130 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



the material acted upon is bedrock or a transported soil, the residual 

 products that remain after weathering are very important. The prin- 

 cipal end product is clay, and the clay minerals may be divided into 

 three groups: montmorillonites, kaolinites, and illites (also called 

 hydrous micas and bravisites) (Grim, 1942). In illites and montmoril- 

 lonites a portion of the aluminum is replaced with iron, magnesium, 

 or both. Potassium is an important element of the illites, which are 

 thought to be the result of alterations occurring on the sea floor rather 

 than after uplift above sea level. The montmorillonites and kaolinites 

 are products of subaerial weathering but also may be produced by the 

 action of hot solutions. The importance of determining which clay 

 may be present is indicated in one respect by the characteristic tend- 

 ency of montmorillonites to swell. Consequently the identity of a 

 clay underlying a structure or a fill should be ascertained. If ordinary 

 soil mechanics and petrographic methods do not suffice, the sample 

 should be submitted to the geological clay specialist for identification. 



The depth to which weathering may penetrate in the formation of 

 residual soils depends on many factors, but the principal one is time. 

 In recent glacial deposits the effects of weathering may be negligible, 

 but in the humid tropics it may extend to depths of 100 feet or more. 

 It may be unusually deep along joints and faults in the bedrock. 



The structural attitude of the bedrock may profoundly affect, and 

 cause extreme variations in, a residual soil cover. In flat-lying strata, 

 if it is assumed that secondary structures and surface conditions 

 exert no local control, the thickness of the overburden in a section 

 may be more or less constant. A concentration of vertical joints, or 

 even a single joint, may allow a "seamy" condition to develop. In a 

 limestone, this can be serious inasmuch as these weathered joint areas, 

 filled with residuum, may be from inches to many feet in width and 

 can appreciably increase the costs of excavation. In steeply inclined 

 strata, similar irregularities may exist. 



A series of limestone beds, steeply dipping, may have layers differ- 

 ing greatly in solubility. Hence it is no uncommon condition to find 

 limestone cropping out under one wing wall of a bridge, but at a depth 

 of many feet under the adjacent wing wall. Recent electrical resistiv- 

 ity exploration in Cumberland County, Pennsylvania, checked by bor- 

 ings and later proved by excavation, showed beneath a single bridge 

 abutment overburden thicknesses varying from 2 to 46 feet. Such 

 irregularities proved to be the rule in this area and were attributable 

 solely to the solubility of individual limestone beds often only a few 

 feet thick. 



Similar conditions were found in road sections crossing steeply dip- 



