10 BULLETIN 1486, TJ. S. DEPARTMENT OF AGRICULTURE 
With classes 4 and 5 where the rock surface is bare, the cofferdam 
construction becomes more costly. Even with a permeable cover, it 
is generally possible to get enough sealing effect outside the dam for 
unwatering for the short time sufficient to inspect the rock surface. 
For bare rock, however, this can be accomplished only by the con- 
struction of a double-walled puddle dam, as shown in Plate 1, D, 
which is very expensive, or by the use of a submarine diving outfit, 
which is also costly. It is very essential that bare rock foundations 
be inspected in the dry, if possible, and also that the masonry be well 
"heeled in" — generally from 8 to 30 inches into the solid rock if the 
current is swift. 
TYPE C. GRAVEL OR BOWLDERS AT A REASONABLE DEPTH, WHERE A PENETRATION 
OF A PEW FEET INTO THE SAME IS SUFFICIENT TO GUARD AGAINST EROSION 
Footings on this material must, in general, be larger than on solid 
rock; on the other hand the placement of sheeting or cofferdams is 
many times easier than on solid rock, since it is possible to drive the 
sheeting into the substrata a sufficient distance to brace it at the 
bottom. 
TYPE D. MATERIAL WHICH IS SOFT ENOUGH TO PERMIT THE DRIVING OF PILES, 
YET STIFF ENOUGH TO AFFORD LATERAL STABILITY 
This type of foundation requires an area of not less than 2.5 by 2.5 
feet, or 6.25 square feet for each pile used. For a 20-ton bearing per 
pile this is equivalent to that of a natural soil of about 3 tons per 
square foot bearing capacity. For locations exposed to excessive 
erosion, this type of foundation affords a greater insurance against 
scour than does one on a gravel or bowlder formation, unless the 
latter is carried to a depth as low or nearly as low as the bottom of the 
piling which would in general result in a much higher first cost than 
the pile foundation. 
TYPE E. VERY SOFT MATERIAL OF A CONSIDERABLE DEPTH OVERLAYING A HARD AND 
IMPENETRABLE SUBSOIL 
This type of foundation precludes the use of piling on account of the 
lack of lateral stability. Resort must be had to the sinking of 
individual tubes or piers well heeled into the solid or to the carrying 
of the entire foundation down to solid material. (Fig. 7.) 
TYPE F. BOWLDERS OR LOOSE ROCK AT A GREAT DEPTH OF WATER 
This is one of the most difficult foundation conditions encountered. 
Piling can not be driven. Difficulty is almost always encountered in 
excavating within a crib and in founding a crib in such manner as to 
make it possible to unwater the cofferdam. On the other hand, it is 
not the part of prudence to found on such material without a thorough 
inspection, preferably in the dry, and the removal of all loose material. 
Where the water is swift, footings must be anchored against lateral 
movement or sliding along inclined rock or bowlder surfaces, and this 
anchorage can be accomplished best only in the dry. 
In weighing the relative merits of the various types of foundation 
conditions, as above outlined, the following facts should always be 
borne in mind: 
A sheet pile cofferdam is generally cheaper than a crib of like dimensions if 
it can be used. 
