Basic Facts Concerning Strength of Sandstone

ＢａｓｉｃＦａｃｔｓＣｏｎｃｅｒｎｉｎｇＳｔｒｅｎｇｔｈｏｆＳａｎｄｓｔｏｎｅ￥ＺａｖｏｄｃｈｉｋｏｖａＭａｒｉａ（Ｓａｎｋｔ－ＰｅｔｅｒｓｂｕｒｙＭｉｎｉｎｇＩｎｓｔｉｔｕｔｅ，Ｓａｎｋｔ－Ｐｅｔｅｒｓｂｕｒｇ１９９０２６；）（Ｅｅｐａｒｔｍｅｎｔ...     

Strength Properties of Sandy Soil–Cement Admixtures

Soil stabilization with cement is a good solution for the construction of subgrades for roadway and railway lines, especially under the platforms and mostly in transition zones between embankments and rigid structures, where the mechanical properties of supporting soils are very influential. These solutions are especially attractive in line works where other ground improvement techniques are extensive and, therefore, very expensive. On the other hand, the economic and environmental costs of such works should be optimized with good balances between excavation and embankment volumes. For this purpose, the improvement of locally available soils can bring great advantages, avoiding a great amount in borrowing appropriate material, as well as the need of disposing huge volumes in deposits. This paper focus on the characteristics of two soils, Osorio sand and Botucatu residual sandstone, which can be converted to well acceptable materials for this purpose, if stabilized with cement. The study of soil stabilization with cement relies on the quantification of the influence of percentage of cement and porosity adopted in the admixing process for different state and stress conditions. This influence will be evaluated from the analysis of unconfined compression strength (UCS or q _u ) test results. This experimental framework will enable a good definition of mechanical parameters used in design of foundations and subgrades of railways platforms and for their execution quality control.     

Strength characteristics and mechanisms of salt-rich soil–cement

Salt-rich soft soils have not only general characteristics of common soft soils, but also contain high contents of Mg²⁺, Cl^?, and SO₄^2?, which have negative effects on deep mixing method using cement to treat soft soils. Laboratory and field tests were conducted to investigate the effects of changing cement incorporating ratio, water content, cement mixing ratio, and contents of Mg²⁺, Cl^?, and SO₄^2? on the unconfined compressive strength of the salt-rich soil–cement. The microstructure of soil–cement and the mechanism for the strength change of salt-rich soil–cement were investigated using X-ray diffraction, scanning electronic microscopy (SEM), and backscattered diffraction technology. It was found that an increase of cement incorporating ratio enhanced the strength of soil–cement but reduced its strength when water is added. Different amounts of Mg²⁺, Cl^?, and SO₄^2? not only caused the difference in the microstructures of salt-rich soil–cement but also influenced the soil–cement strength.     

Prediction of Unconfined Compressive Strength of Soil–Cement at 7?Days

Unconfined compressive strength (UCS) of cement stabilized bases was collected from a number of highway construction projects in Thailand. Results from the statistical analysis indicated that the most important factors affecting the UCS were the CBR and the water to cement ratio. The UCS was however independent on the dry density. A statistical model was developed in the study to predict the UCS of cement stabilized bases. A model was developed based on the following criteria: (1) the dry density of the sample shall be greater than 95 percent of the maximum dry density based on the modified Proctor compaction, (2) samples shall be soaked for at least 2 h prior to testing, and (3) the CBR shall be measured at 0.1 inch (2.5 mm) penetration.     

Some Problems Related to the Strength of Loess at Gubaizui

The paper presents the results of the laboratory research about the shear strength and the structural strength of loess at Gubaizui. Gubaizui loess behaves like creep, and its long time shear strength is lower than its peak value(cq) during direct shear tests, the ratio between these value is about 0.51~0.75(when normal stress σ=0.1~0.5MPa). It is found that the structural strength of Gubaizui loess is very high, from the compression curves(one-dimensional consolidation test) it is obtained that strength is about 0.85~2.38MPa.     

