Principal Stress Orientations Research Articles

Using Syntectonic Calcite Veins to Reconstruct the Strength Evolution of an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

AbstractQuantifying the strength evolution of faults that cut the lithosphere is essential to better understand seismicity in continental regions. We estimate differential stresses and principal stress orientations driving rapid slip of ∼10 mm/yr on the active Mai'iu low‐angle normal fault (LANF), SE Papua New Guinea. The fault's mafic footwall hosts a well‐preserved sequence of mylonite, (ultra‐)cataclasite, and gouge. In these fault rocks, we combine stress inversion of fault‐slip data and paleostress analysis of syntectonically emplaced calcite veins with microstructural and clumped‐isotope geothermometry to constrain a syn‐exhumational sequence of deformation stresses and temperatures, and to construct a stress profile through the exhumed footwall of the active Mai'iu LANF. This includes: (a) at ∼12–20 km depth (T ≈ 275–370°C), mylonites accommodated slip on the Mai'iu fault at low differential stresses (>25–135 MPa) before being overprinted by localized brittle deformation at shallower depths; (b) at ∼6–12 km depth (T ≈ 130–275°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on a mid‐crustal portion of the fault (dipping 30–40°), and to trigger brittle yielding of mafic footwall rocks in a zone of mixed‐mode seismic/aseismic slip; and (c) at shallower crustal depths (T < 150°C; depth <6 km) on the most poorly oriented segment of the Mai'iu LANF (dipping ∼22°), slip occurred on frictionally weak clay‐rich gouges (μ ≈ 0.15–0.38). Subvertical σ1 and subhorizontal σ3 parallel to the extension direction, with σ1 ≈ σ2 (constriction), reflect vertical unloading and 3‐D bending strain during rolling‐hinge style flexure of the footwall.

A new methodology for measuring residual stress using a modified Berkovich nano-indenter

In this paper, a new methodology, based on nanoindentation, is proposed to estimate the magnitude and orientation of the principal residual stress components, in a non-equi-biaxial residual stress field at the nanoscale. The proposed method exploits the force-penetration response provided by a modified Berkovich tip that, compared to the most traditional one, is characterized by a preferential elongated size. This feature allows in estimating non-equibiaxial residual stress field since the force-penetration response changes according to the indenter orientation. No observation of the residual imprint is required. Principal stress components and orientations are estimated thanks to theoretical and finite element analysis, establishing a linear relation between stress components and load variations respect to the stress-free sample. Finite element analysis is used to calibrate the model coefficients and to verify the accuracy of this approach for isotropic metals. Obtained results revealed that the method is well suitable in estimating both tensile and compressive residual stresses with an average error lower than 6% for the first case and lower than 10% for the second one.

Stress heterogeneity in the Changning shale-gas field, southern Sichuan Basin: Implications for a hydraulic fracturing strategy

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (SHmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the SHmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the SHmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity.

Numerical and experimental study of the presplit blasting failure characteristics under compressive stress

Recently, the application of deep hole presplit blasting (DHPB) in underground-engineering excavation has increased, but the research on deformation characteristics and failure mechanism of DHPB is relatively less, and lack of relevant numerical simulation verification. Owing to the above situation, the deformation characteristics and failure mechanism of DHPB are obtained through numerical simulation and model testing of samples under horizontal compressive stress, compressive vertical stress and no compressive stress, and the shape analysis of presplit cracks after presplit blasting. This work demonstrates when presplit blasting is performed in underground engineering, the angle between the connection orientations of the blasthole and the compressive stress orientation should be avoided at 90° as far as possible. When the deep confining pressure is not uniform, the orientation of the minimum principal stress should be at an angle of 90° to the orientation of the blasthole connecting line as far as possible. Furthermore, the orientation of the maximum principal stress should be consistent with the orientation of the blasthole connecting line as far as possible.

