Development of a transparent triaxial cell and observation of rock deformation in compression and creep tests

A transparent triaxial cell was designed and manufactured using acrylic resin. The cell was used to conduct strength and creep tests. Photographs were taken of the specimens at constant time intervals during the constant strain rate test. Photographs were also taken at constant intervals of strain during the creep test, but this rate was changed to one image per second when the specimens first showed tertiary creep. Comparison of the axial and lateral strains during the constant strain-rate and creep tests indicated no significant differences between the two tests. It is well known that the axial creep strain rate is inversely proportional to remaining life in tertiary creep. This study showed that the lateral creep strain rate is also inversely proportional to remaining life. The constant strain-rate tests were conducted with transparent end pieces attached firmly to the upper and lower ends of the specimens. Three holes were drilled into the end pieces, and water was expelled into the holes when the specimens were compressed. It was clearly observed that the water began flowing from the holes back into the specimen during the volumetric expansion of the specimen. The transparent triaxial cell permitted easy observation of water ejection and re-absorption into the specimens.     

Undrained creep behavior of CO2 hydrate-bearing sand and its constitutive modeling considering the cementing effect of hydrates

A technique for carbon dioxide (CO₂) capture and storage using CO₂ hydrates where CO₂ is stored as solid hydrates in the seabed ground, is attracting attention. Shallow sediments may be the most suitable seabed ground for CO₂ hydrate storage because these unconsolidated soil sediments satisfy the limitation for the low-temperature condition. Hence, the deformation properties and long-term stability of gas hydrate-bearing sediments during and after gas storage must be investigated. In this study, a series of undrained triaxial creep tests were conducted on artificially made CO₂ hydrate-bearing sand specimens to study the fundamental time dependent property of hydrate-bearing sediment. We extended an elasto-viscoplastic constitutive model by introducing a cohesion component and its degradation on surfaces and applied the proposed model to creep tests on gas hydrate-bearing sand.Three findings were obtained from the experiments and modelling. First, CO₂ hydrate-bearing sand specimens showed accelerated creep behavior, which was characterized by the creep stress ratio level, regardless of the hydrate saturation. Second, creep accelerated under undrained conditions before the stress reached the critical state line obtained from the monotonic loading tests, and the stress ratio at the occurrence of acceleration creep was higher for specimens with a higher hydrate saturation. Third, the elasto-viscoplastic constitutive model which considered the cementing effect of hydrates was able to well reproduce the undrained creep behavior of hydrate-bearing sand with different hydrate saturations under relatively high creep stress levels.     

Evaluation of liquefaction strength of Japanese natural sandy soil using triaxial and simple shear tests

The aim of this note is to reexamine the stress and strain conditions of cyclic triaxial and cyclic simple shear tests for evaluating the liquefaction strength of natural sandy soil. The conventional method of determining the liquefaction strength relies on the shear stress components. However, the use of stress invariants eliminates the need to specify the stress components, thereby making it a more rational approach for determining the liquefaction strength. A comparison of the results using the conventional stress ratio and the stress invariant ratio is done in this study. A strong correlation is seen between the liquefaction strengths obtained from the triaxial and the simple shear tests when using the stress invariant ratio. Despite this strong correlation, large scattering still remains due to the effects of the soil fabric of the natural sandy soil. A torsional hollow cylindrical device is used here to perform simple shear tests on undisturbed natural sandy soil.     

Triaxial strength of a fault breccia of weak rocks in a strong matrix

Triaxial strengths are difficult to obtain when the rock is very heterogeneous with respect to particle size and particle lithology and the samples are difficult to obtain and prepare. A fault breccia with clasts weaker than the matrix was investigated. It was found that its strength was related to the proportion of the blocks and the size of the specimen. The volumetric block proportion and texture coefficient were evaluated by core scanning and image processing. Triaxial compressive strength tests were conducted on different sized cores and different proportions of blocks. The most accurate estimations of the lower bound strength were obtained using larger diameter cores and samples with a higher volumetric block proportion.      

