Cyclic threshold shear strain in pore water pressure generation in clay in situ samples

The threshold level of the cyclic shear strain required for pore water pressure generation in clay samples is examined through the results of torsional hollow cylinder cyclic shearing tests according to JGS 0543-2009. The study confirms the previous results, namely, that the threshold cyclic shear strain is dependent on the effective consolidation stress and plasticity index (I_p). The average and standard deviations in the estimated threshold strain levels are 0.038?±?0.023% (I_p?<?30, σ′_c?$\leqq$?100?kN/m²), 0.047?±?0.016% (I_p?<?30, σ′_c?>?100?kN/m²), 0.079?±?0.028% (30?$\leqq$?I_p?<?5?0), and 0.143?±?0.041% (I_p?$\geqq$?50). As was found in past research, the levels of threshold strain for pore water pressure generation for clay are larger than those for clean sand. An increase in pore water pressure is only observed when the stiffness is reduced to around 80% of its initial value. This delay occurs because there is a difference between the cyclic threshold strain of the pore water pressure generation, γ_tp, and the cyclic threshold strain of the stiffness degradation, γ_td. Since the test procedure of JGS 0543-2009 is a standard scheme in the practical design process, it is expected that more data will become available in the near future which will allow for further discussions on threshold strain.     

A simplified three-dimensional extension of Hoek-Brown strength criterion

The Hoek-Brown(HB) strength criterion has been applied widely in a large number of projects around the world.However,this criterion ignores the intermediate principal stress σ_2.Many evidences have demonstrated that the rock strength is dependent on σ_2. Thus it is necessary to extend the HB criterion into a three-dimensional(3D) form.In this study,the effect of σ_2 on the strength of rocks is identified by reviewing the true triaxial tests of various rock types reported in the literature.A simple 3D strength criterion is developed.The modified criterion is verified by the true triaxial tests of 13 rock types.The results indicate that the modified criterion can achieve a good fit to most of rock types.It can represent a series of criteria as b varies.For comparisons,several existing 3D versions of the HB criterion are selected to predict the strengths of these rock types.It is indicated that the proposed criterion works better than other criteria.A substantial relationship between parameter b and the unconfined compressive strength is established,which guarantees that the proposed criterion can still work well even in the absence of true triaxial test data.     

Ultimate capacity prediction of axially loaded CFST short columns

Composite columns have superior strength and ductility performance, and they have become more widely accepted in the engineering applications. However, the filled tubular columns require more attention. This study aims to present a new formulation for the axial load carrying capacity (N _u ) of circular concrete filled steel tubular (CFST) short columns having various geometrical and material properties. Although there have been some empirical relations for predicting N _u in the literature, genetic algorithm based explicit formulation is not available. In the current study, 314 comprehensive experimental data samples presented in the previous studies were examined to prepare a data set for training and testing of the prediction model. The prediction parameters were selected as outer diameter of column (D), wall thickness (t), length of column (L), compressive strength of concrete (f _c ), and yield strength of steel (f _y ). The prediction model was obtained by means of gene expression programming (GEP). The proposed model was compared with available ones presented in the current design codes (ACI, Australian Standards, AISC, AIJ, Eurocode 4, DL/T, and CISC) and some existing empirical models proposed by researchers. The prediction performance of all models were also evaluated by the statistical parameters. The results indicated that the GEP model was much better than the available formulae, yielding higher correlation coefficient and lower error.