•DRAINAGE* METHODS EOR COUNTY ROADS. 6 
when a soil contains as much as -±0 or 50 per cent of gravel and suffi- 
cient clay or other bonding material to hold the particles of stone 
together it proves very satisfactory as a material for a roadbed, be- 
cause it is drained easily and is very stable when compacted. 
Marl. — Marl is a clay containing a large percentage of carbonate 
of lime. Frequently it contains shell fragments and nodules of 
chalk or silica, and other materials. It grades into calcareous clay 
on the one hand and into argillaceous limestone on the other, and its 
quality for road-building purposes varies to a considerable extent. 
In general this material has few advantages over ordinary clay for 
use in constructing a roadbed unless the percentage of lime carbo- 
nate be sufficiently high to give the material, when compacted, ap- 
proximately the character of limestone. 
Gumbo. — Gumbo is essentially clay or silt containing a high per- 
centage of decayed vegetable matter and a very low percentage of 
sand. It has a very strong tendency to absorb and hold water, and 
therefore is hard to drain. If drained properly it may make a fair 
subgrade on which to place the foundation of a road, but it has 
comparatively a very low bearing value, and where used as the 
foundation the design should be such as to give a greater distri- 
bution of concentrated loads than is necessary with most other soils. 
DRAINAGE. 
In the design of a road the drainage structures are planned 
to take care of water under three general conditions: (1) The rain 
that falls on the road surface or grade, (2) the rain that falls on 
contiguous land and flows in accumulated volume toward the road, 
and (3) ground water from any mediate or immediate source. The 
structures commonly used to provide drainage and meet these con- 
ditions are longitudinal side ditches, usually parallel or approxi- 
mately parallel to the center line ; intercepting ditches to accommo- 
date water whose approach so close to the roadway as the side ditches 
is likely to cause injury; lateral ditches or culverts to conduct 
accummulatecr water away from or under the road ; and subdrains. 
The side ditches are primarily to care for water flowing from 
the road surface and consequently are placed close to the surface 
or made a part of it, and their use is so general that it is universal 
practice to incorporate them into the design of the road cross 
section. If they actually are made a part of the surface, or an 
extension of it, they take the form of gutters. As gutters, their 
effect is not entirely the same as when constructed in the form of 
a ditch, as in the former case they do nothing toward draining the 
soil of the roadway, Avhile as ditches they frequently reach such 
effective depth, as the result of cutting and filling along the grade, 
that they not only serve to carry away surface water but also do 
more or less toward lowering the water table under the traveled way. 