Creep and time-dependent damage in argillaceous rocks

This paper presents the results of laboratory tests on the time-dependent behaviour of three rocks characterized by a high proportion of clay particles. The viscosity of these sedimentary rocks was studied under different loading conditions in uniaxial compression: static or cyclic creep tests and quasistatic tests (low-loading strain rate) were performed across various orientations of fabric planes.The quasistatic tests showed similarities in the mechanical response of these three argillaceous rocks: a late phase of dilation and a linear development of volumetric deformation before the beginning of unstable crack propagation. The development of secondary and tertiary creep phases during the creep tests highlighted the existence of a deviatoric stress threshold, below which only primary creep is observed. Long-term creep tests also showed that the volumetric variation is not constant during the development of viscoplastic deformations.A microstructural analysis of thin sections extracted from specimens after the tests, gave evidence of cataclastic and granular creep. Damage to the argillaceous matrix occurs and no cracks were observed in the quartz and carbonate grains. This evidence was also demonstrated for tests with loading at a high strain rate.Finally, this study highlights the significant viscoplasticity of argillaceous rocks. The mechanical properties deteriorate rapidly when crack propagation becomes unstable and the viscoplastic strains seem to be due to clay particle slips, known as granular creep.     

Generic creep behavior and creep modeling of an aged surface support liner under tension

Polymer-based materials have been motivated to be an alternative support system element in the mining/tunneling industry due to their logistic and geotechnical benefits. Thin spray-on liner (TSL), a term to define the application of the material on the rock surface with a layer ranging from 2 mm to 10 mm in thickness, shows some promising results. TSLs are mainly composed of plastic, polymer, or cement-based ingredients to a certain proportion. This study intends to reveal the time-dependent response of TSL specimens, cured throughout 500 d, under four constant stress levels for stable laboratory conditions. The results were correlated using two interrelated equations to predict the material's service life (creep-rupture envelopes). The proposed correlations offered an insight into both the effective permanent support time and the strain amount at the liner failure. The time-dependent deformation of TSL, whose performance is highly responsive to creep behavior, was obtained so that the design engineers may use the findings to avoid the severe problems of material creep. Experimental data were also used to develop a Burgers (four-element) creep model. Since the liner has a nonlinear time-dependent behavior, creep models were built for each stress level separately. Subsequently, a generic equation was obtained using the nonlinear parametric dependencies. There is a good agreement between the proposed model and the experimental results. The proposed model can be used as a basis for future numerical studies related to the support behavior of aged surface support liners.     

Shear strength characteristics of geosynthetic reinforced rubber-sand mixtures

Shear strength characteristics of the geosynthetic-reinforced rubber-sand mixture (RSM) has been investigated by conducting Unconsolidated Undrained (UU) triaxial test. In the first part, a series of UU triaxial tests have been carried out to know the size effect of granulated rubber/tyre chips from seven different rubber sizes. RSM sample that provides higher strength, energy absorption capacity and stiffness is considered as the optimal size and has been used in the investigation on geosynthetic-reinforced RSM. In the second part, shear strength characteristics of geosynthetic-reinforced RSM has been investigated by varying proportions of rubber content (50% and 75% rubber by volume), type of geosynthetic (geotextile, geogrid and geonets), number of geosynthetics (1–4) layers, geosynthetic arrangement and confining pressure. The results demonstrate that RSM reinforced with geosynthetic has enhanced peak strength, failure strength and corresponding axial strain at failure. Fifty percent RSM reinforced by geotextile and 75% RSM reinforced by geonets with 4 layers of reinforcement, led to a maximum increase in shear strength. The strength and energy absorption capacity are doubled for the reinforced RSM's, and reduced the brittleness index values as close to zero, which depends on the type, number of layers and arrangement of geosynthetic.     

Numerical study on tensile failures of heterogeneous rocks

Many experimental results have demonstrated the apparent discrepancy of a rock material between its flexural tensile strength measured using various bending methods and its tensile strength measured using direct tension method or Brazil disc(BD) method.To understand the physical mechanism for such discrepancy,numerical simulation using the realistic failure process analysis(RFPA) is carried out in this work to simulate the tensile failure of heterogeneous rocks.Direct tension and semi-circular bend(SCB)tests are simulated using RFPA for rock materials with different levels of inhomogeneity,which is characterized by the homogeneity index of the Weibull distribution used in RFPA.The numerical results show that the discrepancy in the tensile strength values is caused by the inhomogeneity of the rock material.Furthermore,non-local failure criterion is adopted to calculate the characteristic length of the rock materials used in the simulation.It is shown that below a certain value of the homogeneity index,both the characteristic length and discrepancy between two types of tensile strengths of rock decrease with increase of the homogeneity index up to a critical value,at which the discrepancy disappears and the rock material is essentially homogeneous.     

