Low Normal Stress Research Articles

Disturbed State Concept-Based Constitutive Model for Lignosulfonate-Treated Silty Sand

AbstractIn this paper, a new behavioral (plasticity) model for lignosulfonate (LS)–treated silty sand based on the disturbed state concept is proposed. Here, the relative intact (RI) responses for both the LS-treated and untreated soils are simulated using the δ0 version of the hierarchical single-surface plasticity models. The nonassociativeness is incorporated through the disturbance function, which shows the amount of deviation of the actual response from the reference states. In the analysis, the fully adjusted (FA) response of the untreated soil was considered similar to the shear behavior at 20% of the shear strain. Also, for the LS-treated behavior, the FA response was taken as the response of the LS bonds. In this study, the bond response was modeled separately using a linear elastic RI response and zero strength condition as the relative states. A series of laboratory direct shear tests were carried out at low effective normal stresses to determine the model parameters. The proposed model has bee...

Contact mechanism of a rock fracture subjected to normal loading and its impact on fast closure behavior during initial stage of fluid flow experiment

SummaryFast closure of rock fractures has been commonly observed in the initial stage of fluid flow experiments at environmental temperatures under low or moderate normal stresses. To fully understand the mechanisms that drive this fast closure, the evolution of local stresses acting on contacting asperities on the fracture surfaces prior to fluid flow tests needs to be evaluated. In this study, we modeled numerically the asperity deformation and failure processes during initial normal loading, by adopting both elastic and elastic–plastic deformation models for the asperities on a real rock fracture with measured surface topography data, and estimated their impact on initial conditions for fluid flow test performed under laboratory conditions. Compared with the previous models that simulate the normal contact of a fracture as the approach of two rigid surfaces without deformations, our models of deformable asperities yielded smaller contact areas and higher stresses on contacting asperities at a given normal stress or normal displacement. The results show that the calculated local stresses were concentrated on the contacts of a few major asperities, resulting in crushing of asperity tips. With these higher contact stresses, however, the predicted closure rates by pressure solution are still several orders of magnitude lower than that of the experimental measurements at the initial stage of fluid flow test. This indicates that single pressure solution may not likely to be the principal compaction mechanism for this fast closure, and that the damages on contacting asperities that occur during the initial normal loading stage may play an important role. Copyright © 2015 John Wiley & Sons, Ltd.

An experimental investigation of the mechanical behavior and microstructural features of a volcanic soil (Isparta, Turkey) and stability of cut slopes in this soil

Volcanic soils are widely distributed group of soils, which cover significant parts of the world's surface and include areas occupied by urban settlements, structures and infrastructures, and may create geo-engineering problems. These soils exhibit a distinctive behaviour that is a consequence of their formation history, mineralogy and structure. Some part of the subsoil of a large area surrounding the city of Isparta (western Turkey) mainly consists of volcaniclastic deposits. This paper presents the results of the part of a research programme aiming at geotechnically characterizing the uppermost layer of this volcaniclastic sequence, particularly focusing on its collapse potential, shear behaviour and the influence of microstructure on these behaviours. In addition, its main futures were compared with those of some similar volcanic soils at different parts of the world, and the stability of sub-vertical cuts in this soil was also assessed. The experimental investigation mainly consisted of index and classification tests, double oedometer and direct shear box tests to assess the collapse potential and shear behaviour of the soil, respectively. Matric suction of the samples was also determined for preliminary evaluation of the influence of suction on collapse and shear behaviours of the soil. In addition, thin-section studies, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were also conducted to determine mineralogical and microstructural features and to evaluate their influences on the mechanical behaviour of the volcanic soil. The deposit is composed of weakly bonded firm silty sand with an open structure, low unit weight and high porosity, generally unsaturated in-situ. The results of the double oedometer tests suggest that the volcanic soil generally falls in the category of slight collapse potential at normal stresses ranging between 5 and 100kPa, while at normal stresses greater than 100kPa its collapse potential increases and ranges between moderate and moderately severe collapse potential classes. At normal stresses lower than 100kPa, matric suction may have an effect on collapse behaviour. The direct shear box test results indicated that the volcanic soil has two different types of shear strength behaviour represented by bi-linear failure envelopes. In other words, the shear strength parameters of the volcanic soil within a low normal stress range (generally between about 6 and 200kPa) exceed those of determined at higher normal stresses. Since no matric suction was measured during the direct shear box tests, it could not be possible to evaluate the effect of suction on the bi-linear failure envelopes. However, some differences between the suction and degree of bonding of the samples taken from different locations might probably have been contributed to the separation of the bi-linear behaviour at different normal stress levels. The destruction of the bonding (cementation) between the particles is probably more responsible from the bi-linear form of the failure envelope. Comparisons between the SEM micrographs taken from natural and tested specimens revealed that the mechanical behaviour of the volcanic soil is strongly influenced by the bonding material. The results of the stability analyses showed a good agreement with the current situation of the cut slopes observed in the study site and suggested that vertical cut slopes with a height of 14–15m will maintain their stability based on a reasonable factor of safety such as 1.5.

