Improvement of Pavement Subgrade by Adding Cement and Fly Ash to Natural Desert Sand

Soil characteristics are paramount to design pavements and to assess the economic viability of a road. In the desert, such as that found in southern Libya, the very poor quality of soils leads to important pavement distress such as cracks, rutting, potholes, and lateral shear failure on the edges. To improve the strength of desert sand, an innovative approach is proposed, consisting of adding manufactured sand, ordinary Portland cement (OPC), and fly ash (FA) as a binder. OPC and FA improve the characteristics of mixes of crushed fine aggregate (CFA) and natural desert sand (NDS). These results are based on a gradation of two sand sources to determine the particle distribution and X-ray fluorescence (XRF) to determine their chemical and physical properties, respectively. This research assesses the effect of cement and fly ash on the geotechnical behavior of two mixtures of fine desert and manufactured sands (30:70% and 50:50%). The mix composed of 26% of CFA, 62% of NDS, 5% of OPC, and 7% of FA shows optimal results in terms of strength, compaction, and bearing capacity characteristics.

Lifecycle of a Submesoscale Front Birthed from a Nearshore Internal Bore

The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entail complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for ≈4 hours, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of along-shore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Structures and Kinematics of the Huanghua Depression in Bohai Bay Basin, East China: Implications for the Formation Mechanism of a Transtensional Basin

The Bohai Bay Basin in East Asia is a rift basin created by Cenozoic subduction of the oceanic Pacific plate beneath the Asia continent. Many prior studies suggest that the basin was initially formed in the Paleocene with the development of several NNE-trending extensional grabens, but subsequently impacted by right-lateral shear along these existing NNE-trending structures in the middle Eocene, transforming the Bohai Bay Basin into a transtensional basin and producing EW-trending grabens in the Bozhong and the northeastern Huanghua depressions. However, how this transformation occurred remains to be fully understood. Based on seismic and drilling data, we herein investigated the fault structures, basin architecture, and evolutionary stages of the Huanghua Depression in the central-west Bohai Bay Basin to examine the strain partitioning and evolution mechanism during the Paleogene syn-rifting stage. The results reveal that the Huanghua Depression is composed of three structurally distinctive zones, namely, a dextral transtensional, a NW-SE extensional, and a N-S extensional zones from southwest to northeast, which are separated from each other by two transfer zones. The NW-SE extensional zone is interpreted as a horsetail structure on the northern termination of the dextral transtensional zone. This dextral transtensional zone and the Tan-Lu Fault zone to the east served as strike-slip boundaries within which EW-trending depressions such as the northeastern Huanghua and Bozhong depressions formed in the middle Eocene.

Formation of ribbed bedforms below shear margins and lobes of palaeo-ice streams

Conceptual ice stream land systems derived from geomorphological and sedimentological observations provide constraints on ice–meltwater–till–bedrock interactions on palaeo-ice stream beds. Within these land systems, the spatial distribution and formation processes of ribbed bedforms remain unclear. We explore the conditions under which these bedforms may develop and their spatial organization with (i) an experimental model that reproduces the dynamics of ice streams and subglacial land systems and (ii) an analysis of the distribution of ribbed bedforms on selected examples of palaeo-ice stream beds of the Laurentide Ice Sheet. We find that a specific kind of ribbed bedform can develop subglacially through soft-bed deformation, where the ice flow undergoes lateral or longitudinal velocity gradients and the ice–bed interface is unlubricated; oblique ribbed bedforms develop beneath lateral shear margins, whereas transverse ribbed bedforms develop below frontal lobes. We infer that (i) ribbed bedforms strike orthogonally to the compressing axis of the horizontal strain ellipse of the ice surface and (ii) their development reveals distinctive types of subglacial drainage patterns: linked cavities below lateral shear margins and efficient meltwater channels below frontal lobes. These ribbed bedforms may act as convenient geomorphic markers to reconstruct lateral and frontal margins, constrain ice flow dynamics, and infer meltwater drainage characteristics of palaeo-ice streams.

