Drilling and blasting is widely used in underground excavation projects. Timing is considered to be the most important factor in construction projects. In cyclic operations such as drilling and blasting, losing time in each cycle will cause a delay in operation for all cycles and can impose huge amounts of budget loss because of the significance of fixed costs. Therefore, this investigation tries to minimise the mucking time in drilling and blasting operations of the Alborz Tunnel in Iran via controlling the topsize of muckpile in order to eliminate the need for time consuming secondary blasting. Using the Split-Desktop system, the size distribution curve for 25 blasting rounds in Alborz Tunnel were obtained from which the topsize of the muckpile for each round was calculated. 16 datasets were used to develop a multiple linear regression model. The other nine datasets were used to validate the model. Comparing the actual and predicted values of topsizes, R2 and RMSE for the model were obtained as 0.73 and 0.14 respectively, showing that the proposed model can be used for controlling topsize of muckpile. Specific drilling and the ratio of amount of charge to the burden in contour holes are revealed to be the most important parameters in controlling the topsize of the muckpile in this particular case. The proposed model was successfully used and can be used in future excavations as long as the condition of rock mass is not changed.