Stress changes associated with the significant first subevent of the 2008 Wenchuan earthquake and implications for the rupture behavior transition

The rupture process of the Wenchuan earthquake demonstrated a transition from thrust-dominated slip to northeastward strike-slip motion along the Longmen Shan Fault Zone. The initial stress has been reported as playing a critical role in this process; however, the stress changes, especially those caused by the significant first subevent of the Wenchuan earthquake are not well understood. Here, we employ a three-dimensional finite element model of the Sichuan-Yunnan region to analyze the stress change caused by the significant first subevent and explore the possible influence on the following ruptures. The results indicate that the auxiliary maximum principal compressive stress (SH) associated with the significant first subevent was horizontal and that the auxiliary stress regime was SH>Sh>Sv, supporting the ongoing regional thrust motion near the southwestern segment of the rupture plane. However, in the northeastern segment, the auxiliary stress regime transitioned to SH>Sv>Sh, demonstrating that the stress changes promoted the transition of the rupture behavior from predominantly thrust motion in the southwest to right-lateral strike-slip motion in the northeastern segment, which was also supported by the dominant shear stress change and the subtle normal stress change along the fault plane in the northeastern segment. In addition, our modeled results also indicate that the orientation of the maximum principal compressive stress changed from SEE to northeastward NEE along the strike of Longmen Shan Fault Zone. This anticlockwise rotation hastened the rupture behavior transition, suggesting that both the initial stress and the stress changes associated with the first subevent jointly controlled the following northeastward rupture of the Mw7.9 Wenchuan earthquake.

Neogene Kinematics of the Potwar Plateau and the Salt Range, NW Himalayan Front: A Paleostress Inversion and AMS study

ABSTRACT We provide new kinematic data from the Potwar Plateau (Pakistan) to evaluate the tectonic evolution of the region during the Neogene. The plateau is bound by two major strike-slip faults in the west and the east, accommodating its southwards translation. We have recognized two Neogene deformation phases in the plateau, based on paleostress inversion and Anisotropy of Magnetic Susceptibility (AMS) tensors. The first phase lasted until the early Pliocene and was characterized by vertical minor stress and N-S compression, implying thrust tectonics. The second deformation phase is characterized by a near-vertical intermediate principal stress and near-horizontal major and minor stresses, interpreted to be associated with strike-slip tectonics since the late Pliocene. Kint vectors from 21 sites are relatively compatible with the major principal stress orientations (σ1) and indicate two distinct domains. This is possibly because Kmin orientations are related to compaction, whereas Kint orientations were always parallel to tectonic shortening and hence compression direction during both strike-slip (post-late Pliocene) and thrusting (pre-late Pliocene) phases. These phases are characterized by swapping of (σ2) and (σ3) orientations while (σ1) maintained its orientation. The most prominent change occurs at the western part of the Potwar Plateau, where major principal stress directions (σ1) and Kint axes fan out south-westwards. The eastern domain is dominated by NE-SW trending folds and thrust faults, which are absent in the western domain. These structural features are interpreted to be the result of the distribution of deposits of the Neoproterozoic Salt Range Formation as a substratum below the Potwar Plateau. The Salt Range Formation is very thick and widespread in the west area and almost absent in the east. This factor led to unconstrained southwards gliding of the Potwar Plateau over the salt deposits in the west as opposed to frictional sliding and substantial internal deformation in the east.

Stress Reversals near Hydraulically Fractured Wells Explained with Linear Superposition Method (LSM)

Prior studies have noted that the principal stress orientations near the hydraulic fractures of well systems used for energy extraction may wander over time. Typically, the minimum and maximum principal stresses—in the horizontal map view—swap their respective initial directions, due to (1) fracture treatment interventions, and (2) pressure depletion resulting from production. The present analysis shows with stress trajectory visualizations, using a recently developed linear superposition method (LSM), that at least two generations of stress reversals around hydraulic fractures occur. The first generation occurs during the fracture treatment; the second occurs immediately after the onset of so-called flow-back. During each of these stress swaps in the vicinity of the hydraulic fractures, reservoir directions that were previously in compression subsequently exhibit extension, and directions previously stretching subsequently exhibit shortening. The pressure change in the hydraulic fractures—from over-pressured to under-pressured (only held open by proppant packs)—caused the neutral points that separate domains with different stress states to migrate from locations transverse to the fracture to locations beyond the fracture tips. Understanding such detailed geo-mechanical dynamics, related to the pressure evolution in energy reservoirs, is extremely important for improving both the fracture treatment and the well operation, as future hydrocarbon and geothermal energy extraction projects emerge.