Residual shear strength of fine-grained soils and soil–solid interfaces at low effective normal stresses

A laboratory research program was undertaken to study the large-strain shear strength characteristics of fine-grained soils under low effective normal stresses (∼3–7 kPa). Soils that cover a wide range of plasticity and composition were utilized in the program. The interface shear strength of these soils against a number of solid surfaces having different roughness was also investigated at similar low effective normal stress levels. The findings contribute to advancing the knowledge of the parameters needed for the design of pipelines placed on sea beds and the stability analysis of shallow soil slopes. A Bromhead-type torsional ring-shear apparatus was modified to suit measuring soil–soil and soil–solid interface residual shear strengths at the low effective normal stresses. In consideration of increasing the accuracy of assessment and depicting the full-scale field behavior, the interface residual shear strengths were also measured using a macroscale interface direct shear device with a plan interface shear area of ∼3.0 m2. Correlations are developed to estimate the soil–soil and soil–solid interface residual shear strengths at low effective normal stresses. The correlations are compared with soil–soil and soil–solid interface drained residual shear strengths and correlations presented in the literature.

Strain localization and the onset of dynamic weakening in calcite fault gouge

To determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates, single-slide and slide–hold–slide experiments were conducted on 2–3-mm thick layers of calcite gouge at normal stresses up to 26 MPa and slip rates up to 1 m s−1. Microstructures were analyzed from short displacement (<35 cm) experiments stopped prior to and during the transition to dynamic weakening. In fresh calcite gouge layers, dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness. Strain is initially distributed across the full thickness of the gouge layer, but within a few millimeters displacement the strain becomes localized to a boundary-parallel, high-strain shear band c. 20 μm wide. During the strengthening phase, which lasts between 3 and 30 cm under the investigated conditions, the shear band broadens to become c. 100 μm wide at peak stress. The transition to dynamic weakening in calcite gouges is associated with the nucleation of micro-slip surfaces dispersed throughout the c. 100 μm wide shear band. Each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite, interpreted to result from grain welding driven by local frictional heating in the shear band. By the end of dynamic weakening strain is localized to a single 2–3-μm wide principal slip surface, flanked by layers of recrystallized gouge. Calcite gouge layers re-sheared following a hold period weaken nearly instantaneously, much like solid cylinders of calcite marble deformed under the same experimental conditions. This is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide, reducing the effective gouge layer thickness to a few microns. Our results suggest that formation of a high-strain shear band is a critical precursor to dynamic weakening in calcite gouges. Microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate. The delayed onset of dynamic weakening in fresh calcite gouge layers, particularly at low normal stresses, may inhibit large coseismic slip at shallow crustal levels in calcite-bearing fault zones.

Effect of stress state on slow rupture propagation in synthetic fault gouges

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (V r = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.

Investigation of the acoustic emission characteristics of artificial saw-tooth joints under shearing condition

Shear failure of rockmass along a weak plane occurs frequently in rock slopes and in underground tunnels. To study the shear behaviour and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed. The results demonstrated that the strength of the joints increased with increases in normal load and asperity height, while the strength of the joints first increased and then decreased when the shear rate was elevated. The ideal curve of the cumulative hits could be divided into a quiet period, a slow rise period, and a sharp growth period, which could be used to monitor and predict the potential shear failure of the joints. The higher the asperity was, the higher the peak energy rate and the lower the peak hit rate, and cumulative hits at failure were because of differences in asperity size and number. The peak hit rate and cumulative energy tended to increase gradually with the decrease in shear rate, and the minimum peak hit rate and energy rate at failure were both attained at the maximum shear rate. In addition, curves of the energy rate and hit rate showed large fluctuations at higher shear rates. The peak energy rate and cumulative energy under low normal stress could be greater than the peak energy rate and cumulative energy under high normal stress, and the peak hit rate and cumulative hit number under high normal stress could be larger than that of under low normal stress.

