Seismically Induced Settlement of Partially-Saturated Sand

Deformation of soil layers during earthquake shaking is a major cause of damage to buildings and geotechnical structures. Over the past 40 years, several design methods have been proposed to quantify and estimate the settlement of sand layers due to seismic loads. Most of these methods have been focused on sands in dry or water-saturated conditions, leaving a gap in the basic understanding of the mechanisms of seismically induced settlements of partially-saturated sands. Design approaches for partially-saturated soil layers deserve further development because these soil layers may have different deformation mechanisms during earthquake loading than water-saturated or dry soil layers. In this research an effective stress-based empirical methodology is proposed to predict the earthquake induced settlement of a free field partially-saturated sand layer. This approach uses a complex coupling between stress state, seismic compression, and pore water pressure generation during earthquake loading. Accordingly, an experimental study on partially saturated soils was used in tandem with this design model to quantify input relationships for the different variables during earthquake loading and also to verify the induced settlement estimated using the proposed empirical approach. A new centrifuge physical modeling technique was developed that involves a soil specimen within a laminar container mounted atop a hydraulic servo-controlled shake table in a geotechnical centrifuge. Steady state infiltration of water was used to control the stress state in the partially-saturated sand layer during centrifugation. Specifically, infiltration was found to lead to a relatively uniform degree of saturation with depth in the sand layer, simplifying interpretation of the deformation response. Sand layers with a wide range of degrees of saturation were evaluated using this approach to assess the degree of saturation on their deformation response during cyclic loading. A nonlinear trend was observed in the variation of the surface settlement with degree of saturation, with a minimum value obtained for sand with a degree of saturation of about 0.4. This trend is consistent with the relationship between small-strain shear modulus and degree of saturation measured using bender elements and resonant column testing techniques and also with the predicted trend using the developed empirical methodology

Bio-inspired geotechnical engineering: Principles, current work, opportunities and challenges

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Bio-inspired geotechnical engineering: principles, current work, opportunities and challenges

A broad diversity of biological organisms and systems interact with soil in ways that facilitate their growth and survival. These interactions are made possible by strategies that enable organisms to accomplish functions that can be analogous to those required in geotechnical engineering systems. Examples include anchorage in soft and weak ground, penetration into hard and stiff subsurface materials and movement in loose sand. Since the biological strategies have been ‘vetted’ by the process of natural selection, and the functions they accomplish are governed by the same physical laws in both the natural and engineered environments, they represent a unique source of principles and design ideas for addressing geotechnical challenges. Prior to implementation as engineering solutions, however, the differences in spatial and temporal scales and material properties between the biological environment and engineered system must be addressed. Current bio-inspired geotechnics research is addressing topics such as soil excavation and penetration, soil–structure interface shearing, load transfer between foundation and anchorage elements and soils, and mass and thermal transport, having gained inspiration from organisms such as worms, clams, ants, termites, fish, snakes and plant roots. This work highlights the potential benefits to both geotechnical engineering through new or improved solutions and biology through understanding of mechanisms as a result of cross-disciplinary interactions and collaborations

Suction-controlled dynamic simple shear apparatus for measurement of dynamic properties of unsaturated soils

State variables such as confining pressures and the degree of saturation can have a profound effect on the dynamic properties of a soil. These dynamic properties are essential when performing seismic response analysis of geotechnical systems in the phreatic zone. In order to accurately obtain these properties, laboratory testing using a Dynamic Simple Shear system is often implemented. This paper concentrates on the advancements and modifications of a custom-built dynamic simple shear system at the University of New Hampshire to accommodate soils with unsaturated conditions by employing axis translation technique. Tests could be performed under drained (constant suction) or undrained (constant water content) conditions. The procedure for preparing an unsaturated soil sample and testing is discussed, followed by the methods for interpreting the data and the challenges involved. Preliminary data confirms the ability of the system to control and track suction during the cyclic simple shear test. Suction in unsaturated soil increased the shear modulus and decreased the damping ratio comparing with those in dry and saturated conditions

A semi-empirical model to predict excess pore pressure generation in partially saturated sand

Past studies revealed that excess pore pressure generation due to cyclic loading is highly governed by induced strains, volumetric deformation potential of soil, number of cycles, and bulk stiffness of pore fluid. It is well established that partial saturation can significantly reduce bulk stiffness of pore fluid and consequently excess pore pressure generation during seismic loading. On the basis of that, a number of researchers have investigated induced partial saturation as an effective soil improvement technique to increase the liquefaction resistance of fully saturated soils. This paper focuses on development of a semi- empirical model to interpret the effects of partial saturation on the excess pore pressure generation in sands. In this regard, an existing strain based excess pore pressure ratio (ru) prediction model originally developed for fully saturated soils was modified to incorporate the effect of partial saturation on the excess pore pressure generation. The literature data as well as data from a series of strain-controlled direct simple shear test were used to evaluate the reliability of the proposed equation in predicting the excess pore pressure ratio in partial saturation condition

Impact of Suction on the Near Surface Lateral Soil Response using Centrifuge Modeling

Recent studies have shown that unsaturated soils lead to greater lateral pile capacity. This study aims to experimentally assess how suction stress affects the lateral response of piles in unsaturated cohesionless soil. Two centrifuge tests were performed at 50 g to evaluate the effect of suction stress in the soil. Lateral loads were applied monotonically on a single free-head pile in a displacement-controlledmanner to a maximum pile head displacement of 0.44 m. The first test was conducted on fully saturated cohesionless soil, while the second test was performed in an unsaturated state with a mixed unsaturatedsaturated soil layer. The water table was lowered to about 0.12 times the embedded pile depth to ensure an unsaturated condition in the zone closer to the surface of the soil. Lateral response assessment indicates that the unsaturated soil influenced the pile head response, leading to larger applied lateral loads for similar pile displacements in comparison to the fully saturated soil test. Experimental findings reveal that suction stress played a meaningful role in magnitudes of pile bending moments and lateral resistances for unsaturated cohesionless soils