Increase In Lateral Stress Research Articles

Stress Analysis and Spalling Failure Simulation on Surrounding Rock of Deep Arch Tunnel

To study the stress distribution characteristics of surrounding rock and the spalling mechanism of deep hard rock tunnels with different arch heights, the complex variable function and angle-preserving transformation method in elasticity theory were applied to the analytic solution of tangential stress distribution of arch tunnels during stress adjustment. In addition, true triaxial tests were conducted on granite cube specimens (100 mm × 100 mm × 100 mm) containing holes with three arch heights (including the 25 mm semi-circular arch, 16.7 mm three-centered arch, 12.5 mm three-centered arch) to simulate the spalling process under different initial ground stresses. The stress distribution solution and experimental results show that the initial failure stress of arch holes is 0.39–0.48 times the uniaxial compressive strength (UCS) of the rock. The initial failure location occurs at the arch foot, where tangential stress maximizes. When the lateral pressure coefficient is in the range of 0.38–0.50, the tangential stress is 3.2–3.5 times the UCS. The rock debris of the hole wall are in thin flake shapes. Symmetrical V-shaped or curved failure zones occurred on hole sidewalls. The stress distribution resolution of the surrounding rock of tunnels with different arch heights shows that with the increasing burial depth, the bearing performance of the semi-circular arch tunnel is optimal. In addition, the maximum tangential stress increases as the height of the arch decreases or the lateral stress increases, making it easier for the initial failure to occur at the foot of the arch.

A DEM study on microstructural behaviour of soluble granular materials subjected to chemo-mechanical loading

Experimental works provide a mere understanding of the behaviour of granular material subjected to chemo-mechanical weathering, while the microstructural behaviour is of key importance due to the presence of coupling between the chemical and mechanical loading at the grain scale. The Discrete Element Method (DEM) has proven to be a viable tool in characterizing such microstructural behaviour of granular material in the recent past. In the present study, a DEM based numerical model is proposed to simulate chemo-mechanical degradation as observed in experiments. The aim is to understand how the dissolution rate alters during the dissolution of fully soluble reactive granular assembly like calcite under flowing acetic acid solution. The DEM contact parameters are calibrated for calcite granules, while the grain dissolution due to acid flow is mimicked by reducing particle size at a specific rate. The numerical results reveal that a gradual rise in dissolution rate or particle size reduction rate leads to a drop in lateral stress followed by a steady stress state condition. Furthermore, the study demonstrates that a reduction in dissolution rate leads to an increase in lateral stress. The microstructural analysis utilizing average particle velocity indicates that the increasing dissolution rate does not enhance particle rearrangement of the same order. Hence, more voids are created within the sample, and particle contact reduces, which promotes a reduction in lateral stress within the sample.

Failure process and characteristics of three-dimensional high-stress circular tunnel under saturated water content

True-triaxial compression tests were carried out on cubic granite samples with a circular through hole using a true-triaxial testing system to investigate the influence of saturated water content (SWC) on the failure process and characteristics of a circular tunnel of surrounding rocks. The spalling failure under SWC can be divided into four periods: calm period, buckling deformation period, period of rock fragment gradual buckling and exfoliation, and period of formation of symmetrical V-shaped notches. When the horizontal axial and vertical stresses were constant, the spalling failure severity was reduced with the increase in lateral stress. Under natural water content, a strong rockburst with dynamic failure characteristics occurred on the circular hole sidewall. Under SWC, the failure severity was reduced and the circular hole sidewall experienced spalling failure, exhibiting progressive static failure characteristics. Therefore, water can reduce the failure severity of surrounding rocks in deep underground engineering, which has a certain guiding significance for the prevention and control of rockbursts.

