DEAINAGE* METHODS FOR COUNTY ROADS. 6 



when a soil contains as much as 40 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. 



GiLmbo. — 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 Avhich 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 commonlj^ 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 

 accummulated 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, while 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. 