Stability of basement-rooted faults in the Delaware Basin of Texas and New Mexico, USA

Since 2009 the Delaware Basin of Texas and New Mexico has experienced increased seismicity related to oilfield operations. Available, near present-day principal stress orientations, relative magnitudes, and vertical stress estimates are integrated to characterize 20 stress domains in the Delaware Basin. Data density, variability, and quality inform classification of most-likely, less likely, and least likely stress tensor fields. A new interpretation of Delaware Basin, 3D basement-rooted faults is analyzed for stability using most-likely stress tensors. Fault stability is expressed in terms of slip tendency (Ts) and critical pore pressure (ΔPfc). Individual faults are compared in terms of their stability and potential seismic capability, and stress scenarios can be compared for their effects on fault stability. Faults range from stable to critical under the conditions investigated, 8% of total mapped fault area may become critically stressed by small pore-pressure increases (1 MPa, 145 psi) above ambient. Comparison of stability measures with focal mechanisms of recent pore-pressure-induced seismicity indicates that Ts and ΔPfc have predictive value. This work provides critical information for earthquake hazard and mitigation studies in the Delaware Basin.

Mechanomic Engagement Profile: Integrative Mapping of the Mechanical Properties that Inform Endothelial Cell Motion

Background The mechanical properties that govern endothelial cell motion remain poorly understood, especially when these cells are part of a larger cellular community. To address this gap, we have developed a toolkit to quantify wide-ranging mechanical activities represented by cellular morphology, motion, and forces within individual cells and their neighboring region. Using this data, we generated Mechanomic Engagement Profile (MEP) maps that seek to define the functional identity of a cell type based on its degree of correlated engagement in multiple mechanical activities. Methods We cultured radially expanding more than 5 mm diameter colonies of endothelial cells from the pulmonary arteries (AEC), veins (VEC), and microvessels (MEC) of rat lungs. The substrate was 1.25 kPa, 100 µm thick, type-I collagen-coated polyacrylamide hydrogels. Building on the Monolayer Stress Microscopy technique and recent developments, we quantified various physical properties of advancing AEC, MEC and VEC monolayers1,2. MEP maps were generated using nonparametric Spearman covariance between mechanical activities. The data analysis was conducted using custom ImageJ and Python programs. Results MEP maps revealed systematic differences in the mechanical behavior of AECs, MECs and VECs. Consistent with MECs having tighter intercellular connections than AECs and VECs, the activities of individual MECs were better correlated with those in their neighboring region. Consistent with their relatively mature and stable intercellular junctions, the cells far from the migration front engaged more widely correlated activities than the cells near the migration front. Plithotaxis, a physical mechanism where local cellular motion is steered along local maximum principal stress orientation, was found to be enhanced not only with local maximum shear stress (as reported before) but also with strain energy in the substrate. Such enhancement of plithotaxis was strongest in MECs but absent in AECs, potentially suggesting MECs exhibit the most fluid-like behavior of the three cell lines3. Higher cytoskeletal tension was associated with faster motion in AECs and VECs but slower motion in MECs. In addition, stronger plithotaxis was associated with faster motion in AECs and MECs but unrelated to the motion of VECs. These observations indicate distinct mechanisms of migration in AECs, MECs and VECs. Conclusion By using the degree of correlation between various mechanical activities as an identity, the MEP maps provide a means to define the mechanistic fingerprint of AEC, MEC and VEC monolayers. MEP maps would help biologists design strategies for tissue engineering applications, examine functional consequences of molecular changes in the cells, or guide the search for a molecular source of a phenotypic change in the cells.

