Reduction In Factor Of Safety Research Articles

Impact of Climate Change on the Stability of the Miacher Slope, Upper Hunza, Gilgit Baltistan, Pakistan

Especially in recent years, the study of landslide phenomena is considered as very important because of the effects of climate change. The aim of this paper is to examine the stability of the slope located in Miacher Nagar village along the Hunza River (HR), using the Limit Equilibrium Method (LEM). The Miacher slope rises to a height of 900 m from the foot of the Hunza River and has a base angle of 50 degrees. Meta-sediments and quaternary recent glaciated deposits make up the majority of the slope’s composition. The slope movement of Miacher was first triggered in 1995, and was further triggered in 2010 and 2013. The slope was geologically, geomorphologically, geotechnically and geochemically investigated as well as modeled by Slope/w to determine the safety factor. Soil samples were analyzed for their geotechnical, geological and geomorphological properties. The Limit Equilibrium Method (LEM) was employed in this study to analyze the Factor of Safety (FOS) of the slope, based on assumptions of the Morgenstern and Price, Ordinary, Janbu and Bishop Methods, using the Slope/w software. Various factors, including pore water pressure, unit weight, cohesion, angle of internal friction and overburden, were examined by analyzing different scenarios. The findings showed that an increase in cohesion and angle of internal friction resulted in an increase in FOS, whereas an increase in unit weight and overburden caused a decrease in FOS. The influence of pore water pressure was positive to a certain extent, but a further increase led to a significant reduction in FOS. The results showed that the Miacher slope is currently stable, as all FOS values were greater than one, based on the existing strength parameters and simulated results obtained using Slope/w.

Probing the stability landscape of cylindrical shells for buckling knockdown factors.

The buckling response of axially compressed cylindrical shells is well known for its imperfection sensitivity. Mapping out a stability landscape by localized probing has recently been proposed as a rational means for establishing shell buckling knockdown factors. Probing using a point force directed radially inwards and perpendicular to the cylinder wall is based on the insight that a localized single dimple exists as an edge state in the basin boundary of the stable prebuckling equilibrium. Here, we extend the idea of probing to bi-directional inwards and outwards forces to trigger both single-dimple and double-dimple edge states. We identify key features of the ensuing probing stability landscape and generalize these to derive three design curves of varying conservatism that are a function of the non-dimensional Batdorf parameter only. Interestingly, the most conservative of the three knockdown curves bounds a large dataset of experimental buckling results from below, despite being derived from probing features of geometrically perfect cylinders. Overall, the three design curves permit a more nuanced design approach than legacy knockdown factors, as different levels of conservatism can be chosen based on expected manufacturing quality. For instance, the most and least conservative of the three design guidelines differ by a factor of 3 for the most slender cylinder geometries, and the associated reduction in safety factor has profound implications for efficient structural design. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'.

Effect of reinforcement stiffness on response of back-to-back MSE wall upon infiltration

Back-to-back mechanically stabilized earth (MSE) walls are being widely utilized as bridge abutments and highway ramps, considering their cost effectiveness and ease of construction. Though high quality well-graded granular fill is the most appropriate backfill material, owing to its scarcity, locally available low-quality soils are often used as backfill. Nevertheless, numerous failure cases of such walls with low-quality soil are reported, especially subsequent to rainwater infiltration. The stiffness of reinforcement is a key parameter affecting the overall behaviour of MSE walls. The present study investigates the influence of reinforcement stiffness on the response of select fill, marginal fill, and hybrid-fill back-to-back MSE walls using a finite element (FE) based approach. Results show that stiffness of reinforcement has a significant effect on the overall performance of marginal fill wall upon rainfall infiltration. Specifically, with the increase of reinforcement stiffness, the infiltration-induced facing displacement decreases and reinforcement tension increases. However, the performance of select fill and hybrid-fill walls following rainfall infiltration was found to be satisfactory even with low stiffness reinforcements, and further improvement with increase of reinforcement stiffness was insignificant. The reduction of safety factors during infiltration was shown to be independent of reinforcement stiffness for all wall types.

