The research team, led by Wei Sun of The 1st Geological Brigade of Sichuan along with colleagues from Xihua University, focused on the No. 1 high slope of the Maoniup mine, where elevations range from 2,800 to 3,200 meters and open-pit excavation depths reach 30 to 60 meters. The slope consists of granite masses intersected by multiple sets of diabase dikes—intrusive igneous formations created when deep basaltic magma pushed into shallow crustal strata and crystallized. Field surveys identified 25 such dikes across the study area, with lengths ranging from tens of centimeters to tens of meters, most between 5 and 30 meters. These dikes are classified as extremely soft rock with highly fragmented integrity, exhibiting pronounced water-softening behavior: their strength drops drastically after saturation and mechanical disturbance.

The urgency of the problem is underscored by the mine’s recent history. In May 2021, a landslide with a volume of approximately 1.08 million cubic meters occurred on the slope below the 3,080-meter platform. Repeated deformation and failure events have recurred since May 2023, and the Maoniup Rare Earth Mine has been listed as one of China’s key regulated mines, with comprehensive remediation measures urgently required.

Field investigations revealed extensive tensile cracks across all platforms of the slope, with visible cracks concentrated precisely in zones where diabase dikes are distributed—while areas without diabase outcrops remained crack-free. On the 3,056-meter platform, tensile cracks stretched approximately 122.8 meters in length, with widths of 20 to 40 centimeters and depths of 30 to 60 centimeters. Similar cracks appeared on the 3,032-meter and 3,020-meter platforms, and local slumps developed on the slope section between the 3,032-meter and 3,056-meter platforms. Emergency displacement-monitoring instruments installed at five points confirmed a progressive-failure pattern propagating from the 3,056-meter platform toward the 3,080-meter platform, with the most intense deformation recorded at monitoring point M2, where displacement increased from below 20 millimeters to approximately 160 millimeters between mid-July and November 2023.

To quantify the mechanical properties of the fractured diabase, the team conducted in-situ shear tests at two field locations. The results were striking: diabase cohesion measured less than 28 kilopascals and its internal friction angle fell below 30 degrees—values that place it between completely weathered and highly weathered granite in shear strength. These extremely low saturated shear strength parameters confirm that the dikes deteriorate sharply after rainwater softening, making them the dominant internal weak structural planes triggering instability.

With field data in hand, the researchers built a three-dimensional FLAC3D finite-difference model matching the actual four-layer stratigraphy and platform topography of the slope. Each model measured 670 meters in length, 0.5 meters in width, and 406 meters in depth, discretized with eight-node hexahedral elements and local mesh refinement down to 1-meter element size within diabase weak zones. The Mohr–Coulomb strength reduction method was adopted to simulate progressive slope instability by iteratively reducing rock and soil shear strength parameters via scaling factors until the slope reaches a critical unstable state. The final reduction factor at this threshold defines the slope safety factor—a dimensionless measure of how much strength reserve remains before catastrophic failure.

The simulation results reproduce the complete deformation-failure sequence with remarkable fidelity. At a strength reduction factor of 1.0, the slope remains essentially stable with negligible displacement. When the factor increases to 1.25, a deformation concentration zone first emerges on the upper 3,056-meter platform where diabase dikes outcrop, with maximum displacement reaching 0.0304 meters. At 1.5, abrupt displacement jumps occur as high-displacement regions propagate downward along the diabase dikes to the 3,032-meter platform, linking deformation zones and generating a localized potential sliding block—matching the field-recorded small slump between these two elevations. By a factor of 1.75 to 2.0, a continuous displacement band develops from the slope crest down to mid-slope, and shear strain bands extend downward following diabase dip directions to form interconnected through-going slip surfaces across multiple platforms. At 2.25, a fully connected through-going slip surface develops from crest to toe along the diabase interlayers, triggering multi-bench staged sliding that corresponds precisely to the progressive failure observed in monitoring data and field cracks.

Under rainfall conditions, the researchers used a saturated-parameter-replacement approximation rather than rigorous hydro-mechanical coupling, assigning saturated mechanical parameters only to shallow weathered rock units—including diabase dikes, completely weathered granite, and highly weathered granite—while deep granite retained natural-state parameters. This calculation yielded a slope safety factor of 1.15, with large-deformation sliding failure occurring at the 3,080-meter, 3,056-meter, 3,032-meter, and 3,007-meter platforms. Critically, the deformation-failure pattern under rainfall closely matched simulation results obtained with strength reduction factors ranging from 1.75 to 2.0, verifying that the strength reduction method can reliably predict progressive slope failure even when accurate rainfall-induced rock-soil mechanical parameters are unavailable.

To quantify how diabase dike orientation controls stability, the team constructed six numerical models with varying dip angles and dip directions. The results demonstrate that diabase dikes subparallel to slope platforms deliver the highest slope safety reserve at 1.4, because shallow weak layers cannot form deep through-going slip surfaces. Steeply down-dipping dikes at 70 degrees block deep shear band expansion and maintain a safety factor of 1.3. By contrast, anti-dipping diabase dikes with a 45-degree dip angle generate the largest sliding volume and the highest landslide risk, with a safety factor of only 1.2. All remaining configurations also yielded safety factors of 1.2, confirming that dike occurrence fundamentally shapes the failure pathway.

The study clarifies the staged progressive failure evolution law of open-pit multi-stage high slopes containing diabase weak interlayers and provides theoretical references for stability evaluation, excavation optimization, and geohazard prevention at analogous high-altitude rare earth mines. The authors note that their thin-extruded three-dimensional model cannot capture along-strike spatial variations of diabase dike orientation, and that fully coupled hydro-mechanical analysis will be performed in future work once sufficient field seepage-monitoring data become available. For now, the findings offer a practical workflow: prioritize monitoring at dike outcrop positions on slope platforms and design excavation schemes that avoid simultaneous exposure of multiple groups of water-sensitive diabase dikes—lessons that could prove vital for mines worldwide facing similar geological conditions.

Subject of Research: Progressive deformation and failure mechanisms of multi-stage high slopes in open-pit mines containing weak diabase dikes, analyzed using the strength reduction method

Article Title: Analysis on deformation and failure of multi-stage high slopes in open-pit mines based on strength reduction method

Article References: Sun, W., Tang, Z., Ye, L., Fan, X., Tang, J., & Pan, H. (2026). Analysis on deformation and failure of multi-stage high slopes in open-pit mines based on strength reduction method. Frontiers in Earth Science, 14, Article 1970385. https://doi.org/10.3389/feart.2026.1970385

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1970385

Keywords: open-pit mining, slope stability, diabase dikes, strength reduction method, FLAC3D, progressive failure, landslide, rare earth mine, rainfall softening, geohazard, Sichuan, rock mechanics