To elucidate the wear mechanisms of the scraper in shield tunneling through sandy pebble strata, this study aims to achieve high efficiency and low wear during the tunneling process. We evaluate the operational parameters and tool wear characteristics of a 9-m diameter spoke-type shield machine used on the Beijing Daxing Airport Line. The analysis focuses on the wear values of the scrapers and rippers, wear of the scraper in different wear forms, and scraper wear relative to the position of the rippers obtained from the field. The study yielded the following conclusions. The wear values of scrapers on different spokes vary significantly owing to ripper protection. The wear of the scrapers can be categorized into six types: tooth chipping, local damage of teeth, wear of side teeth, wave-type of wear, wear on intermediate teeth, and flat wear, with the majority exhibiting wear on the side and intermediate teeth. The 0 degrees spoke maintained the initial shape of the scrapers, making it more suitable for tunneling in sandy pebble strata. Based on the differences in the relative positions of the ripper and scraper, a model is proposed to determine the ripper plowing influence area. It was found that this area depends on the geological conditions of the soil; thus, the influence angle of ripper plowing in the considered sandy pebble strata is determined to be between 35 degrees and 50 degrees. The results obtained in this study provide a theoretical reference for optimizing scraper layouts in shield construction, even when operating under varying geological conditions.

Gravelly soil strata exhibit heterogeneity and nonlinearity in their physical and mechanical properties, leading to volatile fluctuations of shield scraper force. Understanding the performance of shield scrapers in gravelly soils is significant for the safe and efficient excavation of tunnel boring machines. This paper conducts unconsolidatedundrained triaxial tests to obtain the mechanical properties of gravelly soils with gravel content ranging from 0 % to 30 %. The process of shield scrapers cutting through gravelly soils is analyzed by combining the varying mechanical properties of gravelly soils with the limit equilibrium analysis method. Subsequently, modified models for predicting the scraper force and specific energy in gravelly soils are established. Based on these models, the impact of key factors on the scraper performance is analyzed. Laboratory experiments are further performed on a rotary test bench to validate these models. The experimental results demonstrate that the horizontal cutting force and specific energy of shield scrapers in gravelly soils can be predicted with mean average errors of 8.8% and 7.3%, respectively, with the errors of all predicted values falling within +/- 20 % of the experimental results. These established models can serve as useful references for the structural and operational design of shield scrapers in gravelly soil strata.

Tunnel boring machine serves as a piece of crucial excavation equipment in cross-river and cross-sea tunnel projects. However, the presence of gravelly soil strata in these projects poses significant challenges, resulting in substantial damage to the shield scraper. This study establishes discrete-element numerical models for gravel soils by inversing the macroscopic parameters obtained from triaxial tests on gravelly soils. Numerical simulations were subsequently carried out, and the impacts of different key factors on the cutting process of shield scrapers were investigated, and a force prediction model of the shield scraper was established using the multivariate adaptive regression spline. Laboratory experiments were conducted on a rotary test bench, and the experimental results show that the predicted cutting force of the shield scraper has an average error of 14.2% compared with the experimental results. As the gravel content escalates from 30% to 70%, the cutting force initially rises to a peak and then declines, with the peak occurring at about 62%. The penetration influences the cutting force more than the front rake angle and blade width. The impact of the front rake angle on the scraper performance is less apparent than other factors in high gravel content conditions.