Non-destructive monitoring methods are very important to avoid or reduce the effect from measurement disturbance in both laboratory and field tests during geotechnical engineering processes. The main objective of this research is to design and apply non-destructive methods to ease and enhance the understanding on soil property changes in some common laboratory studies, such as SWCC monitoring, pile installation process and pile setup mechanism. Electrical resistivity and mechanical wave are used in this research to aid non-destructive monitoring in this study.

Electrical resistivity is often utilized to estimate different flow-related properties of unsaturated soil, such as permeability. To be better utilized for such a purpose, the electrical resistivity anisotropy has to be c...[ Read more ]

Non-destructive monitoring methods are very important to avoid or reduce the effect from measurement disturbance in both laboratory and field tests during geotechnical engineering processes. The main objective of this research is to design and apply non-destructive methods to ease and enhance the understanding on soil property changes in some common laboratory studies, such as SWCC monitoring, pile installation process and pile setup mechanism. Electrical resistivity and mechanical wave are used in this research to aid non-destructive monitoring in this study.

Electrical resistivity is often utilized to estimate different flow-related properties of unsaturated soil, such as permeability. To be better utilized for such a purpose, the electrical resistivity anisotropy has to be characterized to account for the anisotropic nature of soils. Thus, a testing cell equipped with two individual four-probe arrays was designed and manufactured using the 3D printing technique. The testing cell was integrated with SWCC device and therefore both the soil resistivity anisotropy and SWCC can be measured simultaneously in the same sample. While considering the boundary effect from the finite sample size and the influence of the electrode sizes, numerical simulations using the finite element method (FEM) were carried out to determine the associated correction factors, which were used to eliminate the associated measurement biases. The validity of the correction factors was also experimentally verified. The experimental results on the mica samples reveal that the electrical resistivity in the vertical and horizontal directions (i.e., ρ_? and ρ_?) continues to increase as the suction increases or the degree of saturation decreases. In addition, the anisotropic factor, ? = √ρ_?/ρ_?, was found to be gradually increasing with increasing suction or decreasing water content.

Beside electrical resistivity, the utilization of mechanical wave is applied in a flexible, multiple-layer, economical but robust, and automated high-speed shear-wave (V_s) tomographic system for process monitoring in the laboratory, and the associated validation for monitoring the dynamic process of model pile installation. This tailor-made tomographic system consists of four major parts: (i) the control unit built with the microcontroller board, Arduino Leonardo, and computer, (ii) the signal generating part, (iii) three layers of bender element sensing arrays (including 14 source and receiver bender elements per layer), and (iv) the signal receiving part, including the tailor-made printed circuit boards (PCBs) and PicoScopes. For each bender element pair, the sampling rate is up to 1 GS/s (20 MS/s was used in this study); the required total measurement time is 2.94 seconds (i.e., for 196 ray path measurements) to facilitate dynamic process monitoring. The obtained time-lapse V_s tomographic images during pile installation show that when the pile toe is approaching the bender element sensing layer, the influence zone gradually enlarges; within the influence zone, the increase in V_s decreases with increasing distance to the pile shaft. For a given depth and fixed distance to the pile shaft, V_s first increases gradually when the pile toe is approaching to the sensing layer and gradually decreases afterwards as the pile toe penetrates through the layer. The validity of the V_s tomographic images is supported by the similar responses in variations of the circumferential stress ?_?′ and the radial stress ?_?′, measured by the tactile pressure sensors.

To solve the data analysis problem due to massive measurements in V_s tomographic monitoring, two new methods, i.e., the Stockwell transform based (ST) method and self-healing cross correlation (SC) method, are proposed in determining the S-wave arrival time for bender element tests, especially for V_s tomographic imaging. In the ST method, the Stockwell transform is first carried out to obtain a high resolution time-frequency representation of the receiving signal; then, the energy in the frequencies around the resonant frequency is summed, followed calculation of the associated energy gradient. The maximum energy gradient is selected as the objective criterion to determine the arrival of the S-wave, since its arrival leads to a distinct amplitude increase and a notable change in the associated energy. The accuracy of this ST method is validated using both numerical and physical experiments. The ST method requires high computing power for signal processing; hence, the SC method is proposed to tackle this practical problem. Considering two consecutive measurements made by the same pair of bender elements in a time interval, subjected to only small changes in the stress states, both measurements should exhibit a similar waveform but with a minute time-shift. Hence, the cross correlation peak of the two consecutive measurements gives the travel time difference between them. The validity of the SC method is verified by a laboratory pile installation test equipped with a bender element sensing layer; and good agreement is found between the results obtained from the ST and SC methods. The strengths of these two methods enable us to objectively and automatically process the tremendous amount of bender element signals produced by the high-resolution time-lapsed V_s tomographic images, as demonstrated by process monitoring of the pile installation.

Finally, model pile tests were designed to characterize the underlying mechanisms and behavior of a driven pile setup in dry sand. The tests involved stress measurement with the tactile pressure sensors and spatio-temporal, shear-wave velocity V_s distributions using an automated high-speed tomographic imaging system. The results of the pile load tests demonstrate a distinct increase in the pile shaft resistance after pile setup. The measured stress and V_s in the soil surrounding the pile suggest that the increase in the radial effective stress during pile loading ??_??′ , resulting from the aging-induced stiffness increase, is the dominant mechanism in pile setup. The associated at-rest radial effective stress only slightly increases during the setup period and therefore, plays a very minor role. The spatio-temporal evolution of the V_s distributions reveal that during initial aging (before pile installation), V_s is similar at any distance from the pile shaft and exhibits a similar aging rate ?₀ in terms of stiffness increase. After pile installation, soil exhibits a higher aging rate ?₁, regardless of the soil depth and the distance from the pile shaft. In addition, the ratio ?₁/?₀ decreases with increasing distance from the pile shaft. This suggests that the action in a driven pile installation initiates a new aging process with a relative higher aging rate (i.e., ?₁/?₀ > 1) for pile setup; the associated effects gradually diminish with increasing distance from the pile shaft. A lower initial V_s and a higher aging rate ?₁ are also observed for measurements at the three sensing layers at different depths. This suggests that the disturbed soil due to pile installation tends to recover at a relative higher aging rate. However, a similar trend in the aging rate is observed immediately before and after load test, which suggests that the action of the load test might not be strong enough to trigger another aging process with a relatively higher aging rate.