Minsu Cha

Long-wavelength P-wave and S-wave propagation in jointed rock masses

Field data suggest that stress level and joint condition affect shear-wave propagation in jointed rock masses. However,thestudyoflong-wavelengthpropagationinajointedrock mass is challenging in the laboratory, and limited data are available under controlled test conditions. Long-wavelength P-wave and S-wave propagation normal to joints, using an axially loaded jointed column device, reproduces a range of jointconditions.Theeffectsofthenormalstress,loadinghistory, joint spacing, matched surface topography i.e., joint roughness, joint cementation e.g., after grouting, joint opening, and plasticity of the joint filling on the P-wave and S-wavevelocitiesandonS-waveattenuationarenotable.The ratio VP / VS in jointed rock masses differs from that found in homogeneous continua. The concept of Poisson’s ratio as a functionof VP / VSisunwarranted,and VP / VScanbeinterpreted in terms of jointed characteristics. Analytic models that consider stress-dependent stiffness and frictional loss in joints as well as stress-independent properties of intact rocks can model experimental observations properly and extract joint properties from rock-mass test data. Thus, joint propertiesandnormalstresshaveaprevalentroleinpropagationvelocityandattenuationinjointedrockmasses.

Shear Strength Estimation of Sandy Soils Using Shear Wave Velocity

Typically, shear strength is associated with large strain phenomena, while shear wave propagation is associated with small strain phenomena. Yet, the effective stress and void ratio, both key determinants of sandy soil shear strength, are also the primary factors affecting shear wave velocity. This study presents a shear wave velocity-void ratio-shear strength correlation through experimental tests. Natural sands taken from various reclaimed or recently deposited sandy fields are used for reconstituting specimens at different void ratios in an oedometer cell. Shear wave velocities are measured while changing the state of the stress in the cell for each specimen prepared at a specific void ratio. The relationship between shear wave velocity and vertical effective stress is found at extreme values of void ratios (emin and emax). Direct shear tests are also performed on specimens with various void ratios. Experimental results show that the internal friction angle of each sand type increases with decreasing void ratio, rendering a unique relationship between friction angle and void ratio. Finally, a procedure is suggested to evaluate the in-situ shear strengths of a sandy soil based on in-situ shear wave velocities. Results show that the suggested method effectively estimates in-situ shear strength.

Small-Strain Stiffness, Shear-Wave Velocity, and Soil Compressibility

AbstractThe small-strain shear modulus depends on stress in uncemented soils. In effect, the shear-wave velocity, which is often used to calculate shear stiffness, follows a power equation with the mean effective stress in the polarization plane Vs=α(σm′/1 kPa)β, where the α factor is the velocity at 1 kPa, and the β exponent captures the velocity sensitivity to the state of stress. The small-strain shear stiffness, or velocity, is a constant-fabric measurement at a given state of stress. However, parameters α and β are determined by fitting the power equation to velocity measurements conducted at different effective stress levels, so changes in both contact stiffness and soil fabric are inherently involved. Therefore, the α and β parameters should be linked to soil compressibility CC. Compiled experimental results show that the α factor decreases and the β exponent increases as soil compressibility CC increases, and there is a robust inverse relationship between α and β for all sediments: β≈0.73−0.27⁡log...

Compression Wave Velocity of Cylindrical Rock Specimens: Engineering Modulus Interpretation

In this study, we experimentally assess which elastic modulus – Youngs modulus or the constraint modulus – is appropriate for application to the compression wave velocity of rock cores measured via an ultrasonic pulse technique and a point-source travel-time method. Experimental tests are performed at pulse frequencies between 50 kHz and 1 MHz, the ratio of diameter (D) to wavelength (λ) is between 0.6 and 25.6, and the specimen length is between 10 and 70 cm. It is found that compression wave velocities obtained from the two methods are constrained wave velocities, and thus the constraint modulus should be applied in the wave equation. Also, the effect of the frequency of the ultrasonic pulse, D/λ, and specimen length on compression wave velocity is negligble within the ranges explored in this study.

Predissolution and Postdissolution Penetration Resistance

AbstractMineral dissolution is a common chemomechanical digenetic process in geological systems. The penetration resistance in sediments that have experienced dissolution is studied using a laboratory-scale cone penetration test device and a calibration chamber. Variables include the initial sediment density and mass fraction of soluble grains. Results show that the void ratio increases with the extent of mineral dissolution; the magnitude of the void ratio change is higher in initially dense sediments. A terminal void ratio is found for dissolution; the void ratio after dissolution will not exceed this terminal void ratio regardless of the extent of dissolution. For boundary conditions applied in this study, the terminal void ratio for dissolution corresponds to a relative density of Dr≈15% , which is attained when dissolution exceeds a mass fraction loss of 10%. While the tip resistance decreases after dissolution, the drop in tip resistance is most pronounced in initially dense sands. A single penetrat...