Judgement of rapid drawdown conditions in slope stability analysis

The rapid drawdown of reservoir may have a significant impact on the stability of adjacent slopes. In order to avoid potential risks, it is important to calculate the change of slope safety factor prior to the reservoir operation. The finite element method is used to analyze the transient seepage during drawdown, and then the pore water pressures are introduced into the stability computation based on limit equilibrium to obtain the transient safety factor. Computations show that for slopes with a specific geometry, the safety factor ratio depends on the parameters K/(Syv) (where K is the permeability coefficient, v is the drawdown speed, and Sy is the specific yield) and c′/(γHtan ϕ′) (where c′ and ϕ′ are the effective cohesion and friction angle, γ is the soil unit weight, and H is the slope height). By considering a wide range of K/(Syv) and c′/(γHtan ϕ′) values and different slope geometries, the percent reduction in critical safety factor of slope during drawdown relative to that during steady-state seepage is obtained. The drawdown condition that causes a large percent reduction in safety factor is judged as a rapid drawdown, and the opposite is a slow drawdown, which does not affect the slope design. This paper presents a series of charts for engineers and designers to judge rapid and slow drawdown conditions. Before the reservoir operation, the appropriate drawdown speed is selected according to the charts to ensure a slow drawdown for adjacent slope, while in the slope stabilization design, only the rapid drawdown stability analysis needs to be performed.

Risk chart for identification of potential landslide due to the presence of residual soil

The incidence of slope failure involving residual soils in the Himalayan Region has increased in recent times. The prevalence of subtropical climatic zone has led to rampant weathering of varying degrees, resulting in the occurrence of residual soil over the bedrock. The study aims to give an overview of residual soil and its effect on the overall stability of the slope. Finite element-based numerical simulations have been performed utilising the results form field visits, laboratory experiments and detailed literature survey, and a landslide risk chart has been proposed. The results indicate the significance of considering the residual soil layer while performing stability analysis. A substantial decline in the factor of safety (FOS) was observed with an increase in the depth of soil layer up to 4 m, and a further reduction in FOS is not observed beyond this depth. Simulation results also indicate limited possibilities of the presence of a thick layer of residual soil on slopes greater than 45°, which is also established through field study. The overall stability of a slope depends on the critical combination of slope inclination, slope height and depth of residual soil layer. The proposed risk chart can be utilised for quick identification of vulnerable slope profiles, thereby indicating priorities for landslide risk management.

Establishing the need to model the actual state of stress along rock bolts

Proper design of rock bolt support in underground mines is critical to avoid incidents, accidents and loss of production. The traditional design approach only considers the axial (tensile) capacity and this is clearly not the situation in situ, where a rock bolt is subjected to both axial and shear/bending loads which determines its overall performance and failure behaviour. To demonstrate and analyse the shear displacement in bedded roof, scaled physical models of underground excavation were created. From the models it was found that the shear displacement between the layers depends on the vertical roof deformation and thickness of beds. To analyse the effect of combined loading on rock bolt design for suspension and beam building models, analytical methods were used to calculate the required spacing of rock bolt for a given safety factor. Numerical models were then created using Rocscience RS2 software to establish the stresses on the rock bolt. The results show a significant reduction in safety factor for suspension as demonstrated in an example (reduced from 3.5 to 2.0) and beam building (2.0 to 1.36) when the rock bolt capacities are calculated considering the effect of combined loading as opposed to just the axial or shear loads.

Numerical Investigation of Damage in Single-step, Two-step, and Reverse Deep Drawing of Rotationally Symmetric Cups from DP800 Dual Phase Steel

Damage evolution in deep drawing processes has an influence on the service properties of manufactured components. Knowledge and control of the damage state will therefore allow a reduction in safety factors and, consequently, in sheet thickness. To investigate the damage after deep drawing of rotationally symmetric cups, a Lemaitre type damage model calibrated in previous works was used in the Finite Element (FE) simulation of the deep drawing process. Three process variants were investigated: single-step deep drawing, two-step deep drawing, and reverse deep drawing. The single-step deep drawing process was used as a reference to evaluate the damage evolution in two-step and reverse deep drawing. In both two-step and reverse deep drawing, the overall damage values did not decrease when compared to single-step deep drawing. However, in reverse deep drawing specifically, the location of maximum damage in the cup wall was moved from an area near the cup floor towards the edge of the cup. In real-life components, this control over the location of maximum damage will allow a tailoring of the damage state in the component to the expected service loads, thereby increasing service life or allowing a reduction in sheet thickness, respectively.

