Neural excitability increases with axonal resistance between soma and axon initial segment

The position of the axon initial segment (AIS) is thought to play a critical role in neuronal excitability. Previous experimental studies have found that a distal shift in AIS position correlates with a reduction in excitability. Yet theoretical work has suggested the opposite, because of increased electrical isolation. A distal shift in AIS position corresponds to an elevation of axial resistance Ra. We therefore examined how changes in Ra at the axon hillock impact the voltage threshold (Vth) of the somatic action potential in L5 pyramidal neurons. Increasing Ra by mechanically pinching the axon between the soma and the AIS was found to lower Vth by ∼6 mV. Conversely, decreasing Ra by substituting internal ions with higher mobility elevated Vth. All Ra-dependent changes in Vth could be reproduced in a Hodgkin–Huxley compartmental model. We conclude that in L5 pyramidal neurons, excitability increases with axial resistance and therefore with a distal shift of the AIS.

Pipe Clamping Mattresses to Mitigate Flowline Walking; Physical Modelling Trials on Three Offshore Soils

Pipe clamping mattresses (PCMs) are a relatively new system for providing anchoring force to pipelines, to mitigate offshore flowline ‘walking’. They represent a cost-effective and highly efficient alternative to anchor piles, rock dump and conventional concrete mattresses. The system comprises a hinged concrete structure that clamps onto a section of laid pipeline, with concrete ballast logs securing the clamping action – with the benefit that 100% of the submerged weight of the PCM contributes to axial friction. PCMs have been applied successfully to one deepwater project, but performance data showing the influence of soil type, and allowing a general design framework to be established, has not yet been available. This paper addresses this gap by investigating the performance of PCMs through three series of centrifuge tests, supported by three Operators. Each series comprises tests on a different reconstituted deepwater soil as follows: (a) West African clay; (b) Gulf of Mexico clay; and (c) carbonate silty sand. In each test, a scaled pipeline is installed in-flight and cycled axially to represent its prior operating life. Scaled PCM models and ballast units are then installed onto the pipe in-flight, mimicking the use of PCMs to mitigate pipeline walking during operation. After installation of the PCMs, further axial cycles are applied, with the system settlement and changes in axial resistance and excess pore pressure measured. The paper shows the performance and applicability of PCMs for a range of soil types, highlighting variations in axial resistance and settlement. The suite of results will help to calibrate design tools for industry, removing unnecessary conservatism and enabling an optimised pipeline anchoring solution to be designed. Key results are equivalent friction factors for the combined pipe-PCM system and PCM settlement, which both show behaviour dependent on soil type. In the clay soils, friction increases significantly over time due to ‘consolidation hardening’. This provides validation of an important effect that has only recently been recognised in pipeline design. In contrast, hardening behavior is not evident in silty sand – although the study suggests there is potential for increasing resistance associated with settlement, which appears to mobilize additional (wedging) stress around the pipeline. Upon PCM installation, the pipelines embed further due to the added weight. Additional settlement occurs during cycling of the system, due to immediate soil deformation and consolidation-related compression. The magnitude of embedment is greater for the clay soils, but in all cases does not cause the clamping action to release. Overall, the efficiency of the PCM system in providing a high level of anchoring force per unit weight placed on the seabed is confirmed. Long term anchoring forces in the range 50-100% of the submerged weight of the PCM are demonstrated. This is several times more efficient than the commonly used alternative of a rock berm.