Effect of Strain Rate on the Strength Characteristics of Soil–Lime Mixture

The effects of rate of strain on strength and deformation characteristics of soil–lime were investigated. Five strain rates (0.1, 0.8, 2.0, 4.0 and 7.0 %/min), five lime contents (0, 3, 6, 9 and 12 %) by dry soil weight and three cell pressures (100, 200 and 340 kN/m²) were carried. Triaxial tests, under unconsolidated condition, were used to study the effect of strain rate on strength and initial modulus of elasticity of soil and soil–lime mixture after two curing periods 7 and 21 days, respectively. A total of 405 triaxial specimens have been tested, where 225 specimens have been tested with first curing period (7 days). The testing program includes nine specimens for each strain rate, and each lime content was carried out, including natural soil with zero lime content. Another set of triaxial tests with a total of 180 specimens for the second curing period (21 days) was prepared at optimum moisture content, and the corresponding maximum dry density was also tested. The effects of strain rate and curing period on each of stress–strain behavior, type of failure, deviator stress at failure, cohesion and angle of internal friction and initial modulus of elasticity were studied thoroughly for the natural soil as well as soil–lime mixtures. For natural soil, the test results showed that the undrained shear strength, the initial modulus of elasticity and the cohesion increase significantly as the strain rate increase, while for soil–lime mixture at different curing periods, the undrained shear strength, initial modulus of elasticity and the cohesion increases to a maximum and then decreases as the strain rate and lime content increase. Also, the same variables and angle of internal friction increase with increasing curing period.     

Strength and stiffness of compacted chalk putty–cement blends

Chalk breaks easily when subjected to human action such as mechanical handling, earthworks operations or pile installation. These actions break the cemented structure of chalk, which turns into a degraded material known as putty, with lower strength and stiffness than the intact chalk. The addition of Portland cement can improve the behaviour of chalk putties. Yet, there are no studies determining the tensile strength of chalk putty–cement blends, the initial stiffness evolution during the curing time and other design parameters such as friction angle and cohesion of this material. This paper addresses this knowledge gap and provides an interpretation of new experimental results based on the dimensionless index expressed as the ratio between porosity and volumetric content of cement (η/C_iv) or its exponential modification (η/C_iv^a). This index aids the selection of the amount of cement and density for key design parameters of compacted chalk putty–cement blends required in geotechnical engineering projects such as road foundations and pavements, embankments, and also bored concrete pile foundations.

     

Sandbox Experimental Study on the Influence of Rock Strength and Gravity on Formation of Thrusts

INTRODUCTIONRock deformation is normally explained by tec-tonic stress as rock deformation results fromthe tec-tonic stress field. The classic tools that explainedfracture mechanisms were the Coulomb shear fracturerule and the Anderson mode derived fromit (Zhu,1999) . More and more studies have shown that it isdifficult to explain rock deformation in a large strainrange using only the Coulomb shear fracture rule( Waltham,2002 ; Gutscher et al .,2001 ; Tikoff andWojtal ,1999) . As a ver…     

Rheological Strength of UHP Eclogite from Dabieshan: Evidences from High p-T Experiments

ＴｈｅｈｙｐｏｔｈｅｓｉｓｏｆｔｒａｎｓｆｏｒｍａｔｉｏｎｏｆｂａｓａｌｔｔｏｅｃｌｏｇｉｔｅａｔｔｈｅｃｏｎｔｉｎｅｎｔａｌＭｏｈｏｄｉｓｃｏｎｔｉｎｕｉｔｙｉｎ 196 0ｓｅｖｅｒｂｒｏｕｇｈｔｂｒｏａｄｉｎｔｅｒｅｓｔｓｔｏｇｅｏｓｃｉｅｎｃｅｃｏｍｍｕｎｉｔｙ (ＲｉｎｇｗｏｏｄａｎｄＧｒｅｅｎ ,1996 ;ＧｒｅｅｎａｎｄＲｉｎｇｗｏｏｄ ,1972 ;ＩｔｏａｎｄＫｅｎｎｅｄｙ ,1971;ＫｕｓｈｉｒｏａｎｄＡｏｋｉ,196 8) .Ｔｈｉｒｔｙｙｅａｒｓｌａｔｅｒ ,ｗｉｔｈｔｈｅｄｉｓｃｏｖ ｅｒｉｅｓｏｆｃｏｅｓ…     

Ionic Strength Dependence of Rare Earth – NTA Stability Constants at 25°C

Observations of competitive complexation of NTA by Cu2+ and rare earth element (REE) ions are used to determine REE-NTA stability constants at ionic strengths between 0.1 and 5.0 molar. Although REE stability constants change markedly with ionic strength, differences in the ionic strength dependence of REE-NTA stability constants across the rare earth element series are small. The ionic strength dependence of log1 for Y and REEs with NTA at 25 °C can be described as: log1(M) = log1(M)0 - 9.198 I1/2/(1+B I1/2)+C I + D I3/2, where 1(M) = [MNTA°][M3+]-1[NTA3-]-1, I is ionic strength, B = 1.732, C = 0.1596, D = 0.0816, and log1(M)° is the metal-NTA stability constant at zero ionic strength.     

