322 
MALARIA 
lateral inverts, French drains, or ditches 
with sodded bottoms. Where the flow is 
constant, French drains or paved laterals 
are utilized, and where the flow is inter¬ 
mittent, a ditch with sodded sides and bot¬ 
tom has been found satisfactory to prevent 
caving of the sides and the formation of 
bars. 
General specifications. The sand and 
gravel should be clean, well graded, and 
free from clay. The three-inch monolithic 
slab is poured of 1:3:5 concrete. In the 
construction of brick inverts, the brick are 
laid flat with half-inch voids in the same 
manner as in constructing a brick wall. 
The brick are set in and covered with one- 
fourth inch of 1: 3 mortar. If soft brick 
are used, it is imperative that they be com¬ 
pletely covered with mortar to prevent 
freezing and cracking during extremely 
cold weather. All brick must be soaked in 
water at least 12 hours before being used 
in the invert. 
All inverts, either monolithic or precast, 
should be cured for at least 72 hours, by 
flooding, covering with a five-inch layer of 
moist dirt, by burlap mats, or by painting 
with an impervious material designed for 
this particular purpose. 
Curtain walls should be constructed 
across the ditch under the invert to prevent 
any damage to the permanent lining by 
underflow. The depth of these walls in a 
100-foot section of ditch must be greater 
than the total fall. They should not be 
more than 3 inches in width. Weep holes 
are placed in the bottoms and sides imme¬ 
diately above the curtain walls at intervals 
not to exceed 50 feet; where much ground 
water is encountered the spacing may be 
reduced to 8,or 10 feet. In sandy soil it 
may be necessary to construct a filter of 
graded gravel behind the weep hole to pre¬ 
vent the loss of sand. * 
Expansion joints should be left in the 
invert at intervals not to exceed 75 feet. 
These joints are made of burlap ropes 
soaked in liquid asphalt or of a soft mate¬ 
rial such as masonite. Construction joints 
are made in the riprap by pouring a thin 
cotmtersunk slab about one foot wide up 
the bank at points where expansion joints 
fall. The bond between this slab and the 
riprap is broken by covering the slab with 
a sheet of newspaper. 
Where round-bottomed inverts must join 
wide, flat-bottomed culverts or bridges, 
steps must be taken to reduce the width 
through the structure in order to increase 
the velocity sufficiently to prevent mosquito 
breeding and the deposition of silt. Many 
engineers are now building culverts with 
bottoms having the same curvature as the 
inverts. 
Prior to construction work all water and 
gas pipes should be lowered below the in¬ 
vert in order to prevent the pipes from 
freezing and to remove an obstruction 
Fig. 5. Tunica, Mississippi. Left, before malarial 
control drainage; right, after construction of rein¬ 
forced concrete invert with sodded banks. 
which will block the flow, causing eddies 
and resulting in erosion to the banks. 
Where fences cross inverts, light swinging 
water gates made the same shape as the 
ditch should be swung across the invert. 
Construction methods. The use of two 
templates on invert projects is well worth 
while. The subgrade template having a 
radius 3 inches greater than the standard 
enables the foreman to construct the sub¬ 
grade accurately. The finishing template 
is used as a screed. By using it constantly 
in finishing the invert, it is possible to 
obtain an accurate, circular shape and its 
use also tends to result in a denser con¬ 
crete. 
