Corroding Anchors and Earthquakes Combine to Threaten Aging Slopes

Deep beneath the surface of countless hillsides, highway cuts, and reservoir banks, steel anchor cables are quietly fighting a losing battle against time. These prestressed tendons, grouted into rock and soil, hold back slopes that would otherwise slide, and in earthquake-prone regions their silent service is a matter of life and death. A new study published in Discover Geoscience by Haitao Wang of Dalian Jiaotong University and colleagues now offers one of the most detailed quantitative portraits yet of how corrosion and seismic shaking conspire to erode the safety of anchored slopes, revealing a troubling acceleration of risk after roughly fifty years of service.

The research team focused on tension-type prestressed anchors, the workhorse of early slope reinforcement in China. Unlike compression anchors, whose grout stays squeezed and therefore rarely cracks, tension anchors place their grout in a state that invites cracking. Those cracks become highways for groundwater, oxygen, chloride, and sulfate ions to reach the steel tendon, which is itself held under high stress. The result is a particularly vicious form of deterioration: stress corrosion that can transform a ductile steel strand into a brittle one, quietly stripping away the load-carrying capacity that engineers counted on at design time.

To capture this decay mathematically, the researchers built on an electrochemical corrosion rate model originally developed by Hong, adapting it to anchor cables. Their equation links the corrosion rate to ambient temperature, relative humidity, the thickness of the grout cover wrapping the tendon, and the compressive strength of the grout itself. The physics is intuitive once unpacked: warmer conditions accelerate the electrochemical reactions through Arrhenius-type kinetics, moisture drives the cathodic half of the corrosion cell, and a thicker, stronger grout cover slows the ingress of aggressive species. Because fully enclosed anchors corrode far more slowly than those exposed to wet-dry cycling, the team applied a correction factor, effectively assuming the grout remains intact until corrosion products expand and crack it from within.

From this corrosion rate, the researchers derived time-varying expressions for three competing failure modes. The first is debonding at the mortar-rock interface, where the entire grout column pulls out of the borehole. The second is slippage at the tendon-grout interface, where corrosion products and chemical degradation of the cement matrix erode the bond that transfers load from steel to rock. The third is tensile yielding of the free segment, as the effective cross-section of the steel shrinks and its yield strength drops. Crucially, the anchorage system behaves as a series, or weakest-link, system: whichever of the three resistances is smallest at any given moment governs the anchor’s capacity, and failure of that single link means failure of the whole.

The study’s most striking mechanistic finding is that this weakest link does not stay put. In the early decades of service, the mortar-rock interface typically controls the anchoring force. But as corrosion progresses, expanding rust products crack the grout, allowing water, oxygen, chloride, and sulfate ions to flood the tendon surface. The tendon-grout bond then collapses far faster than the other resistances, and control of the anchoring force shifts to that degrading interface. In the case study, a twelve-meter anchor’s tendon-grout bond resistance plummeted from 2,714 kilonewtons to just 291 kilonewtons over a century, while the free segment’s yield resistance fell more gently from 1,316 to 1,033 kilonewtons. This mechanism transition is what produces the alarming acceleration of anchoring-force decay in the later service life.

To translate anchor degradation into slope-scale risk, the team embedded their time-dependent anchor capacities into a pseudo-static limit equilibrium framework based on the horizontal slice method and the Mohr-Coulomb strength criterion. Seismic demand was not applied uniformly: following the Chinese seismic design code for hydraulic structures, the horizontal acceleration coefficient was distributed trapezoidally along the slope height, rising from the toe to the crest to reflect the well-documented topographic amplification of ground motion. A genetic algorithm searched for the critical slip surface, and the First-Order Reliability Method, implemented through the JC algorithm with equivalent normalization of non-normal variables, computed failure probabilities at each service age. Peak ground acceleration was treated as a Gumbel-distributed extreme-value random variable, consistent with standard probabilistic seismic hazard practice.

The case study examined a 28-meter rock slope cut into homogeneous slate at 47 degrees, reinforced with seven rows of high-strength strands and analyzed under four seismic intensity levels ranging from 0.1 g to 0.3 g. The results trace a clear three-act drama. For the first forty years, failure probability stays essentially flat, governed mainly by seismic parameters rather than corrosion. Then, between roughly fifty and seventy years, the curves bend upward: under 0.1 g shaking, failure probability jumps from 8.3 percent to 17.5 percent, a 110 percent increase, while under 0.3 g it climbs from about 50.7 to 59.1 percent. In the final decades, the system enters what the authors call a high-risk state, with failure probability exceeding 38.5 percent at 0.1 g and reaching 67.8 percent at 0.3 g by year one hundred.

Perhaps the most counterintuitive insight concerns how seismic intensity and corrosion interact. The absolute gap in failure probability between the 0.3 g and 0.1 g scenarios over a century is about 29.3 percentage points, but the relative difference shrinks dramatically, from 7.7-fold to 1.8-fold. In other words, long-term corrosion accumulation erodes system reliability most severely in low-seismicity environments, where a corroded anchor system has little residual margin to absorb even modest shaking. The authors also benchmarked their results against Chinese code target reliability indices, finding that under a 0.2 g scenario the failure probability surpasses 40 percent after about 85 years, a reliability index below 0.25 that would be unacceptable for most structures, while early-life indices above 1.0 are only marginally adequate even for low-consequence applications.

The sensitivity analysis delivers a rare piece of genuinely actionable engineering guidance. Increasing the grout cover thickness over the tendon from 20 millimeters to 50 millimeters cuts the hundred-year failure probability by 42 percent, raising the reliability index from roughly 0.31 to 0.82. Thin covers show accelerated failure probability growth beginning around thirty years, whereas thicker covers delay that inflection to around fifty years. Conversely, every 10-degree-Celsius rise in average temperature or 20 percent rise in relative humidity boosts the failure probability growth rate by 15 to 25 percent, a sobering prospect for coastal and monsoon-climate regions and for a warming world. Among all parameters examined, the environmental corrosion coefficient exerts the strongest influence on reliability, followed by humidity and temperature, confirming that durability, not seismic demand alone, dominates long-term performance.

The authors are candid about the limits of their framework. They assume uniform rather than pitting corrosion, treat corrosion across the anchor array as statistically independent rather than spatially correlated, and model fully saturated conditions without the strengthening effect of matric suction in unsaturated soil, all choices that make their baseline estimates conservative. Even so, the practical implications are hard to ignore. The fifty-year inflection point offers a quantitative target for maintenance scheduling, pointing to interventions such as secondary grouting or anchor supplementation well before reliability collapses. For major projects with design lives beyond half a century, the study argues, durability design must incorporate environmental acceleration factors and hygrothermal coupling, and slopes in hot, humid regions with inadequate grout cover may fall below acceptable reliability thresholds well within their intended service life. What emerges is a clear message for infrastructure owners: the anchors holding back the world’s slopes are aging on a schedule that static, time-invariant safety checks were never designed to see.

Subject of Research: Time-variant reliability analysis of corroded prestressed anchor-reinforced slopes under seismic loading

Article Title: Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects

Article References: Wang, H., Meng, X., Yang, X., Jia, J., Tu, B., Zhang, L., Pan, X., & Han, Q. (2026). Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects. Discover Geoscience, 4(1), Article 339. https://doi.org/10.1007/s44288-026-00700-5

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00700-5

Keywords: anchored slopes, prestressed anchors, corrosion, seismic loading, time-variant reliability, slope stability, geotechnical engineering, failure probability, grout cover thickness, limit equilibrium method, FORM, infrastructure durability