The Mechanical Behaviour of a Pyroclastic Rock: Yield Strength and “Destructuration” Effects

Summary   A research programme on the mechanical behaviour of a homogenous volcanic tuff found in the centre of the city of Naples (Italy) was carried out at the University of Naples a few years ago. Isotropic and drained triaxial tests were performed in a very wide range of confining pressures (up to 60 MPa). After presenting the stress-strain curve pattern and the mean stress influence on the shear behaviour, the paper focuses on the definition of a strength criterion and of the yield surface for this material. Some tuff samples were subjected to isotropic compression tests up to a confining pressure approximately twice as high as the isotropic yield stress; they were subsequently unloaded and subjected to drained triaxial tests. Partial loosening of the interparticle bonds (“destructuration”) was observed. The paper also compares the mechanical behaviour of intact and “destructured” samples, emphasising the effects of the structure on strength and yield.     

An Integrated Numerical Modelling–Discrete Fracture Network Approach Applied to the Characterisation of Rock Mass Strength of Naturally Fractured Pillars

Naturally fractured mine pillars provide an excellent example of the importance of accurately determining rock mass strength. Failure in slender pillars is predominantly controlled by naturally occurring discontinuities, their influence diminishing with increasing pillar width, with wider pillars failing through a combination of brittle and shearing processes. To accurately simulate this behaviour by numerical modelling, the current analysis incorporates a more realistic representation of the mechanical behaviour of discrete fracture systems. This involves realistic simulation and representation of fracture networks, either as individual entities or as a collective system of fracture sets, or a combination of both. By using an integrated finite element/discrete element–discrete fracture network approach it is possible to study the failure of rock masses in tension and compression, along both existing pre-existing fractures and through intact rock bridges, and incorporating complex kinematic mechanisms. The proposed modelling approach fully captures the anisotropic and inhomogeneous effects of natural jointing and is considered to be more realistic than methods relying solely on continuum or discontinuum representation. The paper concludes with a discussion on the development of synthetic rock mass properties, with the intention of providing a more robust link between rock mass strength and rock mass classification systems.     

Upper Limit for Rheological Strength of Crust in Continental Subduction Zone： Constraints Imposed by Laboratory Experiments

The transitional pressure of quartz-coesite under the differential stress and highly-strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega-pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi-brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle-plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10-12 s-l) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10-14 s-1). The average rheological stress (1 600 MPa) at the strain rate of 10-12 s-1 stands for the ultimate differential stress of UHPM rocks in the semi-brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0-14 -10-13 s-l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pressure condition due to the effect of the differential stress.     

Strength estimation and stress–dilatancy characteristics of natural gas hydrate-bearing sediments under high effective confining pressure

Acta Geotechnica - Gas production by depressurization can significantly increase the effective stress in hydrate-bearing sediments. Therefore, strength and deformation characteristics of sediments...     

A Comparative Study of Different Approaches for Factor of Safety Calculations by Shear Strength Reduction Technique for Non-linear Hoek–Brown Failure Criterion

The shear strength reduction technique is becoming more and more popular to determine the factor-of-safety for geotechnical constructions, especially for slopes. At present, two in principal different procedures are used to apply the numerical shear strength reduction technique for materials characterised by non-linear failure envelopes, like the Hoek–Brown criterion. One procedure is based on the determination on local stress and strength values, whereas the other is based on a global linearization of the non-linear failure envelope. This article shortly describes and discusses these two different procedures and compares results for a broad spectrum of parameter constellations based on slope stability calculations. The local approach is physically more correct. The global approach can be considered as a first approximation. A comparison of both methods reveal that the global approach in comparison to the local approach, can leads to a deviation of up to 15?% in both directions. If one considers the local approach as the ‘correct’ one, depending on the parameters the results of the global approach can lie on the safe or unsafe site. The practical conclusion is that evaluation of slope stability using the global approach can result in uneconomic slope design or overestimation of safety margin. The use of the local approach instead of the global should be preferred. In case of small safety margins (e.g. 20?% or less) the use of the local approach is strictly recommended.