A hybrid interval prediction model for the PQ index using a lower upper bound estimation-based extreme learning machine

The PQ index is an integrated grouting process parameter that represents the relationship between grouting pressure and grout flow. By tracking the variation in the PQ index, operators can understand and control the grouting process. Therefore, accurate and reliable PQ index prediction is important for grouting process guidance. However, because the original grouting process parameter time series has chaotic characteristics and noise interference, traditional point prediction suffers from inevitable prediction errors and cannot quantify the uncertainties associated with the prediction. In this study, to cope with these deficiencies, a hybrid interval prediction model for the PQ index based on an improved extreme learning machine with a lower upper bound estimation method is proposed. In this model, the lower upper bound estimation method is used to construct an advanced extreme learning machine with interval prediction ability to perceive the uncertainties of the prediction. The improved grey wolf optimizer algorithm is applied to optimize the parameters of the extreme learning machine. Moreover, to enhance the prediction ability, cooperating with the hybrid data preprocessing method based on a noise reduction and decomposition strategy, a feature selection method is developed to identify the chaotic characteristics of the PQ index time series and determine the optimal input form of the interval prediction model. The proposed model is applied to predict the PQ value in the grouting process at a hydropower project in China. The results reveal that the proposed model can construct high-quality prediction intervals, which is superior to the performance of other benchmark models and has high potential for practical applications in grouting projects.