An experimental study and failure mechanism analysis on dynamic behaviors of plain concrete under biaxial compression-compression

In order to explore the dynamic mechanical properties of concrete under biaxial compression-compression, a true triaxial instrument was used to conduct an experimental study on the dynamic mechanical properties of plain concrete under biaxial compression-compression. Eight different lateral compressive stresses (0 MPa~14 MPa) and four strain (10−5/s, 10−4/s, 10−3/s and 10−2/s) rate were considered. From this, the effects of lateral compressive stress and loading strain rate on the biaxial compression-compression properties of concrete were compared and analyzed by obtaining the failure modes, principal compressive stress-strain curves and related mechanical characteristic parameters of plain concrete under different loading conditions. The research results show that: with the increase of lateral compressive stress, the failure modes of concrete were developed from columnar failure at low lateral compressive stress to sheet-like failure at high lateral compressive stress. With the increase of lateral stress, the development trend of concrete failure mode decreased gradually under the influence of strain rate. With the increase of lateral compressive stress, the variation range of concrete principal compressive stress decreased first and then tended to be relatively stable under the influence of strain rate. With the increase of strain rate, the increase range of concrete principal compressive stress decreased under the influence of lateral compressive stress. The influence mechanism of lateral compressive stress and strain rate on biaxial compression-compression performance of concrete was analyzed. Meanwhile, based on Kupfer biaxial compression failure criterion, a biaxial compression dynamic failure criterion model considering the effect of loading strain rate on plain concrete was proposed. The research results provided an important theoretical basis for the application and development of concrete engineering.

Effect of Lateral Stress on the Mechanical Properties of Rock Fracture and Its Implication on the Stability of Underground Oil Storage Caverns

It is of great significance to study the mechanical properties of rock fracture for the stability of rock engineering projects. The mechanical properties, including the normal compression and shear properties of rock fracture, are studied by a series of shear tests considering the effect of lateral stress using the self-developed true triaxial test apparatus. The test results show that the initial normal stiffness and the maximum normal closure value of rock fracture increase with the increase of lateral stress, and the peak shear strength and the peak dilatancy angle increase with the increase of lateral stress, whereas the peak shear displacement decreases with the increase of lateral stress. Considering the effect of lateral stress, the improved normal loading model, peak shear strength model and peak dilatancy angle model of rock fracture are established. Using the equivalent parameters of rock fracture obtained based on the test, the hydro-mechanical coupling analysis considering lateral stress is carried out for an underground water sealed oil storage cavern project. It can be concluded that with the increase of lateral stress, the displacement of surrounding rock decreases, and the surrounding rock tends to be more stable.

Numerical modelling of rammed aggregate piers (RAP) in liquefiable soil

Liquefaction poses a significant risk to the built environment, and as a result, ground improvement (GI) techniques are commonly used to mitigate this risk. Liquefaction mitigation strategies continually evolve, and several relatively new techniques, such as Rammed Aggregate Piers® (RAP), have shown promise. While the densification mechanism associated with many of the GI techniques is generally well known, other mechanisms such as reinforcement, lateral stress increase, and improved drainage that are thought to enhance liquefaction mitigation are still not completely understood. Moreover, field performance data for these GI schemes during actual earthquakes is limited. To fill this gap and evaluate the performance of RAP, this study presents the first numerical model on a specific GI technique that uses detailed site characterization, large-scale field test data, and post-earthquake field performance observations to calibrate and qualitatively validate the model. The in-situ characterization and full-scale field test data collected from the Ground Improvement Programme (GIP) performed following the 2010–2011 Canterbury Earthquake Sequence (CES) in New Zealand are used in this study. A set of fully-coupled hydro-mechanical finite difference (FD) models are performed in natural and reinforced conditions. For the unimproved soil profile, the results predict shear strains and zones of high excess pore water pressures reasonably well. For the profile reinforced with RAP, the analyses indicate that the stiffness properties of the RAP have the greatest influence on the reduction of generated shear strains in the soil profile. Finally, the calibrated models are subjected to a set of ground motions with different intensities to assess the efficacy of the RAP under different loading conditions.

