and serves as a cushion for the incoming water on the opposite stroke. The pressure of the steam depresses the surface of the water without agitation and forces the water through the check valve _F_ into the discharge chamber _O_. When the water falls to the level of the discharge chamber the even surface is broken up and the intimate contact of the steam and water condenses the former instantaneously. This forms a vacuum in chamber _A_ which, assisted by a slight upward pressure in chamber _B_ caused by the incoming water, immediately pulls the ball _C_ over to the other seat and directs the steam into chamber _B_. The vacuum in chamber _A_ now draws up a new charge of water through the suction pipe into the chamber.

FIG. 99.—Emerson Steam Vacuum Pump.
]

A section of the Emerson pump is shown in Fig. 99. The pump consists of two vertical cylinders _B_ and _C_. Each chamber has a suction valve _L_ at the bottom, opening upward from a common chamber from which the discharge pipe _U_ extends. On the top of each chamber is a baffle plate _G_ which operates to distribute the steam evenly to the two chambers and to prevent it from agitating the surface of the water in the chambers. A condenser nozzle _F_ is connected with the bottom of the opposite chamber by a pipe into which a check valve opens upward. As the pressure in the chamber alternates water will be injected through _F_ into the opposite chamber and condense the steam therein, promptly forming a vacuum. An air valve _P_ admits a small quantity of air while the chamber is filling with water, the air acting as an insulating cushion as in the Pulsometer. Valve _O_, just above the top connection _S_ is used to regulate the amount of steam that enters the pump. The top connection _S_ has two ports, one leading to each chamber. An oscillating valve enclosed in it admits the steam through these ports to the two chambers alternately. This valve is driven by a small three-cylinder engine, the crank shaft of which extends into the top connection in the center of the bearing on which the valve oscillates. A positive geared connection is made between the valve and the engine and so arranged that the engine will run faster than the valve.

The action of these pumps consists of alternately filling and emptying the two chambers. They will continue operation without attention or lubrication so long as the steam is turned on. In view of the simplicity of their operation and make-up, their ability to handle liquids heavily charged with solids, and their reasonable steam consumption these pumps are widely used for pumping water in construction work. They have an added advantage that no foundation or setting is required for them as they can be hung by a chain from any available support.

These pumps are manufactured in sizes varying from 25 to 2500 gallons per minute at a 25–foot head, and with a steam consumption of about 150 pounds per horse-power hour. They reduce about 4 per cent in capacity for each 10 feet of additional lift. They will operate satisfactorily between heads of 5 to 150 feet, with a suction lift not to exceed 15 feet. Lower suction lifts are desirable and the best operation is obtained when the pump is partly submerged. The steam pressure should be balanced against the total head. It varies from 50 to 75 pounds for lifts up to 50 feet, and increases proportionally for higher lifts. The dryer the steam the lower the necessary boiler pressure.

=141. Centrifugal and Reciprocating Pumps.=—The details of these pumps, their adaptability to various conditions, and their capacities are given in Chapter VII. The centrifugal is better adapted to trench pumping as it is not so affected by water containing sand and grit, but for clear water, high suction lifts and fairly permanent installations, reciprocating pumps can be used with satisfaction.

=142. Well Points.=—In dewatering quicksand a method frequently attended with success is to drive a number of well points into the sand and connect them all to a single pump. Figure 100 shows a well point system used on sewer work in Indiana. The well points are 3 feet apart and are connected to a 2½-inch header which in turn is connected to six Nye pumps, each with a capacity of 200 gallons per minute for a lift of 50 feet. The number and size of well points and pumps to use will depend on conditions as met on the job. On a piece of work in Atlantic City[88] the equipment consisted of two complete outfits each comprising one hundred 1½ inch by 36–inch No. 60 well points, one hundred 6–foot lengths of rubber hose, about 600 feet of suction main, one hundred valved T connections, and a 7 × 8–inch Gould Triplex Pump with a capacity of 200 gallons per minute, belted to a 7½ horse-power motor.

FIG. 100.—Well Points Pumped by Nye Steam Vacuum Pump.
]

=143. Rock Excavation.=—A common definition of rock used in specifications is: whenever the word Rock is used as the name of an excavated material it shall mean the ledge material removed or to be removed properly by channeling, wedging, barring, or blasting; boulders having a volume of 9 (this volume may be varied) cubic feet or more, and any excavated masonry. No soft disintegrated rock which can be removed with a pick, nor loose shale, nor previously blasted material, nor material which may have fallen into the trench will be measured or allowed as rock.

