Kouki Zen

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.

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

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.

Finite Element Modeling of Compaction Grouting on its Densification and Confining Aspects

Compaction grouting method called as CPG is becoming popular as a ground improvement technique due to its wide variety of applications and several advantages. However, so far, the application of the method has been mainly dependent on field tests, practical experience and empiricism. This paper presents a 2-D finite element model of CPG based on the elasto-plastic theory of strain hardening that takes variations in stress–strain fields along the depth and the impact of the ground surface conditions on the grouting process. This reveals densification and confining aspects of CPG with relative contribution of variables like injection pressure, spacing of injection points, overburden pressure and initial relative density of ground. This paper concludes that the ground improvement by CPG should not be taken as increased ground density alone but also as increased lateral earth pressure. Although no densification can be achieved for medium or dense soil, CPG process is found to be effective in increasing the lateral earth pressure of medium or dense soil significantly. The lateral earth pressure increases with increase in injection pressure only up to the some extent but with further increase in injection pressure its effect reaches to the farther radial distances.

An Experimental Study of Compaction Grouting on Its Densification and Confining Effects

Compaction grouting (CPG) involves the injection of high viscosity mortar-type grout under relatively high pressure that displaces and compacts the soil in-place. Many case studies around the world prove the effectiveness of CPG to treat liquefiable soils. In these studies, mostly CPG has been developed and used on the basis of practical experiences, its densification and confining effects are not well understood until now. The objective of the present study is to bring out densification and confining effects of CPG. This paper presents a laboratory experimental study carried out to model a CPG treated ground. The study includes the experiments performed to decide appropriate consistency of grout material and appropriate relative contribution of factors such as overburden pressure, grout volume, injection speed and injection pressure that ensures efficient grouting. The effects of CPG are quantified in terms of lateral earth pressures and lateral displacements that are further defined in terms of coefficient of lateral earth pressure, K, and densification factor in the soil around grout column. This leads to a better understanding of its densification and confining effects. Although in practice, majority of the works with CPG have been related to its densification effect and lesser attention has been given to its confinement effect, in present experimental study, its confinement effect were prominent. Also at the farthest locations usually where field tests are carried out, its densification effect could not be found experimentally but its confinement effect was clear.

Controlled Permeation Grouting Method for Mitigation of Liquefaction

AbstractThe application of silica stabilizers to a liquefiable ground covered by existing structures is a challenge to engineers. This paper provides details of ground improvement under the runway and around the common ducts of Fukuoka International Airport. The ground was improved via a newly developed controlled curved drilling machine to enable controlled permeation grouting under the runway without any disturbance to its operation. At this stage approximately 100 m of the runway has been improved. Experiments on the gel time and the form of chemical diffusion at different injection times secured the improvement of ground in the desired zone, and a posttreatment investigation confirmed this improvement.

Shaking table test on the seismic behavior of caisson type quay-wall in application of ground solidification technique

In order to investigate the effect of ground solidification method for reducing the seismic damage of caisson type quay-wall, a series of shaking table tests in 1g gravitational field were performed in terms of the shear strength and the improvement width of ground solidification. The main conclusions obtained from this study are as follows: 1) Ground solidification behind caisson type quay-wall is effective for reducing the seismic damage of caisson type quay-wall and, however, setting backfill stones in solidified ground is not useful for the stability of quay-wall in current experimental condition for this study. 2) For solidified ground with large shear strength, the seismic behavior of quay-wall sympathizes with solidified ground as if both of solidified ground and quay-wall are a combined structure. Consequently, the seismic earth pressure from liquefied sandy ground is absorbed by solidified ground reducing the seismic earth pressure to quay-wall. 3) A formula to evaluate the sliding safety of caisson type quay-wall with solidified ground is proposed considering the cohesive component of shear strength of solidified ground. Based on the proposed formula, the improvement width for ground solidification method can be reduced from a conventional width in current design code.Copyright

Shaking table test on the improvement dimension of permeable grouting method for liquefaction countermeasure

In order to clarify an optimum improvement dimension for the permeable-grouting method as a liquefaction countermeasure, a series of shaking table tests have been conducted for improved model grounds with various improvement dimensions. To express the variety of improvement dimensions in field, the model ground was created by dividing it into two types of mesh elements, in which each mesh element was simplified as a liquefiable element (unimproved element) or non-liquefiable element (improved element) respectively. Improvement ratio defined by the volume ratio of improved elements in total elements was set for 0% or 50% and the width of mesh element was 50mm. The acceleration of shaking table was applied by step loading; 100, 200, 300 and 400gal with the sinusoidal wave form of 3Hz. In order to investigate the seismic behavior of the improved ground, pore water pressure transducers and acceleration meters were set in the model ground. The main conclusions obtained from this study are as follows; 1) Both of the ground settlement induced by liquefaction and the acceleration response during seismic loading are greatly affected by the generated excess pore water pressure depending on the improvement dimension. Therefore, the “liquefiable region” in which the excess pore water pressure ratio is more than 0.75 is newly defined to evaluate the effect of improvement dimension on the acceleration response of ground, excess pore water pressure and vertical settlement, 2) As improved element increases in the liquefiable region, both of the excess pore water pressure in liquefiable element and the acceleration response of ground surface decrease. Namely, the improvement ratio in the liquefiable region could be an important index to evaluate the effect of improvement, 3) From the experimental conditions in this paper, it is suggested that 300mm is the best vertical interval and 150mm is the worst one to reduce the ground settlement induced by liquefaction and the vibration of ground.Copyright

THE RESERCH ON CHARACTERISTIC OF BEARING CAPACITY OF THE SUCTION FOUNDATION OVER DRAWING-OUT

Suction foundation is a new type of foundation for offshore structures. It forms like a tea caddy. It is built by installing hollow cylindrical caisson upside-down, into sea bottom, and carrying out forcible drainage of the internal water using a pump after the inner side of foundation is sealed by the submarine side. The difference of water pressure (suction) between inner and outer side of caisson and it will use this suction force as pushing power into the ground. Using suction foundation is useful for working term and constructing cost.This study pays attention to the characteristics of bearing capacity of the suction foundation over drawing-out, and bearing capacity formula.