[Objectives] River sediment mining is one of the major causes of bridge scour-induced failures. To address the growing threat to bridge safety posed by increasing global river sediment extraction, this study aims to investigate the effects of sediment mining pits on scour at bridge piers. The goal is to quantitatively reveal the impacts of mining pit location and geometry on clear-water and live-bed scour depths, and to develop reliable predictors for the maximum scour depths at bridge piers in mining-affected reaches. [Methods] Generalized flume experiments were conducted in a water-sediment recirculating flume with a uniform sediment bed and a circular bridge pier model. Two series of tests were performed under clear-water and live-bed scour conditions. A total of 32 runs were designed by systematically varying the pit-pier spacing, pit depth, and pit length. The temporal evolution of pier scour depth was continuously measured using ultrasonic transducers installed around the pier, and the maximum scour depths were extracted from the measured time series. The responses of these scour parameters to mining pit location and geometry were quantitatively evaluated to establish empirical relationships between the pit parameters and the maximum scour depths. [Results] The experimental results show that upstream mining pits exert a greater influence on pier scour depth than downstream pits. Under clear-water scour conditions, an upstream pit increases both the temporal scour depth and the time required to reach equilibrium stage, with the influence intensifying as the pit-pier distance decreases and as the pit depth or length increases. When the upstream pit is within 8 times the pier diameter, or when its depth exceeds 4 times or its length exceeds 20 times the pier diameter, the temporal evolution of scour depth exhibits an oscillatory pattern before reaching equilibrium, indicating that the flow structure around the pier is substantially disturbed by the nearby pit. Under live-bed conditions, the scour depth exhibits a distinct initial peak due to sediment interception by the upstream pit at the early stage, which is approximately 120% larger than that without mining pit; after the pit is refilled, the scour depth decreases substantially and gradually approaches a dynamic equilibrium. The refilling process is accompanied by sediment supply from upstream, which gradually restores the sediment feed to the pier scour hole. Once the pit is completely filled, the scour depth fluctuates around a relatively stable mean value, indicating that the influence of the pit on pier scour becomes substantially weakened. The maximum clear-water and live-bed scour depths both increase with increasing pit depth and length, and decrease with increasing pit-pier distance; when the pit-pier distance exceeds approximately 64 times the pier diameter, the upstream pit effect becomes negligible. Predictors for estimating the maximum pier scour depth considering the impacts of mining pit location and geometry are proposed. The fitting errors for the present experimental data are within ±5% and ±15% for clear-water and live-bed scour conditions, respectively. In addition, two correction factors are proposed to adjust existing predictors of maximum scour depth at bridge piers under non-mining conditions for application in mining-affected reaches. [Conclusions] This study highlights that upstream mining pits are the primary concern for pier scour in mining-affected reaches. In the presence of an upstream pit, the initial peak scour under live-bed scour conditions, though transient, can be substantially larger than the long-term equilibrium scour and should be considered in bridge safety assessment during flood events. The proposed predictors and correction factors provide technical support for scour-resistant design of bridge piers in mining-affected reaches. Future work may extend the experimental conditions to non-uniform sediments, different pier shapes, and unsteady flows, and compare the proposed formulas with field observations to further validate their general applicability.