A new method for measuring internal pore water pressure (PWP) is introduced to determine the critical state line (CSL) in partially frozen sand, investigating the influence of temperature and strain rate on the critical state parameters. A series of consolidated undrained and drained triaxial tests, along with internal PWP measurements, were conducted on both dense and loose specimens under different temperatures and strain rates. Similarly to unfrozen sand, a unique CSL was established for the partially frozen sand at -3 degrees C in both stress (q-p ') and void ratio (e-p ') space. The results show that the critical state friction angle (phi cs ') is not affected by temperature (warmer than -5 degrees C) and strain rate, while the critical state cohesion (ccs ') varies with temperature, strain rate and failure mode. The ccs ' increases with decreasing temperature from 23 degrees C to -3 degrees C and to -10 degrees C, but decreases to zero when the strain rate was reduced from 1%/min to 0.1%/min. In e-p ' space, the slope of CSL could be associated with the dilation of partially frozen sand, which increases with decreasing temperature and increasing strain rate, potentially due to the increased contact area between the pore ice and sand grains.

The complex distribution characteristics of root-soil composites pose challenges in understanding their mechanical behaviour during conservation tillage. This study aims to analyse mechanical parameters of root-soil composites at different soil depths, considering root distribution, and establish an empirical critical state model. Three layers were defined based on root density distribution: Shallow Aggregated Root Zone (SARZ: 0-60 mm), Middle Enriched Root Zone (MERZ: 60-150 mm), and Deep Extended Root Zone (DERZ: 150-210 mm). Triaxial tests revealed varying shear strengths, with MERZ exhibiting the highest and SARZ the lowest. The Duncan-Chang model parameters, initial modulus of deformation, and initial Poisson's ratio were significantly influenced by soil depth, mirroring shear strength trends. An empirical formula incorporating soil layer depth into the Duncan-Chang model was proposed. Critical state stress ratios for SARZ and MERZ were determined as 0.93 and 1.11, respectively, quantifying their relationship with soil depth and root distribution. This study provides theoretical and parameter support for understanding the failure mechanism of root-soil composites.

The critical state line (CSL) plays an essential role in the constitutive modelling of granular soils. It serves as a reference line for the measurement of the state parameter. The critical state of crushable soils cannot accurately be determined from experimental data because of the ever-changing soil properties due to particle breakage. In this study, two new parameters eB and eb are introduced, which account for the final position and the evolution of CSL of crushable soils during shearing, respectively. To identify the optimal CSL-related parameters from experimental data, a hybrid genetic algorithm combining artificial immune system (AIS) and real-coded genetic algorithm (RCGA), namely AIS-RCGA is adopted. The fitness is defined to minimize the prediction error of both void ratio (or pore water pressure) and deviatoric stress. We have refined the tuning methods for several hyperparameters of AIS-RCGA and proposed a novel method to assess the similarity of individuals within AIS-RCGA. Results show that the new method is more efficient in finding the global optimal for our problem. With optimized model parameters, the new constitutive model can accurately predict the response of crushable soil, outperforming other constitutive model reported in the literature.

The main objective of this study was to investigate the response of uniform sand under constant volume (i.e., undrained) conditions and how it is influenced by the initial anisotropy induced in the soil fabric due to preshearing stress history. The experimental program explored a range of parameters, including stress-strain response, tendency to volume change, phase transformation, flow instability, noncoaxiality between stress and strain rate, and the critical state line. To induce initial anisotropy, samples were presheared along different directions and subsequently tested using an Swedish Geotechnical Institute (SGI)-type bidirectional direct simple shear apparatus. The testing program focused on the effects of initial anisotropy that were induced by preshearing, resulting from the application of initial shear stress in various directions relative to the subsequent shearing direction. To interpret the variations of stresses within the samples, Budhu's approach for stress state determination in simple shear specimens was adopted. The results demonstrate that the stress-strain behavior and global volume change tendency of the soil are heavily influenced by the magnitude and direction of the preshearing stress history. Furthermore, the study reveals that the effects of stress history significantly diminish at large shear strains as the samples approach the critical state.

The presence of fines can significantly influence the mechanical behavior of soils. In this study, a hypoplastic model is extended to simulate the stress-strain relationship of sand-fines mixtures. Firstly, three modifications are incorporated into the model to accurately simulate the effective stress path, hardening rate, and limited flow type response of sand during undrained loading. Additionally, a novel formulation is proposed to capture the critical state line of soil mixtures across a wide range of fines content. This formulation is then integrated into the characteristic void ratios of the hypoplastic model, enabling it to effectively consider the combined influence of void ratio, confining pressure, and fines content on the density state of the sand-fines mixtures. The predictive capability of the model is demonstrated through a comparison of simulation results and experimental data for undrained triaxial tests conducted under various conditions.

The mechanical behaviour of rockfill materials is strongly affected by particle breakage, which causes a continuous variation of the grain size distribution (GSD) until a stationary condition. Although the evolution of grain crushing caused among others by the initial grading, the relative density and stress level has been extensively studied, the relationship describing the influence of the current GSD on other soil properties such as the critical state line is not uniquely defined. In this study, a series of triaxial compression tests with various stress paths under monotonic loading were performed on a rockfill to examine the effect of particle breakage on the position of the critical state lines (CSLs) in the compression plane. Specimens were prepared with the same initial grading and relative density to investigate the evolution of the GSD as a result of grain breakage determined by different stress paths applied in a large triaxial apparatus. In addition, a recently proposed simplified procedure is employed to capture the evolution of the CSLs of the tested crushable rockfill as a function of a breakage parameter related to the breakage index Bg. Experimental results obtained for the tested rockfill demonstrate that the abovementioned procedure is capable to predict the position of the CSL linked to the GSD reached at the end of the triaxial test, also when the critical state condition, defined as the ultimate condition in which shearing could continue indefinitely without changes in volume or effective stresses, is not reached.