Page 256 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

5.58 THE WORK

because silt and organic matter deposited from the water have sealed all spaces in a thin layer on
its bottom and sides. Breaking this layer by any means will cause the pond to drain, unless suffi-
cient silt is stirred up to seal it again.
Vertical drains, through which water rises because of being displaced by the weight of fill, are
used to stabilize deep layers of saturated peat. Such soils may contain 50 percent or more water,
and behave as viscous fluids under pressure. Fills placed on them sink, owing in part to water
being squeezed out of the mud, and in part to the mud being displaced to each side. This settling
of the fill may continue for a great many years, and become so uneven as to make pavements or
other structures on the fill become unusable.
The squeezing out of water may be accelerated, and the sideward movement of mud practically
eliminated, by making vertical holes in the mud, filling them with clean sand, and connecting their
tops with a drainage system, as indicated by diagrams in Fig. 5.49. The fill is then placed, and its

FIGURE 5.49 Vertical sand drain installation.