HIGHWAY BKIDGE LOCATION 11 
For any type of foundation, except piling, the cost increases nearly as the 
square of the depth- below water surface. 
Crib work on a pile foundation need not be unwatered, but can be sealed 
with concrete placed with a tremie or bottom-dump bucket. Footings through 
semisolid material, such as stiff clay or gravel resting on solid rock overlaid with 
such material may be similarly placed, since the overburden affords the necessary 
lateral support. Footings on bare rock or on rock overlaid with soft material 
must be anchored laterally, for which purpose it is generally necessary to exca- 
vate into the rock a short distance. For this reason, and also to avoid the 
danger of founding upon inclined rock surfaces or large loose bowlders, it is 
generally necessary to unwater foundations of this kind. 
For relatively great depths, the cost of a concrete seal of sufficient thickness 
to resist the hydrostatic head during construction is a large item. Considerable 
saving may then accrue if an impermeable or nearly impermeable material is 
encountered, as in this case the water surface surrounding the cofferdam may be 
kept lowered by means of a double wall dam or an auxiliary pumping pit, until 
the concrete in the footing base has been placed. 
In making a bridge reconnaissance, the engineer should bear in 
mind that exposed rock at the water's edge, even if both banks of 
the stream disclose the same formation, does not always indicate a 
rock bottom extending across the channel. There are numerous 
instances where a stream 
during a former geologic 
period has cut a deep gorge 
which during a subsequent 
period has been filled with a 
lava flow or with glacial, or 
alluvial debris. The Hud- 
son River is a classic ex- 
ample of this action. Here 
the river gorge has been 
filled to a depth of a thou- 
sand feet due to a general 
land subsidence which con- 
verted the lower reaches of 
the river into an estuary or 
bay in the comparatively 
still water of which a general 
Sedimentation OCCUrred. A fig. lO.-A buried channel or fissure formation 
later reelevation of the land 
reconverted the bay into a river and the flow during the present 
period is through and over the sedimentary deposit which overlies 
the original gorge floor for a depth of a thousand feet or more. In 
certain areas in the West, deep rock-bound gorges have been partially 
filled through subsequent lava flows and in the glaciated sections 
glacial gravels or glacial till is found filling up old buried channels 
in the original rock formation. 
In certain instances river gorges have had their origin in a fissure 
formation or in the comparatively rapid erosion of intrusive dikes 
of material softer than the surrounding rock. 
In view of the foregoing, a condition as indicated in Figure 10 may 
quite possibly be encountered. In such a case, it might easily be pos- 
sible to mistake the overhanging edge of the buried channel for solid 
rock bottom, and to form the conclusion that this formation extended 
clear across the stream. A careful investigation of the general char- 
acteristics of the river gorge should be made before judgment is passed 
upon foundation conditions. 
FISSURE LIKE 
