Effects of Barrier Stiffness on Debris Flow Dynamic Impact—I: Laboratory Flume Test

Debris flows often cause local damage to engineering structures by exerting destructive impact forces. The debris-flow–deformable-barrier interaction is a significant issue in engineering design. In this study, a large physical flume model test device was independently designed to repeatedly reproduce the flow and impact process of debris flow. Three physical flume tests were performed to investigate the effect of barrier stiffness on the debris flow impact. The flow kinematics of debris flow with three barrier stiffness values are essentially consistent with the process of impact–run-up–falling–pile-up. The development of a dead zone provided a cushion to diminish the impact of the follow-up debris flow on the barrier. The peak impact forces were attenuated as the barrier stiffness decreased. The slight deflections of a deformable barrier were sufficiently effective for peak load attenuation by up to 30%. It showed that the decrease of the barrier stiffness had a buffer effect on the debris flow impact and attenuated the peak impact force. And with the decrease of the barrier stiffness, when the barrier was impacted by the same soil types, the recoverable elastic strain will be larger, and the strain peak will be more obvious.

Scour Protection of Submarine Pipelines Using Ionic Soil Stabilizer Solidified Soil

A novel scour protection approach for pipeline using the Ionic Soil Stabilizer (ISS) solidified soil was proposed in this study. The ISS-solidified slurry can be poured adjacent to the pipeline immediately after it was placed, or in the growing scour holes. In the present study, the first type was utilized as the scour protection layer around the pipeline. A series of laboratory flume tests were conducted to validate the protective capacity of ISS-solidified slurry for the pipeline in waves and combined waves and current. Then, the scanning electron microscope (SEM) tests and pore size tests were carried out, respectively, to investigate the mechanism of ISS-solidified slurry for scour protection around the pipeline. Finally, the effects of the ISS-solidified layer for liquefaction stability of non-cohesive subsoil were evaluated. The results indicated that the ISS-solidified slurry is a reliable, economic approach for scour protection around pipelines in the ocean environment. It is noteworthy that if a non-cohesive soil layer underlies the ISS-solidified slurry, it is vulnerable to suffer accumulated liquefaction due to the dense crust structure of the ISS-solidified layer, so the adverse effects for accumulated liquefaction should be considered carefully due to the set of the ISS-solidified layer.

Experimental Study on Local Scour Depth around Monopile Foundation in Combined Waves and Current

Local scour is one of the key factors that cause the collapse of structures. To avoid structure failures and economic losses in water, it is usually essential to predict the equilibrium scour depth of the foundation. In this study, several design models which were presented to predict the equilibrium scour depth either under steady clear water conditions and combined waves and current conditions were recommended. These models from China, the United States and Norway were analyzed and compared through experiments. Moreover, flume tests for monopile foundation embedded in sand under different flow conditions were carried out to observe the process and gauge the maximum depth around the pile. Based on this study, for predicting the equilibrium scour depth around bridge piers, the computational results of three design methods are all conservative, as expected. For the foundation of offshore structures in marine environment, most of the predicted scour depths by design methods are different from field data; in particular, the mean relative error with these design methods proposed may reach up to 966.5%, which may lead to underestimation of the problem, overdesign and consequently high construction cost. To further improve the ability of the scour prediction in a marine environment, data from flume tests and some field data from a previous study were used to derive the major factors of scour. Based on the dimensional analysis method, a new model to estimate the equilibrium scour depth induced by either current or waves is proposed. The mean relative error of the new formula is 49.1%, and it gives more accurate scour depth predictions than the existing methods.

