Page 213 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.15

FIGURE 5.9 Manufactured trench shield. (Courtesy of Efficiency Production, Inc.)

from between them afterward. Penetration and control of direction are usually best if the planks
are driven only a short distance below the digging. However, mud may flow so readily that the
sheeting must be down several feet below excavation level to prevent it from swelling on the bottom.
All gradations between this condition and stable banks may be encountered in a short distance.
Washout Failures. Only wet ground or loose sand exerts very heavy thrusts against the shoring.
Water draining down the sides of a braced trench may erode them so the sheeting moves outward,
thus loosening the crossbeams or jacks and allowing them to fall, after which the sheeting can be
pushed in by any movement of the banks.
Stabilization. Soft ground may be stabilized by chemical treatment, well-point pumping,
drainage, or freezing, by procedures described elsewhere in this book.

STREAM CROSSING

Pipelines must often be built across both large and small streams.
If a wide stream or a pond has a soft bottom and is not used by ships or barges, the pipe may
be laid directly on the river bottom. However, it is often necessary to protect it by burying it.
Excavation may be done by a grapple dredge; by a clamshell, dragline, or hoe on a barge made
up for the purpose; by a cable excavator on the bank; or from a temporary jetty.