Channeling consists in cutting long narrow channels in the rock to free the sides of large blocks of stone. The block is then loosened by driving in wedges or it is pried loose with bars. It is a method used more frequently in quarrying than in trench excavation where it is not necessary to preserve the stone intact. In blasting, a hole is drilled in the rock, and is loaded with an explosive which when fired shatters the rock and loosens it from its position.

FIG. 101.—Plug and Feathers for Splitting Rock.
]

In drilling rock by hand the drill is manipulated by one man who holds it and turns it in the hole with one hand while striking it with a hammer weighing about 4 pounds held in the other hand, or one man may hold and turn the drill while one or two others strike it with heavier hammers. In churn drilling a heavy drill is raised and dropped in the hole, the force of the blow developing from the weight of the falling drill. Hand drills are steel bars of a length suitable for the depth of the hole, with the cutting edge widened and sharpened to an angle as sharp as can be used without breaking. The drill bar is usually about ⅛th of an inch smaller than the diameter of the face of the drill.

Wedges used are called plugs and feathers. They are shown in Fig. 101 which shows also the method of their use. The feathers are wedges with one round and one flat face on which the flat faces of the plug slide.

=144. Power Drilling.=—In power drilling the drill is driven by a reciprocating machine which either strikes and turns the drill in the hole, or lifts and turns it as in churn drilling, or the drill may be driven by a rotary machine which is revolved by compressed air, steam, or electricity. There are many different types of machines suitable for drilling in the different classes of material encountered and for utilizing the various forms of power available.

A jack hammer drill is shown in Fig. 102. In its lightest form the drill weighs about 20 pounds and is capable of drilling ⅞-inch holes to a depth of 4 feet. Heavier machines are available for drilling larger and deeper holes. The same machine can be adapted to the use of steam or compressed air. When in use the point of the drill is placed against the rock and a pressure on the handle opens a valve admitting air or steam. The piston is caused to reciprocate in the cylinder, striking the head of the drill at each stroke. The drill is revolved in the hole by hand or by a mechanism in the machine. A hollow drill can be used by means of which the operator admits air or steam to the hole, thus blowing it out and keeping it clean. These machines have the advantage of small size, portability and simplicity. They can be easily and quickly set up and the drills can be changed rapidly. Their undesirable features are the vibration transmitted to the operator and the dust raised in the trench.

FIG. 102.—Jack Hammer Rock Drill.
]

FIG. 103.—Tripod Drill.
]

A type of drill heavier and larger than the jack hammer drill is shown in Fig. 103. It requires some form of support such as a tripod, or in tunnel work it can be braced against the roof or sides. Some data on steam and air drills are given in Table 56. The effect of the length of the transmission pipe, temperature of the outside air, pressure at the boiler or compressor, etc., will have a marked effect on the amount of steam or air to be delivered to the drill. Compressed air is affected more than steam by these outside factors, but it has an advantage in that as it loses in pressure it increases in volume so that the loss of power is not so marked. Gillette states:

We may assume that a cubic foot of steam will do practically the
same work in a drill as a cubic foot of compressed air at the same
pressure, because neither the steam nor the air acts expansively
to any great extent in a drill cylinder, due to the late cut-off.
This being so ... one pound of steam is equivalent to nearly 30
cubic feet of free air ... all at the same pressure of 75 pounds
per square inch. If a drill consumes at the rate of 100 cubic feet
of free air per minute ... it would therefore consume 240 pounds
of steam (at 75 pounds pressure) per hour.... Where not more than
three or four drills are to be operated, probably no power can
equal compressed air generated by gasoline. It will require 12
horse-power to compress air for each drill, hence 1½ gallons of
gasoline will be required per hour per drill while actually
drilling.