Particle Flow Simulation of Failure Characteristics of Deep Rock Influenced by Sample Height-to-Width Ratios and Initial Stress Level under True-Triaxial Unloading

The micromechanism of the effects of different height/width ratios (H/W) and initial stress levels on unloading characteristics of deep rock was investigated based on PFC3D true-triaxial unloading simulation. The results show that the increase of H/W will increase the movement speed of rock particles and intensify the acoustic emission (AE) activity inside the rock. With the increase of H/W, the failure mode of rock changes from splitting failure to tensile-shear failure. With increasing initial stress level, the particle velocity and overall fragmentation degree of rock increase. However, the increase of lateral stress will limit the coalescence of microfractures and weaken AE activity in the rock. Under unloading condition, the bonds between particles generally crack along the unloading direction, and the tensile effect is more pronounced under the condition of low initial stress level and high H/W. Under unloading condition, the variable energy of rock increases with increasing H/W and initial stress level, and the kinetic energy of rock particles increases with increasing H/W. The increase of initial stress level will increase the kinetic energy of rock particles when H/W is high.

Cracking process and acoustic emission characteristics of sandstone with two parallel filled-flaws under biaxial compression

Discontinuities widely exist in natural rocks. To investigate the progressive micro-cracking process and failure mechanism of fissured rocks, a series of biaxial compression tests were conducted on sandstone specimens containing two parallel filled flaws using acoustic emission (AE) analysis synchronized with digital image correlation (DIC) monitoring. Experimental results show that the peak strength and elastic modulus of sandstone decrease first and then increase with the change in the ligament angle from 0° to 150°, achieving a minimum at 60°. The flaws remarkably facilitate crack coalescence under low lateral stress, such as at 2.5 MPa and 5 MPa. However, with an increase in lateral stress to 10 MPa, the crack coalescence is less influenced by the presence of pre-existing flaws. The AE events produced by flawed sandstone during the loading process conform to the Hurst statistical law. Fractal analysis shows that the lateral confinement reduces the irregularity of ultimate fracture geometry. Based on the AE dominant frequency features, the micro-tensile cracks, micro-shear cracks and micro-tensile-shear cracks are distinguished. The results show that with an increase in lateral stress, the percentage of micro-tensile cracks are constrained, but the number of micro-shear and mixed tensile-shear cracks increases. In addition, the micro-shear cracks preferentially appear in flawed sandstone specimens under high lateral stress as compared with specimens subjected to low stress.

Study on Transient Unloading Response Characteristics of Brittle Coal Samples Under Different Stress Paths

During the unloading process of engineering excavation, the high energy storage in brittle coal is induced to release, which leads to the failure of surrounding rock. The response characteristics of brittle coal samples under instantaneous unloading are analyzed by numerical simulation of different unloading stress paths in this paper. The results show that under the condition of limited lateral displacement, there is a certain linear relationship between the lateral stress and the vertical stress in a certain range before unloading. With the increase of the level of vertical stress loading, the maximum displacement increases obviously after the instant unloading and the failure is more serious. The critical impact stress value of the brittle coal sample is 17 MPa. Under the condition of equal lateral pressure on both sides, the maximum displacement and the vertical stress increase in the form of quadratic polynomial after instant unloading. The relationship between vertical stress and lateral stress on the critical failure envelope is a fourth-order quartic polynomial. With the increase of lateral stress, the impact liability of brittle coal samples is changed from being controlled by vertical stress to being controlled by lateral stress. Under the condition of unequal lateral pressure on both sides, the burst liability of brittle coal samples after instantaneous unloading is most affected by the vertical stress, followed by the minimum stress, and least affected by the intermediate stress.

