FAQ >> Backhoe Excavator

Backhoe Excavator

The backhoe is a principal competitor for small basement work. There arc two typos till-revolving hydraulic or tractor-mounted part-swing hydraulic. Tractor mountings gel into difficulties because of restricted swing. and ollcn limited roach. But they can dig a basement efficiently. Any hoc is capable of shallow digging. but compares most favorably with the dozcrs when the hole is to be over 6 feet (1.8 m) deep. or when unfavorable bottom conditions, such as water. mud. boulders, or ledge, are encountered. It is able to take cart of anv necessary ditching without change of attachments.

It is recommended that the digging lines be set a few inches outside of the required excavation. although in even-textured soil the backhoe can do a very exact job. In addition to the corner stakes, intermediate guide pegs should he set at short intervals along the digging lines. as the operator cannot sight along these lines without getting down from the shovel. and the finished wall is established with the first cut.

Figure 4.11 shows the 20 by 30 foot (6 x 9 m) basement with the depth increased to 8 feet (2A m) Lining Cp. Accurate lining up of the machine is essential for a clean job If the cut is to begin along the south line, the shovel is placed as in (A), with the bucket about three-quarters extended and testing a few inches beyond the southwest corner. The boom and the tracks are parallel with the south digging line. Lining it up in this manner is greatly simplified by marking the width of the bucket (including side cutters. of used). centered. on the bottom of both dead axles. with paint. or better yet, with stubs of welding rod. Sighting across the outside pair of these marks from the rear, the outer edge of the bucket should he exactly in line with them. and all three points should be on the digging line.

A ditch is now dug to bottom grade with its bell edge on the digging IineI and the spoil is dumped to the south The far end of the ditch will curve rather sharply inward. When the hackhoe has dug as much of the south wall as it can from its position. it reaches to the center of the west line and digs a trench back from there. The triangle included in these ditches is dug in layers to bottom grade. which may be found by a measuring stick, and as near to straight down from the west line as possible.

The shovel is then backed up a few feet (meters) to position (B). It can now cut the west end of the ditch to he almost vertical, because of the more extended position of the bucket. The south wall ditch is then extended as near the shovel as possible. and material between it and the center cut down in Iavcrs to the bottom. The ccntcrdinc will be irregular.

The spoil pile will tend to build up too sharply at the edge of the hole unless pushed back. This pushing may be done by regulating the outward swing of the bucket during the dump, so that it strikes the pile at a spot where its momentum will push a considerable quantity of dirt outward, without stopping its own motion. Knocking dirt back should be started early. before the pile gets high. The hoist clutch must be engaged during this operation The quantity of dirt that can he put in a pile is greatly increased in this way.

Digging is continued in the same manner, with careful attention to a clean, level bottom until the east end is reached. The hackhoe can probably cut this to a nearly vertical wall immediately in front of it. but will leave a ragged edge. as (C) farther north The hackhoe is then turned and walked into the unexcavated north section. When its center is a half-bucket width inside the cast line. as in (D). it stops and shaves the end of the excavation. then trenches to the north edge. 11 next straddles the north line, and is lined up in the same manner as before, with the bucket resting in the hole in the northeast corner.

The north section is excavated in the same manner as described for the south, and complctci the excavation. The west edge may be cleaned up, if necessary, by turning the backhoe to walk parallel to the edge, so that the bucket can dig straight up The backhoe should not be put in thie position. Iwvvever. unless the soil is firm and is known to have good load-bearing qualities, as a crawler machine is vulnerable to cave-ins or slumping under one track.

This edge may also be trimmed from the north and south banks.

The completed excavation and spoil piles arc shown in Fig. 4 12. It will he noticed that the piles are somewhat offset from the hole, so that the south pile can easily he used for fill on the cast end. and the north pile on the west end Both ends are let) open for access and storage The north cut could have been made in the same direction as the south cut. if the east ditch were shorter. but the till would then have been concentrated toward the cast end.

A more finished hole could he made by' starting the digging with a ditch along the west edge, dug from the south. The spoil pile would largely block the access to that side. unless the soil were piled in the basement area for rehandling Ifacccas were not important, this ditch could he widened toward the center, reducing the amount to he piled to south and north. Existence of such a ditch wv)uld make it necessary to work the north section toward the cast.

