Ch. 7] THE SUBGRADE AND BASE COURSES 141 



of the ground-water table by natural causes or from excessive domes- 

 tic and public usage in restricted areas. In regions of crystalline or 

 otherwise hard rock, settlement may be negligible, but in soils and 

 weakly cemented strata it can be a serious problem. In any area 

 where liquids are withdrawn from the pore spaces in sediments, re- 

 adjustments are inevitable and must be planned for in construction. 



When marsh or swamp lands are floored by impervious material 

 which in turn is underlain by free-draining strata, drainage by means 

 of vertical sand drains is feasible provided that the free-draining layer 

 is above the water table. These sand drains may be in the order of 

 28 inches in diameter, spaced 10 to 20 feet apart, and placed to depths 

 of 40 or 50 feet or more (Hewes, 1942, p. 181). Sand drains of smaller 

 diameter may be used. This method may also be used in deep soil 

 cuts for the prevention of slides. However, when the base of the cut 

 is close to bedrock, or the water table is high, the vertical sand drains 

 must feed into a longitudinal drainage tunnel or into drains leading 

 from the base of the sand drain to the gutter or longitudinal sub- 

 drains beneath the highway shoulder. Peat and organic silts, which 

 are highly compressible, should be entirely removed when feasible, 

 by the use of either conventional excavation equipment or explosives 

 (du Pont de Nemours, 1939). Settlement otherwise may be antici- 

 pated, even though a prism of suitable material is used to replace 

 the upper few feet. 



Subsidence of the surface over shallow oil fields (Johnson and Pratt, 

 1926) or sulphur mines, where the oil or sulphur is brought to the sur- 

 face in a liquid state, may cause subsidence in the form of long, gentle 

 sags, but sometimes with offsets of a foot or two at the margins. 

 Here, again, ponding is the chief danger, because the subsidence may 

 be so gradual over a relatively long time that highway and railway 

 services may not be interrupted. In extreme cases regrading may 

 be necessary. 



THE SUBGRADE AND BASE COURSES 



The modern heavy-duty highway is subject to more frequent and 

 heavier wheel loadings than ever before; hence it is essential that most 

 critical attention be paid to the underlying soils and base courses. 

 The soil beneath the road surface is known as the subgrade. Base 

 courses or foundation courses are often placed between the road sur- 

 face and the subgrade. The load-carrying ability of the subgrade is 

 of vital importance, and it is often necessary to prepare these soils 

 for the types of loads anticipated. The thickness, gradation, and 



