Kiyonobu Kasama

Effect of spatial variability on the slope stability using Random Field Numerical Limit Analyses

ABSTRACT This paper presents a probabilistic approach to evaluating the geotechnical stability problem by incorporating the stochastic spatial variability of soil property within the numerical limit analyses (NLAs). The undrained shear strength and unit weight of soil are treated as a random field which is characterized by a log-normal distribution and a spatial correlation length. The current calculations use a Cholesky Decomposition technique to incorporate these random properties in NLAs. The Random Field Numerical Limit Analyses are applied to evaluate the effects of spatial variability of soil property on the slope stability and failure mechanism of slope. Monte Carlo iterations are then used to interpret the slope reliability and the dimension for collapsed slope for selected ranges of the coefficient of variation in soil property and the ratio of correlation length to slope height. Finally, the variation in the dimension of collapsed slope is examined in terms of the variability of slope reliability.

The slope modeling method with GIS support for rockfall analysis using 3D DDA

Rockfall is the most frequent major hazard in mountainous areas. For hazard assessment and further countermeasure design, realistic and accurate prediction of rockfall trajectory is an important requirement. Thus, a modeling method to represent both geometrical parameters of slope and falling rock mass is required. This study, suggests taking the advantages of discontinues deformation analysis (DDA) and geographical information system (GIS). In this study, after developing a three dimensional (3D) DDA program, firstly a special element named contact face element (CFE) was introduced into 3D DDA; secondly, effectively modeling tools with GIS support were developed. The implementation of CFE also improves the efficiency of both the contact searching and solution process. Then a simple impact model was devised to compare the 3D DDA implemented directly with a sliding model with theoretical analysis to verify the reliability of the modified 3D DDA program and investigate the parameter settings. Finally, simulations concerning rock shapes and multi-rocks were carried out to show the applicable functions and advantages of the newly developed rockfall analysis code. It has been shown that the newly developed 3D DDA program with GIS support is applicable and effective.

Comparing data-driven landslide susceptibility models based on participatory landslide inventory mapping in Purwosari area, Yogyakarta, Java

There are different approaches and techniques for landslide susceptibility mapping. However, no agreement has been reached in both the procedure and the use of specific controlling factors employed in the landslide susceptibility mapping. Each model has its own assumption, and the result may differ from place to place. Different landslide controlling factors and the completeness of landslide inventory may also affect the different result. Incomplete landslide inventory may produce significance error in the interpretation of the relationship between landslide and controlling factor. Comparing landslide susceptibility models using complete inventory is essential in order to identify the most realistic landslide susceptibility approach applied typically in the tropical region Indonesia. Purwosari area, Java, which has total 182 landslides occurred from 1979 to 2011, was selected as study area to evaluate three data-driven landslide susceptibility models, i.e., weight of evidence, logistic regression, and artificial neural network. Landslide in the study area is usually affected by rainfall and anthropogenic activities. The landslide typology consists of shallow translational and rotational slide. The elevation, slope, aspect, plan curvature, profile curvature, stream power index, topographic wetness index, distance to river, land use, and distance to road were selected as landslide controlling factors for the analysis. Considering the accuracy and the precision evaluations, the weight of evidence represents considerably the most realistic prediction capacities (79%) when comparing with the logistic regression (72%) and artificial neural network (71%). The linear model shows more powerful result than the nonlinear models because it fits to the area where complete landslide inventory is available, the landscape is not varied, and the occurence of landslide is evenly distributed to the class of controlling factor.

Failure of rubble mound beneath caisson due to earthquake-induced Tsunami

The 2011 off the Pacific Coast of Tohoku Earthquake on the 11th of March, 2011 had induced a gigantic Tsunami and caused the catastrophic damage in the northeast coast area of Japan. Lots of breakwaters have been seriously damaged in this disaster, including the world deepest breakwaters, Kamaishi Harbor Mouth Breakwaters. According to a wave height recorder, a large hydraulic head difference between the seaside and harbor side of breakwaters was observed for long duration of several minutes or more. Such a hydraulic head difference as observed generates the seepage flow in the rubble mound underneath the caisson. The seepage flow is considered one of the influential factors causing the instability of caisson type composite breakwaters.In order to find out the instability mechanism of caisson type composite breakwaters against tsunami, 1) the pop-out phenomena of armored blocks on the mound, 2) the possibility of shear failure of rubble mound and 3) the reduction of bearing capacity of rubble mound due to seepage flow are investigated. Both theoretical analysis and laboratory experiment are represented in this paper.The safety factors for the pop-out failure and shear failure of rubble mound were theoretically derived. A scale of 1/100 model experiment was performed in laboratory. Loading tests were carried out to investigate the reduction of bearing capacity under the existence of seepage flow. From the results of laboratory experiment and theoretical analysis, it was confirmed that the pop-out phenomenon happens and the bearing capacity of rubble mound can considerably decrease due to the tsunami-induced seepage flow. It is concluded that the effect of seepage flow in the rubble mound should be taken into account when making a design of the caisson type composite breakwater against tsunami.Copyright

