Strain-controlled Triaxial Tests Research Articles

Utilization of Different Additives in Improving Sandy Soil against Liquefaction

One of the main risks in low-densified sandy soils with the presence of water and an external force such as an earthquake is the generation of liquefaction. The influence of several types of reinforcement on liquefaction resistance, such as polypropylene fibers, geofibers, cement and polypropylene fibers with cement is shown in this study. Cyclic stress-controlled triaxial tests and cyclic strain-controlled triaxial tests were performed on saturated samples with and without reinforcements under undrained conditions. Cemented specimens were prepared with cement contents ranging from 0% to 3% by weight of dry sand and then cured for 3 days. The lengths of polypropylene fibers are 10 mm and 20 mm, respectively. The fibers were mixed with dry sand– cement mixes containing 0.50% and 1.00% by weight, respectively. Geofiber specimens were prepared in various arrangements. It was found that the liquefaction improvement factor (LIF) increased when fiber content and fiber length increased. The addition of geofibers increased the liquefaction resistance, as the number of layers increased. The addition of 3%C+1%F provided the best liquefaction resistance in this study compared with other additives. Finally, the reinforcement with cement and fibers is crucial for liquefaction resistanceof bitumen mastic should be considered beside the asphalt mixture performance and the bitumen rheological behavior. KEYWORDS: Liquefaction, Shear modulus, Cyclic stress, Geofiber, Polypropylene fiber.

Constitutive Modeling of Monotonic Behavior of Clays: Mathematical Formulation, Numerical Implementation and Experimental Verification

Foundations constitute a significant part of the design of civil engineering systems. Geotechnical considerations are particularly important in identifying the conditions leading to instability of shallow and deep foundations under various loadings. In the case the foundation layer is clay, one should identify the conditions leading to failure of clay soil upon loading. The most common way of doing so is to theorize the constitutive behavior of the soil using mathematical equations. In this study, constitutive modeling of clays under monotonic loadings is presented using the Generalized Plasticity Theory. Numerical formulation is summarized in terms of governing equations which are solved for each load step by an explicit integration method which is implemented into a computer program. Elasto-plastic constitutive matrix is derived based upon the inversion of strain-stress relationship without using a yield or a potential function in the model which is used to get the stress-strain incremental relationship. Plastic strains are then calculated using a non-associative flow rule. Subsequently, a number of drained and undrained strain-controlled triaxial tests are simulated to verify the model and its implementation. Simulation results demonstrate the effectiveness and the capability of the model to capture static behavior of normally and overconsolidated clays.

Anisotropically Consolidated Undrained Compression Test on Residual Soil

In Malaysia, the shear strength and corresponding deformation characteristics of residual soils are normally investigated by isotropically consolidated undrained (CIU) triaxial test. In the present study, anisotropically consolidated undrained compression (CAU) tests were carried out and explored in an attempt to facilitate understanding on the fundamental behaviours of a selected residual soil. A triaxial apparatus setup which was capable of performing the CAU test was first developed. Through a monotonic strain-controlled triaxial test apparatus, soil properties and stress paths could be obtained experimentally. It was confirmed that the newly fabricated triaxial apparatus could provide reasonable experimental results. As distinguished from the isotropically consolidated undrained compression (CIU) test, anisotropic stresses were induced in the stress path test. It follows that the soil responses were different between the CIU and CAU tests. The coefficient of earth pressure at rest (Ko) values as obtained in the present experiment showed a good agreement with the values estimated using equation reported by Jaky (1948).

A Constitutive Model of Sandstone Considering the Post Peak Behavior

AbstractIn rock engineering, evaluating the post-peak strength and deformation of rock is necessary. To explore the elasto-plastic behavior of sandstone in the post-peak stage, a series of strain-controlled triaxial tests were conducted under different confining pressures. According to the post-peak characteristics, a constitutive model based on nonlinear elasticity and generalized plasticity is proposed. This proposed model is characterized by the following features: (1) Nonlinear elasticity is observed under hydrostatic and shear loading; (2) the associated flow rule is followed; (3) substantial plastic deformation occurs during shear loading; and (4) post-peak softening behavior is accurately predicted. This model requires twelve material parameters, three for elasticity and nine for plasticity. The proposed model was validated by comparing the triaxial test results of Mushan sandstone at different hydrostatic pressures under dry and saturated conditions. In addition, the model is versatile; it can simulate the deformational behavior of two other sandstones. In summary, the proposed model can reasonably predict the complete stress–strain curve of sandstone.

A new bounding-surface plasticity model for cyclic behaviors of saturated clay

A new combined isotropic–kinematic hardening rule is proposed based on the concept of the generalized homological center and the generalization of Masing’s rule. The key point of the new hardening rule is that the unloading event can be treated as if it were virgin loading through taking the stress reversal point as the new generalized homological center of the bounding surface. Therefore, a new simple bounding-surface plasticity model with three important features for the cyclic behaviors of saturated clay is developed. Firstly, according to the movement of the generalized homological center, the model can harden not only isotropically but also kinematically to account for the anisotropy and memory the particular loading events. Secondly, the continuous cyclic loading is divided into the first loading, unloading and reloading processes and they are treated differently when calculating the hardening modulus to describe the soil responses accurately. The third feature is taking the generalized homological center as the mapping origin in the mapping rule to reflect the plastic flow in the unloading event. The behaviors of saturated clay for the monotonic and cyclic stress-controlled and strain-controlled triaxial tests are simulated by the model. The prediction results show an encouraging agreement with the experimental data.

On the role of grain shape in static liquefaction of sand–fines mixtures

This technical note presents new experimental evidence for the role of grain shape in the static liquefaction behaviour of sand–fines mixtures. The laboratory experiments comprise a series of strain-controlled triaxial tests on several types of sand–fines mixtures that are intended to clarify a concern remaining in a previous study by the same authors: whether the observed effect of grain shape was due to the differences in hardness of the added fines. The comparison of new test results with the previous ones confirms that it is grain shape rather than grain hardness that is responsible for the observed differences in overall behaviour. Adding fines of rounded shape into a clean sand indeed results in a marked increase of liquefaction potential compared with adding fines of more angular shape into the same sand. It is also shown that the concept of the combined roundness is effective and useful for quantifying in an integrated manner the influence of grain shape and fines content on the overall friction angle of sand–fines mixtures at critical state.