ANALISIS BAHAYA, LINTASAN, DAN SISTEM PROTEKSI TERHADAP POTENSI LONGSORAN TIPE JATUHAN BATU PADA LERENG BANGUNAN PELIMPAH BENDUNGAN TUGU, JAWA TIMUR

The construction of the Tugu Dam spillway does not escape the problem of slope instability, especially the rock fall type landslide as a result of the rock slope cutting work at STA+80. The purpose of this study was to determine the characteristics of the rock discontinuity area and the solutions needed to address the potential hazards of rock fall on the slopes of spillway structure. In this study, a semi-quantitative method conducted based on the Rockfall Hazard Rating System (RHRS) which is carried out by identifying outcrops on rock slopes. Determination of the rock fall trajectory, was conducted by statistical methods on rock mass based on changes in velocity when rocks roll, slide, and bounce. Geologically, the research area belongs to the Mandalika Formation. Based on the RHRS weighting, the total score on the STA+80 slope is 399, which means that the slope needs to be repaired or given safely with a moderate level of urgency. The rock fall trajectory modeling at the measurement location X = 121,875 has a kinetic energy of 973.14 kJ andesite and 72.59 kJ of volcanic breccia, for high results of 0.43 meters of andesite reflection and 2.04 meters of volcanic breccia, and velocity results translational velocity obtained at 33.8 m/s andesite and 8.67 m/s volcanic breccia. The potential for rock fall requires a safety system with a type of retained flexible barriers with a height of 5 meters that can be applied to the toe of the slope.Keywords: rock fall, discontinuity, trajectory, protection system, Tugu Dam

ENERGY DISSIPATING MODES AND DESIGN RECOMMENDATION OF H-SHAPED STEEL BAFFLES SUBJECTED TO BOULDER IMPACT

Flexible barriers are one of the most effective protective structures, which have been widely used for the mitigation of rockfalls. As the only compression members in a flexible barrier system, steel posts maintain the integrity of the interception structure to keep the function of the system. Due to the random trajectories of rockfalls, steel posts may be impacted by boulders directly. The impact scenario may result in the failure of the post and even the collapse of the system. In this paper, firstly, steel baffles were proposed to be an additional structural countermeasure to avoid the direct impact of posts. Secondly, numerical method was adopted to study the structural behaviour of steel baffles under direct boulder impact. Then, an available published experimental test of H-shaped steel beams under drop weight impact loading by others was back analyzed to calibrate the finite element model. Finally, numerical simulations were carried out to investigate the energy dissipating modes and energy dissipating efficiency of the H-shaped steel baffles. The simulation results show that there are three typical energy dissipating modes of H-shaped baffles subjected to boulder impact, namely flexural, local compression buckling and shear buckling. Local compression buckling is the most efficient energy dissipating mode. The thickness of the web of an H-shaped baffle is suggested to be 4 mm and 6 mm for the rated dissipating energy of 50 kJ and 100 kJ, respectively.

The Effects of Upstream Flexible Barrier on the Debris Flow Entrainment and Impact Dynamics on a Terminal Barrier

The destructive nature of debris flows is mainly caused by flow bulking from entrainment of an erodible channel bed. To arrest these flows, multiple flexible barriers are commonly installed along the predicted flow path. Despite the importance of an erodible bed, its effects are generally ignored when designing barriers. In this study, three unique experiments were carried out in a 28 m-long flume to investigate the impact of a debris flow on both single and dual flexible barriers installed in a channel with a 6 m-long erodible soil bed. Initial debris volumes of 2.5 m³ and 6 m³ were modelled. For the test setting adopted, a small upstream flexible barrier before the erodible bed separates the flow into several surges via overflow. The smaller surges reduce bed entrainment by 70% and impact force on the terminal barrier by 94% compared to the case without an upstream flexible barrier. However, debris overflowing the deformed flexible upstream barrier induces a centrifugal force that results in a dynamic pressure coefficient that is up to 2.2 times higher than those recommended in guidelines. This suggests that although compact upstream flexible barriers can be effective for controlling bed entrainment, they should be carefully designed to withstand higher impact forces.

