66 BULLETIN 124, IT. S. DEPARTMENT OF AGRICULTURE. 
concrete foundation. An inspection after failure showed that the 
foundation of this road was made of rather poor grade concrete, and 
was not uniformly of the required depth, but the completeness of the 
failure would seem to indicate that even full 4 inches of good con- 
crete hardly would have been sufficient. In this case the subgrade 
conditions were fair and the road had held up under light traffic for 
about one year, when failure was caused by a few trips of a loaded 
motor truck. 
The maximum depth of 8 inches is sufficient for all ordinary sub- 
grade and traffic conditions, provided a good quality of concrete is 
used, and in most cases a depth of 6 or 7 inches has proved entirely 
sufficient. 
Though the strength of a concrete slab increases rapidly with an 
increase in depth, the conditions affecting the concrete road founda- 
tions are so involved that no data now available furnish reasons for 
a definite conclusion as to the rate of increase. When the probable 
effects of an increase of depth are all considered, such as greater 
strength as a simple beam, extra dead- weight to act against frost, 
additional resistance to cracking under changes of temperature and 
moisture, additional strength to resist impact, and the lower per- 
centage of scant thickness produced locally by the unavoidable ir- 
regularities of subgrade, it is probable that a 6-inch slab foundation 
will, give at least twice the effective service that a 4-inch slab will 
give. 
If it be deemed advisable to employ a greater depth than 6 inches, 
it usually is permissible to decrease the depth from the center toward 
the edges. This is done by flattening the crown of the subgrade. 
CUKBS. 
The types of road crust ordinarily requiring concrete foundations 
also usually require curbs of some kind to protect the marginal por- 
tions of the crust from disintegrating under traffic. The curbs may 
be made of either concrete or stone, and, for country roads, are de- 
signed to come flush with the road surface. Stone curbs should have 
a depth of from 12 to 18 inches, according to the depth of the frost 
line, and should be set before the foundation is laid. Concrete curbs 
may ' be constructed either separately or in combination with the 
foundation. In the former case they should have a cross section 
similar to stone curbs and should be constructed in place ahead of 
the foundation, while in the latter they may be simply flanges on the 
edges of the foundation, formed of rich concrete poured before the 
concrete of the foundation has taken its initial set. The proper thick- 
ness for both stone and concrete curbs varies somewhat with the con- 
ditions involved, but 6 inches is the thickness employed most com- 
monly. Four inches is the minimum- likely to prove satisfactory 
