Constructing rigid barriers along a predicted flow path is a widely used countermeasure against debris flow hazards. In particular, boulders are accumulated and transported at the flow front through particle-size segregation during surging down the slope. It is vital to shield and protect the barriers to meet safety requirements since large boulders can destroy the resisting structures along flow path and cause possible loss of life. Therefore, installation of cushion layers in front of rigid barriers to mitigate the impact force is a common engineering measure. Although previous researchers focused on a single boulder impact on different cushion materials shielding rigid barrier. Cushion layers attenuating the impact force by a cluster of boulders is not well known. The main objectives...[ Read more ]

Constructing rigid barriers along a predicted flow path is a widely used countermeasure against debris flow hazards. In particular, boulders are accumulated and transported at the flow front through particle-size segregation during surging down the slope. It is vital to shield and protect the barriers to meet safety requirements since large boulders can destroy the resisting structures along flow path and cause possible loss of life. Therefore, installation of cushion layers in front of rigid barriers to mitigate the impact force is a common engineering measure. Although previous researchers focused on a single boulder impact on different cushion materials shielding rigid barrier. Cushion layers attenuating the impact force by a cluster of boulders is not well known. The main objectives of this study are to investigate the attenuation mechanism of various cushion materials under bouldery flow impact and determine the behaviour of gabions subjected to flow impacts up to 1000 m³ of boulders.

Two types DEM-FEM numerical flume modelling of 28-m-long in Hong Kong and 172-m-long in Kunming, respectively, were used to investigate the bouldery flow impact on the cushion layers. After calibration with large-scale physical tests, three kinds of cushion materials, gabion, EVA and cellular glass were examined for the cushioning performance under the bouldery flow impact up to 10 m³ using 28 m long flume. Additionally, a series of simulations using the 172-m-long flume were investigated for the effects of flow volume up to 1000 m³ and gabion thickness of 0.5 m and 1.0 m, respectively. Furthermore, a 28-m-long physical flume model is used to investigate the effects of higher boulder ratio and the cushioning influences of debris on a cluster of boulders are discussed.

It was found that flexible, low density and elastic EVA foam cushions are at least 1.5 times as effective at reducing peak impact force on rigid barrier compared to stiffer rock filled gabion. When flow volume up to 1000 m³, although increasing gabion thickness to 1.0 m can reduce peak barrier impact force significantly, a 0.5 thick gabion is sufficient to meet the requirement of peak barrier impact force. Gabion deformation due to bouldery flow impact is localized to bottom half of corresponding disposition height and can reach 88% of original gabion thickness. Gabion thickness along the barrier can be redesign by using a 1.0 m thick gabion in the bottom half and a 0.5 m thick gabion in the top half based on current international guideline. A newly volume-dependent allowable gabion deformation envelope is used to describe the relationship between bouldery flow volume and normalized peak gabion deformation using a 0.5 m thick gabion. The impact effects of a cluster of boulders can be taken into account in the modified equation for predicting the load reduction factor K_c with the reflected debris length.