Geotechnical characterization of slump scars,eroded slopes,and correlated deposits on continental margins: Gulf of Lions,Western Mediterranean sea

Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.     

Behavior of soil plugs in open‐ended model piles driven into sands

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.     

Role of piles in mitigating the movement of pipes in soft grounds during embankment loading

Abstract

Pipes buried in soft ground can be damaged due to the vertical and lateral movement of the ground during the construction of the embankment. To investigate such a movement of the soft ground, full-scale tests using embankment piles and stabilizing piles were conducted for 70?days. A pile-supported embankment has been used to reduce the deformation of soft ground by transferring the embankment load through piles to the firm layer below the soft ground, whereas stabilizing piles have been employed to resist the lateral earth pressure that is induced in soft ground by embankment loads. The Coupling Area (CA), which was defined as the quantitative index to determine the resistance effect of both settlement and lateral flow of the soft ground when the embankment was reinforced, is adapted. The analysis results of the CA indicate that the piled embankment was more effective for preventing the damage to buried pipe installed near the embankment, while the stabilizing piles had almost the same effect as the piled embankment when the pipe was buried far away from the embankment.     

Movement patterns of individual migrating western rock lobster,Panulirus cygnus,in Western Australia

Abstract

Migrating and pre‐migrating western rock lobster Panulirus cygnus were tagged with datastorage tags that recorded temperature and pressure, which was converted to depth (Pressure (kPa) — surface pressure (kPa)/10)) at Dongara and Jurien Bay in Western Australia between December 2005 and December 2007. All lobsters were fitted with tag flotation devices, and returns were made by either commercial fishers or beachcombers who located detached tags. A total of 135 lobsters were released with “backpack” flotation tags, but only 84 (62.2%) of the backpacks carried data‐storage tags. Depths of release ranged from 5 to 113 m. Of the tagged lobsters released, commercial fishers recaptured 52 (38.5%), whereas 11 tags (8.1%) were found by beachcombers. At least 33%, and possibly up to 63%, of animals identified by their pale coloration as pre‐migrating individuals, failed to migrate. Those that did migrate (n = 11) were at liberty from 1 to 94 days and showed generally similar movement patterns in that they migrated only at night from darkness (after 2000 h) until after moonrise. However, their movement patterns were less constrained by the rising of the moon in deep water. Only 27% migrated nightly, compared with 73% that skipped migrating on one or more nights, to restart some days later. This latter proportion would likely have been considerably greater, but some migrating animals were only at large for short periods before recapture, and therefore had little time to show any variation to the nocturnal migration pattern. Individual speeds of migration during periods of activity were estimated for nine lobsters as 0.20 to 0.68 km h^?1, with a mean speed of 0.44 km h^?1, or 7.4 m min^?1. Improved knowledge of daily movement patterns resulting from this study provides a potentially important input into technological improvements in bait and pot design.     

Trophic ecology of Pomatoschistus microps within an intertidal bay (Roscoff,France), investigated through gut content and stable isotope analyses

The diet of Pomatoschistus microps has been studied using both gut content and stable isotope analyses. In the Roscoff Aber Bay (Brittany, France), this fish is commonly found on sandy muddy intertidal flats. Gut content analyses were also interpreted using trophic indices. Owing to the large diversity of prey consumed, these indices emphasised the opportunistic feeding behaviour of P. microps. Here, this species fed mainly on endofauna with meiofauna being of high relative importance. The main biotic components of its trophic habitat, characterized by δ¹³C and δ¹⁵N, provided evidence of a major trophic pathway based on drift Enteromorpha sp. Trophic positions estimated by both diet analyses and isotopic analyses led to similar results. In this bay, P. microps is a first‐order predator with a low degree of omnivory. Despite a preferential consumption of the amphipod Corophium arenarium, we assumed that this goby behaves as a generalist feeding on a uniform variety of endofauna taxa.     

Diversity and temporal fluctuations of epiphytes and sessile invertebrates on the rhizomes Posidonia oceanica in a seagrass meadow off Tunisia

The aim of this work is to study the temporal dynamics of rhizome epiphytes and sessile animals living on the rhizomes of the seagrass Posidonia oceanica in the east of Tunisia. Surveys were conducted in October 2009, and in January, April and August 2010 on a fringing reef located in Chebba. Rhizomes were sampled by SCUBA diving at three stations. Samples were examined with a microscope to estimate the cover of macroinvertebrate and macroalgal organisms on the top 10 cm of each rhizome. Results revealed a high diversity of epiphytes on P. oceanica rhizomes with a dominance of red and brown algae, ascidians, and bryozoans. Distinct temporal changes were observed in Oued Lafrann, with a high January cover (winter period) for all groups. These winter increases can be attributed to: (i) the low phenological parameters of P. oceanica in winter that reduce the effects of shading, (ii) life cycles of the epiphytes and invertebrates, (iii) water motion and (iv) grazing.     

