50 BULLETIN" 724, U. S. DEPARTMENT OF AGRICULTURE. 
back fill in layers not over 8 inches thick and thoroughly tamping 
each layer with a hand tamper. 
The thickness of the successive layers and the weight and face of 
the tamper should be adjusted to produce the most uniform and solid 
compression reasonably possible. Common practice often is inade- 
quate, sometimes the layers being made a full 8 inches thick and a 
light one-man tamper being used. On loose and friable soils a broad 
face is to be preferred, say 8 by 8 inches, and the weight may be raised 
to 50 pounds if double handles are provided and the work is done by 
two men. 
Where the soil is essentially clay and the trench is deep, the back 
fill should be deposited preferably in courses and a tamper having 
a smaller face and weighing 10 to 15 pounds used. 
If a clay or silt loam that readily breaks down in water is used, the 
trench should be filled at first to a depth of not more than 2 or 3 feet, 
the material being tamped thoroughly as above described, and the 
partial back fill then may be kept covered with water for several 
days until the material has become thoroughly saturated and has 
finished settling. Subsequent layers should be added and " puddled " 
until the trench is filled completely. Then it should be permitted to 
stand for several weeks, until the back fill has dried out, before the 
subgrade is prepared and the pavement constructed. Not infre- 
quently it is cheaper to haul in gravel, or other material which may 
be compacted readily for use in back fills, rather than to replace clay 
according to the method above described. In some cases, where con- 
ditions were unusually bad, concrete has been used as a back-filling 
material. Plate VI shows a characteristic failure of a pavement 
over a culvert where the back fill had been poorly made. 
The exclusion of vegetable matter from embankments is nearly 
always provided for in specifications covering road-improvement 
work, but in a great many cases the importance of this detail does 
not seem to be realized. As a rule, no especial harm results to a road 
from vegetable matter being placed in the slopes of embankments, 
because unequal settlement of the slopes would not necessarily injure 
the road surface. When sod and roots are placed in the subgrade, 
however, even in very small quantities, serious damage to the sur- 
face may result from the porosity caused by the eventual decay and 
shrinkage of the vegetable matter. Further, roots near the edges of 
the surface are very likely to live and grow, and the result is a weak- 
ening of subgrade on a line where the pavement is especially ex- 
posed to damage. 
Wet-weather springs are not always in evidence at the time a road 
is being graded, and it is therefore important that the road be exam- 
ined carefully during wet seasons in order to locate troubles of this 
kind. Such springs usually may be drained successfully by means of 
