Cyclic Resistance Of Sand Research Articles

Deformation and cyclic resistance of sand in large-strain undrained torsional shear tests with initial static shear stress

This paper presents the findings from an experimental study focusing on the undrained cyclic behavior of sand in the presence of initial static shear stress. A series of undrained cyclic torsional shear tests was performed on saturated air-pluviated Toyoura sand specimens up to single amplitude shear strain (γSA) exceeding 50%. Two types of cyclic loading conditions, namely, stress reversal (SR) and stress non-reversal (SNR), were employed by changing the amplitude of the combined initial static shear and cyclic shear stresses. The tests covered a broad range of initial states in terms of relative density (Dr = 20–74%) and the initial static shear stress ratio (α = 0–0.30). The following five distinct modes of deformation were identified from the tests based on the density state, the transient undrained peak shear stress, and the combined cyclic and static shear stresses: 1) static liquefaction, 2) cyclic liquefaction, 3) cyclic mobility, 4) shear deformation failure, and 5) limited deformation. Of these, cyclic liquefaction and static liquefaction are the most critical. They occur in very loose sand (Dr ≤ 24%) under SR and SNR, respectively, and are characterized by abrupt flow-type shear deformation. Cyclic mobility occurs under SR in loose to dense sand with Dr ≥ 24%. Contrarily, shear deformation failure typically occurs under SNR in sand with 24 < Dr < 65%, and limited deformation may take place in dense sand with Dr ≥ 65%. In this paper, a stress-void ratio-based predictive method is proposed to identify the likely mode of deformation/failure in sand under undrained shear loading with static shear. Furthermore, the cyclic resistance is evaluated at three different levels of γSA (i.e., small, γSA = 3%; moderate, γSA = 7.5%; and large, γSA = 20%). The results show that, independent of the density state, the cyclic resistance continuously decreases with an increase in α at the small γSA level, while it first decreases and then increases for both loose and dense sand at the moderate and large γSA levels.

Effect of vertical effective and initial static shear stresses on the liquefaction resistance of sands in cyclic direct simple shear tests

To investigate the effect of the vertical effective and initial static shear stresses on cyclic resistance under cyclic loading with principal stress rotation, a series of cyclic direct simple shear tests were conducted on loose and dense Nakdong River sands. The results revealed that the increasing vertical effective stress (σ′v0) often decreased the cyclic resistance, and the decrease was more evident for a high initial static shear ratio (α = 0.2). The degree of reduction of the correction factor Kσ predicted by recommended methods in practice mostly showed more conservative results than the measured Kσ values in this study. Increasing the initial static shear stress decreased the cyclic resistance of the loose sand. For the dense sand, the cyclic resistance either increased or decreased, depending on the combined effect of the σ′v0 and α levels. The correction factor Kα decreased minimally to 0.689 for the loose sand and rose maximally to 1.175 for the dense sand. When the relative density (Dr) increased from 40% to 80%, the increase in the cyclic resistance for a high α (=0.2) was considerably greater than that for α = 0. The cyclic response of the loose sand was due to sudden flow deformation without any apparent shear strain accumulation, regardless of the α value; while the cyclic response of the dense sand indicated plastic strain accumulation. Thus, the cyclic resistance of sand is influenced by the combined effect of Dr, σ′v0, and α, with the potential influence of principal stress rotation.

Cyclic response of coir fibre-reinforced sand

The reinforcement technique is a better choice to the conventional ground improvement technique under appropriate conditions to enhance the bearing capacity of soil. The behaviour of reinforced earth was studied by considering generally static loads. In some situations, structures are also subjected to cyclic loads; such as those occurred by the machine foundations, traffic loads, earthquake, etc. Review of earlier studies has indicated that very few literatures are available with regard to performance of coir reinforcement under cyclic loading. The objective of the present study is to assess the behaviour in terms of load-carrying capacity and cyclic settlement using coir fibres as reinforcement and by conducting cyclic load settlement tests using model footings resting on sand reinforced with coir fibres. The present experimental study indicated that, coir reinforcement shows decrease in cyclic resistance ratio as well as cyclic stiffness with increase in u/B ratio. In the present study, better cyclic resistance of sand was observed in coir fiber-reinforced sand compared to unreinforced sand under all excitation parameters tested.

Effect of stress direction on the cyclic resistance of sand

In total, 55 undrained cyclic torsional shear tests were conducted at a constant mean confining stress, σ0m, constant intermediate principal stress ratios, b = (σ2 − σ3)/(σ1 − σ3), and principal stress directions, α. The cyclic resistance of loose, isotropically consolidated specimens exhibited a dip in strength, with the lowest values for α being 45°, an angle at which α was oriented such that the potential shear band directions dovetailed with the plane having the weakest strength (i.e. horizontal plane). For a given α, the cyclic resistance decreased to the lowest values for b = 0·5, at which the value of the principal stresses was different in the three major directions. For dense specimens, the direction of the major principal stress had no significant influence on the cyclic resistance. For loose, anisotropically consolidated specimens, higher cyclic resistances were observed at the point of approaching the direction of the major principal stress during the consolidation and loading stages.

Undrained behavior of sand under cyclic paths that match storm-wave loading conditions

ABSTRACTIn ocean engineering, storm-wave loading imposes irregular cycles in the magnitudes acting on the coastal or seabed cohesionless soils, giving rise to more complex consequences on the their liquefaction susceptibility, compared with those under uniform cyclic loadings that were considered in most routine engineering designs. This paper presents a systematic experimental investigation into the undrained behavior of saturated loose sand subjected to irregular cyclic triaxial paths that match the storm-wave loading conditions, with the presence of static shear stress that mimics the initial condition of sand deposits in the field, along with the conventional uniform cyclic triaxial tests for comparison. The results indicate that both the initial static stress and the cyclic irregularity influence the pore pressure development and cyclic resistance of sand. When the static shear stress presents, the sand samples under storm-wave loading show a monotonic increase in cyclic resistances to liquefaction. In addition, the concept of strain energy is used to delineate the accumulation of pore pressure during cyclic loading. A unique relationship between the normalized pore pressure and the strain energy is observed, irrespective of the static stress level, cyclic stress amplitude, and applied cyclic loading patterns.

Cyclic Resistance of a Loose Sand Subjected to Rotation of Principal Stresses

An experimental study aimed at providing insights into the effects of the inclination of principal stress directions with respect to the bedding plane and the role of principal stress rotation on the cyclic resistance of sands is presented. A new hollow cylinder torsional shear device commissioned at Carleton University was used to conduct the experiments in water-deposited Fraser River sand. Specimens at identical initial states were subjected to a constant amplitude cyclic shear stress along loading paths that impose different levels of stress rotation. Cyclic resistance decreased significantly as the maximum orientation of the major principal stress increased to approximately 45°–60°, but it increased with further rotation. This is contrary to the conventional idea according to which the weakest response is expected when the loading causes larger principal stress rotation and/or when the major principal stress is aligned with the bedding planes. The alignment of the plane of maximum shear stress toward and stronger time history of shear stress on the horizontal bedding planes lead to increased liquefaction susceptibility. The nature of shear stress on the bedding planes appears more critical than the alignment of the major principal stress along the bedding planes during cyclic loading. Test results also suggest that cyclic simple shear tests on the basis of stress rotation would represent a lower bound or closer to lower bound resistance. Thus, cyclic simple shear tests provide a convenient means of determining the cyclic resistance and the obtained resistance can be considered a conservative measure if field loading conditions deviate from that in simple shear.