Nonuniform Stress Field Determination Based on Deformation Measurement

Abstract In this investigation, we demonstrate a technique that, under certain circumstances, will determine stresses associated with a nonuniform deformation field without knowing the detailed constitutive behavior of the deforming material. This technique is based on (1) a detailed deformation measurement of a domain (currently in 2D) and (2) the observation that for isotropic materials, the strain and the stress, which form the so-called work-conjugate pair, are co-axial, or their eigenvectors share the same directions. The particular measures for strain and stress chosen in this study are the Lagrangian (or Green-Lagrangian) strain and the second Piola–Kirchhoff stress. The deformation measurement provides the field of the principal stretch orientation θλ and since the Lagrangian strain and the second Piola–Kirchhoff stress are co-axial, the principal stress orientation θs of the second Piola–Kirchhoff stress is then determined. The Cauchy stress is related to the second Piola–Kirchhoff stress through the deformation gradient tensor, which can be measured experimentally. We then show that the principal stress orientation θσ of the Cauchy stress is the sum of the principal stretch orientation θλ and the local rigid-body rotation θq, which is determinable by the deformation gradient through polar decomposition. Such a relationship is valid for finite deformations. With the principal stress orientation θσ known, the equation of equilibrium, now in terms of the two principal stresses, σ1 and σ2, and θσ, can be solved numerically with appropriate traction boundary conditions. The stresses determined using this technique obviously satisfy the equation of equilibrium, in contrast to those obtained from a constitutive model with input from deformation measurement. The technique and the associated numerical scheme are verified and validated through two virtual test cases representative of the simply-connected and multiply-connected domains, where exact solutions are available. The technique is then applied to an experimental case of nonuniform deformation of a polyvinyl chloride (PVC) sheet with a circular hole and subject to uniaxial tension. In this case, the associated stress field is also determined through a constitutive model of hyperelasticity, the generalized neo-Hookean (GNH) model, calibrated for the particular PVC sheet. Limitations and restrictions of the technique and the associated numerical scheme, as well as possible extensions will be discussed.

Inversion for the static friction coefficient of seismogenic faults: Application to induced seismicity of the Basel Enhanced Geothermal System, Switzerland

The static coefficient of friction describes the strength of faults and is considered to vary in nature between typical laboratory values measured on a large variety of rocks (0.6-0.85) and very low values for fault rocks containing talc, chlorite and kaolinite (<0.3). The effective frictional strength of a seismically active fault is the result of a combination of frictional properties of the fault rocks and the prevailing pore fluid pressures, which can reach supra-hydrostatic values permitting unfavourably oriented faults to slip. Here, we present an iterative inversion procedure to obtain the average static friction coefficient of a population of seismic faults, provided that stress tensor, geometry of faults and fluid pressures are adequately constrained. The application to a dataset of 48 seismically activated faults (earthquake moment magnitude, MW≤3.0) by fluid injection in crystalline basement in the Basel Enhanced Geothermal System (Switzerland) yields an average coefficient of friction of 0.66±0.11. The inversion results, which are well constrained by high-quality focal mechanisms and robust above-hydrostatic fluid pressure estimations, have uncertainty primarily dependent on the principal stress and fault plane orientations. Values of the friction coefficient in crystalline rocks in the continental basement are compatible with experimentally determined values but require supra-hydrostatic fluid pressures. A comprehensive characterization of all the frictional parameters should be required to better assess rheological and stability fault models, both for natural and human induced earthquakes.

Сопоставление различных методов сведения сложного напряженного состояния к простому при непропорциональном нагружении

Object and purpose of research. This paper discusses ship structures. Its purpose is to compare the most common methods for transition from complex strained state to the simple one in calculations of fatigue strength under non-proportional loading. Materials and methods. This paper describes the method of absolute maximum principal stresses, the method of signed von Mises stresses, Sins method and critical-plane method. Main results. Analysis of time histories for design stresses obtained as per above-mentioned methods for the simplest examples of non-proportional loading. The comparison of these results was illustrated by a case study (loading of an icebreaker propulsion shaft). Conclusion. It is shown that, of all the methods discussed in this paper, only critical-plane method can fully take the loading process into account. None of the other methods can take into account the changes in orientation of principal stresses, so their results could be both somewhat too high and too low.

Design, preparation and mechanical properties of full-field aligned steel fiber reinforced cementitious composite

In this study, a full-field aligned steel fiber reinforced cementitious composites (FASFRC) prism specimen was designed, prepared and tested. The special feature of the FASFRC is the orientation of steel fibers in the specimen is consistent with the principal stress when apply flexural loading to the specimen. In order to optimise the design of the FASFRC prism, the orientation of the major principal stress of the prism subjected to flexural load was determined by finite element analysis, which is the expected orientation of steel fibers in a true specimen. Then, in the preparation of the FASFRC prisms, an external magnetic field with magnetic line consisting with the direction of the principal stress was used to control the direction of steel fibers to be consistent with the designed orientation when the mixtures were compacted. Finally, the flexural strength, toughness and residual strength of FASFRC, conventional steel fiber reinforced cementitious composite (SFRC) and aligned steel fiber reinforced cementitious composite (ASFRC) prism specimens were tested by four-point bending tests. Through the tests, it is found that the mechanical properties of the FASFRC specimens were significantly higher than other specimens mainly due to the enhancement of the reinforcement of steel fibers.

