HIGHWAY BEIDGE LOCATION 
9 
In other words the encountering of rock or hardpan at elevation 
-30.0 increased the cost of this work from $1,224 to $3,808.45. Of 
course, if the foundation encountered had been hard rock, a higher 
unit load on the foundation could have been employed and some 
saving made thereby. The cost, however, could not have been 
brought down to the cost of the piling foundation. 
Subaqueous rock foundations are grouped in the order of their 
desirability as follows: 
(1) Exposed rock in still water at shallow depths. 
(2) Rock overlaid with a stiff impermeable cover. 
(3) Rock overlaid with a stiff permeable cover. 
(4) Bare rock in still water and comparatively deep. 
(5) Bare rock in swift current water and comparatively deep. 
PUMP-^ 
SACK DAM - -^ 
ROCK CUT TO HERE FOR PIER 5EAT AND ANCHORAGE 
Fig. 8.— Sack dam used in foundation work on rock at shallow depths and where the current is not strong 
Class 1 admits of the construction of cheap sack dams as shown in 
Figure 8. After these are constructed they may be unwatered easily 
and calked or plastered either from the inside or outside to stop any 
large leaks. The rock sur- 
face can be worked to 
roughen and to "heel in' 7 
the pier base and back forms 
may be built and easily 
sealed at the bottom as 
shown. This type of con- 
struction may be used for 
depths such as can be held 
by sack dams and in cur- 
rents which are not too 
swift to permit working 
around the sack dam. 
Classes 2 and 3 are 
illustrated in Figure 
9. Where the cover is 
impermeable, as indicated 
at the left, the foundation may be kept dry by intermittent 
pumping from a sump located between the sheet piling or crib 
and the pier forms and no seal is necessitated at the base. 
Where the cover is permeable, as shown at the right of Figure 9, 
a tremie or bottom-dump bucket seal of thickness sufficient to 
withstand the hydrostatic head 7i must be placed before the dam 
can be unwatered. This seal must, in general, be of a thickness equal to 
62 5 
~^~ 7t, or about 0.4 Ti. For great depths, the cost of this seal becomes 
very high and for very great depths the use of the pneumatic caisson 
is more economical. 
32903°— 27 2 
Fig 
—Methods of pier construction where rock is over- 
laid by permeable and impermeable cover 