Coupling Depth-Averaged and 3D models for debris flow: a multi-domain strategy

<p>Debris flows are landslide phenomena which occur worldwide, posing a major threat to mountain settlements. They consist of flowing fine and coarse sediment saturated with water, which propagate mainly in channelized paths. Because of their high velocity and unpredictability, the evacuation of local populations is often impossible. Losses of human lives and economical damages can be avoided if a correct risk mitigation procedure is adopted. Hence, mitigation structures, such as filter barriers or flexible barriers are often installed in high-risk areas. The primary goal of these structures is to reduce the flow energy and to retain the coarsest boulders. Their design process, which is still frequently based only on empirical or simplified models, would greatly benefit from the support of a reliable numerical model.</p><p> </p><p>In this framework, continuum-based Depth-Averaged Models (DAMs) have been the dominant numerical tool since the 90s. DAMs can simulate events propagating over a wide area while keeping the computational time low, even on complex topographies (Pirulli, 2010). Nevertheless, the averaging process applied to velocity and pressure causes a loss of information, which is critical when the flow impact against structures is evaluated. A full 3D model would allow for a more accurate resolution of fluid-structure interaction (Leonardi et al., 2016). However, debris flows may propagate up to kilometres, and a complete 3D analysis would therefore require exceedingly long computational times.</p><p> </p><p>To bypass the shortcomings mentioned above, this work aims to couple DAMs to a 3D model based on the Lattice Boltzmann Method (LBM). Thus, the domain is split into two parts. First, DAMs describes the flow evolution from its initialization to the transport phase. In this portion of the domain, no structures are present. When the flow approaches a structure, DAMs is coupled to a 3D model. To verify the coupling procedure accuracy, the model is benchmarked on the laboratory tests conducted by Moriguchi et al. (2009). These laboratory tests targeted the flow of dry sand on a steep chute, evaluating the flow impact on a barrier. Preliminary results suggest that the coupled model reproduces the laboratory results reasonably well.</p><p> </p><p><strong>Keywords: </strong>debris flow, coupled numerical modelling, depth-averaged method, 3D Lattice-Boltzmann Method</p><p> </p><p>REFERENCES</p><p>Leonardi, A., Wittel, F. K., Mendoza, M., Vetter, R., & Herrmann, H. J. (2016). Particle-Fluid-Structure Interaction for Debris Flow Impact on Flexible Barriers. Computer-Aided Civil and Infrastructure Engineering, 31(5), 323–333.</p><p>Moriguchi, S., Borja, R. I., Yashima, A., & Sawada, K. (2009). Estimating the impact force generated by granular flow on a rigid obstruction. Acta Geotechnica, 4(1), 57–71.</p><p>Pirulli, M. (2010). On the use of the calibration-based approach for debris-flow forward-analyses. Natural Hazards and Earth System Science, 10(5), 1009–1019.</p>

Assessing the performance of flexible barrier subjected to impacts of typical geophysical flows: a unified computational approach based on coupled CFD/DEM