The present‐day stress field in the Zagros Fold-Thrust Belt of Iran, from inversion of focal mechanisms

The knowledge about spatial variation of stress field is an important step in the identification of geodynamic and seismotectonic characteristics. Inversion of focal mechanism solutions is one of the most common methods for determination of stress field and the shape ratio in a tectonic region. In this paper, Iterative Joint Inversion method is used to determine principal stress axes from earthquake focal mechanisms in the Zagros continental collision zone of Iran. In order to determine the influence of dependent events on the inversion results, the declustering process is carried out by the Urhammer's method. The area is divided into seven subzones according to the stress field heterogeneity in the Zagros region. In each subzone, the trends of principal stress axes, the orientation of maximum horizontal stress, stress regime and the shape ratio are determined. The results show that the compressional stress direction averages NNE-SSW that accord with the current convergence between the Arabian plate and Iranian micro-continent. A gradual counterclockwise rotation of the compressional axis is also concluded which is compatible with spatial changes in the Arabian–Eurasian convergence paths along the Zagros. The stress regime of most of the subzones is deduced as the Transpression that reflects oblique convergence. The orientation of the maximum principal stress (σ1) show the lowest uncertainty in respect to the intermediate (σ2) and minimum (σ3) principal stress axes. The shape ratio error in all zones are less than 6 and 7.2 percent before and after implementing exclusion respectively, which indicate this error increases with removing the dependent events in all zones except for zone 3.

Reverse migratory behaviour of the earthquakes aftershock sequences along Himalayan Seismic Belt, Northwest Himalaya

The Anderson's theory (Anderson, 1951) can be utilized to understand the allocation of principal stresses. For various tectonic regimes, the global seismicity trend for crustal earthquakes of less than 20 km depth suggests that the source fault inclination also plays a critical part in the propagation of the aftershocks in the subsurface rupture with respect to the mainshock earthquake. It has also been observed that majority of the compression earthquakes that display upward seismicity migration typically occur along steep faults. However, in the context of Himalaya seismicity, the opposite migratory behaviour of the aftershock sequence is observed even along the gentle dipping source faulting. The failure envelope Mohr's diagram is used to understand the alteration in the dynamics of the subsurface locked zone of Himalayan Seismic Belt (HSB). This study discusses the alteration in the effective strength of the rock mass which imply the variation in the crustal hydrostatic fluid pressure at shallow depth (0–15 km). Different earthquakes and their aftershocks sequence of Himalayan terrain have been incorporated to corroborate the role of orientation of tectonic principal stresses, subsurface morphology and fluids/partial melts if present. In addition, topography contributes to the vertical loads and development of the subsurface geometry, therefore its role and its influence on seismicity has been discussed. This study highlights the controlling factors and the trend of the principal applied stresses and subsurface fluids in the distribution of aftershock sequences along tectonic regime of the Himalayan arc.

Revisit on the analytic formulations of principal stress orientation in fracture mechanics

In fracture mechanics, it is important to understand the fracture parameters, several strides in these studies have resulted in various analytical formulations. In this work, important analytical formulations of stress field data are revisited. Though the interpretation of stress field data is well studied, the possibility of utilizing principal stress orientation in order to estimate fracture parameters have not been properly investigated. This work attempts to shed some light on that possibility by studying the principal stress orientations for fracture problems using different formulations. In this work, only mode-I problems are considered and whole field stress fields are generated for different analytical formulations. The principal stress values and orientations are determined using Eigenvalue and Eigenvector approach and the principal stresses are studied. It is found that the principal stress orientations are insensitive towards the change in stress field parameter in the case of single parameter equation. In the case of multi parameter equations it was found that principal stress orientations have variation with respect to the stress field parameters.

