Due to atmospheric scattering, solar radiation, and other factors, the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) product data suffer from a substantial amount of background noise. This poses a significant challenge when attempting to directly utilize the raw data. Consequently, data denoising becomes an indispensable preprocessing step for its subsequent applications, such as the extraction of forest structure parameters and ground elevation data. While the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is currently the most widely used method, it remains susceptible to complexities arising from terrain, low signal-to-noise ratio (SNR), and input parameter variations. This paper proposes an efficient Multi-Level Auto-Adaptive Noise Filter (MLANF) algorithm based on photon spatial density. Its purpose is to extract signal photons from ICESat-2 terrestrial data of different ground cover types. The algorithm follows a two-step process. Firstly, random noise photons are removed from the upper and lower regions of the signal photons through a coarse denoising process. Secondly, in the fine denoising step, the K-Nearest Neighbor (KNN) algorithm selects the K photons to calculate the slope along the track. The calculated slope is then used to rotate the direction of the searching neighborhood in the DBSCAN algorithm. The proposed algorithm was tested in eight datasets of four surface types: forest, grassland, desert, and urban, and the extraction results were compared with those from the ATL08 datasets and the DBSCAN algorithm. Based on the ground-truth signal photons obtained by visual inspection, the classification precision, recall, and F-score of our algorithm, as well as two other algorithms, were calculated. The MLANF could achieve a good balance between classification precision (97.48% averaged) and recall (97.96% averaged). Its F-score (97.69% averaged) was higher than that of the other two methods. This demonstrates that the MLANF algorithm successfully obtained a continuous surface profile from ICESat-2 datasets with different surface cover types, significant topographic relief, and low SNR.

Studies on grout consolidation after synchronous grouting around shield tunnels have ignored the expansion of the grout–groundwater interface and the difference in the permeabilities of the grout and water in the stratum. The process of grout pressure dissipation and diffusion in the stratum was solved with a two-dimensional two-phase seepage model, with which the head losses caused by grout diffusion in the stratum and groundwater seepage in the stratum under the ‘push-displacement effect’ were studied together. In addition, the influence of the temporal variation in viscosity and the infiltration effect were considered. Then, a two-phase analytical seepage model of consolidation for shield grouting considering the temporal variation in viscosity and the infiltration effect was established. A numerical algorithm was designed to solve the theoretical model with the difference method. The analytical model was verified by comparison with the results of numerical simulation. Furthermore, the main parameters in the analytical model were analysed. The results show that the temporal variation in grout viscosity and the infiltration effect greatly influence the stable pressure value and the loss rate of the volume of grout after consolidation. Among the main parameters in the model, the permeability coefficient of the stratum is the most sensitive.

This article outlines the merits of the proposed synchronous propulsion and assembly method in the construction of a long-distance tunnel. In traditional shield construction, the stress of the unlooped segment during the assembly process is systematically overlooked. However, in the novel tunnel construction method, the advancing force of the shield directly acts on the unlooped segment, so the safety of the unlooped segment is unknown. In this paper, focusing on the safety of the segment assembly process, the segment interactions and stress concentrations under asymmetric force effects are analysed in detail via a suite of finite element models. The results show that in the synchronous propulsion and assembly mode the segments will rotate inward. A clamping effect will gradually appear during the assembly process, which makes segment deflection decrease and the stress distribution more uniform. Under asymmetrical stress, the damage to longitudinal segments is highly correlated with the types of assembly errors. The damage position that is deflected radially inward will change with the deflection angle, and the outer joint of the segment is the largest. Based on the numerical outputs, guidelines for the application of synchronous propulsion and assembly technology in practical engineering are provided.