Page 212 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.14 THE WORK

Design. The OSHA standards contain many design requirements that must be satisfied to meet the
regulations for worker safety in the excavation. The standards give the sizes and spacing of timber cross
braces (struts), wales, posts, and sheathing for various soil types and trench depths. The timbers are to
be oak or the equivalent, with a bending strength not less than 850 pounds per square inch. The soil
types are cohesive soils with certain unconfirmed compressive strength (q ), type A having high q u
u
as compared to type C which has low q . These types are similar to type 2 and type 4 in the above
u
illustrations from the Construction Safety Association of Ontario. Their type 1 is hard ground to dig,
in fact, close to rock. There are other standards for aluminum hydraulic shoring that are followed by
the manufacturers of that form of shoring. OSHA requires a ladder, stairway, ramp, or other means
of access or egress for any excavation deeper than 4 feet. Whatever safe means of access or egress is
used, it cannot be located farther than 25 feet away from any employee.
OSHA requires the contractor to sign a Competent Person who is capable of identifying existing and
predictable hazards dangerous to employees, and who has authority to take prompt measures to eliminate
them. This person does not need to be an engineer, but must have training in, and be knowledgeable
about, soil analysis, the use of protective systems, and the requirements of this standard.
Immediate Bracing. With any bracing the ditch is made sufficiently wider than the bucket that
it can work between the struts. The ditch is dug to a depth of about 2 feet, full width, and the top
pair of walers placed, and cross braces set with such spaces that the bucket can get down between
them. Planks are set vertically touching each other outside the walers.
The shovel, preferably a clamshell, now digs a foot or two below the waler. Workers with hand tools
dig the dirt out from under the vertical planks, allowing them to settle, and also remove dirt under the
crossbeams which the buckets cannot reach. This dirt is piled in the middle of the ditch and is taken out
by the bucket when the laborers are out of range. The shovel then digs more deeply and is followed by
hand tool work. At a depth of 2 to 5 feet below the top waler, another pair of walers is set inside the planks
and braced across the ditch. Alternate excavation by shovel and hand tools, undermining and dropping
of side planks outside the walers, and installation of additional beams are continued until bottom grade is
reached. Ordinarily, the walers and crossbeams (struts) are either heavier or more closely spaced with
increasing depth as the potential pressure increases.
If the ditch is deeper than the length of available planks, those started at the surface should be
of variable length. As each one drops below the top waler, another plank is placed on top of it to
follow it down. Mixed lengths make this possible without weakening the structure by having a
row of these joints together.
The waler beams are also of different lengths so that both members of a pair do not end together.
The joint between any two can therefore be braced against a solid beam on the other side.
Two- or 3-inch sheeting, and 6 6 walers and crossbeams, spaced 5 to 8 feet apart, are strong
enough for moderate depths in most soils. If more protection is needed, heavier wood may be
used, additional planks can be driven outside the sheeting, or inserted inside by a complicated
process of removing and replacing walers. Steel sheet piling is much stronger than wood.

Movable Bracing. When the work which is to be done in the ditch can be completed in short
sections so that the ditch can be backfilled a few yards behind a backhoe, a portable bracing structure
can be used. See Fig. 5.9.
It may be made up of steel or wood, and should be equipped with a tow bar or chain at the front
bottom, which can be gripped by the bucket teeth. It is lowered into the first section dug, and the
pipe laying or other work is done inside it while another section is dug. The shovel drags it along
in the ditch whenever sufficient digging or pipe laying has been completed to justify moving it.
Such a device can result in tremendous savings. However, it cannot be used on many jobs
because of the necessity of checking the work, or having it inspected. Also, if the sides should
close in on it, it might be very difficult to free up for moving.
Backfilling should be done as soon as possible, as allowing the sides to cave may damage the
pipe, or shift it out of line.

Flowing Banks. If the sides are so unstable that they cave or flow immediately upon being cut, the
sheeting planks or sheet piling must be driven down by air hammers or pile drivers, and the dirt dug