Detection and analysis of mass wasting events in chalk sea cliffs using UAV photogrammetry

Coastal cliff recession represents a significant risk to both people and infrastructure, it is therefore important that we are able to efficiently monitor these environments to inform future management decisions. Through the use of UAV digital photogrammetry, we obtain point clouds to develop monthly models of the sea cliffs at Telscombe, East Sussex, UK between August 2016 and July 2017. The models captured were accurate to 0.05 m and had an average point density of 351 pts./m2. Using the methodology presented we were able to automatically detect rockfalls by undertaking a 2.5D surface change detection which populated monthly inventories through volumetric estimations. A total of 10,085 failures were observed with an estimated volumetric flux of 3889.4 m3 over the 12 month period of data collection. Due to the high frequency of data capture, successive block failures in the Newhaven Chalk formation were observed. The largest failure within the 12 month period was estimated at 2546.8 m3 and followed significant toe erosion due to wave action. The steepening of the cliff face was modelled through limit equilibrium analysis to determine the reduction in factor of safety for the months preceding failure. We then present a magnitude-frequency analysis using negative power laws from the monthly rockfall inventories for the entire study area. The negative power law models produced a strong correlation across all months with r2 values ranging from 0.97 to 0.99. The normalised power law scaling parameters ranged from 1.421 to 1.955 for β and from 33.79 to 904.14 for s. The observed rollover in power laws presented in previous research matches the resolution of the data presented in this study. Our results show that this method of data capture is comparable to existing methods whilst offering significant benefits in field surveying time and cost.

Effect of variations in rainfall intensity on slope stability in Singapore

Numerous scientific evidence has given credence to the true existence and deleterious impacts of climate change. One aspect of climate change is the variations in rainfall patterns, which affect the flux boundary condition across ground surface. A possible disastrous consequence of this change is the occurrence of rainfall-induced slope failures. This paper aims to investigate the variations in rainfall patterns in Singapore and its effect on slope stability. Singapore's historical rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 were obtained and analysed by duration using linear regression. A general increasing trend was observed in both weather stations, with a possible shift to longer duration rainfall events, despite being statistically insignificant according to the Mann-Kendall test. Using the derived trends, projected rainfall intensities in 2050 and 2100 were used in the seepage and slope stability analyses performed on a typical residual soil slope in Singapore. A significant reduction in factor of safety was observed in the next 50 years, with only a marginal decrease in factor of safety in the subsequent 50 years. This indicates a possible detrimental effect of variations in rainfall patterns on slope stability in Singapore, especially in the next 50 years. The statistical analyses on rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 indicated that rainfall intensity tend to increase over the years, with a possible shift to longer duration rainfall events in the future. The stability analyses showed a significant decrease in factor of safety from 2003 to 2050 due to increase in rainfall intensity, suggesting that a climate change might have existed beyond 2009 with possibly detrimental effects to slope stability.

Huangtupo landslide stability under water level fluctuations of the Three Gorges reservoir

The Huangtupo riverside slope, which extends an area of 1.35 × 106 m2 and has a total volume of 6.934 × 107 m3, is one of the most noted large-scale landslides with great concern about the safety of immigrants in the Three Gorges reservoir area of China. During the normal operation, the reservoir water level fluctuates periodically. Water level fluctuation will soften the strength of the geologic materials on the banks, induce underground water fluctuation, and in turn affect the slope stability. On the basis of the thorough engineering geological survey, the analysis of engineering geological conditions and deformation mechanism is conducted primarily. Then, the Huangtupo landslide is divided into four minor landslides, all of which have typical three-dimensional characteristics. To these landslides, a three-dimensional rigorous limit equilibrium method is proposed, which can accommodate failure surfaces of any shape, and satisfies all the equilibrium conditions. In terms of the strength parameters of geologic materials, the stability evolution of the four minor landslides at the Huangtupo landslide under periodical water level fluctuation is investigated in detail. From the results, the 3D safety factors turned out to be somewhat bigger than the 2D results even though they displayed the same trends. The differences between 2D and 3D results are about 2.92–6.11 %. The maximum reduction in safety factors with the rapid down of water level is 0.11. It is suggested that the safety margin of the two minor landslides near the riverside is insufficient and might become instable under periodical water level fluctuation.