Blast-induced liquefaction in silty sands for full-scale testing of ground improvement methods: Insights from a multidisciplinary study

In the engineering geology field increased attention has been posed in recent years to potential liquefaction mitigation interventions in susceptible sand formations. In silty sands this is a major challenge because, as the fines content increases, vibratory methods for densification become progressively less effective. An alternative mitigation technique can be the installation of Rammed Aggregate Pier® (RAP) columns that can increase the resistance of the soil, accounting for its lateral stress increase and for the stiffness increase from soil and RAP composite response. To investigate the influence of these factors on liquefaction resistance, full-scale blast tests were performed at a silty sand site in Bondeno (Ferrara, Italy) where liquefaction was observed after the 2012 Emilia-Romagna earthquake. A multidisciplinary team of forty researchers carried out devoted experimental activities aimed at better understanding the liquefaction process at the field scale and the effectiveness of the treatment using inter-related methods. Both natural and improved areas were investigated by in-situ tests and later subjected to controlled blasting. The blast tests were monitored with geotechnical and geophysical instrumentation, topographical surveying and geological analyses on the sand boils. Results showed the RAP effectiveness due to the improvement of soil properties within the liquefiable layer and a consequent reduction of the blast-induced liquefaction settlements, likely due to soil densification and increased lateral stress. The applied multidisciplinary approach adopted for the study allowed better understanding of the mechanism involved in the liquefaction mitigation intervention and provided a better overall evaluation of mitigation effectiveness.

Shaft capacity of pre-bored grouted planted nodular pile under various overburden pressures in dense sand

This paper presents the results of a series of model tests performed to study the shaft capacity of pre-bored grouted planted nodular (PGPN) pile in dense sand. The influence of the vertical overburden pressure on the shaft capacity of the PGPN pile is also investigated based on the test results. The test piles were equipped with strain gauges to measure the axial loads during the loading process, moreover, a foam plate was buried beneath pile tip to eliminate the influence of tip resistance on the shaft capacity. Some conclusions can be drawn based on the test results: the peak skin friction of PGPN pile increases with the increase of vertical overburden pressure applied on the foundation soil, while the rate of increase decreases with the increasing overburden pressure; the surface of the pile–soil interface of PGPN pile is relatively rough, and significant dilatant increase in lateral stress occurs during the loading process.

Shear relaxation behind the shock front in [formula omitted] molybdenum – From the atomic scale to continuous dislocation fields

In this work we study shock-induced plasticity in Mo single crystals, impacted along the 〈1 1 0〉 crystal orientation. In particular, the shear relaxation behind the shock front is quantitatively inspected. Molecular dynamics (MD) simulations are employed to simulate the deformation during shock, followed by post-processing to identify and quantify the dislocation lines nucleated behind the shock front. The information on the dislocation lines is ensemble averaged inside slabs of the simulation box and over different realizations of the MD simulations, from which continuous dislocation fields are extracted using the Discrete-to-Continuous method. The continuous dislocation fields are correlated with the stress and strain fields obtained from the MD simulations. Based on this analysis, we show that the elastic precursor overshoots the shear stress, after which dislocations on a specific group of slip planes are nucleated, slightly lagging behind the elastic front. Consequently, the resolved shear stress is relaxed, but the principal lateral stress increases. The latter leads to an increase in the resolved shear stress on a plane parallel to the shock wave, resulting in an additional retarded front of dislocation nucleation on planes parallel to the shock front. Finally, the two-stage process of plasticity results in an isotropic stress state in the plane parallel to the shock wave. The MD simulation results are employed to calculate the dislocation densities on specific slip planes and the plastic deformation behind the shock, bridging the gap between the information on the atomic scale and the continuum level.

Deep Compaction Control of Sandy Soils

Abstract Vibroflotation, vibratory compaction, micro-blasting or heavy tamping are typical improvement methods for the cohesionless deposits of high thickness. The complex mechanism of deep soil compaction is related to void ratio decrease with grain rearrangements, lateral stress increase, prestressing effect of certain number of load cycles, water pressure dissipation, aging and other effects. Calibration chamber based interpretation of CPTU/DMT can be used to take into account vertical and horizontal stress and void ratio effects. Some examples of interpretation of soundings in pre-treated and compacted sands are given. Some acceptance criteria for compaction control are discussed. The improvement factors are analysed including the normalised approach based on the soil behaviour type index.

