Page 193 - Moving the Earth_ The Workbook of Excavation

BASEMENTS

BASEMENTS 4.33

FIGURE 4.27 Bracing, trench method.

Porous soil under the bottom of the wall may permit excessive quantities of water and sand to
boil up in the bottom of the excavation as final grade is approached. If the bottom is in clay, but
porous soil is immediately below, the job may proceed easily, and then suffer from a sudden and
disastrous blowup of the bottom.
An area protected by a cofferdam may be dug wet, in which case the structure serves to prevent
soil from slumping into it.
Caissons. A caisson is a structure which serves to keep soil and water out of an excavation, and
forms part of the permanent structure for which the excavation is made.
A simple type of open-top caisson and stages in its growth are shown by cutaways in Fig. 4.28.
A hollow square, ring, or other shape is made of reinforced concrete, with the bottom tapered to
an inside edge. If the work starts on dry ground it may be built in a shallow excavation where it
is to be used. If the start is underwater, it is made elsewhere with walls high enough to keep out
water when it is lowered into place. Transportation is usually by barge.
The caisson is lowered by digging inside to undermine it, and building the top to provide more
weight, and to keep it above ground or water as it descends. Most of the digging is done under-
water, and it is a very ticklish job to do it accurately enough that the caisson will sink straight.
When it comes to the bottom, investigation must be made to determine whether it is on bedrock
or boulders. If the rock surface slopes, concrete must be pumped underneath to give it firm bear-
ing on the low side.
The pneumatic caisson has an airtight cap above the bottom, with sufficient air pressure main-
tained under it to keep water out. Air locks and chambers are provided for entrance and exit of