Effect of Confining Pressures on the Dynamic Response Characteristics of Niger Delta Clay Soils

The study investigated the earthquake potential in Niger Delta region of Nigeria. A series of resonant column and bender element test was performed on compacted clay soil samples across the investigated Niger Delta States, which showed the influence of confinement on frequency, shear modulus, shear velocity and damping ratio. The confinement in clay was high. The frequency response increases with pressure increase. Also, the resonance column test at various confinements revealed changes in shear modulus, accelerometer output and damping ratio. Thus, there was high variation in the test parameters as confinement pressure was increased. Similarly, the bender element tests also showed that pressure has effect on shear wave-velocity, shear modulus and damping ratio confinement. Although, unlike Resonance Column tests, the shear modulus and shear wave-velocity generally increased as confinement pressure was increased, while for damping ratio it decreases as confinement pressure was increased. The variations in resonance column/binder element test parameters showed that the Niger Delta region, as an oil and gas area, is susceptible to earthquake. Therefore, continuous monitoring of oil exploration activities must be put in place.

Effect of Confining Pressures on Dynamic Response Characteristics of Silty Soils in the Niger Delta

The probability of earthquake occurrence in the Niger Delta region of Nigeria was studied in this research. The resonant column/bender element tests were used for the study. Series of analysis were carried out on compacted silt in subsoil strata obtained from various locations in Rivers, Bayelsa, Delta and Akwa Ibom States. The effects of confinement on frequency, shear modulus, shear velocity and damping ratio were studied. The tests results revealed that confinement has effects on the investigated parameters. Thus, frequency response increases with increase in confinement pressure. Also, the resonance column test at various confinements revealed changes in shear modulus, accelerometer output and damping ratio. Accordingly, there was high disparity in the tested parameters as confinement pressure was increased. Similarly, the bender element tests also showed that pressure has effect on shear wave-velocity, shear modulus and damping ratio confinement. The shear modulus and shear wave-velocity generally increased as confinement pressure was increased, while damping ratio decreases as confinement pressure was increased. The variations in Resonance Column/Bender Element test parameters showed that the silty soil in the Niger Delta region, an oil and gas rich area, is likely to experience earthquake in the future. Therefore, geological data should be collated for monitoring, especially as several geological activities take place in the region.

Sandy Soil Improvement Using MICP-Based Urease Enzymatic Acceleration Method Monitored by Real-Time System

This paper aims at monitoring the improvement of sandy soil properties with biocementation through the microbially induced calcite precipitation (MICP) method with reaction accelerations by self-developed soybean urease enzymes. In this study, the concentration of calcium ions (Ca2+ ions as CaCl2) is varied at 50, 100, 250, and 500 mM to determine an optimum shear strength. The self-developed soybean urease enzymes of 20% by volume (v/v) are used to accelerate the MICP reaction to finish within 7 days. Based on real-time monitoring bender element system and direct shear tests, the optimum Ca2+ concentration is found as 250 mM. However, a detrimental effect occurs in case of high concentration of Ca2+ as CaCl2 (500 mM) because of solution acidification from high Cl− concentration. This condition lowers CaCO3 precipitation causing the reduction of biocementation process. At equivalent shear modulus, the biocementation time of MICP-based sand with acceleration by urease enzymes is about 10 times faster than that without. Using spectrophotometer and pH meter, the ammonification rate and the solution pH of biocemented sand with acceleration by urease enzymes for 3 days are found relatively higher than those without urease enzymes for 40 days. The analyses by scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirm not only the occurrence of CaCO3 binding sand particles together but also the improvement of physical strengths of sandy soil samples with the MICP-based urease enzymatic acceleration method. These results introduce an option to accelerate biocemented sandy soil improvement.

Laboratory-Based Correlation between Liquefaction Resistance and Shear Wave Velocity of Sand with Fines

This paper presents the results of a laboratory investigation into the effect of non-plastic fines on the correlation between liquefaction resistance and the shear wave velocity of sand. For this purpose, undrained stress-controlled cyclic triaxial and bender element tests were performed on clean sand and its mixtures with non-plastic silt. It is shown that the correlation between liquefaction resistance and shear wave velocity depends on fines content and confining effective stress. Based on the test results, correlation curves between field liquefaction resistance and overburden stress corrected shear wave velocity for sand containing various contents of fines are derived. These curves are compared to other previously proposed by field and laboratory studies.

Influences of initial anisotropy and principal stress rotation on the undrained monotonic behavior of a loose silica sand

Triaxial compression and extension tests have been conducted under different initial anisotropy conditions to investigate the undrained response of a crushed silica sand. The loose to medium specimens were prepared using the moist tamping method. Five stress paths with different stress ratios (q/p^') were employed to prepare anisotropically consolidated specimens. Several specimens were consolidated under a specific condition in which a stress rotation occurred under undrained monotonic shearing similar to a reversed cyclic shear stress loading during an earthquake. The effects of initial induced anisotropy at consolidation on the onset of liquefaction, phase transformation, and critical state are investigated within the framework of Anisotropic Critical State Soil Mechanics (ACSSM). In addition, fabric evolution during shearing towards the critical state is evaluated using bidirectional bender element tests. The results illustrate the fact that there is a unique anisotropic critical state representing anisotropic fabric, irrespective of initial anisotropy, and the states of stress. Similar to the critical state line, the phase transformation line has the same loci for different initial anisotropies.

Properties of Mine Tailings for Static Liquefaction Assessment

Static liquefaction has been associated with numerous recent failures of tailings storage facilities (TSFs) around the world (e.g., the 2019 Brumadinho failure). These failures lead to devastating consequences for the environment and civil infrastructure, as well as the loss of human lives. In this study, we present trends for the mechanical response of mine tailings considering a) triaxial tests, b) bender element tests, and c) consolidation tests on 53 mine tailings materials (including recent case histories). These materials have a broad range of states, particle size distributions, and compressibility. The trends are evaluated in the context of static liquefaction using critical state soil mechanics concepts, focusing on the variation of the shear strength (residual and peak), state and brittleness soil indexes, excess pore pressure indexes, instability stress ratios, and dilatancy. In particular, we highlight that mine tailings mechanical properties reflect both the properties of the particles themselves and the relative proportions of different particle sizes. For instance, the observed trends suggest that particle gradation influences the small strain stiffness and dilatancy; the proportion of voids to the size of fine particles influences strength, and particle shape affects dilatancy. Finally, we propose static liquefaction screening indexes based on the observed trends.