Geologic Techniques in Civil Engineering 1125 



Faulted sandstone may be brecciated and unless recemented may have 

 many open channels. Faults in shale or claystone have a tendency to fail 

 by plastic flow and therefore usually are relatively impermeable. 



Extreme caution must be used when dipping strata are involved in 

 foundation areas. Many landslides have occurred where soft, incompetent 

 beds dipping in the direction of the ground surface become saturated or 

 overloaded as a result of construction. Excavations that remove the sup- 

 port from dipping strata are potentially dangerous. Heavy structures such 

 as retaining walls or building foundations may slide downdip if not 

 properly anchored or keyed into the foundation. 



Joints may provide zones of structural weakness if the trend is in the 

 same direction as the stress pattern of the structure. More often, however, 

 joints provide paths of leakage. Jointed rocks are difficult to excavate, 

 as the limits of the excavation are controlled by the joint system and not 

 by the arbitrary limits set by the engineers. 



Ground Water 



Ground-water investigations are those in which ground-water hydrol- 

 ogy is studied in conjunction with the climate and the properties of the 

 rock and soil as they influence the hydrology of an area. A discussion 

 of the measurement and calculation of ground-water reservoirs would be 

 too lengthy for this chapter. It is sufficient to say that porosity, permea- 

 bility, hydraulic gradient, discharge, and recharge must be taken into 

 account. Most of these are similar to petroleum-reservoir calculations. 



There are many other 'applications of ground-water geology to en- 

 gineering geology. The construction of a dam and reservoir will influence 

 the ground-water levels in the vicinity of the structure. The effect of the 

 change is often deleterious to water-supply sources, sewage disposal, arable 

 soils, foundations, and the like. Attention to the ground-water level should 

 be given during the investigational stages, the construction, and the opera- 

 tion of a structure, so that the status and future efi"ect of ground-water 

 conditions may be fully appraised. 



Ground-water conditions may influence the actual design and con- 

 struction procedures. Problems relating to the load capacity of saturated 

 foundation materials may be encountered. Ground-water conditions may 

 aff"ect construction-material sources. Artesian uplift pressure may aff^ect 

 load distribution. The buoyant eff"ect of a raised water table on water- 

 tight structures may cause damage. Slope stability due to saturation must 

 be considered. Mineralized ground water may have a deleterious eff"ect on 

 the materials with which a structure is constructed. 



Excavations for tunnels, bridges, canals, or structure foundations that 

 extend below the water table may involve ground-water disposal or de- 

 watering problems. The possibility of damage to ground-water reservoirs 

 by lowering water table or the cutting of recharge channels or beds must 

 be investigated. 



