Yu Otake

A Simple Method to Assess the Effects of Soil Spatial Variability on the Performance of a Shallow Foundation

ABSTRACT: The performance of a geotechnical structure is controlled not by the properties of soil at a point but by the average over a certain line, area or volume, as pointed out by Vanmarcke (1977) who proposed the variance function. This paper develops his idea, and attempts to specify the size of the local averaging of soil parameters to evaluate the resulting variability in performance of a geotechnical structure. In other words, the mean and variance of a local average of a geotechnical parameter for an appropriate volume are used to evaluate the effects of soil spatial variability on the performance of a geotechnical structure. The size seems to be highly dependent on the type of geotechnical problem under consideration. This paper examines the deformation and stability of a shallow foundation to specify the averaging volume size. The results suggest that the concept can be extended to many other geotechnical reliability analyses.

Continuous River Levee Safety Assessment based on a Reliability Analysis

A design scheme is proposed to evaluate the safety of continuous linear structure i.e. river levee. The scheme is developed to establish a procedure to continuously evaluate the stability failure of embankment during flood runoff by seepage, and to identify locations for reinforcement of river levee. A 20km stretch of a fairly large river running through one of the major cities in Japan is chosen for a case study. First all the information available for this river levee for the both side of river have been collected and complied. The data include river levee configurations for every 200m, all the soil investigation results done in the past and the results of levee safety inspection by MLIT. The strength parameters of the embankment and the foundation are obtained from about 50 triaxial test results (CUbar). The permeability coefficient is estimated from about 300 grain size distribution and 15 permeability test results. The configurations of the levee sections are analyzed, and the ranges of various dimensions, such as slope inclinations, crown width, height of retaining wall at edge of slope etc., are obtained. Based on these information, detailed seepage and stability analyses for the possible dimensions of levee sections and material properties are carried out to get the response function (RS) for the safety assessment. Based on this information, the response surface (RS) for the river levee is developed. In developing RS, the experimental design technique is applied for efficiency. MCS is applied to evaluate the failure probability of the levee based on the parameters obtained from the all the analyses explained above for every 200m interval along the levee because all the configuration dimensions of the levee is available at these sections. Results of MCS are presented along the levee, which immediately shows less safe sections.

Reliability Analysis of Long River Dike Against Liquefaction Failure

The authors proposed a reliability analysis method for long continuous structures considering investigation sites to counter liquefaction failure. This method is characterized by continuously calculating the failure probability caused by liquefaction at an arbitrary point and quantifying the uncertainty concerning the investigation site. The purpose of this study is to verify its effectiveness based on application to an actual river dike that suffered liquefaction failure due to the 2011 Earthquake off the Pacific coast of Tohoku.

Consideration on Major Uncertainty Sources in Geotechnical Design

The authors have been proposing a reliability based design (RBD) scheme for practicing geotechnical engineersthat is termed GRASP (Geotechnical Reliability Analysis by a Simplified Procedure). The essence of GRASP is its simplified way to evaluate the effects of the soil spatial variability on structure performance, i.e. the local average (LA) of soil property for an appropriate volume is used to evaluate the effects. Furthermore, by focusing on the LA, it is possible to establish a clear-cut statistical estimation theory which gives the bases for the sampling strategies. In this paper, the outline of GRASP is summarized in the first half of this paper. The latter half of the paper is devoted to the consideration onthe major uncertainty sources in geotechnical design based on results of analyses by GRASP on various typical geotechnical structures, such as piles, shallow foundations and embankments on soft ground.

Statistical Study on Evaluating Bridge Health Index based on Bridge Inspection Database

This paper presents results of an investigation of evaluating the bridges health degree of 54 bridges managed by Gifu Prefecture in Gifu City. Bridge health degree is statistically calculated with periodic inspection data conducted by Gifu Prefecture. This inspection results consist of many items and evaluate the state of bridge details, but a currently index is insufficient as one used to maintenance. Accordingly, the index is required that can evaluate overall of each bridge elements or entire bridge. So, this study aims to express health degree of bridge elements by one or two overall index with principal component analysis, and grasped relationship to calculated health index, age, specification, design standard and environmental condition by multiple regression analysis.

STUDY ON A LIQUEFACTION COUNTERMEASURE FOR FLUME STRUCTURE BY SHEET-PILE WITH DRAIN

Canals are important as lifeline facilities that supply water to urban areas. However, the countermeasure in popular use, such as improvement of entire liquefiable ground of the canal by chemical feeding, is hugely expensive. Against this background, the authors investigated a reasonable liquefaction countermeasure for an existing flume canal using sheet-pile with drain. In this study, several shaking table tests in the 1g gravitational fields were conducted, and the experimental reproducibility of the analysis was checked to confirm the effectiveness of the proposed countermeasure. The results indicated that the proposed method was effective in reducing the deformation (i.e., uplift/sinking dis- placement and inclination) of the canal, and good reproducibility for dynamic effective stress FEM analysis using LIQCA code with respect to the results of the experiment was confirmed.