Scaling of Haversian canal surface area to secondary osteon bone volume in ribs and limb bones

Studies of secondary osteons in ribs have provided a great deal of what is known about remodeling dynamics. Compared with limb bones, ribs are metabolically more active and sensitive to hormonal changes, and receive frequent low-strain loading. Optimization for calcium exchange in rib osteons might be achieved without incurring a significant reduction in safety factor by disproportionally increasing central canal size with increased osteon size (positive allometry). By contrast, greater mechanical loads on limb bones might favor reducing deleterious consequences of intracortical porosity by decreasing osteon canal size with increased osteon size (negative allometry). Evidence of this metabolic/mechanical dichotomy between ribs and limb bones was sought by examining relationships between Haversian canal surface area (BS, osteon Haversian canal perimeter, HC.Pm) and bone volume (BV, osteonal wall area, B.Ar) in a broad size range of mature (quiescent) osteons from adult human limb bones and ribs (modern and medieval) and various adult and subadult non-human limb bones and ribs. Reduced major axis (RMA) and least-squares (LS) regressions of HC.Pm/B.Ar data show that rib and limb osteons cannot be distinguished by dimensional allometry of these parameters. Although four of the five rib groups showed positive allometry in terms of the RMA slopes, nearly 50% of the adult limb bone groups also showed positive allometry when negative allometry was expected. Consequently, our results fail to provide clear evidence that BS/BV scaling reflects a rib versus limb bone dichotomy whereby calcium exchange might be preferentially enhanced in rib osteons.

Numerical Analysis on the Evolutionary Features of Deformation and Failure Modes of Slope

The failure of slope is a gradual accumulation process. Under the effect of many interior and exterior factors, some parts in the slope reach yield with the increase of stress; sliding surface forms gradually till complete transfixion; with the plastic strain continuous increases, overall failure happens on the slope. Traditional analysis method cannot display the mechanic conditions and the whole process of deformation, transfixion of sliding surfaces and failure. Meanwhile, FEM strength reduction can quantitatively show the deformation features and the process of occurrence and development of sliding surface. Based on the previous researches, the paper classifies slopes according to the features of rock and soil and the slope structure. Through analyzing the graphs of deformation and the nephograms of plastic strain under different reduction factors or safety factors, the researchers can directly find the deformation tendency of slopes and the whole process of the extension, transfixion and failure of sliding surface with the reduction of safety factor. So, the failure mechanism of slope can be found intuitively, which can provide effective basis for the prevention and governance of slopes.

An analytical solution for rainfall infiltration into an unsaturated infinite slope and its application to slope stability analysis

SUMMARYSurficial slope failures in residual soils are common in tropical and subtropical regions as a result of rainfall infiltration. This study develops an analytical solution for simulating rainfall infiltration into an infinite unsaturated soil slope. The analytical solution is based on the general partial differential equation for water flow through unsaturated soils. It can accept soil–water characteristic curve and unsaturated permeability function of the exponential form into account. Numerical simulations are conducted to verify the assumptions of the analytical solution and demonstrate that the proposed analytical solution is acceptable for the coarse soils with low air entry values. The pore‐water pressure (pwp) distributions obtained from the analytical solution can be incorporated into a limit equilibrium method to do infinite slope stability analysis for a rain‐induced shallow slip. The analysis method takes into account the influence of the water content change on unit weight and hence on factor of safety. A series of analytical parametric analyses have been performed using the developed model. The analyses indicate that when the residual soil slope, consisting of a completely decomposed granite layer underlain by a less permeable layer, is subjected to a continuous heavy rainfall, the loss of negative pwp and the reduction in factor of safety were found to be most significant for the shallow soil layer and during the first 12 h. The antecedent and subsequent rainfall intensity, depth of a less permeable layer and slope angle all have a significant influence on the pwp response and hence the slope stability. Copyright © 2012 John Wiley & Sons, Ltd.

