What is the purpose of the Q-system in rock mass classification?

The Q-system, also known as the NGI (Norwegian Geotechnical Institute) Q-system, is a rock mass classification system used to assess the quality of rock masses for tunneling and other underground construction projects. Its primary purpose is to quantitatively describe the rock mass characteristics so that engineers can estimate the rock's strength, stability, and support requirements. The Q-system achieves this by assigning a numerical value, the Q-value, based on six parameters: Rock Quality Designation (RQD), joint set number (Jn), joint roughness number (Jr), joint alteration number (Ja), stress reduction factor (SRF), and water reduction factor (Jw). These parameters account for the degree of fracturing, the condition of the joints, and the influence of stress and water on the rock mass. The Q-value is then used to estimate the appropriate type and amount of rock support needed to ensure the stability of the tunnel. The system helps in selecting suitable tunnel boring machines and planning excavation sequences. It also provides a framework for monitoring and adjusting support measures as tunneling progresses. The Rock Quality Designation (RQD) is a measure of the percentage of intact core pieces longer than 10 cm in a borehole. The joint set number (Jn) represents the number of joint sets in the rock mass. The joint roughness number (Jr) describes the roughness of the joint surfaces. The joint alteration number (Ja) quantifies the degree of alteration or weathering of the joint surfaces. The stress reduction factor (SRF) accounts for the influence of in-situ stress on the rock mass. The water reduction factor (Jw) considers the effect of water pressure on the rock mass stability.