Reliability assessment of cement-mixed ground bearing capacity considering spatial variability

Cement-mixing method is gaining popularity as a method for stabilizing soft soils in applications ranging from the improvement of foundation properties to mitigation of liquefaction. However, spatial variability in the shear strength of the cement-mixed ground introduces uncertainties in estimating the bearing capacity for design. This paper presents a reliability assessment for the bearing capacity of cement-mixed ground based on the results of a probabilistic study in which the shear strength of the cement-mixed ground is represented as a random field in Monte Carlo simulations of undrained stability for a surface foundation using numerical limit analyses. The results show how the bearing capacity is related to the coefficient of variation and correlation length scale in the shear strength of cement-mixed ground. Finally, the reduction of bearing capacity due to spatial variability of shear strength in cement-mixed ground is evaluated based on the result of the application to construction sites by cement-mixing in Japan in terms of probabilistic point of view. ABSTRACT: Cement-mixing method is gaining popularity as a method for stabilizing soft soils in applications ranging from the improvement of foundation properties to mitigation of liquefaction. However, spatial variability in the shear strength of the cement-mixed ground introduces uncertainties in estimating the bearing capacity for design. This paper presents a reliability assessment for the bearing capacity of cement-mixed ground based on the results of a probabilistic study in which the shear strength of the cement-mixed ground is represented as a random field in Monte Carlo simulations of undrained stability for a surface foundation using numerical limit analyses. The results show how the bearing capacity is related to the coefficient of variation and correlation length scale in the shear strength of cement-mixed ground. Finally, the reduction of bearing capacity due to spatial variability of shear strength in cement-mixed ground is evaluated based on the result of the application to construction sites by cement-mixing in Japan in terms of probabilistic point of view.

Rockfall susceptibility zoning based on back analysis of rockfall deposit inventory in Gunung Kelir, Java

A rockfall susceptibility based on trajectory-energy/velocity approach needs release area or rockfall source. However, identification of rockfall source is not always possible for some areas in Indonesia. This paper presents a rockfall susceptibility zoning based on back analysis technique of rockfall deposit inventory in Gunung Kelir, Java. There were several steps in the rockfall susceptibility zoning: (1) rockfall deposit inventory, (2) rockfall simulation based on back analysis of rockfall deposit inventory, (3) sensitivity analysis, and (4) rockfall susceptibility zoning. The result suggests that the travel distance is affected by the spatial distribution of rockfall source, lithology or surface material, and topography (angle of slope and angle of aspect). Final trajectories were employed to generate landslide susceptibility map which may allow a policy maker to have an advanced consideration to achieve specified risk measures and evaluation of their cost efficiency to optimize budget and design. Application of rockfall susceptibility zoning based on back analysis of rockfall deposits is efficient where rockfall source information is unavailable.

Effect of relative density on the wave-induced liquefaction in seabed around a breakwater