TABLE 56

DATA ON ROCK DRILLS

(From H. P. Gillette)
───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬─────
Diameter of cylinder in inches │ 2¼│ 2½│ 2¾│ 3⅛│ 3¼│ 3⅜
Length of stroke in inches │ 5│ 6│ 6½│ 6⅝│ 6⅝│ 7¼
Length of drill from end of crank │ │ │ │ │ │
to end of piston │ 36│ 43│ 50│ 50│ 50│ 52
Depth of hole drilled without │ │ │ │ │ │
change of bit, inches │ 15│ 20│ 24│ 24│ 24│ 24
Diameter of supply inlet. Standard │ │ │ │ │ │
pipe, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Approximate strokes per minute with│ │ │ │ │ │
60 pound pressure at the drill │ 500│ 450│ 375│ 350│ 325│ 300
Depth of vertical hole each machine│ │ │ │ │ │
will drill easily, feet │ 6│ 8│ 10│ 14│ 16│ 20
Diameter of holes drilled, inches │ ¾ to 1½ as desired
Diameter of octagon steel, inches │ ¾ to│ ⅞ to│ 1 to│1⅛ to│1⅛ to│1¼ to
│ ⅞│ 1│ 1⅛│ 1¼│ 1¼│ 1⅜
Best size of boiler to give plenty │ │ │ │ │ │
of steam at high pressure, │ │ │ │ │ │
horse-power │ 6│ 8│ 8│ 9│ 10│ 12
Best size of supply pipe to carry │ │ │ │ │ │
steam 100 to 200 feet, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Weight of drill unmounted, with │ │ │ │ │ │
wrenches and fittings, hot boxed,│ │ │ │ │ │
pounds │ 128│ 190│ 265│ 315│ 385│ 390
Weight of tripod, without weights, │ │ │ │ │ │
not boxed, pounds │ 80│ 160│ 160│ 160│ 210│ 275
Weight of holding down weights, not│ │ │ │ │ │
boxed, pounds │ 120│ 270│ 270│ 285│ 330│ 375
Cubic feet of free air per minute │ │ │ │ │ │
required to run one drill at 100 │ │ │ │ │ │
pounds │ 92│ 104│ 126│ 146│ 154│ 160
───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────
For more than one drill, multiply the value in the above line by the
following factors: For 2 drills, 1.8; 5 by 4.1; 10 by 7.1; 15 by 9.5;
20 by 11.7; 30 by 15.8; 40 by 21.4; 70 by 33.2.

Since gasoline air compressors are self regulating, when the drill
is not using air very little gasoline is burned by the gasoline
engine driving the compressor. A gasoline compressor possesses
other very important economic advantages over a small steam-driven
plant. First, there is the saving in wages of firemen and second,
there is the saving in hauling and pumping of water and the
hauling of fuel. The cost of gasoline is often less than the cost
of coal for operating a small plant.

An electric drill[89] operated on the principle of the solenoid does away with motor, valves, pipes, vapor, freezing, and other difficulties attendant on the use of steam or air.

The rates of drilling in different classes of rock are shown in Table 57. Frequent changes of drills and relocation of tripods will materially reduce the performance of a drill, for as much as 45 minutes may be lost in making a new set up. In this the jack hammer drills show their advantage as no time is lost in a set up.

TABLE 57

RATES OF ROCK DRILLING

Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep.
(From Gillette)

Hard Adirondack granite 48
Maine and Massachusetts granite 45–50
Mica-schist of New York City. Possible 60–70
Mica-schist of New York City. Average 40–50
Hard, Hudson River trap rock 40
Soft red sand stone of Northern New Jersey 90
Hard limestone near Rochester, N. Y 70
Limestone of Chicago Drainage Canal 70–80
Douglass, Indiana, syenite. Difficult set ups 36
Canadian granite on Grand Trunk R. R 30
Windmill point, Ontario limestone:
3⅝-inch drills 75
2¾-inch drills 60
2¼-inch drills 37

=145. Steam or Air for Power=.—The choice between steam or air is dependent on the conditions of the work. Steam is undesirable in tunnels on account of the heat produced. In open cut work it is at a disadvantage because of the loss of power due to radiation from the hose or pipe. The life of the hose is not so long as when air is used, escaping steam causes clouds of vapor which obscure the work, and serious burns may occur due to hot water thrown from the exhaust. It is advantageous since leaks may be easily discovered and remedied, it requires less machinery than air, and it is sometimes less expensive. With compressed air, gasoline or electric motors can be used for operating the compressors.

