Seafloor Stability Research Articles

Pore-Pressure Dissipation During Offshore Cyclic Loading

For cohesionless seafloor soils, the magnitude of pore pressures have a significant influence on design. As effective stresses control stiffness and strength characteristics for cohesionless soils, the rate of dissipation of pore pressures induced by cyclic loading will have a major influence on foundation deformations and seafloor stability. Four problems are examined: (1)Freefield earthquake response; (2)lateral soil-pile interaction; (3)wave loading of offshore gravity structures; and (4)wave induced seafloor pore-pressure response. Both published examples and new analytical solutions are used to show that significant pore-pressure dissipation may occur in many field loading problems, the extent of dissipation depending on induced pore-pressure gradients, the permeability of the soil, and the frequency of loading.

Environmental Geology of Continental Margin Off Central and South Texas in Atlas Format: ABSTRACT

Integrated environmental studies keyed to understanding fundamental processes and their interrelations were conducted from 1974 through 1977. Most of the aspects investigated were sampled seasonally to provide data on variations within each year and among the 3 years studied. The geologic and related hydrographic, biologic, and chemical data have been compiled in an atlas format consisting of topical overlays so that interrelations are apparent. Three atlases, each covering a geographic area of 2° × 1°, have been completed: Port Isabel, Corpus Christi, and Beeville. Each, in addition to bathymetry and the lease-block grid, consists of seven topical maps: location of oil and gas fields and production installations; water circulation and rates of sedimentation; trace-metals content and texture of surficial bottom sediments (plus seasonal variations); nature of shallow subsurface sediments and biogeology; post-Wisconsin sedimentation patterns and tectonism; late Pleistocene and Holocene depositional environments; and structure of the continenta terrace with emphasis on the chronology of faulting and seafloor stability. Results of the study indicate that water circulation is strongly seasonal, but both local variations and differences in surface and bottom movement are evident. Sedimentation rates have been relatively high throughout the Holocene, averaging almost 1 m/1,000 years; rates now are as much as 9 mm/year locally, based on 210Pb dating. Seasonal differences in both the texture and trace-metals content of surficial bottom sediments are characteristic and probably can be related to variations in infaunal activity and movement of bottom sediments. Faulting has migrated progressively seaward across the shelf with time; movement during the Holocene has been mainly near the shelf edge. Slumping has been extensive along parts of the outer shelf and on the upper continental slope. End_of_Article - Last_Page 420------------

Shoreface-connected sand ridges on American and European shelves: A comparison

Large scale sand ridges, 10 m or more in height and 2–4 km apart have long been accepted as characteristic of shelves experiencing strong tidal flow. However, they also occur on inner continental shelves whose strongest flows are wind-induced. The Middle Atlantic Bight of North America is the best known example, but inner shelf ridge topographies occur extensively on other Atlantic shelves. We wish to call attention to similarities and differences among the inner shelf ridges of North America, South America, and Europe, as a necessary step prior to the framing and testing of hypotheses concerning the hydrodynamics of ridge formation. Inner shelf ridge fields occur primarily on low, unconsolidated coasts whose substrates provide an ample supply of sediment for ridge formation. The constituent materials are commonly coarse to fine sand, but on the Guiana coast of South America, similar ridges appear to be forming in a mud substrate. Here ridges have much greater spacing (tens of km versus km), less relief, and much flatter slopes (1,3000 versus 1,500). Inner shelf ridges are characteristically aligned obliquely to the shoreline, with acute angles opening into the prevailing flow direction. Downcurrent slopes tend to be steeper and finer grained. The ridges tend to migrate downcoast and offshore, extending their crestlines so as to maintain contact with the shoreface. Where ridges are nearly shore-parallel (current parallel), the movement is expressed as downcurrent growth of ridges and downcurrent extension of swales between ridges. The textural and morphologic asymmetry of inner shelf sand ridges can be explained in terms of existing models for sand wave formation. The oblique orientation with respect to the shoreline is also possibly compatible with these models. An alternative, or perhaps complementary explanation for sand ridge genesis requires helical flow structure in the shelf flow field during peak events, but both theory and observation are less well developed for this model. p]Inner shelf sand ridges appear to be responses to periods of intense flow induced by the passage of storms. The extent to which a given shelf sector develops ridge topography may depend on the efficiency with which the local water mass responds to storm passage. Ridge behavior, and especially ridge migration rates, are necessary information for marine environmental management, since ridges are most active in the nearshore zone where the shelf floor is used for sewage outfalls, deep water ports, nuclear reactor sites, and other structures requiring a measure of sea floor stability.

Progress in Marine Geotechnical Engineering

This paper highlights the progress during the past 30 yr in several subareas of marine geotechnical engineering, which includes pipelines, small footing structures such as jack-up rigs, gravity structures, piles, object penetration and breakout resistance, seafloor stability, and geotechnical data acquisition. The progress is illustrated using a number of significant events subsequent to World War II, such as hurricanes, planning for projects in new geographic areas, installation of specific structures, and initiation of major research.