A large direct simple shear device for the testing of peat at low stresses

A large direct simple shear device has been developed for the testing of peat at low normal stress. The large size allows adequate representation of the often coarse structure of peat, increases the accuracy with which small loads can be applied and diminishes device resistances to acceptably small proportions. A plane strain configuration between glass plates is used, allowing observation of the deformation field as it develops. The sample is gripped by pins along all four in-plane boundaries to reduce slippage during shear. Interesting results have been obtained in an initial series of tests. Strengths appear to be some 10% higher than those in standard simple shear tests on small-diameter samples, while the post-peak softening often measured in the latter appears to be lacking.

Failure strain formulation via average stress triaxiality of an EH36 high strength steel

The paper deals with formulation of failure strain according to average stress triaxiality of a low temperature high-tensile steel, EH36, used for mainly arctic marine structures. Stress triaxiality is recognized as one of the important factors for prediction of failure strain of ductile metals. A number of tensile tests are carried out for flat specimens with different notches from relatively smooth to very sharp levels. Numerical simulations of each specimen are performed via nonlinear finite element analysis (FEA). By means of comparing engineering stress versus strain curves obtained from the tests with simulated ones, failure initiation in numerical simulations are identified. Equivalent plastic strains to fracture are plotted in average stress triaxiality domain, then a new failure strain formula is proposed in relatively low average stress triaxiality region.

Clast-cortex aggregates in experimental and natural calcite-bearing fault zones

We investigated the formation mechanisms of rounded clast-cortex aggregates, a composite grain found in the slipping zones of faults hosted in calcite- and clay-rich rocks. The natural aggregates contain a central clast commonly made of host-rock fragments or reworked cataclasite from the slipping zone. The central clasts are surrounded by an outer cortex of calcite or clay grains a few μm or less in size. In laboratory experiments on calcite gouges using two rotary-shear apparatus we investigated the dependence of clast-cortex aggregate formation on the applied slip rate, normal stress, total displacement and ambient humidity. Clast-cortex aggregates formed at all investigated slip rates (100 μm/s to 1 m/s) but only at relatively low normal stresses (≤5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that clast-cortex aggregates in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces.

Direct Measurement of the Contact Normal Stress Distribution between Two Smooth Joint Surfaces

The shear strength of rock joints is highly depended upon the failure mode of joint asperity. At lower normal stress slide-up of one asperity up over another mode, however at high normal stress the joint asperities are sheared off at the base. This research uses pressure measurement film to directly measure the contact normal stress between smooth joint surfaces. It demonstrates that the density of color impression is capable of capturing the normal stress distribution behavior. The contact normal stress distribution during shearing is changed. After shearing, the contact stress becomes large. This increase in contact normal stress is to fracture the joint wall material.

Traction force microscopy in rapidly moving cells reveals separate roles for ROCK and MLCK in the mechanics of retraction

Retraction is a major rate-limiting step in cell motility, particularly in slow moving cell types that form large stable adhesions. Myosin II dependent contractile forces are thought to facilitate detachment by physically pulling up the rear edge. However, retraction can occur in the absence of myosin II activity in cell types that form small labile adhesions. To investigate the role of contractile force generation in retraction, we performed traction force microscopy during the movement of fish epithelial keratocytes. By correlating changes in local traction stress at the rear with the area retracted, we identified four distinct modes of retraction. “Recoil” retractions are preceded by a rise in local traction stress, while rear edge is temporarily stuck, followed by a sharp drop in traction stress upon detachment. This retraction type was most common in cells generating high average traction stress. In “pull” type retractions local traction stress and area retracted increase concomitantly. This was the predominant type of retraction in keratocytes and was observed mostly in cells generating low average traction stress. “Continuous” type retractions occur without any detectable change in traction stress, and are seen in cells generating low average traction stress. In contrast, to many other cell types, “release” type retractions occur in keratocytes following a decrease in local traction stress. Our identification of distinct modes of retraction suggests that contractile forces may play different roles in detachment that are related to rear adhesion strength. To determine how the regulation of contractility via MLCK or Rho kinase contributes to the mechanics of detachment, inhibitors were used to block or augment these pathways. Modulation of MLCK activity led to the most rapid change in local traction stress suggesting its importance in regulating attachment strength. Surprisingly, Rho kinase was not required for detachment, but was essential for localizing retraction to the rear. We suggest that in keratocytes MLCK and Rho kinase play distinct, complementary roles in the respective temporal and spatial control of rear detachment that is essential for maintaining rapid motility.

