The eastern Himalayan syntaxis is one of the regions having the most intense tectonic activities, the most complex geological conditions, and the most frequent geohazards in the world. The planning and construction of engineering projects are faced with four types of catastrophic geo-safety risks, including tectonic faulting in the plate tectonic belt, disaster occurrence of the deep-buried tunnels, instability of loose mountains, and regional geological disaster chain. It is a critical topic in the field of engineering geology how to select relatively stable and safe sites in active tectonic zones to minimize the geo-safety risks of planning, construction, and operation of engineering projects. This paper summarized the major geo-safety problems in the eastern Himalayan syntaxis. Accordingly, it revealed that traditional site selection theories hardly satisfy the requirements of the engineering projects in the eastern Himalayan syntaxis area. The site selection encounters geo-safety challenges caused by unclear geological evolution process and construction, prominent disaster risk of tectonic activity and strong earthquake, weak research on the deep tectonic stress field and disaster evaluation, and severe ultra-high-elevation and ultra-long-runout geological disaster chain. Thus, this paper suggested the main research directions of the site selection from five aspects: (1) regional geological evolution and engineering geological problems, (2) active fault and engineering safety risk, (3) complex in-situ stress field and engineering disaster risk, (4) engineering risk of regional geological disaster chain, and (5) theory and method of site selection in eastern Himalayan syntaxis. This paper provides ideas for improving the risk assessment and prevention methods of site selection of engineering projects.

The Namjag Barwa syntaxis is in the eastern Himalayan syntaxis area with the most intensive neotectonic activity. There are many late Quaternary active fault belts and strong seismicities. The tectonic stability of these active fault belts, such as the Jiali, Dongjiu-Milin, and Motuo fault belts, may influence the project's construction. In-situ stress is a critical parameter for estimating regional tectonic stability. Currently, there is a lack of abundant in-situ stress results about the Namjag Barwa syntaxis. It is challenging to assess geological safety risks for major projects. Based on focal mechanism solutions, the paper reveals the orientation of the maximum principal stress surrounding the Namjag Barwa syntaxis using the stress tensor inversion method. According to the critical condition of fault instability, the magnitudes of principal stresses around the Namjag Barwa syntaxis are also estimated by combining the inversion of the stress shape ratio and the frictional coefficient. The results indicate that the maximum principal stress direction in the Namjag Barwa syntaxis area is NE-NNE. The maximum and minimum horizontal principal stresses increase linearly with depth at a gradient of 0.032~0.0355 MPa/m and 0.0227~0.0236 MPa/m, respectively. Heterogeneous features of the in-situ stress field still exist. Generally, the results estimated in this study are in good concordance with the in-situ stress measurements. They can provide reliable in-situ stress parameters for evaluating the tectonic stability in the Namcha Barwa region.

The southwest mountainous region in China is the worst-hit area due to the most developed geohazard chains. In order to better understand the pattern of the geohazard chains in this region, we analyzed the main control factors and characteristics of the geohazard chains based on 19 typical geohazard events in history. Three classification patterns, namely landslide-type, collapse-type, debris flow-type, and five typical hazard-inducing processes, were summarized. A typical hazard-inducing chain process in each model was selected for analysis. On this basis, we discussed the mechanism of the geohazard chain, the construction of a database and technical standard system, and the measures for preventing and controlling transboundary basin chain-type geohazards, aiming to provide a reference for the regional geohazard prevention and mitigation plan, and the major engineering projects' construction and people's safety in town.

The lower stream of the Yarlung Zangbo River is in the front zone of the collision between the Indian and Eurasian plates with active neotectonics movements and many high mountains in this region. It is a typical mountain-valley area. Due to the unique geological structure and the influence of climate change, geohazards such as collapses, landslides, and mudslides frequently happen in this area. We used Sentinel-1 and ALOS/PALSAR-2 images to identify the high-elevation geohazards in the region from 2014 to 2020 by combining multiple time-series InSAR techniques and SAR offset-tracking techniques. The identification results show that there are 260 geohazard-induced deformed areas in the study area, and most of them are located in gullies and peaks at higher elevations. The rock avalanche deformations in the Zebalongba glacier gully have formed several large tension cracks, and once the avalanche falls, they are most likely to form a dam. The back edge of the Dabo landslide, which was reactivated by the Milin earthquake, has completely been detached, and the cracks fully penetrate the left and right sides. Once the landslide destabilizes, it will completely block the Yarlung Zangbo River. This study provides a general method for identifying high-elevation geohazards in high mountain-valley areas and a reference for similar geohazards identification.

The pile-beam composite structure in high-elevation debris flow areas is selected as the research object. Based on characterizing the pile-beam composite structure, the particle-flow simulation analysis program and the explicit dynamic analysis program were used to study comparatively the blocking effects of single-row piles and two-row piles, as well as that of a pile-beam composite structure at different positions. Besides, We simulated the mechanical characteristics of the pile-beam composite structure and discussed debris flow accumulation and structural stress transfer after the blocking. The calculation results show that the blocking effect of the pile-boulder force chain formed by the contact between large-size particles in the debris flow with the blocking structure and side boundaries on both sides of the gully could effectively block and delay the subsequent debris flow movement. The blocking effect of the pile-beam composite structure is the best. Meanwhile, the transition zone between the two-row piles further suppressed the flow velocity. When choosing the position for a pile-beam composite structure, we should consider suppressing the debris flow velocity as early as possible at the beginning and the potential energy-kinetic energy conversion process. Meanwhile, we also need to emphasize the design of the reservoir capacity, beware of the escape of debris flow due to a low-head barrier, and choose the optimal solution for the layout. The impact stress by debris flow boulders will be transmitted to the rear pile through the connecting beams, and the connecting parts at both ends of the beam almost reach the yield strength, which needs reinforcement to strengthen.