The team, led by researchers at the Guangzhou Urban Planning and Design Survey Research Institute and affiliated laboratories in Guangzhou, China, prepared blocks in four shape classes: elongated, cubic, spherical, and flaky. Each class was defined by the ratios among the block's three principal dimensions, with spherical and cubic blocks sharing similar aspect ratios but differing in edge rounding. Six mass groups were tested, spanning 6.5 to 55 kilograms, and every shape–mass combination was replicated six times, yielding 144 individual releases.

Each block was weighed to within 0.1 kilogram, measured, photographed, and given a unique identifier before its single trip down the slope, a precaution that prevented repeated impacts from altering geometry or surface texture. The test site was a constructed slope with a vertical height of 30 meters and a uniform inclination of about 35 degrees, cleared of loose debris and vegetation to expose bedrock with minor irregularities. At the toe, the team built a horizontal receiving platform 10 meters wide and 20 meters long, surfaced with a 0.3-meter-thick compacted layer of crushed stone.

Keeping the slope geometry and platform condition identical across all tests was the point: it allowed shape and mass effects to be compared under one common field configuration. Blocks were placed at the crest and released from rest without any push, and all trials were conducted in dry, windless conditions. Two high-speed cameras recorded each trajectory from release to rest, one viewing the slope and the other the platform.

Frame-by-frame analysis yielded the impact velocity at the slope toe, while tape measurements to within 0.01 meter gave the horizontal displacement from the toe to the final stopping point. Lateral offset was defined as the nonnegative transverse distance from the slope centerline to the stopping point, and each block received a dimensionless offset ratio, calculated as lateral offset divided by horizontal displacement, before any group averaging. The longest recorded run reached 15.58 meters, still 4.42 meters short of the platform end, so the stopping-distance records show no truncation at the downstream boundary.

The longitudinal results were strikingly consistent. Mean horizontal displacement followed the same ordering at every mass group: spherical blocks traveled farthest, followed by cubic, then elongated, with flaky blocks stopping shortest. In the heaviest, 55-kilogram class, spherical blocks averaged 13.26 meters while flaky blocks averaged 9.47 meters, a difference of 3.79 meters, or 28.6 percent relative to the spherical mean.

In the lightest class the gap was only 1.33 meters, hinting that geometry and scale may act jointly. Mean impact velocity mirrored this pattern, with spherical blocks fastest and flaky blocks slowest at all six masses, and cubic and elongated blocks occupying intermediate positions whose ordering varied across classes. The transverse results told the opposite story.

Spherical blocks stayed remarkably close to the centerline, with mean lateral offsets of only 0.177 to 0.183 meters across all mass groups, and cubic blocks remained modest at 0.263 to 0.373 meters. Elongated and flaky blocks, despite their shorter runout, spread far more widely: in the 6.5-kilogram class their mean lateral offsets reached 1.345 and 1.540 meters respectively. Offset ratios preserved the same hierarchy, flaky highest, then elongated, cubic, and spherical, at every mass group; in the lightest class the mean ratios were 0.655, 0.514, 0.133, and 0.050 respectively.

In other words, the shapes that rolled farthest also stayed closest to the fall line, while the irregular shapes that stopped early wandered widest, a dissociation the authors emphasize as the study's central practical finding. Mass left its own fingerprint. Mean impact velocity rose in every shape class between the lightest and heaviest groups, from 11.14 to 13.02 meters per second in the 6.5-kilogram class to 17.23 to 18.45 meters per second at 55 kilograms, with all four endpoint contrasts highly significant.

Horizontal displacement likewise increased for all shapes, by 8.24 meters for elongated, 9.38 meters for cubic, 9.47 meters for spherical, and 7.01 meters for flaky blocks. But absolute lateral offset declined significantly with mass only for the irregular classes: elongated blocks lost a mean of 0.767 meters and flaky blocks 0.800 meters between the lightest and heaviest groups, while cubic and spherical offsets barely changed. A blanket claim that heavier blocks always spread less is therefore not supported; the reduction is concentrated in elongated and flaky geometry.

The statistical treatment was deliberately cautious. A two-factor fixed-effects model with shape, mass group, and their interaction showed significant marginal effects for all four outcomes, with conventional partial eta-squared values reaching 0.921 for shape in lateral offset. Heteroscedasticity-robust HC3 tests, Holm-adjusted across twelve omnibus tests, confirmed every main effect.

Interaction evidence was strong for lateral offset and offset ratio, absent for impact velocity, and method-sensitive for horizontal displacement: the conventional test was not significant while the robust counterpart was, a discrepancy the authors retain in their interpretation rather than resolve. Among the 36 within-class pairwise displacement comparisons, only six survived multiplicity adjustment, and the headline 55-kilogram spherical-versus-flaky contrast, with a Bonferroni confidence interval spanning −0.47 to 8.06 meters, did not remain significant after correction. Mean rankings, the team stresses, should not be read as universal pairwise separation.

An exploratory regression tied the findings together. Using measured rock mass and categorical shape codes with shape-by-log-mass interactions, fitted by ordinary least squares on the logarithm of displacement, the model achieved an in-sample log-scale R-squared of 0.902. Six-fold internal cross-validation produced a held-out meter-scale R-squared of 0.876 and a root-mean-square error of 1.18 meters, and leaving out an entire mass group at a time gave comparable performance.

Yet the authors are explicit that this is not a design equation: the shape-by-log-mass slope interactions were not significant, the validation is entirely internal, and performance outside the tested 6.5-to-55-kilogram range or at another site remains unknown. The mechanistic interpretation stays deliberately qualitative. A rounded, equant body may maintain a stable rolling path and preserve a large downslope velocity component, while elongated or flaky geometry produces orientation-dependent face, edge, and corner contacts that can redirect translation into rotation, deformation, or rebound and deflect the block off the fall line.

Because angular velocity, contact impulses, and energy partitioning were not measured, these explanations remain hypotheses. The limitations are equally candid: one slope, one platform material, dry conditions only, six observations per factorial cell, no individual dimension or density records to separate mass from size effects, and departures from normality in several outcomes. What the experiment does establish is a workflow: inventory the shape mix in a source area, size the longitudinal envelope using the more mobile equant cases, check the transverse envelope with the irregular classes that spread widest, and test combinations of shape and mass in a calibrated trajectory analysis before fixing barrier alignment or protection-zone width.

Runout and lateral spreading, the authors conclude, must be assessed together, because their shape rankings point in opposite directions.

Subject of Research: Effects of rock block shape and mass on rockfall runout and lateral spreading in controlled field tests

Article Title: Influence of rock block shape and mass on runout and lateral spreading: evidence from controlled field tests

Article References: Huang, K., Liu, G., Huang, J., Wen, Z., Sun, H., Chen, H., Zeng, Q., & Liang, Z. (2026). Influence of rock block shape and mass on runout and lateral spreading: evidence from controlled field tests. Frontiers in Built Environment, 12, Article 1987413. https://doi.org/10.3389/fbuil.2026.1987413

Image Credits: AI Generated

DOI: 10.3389/fbuil.2026.1987413

Keywords: rockfall, block shape, rock mass, runout, lateral spreading, field experiments, geotechnical engineering, impact velocity, hazard assessment, trajectory analysis, protective barriers, Influence