Bankim Mahanta

Stability analysis of potential failure zones along NH-305, India

The national highway (NH-305) is crucial since it is used as the only alternative connectivity for transportation of goods and other materials for military purpose during the closure of other highway. A number of slope failures were reported in the past therefore hazard zonation map is prepared using five commonly used parameters to identify the potential susceptible areas of failure. This was followed by detailed field investigations for collection of rock engineering parameters and geomechanics classification. Several locations were identified from hazard zonation map and subsequent field investigations for stability analysis. Hazard zonation clearly demarcates the steep cut slopes along the right bank of river Sutlej as potential failure zones, which is also confirmed by low values of slope mass rating. Finite element method was later used to investigate the deformation mechanism associated with such slope failures. The safety factor value is on a higher side indicating the slopes to be stable but the displacement contours and shear strain concentration near the toe of the slope suggests otherwise.

Indirect estimation of compressive and shear strength from simple index tests

Uniaxial compressive and shear strength are two of the very important parameters, commonly required in the initial stages of planning and design of rock engineering projects. So, an attempt has been made in this study to predict compressive and shear strength (output) of rocks from some simple and easily determinable parameters in laboratory viz., point load index, tensile strength, unit weight and ultrasonic velocity (input). Failure modes have also been studied and correlated with their ultrasonic velocity. The study uses two of the most commonly used predictive mathematical techniques: statistical analysis and neural networks to predict the strength parameters. The regression analysis shows that the rock quality parameters are very well correlated with the ultrasonic velocity except for the unit weight. Unlike few researchers in the past, a linear correlation was best suited for the rock quality parameters in this study. On the other hand, Artificial Neural Network (ANN) was able to predict the same strength parameters with a better reliability than regression analysis. The study shows that the proposed method for the prediction of UCS and shear strength is acceptable and can be reliably applied in various rock engineering problems.

The Effect of Lineation on Anisotropy in Dry and Saturated Himalayan Schistose Rock Under Brazilian Test Conditions

The paper presents experimental observations of the anisotropic effect of dry and saturated lineated schistose rock on deformational behaviour and fracture patterns under indirect tensile loading conditions. Dry mylonitic quartz–mica schist (M-QMS) shows strength minima in between lineation orientation of 20°–30° with loading direction, whereas dry crenulated quartz–mica schist (C-QMS) shows strength minima in between 45° and 60°. Water saturation significantly reduces the failure strength and shifts the strength minima towards a higher angle. In addition, the water saturation effect induces plasticity along the lineation direction. P-wave velocity is found to be maximum along lineation direction, which continuously decreases to a minimum value across the lineation. Three types of fracture patterns, namely layer activation, central and non-central, are observed for both the rock types. For both the rock types, tensional fractures occur when the lineation is inclined at 0° and 90° to the loading direction, whereas shear and mixed-mode fractures occur at all other orientations. Fracture through the layer activation is prominent in saturated specimens compared to the dry specimens for β angle range of 15°–60° and 15°–45° in the case of M-QMS and C-QMS, respectively.

Assessment of dynamic material properties of intact rocks using seismic wave attenuation: an experimental study

The mechanical properties of any substance are essential facts to understand its behaviour and make the maximum use of the particular substance. Rocks are indeed an important substance, as they are of significant use in the energy industry, specifically for fossil fuels and geothermal energy. Attenuation of seismic waves is a non-destructive technique to investigate mechanical properties of reservoir rocks under different conditions. The attenuation characteristics of five different rock types, siltstone, shale, Australian sandstone, Indian sandstone and granite, were investigated in the laboratory using ultrasonic and acoustic emission instruments in a frequency range of 0.1–1 MHz. The pulse transmission technique and spectral ratios were used to calculate the attenuation coefficient (α) and quality factor (Q) values for the five selected rock types for both primary (P) and secondary (S) waves, relative to the reference steel sample. For all the rock types, the attenuation coefficient was linearly proportional to the frequency of both the P and S waves. Interestingly, the attenuation coefficient of granite is more than 22% higher than that of siltstone, sandstone and shale for both P and S waves. The P and S wave velocities were calculated based on their recorded travel time, and these velocities were then used to calculate the dynamic mechanical properties including elastic modulus (E), bulk modulus (K), shear modulus (µ) and Poissons ratio (ν). The P and S wave velocities for the selected rock types varied in the ranges of 2.43–4.61 km s−1 and 1.43–2.41 km h−1, respectively. Furthermore, it was observed that the P wave velocity was always greater than the S wave velocity, and this confirmed the first arrival of P waves to the sensor. According to the experimental results, the dynamic E value is generally higher than the static E value obtained by unconfined compressive strength tests.