TABLE 58

ROCK BLASTING

(From Gillette)
────────────────────┬────────────────────┬─────────┬─────────┬─────────
Character of │Powder Used per Hole│ │Distance │Distance
Material │ │Depth of │ Back of │ Hole to
│ │ Hole, │ Face, │ Hole,
│ │ Feet │ feet │ feet
────────────────────┼────────────────────┼─────────┼─────────┼─────────
Limestone of Chicago│40 per cent dynamite│ │ │
Drainage Canal │ │ 12│ 8│ 8
Sandstone │200 pounds black │ │ │
│ powder │ 20│ 18│ 14
Granite │2 pounds 60 per cent│ │ │
│ dynamite │ 12│ 1½│ 4½ to 5
Pit mining, │ │ │ │
Treadwell, Mine, │ │ │ │
Alaska │ │ 12│ 2½│ 6
────────────────────┴────────────────────┴─────────┴─────────┴─────────

=146. Depth of Drill Hole.=—The depth of the hole is dependent on the character of the work. The deepest holes can be used in open cut work where the shattered rock is to be removed by steam shovel. The face can be made 10 to 15 feet high. The depth of the hole in center cut tunnel facings are from 6 to 10 or even 12 feet. In the bench the depth is equal to the height of the bench. In narrow trenches where the rock is to be removed by derrick or thrown into a bucket by hand, the hole should be sufficiently deep to shatter the rock to a depth of at least 6 inches below the finished sewer. Frequently shooting to this depth at one shot cannot be done due to the built up condition of the neighborhood or other local factors. The depth of the hole in trench work should not much exceed the distance between holes. Deep holes are usually desirable as a matter of economy in saving frequent set ups, but the holes cannot be made much over 20 feet in depth without increasing the friction on the drill to a prohibitive amount.

=147. Diameter of Drill Hole.=—The diameter of the hole should be such as to take the desired size of explosive cartridge. The common sizes of dynamite cartridges are from ⅞ inch to 2 inches in diameter. In drilling, the diameter of the hole is reduced about one-eighth of an inch at a time as the drill begins to stick. This reduction should be allowed for, and experience is the best guide for the size of the hole at the start. In general the softer or more faulty or seamy the rock, the more frequent the necessary reductions in size of bit.[90] For hard homogeneous rock the holes can be drilled 10 feet or more without changing the size of the drill bit.

=148. Spacing of Drill Holes.=—The spacing of holes in open cut excavation is commonly equal to the depth of the hole. The character of the material being excavated has much to do with the spacing of the holes. The spacing, diameter and depth of holes used on some jobs is shown in Table 58. Gillette states:

It is obviously impossible to lay down any hard and fast rule for
drill holes. In stratified rock that is friable, and in traps that
are full of natural joints and seams, it is often possible to
space the holes a distance apart somewhat greater than their
depth, and still break the rock to comparatively small sizes upon
blasting. In tough granite, gneiss, syenite, and in trap where
joints are few and far between, the holes may have to be spaced 3
to 8 feet apart regardless of their depth for with wider spacing
the blocks thrown down will be too large to handle with ordinary
appliances. Since in shallow excavations the holes can seldom be
much further apart than one to one and one-half times their depth
we see that the cost of drilling per cubic yard increases very
rapidly the shallower the excavation. Furthermore the cost of
drilling a foot of hole is much increased where frequent shifting
of the drill tripod is necessary.

The common practice in placing drill holes is to put down holes in
pairs, one hole on each side of the proposed trench; and if the
trench is wide one or more holes are drilled between these two
side holes[91] but in narrow trench work, such as for a 12–inch
pipe, one hole in the middle of the trench will usually prove
sufficient.

The holes are spaced about 3 feet apart longitudinally. After the holes have been completed they should be plugged to keep out dirt and water.

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 − √(cos^2 θ − cos^2 φ))⁄(cos θ + √(cos^2 θ −
cos^2 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 φ)_wh_.

=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_ = 36.7_h_.

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_ = 18_h_.

