Browsing by Subject "Geophysical engineering"

This dissertation aims to understand the physics of geomaterial under different loading rates, employing a combination of experimental analysis and theoretical modeling. After obtaining a comprehensive understanding of geophysics, the objective shifts to exploring methods for enhancing the mechanical properties of materials to mitigate or prevent structural failures.

One particular focus in experimental studies is placed on understanding the phenomenon of desiccation, where the volumetric shrinkage rate of the geomaterial can be manipulated by adjusting atmospheric conditions. The experimental results from desiccation tests, supported by triaxial tests, reveal that the behavior of geomaterials is dependent on the loading rate, indicating a rate-dependent response. This observation highlights the need to consider viscoplasticity in the mechanical analysis of these geomaterials. Subsequently, a theoretical mathematical model incorporating viscoplasticity is utilized to describe the stress distribution within the geomaterial. By comparing the predicted locations and the number of stress singularities obtained from the model with the observed locations and the number of cracks in desiccation tests conducted under controlled atmospheric conditions, the effectiveness of the model in capturing the mechanical behavior of the geomaterial is assessed.

Once the mechanical behavior is understood and the corresponding theoretical model is validated, modifications in soil properties can be achieved through adjustments to viscosity. Initially, increasing the viscosity results in the formation of more cracks with narrower spacing. However, as viscosity continues to increase, it eventually leads to the complete prevention of failure. Desiccation experiments containing fluids with varying viscosities were conducted to validate the predicted failure pattern. The experimental results align with the theoretical predictions, providing confirmation of the anticipated behavior.

During desiccation tests conducted on amended soil samples, it was observed that crack development was mitigated, indicating that cracks initially appeared but remained suspended during the dehydration process. Due to the complexity of solving time-dependent partial differential equations with shifting boundary conditions, capillary experiments were introduced to provide insights into the force development within soil particles with the loss of water. The morphology and force development from capillary tests revealed distinct outcomes during dehydration: in the capillary system with distilled water and low viscosity fluid, a rapid force reduction (drop to zero) occurred as the capillary bridges broke, while the presence of high viscosity fluid resulted in a rebound followed by a high attraction force due to bonding formation in the capillary system.

In all, this dissertation offers a novel perspective on describing soil behavior and provides a micro-scale explanation of force development in soil dehydration.

Knowledge of the drying process of geomaterials is meaningful and helpful in the field of geotechnical and geo-environmental engineering. This experimental study focuses on drying tests on geomaterial samples with monitored surface moisture contents and controlled environmental conditions. Using the digital image correlation (DIC) method to analyze the sample’s displacement, the 3D displacement plots and volumetric strain maps are obtained after calculation. By combining the monitored moisture content with analyzed displacement and volumetric strain plots, the phenomenon and characteristics of a geomaterial’s drying process are discussed and concluded. This study offers a better understanding of the deformation of geomaterials during the drying process in 3D.

Low frequency noise in measured sensor data is amplified when integrated. In the integration of measured acceleration data to displacement, such low frequency noise can lead to significant drift errors. In non-real-time applications, time domain and frequency domain detrending methods can be employed to remove bias and drift errors. For real-time applications, recursive high-pass digital filters, such as Butterworth filters, are computationally simple to implement. This research focuses on developing a discrete time state-space model to simultaneously filter out low frequency noise, deconvolve, and integrate voltage measurements from a geophone sensor. A circuit model for the sensor was chosen. Forward and inverse dynamical systems describing the circuit were derived utilizing the theory of linear time-invariant systems. The stability and accuracy of Butterworth filter design using the bilinear transformation method can be affected by the filter order and cut-off frequency. This research reveals the root cause of the numerical instability of high-pass digital Butterworth filters having low cutoff frequencies (less than half a percent of the sampling frequency) and high filter orders (greater than 6). These instabilities arise when filter coefficients are computed from discrete time poles and can be avoided by converting a continuous-time state-space model for the filter to discrete time via a matrix exponential. The method is demonstrated using measured geophone data.

This work advances a phase-field method for fluid-driven fractures and proposes arobust and efficient discretization framework. It begins by addressing a modeling challenge related to the application of pressure loads on diffuse crack surfaces. Along the way, a new J-Integral for pressurized fractures in a regularized context is devel- oped.

Then, the focus turns to a hybrid method to model fluid-driven fracture propaga-tion. A so-called multi-resolution method is presented that uses a combination of en- richment schemes with the phase-field method to address the complex fluid-fracture interaction that occurs during hydraulic fracture. On one hand, the phase-field method alleviates some of the difficulties associated with the geometric evolution of the fracture, which are usually the limiting aspect of purely enrichment-based schemes. On the other hand, the discrete representation allows for a better treat- ment of the fluid loads and crack apertures, which are the main challenges associated with phase-field approaches.

The multi-resolution method is first presented in a simplified scheme to treat two-dimensional problems. Various benchmark problems are used to verify the framework against well-known analytical solutions. The method is then extended to three- dimensions. A robust algorithm to handle planar cracks in 3D is developed and its extension to non-planar cases is discussed. Finally, opportunities for improvements and extensions are discussed, paving the road for future work in this area.