Strain-controlled Cyclic Triaxial Tests Research Articles

Evaluation of dynamic properties of sandy soil at high cyclic strains

Dynamic loading conditions, such as earthquakes, may result in the generation of high shear strain (>5%) in the soil. Conventionally, dynamic properties of soils are estimated from the tests conducted up to a shear strain of 1% by considering Symmetrical Hysteresis Loop (SHL). However, it is commonly observed that the hysteresis loops become progressively asymmetric with increasing shear strain, which leads to the over- or under-estimation of the conventionally evaluated dynamic properties. Hence, it is necessary to adopt a modified methodology of evaluating the dynamic properties of saturated sands based on the actual Asymmetrical Hysteresis Loop (ASHL). Strain-controlled cyclic triaxial tests have been conducted, for a peak shear strain range of 0.015–4.5% at 1Hz loading frequency, on test specimens prepared at different relative density (30–90%) and confining stress (50–150kPa). Although, the shear modulus evaluated considering SHL and ASHL are on close agreement, the damping ratio evaluated considering SHL is approximately 40–70% lesser than that obtained by considering ASHL. Moreover, in contrast to the classical curves as largely applied in geotechnical engineering, a noticeable decrement of the damping ratio is observed beyond 0.75% shear strain.

Cyclic Behavior and Liquefaction Potential of Renusagar Pond Ash Reinforced with Geotextiles

AbstractIn this study, cyclic behavior and dynamic properties of Renusagar pond ash samples reinforced with geotextiles have been studied. Pond ash samples were collected from the upstream side of the pond ash embankment. Laboratory tests have been carried out to obtain the material properties of the pond ash. Strain-controlled cyclic triaxial tests have been carried out on reconstituted pond ash samples with and without geotextiles, at varying confining pressures (70, 100, and 150 kPa), loading frequencies (1 and 0.5 Hz), and cyclic shear-strain amplitudes (0.5, 0.8, and 1.05%). The woven geotextiles were used as reinforcing materials, which were placed in the pond ash samples in one and three layers. Results show that the pond ash samples reinforced with geotextiles exhibit better friction angle, drainage properties, and liquefaction resistance. It may be concluded that pond ash reinforced with geotextiles can be used effectively as embankment construction material.

Behavior of Expansive Soil Treated with Steel Slag, Rice Husk Ash, and Lime

AbstractThis paper presents the stabilization of expansive soil (C) with steel slag (SS), rice husk ash (RHA), and quick lime (L). The geotechnical properties, shearing strength characteristics, and dynamic properties of stabilized soil have been studied. The optimum mix is found to be in the proportion of 65C+20SS+5L+10RHA. The unconfined compressive test shows that strength of treated soil increases about 45 and 90% for uncured and cured samples at 30 days. The soaked California bearing ratio (CBR) test shows that the strength increases with increasing curing periods from 7 to 30 days. Laboratory strain-controlled cyclic triaxial tests have been carried out under undrained conditions to study the dynamic properties of stabilized soil for different strain amplitudes and frequencies. The stiffness of stabilized soil increases to 58–78% as compared to clay soil. This study discussed the suitability of stabilized material for subgrade pavement.

Static and cyclic properties of clay subgrade stabilised with rice husk ash and Portland slag cement

In the present study, clay soil collected from new Banda, Uttar Pradesh, India has been treated with rice husk ash (RHA) and Portland slag cement (PSC). Based on unconfined compressive strength test results, the optimum mix obtained is of 82.5%Soil+7.5%PSC+10%RHA. The increase in strength of the optimum mix is about 29.8%, 37.2% and 48.55% for a curing period of 7, 14 and 30 days, respectively. The soaked California bearing ratio (CBR) test gives about 91.75% higher values as compared to unsoaked CBR test for a curing period of 30 days. Strain-controlled cyclic triaxial tests were conducted to study the variation of degradation index, shear modulus and damping ratio of the optimum mix with number of cycles for strain amplitudes of 0.4%, 0.6%, 0.8% and 1% and for frequencies of 0.2 and 1 Hz at an effective confining pressure of 100 kPa. It is observed that the degradation index decreases at a fast rate for the first 25–50 cycles. From the study, it is concluded that the aforementioned mix may be suitable for pavement subgrade material.

MODULUS REDUCTION AND DAMPING CURVES FOR SAND OF SOUTH-EAST COAST OF INDIA

Adequate information on dynamic soil properties, especially strain dependent shear modulus (G) and damping ratio (ξ) for each soil layer are the essential input data for seismic ground response analysis and soil-structure interaction studies. In the present study, the shear modulus and damping ratio of sand are estimated for a wide range of strains based on undrained strain-controlled cyclic triaxial tests. The bender elements are also utilized in the cyclic triaxial test to estimate the low strain shear modulus. For this purpose, the soil samples are taken from a nuclear power plant site located at the south-east coastal region of India. Based on the experimental results, an empirical expression is developed to calculate the maximum shear modulus, Gmaxas function of void ratio and effective confining stress. Predictive relationships are also developed for estimating normalized shear modulus and damping ratio curves for the sand. The predictive relationships are based on the hyperbolic model and cyclic triaxial test results. The developed modulus reduction and damping ratio curves from the predictive relationships are compared with the previously available curves in the literature.

