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

Rock block shape and mass influence rockfall mobility, but their combined effects on runout and lateral spreading are not well quantified. We conducted 144 controlled field tests using elongated, cubic, spherical, and flaky blocks in six mass groups (6.5–55 kg; n = 6 per shape–mass combination). Blocks were released from rest on a 30-m-high, 35° slope and traveled onto a 20-m-long horizontal platform. Impact velocity, horizontal displacement, lateral offset, and the offset ratio (lateral offset/horizontal displacement) were measured. Spherical blocks generally had the highest impact velocities and longest horizontal displacements, whereas flaky blocks had the lowest; cubic and elongated blocks showed intermediate responses. Increasing mass was associated with greater impact velocity and horizontal displacement, while lateral offset decreased mainly for elongated and flaky blocks. An exploratory log-linear model using measured rock mass and categorical shape codes achieved an in-sample log-scale R 2 of 0.902 and a six-fold cross-validated displacement RMSE of 1.18 m. Independent validation is required before design application. Under the tested conditions, the mean horizontal displacement of 55-kg flaky blocks was 28.6% shorter than that of spherical blocks, while irregular blocks showed substantially greater lateral spreading. These results show that both block geometry and mass should be considered when estimating rockfall runout and protection-zone width.