Safety Factor for Neuromuscular Transmission Is Reduced in Mild and Moderate Myasthenia Gravis Due to Loss of Both Sodium Channels and Acetylcholine Receptors (S35.006)

Objective: To evaluate the contributions of loss of Na + channels and acteylcholine receptors (AChR) to reduction in safety factor for neuromuscular transmission (SF) in patients with MGFA class IIa and IIIa myasthenia gravis (MG). I hypothesized that loss of endplate Na+ channels would contribute to the reduction of SF in patients with MG. Background Endplate Na + channels improve neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential (E AP ). In MG, AChRs and Na + channels are lost from the endplate. Na+ channels as well as AChRs were lost from the endplate membrane. Design/Methods: I used standard electrophysiological techniques to compare membrane currents and potentials in intercostal nerve-muscle preparations from 5 MGFA IIIa, 2 MGFA IIa and 7 control adult male subjects. Na + currents recorded using loose patch voltage clamp. Action potential (AP) characteristics measured with a 2 electrode clamp. Endplate potentials (EPPs) studied using μ-conotoxin GIIIb to block muscle Na + channels. The motor nerve was stimulated using a suction electrode. I report the findings for recordings from fast-glycolytic muscle fibers. Results: In MG, EPP was reduced from 40.2 ±1.3 to 30.1±1.3 mV (IIa) and 23.5 ±1.7 mV (IIIa) (p + channel loss increased E AP from −71.9 ±2.2 mV to -66.1 ±2.7 mV (IIa; p AP was the same for MG fibers and controls. The SF for neuromuscular transmission was reduced from 2.98 to 1.58 (IIa) and 1.09 (IIIa). Loss of Na + channels accounted for 53% (IIa) or 41% of the SF reduction. Reduction of EPP produced 47% (IIa) or 59% (IIIa) of the SF decrease. Conclusions: Loss of endplate Na + channels contributes to the reduced SF in MG to a similar extent as does the loss of endplate AChRs. Supported by: Department Veterans Affairs. Disclosure: Dr. Ruff has nothing to disclose.

Selection of indicators for assessment of overall, computed, and remaining service lives of production equipment

Vessels and apparatus employed in the chemical and petrochemical industry (containers, heat exchangers, towers, scrubbers, reactors, tanks, and pipelines) do not contain redundant components; safety factors assigned during design for cases of wear, degradation of material properties, and other unforeseen situations therefore ensure their safe-operating lives. A reduction in safety factors will make it possible to judge how closely structural components and production equipment as a whole approach the limiting state. The standard safety factor serves as a reference point indicating just how closely the actual safety factor comes to the limiting state, and in the author’s opinion, can be adopted as the limiting allowable safety factor ensuring the safe operation of production equipment. Reserve-strength investigations conducted by the Angarsk Petrochemical Company over a number of years have demonstrated the versatility of dimensionless values of design, actual, and standard safety factors, and the possibility of their use as indicators for evaluation of overall, computed, (allowable) and remaining service life. Evaluation of the overall, computed (allowable), and remaining service lives is acceptable for design, fabrication, and appraisal of industrial safety as a technico-economic indicator serving to define safe-service lives assigned to production equipment.

The Definition of Hormesis and its Implications for In Vitro to In Vivo Extrapolation and Risk Assessment

This article comments on some of the basic questions put forward in state-of-the-art discussions on hormesis. There seems to be a need for a better definition of the concept itself and reconsideration of whether all biphasic dose-response curves should be considered representative for hormesis. Hormesis may be restricted to phenomena that proceed by mechanisms that are broadly generalizable and represent possibly beneficial overcompensation in response to an adverse stimulus. Using the concept that hormesis is defined as such, the biphasic effect of quercetin on cell proliferation, but also several other receptor-mediated biphasic dose-response phenomena, should not be related to hormesis. Taking into account hormesis in the procedures for risk assessment on compounds characterised by a threshold for the adverse effect is another matter for considerable debate. In our opinion, this would require the reduction of safety factors, providing the possibility for beneficial hormesis-type effects for some people, at the cost of increased chances on adverse effects for other parts of the population. Whether this is a proper way forward remains to be discussed. Improvement of risk assessment strategies may include taking into account biphasic dose-response curves, but should rather start with the consideration of proper physiologically based pharmacokinetic (PBPK) models for better extrapolation of differences in toxicokinetics going from high- to low-dose exposure, as well as taking into account kinetics for gene repair systems. Without considering in vivo toxicokinetics in the in vitro models, extrapolation from in vitro biphasic dose-response curves on cell proliferation to in vivo cell proliferation is difficult to do. Altogether, it is concluded that hormesis is an important phenomenon, especially from the scientific point of view, but that its consequences for risk assessment and the possibilities for in vitro to in vivo extrapolation may remain limited without additional mechanistic insight.

