Influence of anisotropic stress path and stress history on stiffness of calcareous sands from Western Australia and the Philippines

Investigation of dynamic properties of carbonate/calcareous soils is important in earthquake and offshore engineering as these soils are commonly encountered in large-scale projects related with energy geomechanics and land reclamation.In this study,the stiffness and stiffness anisotropy of two types of calcareous sands(CS)from the Western Australia and the Philippines were examined using bender elements configured in different directions in stress path setups.Stiffness measurements were taken on specimens subjected to constant p’compression/extension and biaxial stress paths and additional tests were performed on three types of silica sands with different geological origins and particle shapes,which were used as benchmark materials in the study.Compared with the three brands of silica sands,the stiffness of the CS was found to be more significantly influenced by anisotropic loading;an important observation of the experimental results was that stress anisotropy had different weighted influences on the stiffness in different directions,thus influencing stiffness anisotropy.Comparisons were made between the specimens subjected to complex loading paths,and respected model parameters as suggested from published expressions in the literature.These comparisons further highlighted that calcareous soils have different responses in terms of stiffness,stiffness anisotropy and loading history,compared with that of silica-based sands.

Experimental study of seepage characteristics of single rock fracture based on stress states and stress history

Through seepage tests under different loading and unloading confining pressures and different hydraulic gradients,the authors studied the effects of stress states and stress history on fracture permeability evolution for single granite fracture and sandstone fracture. The results show that there exists a linear relationship between the seepage discharge and osmotic pressure in sandstone fissure under each level of confining pressure. With the increasing in the confining pressure,the permeability of the fracture decreases,but the decreasing rate is changeing. During the unloading process,the fracture seepage velocity cannot be fully recovered to the size of the loading process. Therefore,in the unloading process of the confining pressure,the recovery of fracture permeability shows obvious hysteresis effects. The flow rate of the fracture remains unchanged during five cycles of loading and unloading processes of the confining pressure. In each cycle,the evolution character of the flow rate with the confining pressure remains unchanged. These experiments show that the seepage characteristics of sandstone and granite fractures are not the same under the same stress state.

Influence of excavation beneath existing building on dynamic impedances of underpinning pile considering stress history

The addition of basement beneath existing building changes the underpinning pile from fully embedded to partially embedded,and thus influences the mechanical properties of pile.In the past,scholars paid attention to the change in the bearing capacity of pile but neglected the difference of dynamic characteristics before and after construction,and potential changes in stress history of remaining soil are also ignored.In this work,a calculation model is built to investigate the influence of excavation on dynamic impedance of underpinning pile considering the effect of stress history.The soil is simulated by the dynamic Winkler foundation,which is characterized by springs and dashpots.Properties of remaining soil after excavation are updated to consider the effect of stress history through modifying the initial shear modulus and related parameters.The dynamic impedance of pile after excavation is obtained based on the transfer matrix method.The parameter study is carried out to evaluate the dynamic impedance with various excavation depths,considering or ignoring stress history effect,and various element lengths.The results show that shallow soil plays an important role to dynamic impedance,and overestimated dynamic impedance is obtained if not considering the stress history effect.

An accelerated life test model for harmonic drives under a segmental stress history and its parameter optimization

Harmonic drives have various distinctive advantages and are widely used in space drive mechanisms. Accelerated life test （ALT） is commonly conducted to shorten test time and reduce associated costs. An appropriate ALT modet is needed to predict the lifetime of harmonic drives with ALT data. However, harmonic drives which are used in space usually work under a segmental stress history, and traditional ALT models can hardly be used in this situation. This paper proposes a dedicated ALT model for harmonic drives applied in space systems. A comprehensive ALT model is established and genetic algorithm （GA） is adopted to obtain optimal parameters in the model using the Manson fatigue damage rule to describe the fatigue failure process and a cumulative dam- age method to calculate and accumulate the damage caused by each segment in the stress history. An ALT of harmonic drives was carried out and experimental results show that this model is acceptable and effective.

A Note on Soil Structure Resistance of Natural Marine Deposits

It has been well documented that natural normally-consolidated marine soils are generally subjected to the effects of soil structure. The interpretation of the resistance of soil structure is an important issue in the theory study and engineering practice of ocean engineering and geotechnical engineering. It is traditionally considered that the resistance of soil structure gradually disappears with increasing stress level when the applied stress is beyond the consolidation yield stress. In this study, however, it is found that this traditional interpretation of the resistance of soil structure can not explain the strength behavior of natural marine deposits with a normally-consolidated stress history. A new interpretation of the resistance of soil structure is proposed based on the strength behavior. In the preyield state, the undrained strength of natural marine deposits is composed of two components: one developed by the applied stress and the other developed by the resistance of soil structure. When the applied stress is beyond the consolidation yield stress, the strength behavior is independent of the resistance of soil structure.