A large number of slope failures (slumps or submarine slides) have occurred in the geologic past along the Sigsbee Escarpment between the Atlantis and Mad Dog Developments. A multidisciplinary study was conducted to define the risk of submarine slide activity in the context of the past historical activity that created 35 large scale slumps. Large diameter piston cores were obtained at 95 sites and deep soil borings were drilled at 8 locations to define the physical and engineering properties of the sediments and to determine the estimated time that these mass movements occurred along the Escarpment.Age dating ( 14 C and δ 18 O) on the cores and biostratigraphic analyses on borehole samples provided stratigraphic age control required to understand the time scales associated with the structural evolution of the Sigsbee Escarpment, the sequence and timing of the major slump events, and the relative activity and occurrence interval of the smaller debris and turbidity flows.The steep slope angles and upward and lateral movement of the underlying salt mass has resulted in slope instabilities producing gravity-driven slides. This study confirmed that each major "slump feature" is typically the product of a large number of individual soil mass failure events. A total of 79 age dates were measured by C-14 techniques on 95 cores taken within and around the slumps to give an historical account of the level of activity. The age dating and core results were correlated with the geophysical data to assess: (1) the state of seafloor activities and processes, (2) historical account of the slide events, and (3) ground truth the geologic/geotechnical properties within each slump area.A significant level of submarine slide activity has occurred along the Sigsbee Escarpment in this region over the last 25,000 years. The soil mass failures have reduced in number, magnitude, and level of activity in the last 8,000 years. The rates of sedimentation accumulation were higher during the sea level low stand and are now lower during the sea level high stands. Thus, the large slide events and the attendant debris flows creating the slumps occurred primarily during the periods of rapid lowering of the sea level. The most recently active slump event occurred in the Atlantis area about 2,970 years ago. Some slumps have not been active in the last 10,000 to 16,000 years.
Synopsis The mechanism of cracking in concrete under sustained axial tension is investigated experimentally. This involves the development of new loading and measuring techniques: in particular, the development of a specimen with shoulder-pieces composed of high alumina cement concrete which permits the direct transmission of ultrasonic pulses along the longitudinal axis. Observations indicate that cracks form in the concrete at an early stage of loading at interfaces between the cement matrix and aggregate but that failure is governed by the growth of cracks within the cement matrix. The results are used to establish a law relating sustained stress to time to rupture, which may prove to be of value in analysing the behaviour of more complex structures under sustained load.
A new deepwater static cone penetrometer system, "CPT Stinger", was used to investigate subsurface conditions at a number of production sites in the deepwater Gulf of Mexico. Same site high-resolution geophysical data and long cores obtained with a Jumbo Piston Core (JPC) system illustrate the excellent correlation obtained with continuous geotechnical and geophysical data for defining the spatial variation in soil properties. The high cost of drilling deepwater borings and sampling at widely spaced intervals imposes a significant constraint on obtaining a sufficient quantity of high quality soils data. Thus, improved methods are desirable for more quickly assessing requisite soil properties without sacrificing accuracy. The "CPT Stinger" system is a new tool that can fill this role. This new system allows the same general suite of JPC coring equipment to be modified for CPT testing and can be deployed from an oilfield supply vessel. By simply replacing the standard piston core liner with a CPT system containing thrusting rods, a power/control module, and CPT data logging system, the field operation can quickly be converted from sampling to in situ testing mode. The results show that the new system provides continuous soundings with centimeter-depth accuracy and stratigraphic consistency. In addition to the acquisition of high-quality static CPT data, the sampling rate of the CPT logger allows the acquisition of dynamic CPT data during free fall that can be adjusted for velocity differences to emulate static data A particularly effective way to use the system is in conjunction with nearby, continuous sampling with JPC cores and subbottom seismic profiles. This allows the correlation of the CPT results with strengths from continuous samples over a significant depth of overlap and with the geophysical cross sections. Correlations are presented in the paper for undrained shear strength data from long cores with in situ CPT data. Following the premise that more information acquired for a given budget tends to reduce the risks associated with foundation design and installation planning, the economic benefits of rapidly acquiring the geotechnical data from a lower cost vessel are also illustrated.
A thick section of sediments was deposited along the lower continental slope in the vicinity of the Atlantis and Mad Dog Developments in the northwest Gulf of Mexico during the time period spanning the last glaciation to the present. Sediments were collected using large diameter piston cores and borings, and studied to understand how variations in sedimentation rate and sediment properties relate to changes in climate and sea level. Sediment bulk density and compressional-wave velocity were measured using a multisensor core-logging device; afterward, the cores were split and the sediment lithologies were described. Based on these data and the interpretation of high-resolution seismic data collected at the core sites, samples were taken from the cores to determine the ages of the sediments using the following methods: radiocarbon dating, downcore changes in the oxygen isotopic composition of foraminifera shells, and nannofossil occurrence.A detailed chronology for the sediments deposited during the period of time spanning the last glaciation to the present was determined and compared to other sediment characteristics. The establishment of this chronology was an essential step in the assessment of potential geohazards because it provides a fundamental framework for understanding how environmental processes affect the geologic and geotechnical characteristics of the seafloor. For example, the ages of regionally persistent high-resolution seismic reflectors were established and related to changes in sediment properties, providing an important temporal framework for the interpretation of the seismic, lithologic and geotechnical data. A significant decrease in the rate of sediment accumulation occurred as sea level rose from the last glacial maximum to the late Holocene. The data supports the suggestion that the sediment accumulation rate is also influenced by changes in the flux of glacial meltwater into the Gulf of Mexico.
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