803 圧力容器規格における安全率引き下げの検討

803 圧力容器規格における安全率引き下げの検討

A transient analysis of slope stability following drawdown after flooding of a highly plastic clay

The stability of slopes at bridge abutments across the Carrot River in east-central Saskatchewan was not influenced significantly by drawdown after flooding in the spring of 1995. Traditional methods of analysis for rapid drawdown predicted the factor of safety of slopes on highly plastic clays of proglacial Lake Agassiz would drop to 0.65 from an initial value of 1.0. Deformation along a well-defined slip plane has persisted at a more or less constant, slow rate since the bridge was constructed in 1975. The river rose approximately 10 m during a flood in the spring of 1995, yet there was only minimal response in piezometers and no measurable increase in the rate of deformation recorded by inclinometers. Pore-water pressures from a steady state seepage model, which was calibrated from piezometer measurements, were integrated into a stability analysis. Changes in pore-water pressures caused by flooding and subsequent drawdown were characterized from a transient seepage model using the flood hydrograph as a flux boundary. The stability analysis integrated with the transient seepage model estimated the factor of safety would drop from 1.0 to 0.91 after drawdown. Field measurements indicated the reduction in factor of safety was even less.

Effect of Imperfect Contact Between Adjacent Layers on the Integrity of Wrapped Vessels

Multilayer vessels are the designer’s solution to the inefficiency involved in operating monoblock vessels at pressures above 50 MPa. Multilayer wrapped vessels are commonly employed in high-pressure applications of up to 550 MPa. They are advantageous to monoblock cylindrical construction in several aspects, one of which is the prestressing of the layers in order to more efficiently distribute the stresses developed due to the inside pressure. Calculation of prestress in each layer, due to shrinkage of the weld, has been reported before. Nearly all such reports are based on the assumption that layers are in perfect contact with one another. In practice, however, occasions may arise, where due to imperfect rolling and/or variations in the thickness of a plate, there remains an isolated local gap between adjacent layers. In this investigation, the size of a gap is defined in terms of its circumferential arc length and maximum radial height. The effects of imperfect contact on prestresses are obtained, in terms of the size of the gap, by calculating the stresses sustained by the layer in order to bring the layer in perfect contact with the next layer. The foregoing task is achieved by deriving expressions for the stresses due to the closure of a gap as well as analysis by finite element approach. A method is proposed to calculate, for a given size gap, the necessary reduction in operating pressure in order to maintain the original factor of safety. Inversely, if the vessel must operate at the original design pressure, one can calculate the maximum allowable size of a gap (for a given percent reduction in factor of safety or strength) that would not affect the integrity of the vessel.

Functional elimination of afferent pathways and decreased safety factor during postembryonic development of cockroach giant interneurons.

The giant interneurons (GIN) from the cockroach CNS undergo two major physiological changes during the postembryonic developmental period: (A) a marked decrease in the number of afferent pathways innervating the GIN at the metathoracic ganglion (Ts); and (B) a gradual decrease in the safety factor for impulse propagation along the intraganglionic segment in T3. In 100% of the experiments (n greater than 100) performed on GIN from early developmental stages, spontaneous postsynaptic potentials (SPSPs) were recorded; in adults, on the other hand. SPSPs have been recorded in only 34% of the experiments (n = 74). Evoked synaptic potentials can be elicited in nymphal stages by stimulation of 8 nerves of T3, the contralateral connectives, ipsi- and contralateral nerve roots 2, 3, 5, and by stimulation of adjacent GINs. In adult, PSPs can be evoked by stimulation of adjacent GINs, and contralateral thoracic connectives, but not from nerves 2, 3 and 5. The functional disappearance of synaptic inputs to the GINs does not reflect a general phenomenon of reduction in synaptic transmission efficacy. In previous studies it was demonstrated that high frequency stimulation of adult GIN leads to blockage of impulse propagation in T3. In nymphal stages, the safety factor for propagation of impulses along T3 is higher. The reduction in safety factor appears gradually during the postembryonic developmental period. From analysis of the mechanisms underlying the elimination of functional afferent pathways and the appearance of low safety factor (see consecutive paper by Yarom and Spira) it is concluded that the functional elimination of afferents is a consequence of decreased transmission efficacy, while the appearance of low safety regions for impulse propagation is a consequence of morphological changes of the GIN segment within ganglion T3.