2. Existing utilities that cross the pipeline trench prevent shields
from being pulled forward. Pulling and resetting shields
around crossing utilities is time-consuming plus the area
around the crossing utility has to be shored in some other
way. In pit construction any existing line that crosses through
the hole is enough to prevent use of a shield.
Because most water, gas, and electric installations are
within the top 8 ft of the surface, it is possible to remove the
top shield and pull lower shields under the utilities. Shoring
or sloping still needs to be provided above the boxes.
S h o r i n g S y s t e m s S e l e c t e d f r o m Ta b u l a t e d D a t a 389
9.6.2 Shoring Shield Size and Nomenclature
The basic elements and nomenclature of a shoring shield are shown in
Fig. 9.30. Shoring shields and boxes are described by height × length ×
wall thickness. The wall thickness is nominal. Most shields are constructed
with 3/16-in plate “skin” over tube steel ribs so the actual
wall thickness is nominal size +3/8 in. Standard heights are 2, 4, 6, and
10 ft. Standard lengths are in 2-ft increments from 10 to 32 ft. Wall
thickness is critical to wall strength and deflection. In terms of strength
efficiency, the ratio of the weight to the psf rating, of the box there is a
practical length to each wall thickness. In other words at certain
lengths a box that weighs less can be built with a higher strength rating
and less deflection by increasing the wall thickness. Thicknesses
and practical lengths are 3 in and 12 ft, 4 in and 16 ft, 6 in and 24 ft, 8 in
and 27 ft, and 10 in and 30 ft. Every additional 2 in of wall thickness
translates into an additional 4 in of excavation and backfill quantity,
and so thinner walls are desirable. Required length, next strength, and
then wall thickness usually control the selection of the box. If boxes
are going to be dug in or used to protect existing facilities, thicker
walls should be used to control deflection. Custom boxes can be
ordered from manufacturers in any size and strength. The sky is the
limit, but the practical limit is hit when the weight of the box exceeds
the handling strength of the excavator being used to move the box.
From the excavations handbook