DISCRETE ELEMENT ANALYSIS OF THE RESPONSE OF GRANULAR MATERIALS DURING CYCLIC LOADING

Soil can experience cyclic or repeated loadings in a range of situations and prediction of cyclic soil response is obviously important to geotechnical engineers. The particulate nature of soil results in highly complex, non-linear response characteristics, and developing models that can capture soil response under cyclic loading is non-trivial. The distinct element method (DEM) can be used to study the fundamental, particle-scale mechanics of granular materials, and offers much promise as a tool to advance understanding of soil response. The first stage in adopting DEM to model cyclic soil response is to quantitatively demonstrate that a DEM model can replicate physical test data, as analytical validation of DEM models for random assemblies of particles under repeated loading is not viable. This paper describes a series of strain-controlled cyclic triaxial tests on an ideal granular material (steel spheres) that were used to validate the capability of an axi-symmetric DEM model to analyse cyclic loading. The DEM model was then used in a parametric study to examine the particle-scale mechanics of the response of specimens of uniform spheres to 50 cycles of loading with various strain amplitudes. The distribution of contact force orientations and magnitudes during testing was examined. The simulations indicate that both the fabric anisotropy and coordination number continued to evolve over the 50 cycles considered. While the variation in the macro-scale response was less marked, there is a clear relation between the micro-scale parameters and the overall specimen response.

Pore pressure generation in silty sands during cyclic loading

Recent developments in studies of soil response to earthquake loadings have made it possible to incorporate the rates of pore water pressure build-up in soils in to nonlinear response analyses of the grounds. Such pore pressure changes help in computing the changes in stress-strain behaviour of soils in the deposit progressively as the earthquake progresses. The rate and magnitude of pore pressure generation in soils during seismic loading will have important effects on the shear strength, stability, and settlement characteristics of a soil mass, even if the soil does not liquefy. The results in terms of pore pressure response in soils from a series of experimental investigations using strain-controlled cyclic triaxial tests on soils samples collected from liquefied sites are presented in this paper. The effect of relative density, amplitude of cyclic shear strain, number of loading cycles, confining pressure and frequency of cyclic loading on the pore pressure build-up are studied. Analytical expressions are proposed using regression analysis to define mean relationships between normalized pore water pressure and normalized cycles for the prediction of pore water pressure build-up in silty sands. Also, the pore water pressure build-up in soils is independent of frequency of loading.

The effect of plastic fines on the pore pressure generation characteristics of saturated sands

Pore water pressure generation during earthquake shaking initiates liquefaction and affects the shear strength, shear stiffness, deformation, and settlement characteristics of soil deposits. The effect of plastic fines (kaolinite) on pore pressure generation in saturated sands was studied through strain-controlled cyclic triaxial tests. In addition to pore pressure generation, this experimental study also focused on evaluating the threshold shear strain for pore pressure generation and the volumetric compressibility of specimens during pore pressure dissipation. The results reveal that specimens having up to 20% plastic fines content generated larger values of pore water pressure than clean sand specimens. At 30% fines content, the excess pore water pressure decreased below that of clean sand. The threshold shear strain, which indicates the strain level above which pore pressures begin to generate, was assessed for different kaolinite–sand mixtures. The threshold shear strain was similar for 0–20% fines ( γ t∼0.006–0.008%), but increased to about 0.025% for 30% fines. The volumetric compressibility, measured after pore pressure generation, was similar for all specimens. The transition of behavior at fines contents between 20% and 30% can be attributed to a change in the soil structure from one dominated by sand grains to one dominated by fines.

Cyclic triaxial tests on frozen sand

Strain-controlled cyclic triaxial tests were performed on a one-size silica (Ottawa) sand artificially frozen into 71.1-mm-diameter cylindrical samples. Ice-saturated samples with three different sand contents were tested under the following conditions: axial strains ranging from 3 × 10 −3 to 3 X 10 −2%, confining pressures from zero to 1.378 MPa, frequencies of 0.05–5.0 cps and temperatures from −1 to −10 °C. Test equipment included (1) an MTS electrohydraulic closed-loop testing system which applies the load to the sample, (2) a triaxial cell completely immersed in a low-temperature coolant for temperature control, (3) a refrigeration unit for control of the coolant temperature and constant coolant circulation and (4) measuring devices including an LVDT and load cell, together with recording devices such as a digital multimeter, an oscilloscope, a strip-chart recorder and a minicomputer. Test results indicate that the dynamic Young's modulus increases with increasing frequency, confining pressure and sand content, but decreases with increasing strain and temperature. The damping ratio decreases with increasing frequency, sand content and lower temperatures. The influence of confining pressures and axial strain on the damping ratio are less explicit for the ranges considered. The experimental results are compared with data from other sources.