V. ENVIRONMENTAL AND ENGINEERING FACTORS 



1. General . 



The planning and design of most coastal and inner shelf engineering 

 works require an assessment of a number of factors related to the geolog- 

 ical environment. Such factors as geologic hazards in the project area, 

 shelf floor and subfloor soil characteristics, and the mobility of the 

 surficial sediment layer are important considerations in most construc- 

 tion, dredging, and disposal operations. In addition, many inner shelf 

 sediment and rock deposits are potential sources of sand for restoration 

 and nourishment of nearby beaches, construction aggregate, fill material, 

 and riprap. 



As a result of the analysis of the ICONS survey data, some general 

 observations can be made on a number of these factors in the Cape Fear 

 inner shelf region. Quantitative data cannot be provided in most cases 

 because of the general nature of the survey and the unsuitability of 

 vibracore samples for many engineering soil tests. Available samples 

 are, however, suitable for classification under the Unified Soils 

 Classification System (see U.S. Department of Interior, 1963) . 



2 . Geological Hazards . 



Of the many geological hazards affecting engineering works (Bolt, 

 et al . , 1975), the following were most pertinent to the study area: 

 ground rupture, ground shaking, soil liquefaction, rapid settlement, 

 slope instability, and tsunami waves. All of these hazards are apt to 

 occur in association with earthquakes but most can also be initiated by 

 other causes. Earthquakes appear to be the principal geological hazard 

 for large-scale or widespread damage, and the assessment of earthquake 

 risk and the predisposition of soils for failure under earthquake load- 

 ing are important planning factors for engineering works. Local and 

 State planning agencies, the U.S. Geological Survey, and the National 

 Oceanic and Atmospheric Administration compile and publish data on 

 earthquake potential and characteristics in the United States (e.g., 

 Algermissen, 1969; Coffman and Von Hake, 1973). 



Earthquakes are most commonly caused by massive movement of earth 

 segments along fault zones. The relative displacement itself may cause 

 severe damage to structures which span the fault and associated phenomena, 

 such as tsunamis and soil failures due to ground shaking, can affect an 

 area extending many miles from the actual epicenter. For that reason, 

 the presence or absence of faults which could be potential areas of 

 massive earth movements is an important consideration in locating and 

 designing offshore and coastal zone structures. 



Faults can be detected on seismic reflection profiles provided three 

 conditions are met. First, there must be sufficient penetration of sub- 

 bottom strata to reach the faulted strata; second, there must be one or 

 preferably more reflective interfaces within the faulted strata to serve 



72 