Modeling of the compaction-induced stress on reinforced soil walls

A new simple analytical procedure (AASHTO modified) that includes the effect of the induced stress due to backfill compaction for use with conventional design methods of geosynthetic reinforced soil (GRS) walls is proposed. The proposed analytical procedure may be used with any conventional design methods that do not take into consideration the effect of the compaction-induced stress in their calculations. This approach is based on an equation suggested by Wu and Pham (2010) to calculate the increase in lateral stress in a reinforced soil mass due to compaction. Additionally, two numerical procedures for modeling of compaction are described. Analyses using these procedures were performed to evaluate the capability of the proposed analytical procedure. The results were compared with values predicted using the Ehrlich and Mitchell (1994) method, the modified version of the K-stiffness method (Bathurst et al., 2008) and the AASHTO simplified method. The results show that the AASHTO modified method and the numerical analyses, in which the compaction-induced stress was modeled using two distributed loads at the top and bottom of each soil layer, resulted in values of the maximum reinforcement tension, Tmax, that agree with those from the full-scale test and those calculated by Ehrlich and Mitchell (1994). On the other hand, the K-stiffness method under-predicts the measured Tmax values. Moreover, numerical modeling of compaction using a distribution load only at the top of each soil layer overestimated the measurements.

Modeling the Effects of Backfilling and Soil Compaction beside Shallow Buried Pipes

Compaction of the soil placed beside culverts (the side-fill) can have a significant effect on the behavior of flexible and rigid structures. This is particularly true for shallow buried structures when the stresses resulting from compaction represent a greater proportion of the total stresses present. Different techniques have been reported in the literature to model soil compaction during finite-element analyses. A new semiempirical technique is proposed, which takes into consideration the increase in lateral stress and soil kneading during compaction. A simple procedure is discussed to incorporate the compaction of granular material in finite-element analysis. The new technique is used to model five different pipe products composed of different materials and dimensions, and the results are compared to measured values reported in the literature. A new factor is proposed to account for soil kneading during compaction to provide an upper limit for the pipe deformations and stresses that result during installation. The technique can be used to estimate peaking in flexible culverts and the additional crown moments and thrust that result in rigid culverts.

Lateral Stress Concentration in Localized Interlayer Rock Stratum and the Impact on Deep Multi-Seam Coal Mining

To explore the impact of lateral stress concentration in interlayer rock stratum on the exploitation of protected coal seam, a field experiment was carried out in a multi-seam mining structure. Lateral stress redistribution and interlayer rock failure behavior were surveyed. Then an assistant numerical investigation was implemented to evolve the effect of liberated seam mining and its influence on stress reconstruction in surrounding rock mass. The cause of lateral stress concentration and its impact were discussed finally. Key findings turn out that a certain lateral stress increases in interlayer rock stratum and concentrates on its lower region. Lateral stress concentration and interlayer rock failure are interactional. The former is an inducing factor of the latter; the latter promotes the increase of concentration degree. Extent of lateral stress concentration increases to the maximum as seam distance is about 50 m. But the efficacy of liberated seam mining decreases as the seam spacing gets larger. Protected seam mining is then classified based upon the impact of lateral stress concentration, which helps to prevent the rock burst hazard and then to achieve a reliable mining in deep mines.