Interface Shear Damage to a HDPE Geomembrane. I: Gravelly Compacted Clay Liner

AbstractAn experimental program of large-scale direct shear tests has indicated that shear displacement of a high-density polyethylene (HDPE) geomembrane (GM) over a gravelly compacted clay liner (CCL) under moderate to high normal stress conditions can cause much greater damage to the geomembrane than static pressure alone. Essentially, no damage was observed at low normal stress. The greatest geomembrane damage occurred at high normal stress (1,658 kPa) and yielded an average of 169 holes/m2, with a maximum hole size of 23 mm. Size, angularity, and hardness of the gravel particles are also important factors with regard to potential damage. Geomembrane damage was greatly reduced when a needle-punched geosynthetic clay liner (GCL) was placed between the geomembrane and gravelly CCL, including one test conducted at very high normal stress (4,145 kPa). The findings suggest that gravelly soils should be viewed with caution for the construction of GM/CCL composite liners for landfill bottom liner systems and ...

Interface Shear Damage to a HDPE Geomembrane. II: Gravel Drainage Layer

AbstractAn experimental program of large-scale direct shear tests has indicated that shear displacement of a gravel drainage layer and nonwoven geotextile protection layer over a high-density polyethylene (HDPE) geomembrane under moderate to high normal stress conditions can cause much greater damage to the geomembrane than static pressure alone. Essentially, no damage was observed at low normal stress. The greatest damage occurred at high normal stress (1,389 kPa) using a lightweight geotextile (335 g/m2) and yielded an average of 31 holes/m2, with a maximum hole size of 29 mm. Surprisingly, geomembrane damage measured using a lightweight geotextile was greater than that measured using no geotextile due to a change in failure surface location. For the same conditions, shear-induced damage was slightly less for a geomembrane placed on a compacted sand subgrade than on a compacted clay subgrade. Interface shear strength increased significantly with decreasing geotextile mass/area due to greater out-of-plan...

New approach to quantifying rock joint roughness based on roughness mobilization characteristics

This study explores the effect of normal stress levels and asperity scales on roughness mobilization characteristics through joint shear tests, and suggests a new method for quantifying rock joint roughness. Two-dimensional joint surface profiles were scanned by using a digital camera-based 3D scanner system, and were classified into a small scale of roughness (unevenness) and a large scale of undulation (waviness). Joint surface profile specimens, duplicated using a wire-cut electric discharge machining method, were tested for shear behaviors. For waviness showing non-stationary signals, the mobilized roughness is not constant but varies nonlinearly at normal stress levels. However, for unevenness showing noise and a stationary signal, the ultimate shear strength appears at a low normal stress level.

상용 유한요소코드 사용자-서브루틴을 이용한 저온용 고장력강 (EH36)의 파단 시뮬레이션

This paper discusses a new formulation for the failure strain in the average stress triaxiaility domain for a low-temperature high-strength steel (EH36). The new formula available at a low average stress triaxiality zone is proposed based on the comparison of two results from tensile tests of flat type specimens and their numerical simulations. In order to confirm the validity of the failure strain formulation, a user-subroutine was developed using Abaqus/Explicit, which is known to be one of the most popular commercial finite element analysis codes. Numerical fracture simulations with the user-subroutine were conducted for all the tensile tests. A comparison of the engineering stress-strain curves and engineering failure strain obtained from the numerical simulation with the user-subroutine for the tensile tests revealed that the newly developed user-subroutine effectively predicts the initiation of failure.

Shearing rate effect on residual strength of landslide soils in the slow rate range

In the present study, we investigate the influence of shearing rate on the residual strength parameters, ϕ r, in landslide soils using ring-shear tests at two shearing rates (0.01 and 0.5 mm/min) under selected effective normal stresses. The landslide soil samples used for this study cover a wide range of soil types and properties. Relationships between the ϕ r and soil properties, such as liquid limit, plasticity index, and the clay fraction show that the ϕ r decreases with increasing soil LL, I p and CF. The differences in the ϕ r at two shearing rates, ϕ r (0.5) − ϕ r (0.01), under different effective normal stresses are either negative or positive values of which the maximum magnitude is generally about 1.0°. The relationships between the ϕ r (0.5) − ϕ r (0.01) and the soil properties (LL, I p, and CF) are not exhibited in a regular pattern. On the other hand, the ϕ r (0.5) and ϕ r (0.01) determined at each effective normal stress suggest that the tendency for increased negative effect of the ϕ r with decreasing effective normal stress is more noticeable in the slow shearing rate range. The absolute value of ϕ r (0.5) − ϕ r (0.01) at lower effective normal stress is found to be greater than 1.0°, with the maximum of about 4.0°. The negative shearing rate effect on ϕ r at low effective normal stress is affected by the undulating shear behavior due to poor or no slickenside development.

