Granite residual soil slopes are highly water-sensitive and prone to rapid collapse, strength degradation, and rainfall-induced erosion. This study investigates the improvement effects and underlying mechanisms of carboxymethyl cellulose (CMC) on the water stability, mechanical properties, and rainfall erosion resistance of granite residual soil from Fuzhou, Fujian Province, China. Laboratory tests, including unconfined compressive strength (UCS) tests, direct shear tests, disintegration tests, slope rainfall scouring model experiments, X-ray diffraction test (XRD) and scanning electron microscopy (SEM) observations, were conducted to evaluate the performance and microstructural behavior of CMC-stabilized soils. The results indicate that the addition of CMC significantly enhances soil resistance to disintegration: the 24 h disintegration ratio decreased to 0.5% at 0.5% CMC content. The incorporation of CMC can significantly enhance the unconfined compressive strength (UCS) of the soil and lead to an increase in cohesion, while its effect on the internal friction angle is limited. Under simulated rainfall conditions (30° slope, 120 mm·h⁻¹ rainfall intensity, 60 min duration), slopes stabilized with 0.5% CMC exhibited suppressed rill formation and a 47.5% reduction in sediment yield, accompanied by delayed moisture increase at different depths and reduced infiltration rates. Microstructural analyses reveal that CMC hydration forms gel-like films and filamentous bridges, promoting particle aggregation and pore filling, thereby constructing a denser particle network without generating new chemical compounds. This microstructure collectively enhances soil disintegration resistance, mechanical strength, and slope erosion resistance.

A growing number of coastal foundation pits are being constructed. Based on an actual coastal deep foundation pit project, this study develops a finite element model that incorporates fluid–soil–structure coupling and dynamic seepage boundaries to simulate tidal fluctuations. The model investigates the influence of seawater and river water on the deformation behavior of the foundation pit. Results demonstrate the feasibility of the proposed modeling approach, which integrates fluid–soil–structure coupling with dynamic seepage boundaries and employs appropriate constitutive models for different soil layers. Under tidal action, deformation of the soil on the seaward side of the pit is significantly greater than at other locations. Pore pressure and pit deformation exhibit periodic fluctuations synchronized with the tidal cycle. Compared to static water conditions, pore pressure and surface settlement increase markedly, whereas horizontal displacement shows no significant final difference. An increase in the mean sea level leads to greater horizontal displacement of the diaphragm wall but reduces ground settlement outside the pit. Although river water level changes affect deformation through a mechanism similar to that of mean sea level, its impact is considerably weaker due to the greater distance from the pit and relatively stable water level. Therefore, tidal effects should be prioritized in the design and risk assessment of coastal foundation pits.

Completely weathered granite residual soil is a weathering-derived, soil-like geomaterial whose shear strength is difficult to characterize using only conventional small-scale laboratory tests. This study evaluated the effects of specimen size and material structure by comparing in situ direct shear tests, conventional laboratory direct shear tests on undisturbed and remolded specimens, and large-scale laboratory direct shear tests on remolded specimens with box sizes of 150, 200, and 250 mm. The results show that undisturbed specimens exhibited higher shear strength than remolded specimens, indicating a clear structural contribution. With increasing specimen size, cohesion decreased from 41.2 to 31.4 kPa, whereas the friction angle increased from 35.3° to 40.6°. Compared with the conventional undisturbed test, the in situ tests yielded lower cohesion but higher friction angles. These results indicate that both size effect and structural disturbance significantly influence the interpretation of shear strength parameters in completely weathered granite residual soil. For engineering design in weathered-granite terrains, strength parameters derived from larger specimens or in situ tests are likely to be more representative than those obtained from conventional small-scale laboratory tests.

Rural cut-slope construction constitutes a typical trigger of geological hazards in mountainous regions of developing countries, a risk exacerbated under climate change with the increased frequency and intensity of extreme rainfall events. This study developed an early identification framework for assessing landslide hazard potential associated with such construction, based on the Comprehensive Index Method (CIM). Using Fujian Province, China, as a case study, seven core influencing factors—including slope-wall distance, cut-slope height, and slope gradient—were selected to establish a differentiated weighting system. By integrating multi-source geospatial data, the framework enables automatic identification of potential hazards and risk classification. Results indicate that of the more than 144,000 potential hazard sites identified across the province, 21.20% were classified as medium-risk or higher-risk. High-risk sites display marked spatial clustering, predominantly located in inland counties of northwestern, central, and western Fujian, characterized by steep topography, frequent cut-slope activities, and extensively distributed clay soil layers—conditions highly sensitive to rainfall infiltration. Structural parameter analysis reveals that the vast majority of potential hazard sites exhibit typical engineering geological characteristics, including narrow slope-wall distance, steep cut-slope gradients, and moderate cut-slope height, collectively increasing the susceptibility to rainfall-induced instability. Validation based on two heavy rainfall events in 2024 (Super Typhoon Gaemi and the 9 June Wuping-Shanghang event) yielded identification match rates of 91.8% and 79.98%, respectively, with Kappa coefficients of 0.85 and 0.72, confirming the reliability and practical applicability of the method under extreme weather scenarios. The proposed framework offers valuable support for regional landslide prevention and climate adaptation planning in the context of ongoing climate change.