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 │ │ │ │ │Shear │Shear
│ │ │ │ │ With │Across
│ │ │Compression│Transverse│Grain,│Grain,
│Tension,│Compression│ Across │ Bending, │ lb. │ lb.
│lb. sq. │With Grain,│Grain, lb. │ lb. sq. │ sq. │ sq.
│ in. │lb. sq. in.│ sq. in. │ in. │ in. │ 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, _bd_^2 = _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, _bd_^2 = 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_ ⪙ _S__{1}(1 − _l_⁄(60_d_)),[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_⁄(60_d_)).
──────────────────────────────┬───────────────────────────────────────────────
Sheeting 2 inches × 12 Inches │ Cross Braces
──────────┬───────────┬───────┼───────────┬──────┬──────┬──────────┬──────────
│ │Maximum│ │ │ │ │
│ │ Fiber │ │ │ │ │
│ │Stress,│ │ │ │ Actual │Allowable
│ Maximum │Pounds │ │ │ │Intensity,│Intensity,
│ Bending │ per │ │Total │ │Pounds per│Pounds per
│ Moment, │Square │ Depth and │Load, │Size, │ Square │ Square
Depth │Inch-Pounds│ Inch │Description│Pounds│Inches│ Inch │ Inch
──────────┼───────────┼───────┼───────────┼──────┼──────┼──────────┼──────────
│ │ │end at 26′│ │ │ │
23′–26.75′│ 9100│ 1140│ 9″│ 6,445│ 4 × 8│ 202│ 784
│ │ │int. at 26′│ │ │ │
19′–23′│ 8800│ 1100│ 9″│12,890│ 4 × 8│ 403│ 784
│ │ │end at 23′│ │ │ │
13′–17.5′│ 8550│ 1070│ 0″│ 6,393│ 4 × 8│ 200│ 784
│ │ │int. at 23′│ │ │ │
8′–13′│ 7160│ 900│ 0″│12,785│ 4 × 8│ 400│ 784
│ │ │end at 19′│ │ │ │
0′–6′│ 3000│ 375│ 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
──────┬──────┬─────────┬──────┬────────────────────┬──────┬───────────┬───────
│ Area │ │ │ │ │ │
│ of │Intensity│ │ │ │ │
│Panel │ of │ │ │ │ │Maximum
│Below │Pressure,│ │ │ │ Maximum │Stress
│ this │ Pounds │Total │ │ │ Bending │Pounds
│Depth,│ per │ Load │ │ │ Moment in │ per
│Square│ Square │ in │Load Transmitted to │Size, │ Thousand │Square
Depth │ Feet │ Inch │Pounds│the Ranger from the │Inches│Inch-Pounds│ Inch
──────┼──────┼─────────┼──────┼──────┬──────┬──────┼──────┼───────────┼───────
│ │ │ │Panel │Panel │ Both │ │ │
│ │ │ │Below │Above │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;

_S__{1} = 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, _S__{1} 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.
]

LINE AND GRADE

=159. Locating the Trench.=—In order to locate a trench a line of stakes should be driven at about 50–foot intervals along the center line of the proposed sewer before excavation is commenced. Reference stakes or reference points to this line are located at some fixed offset or easily described point, or the stakes marking the center line of the trench may be driven at some constant offset distance one side of the trench, in order to avoid danger of loss or disturbance of the stakes. Grade or cut is seldom marked on the line of preliminary stakes, although the approximate cut may be indicated.

For hand excavation the foreman lays out the trench from these stakes. In machine work the operator guides the machine so as to follow the line of the stakes.

=160. Final Line and Grade.=—After the excavation of the trench has proceeded to within a foot or two of the final depth, the grade and line are transferred to markers supported over the center of the trench. The markers are horizontal boards spanning the trench and held in position either by nails driven into stakes at the side of the trench, by nails driven into the sheeting, or by weights holding the boards on the ground. Two stakes driven in the ground at the side of the trench as shown in Fig. 113 are the common method of support. If the banks are too weak to stand under the jarring of the driving of the stakes, or pavement or other causes prevent their use the horizontal cross piece may be weighted down by bricks or a bank of earth. The cross pieces are located about every 25 feet along the trench and at any convenient distance above the surface of the ground. The nearer the ground the stronger the support but the greater the interference with work in the trench. The center line of the sewer is marked on the cross pieces after they are set, and vertical struts are nailed on them with one edge of the strut straight, vertical, and on the center line as shown in Fig. 1. The corresponding edge should be used on all struts in order to avoid confusion. The edge is placed in a vertical position by means of a plumb bob or carpenter’s level.

FIG. 113.—Methods for the Support of the Grade Line.
]

The cut to the invert of the sewer is recorded to an even number of feet where practicable by driving a nail in the upright strut so that the top edge of the nail is at the desired elevation above the sewer, or the upright is nailed with its top at the proper number of feet above the sewer invert. The cut is marked on the upright in feet, tenths, and hundredths from the recorded point to the elevation of the invert.

The inspector should watch these grade markers with care by sighting back along them to see that they are in line and have not moved. In quicksand or caving material the marks may move during the setting of the pipes and the levelman should be on the job constantly.

When excavation is being done by machine the depth of the excavation is controlled by the operator who maintains a sighting rod on the machine in line with the grade marks on the uprights.

