JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year STRATA CONTROL TECHNOLOGY Previous Question Paper (July/August - 2022 - R16)

The Q-system, developed by Barton et al. (1974), is a rock mass classification system used to estimate rock mass quality and support requirements. It is defined by the following six parameters:

1. RQD (Rock Quality Designation): Represents the degree of jointing or degree of rock mass quality.
2. Jn (Joint Set Number): Represents the number of joint sets in the rock mass.
3. Jr (Joint Roughness Number): Describes the roughness of the joint surfaces.
4. Ja (Joint Alteration Number): Describes the degree of weathering or filling material in the joints.
5. Jw (Joint Water Reduction Factor): Accounts for the influence of water pressure and flow on the rock mass.
6. SRF (Stress Reduction Factor): Accounts for the influence of in-situ stress, shear zones, or squeezing ground conditions.

The Q-value is calculated using the formula:

Geological factors significantly influence the stability of underground excavations. Five key factors contributing to strata control problems are:

1. Lithology and Rock Strength: Variations in rock type, such as the presence of weak shale or mudstone layers interbedded with stronger sandstone, create differential stress distribution and potential for roof falls or floor heave.
2. Structural Discontinuities: The presence of faults, joints, and bedding planes creates planes of weakness. These discontinuities reduce the overall shear strength of the rock mass, leading to instability if they intersect the excavation boundary.
3. In-situ Stress Field: High horizontal stresses, often of tectonic origin, can lead to rock bursting, spalling, or squeezing ground conditions, particularly at greater depths, necessitating robust support systems.
4. Groundwater Conditions: High pore water pressure reduces the effective normal stress on joints, lowering their shear strength. Water also facilitates the weathering of clay-rich minerals, leading to time-dependent degradation of the rock mass.
5. Presence of Weak Zones or Shear Zones: Zones of highly fractured or crushed rock associated with past geological movements act as zones of low stiffness. Excavating through these areas often leads to excessive deformation and requires specialized support measures to prevent collapse.