A Bayesian Approach for Uncertainty Quantification in Overcoring Stress Estimation

In civil and mining engineering, three-dimensional in situ stress states are commonly measured using overcoring (OC) techniques. With these techniques, stresses are usually estimated by classical least-squares regression analysis of measured OC data. However, the estimated stresses are uncertain and may even be unreliable, due to factors such as rock heterogeneity, measurement errors and inadequacy of the regression model. Quantifying such uncertainty is crucial, as doing so both permits quantitative assessment of the reliability of the measured stress state and facilitates application of probabilistic design approaches in rock engineering such as reliability-based design. The classical approach to OC stress estimation suffers various limitations in this respect, particularly the failure in quantifying uncertainty in estimates of principal stresses, and the inability to improve stress estimation by incorporating stress information from other sources (say, stress states measured at nearby locations and orientation of the major principal stress as determined from observations of borehole breakouts). To overcome these limitations, this paper proposes a novel Bayesian approach to OC data analysis that probabilistically quantifies uncertainty in stress estimations and permits formal incorporation of additional stress information in forms of prior distributions. It also discusses the challenges faced in developing the informative prior distributions that are required to allow incorporation of additional stress information.

Direct Inversion Method of Fault Slip Analysis to Determine the Orientation of Principal Stresses and Relative Chronology for Tectonic Events in Southwestern White Mountain Region of New Hampshire, USA

The orientation and relative magnitudes of paleo tectonic stresses in the western central region of the White Mountains of New Hampshire is reconstructed using the direct inversion method of fault slip analysis on 1–10-m long fractures exposed on a series of road cuts along Interstate 93, just east of the Hubbard Brook Experimental Forest in North Woodstock, NH, USA. The inversion yields nine stress regimes which identify five tectonic events that impacted the White Mountain region over the last 410 Ma. The inversion method has potential application in basin analysis.

Late Cenozoic paleostress reconstruction and stress regimes in Taiyuan basin of the Shanxi rift, North China

The Taiyuan basin, initiated during the late Cenozoic, is part of the left-stepping en-echelon graben systems of Shanxi. The state of stress in this area and its evolution over time is a significant concern given the existence of high shear strain and seismic risk along the basin boundary faults. In the present study, the inversion of fault-slip data through the right dihedron and the rotational optimization methods led us to identify 21 significant paleostress tensors. The maximum principal stress orientation shows two modes trending NE–SW and ENE–WSW with most of stress tensors having stress ratio values within a wrench regime. The extensional sites reveal NW–SE and NE–SW directions. Three tectonic stages with different kinematics were successfully identified since the late Cenozoic. The oldest event has been identified in our analysis since the late Miocene to late Pliocene and belongs to NW–SE extension/strike-slip stress regime. The next was generated in the early Pleistocene under the NE–SW extension stress regime, and the youngest activity recorded belongs to the NNW–SSE extension/strike-slip regime developed since the late Pleistocene. These events were dynamically driven by the northeastward extrusion of the Tibetan Plateau in response to the plate boundary motion involving the Indian and Eurasian plates and lesser by the NW subduction of the Pacific plate. The present-day stress state is characterized by ENE–WSW compression and NNW–SSE extension derived from earthquake focal mechanisms.

Influence of loading geometry and palm fibres’ orientation on modification of collapsible sand

A review of existing literature suggests that extensive numerical and experimental works have been conducted on modifying soils using randomly distributed fibres. However, the influence of different types of loading geometry and the effect of fibres on modification of sandy soil have not been reported yet. The main objective of the present study was to evaluate the influence of palm fibres’ orientation on the modification of collapsible sand using different types of loading, including constrained, axisymmetric, and plane strain simulation. A laboratory testing program, consisting of 95 tests, was performed using oedometer, direct shear, and triaxial tests. It was observed that the orientation of fibres in the specimens was approximately perpendicular to the direction of the applied compaction effort. The results of oedometer tests for measuring of the collapsibility potential revealed that addition of the fibre content had no significant effect in reducing the collapsibility potential. For the two mould sizes used in the direct shear tests, an increase in fibre content did not cause any changes in the shear strength parameters. In triaxial tests, the results indicated a significant enhancement in strength with increase in the fibre content. The rotation of the principal stress orientation during the shear stage of the tests played an important role in mobilising the tensile strength of the fibres. The results suggested that the effect of fibre orientation and loading geometry is strongly consistent for evaluating the modification of fibre-reinforced sand.