What is the purpose of RQD (Rock Quality Designation) in geotechnical investigations for open pit slope design?

The purpose of Rock Quality Designation (RQD) in geotechnical investigations for open pit slope design is to provide a quantitative estimate of the degree of fracturing in a rock mass, which is then used to assess the rock mass strength and stability. RQD is calculated from rock core samples obtained during drilling. It is defined as the percentage of intact core pieces longer than 10 cm (4 inches) in the total length of the core run. A higher RQD value indicates fewer fractures and a more intact rock mass, while a lower RQD value indicates more fractures and a less intact rock mass. The RQD value is used as an input parameter in several empirical rock mass classification systems, such as the Rock Mass Rating (RMR) and the Q-system. These classification systems combine RQD with other parameters, such as joint spacing, joint condition, and groundwater conditions, to assign a numerical rating to the rock mass quality. This rating is then used to estimate the rock mass strength and deformation modulus, which are essential parameters for slope stability analysis. RQD is particularly useful in open pit slope design because it provides a relatively simple and cost-effective way to characterize the rock mass over large areas. The RQD data can be used to identify zones of weak or fractured rock that may be prone to failure and to optimize slope angles to ensure stability. It is important to note that RQD is just one parameter used in slope stability analysis, and it should be combined with other geotechnical data and engineering judgment to make informed decisions about slope design.