Design drawings and specifications for trench boxes

Design drawings and specifications for trench
boxes must be signed and sealed by the
professional engineer who designed the system
and must be kept on site by the constructor.
Boxes are normally placed in an excavated but
unshored trench and used to protect personnel.
A properly designed trench box is capable of
withstanding the maximum lateral load expected
at a given depth in a particular soil condition.
Trenches near utilities, streets, and buildings
may require a shoring system.
As long as workers are in the trench they
should remain inside the box. Workers must not
be inside the trench or the box when the box is
being moved. A ladder must be set up in the
trench box at all times.
Excavation should be done so that the space
between the trench box and the excavation is
minimized (Figure 14).
The two reasons for this are
1)allowing closer access to the top of the
box
2)limiting soil movement in case of a
cave-in.
Check the drawings and specifications for the
trench box to see if the space between the box
and the trench wall needs to be backfilled and
the soil compacted.
Shoring
Shoring is a system which “shores” up or
supports trench walls to prevent movement of
soil, underground utilities, roadways, and
foundations.
Shoring should not be confused with trench
boxes. A trench box provides worker safety but
gives little or no support to trench walls or
existing structures such as foundations and
manholes.
The two types of shoring most commonly used
are timber and hydraulic. Both consist of posts,
wales, struts, and sheathing.
Figures 15 and 16 identify components,
dimensions, and other requirements for timber
shoring in some typical trenches.
Keep space between trench box and excavation as small as possible.
Backfill
if necessary
to prevent a
cave-in.
Figure 14
Cut back unshored top position with a 1 to 1 slope
(or a 1 to 3 slope for Type 4 soil)
Figure 13
Figure 12
It is good practice to cut a
bench at top of shoring

31 – 7
TRENCHING
“Hydraulic shoring” refers to prefabricated strut
and/or wale systems in aluminum or steel. Strictly
speaking, these may not operate hydraulically.
Some are air-operated or manually jacked. Design
drawings and specifications for prefabricated
shoring systems must be kept on site.
One major advantage of hydraulic shoring over
some applications of timber shoring is safety
during installation. Workers do not have to enter
the trench to install the system. Installation can
be done from the top of the trench.
Most hydraulic systems are
• light enough to be installed by one worker
• gauge-regulated to ensure even distribution
of pressure along the trench line
• able to “pre-load” trench walls, thereby
using the soil's natural cohesion to prevent
movement.
• easily adapted to suit various trench depths
and widths.