FIG. 114.—Diagram Showing the Use of the Grade Rod for Fixing the
Elevation of a Sewer.
]

=161. Transferring Grade and Line to the Pipe.=—In transferring grade and line to the sewer a light strong string is stretched tightly from nail to nail on the uprights marking the line and grade. A rod with a right angle projection at the lower end, as shown in Fig. 114, is marked with chalk or a notch at such a distance from the end that when the mark is held on the grade cord the lower portion of the rod which projects into the pipe will rest on the invert. The pipe is placed in line by hanging a plumb bob so that the plumb bob string touches the grade and center line cord. These marks are taken only as frequently as may be necessary to keep the sewer in line. An experienced workman can maintain the line by eye for considerable distances. Measurements should never be taken to the top of the pipe in order to determine position and grade as the variations in the diameter of the pipe may cause appreciable errors.

The position and elevation of the forms for brick, concrete, and unit block sewers are located by reference to the grade line, or they may be placed under the immediate direction of the survey party, or by specially located stakes. For large sewers requiring deep and wide excavation the grade and line stakes are driven in the bottom of the trench about a foot above the finished grade. This requires the constant presence of an engineer who is usually available on work of such magnitude.

=162. Line and Grade in Tunnel.=—In tunnels, line and grade are given by nails driven in the roof, the progress of excavation or the shield being followed by eye and the forms set by direct measurement to the nails.

TUNNELING

=163. Depth.=—The depth at which it becomes economical to tunnel depends mainly upon the character of the material to be excavated and on the surface conditions. In soft dry material with unobstructed working space at the surface, open cut may be desirable to depths as great as 35 or 40 feet. Tunnels are cut in rock at depths of 15 feet or less. In some very wet and running quicksand encountered in the construction of sewers for the Sanitary District of Chicago it was found economical to tunnel at depths of 20 feet and less. Crowded conditions on the surface, expensive pavements, or extensive underground structures near the surface may make it advantageous to tunnel at shallower depths than would otherwise be economical. Winter is the best season for tunneling as the workmen are protected from the elements and labor is more plentiful.

=164. Shafts.=—In sinking a shaft in soft material, the excavation is usually done by hand, the material being thrown into a bucket which is hoisted to the surface and dumped. The size of the shaft is independent of the size of the sewer and depends principally on the machinery which it is necessary to lower into the tunnel. Ordinarily a shaft 6 feet in the clear is satisfactory. A method of timbering a shaft is shown in Fig. 115. Because of the timbering the shaft must be started sufficiently large at the top to finish with the desired dimensions at the bottom. This excess size is sometimes obviated by driving the sheeting at an angle to maintain the same size of shaft from top to bottom.

In timbering a shaft as shown in Fig. 115 the upper frame is staked securely in position at the surface of the ground. This frame is composed of timbers fastened together in the form of a square with the ends of the timbers extending about 12 inches on all sides. The protruding ends are used to hold the frame in position. Excavation is begun inside the frame, and sheeting is driven around the outside of it as excavation progresses. Only two or three men can work advantageously at one time in these small shafts. The second frame is made up of the same size timbers, but all are cut off flush with the outside of the square. The outside dimensions of this frame are such as to allow sheeting to be slipped in between it and the sheeting already driven. The frame is lowered into position and supported from the upper frame by vertical struts nailed to it. The lower end of the sheeting already driven is held out from the lower frame by blocks of the thickness of the next length of sheeting. These blocks are removed as the next length of sheeting is placed and driven. The driving of the sheeting is facilitated by excavating beneath it as it descends.

FIG. 115.—Section of Shaft Timbering.

Abbot, Journal Western Society of Engineers, Vol. 22.
]

The sizes of sheeting and timbering should be computed on the same basis as that for trench sheeting except that for depths greater than 30 to 35 feet Rankine’s Theory is not applicable and judgment must be relied on for computing the sizes for deep shafts. In stiff dry material the pressures will change very little as the depth increases. Sheeting is needed in shaft excavation in rock only to protect the workmen from falling fragments, but in sand, particularly in quicksand and in wet ground, the pressures increase directly with the depth and the sheeting should be computed accordingly. Care must be taken to prevent the formation of cavities behind the sheeting, to fill them if formed, and to see that all pieces of the sheeting and bracing have a firm bearing. It is difficult to prevent the collapse of the shaft once the movement of earth against the sheeting has commenced.