Dynamic strength of rocks and physical nature of rock strength

Time-dependence of rock deformation and fracturing is often ignored.However,the consideration of the time-dependence is essential to the study of the deformation and fracturing processes of materials,especially for those subject to strong dynamic loadings.In this paper,we investigate the deformation and fracturing of rocks,its physical origin at the microscopic scale,as well as the mechanisms of the time-dependence of rock strength.Using the thermo-activated and macro-viscous mechanisms,we explained the sensitivity of rock strength to strain rate.These mechanisms dominate the rock strength in different ranges of strain rates.It is also shown that a strain-rate dependent Mohr-Coulomb-type constitutive relationship can be used to describe the influence of strain rate on dynamic rock fragmentation.A relationship between the particle sizes of fractured rocks and the strain rate is also proposed.Several time-dependent fracture criteria are discussed,and their intrinsic relations are discussed.Finally,the application of dynamic strength theories is discussed.     

Triaxial unloading test of rocks and its implication for rock burst

 The behaviour of rock deformation and its failure characteristics under loading and unloading conditions are substantially different. In this paper, triaxial unloading tests have been designed to simulate the unloading process during tunnel excavation in three kinds of rock (granite, migmatitic granite and limestone). The results show that elastic moduli obtained under unloading conditions are generally less than under loading conditions. The strength of the rock samples also decreases with an increasing rate of unloading. This study reveals that rock bursts during tunnelling in a high in-situ stress area could be controlled or reduced by lowering the excavation speed or applying precautionary measures to control the displacement of surrounding rocks. Received: 25 February 1999 · Accepted: 20 June 2000     

Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.     

桥用高强混凝土双轴徐变试验研究

为获得大跨度预应力混凝土箱梁腹板在双向预应力作用下的收缩徐变发展规律,分析不同预应力组合对收缩徐变效应的影响,研究了大跨度预应力箱梁腹板的常遇应力组合,并采用十字交叉梁开展了2种应力组合条件下的高强混凝土双轴徐变试验,即应力组合分别为14MPa和2MPa,6MPa和2MPa,对比开展了单轴压应力为6MPa、14MPa的高强混凝土收缩徐变试验,分析单、双轴徐变和应力组合对高强混凝土收缩徐变的影响。研究结果表明：应力组合对高强混凝土徐变影响显著,360d双轴应力条件下的徐变系数仅为相应单轴徐变系数的75%;采用现有收缩徐变预测模型不能较好预测高强混凝土的实际徐变发展过程,而采用指数函数具有较好的拟合精度。建议在大跨度连续箱梁设计和施工中,确保腹板竖向预应力水平控制在设计允许范围内。     

硬脆性岩石的流变长期强度及细观破裂机制分析研究

岩石的流变力学特性是影响水电坝基稳定性的重要因素,而流变长期强度是评价坝基岩体长期稳定性的重要指标 。根据大岗山水电站坝基辉绿岩的三轴流变试验结果,分别用等时应力–应变曲线簇法、非稳定蠕变判别法、流变体积应变法和加卸载流变残余应变法进行了硬脆性辉绿岩流变长期强度的分析。结果表明：对于完整辉绿岩的长期强度, 4 种方法得出的结果较为一致;而对于含裂隙辉绿岩,流变体积应变法得出的结果与实际情况更为相符。通过对破裂辉绿岩的电镜扫描观察,发现辉绿岩的长期宏观强度主要取决于岩体内部矿物颗粒镶嵌组合的牢固程度及矿物之间的胶结程度。     

Numerical evaluation of strength and deformability of fractured rocks

Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the co     

A new testing method for investigating the loading rate dependency of peak and residual rock strength