Technical design and stability analysis procedure for horizontal stability construction of roads and railways

Unstable sections of predominantly vertical roads and railways are usually stabilized by viaducts, while predominantly horizontal unstable sections of the same structures are regularly stabilized by special structures which have a common feature of spaciousness or massiveness, and which proportionally also require peculiarity in all aspects of the construction. The goal of the new solution is to avoid the highlighted structural peculiarity, that is, to apply a solution that will be more of a constructive element of roads and railways, like a viaduct in an approximate sense. There is such a solution, and that is the low-rise stable structure, which in a naturally appropriate way counteracts horizontal instabilities on low-rise objects. The horizontal effect on the object is converted to a vertical direction via this construction by means of pile coupling, while this effect is greatly reduced due to the effect of static interaction between the components of the coupling. If, instead of various vertical structures with horizontal anchors or mass structure retaining walls, we apply the slope-pile coupling at an optimal angle in the range of 15 to 20 degrees, then, by activating the external horizontal effect, i.e. instability, the primary axial resistance in the oblique pile is simultaneously activated through circumferential friction. The vertical component of this resistance decreases the active horizontal component, while the horizontal does the same, provided that the pile has a transverse static EI feature. This approach has not been used thus far in engineering practice and therefore represents a novelty. Therefore, it can be argued that by constructing a low-rise stable structure, we can achieve at least approximately the same structural impression that we enjoy regarding the viaduct construction for predominantly vertical instabilities.

Incorporation of rubber ash as a partial substitute of fine aggregates in concrete

The construction industry is responsible for a high consumption of natural resources, demanding high quantities of aggregate materials for use in construction. In addition, large quantities of rubber waste generated worldwide have emphasized the need to find practical reuse applications. The present study partially replaces fine aggregates by ash from a co-processing of milled and burned conveyor belt rubber waste. Test specimens with ash concentrations of 0%, 5%, 10%, 15% and 20%, were made comparing its workability, mechanical axial resistance and absorption of water by capillarity. It was concluded that the partial replacement of sand by 5% of rubber ash has improved the traditional concrete mixture, with better workability, less amount of water, leading to a greater resistance to axial compressive and acceptable absorption of water. Thus, the results confirm that the concrete with incorporation of rubber ash is a potential alternative technology to achieve sustainable development in the construction industry.

Neural excitability increases with axonal resistance between soma and axon initial segment

The position of the axon initial segment (AIS) is thought to play a critical role in neuronal excitability. In particular, empirical studies have found correlations between a distal shift in AIS position and a reduction of excitability. Yet, theoretical work has suggested that the neuron should become more excitable as the distance between soma and AIS is increased, because of increased electrical isolation. Specifically, resistive coupling theory predicts that the action potential (AP) threshold decreases with the logarithm of the axial resistance (Ra) between the middle of the AIS and the soma. However, no direct experimental evidence has been provided so far to support this theoretical prediction. We therefore examined how changes in Ra at the axon hillock impact the voltage threshold (Vth) of the somatic AP in L5 pyramidal neurons. Increasing Ra by mechanically pinching the axon between the soma and the AIS was found to lower the spike threshold by ~6 mV. Conversely, decreasing Ra by replacing a weakly mobile ion (gluconate) by a highly mobile ion (chloride) elevated the spike threshold. All Ra-dependent changes in spike threshold could be reproduced in a Hodgkin-Huxley compartmental model. We conclude that in L5 pyramidal neurons, excitability increases with axial resistance, and therefore with a distal shift of the AIS.

Formulation of 3D Soil Springs for Pipe Stress Analyses

Interdependence between pipe-soil interaction springs in a pipe stress analysis should be considered. This example focused on a single pipe configuration “wished” in place in a clay soil. A conventional pipe stress analyses often idealizes the pipe soil interaction with a beam-spring finite element model where independence is assumed between reactions in axial, lateral and vertical directions. There is however interdependence between these springs as recognized in recent Pipeline Research Council International (PRCI) guidelines. For a frictional interface, axial resistance can be much higher than indicated by PRCI guidelines when accounting for increased lateral and vertical bearing pressure. At the same time, lateral and vertical capacities are shown to be reduced in comparison to pure vertical and lateral loading directions. This paper highlights the development of a 3D soil-spring interaction model based on a continuum finite element analysis approach. By developing a soil capacity envelope based on 3D continuum modeling, updated soil springs can reflect modified capacities depending on the direction of pipe movement. For the landslide scenarios considered in application of the model, the directional dependency is shown to change the accumulated plastic strain profile in the pipe.