On the Inspection of River Levee Safety in Japan by MLIT

The ministry of land, infrastructures, transportation and tourism (MLIT) of the Japanese government maintains governmental managed river levee for the safety against flood whose total length is more than 10,000 km. The design standard of river levee had been traditionally done based on so called shape based specification, where levee is judged to be safe as long as a section satisfies the specified dimensions. In 2002, this policy had been changed and a new inspection guideline which is based on the modern soil mechanics principles had been introduced. The method mainly consists of the non-stationary seepage analysis followed by the circular slip line stability analysis. The exit gradient and uplift pressure are also checked based on specified safety factors. The way of the safety assessment is that, first a river is divided into so called a continuous strip (CS) which is judged to have similar configurations, geotechnical and hydraulic conditions. The typical length of a CS is half to several km long. Then a representative cross section (RCS) that is considered to represent all the CS section is selected. At RCS, detailed soil investigations are carried out, and the inspection is done to assess the safety of the section for stability and piping. A CS is judged to be safe if all the verification items satisfy the specified safety factor, but judged to be NG (no good) if any of the items could not fulfill the threshold value. The first round assessment at year 2011 indicated, only 60% of the all levee is judged to be safe for all verification items. 25% of the levee could not satisfy the stability requirement, 30% piping requirement, and 14% the both requirements. In the paper, the details of the verification methods are described. Furthermore, some recent flood events are introduced. Finally, further challenges for the second screening of the safety of levee are discussed.

A procedure to determine resistance factors for a newly developed rotation steel pile

The aim of this study is to establish a procedure to determine resistance factors for newly developed pile products through an example of the rotational intrusion steel pile (RI pile). RI pile is a newly developed steel pile product which has the vane type tip structure to resist reliably against vertical load and to intrude easily by rotating itself. Moreover, the RI pile is environment-friendly, i.e. reduce waste soil and less construction noise, because of the rotating intrusion method. The construction can be done with the small pile driving equipment and the pile can be reliably founded into the bearing ground by monitoring and controlling the torque value during installation. For these reasons, the RI-pile is expected to have the superior bearing capacity against vertical loads. However, the RI-pile has not been adopted in many construction projects yet because the current design method requires applying the same safety factor neglecting the level of the uncertainty of each pile type. In this study, 64 loading test data are gathered to quantify the model error of the RI-pile assisted by Public Works Research Institute (PWRI). Then, Reliability analysis is conducted based on our proposed reliability analysis scheme (GRASP) to specify the reliability level of the current design. Furthermore, the partial factor updating procedure according to the newly developed the RI pile is formulated based on the design value method. From the results, the partial factor is determined and applied to compare the RI steel pile to the traditional piles, i.e. cast in pile and driven steel pile. The difference in construction cost among the three piles has become less, and the RI pile is less expensive when the rate of the pile tip resistance is high.

A Group of Deterioration Curves of Bridges Estimated by Extended EM Algorithm

This study proposes a method to separate data into several groups and perform regression analysis for each group at the same time statistically by using EM (Expectation and Maximization) algorithm or its extended formulation to nonlinear regression problem. With using the proposed method, a group of deterioration curves is estimated based on database of health index of 341 bridges by visual inspection in Gifu prefecture, Japan. Ordinary EM algorithm can treat linear regression only, so that the algorithm is extended to nonlinear regression problems. In our proposed method, the hidden parameter and standard deviation of error are estimated following ordinary EM algorithm, whereas nonlinear coefficients are estimated by using Particle Swarm Optimization, which is one of global optimization methods. Deterioration curves of the four bridge groups are estimated with the inspection database by the proposed algorithm.

Analytical Study on Mitigation of Liquefaction-Related Damage to Flume Channel Using Sheet-Pile with Drain

Canals are important facilities that serve as lifelines by supplying water to urban areas. However, a budget of staggering proportions would be necessary to apply the usual countermeasures adopted in preventing liquefaction to an entire canal. Against this background, the application of sheet-pile with drain as a reasonable mitigation method to reduce liquefaction-related damage to existing flume channels (i.e., uplift/sinking displacement and inclination) based on performance-based design has been studied by authors. This paper confirms good reproducibility for dynamic effective stress FEM analysis with respect to the results of several shaking table tests in 1g gravitational fields and the effectiveness of the proposed countermeasure for actual-scale structures based on this analysis.