A new model for the vibration isolation via pile rows consisting of infinite number of piles

In this study, on the basis of the Floquet transform method, a numerical model for the simulation of the vibration isolation via multiple periodic pile rows with infinite number of piles is established. By means of the fictitious pile method due to Muki and Sternberg, the second kind of Fredholm integral equations for the pile rows are developed by using the fundamental solutions for the half‐space and the compatibility conditions between the piles and half‐space. Employing the Floquet transform method, integral equations for the pile rows in the wavenumber domain are then derived. Solution of the integral equations yields the wavenumber domain solution for the pile rows. The space domain solution can then be retrieved by inversion of the Floquet transform. Numerical results show that the proposed model with the Floquet transform method is in a good agreement with those of the conventional direct superposition method. On the basis of the new model, influences of the spacing between neighboring piles, the Young's modulus of the piles, and the pile length on the vibration isolation effect of the pile rows are investigated. Numerical simulations conducted in this study show that compared with the direct superposition method, the efficiency of the proposed model for simulation of the vibration isolation via pile rows is very high. Copyright © 2012 John Wiley & Sons, Ltd.     

Deep tow studies of the Tamayo transform fault

The Tamayo transform fault occurs at the north end of the East Pacific Rise where it enters the Gulf of California. The two deep-tow surveys reported here show that the transform fault zone changes significantly as a function of distance from the spreading center intersections. At site 1, near the intersection, one side of the fault is young and the fault zone is narrow and well-defined. Strike slip occurs in a zone approximately 1-km wide suggesting a correspondingly narrow zone of decoupling between the Pacific and North American plates. On the young side of the strike-slip zone, normal faults occur along shear zones which are 45°–50° oblique to the transform strike. They occur parallel to the short axis of the strain ellipse for transform fault strain here, i.e., perpendicular to the least compressive stress. The transform walls are formed by normal faulting as has been pointed out in previous detailed surveys. Here, however, the age contrast of 2.5 m.y. across the transform valley is apparent in the morphology of the normal fault scarps. While the scarps are steep and well-defined on the young side, the scarps on the older side have gradual 10°–30° slopes and appear to be primarily talus ramps. Apparently, the scarps have been tectonically eroded by continued strike slip activity after the initial stages of normal faulting. Thus, transform valleys should be quite asymmetric in cross-section where there is a significant age contrast and one side is less than approximately 0.5 m.y. old. Also, along older sections of the transform valley walls, normal faulting may not be at all obvious due to degradation of the scarps by tectonic erosion. This phenomenon makes the likelihood of transform faults providing windows into the oceanic crust most unlikely except in special cases.The picture of transform deformation is more complex at site 2 in the central portion of the fault where both sides of the fault are greater than 1 m.y. old. Here the transform valley is wider (25–30 km as opposed to 2–5 km). There is no clear simple zone of strike slip tectonics. In fact, the only clear evidence for deformation is the intrusion of magmatic or serpentinite diapirs through the sediments of the transform valley floor. The diapirs have deformed the turbidite layers flooring the valley and in one carefully studied case the turbidite sequence has been uplifted, perched atop the diapir. The pattern of deformation on this outcropping diapir shows radial and concentric fractures which can be modeled by a vertical intrusion circular in plan view. Magnetic studies limit the possible composition to basalt or serpentinite. A 60-km-long median ridge is also likely to be the product of intrusion along the transform fault. The survey at site 2 pointed out the importance of vertical tectonics in the transform valley floor and in particular the importance of diapiric intrusions of either basaltic or serpentinite composition.Based on initial boundary conditions and present tectonic elements in the Tamayo fault zone, a possible history of the mouth of the Gulf of California is outlined. The median ridge was emplaced starting approximately 0.8 m.y. ago by regional extension across the transform fault, the result of leaky transform faulting. The diapirs occur along a possible relay zone of extension midway along the fault which began approximately 0.15 m.y. ago. The extension in this case is parallel to the trend of the transform fault, is still occurring at present, and may evolve into a true spreading center.Contribution of the Scripps Institution of Oceanography, new series.     

Application of probabilistic neural network to design breakwater armor blocks

This study presents a probabilistic neural network (PNN) technique for predicting the stability number of armor blocks of breakwaters. The PNN is prepared using the experimental data of Van der Meer. The predicted stability numbers of the PNN are compared with those of previous studies, i.e. by an empirical formula and a previous neural network model. The agreement index between the measured and predicted stability numbers by PNN are better than those by the previous studies. The PNN offers a way to interpret the network's structure in the form of a probability density function and it is easy to implement. Therefore, it can be an effective tool for designers of rubble mound breakwaters.     

Molecular phylogeny of the families Pleuronectidae and Poecilopsettidae (PISCES,Pleuronectiformes) from Korea,with a Proposal for a new classification

A new classification of the Korean pleuronectids was proposed based on a molecular phylogeny using specimens collected from Korea (including some Japanese specimens) between 2008 and 2013. A molecular phylogeny based on partial sequences of the two mitochondrial DNA regions (COI and 16S rRNA) supported the reciprocal monophyly of the three genera, Cleisthenes, Pleuronectes and Pseudopleuronectes. We also found that the genus Poecilopsetta is clearly distinct from Pleuronectidae at the family level. Therefore, the previous classification of the Korean pleuronectids should be changed as follows; two families (Pleuronectidae and Poecilopsettidae), 18 genera, and 26 species. Further research is required to resolve the taxonomic uncertainty of the five species in the genus Limanda, which clustered into two clades in our analysis.