James R. Hooper, 1981. "Soil Properties and Case Histories of Structure Siting in Delta Mudflows", Offshore Geologic Hazards: A Short Course Presented at Rice University, May 2-3, 1981 for the Offshore Technology Conference, Arnold Bouma, Dwight Sangrey, James Coleman, David Prior, Anita Trippet, Wayne Dunlap, James Hooper
Download citation file:
A study program for offshore platform location and design near the Mississippi River Delta includes comprehensive analyses of geologic conditions. In addition to site and regional geophysical investigations, samples from borings and drop cores are tested using techniques of soil geochemistry, x-ray radiology, clay mineralogy and paleontology; radioisotope dating of sediments by Pb-210 and Ce-137 analysis is also used in certain regions where soil conditions are appropriate. When results are combined with data on site history from old bathymetric maps of the delta and by comparisons of repeated geophysical surveys, a detailed picture of site and regional depositional history is constructed.
Engineering properties such as in situ soil strength, are measured in deep-penetration borings and laboratory test programs performed on samples recovered from the boring. Near-surface soil variability is estimated from results of in situ strength tests 10 to 20 ft deep, performed at various locations. Borings are also drilled into nearby geologic features that may influence design and analysis. Pore pressures are commonly measured using piezometers placed in granular deposits several hundred feet below the seafloor.
Programs of this type have been pursued in the mudflow region of the delta since the early 1970's. The scope and sophistication of the investigations have changed over the years in response to the availability of new tools and techniques and the growing knowledge of depositional processes. However, regional coverage now includes the full sweep of the active delta front from Southwest Pass in the west to Pass A Loutre in the east and many of the most significant types of seafloor features and stratigraphic variations have been studied to some degree.
It is inherent in development of a project report that comparisons are made between geologic test results and soil engineering measurements. With the passage of time and completion of successive projects, useful models of interrelationships between engineering properties and depositional characteristics have been developed. It is possible, for example, to typify shear strength properties of certain types of mudflows, or to predict pore pressures in buried sand strata. Differences between engineering properties of soils in transport gullies on the seafloor can also be contrasted to adjacent, more stable deposits.
In the discussion, examples are given of geologic and engineering characteristics of stratigraphic conditions encountered in the delta front region of the Mississippi River. Emphasis is placed upon relationships between these soil properties. Methods of inferring (a) soil engineering properties from geologic studies and (b) soil depositional history from engineering tests are presented for sites in the delta. Using results of studies performed within the massive mudflow region of South Pass, the development of engineering design parameters from geologic evidence is outlined.