Stress-cycle driven evolution of faulting in a plate boundary transition zone

<p>Upper plate faults along the Cascadia subduction margin of North America go through a 3 stage evolution over millions of years as a consequence of the migrating Mendocino Triple Junction (MTJ). Initially, NE-directed cyclic shortening produced by the Cascadia subduction earthquake cycle drives reverse dip-slip motion on trench-parallel faults. As the triple junction moves north, NNW-shortening associated with the Mendocino Crustal Conveyor (MCC, Furlong & Govers, 1999) is superimposed on the cyclic subduction-earthquake-cycle regional stress field. As the triple junction migrates further north, and these faults transfer from the subduction to transform plate boundary, they become part of the San Andreas system and are loaded by right-lateral shear. In this work we investigate how the faulting behavior in northern California evolves through time from first being driven by cyclic subduction zone stresses (superimposed on a NNW-oriented shortening field) to eventually forming the primary structures within a dominantly strike-slip stress regime.</p><p>We decompose the observed horizontal GPS velocity field in southern Cascadia to determine a subduction coupling component and a NNW-directed displacement component to separate the subduction cycle effects from other tectonic effects on the behavior of upper plate faulting and its evolution through time. Since the MCC processes acts over millions of years, we assume that the effects associated with the NNW-directed signal can be represented by a constant stress field over subduction earthquake cycle timescales. Early in the subduction earthquake cycle, the principal stresses north of the MTJ are oriented in this NNW-SSE direction and rotate clockwise as the subduction component increases. This stress cycle then resets following each large megathrust event. Coulomb stress analyses indicate that the cyclic nature of the regional stress field, changes the likelihood of faulting and slip behavior on faults in southern Cascadia over time intervals of 100s of years. Trench-parallel faults are most likely to exhibit right-lateral or oblique motion early in the seismic cycle, however by ~100-200 years following a megathrust event, they are more likely to exhibit reverse dip-slip motion as the stress effects from the subduction component increase.</p><p>Though the NNW-oriented displacement field is assumed to be temporally constant over subduction earthquake cycle timescales, the spatial extent of this deformation field constrains strain localization within the upper plate. For example, a steep decrease in GPS velocities from SW to NE in southernmost Cascadia indicates right-lateral strain is accumulating adjacent to the relatively rigid Klamath Mountain Province. This region of localized right-lateral shear coincides with the location of the development of several regional-scale right-lateral strike slip faults. We hypothesize these faults, formed within the subduction regime, evolve to become regional-scale 'San Andreas-type' plate boundary faults. Understanding the implications of such time- and space-variable stress regimes provides insight into interpreting geologic estimates of the slip history of faults along the Cascadia and northern San Andreas margins of North America, and also a framework for understanding how a new plate boundary develops following a major change in plate interaction.</p>

Load-Sharing and Kinematics of the Human Cervical Spine Under Multi-Axial Transverse Shear Loading: Combined Experimental and Computational Investigation

The cervical spine experiences shear forces during everyday activities and injurious events yet there is a paucity of biomechanical data characterizing the cervical spine under shear loading. This study aimed to 1) characterise load transmission paths and kinematics of the subaxial cervical spine under shear loading, and 2) assess a contemporary finite element cervical spine model using this data. Subaxial functional spinal units (FSUs) were subjected to anterior, posterior and lateral shear forces (200 N) applied with and without superimposed axial compression preload (200 N) while monitoring spine kinematics. Load transmission paths were identified using strain gauges on the anterior vertebral body and lateral masses and a disc pressure sensor. Experimental conditions were simulated with cervical spine finite element model FSUs (GHBMC M50 version 5.0). The mean kinematics, vertebral strains and disc pressures were compared to experimental results. The shear force-displacement response typically demonstrated a toe region followed by a linear response, with higher stiffness in anterior shear relative to lateral and posterior shear. Compressive axial preload decreased posterior and lateral shear stiffness and increased initial anterior shear stiffness. Load transmission patterns and kinematics suggest facet joints play a key role in limiting anterior shear while the disc governs motion in posterior shear. The main cervical spine shear responses and trends are faithfully predicted by the GHBMC cervical spine model. These basic cervical spine biomechanics and the computational model can provide insight into mechanisms for facet dislocation in high severity impacts, and tissue distraction in low severity impacts.

Assessment of the impact on the railway track of a special purpose passenger car model 61-934

In this work, theoretical studies have been carried out in order to assess the strength of structural elements of the track superstructure from the impact of a special-purpose passenger carriage. The assessment of the impact of the wagon on the railway track was carried out according to the stresses in the edges of the rail base, stresses in the main area of the subgrade and the stability of the track against lateral shear. The calculations showed that the strength and stability of the structural elements of the track superstructure as a result of the impact on it of a special-purpose passenger carriage model 61-934 meets the regulatory requirements.