The liquefaction of seabed induced by ocean waves is considered to be one of the influential phenomena related to damages of coastal marine structures such as the floating of pipelines, the settlement of concrete blocks and the reduction of pile foundation resistance, etc, since the liquefied seabed loses its shear strength and then easily and drastically deforms. A model flume was newly developed to simulate the wave-induced liquefaction in seabed around a breakwater with a reduced model scale against the caisson type breakwater widely used in Japan. The dimension of developed flume was the length of 6.0m, the width of 0.4m and the depth of 0.9m. As for geotechnical parameters affecting the wave-induced liquefaction of seabed around the model breakwater, the effect of seabed density on liquefaction was highlighted in this paper in terms of the generation of pore water pressure in seabed and the settlement of seabed surface. The experiment was carried out under the following conditions; the wave period was 1.0s, the incident wave height was 55mm, the depth of water was 170mm, the thickness of permeable layer was 350mm and the relative density was between 20% and 60%. In order to satisfy similarity law in 1g gravitational field, the polymer fluid was used to decrease the permeability of model seabed. As the result from this study, the following conclusions were obtained; 1) When water was used as a fluid, the liquefaction due to the residual excess pore water pressure happened in the sand bed with the relative density of 23%. However, the liquefaction did not happened in the sand bed with the relative density more than 30%, 2) When the polymer fluid is used for reducing the permeability of model seabed, the generation of pore water pressure ratio becomes larger and the dissipation time of generated pore pressure becomes longer compared with the case using water, 3) When the relative density of seabed was between 20% and 40%, the liquefaction induced by the residual excess pore water pressure was observed in the deep area of model seabed while the shear failure of seabed was observed in the shallow area of model seabed, 4) When the relative density was between 50% and 60%, the liquefaction due to the residual excess pore water pressure was not observed in the present experimental conditions.Copyright

Behavior of Breakwater Foundation Reinforced with Steel Sheet Piles Under Seismic Loading

Waterfront structures such as breakwater, coastal dike, sea wall, etc., suffer serious damage from the earthquake and tsunami. The breakwaters are designed to protect coastline and seaport from the devastation effect of wave and current of tsunami by absorbing their wave energy and reducing overtopping. The port of Kamaishi (Iwate Prefecture, Japan) suffered heavy causalities due to the Great East Japan Earthquake in March 2011 mainly due to the damage of breakwater mound/foundation which was caused due to collapse of the breakwater. On the other hand, mitigation of compound disaster due to predicted future earthquakes such as Tokai earthquake, Nankai Earthquake, and Tonankai-Nankai Earthquake is a matter of great concern. The stability and safe performance of breakwater is very important for the protection of structures and population living near to coastline. It is, therefore, necessary to develop a new earthquake and tsunami resistant reinforcement technique for breakwater foundations which will make the breakwater resilient against the earthquake and tsunami forces. This paper deals with the development of an effective reinforcement technology for breakwater foundation which provides resiliency to the mound against earthquake. The technique involves use of steel sheet piles and gabion type mound (gravel wrapped up in steel wired mesh), which is effective in preventing breakwater subsidence and horizontal displacement. As a part of the study, a series of shaking table test in 1 g of gravitational field were performed and through the tests, the reinforcement effect by the steel sheet pile and gabion under earthquake loading and its influence on breakwater performance was made clear.

Effects of spatial variability of soil property on slope stability

This paper presents a probabilistic approach to evaluating the geotechnical stability problem by incorporating the stochastic spatial variability of soil property within the numerical limit analyses. The undrained shear strength and unit weight of soil are treated as a random field which is characterized by a log-normal distribution and a spatial correlation length. The current calculations use a Cholesky Decomposition technique to incorporate these random properties in numerical limit analyses. The Random Field Numerical Limit Analyses are applied to evaluate effects of spatial variability of soil property on the slope stability and the failure mechanism. Monte Carlo simulations are then used to interpret the failure probability of slope for selected ranges of the coefficient of variation in soil property and the ratio of correlation length to slope height. Finally, the conventional safety factor of slope stability is evaluated to obtain an objective probability of slope failure. RANDOM FIELD NUMERICAL LIMIT ANALYSES Numerical limit analyses

Effect of Blasting on the Adjacent Underground Tunnels

The blasting method is regarded as a simple, convenient and economical method for constructing the underground structure, so it is advisable method for many underground structures to construct. But the investigation of the effect of the blasting dynamic load on the vicinal tunnel structure is rare, and the effect of blasting on the vicinal structures cannot be ignored either; sometimes, the effect will cause crack and even collapse in the tunnel liner and surrounding rock. So this paper presented the effect of blasting on the vicinal underground structure in differential cases using the finite element software Midas GTS. The investigation in this paper indicated which case will suffer the more severe effect caused by blast and let us know the vibration principle of the underground structure in differential case, and that will provide the knowledge about the vibration caused by blasting to the design and construction by numerical simulation; additionally, this paper has presented the reinforcement method about inserting the bolt into the surrounding rock to analyze how to resist the effect of the blast load. So from this analysis, it can be noted that the blasting method or the explosive energy will be chosen on the basis of different construction shape, and the reasonable location of the bolt will be adopted in order to reduce the effect of the dynamic on the vicinal tunnel structure.