Experimental Investigation of the Outburst Discharge of Landslide Dam Overtopping Failure

Research on the factors and mechanisms that influence outburst floods are essential for estimating outflow hydrographs and the resulting inundation. In this study, large flume tests are conducted to investigate the effects of the upstream inflow and the presence of loose erodible deposits on the breaching flow and the subsequent outburst floods. Experimental results reveal that hydrographs of the breaching flow and outburst flood can be divided into three stages where each stage is separated by inflection points and peak discharges. It is found that the larger the inflow discharge, the larger the peak discharge of the outburst flood and the shorter the time needed to reach the peak and inflection discharges of the outburst flood. The breaching flow decreases along the longitudinal direction at rates that increase with the inflow discharge. The ratio between the length of the upstream dam shoulder and the dam width is inversely related to the ratio of the outburst discharge to inflow discharge. We also show that the presence of loose deposits at the dam toe can amplify the peak discharge of outburst flood by increasing the solids content of the water flow.

Flume Tests to Investigate the Initiation Mechanism of Loess Mudflows on the Chinese Loess Plateau

Loess has a strong water sensitivity, so loess landslides often transform into loess mudflows when water is added on the Chinese Loess Plateau, which results in high casualties and property loss of the Chinese government. In this study, a series of flume tests were designed to study the initiation of loess mudflows. The results reveal that the initiation modes of loess mudflows include large-scale mudflow and retrogressive toe sliding (Type A), and small-scale mudflow and retrogressive toe sliding (Type B). A model was used to analyze the test results that describe the effects of water flow on the potential for hillslope failure and liquefaction. It was found that the soil accumulation was unconditionally stable before a loess mudflow was formed, but as the rainfall continued, the water gradually infiltrated the soil, and the soil accumulation changed from unconditionally stable to unconditionally unstable. Thus, this led to different initiation modes during the tests. For Type A, the water preferentially infiltrated into the area with an uneven density and a large amount of water accumulated. The pore water pressure increased quickly and could not dissipate in time, so the loess liquefied. As the liquefaction area continued to expand and became larger, Type A occurred. Relatively speaking, Type B occurs in soil accumulations with relatively uniform densities. These results provide a certain scientific reference for the study of loess mudflows.

Backward erosion progression rates from small-scale flume tests

Backward erosion piping (BEP) is an internal erosion mechanism by which erosion channels progress upstream, typically through cohesionless or highly erodible foundation materials of dams and levees. As one of the primary causes of embankment failures, usually during high pool events, the probability of BEP-induced failure is commonly evaluated by the U.S. Army Corps of Engineers for existing dams and levees. In current practice, BEP failure probability is quantitatively assessed assuming steady state conditions with qualitative adjustments for temporal aspects of the process. In cases with short-term hydraulic loads, the progression rate of the erosion pipe may control the failure probability such that more quantitative treatment of the temporal development of erosion is necessary to arrive at meaningful probabilities of failure. This report builds upon the current state of the practice by investigating BEP progression rates through a series of laboratory experiments. BEP progression rates were measured for nine uniform sands in a series of 55 small-scale flume tests. Results indicate that the pipe progression rates are proportional to the seepage velocity and can be predicted using equations recently proposed in the literature.

Experimental Wave Flume Tests in ROV-Wave Interaction Effects on the Line Tension for a Work Class ROV in Splash Zone

Integrity and stability of Remotely Operated Vehicle (ROV) when passing through the splash zone is one of the main concerns in the design of an ROV-umbilical system. Due to the lightweight nature of ROV in water, the umbilical experiences repetitive rapid transitions between slack and taut as the ROV travels through the splash zone. These rapid transitions induce tension spikes in the umbilical, namely snap forces, that can endanger the launch and recovery of an ROV. Therefore, it is important to ensure that the tension spikes do not exceed the safe working load of the umbilical. In this study, launch and recovery of a deep-water work class ROV are experimentally investigated using a 1:10 scaled ROV model through a series of wave flume tests. Different regular and irregular waves are generated in the flume while the ROV model is hung over the flume in four different positions. The tension time-history in the line is measured and recorded using a load cell at the top-end of the line. A simplified numerical model for launch and recovery of the ROV is developed and the numerical results are compared with the experimental ones. It is shown that the presented simplified model can be accurately used for analysis of launch and recovery of the ROV.