Influence of wall friction on settling behavior of loose-fill cellulose insulation material dependent on different surface roughness

As a preliminary test regarding the expected influence of wall friction on the settling behavior of loose-fill cellulose insulation (LFCI), a settling test was performed according to the draft ISO CD 18393, B. Two test boxes with different cavity depth were used with acrylic glass and soft board as two plank materials of different surface roughness. The wall elements were filled with LFCI and strained by drop off impacts. After 13 and 20 impacts, settling was measured and documented. Results of all four test variations show a clearly lower settling on the rougher surface consisting of soft board compared with the smooth acrylic glass. Concerning the different cavity depth, no obvious influence on wall friction is visible. Based on these results, a wall friction test with LFCI according to the Jenike test method was performed on three types of wood based panels with different roughness and acrylic glass as reference. Results show only little difference between the two rough surfaces. Under low normal stress, the three wood based surfaces are almost similar. Only acrylic glass as the smoothest surface of the test differs significantly from the others. On acrylic glass and oriented strand board, the coefficient of wall friction is rising with decreasing normal stress.

Changes in the lifetime prevalence of suicidal feelings and thoughts among Norwegian doctors from 2000 to 2010: a longitudinal study based on national samples

BackgroundThinking about suicide is an indicator of suicide risk. Suicide rates are higher among doctors than in the population. The main aims of this study are to describe the changes in the lifetime prevalence of suicidal feelings from 2000 to 2010 and the possible predictors of serious suicidal thoughts in 2010 among Norwegian doctors. Differences in lifetime prevalence of suicidal feelings between Norwegian doctors in 2010 and German doctors in 2006 will be also described.MethodsLongitudinal and cross-sectional study based on questionnaire data from 2000 and 2010, including approximately 1,600 Norwegian doctors. In Germany, cross-sectional study based on questionnaire data from 2006 among a sample of 3,295 doctors. The main outcome measures were the lifetime prevalence of suicidal feelings (felt life was not worth living, wished own death, had thoughts of taking own life).ResultsThe prevalences in 2000 and 2010 of ever had feelings of life not worth living were 48 (44 to 52) % and 45 (41 to 49) %, of ever wished own death 27 (23 to 30) % and 23 (20 to 26) %, and of ever had thoughts of taking own life 29 (16 to 33) % and 24 (21 to 27) %. Paired t-tests among those who responded both in 2000 and 2010 show significant reductions for felt life not worth living (t = −3.4; p = 0.001), wished own death (t = −3.1; p = 0.002) and had thoughts of taking own life (t = −3.5; p < 0.0001). In 2010, significant predictors of serious suicidal thoughts in a multivariate model were low subjective well-being (OR 0.68; 95% CI 0.52-0.90), poor or average self-rated health (2.36; 1.25-4.45) and high psychosocial work stress (1.92; 1.06-3.46), controlled for age, gender, speciality and job satisfaction. Norwegian doctors in 2010 compared with their German counterparts in 2006 reported quite similar prevalences of suicidal feelings.ConclusionsSuicidal feelings among Norwegian doctors decreased from 2000 to 2010. Individual and work-related factors may to certain explain these findings. Compared with other professionals in Norway and doctors in Germany, Norwegian doctors showed no higher risk of suicidal thoughts.

Lack of Additional Triggered Tectonic Tremor around the Simi Valley and the San Gabriel Mountain in Southern California

Abstract We conduct a systematic search of tectonic tremor triggered by distant large earthquakes around the Simi Valley (SV) and the San Gabriel Mountain (SGM) in southern California. Out of 59 large earthquakes between 2000 and 2013, only the 2002 M w 7.9 Denali fault earthquake triggered clear tremor in the region. The observed travel times of the triggered tremors are consistent with theoretical predictions from tremor sources that are spatially clustered in the SV, close to the rupture zone of the 1994 M w 6.7 Northridge earthquake. We also estimate the triggering stress threshold as ∼12 kPa from measuring the peak ground velocities near the tremor source. The lack of clear tremor beneath the SGM provides a “negative” example for a region where tremor is expected to occur because of clear evidence of fluid‐rich zones at the middle crust. The results imply that the necessary conditions for tremor to occur are more than fluid‐induced low effective normal stress. Online Material: Figures of additional tremor inspection, tables of earthquake catalogs, and tremor locations.