Drag line excavators will perform as much work as steam shovels under favorable conditions. They are less expensive in first cost and operation, and are equally reliable but they are not adapted to the more difficult situations where steam shovels can be used to advantage. Drag lines are suitable only for relatively wide trenches in material requiring no bracing, and in a locality where relatively long stretches of trench can be opened at one time.

The bucket excavator differs from the drag line in that the bucket can be lifted vertically only and the types of buckets used in the two types of machine are different. The bucket may be self filling of the orange-peel or clam-shell type, or a cylindrical container which must be filled by hand. A drag line can be easily converted into a boom and bucket excavator. Boom and bucket excavators are well adapted to use in deep, closely braced trenches and shafts.

135. Excavation in Quicksand.^[85]—A sand or other granular material in which there is sufficient upward flow of ground water to lift it, is known as quicksand. Its most important property, from the viewpoint of sewer construction, is its inability to support any weight unless the sand is so confined as to prevent flowing of the sand, or unless the water is removed from the sand.

Excavation in quicksand is troublesome and expensive and is frequently dangerous. The material will flow sluggishly as a liquid, it cannot be pumped easily, and its excavation causes the sides of the trench to fall in or the bottom to rise. The foundations of nearby structures may be undermined, causing collapse and serious damage. These conditions may arise even after the backfilling has been placed unless proper care has been taken. The greatest safeguard against such dangers is not only to exercise care in the backfilling to see that it is compactly tamped and placed, but to leave all sheeting in position after the completion of the work.

The ordinary method of combating quicksand and in conducting work in wet trenches is to drive water-tight sheeting 2 or 3 feet below the bottom of the trench, and to dewater the sand by pumping. When dry it can be excavated relatively easily. A more primitive but equally successful method is to throw straw, brickbats, ashes, or other filling material into the trench in order to hold the excavation once made, or this may supplement the attempts at pumping, or the wet sand may be bailed out in buckets. Successful excavation in quicksand requires experience, resourcefulness. and a careful watch for unexpected developments. The well points described in Art. 142 are used for dewatering quicksand.

136. Pumping and Drainage.—Ground water is to be expected in nearly all sewer construction and provision should be made for its care. Where geological conditions are well known or where previous excavations have been made and it is known that no ground water exists it may be safe to make no provision for encountering ground water. Where ground water is to be expected the amount must remain uncertain within certain rather wide limits until actually encountered.

In order to avoid the necessity for pumping, or working in wet trenches it is sometimes possible to build the sewer from the low end upwards and to drain the trench into the new sewer. The wettest trenches are the most difficult to drain in this manner as the material is usually soft and the water so laden with sediment as to threaten the clogging of the sewer. It is undesirable to run water through the pipes until the cement in the joints has set. This necessitates damming up the trench for a period which may be so long as to flood the trench or delay the progress of the work. If it is not possible to drain the trench through the sewer already constructed the amount of water to be pumped can be reduced by the use of tight sheeting.

Pumps for dewatering trenches must be proof against injury by sand, mud, and other solids in the water. For this purpose pumps with wide passages and without valves or packed joints are desirable. The types of pumps used are: simple flap valve pumps improvised on the job, diaphragm pumps, jet pumps, steam vacuum pumps, centrifugal pumps, and reciprocating pumps. All are of the simplest of their type and little attention is paid to the economy of operation because of the temporary nature of their service.

137. Trench Pump.—A simple pump which can be improvised on the job is shown in section in Fig. 95. Its capacity is about 20 gallons per minute but its operation is backaching work. It is inexpensive, quickly put together and may be a help in an emergency. It is to be noted that the passages are large and straight, that there are no packed joints, and that the velocity of flow is so small that it is not liable to clogging by picking up small objects.

138. Diaphragm Pump.—The type of pump shown in Fig. 96 is the most common in use for draining small quantities of water from excavations. It is known as the diaphragm pump from the large rubber diaphragm on which the operation depends. The pump is made of a short cast-iron cylinder, divided by the rubber diaphragm or disk to the center of which the handle is connected. The valve is shown at the center of the disk. As the diaphragm is lifted the valve remains closed, creating a partial vacuum in the suction pipe and at the same time discharging the water which passed through the valve on the previous down stroke. When the valve is lowered the foot valve on the suction pipe closes, holding the water in place, and the valve in the pump opens allowing the water to flow out on top of the disk to be discharged on the next up stroke. Table 54 shows the capacities of some diaphragm pumps as rated by the manufacturers. The smaller sizes are the more frequently used and are equipped with a 3–inch suction hose with strainer and foot valve. They are not adapted to suction lifts over 10 to 12 feet. Where greater lifts are necessary one pump may discharge into a tub in which the foot valve of a higher pump is submerged.

139. Jet Pump.—The simplicity of the parts of the jet pump is shown in Fig. 97. It has a distinct advantage over pumps containing valves and moving parts in that there are no obstructions offered to the passage of solids as well as liquids through the pump. It is not economical in the use of steam, however. It operates by means of a steam jet entering a pipe at high velocity through a nozzle. This action causes a vacuum which will lift water from 6 to 10 feet. The lower the suction lift, however, the greater the efficiency of the work. The sizes and capacities of jet pumps as manufactured by the J. H. McGowan Co. are shown in Table 55.

140. Steam Vacuum Pumps.—This type of pump depends on the condensation of steam in a closed chamber to create a vacuum which lifts water into the chamber previously occupied by the steam and from which the water is ejected by the admission of more steam. The best known pumps of this type are the Pulsometer, manufactured by the Pulsometer Steam Pump Co., the Emerson, manufactured by the Emerson Pump and Valve Co., and the Nye Pump, manufactured by the Nye Steam Pump and Machinery Co.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.