Estimation of Ultimate Lateral Load Capacity of Piles in Sands Using Calibration Chamber Tests

Abstract Current practice for the estimation of the ultimate lateral load capacity of piles is typically based on the vertical effective stress σ′v, while the effect of the lateral effective stress σ′h is not specifically considered. In the present study, calibration chamber lateral pile load tests are conducted to investigate the load response and ultimate lateral load capacity Hu of laterally loaded piles under various soil and stress conditions. In order to determine Hu from load-deflection curves, different criteria are explored and analyzed. From the test results, it is shown that Hu increases significantly with increasing σ′h for a given σ′v. It is also found that lateral deflection of pile at ultimate state tends to increase as the relative density and lateral stress increase. On the basis of the test results, the lateral stress correction factor reflecting the effect of the lateral effective stress σ′h on Hu is proposed. From the test results, it is seen that the proposed procedure using the lateral stress correction factor produces more realistic estimation of Hu. Case examples are selected from the literature and used to compare results measured and predicted using the proposed approach.

Lateral soil stiffness adjacent to deep integral bridge abutments

Our understanding of the performance of integral bridge abutments subjected to cyclic loading has been improved using well-designed laboratory experiments, and by observation of abutment performance in the field. However, there remains significant uncertainty on how the increase in lateral stress that arises due to cycling of the backfill may be assessed for abutment design. Such uncertainty is addressed in this paper by drawing on results from a series of experiments performed using the University of Western Australia (UWA) beam centrifuge. The experimental results shed light on the effects, for granular backfill, of initial density, retained height, cyclic history and particle shape. These results, as well as observations made from tests involving overconsolidated clays, are used in conjunction with the existing database of lateral stress measurements to assist in the development of recommendations for assessment of design lateral stresses on deep integral bridge abutments.

An analytical model for evaluation of compaction-induced stresses in a reinforced soil mass

When an earth fill is subject to loading and subsequent unloading, it will bring about an increase in the lateral stress, provided that there is sufficient constraint to lateral deformation of the soil mass. The increase in lateral stress is commonly known as the "residual" or "lock-in" lateral stress. The residual stress resulted from compaction operation, which involves a series of loading and unloading onto a soil mass, is referred to as "compaction-induced stress" (CIS). The CIS will increase the stiffness and strength of the soil mass, and is an important factor to be considered in the behavior of compacted soil. A number of studies have been conducted on the CIS in an unreinforced soil mass. With a reinforced soil, the CIS is likely to be much more pronounced because there is a higher degree of constraint to lateral deformation in a reinforced soil mass due to soil-reinforcement friction. An analytical model, referred to as the CIS model, is developed for evaluation of compaction-induced stress in a reinforced soil mass. Correlations for determination of model parameters are given so that the parameters can be estimated through reinforcement spacing and stiffness, and common soil parameters, such as the angle of internal friction (Φ), and overconsolidation ratio (OCR). In addition, the stress paths of typical fill compaction operation are discussed, including compaction with a plant moving toward and away from a section under consideration, and compaction with multiple passes. The CIS model presented in this paper is found to give CIS values very close to those obtained from sophisticated finite element analysis of a 6-m high GRS mass under different values of compaction pressure. The CIS in the reinforced soil mass is significant under compaction pressures typically used in actual fill compaction.

Lateral stress effects on liquefaction resistance correlations

When the sand compaction pile (SCP) method is implemented to improve loose deposits of sandy soils, its effect is evaluated generally in terms of increase in density, which is beneficial for reducing the liquefaction potential of the deposits during earthquakes. An additional advantage can be expected to occur due to concurrent increase in lateral stress. When the resistance to liquefaction is evaluated on the basis of SPT N-value or CPT qc-value, the increased resistance to penetration due to the sand compaction has been interpreted conventionally as being associated mainly with the increase in density. Therefore, in order to properly evaluate the effectiveness of ground improvement in compacted soils, it is necessary to quantify the effect of lateral stresses on the penetration resistance and liquefaction strength. In this paper, based on the results of SPT and CPT performed in a chamber box in the laboratory, the relationships between penetration resistance, liquefaction resistance and relative density were re-examined and the influence of lateral stress, expressed in terms of KC, was investigated. Although the results indicated that generally the resistance to liquefaction increases with increasing KC–value, little difference was noted when the density of the deposit was high. Based on the results, recommended charts incorporating the effect of KC were proposed.