One of the important methods for investigating viscoelasticity is to measure the loading-rate dependency of peak strength; however, no experimental method has been established for accurately measuring the loading-rate dependency of peak strength from a small number of samples. In this study we propose such a method. A single sample is loaded at alternating strain rates to obtain stress–strain curves for both strain rates. The loading-rate dependency of peak strength obtained via this method was compared with the findings of conventional methods. The loading-rate dependency indicated for Tage tuff, Sanjome andesite and Akiyoshi marble was nearly identical to previous results obtained using conventional methods, including results obtained under confining pressure. The loading-rate dependency of peak strength in these experiments shows a close relation with the creep stress-dependency of creep life. We also investigated the loading-rate dependency of the stress–strain curve for the post-failure region for which few results have been published. Under confining pressure, the corrected stress–strain curve, obtained by multiplying the stress of the complete stress–strain curve obtained at the fast strain rate by a constant determined by the ratio between the fast strain rate and slow strain rate, is nearly coincident with the stress–strain curve for the slow strain rate. This is an interesting result and represents new knowledge that may help elucidate failure mechanisms in the post-failure region. The loading-rate dependency of stress in the alternating strain rate experiment proposed here was most clearly observed when the stress–strain curve becomes flat, parallel to the strain axis. Some improvements to the proposed method are required to enable accurate investigations of loading-rate dependency during low stresses immediately after initiation of loading or during the abrupt decreases in stress that occur following peak strength.     

The flattened Brazilian disc specimen used for testing elastic modulus, tensile strength and fracture toughness of brittle rocks: analytical and numerical results

The flattened Brazilian disc specimen is proposed for determination of the elastic modulus E, tensile strength σ_t and opening mode fracture toughness K_IC for brittle rocks in just one test. This paper is concerned with the theoretical analysis as well as analytical and numerical results for the formulas. According to the results of stress analysis and Griffith's strength criteria, in order to guarantee crack initiation at the centre of the specimen, which is considered to be crucial for the test validity, the loading angle corresponding to the flat end width must be greater than a critical value (2α20°). The analysis shows that, based on the recorded complete load–displacement curve of the specimen (the curve should include the ‘fluctuation’ section after the maximum load), E can be determined by the slope of the section before the maximum load, σ_t by the maximum load, and K_IC by the local minimum load immediately subsequent to the maximum load. The relevant formulas for the calculation of E, σ_t, K_IC are obtained, and the key coefficients in these formulas are calibrated by finite-element analysis. In addition, some approximate closed-form formulas based on elasticity are provided, and their accuracy is shown to be adequate by comparison with the finite-element results.     

Accurate time-dependent analysis of concrete bridges considering concrete creep, concrete shrinkage and cable relaxation

This paper proposes a new relaxation model for steel tendons based on the equivalent creep coefficient to enable the accurate estimation of losses of cable forces. The equivalent creep coefficient works not only in the case of intrinsic relaxation but also under various boundary conditions. Based on the proposed relaxation model, an accurate finite element analysis of the time-dependent behavior of concrete bridges considering concrete creep, concrete shrinkage and cable relaxation is devised based on the time integration method. Concrete members are modeled by beam elements while tendons are modeled by truss elements with nodes connected to the beam axis by perpendicular rigid arms. Then the individual and combined effects of concrete creep, concrete shrinkage and cable relaxation on the long-term performance of concrete structures are investigated. It is found that the proposed relaxation model and time integration method can provide a reliable method for time-dependent analysis. The numerical results obtained indicate that the interactions among these factors should be considered carefully in analyzing the long-term performance of concrete bridges.     

复杂层状结构岩石抗剪强度试验方法研究

岩石的抗剪强度是边坡稳定性、地基强度和稳定性评价中的一项非常重要的指标,不同类型的岩体,岩石的抗剪强度特性是不同的,其中复杂层状结构岩石的抗剪强度是岩土工程研究的重点内容之一.工程中经常采用的岩石剪切试验方法包括室内试验和现场原位试验,由于试验约束条件不同,试验结果存在一定的差异.本文以某核电工程为例,该项目分别采用了...     

分级加载条件下岩石流变特性的试验研究

以红砂岩为例,采用重力加载式流变仪,在分级加载条件下对岩石的蠕变特性进行了单轴压缩蠕变试验研究,重点观察和分析了蠕变条件下岩石的弹性模量和泊松比的变形效应,同时对其他时效变形特点进行了分析。试验结果表明,侧向稳定蠕变阶段的应力门槛值低于轴向稳定蠕变的应力门槛值;侧向蠕变有明显的加速蠕变阶段,且比轴向加速蠕变阶段出现得早,而轴向蠕变的第三阶段则不太明显,一经出现试样随即破坏。在单体分级加载条件下,受蠕变的影响,随应力水平和变形量的增加,岩石的瞬时弹性模量有明显的增大,泊松比的变化则更为显著。