<p>Flexible barriers have been increasingly used in the mitigation of destructive geophysical flows, including rock avalanches, debris avalanches, debris flood, muddy debris flows as well as muddy flows. No rigorous analytical tools are available for the design of flexible barriers to resist a wide spectrum of geophysical flows of different natures and over a broad Froude-number range. Responses of a flexible barrier to the impacts of geophysical flows are known to be exceedingly complicated, involving intricate multi-body, multi-phase interactions, mass exchange and transportation and energy transformation/dissipation which are challenging for both numerical and physical modelers. To investigate the complex interactions between channelized geophysical flows and a non-uniform flexible barrier, a unified hydro-mechanical modeling framework was developed based on the coupled computational fluid dynamics and discrete element method (CFD/DEM). Five typical geophysical flows were modeled, for instance, a muddy debris flow was considered as a mixture of a continuous viscous fluid phase and a discrete phase consisting of gap-graded frictional particles. A permeable flexible barrier consisting of deformable meshes, cables and energy dissipators was modeled by applying the DEM accounting for connections and contact in a realistic manner. The coupled CFD/DEM model was well validated by experimental data in the literature. Based on the simulations, we examined the dynamics of flow-barrier interactions, energy dissipation mechanism, regime quantification, peak-static load ratio, momentum reduction and the correlations between flow Froude number/solid fraction and the impact mechanism transitions. It was observed that the peak-static load ratio in a flexible barrier increases while the barrier-induced momentum reduction of overflow decreases with increasing flow Froude-number. The analyses of the peak-static load ratio showed that rock avalanches generate the largest one and muddy flows generate the lowest one. For the first time, the impact mechanism transitions from pile-up to run-up for five geophysical flows impacting on a non-uniform flexible barrier were quantitatively identified according to the approaching flow dynamics and solid fraction. (<em>The study was supported by RGC/HK under T22-603/15N and GRF#16205418.</em>​)</p>

Nonlinear Numerical Modeling of the Wire-Ring Net for Flexible Barriers

To investigate the nonlinear mechanical behavior of the wire-ring net, this paper presents a new numerical model that can collectively consider equivalence between numerical and actual wire rings. Quasi-static tests, including tensile tests on steel wires and one-ring specimens, and puncturing tests on net specimens were conducted. Based on the test results, the axial constitutive curves of steel wires were obtained. The linear correlation equations for the breaking loads of the one-ring specimens and the puncturing strength of wire-ring nets were established, both of which were related to the number of windings. The wire rings were modeled via an equivalent structure with a single winding and a circular cross section. Equivalence between the numerical and actual wire rings in terms of bending and tensile strength, total mass, contact with sliding friction, and rupture behavior were also derived and presented. In particular, the emphasis was on simulating the flattening effect, a phenomenon rarely accounted for in conventional numerical models. All dominant factors were reflected in a model with the material law by the input of material parameters. The proposed mechanical model was calibrated and verified by the data from the tests of the wire-ring net. The calibrated mechanical model is also shown to successfully simulate a full-scale test of a flexible rockfall protection barrier according to the ETAG027 standard.

Finite element analysis for rockfall and debris flow mitigation works

Landslide risks arising from boulder falls and debris flows are commonly mitigated using rigid and flexible barriers. Debris–barrier interaction is a complicated process, so current design methods rely on the use of the pseudo-static force approach. In addition to physical testing, numerical simulations can be used to provide insight into the impact mechanism. This paper presents the applications of numerical models to simulate rigid and flexible barriers subjected to rockfall and debris-flow impacts, respectively. For rigid barriers, rock-filled gabions, a recycled glass cullet, cellular glass aggregates, and ethylene-vinyl acetate (EVA) foam were assessed for their performance as cushioning materials. From the results, empirical equations were established for predicting the boulder impact forces and penetration into the cushion layer. Amongst the materials considered in this study, rock-filled gabions appear to be the most promising for use in practice. For flexible barriers, finite-element models, calibrated using documented case histories, were developed to simulate the debris–barrier interaction. The models were used to investigate the barriers’ behavior under debris impacts from both force and energy perspectives. From the results, the hydrodynamic pressure coefficient was found to be lower than the current recommended value whilst only a small amount of debris energy was transferred to the barrier.

Large-scale physical modelling study of a flexible barrier under the impact of granular flows

Flexible barriers are being increasingly applied to mitigate the danger of debris flows. However, how barriers can be better designed to withstand the impact loads of debris flows is still an open question in natural hazard engineering. Here we report an improved large-scale physical modelling device and the results of two consecutive large-scale granular flow tests using this device to study how flexible barriers react under the impact of granular flows. In the study, the impact force directly on the flexible barrier and the impact force transferred to the supporting structures are measured, calculated, and compared. Based on the comparison, the impact loading attenuated by the flexible barrier is quantified. The hydro-dynamic approaches with different dynamic coefficients and the hydro-static approach are validated using the measured impact forces.