Loading Trucks. A backhixr can load the spoil in trucks instead of dumping it on the ground Where the piles will be so large that they will have to be dragged back, a truck (it tricks may be used to take part of it avtay, the backhoe continuing to dump on the piles when no truck is in loading position. If grading plans have been prepared requiring use of the fill away from the foundation. it may be cheaper to truck it than to push it later with a dozer. Ilowerer. enough of a pile should be lets by the hole for hackfilling between the foundation and the edge of the excavation.

The backhoe can dig tooting trenches below the floor level where it is working parallel to the edge. as along the south. cast. and north walls in Fig. 4.11. The hydraulic backhoe can dig them anywhere. but the parallel position is easiest.

Checking Grade. Cutting the bottom to proper grade is more difficult with a backhoe than with a dozer, as the hackhoe operator looks down at the grade rather than along il: ha: more difficult climbing down to check it, and cannot move the machine back to grade over mistakes.

It is very helpful to the operator to have someone to check the work, although the operator can manage alone if necessary. fhc corner stakes, and preferably some other stakes. nun he marked at a certain height, as 9 feet (2.7 in) above the bottom. In a level field the marks would all be 1 foot (0.3 in) above the ground: in a sloping one the highest stake should be marked a little above the ground. and the others with the aid of an instrument or a string level. A stick should tx: cut 9 feet (2 7 in) long.

if the operator is checking the grades alone, he or slx: nay fasten a taut string between two stakes so that at will go over the spot in question and measure the distance from the floor to the string with the stick. Spots which cannot be crossed b% the string, of measured directly front the height of the wall. may be checked from a known spot by e\ c. or with a hand or carpenter's let cl. See Fig 4 13(A).

If two people arc doing the work. a string may he stretched between stakes on one side. Another string fastencd to another stake across the excavation may be held in any desired position on the first string, as in (B). while the other person holds the stick.

Hand. transit, or laser levels may. be used A long rod is required when the instrument is set outside the hole.

A hackhne cannot cut a perfect floor In a pit. because of the projection of the teeth and the basca. The smoothest grade is obtained when the bottom of the bucket is used for finishing, than the teeth.

Irregular Edge. Figure 4.14 shows a basement of the same irregular shape as that in Fig. 4.7. The principal considerations in doing complicated excavations with a backhue arc to avoid digging it into a trap, to avoid surrounding it or blocking it from other work by piles of spoil; and to work either parallel or at right angles to outside edges.

There arc several ways in which this basement can be dug The north aide can he dug from the cast end, as in (A) and (R) When the jog is reached it is finished off with a vertical cut. the backh(e backed away, and brought back in position to cut along the inner line If the start is made at the west, the cut is brought a little beyond the jog, aril the position then shelled to dig along the outer line The machine may dig the south side by entering from the west and starting at the southeast corner. Excavation is carried back to the west line. first ditching the edge then digging out the center. Care is taken to begin the spoil pile well west of the south room. The backhoe is then moved oll' to the south and up to the room. It is first lined up to the west side of this room and makes a Irregular Edge. Figure 4.14 shows a basement of the same irregular shape as that in Fig. 4.7. The principal considerations in doing complicated excavations with a backhue arc to avoid digging it into a trap, to avoid surrounding it or blocking it from other work by piles of spoil; and to work either parallel or at right angles to outside edges.

There arc several ways in which this basement can be dug The north aide can he dug from the cast end, as in (A) and (R) When the jog is reached it is finished off with a vertical cut. the backh(e backed away, and brought back in position to cut along the inner line If the start is made at the west, the cut is brought a little beyond the jog, aril the position then shelled to dig along the outer line The machine may dig the south side by entering from the west and starting at the southeast corner. Excavation is carried back to the west line. first ditching the edge then digging out the center. Care is taken to begin the spoil pile well west of the south room. The backhoe is then moved oll' to the south and up to the room. It is first lined up to the west side of this room and makes a The first method requires at least pailial hacktilling as soon as the foundations ate up. to provide access to building material. The weight of the till and of the large dozers commonly used is likely to break in uncured foundations, particularly if they are not braced by first-tloot learns.

A large dragline might be able to pile all the spoil on one side to allow access to the other. The other methods give immediate access to the front and hack. and allow space for piling materials. Backtilling can be postponed until the building is completed. Relative cost sill depend partly on the %aluc of the spoil removed.

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