Long-Term Effects of Earthquake-Induced Slope Failures

The earthquake-induced slope failures are now attracting significant concerns among experts of landslides. The reason for this is the complicated causative mechanisms as well as its impact on communities. The present paper states moreover that the onset of seismic slope failure is not only the consequence of a natural disaster but also the beginning of a long-term disaster in which the compound effects of the preceding earthquakes and the following heavy rain falls play significant roles. After addressing cases of this kind of natural disaster together with their classifications, study is made of the material properties of rocks subjected to this long-term proce-dure. Then, because it is difficult to mechanically stabilize the long-term unstable slopes, an early-warning technology as a practical mitigation measure is introduced

New Insight in Liquefaction After Recent Earthquakes: Chile, New Zealand and Japan

Liquefaction has proved to be one of the major geotechnical issues caused by earthquakes. It is one of the most costly phenomena and has affected several cities around the world. Although the topic has been studied since the 1960s, new questions are emerging. The earthquakes of Chile in 2010, New Zealand in 2010 and 2011, and Japan in 2011 had in common not only being some of the largest earthquakes of this decade but also having a problem of extensive liquefaction. Although most seismic codes have provisions against liquefaction, there are still some misconceptions regarding the characteristics of soil susceptibility and the effect of repeated liquefaction. This chapter introduces a detailed report of the damage caused by liquefaction in the cities affected by those earthquakes and also highlights observations in liquefied areas that were unexpected. Advanced geotechnical testing was conducted and compiled to compare them with previous assessment criteria and observations. A more comprehensive framework for the evaluation of liquefaction susceptibility and countermeasures will be presented and a roadmap of future work in the area will be described

Shaking Table Tests on Effiency of New Type of Drains

One method to mitigate liquefaction-induced hazard is the use of a system of vertical drains to dissipate the excess pore water pressure generated by earthquake loading. Performance assessments for these systems require the estimation of vertical drain spacing such that a maximum threshold level of excess pore pressure ratio is not exceeded. The objective of this research is to study efficiency of installing vertical drains on generation and dissipation of pore water pressure. For this purpose series of shaking table tests were performed using a laminar box, in Geotechnical Laboratory on Tokyo University. The ground model consists of two layers of saturated sand with relative densities of 80% and 40%. Two different types of vertical drains were investigated: prefabricated micro drain with diameter 22 mm and gravel drain with diameter of 30 mm. Several shaking table tests were performed with different distribution pattern in order to achieved optimal spacing between vertical drains on dissipation of pore water pressure. The tests were carried out with harmonic loading at frequency of 10 Hz and varying the magnitude of input acceleration in wide range from 0.05 to 0.60 (g). The results from above shaking table tests provided a detail view of efficiency of new type of vertical drains as one of the frequently used remedial measures against liquefaction

Model Tests on Mitigation of Liquefaction-Induced Subsidence of Dike by Using Embedded Sheet-Pile Walls

Several earthquakes in the 1990’s revealed the problem of significant subsidence of river dikes induced by subsoil liquefaction. Although soil improvement is the most reliable solution to avoid the onset of liquefaction, the substantial length of liquefaction-prone dikes together with the economical reasons does not allow this solution. It is aimed, therefore, to mitigate the subsidence to a certain extent so that a fatal flooding is avoided. The present study examines the possibility of installing sheet pile walls at toes of river dikes. Shaking table tests were conducted for this purpose and the effects of sheet pile walls were studied. Moreover, a combination of walls and drains for quick dissipation of excess pore water pressure was investigated

Modelling the Deformation of Sand during Cyclic Rotation of Principal Stress Directions

The paper presents an elastoplastic constitutive model for the deformation of sand during cyclic rotation of principal stress directions. The model employs a plastic potential theory that allows for the dependency of flow on the stress increment direction and a stress-dilatancy relation incorporating the effects of noncoaxiality. The continuous plastic deformation of sand during principal stress rotation at constant shear stress level is allowed for in the model by using a small elastic area in the stress space. The effects of cyclic stress history is modelled by using discrete surfaces of equal hardening modulus which are allowed to move with the stress point during loading. Additionally, the plastic hardening modulus is allowed to stiffen during cyclic loading depending on the amount of accumulated plastic normalized work. The model is used to simulate the deformations in the hollow cylindrical specimen subjected to several cycles of principal stress rotations. The model is shown to be capable of satisfactorily predicting the response of sand during cycles of principal stress rotations

Pseudo- Three-Dimensional Analysis of Cyclic Deformation of Ground Subject to Seismic Liquefaction

A Numerical tool was developed to evaluate the effects of differential movement which occurs at the ground surface during earthquakes. A special emphasis is placed on liquefaction of subsoils. A complicated three-dimensional analysis was avoided by using a pseudo-three-dimensional method of finite element analysis which runs on an element mesh of the ground surface topography as seen from the sky. The present analysis takes into account the nonlinear stress-strain behavior of soils, the ground softening due to pore pressure development, and the irregularity in the topography. The proposed method was applied to several cases in which buried pipelines were damaged by seismic liquefaction. The calculated results showed that the differential movement of the ground in cyclic manners is not significant. Thus, it seems that those pipeline failures were induced not by the cyclic ground movement but by the monotonic or permanent displacement which accumulated to several meters

Post-Liquefaction Torsion Shear Tests on Sand With Various Strain Rates

This text concerns with the deformation characteristics of liquefied sand which undergoes large residual deformation and flow failure. In particular, the nature of sand which can be used in a dynamic analysis of deformation in the time domain is aimed at. Although the former model tests have suggested a rate-dependent behavior of sand, a detailed measurement of stress and strain was not possible. This problem was overcome in the present study in which two types of torsional shear device were employed. A special care was therein taken to achieve such a large and realistic shear strain as 100% in a sample; Moreover, it was attempted to make the stress state within a specimen as uniform as possible after liquefaction by reducing the gravity-induced stress component. It was revealed that a specimen with very small effective stress does not have a rate-dependent nature when large shear deformation takes place

Experimental Study on Mitigation of Liquefaction-Induced Lateral Displacement Deep Soil Mixing

After 1990s\u27 earthquakes in Japan, lateral flow of liquefiable slopes became a serious concern of engineers. Espisally Kobe earthquake (1995) in which high subsidence of river levee as a result of liquefied sand lateral flow was observed, become a turning point in geotechnical engineering approach in dealing with this phenomena. From that time many different kinds of mitigation measures for preventing or at least controlling the extent of lateral flow have been proposed. Improving soil by deep mixing columns is one of the common methods of soil improvement that can also be used for controlling the consequences of liquefied sand flow. For analyzing the factors affecting the efficiency of this method, several shaking table tests have been done. This article is showing the effects of studied factors including columns pattern the length and improvement ratio. Moreover the magnitude of flow inside and outside of improved area are scrutinized. Finally, based on experimental observations, behavior of liquefied sand in existence of deep mixed soil is modeled numerically

Zero-Gravity Triaxial Shear Tests on Mechanical Properties of Liquefied Sand and Performance Assessment of Mitigations Against Large Ground Deformation

This paper concerns prediction of liquefaction-induced large deformation of geotechnical structures that will play major roles in practice of seismic performance design. To do this prediction, it is essential to establish a mechanical model for liquefied sand in which effective stress is null or extremely low. Although past model tests suggested that liquefied sand behaves similar to viscous liquid, there is an opinion against it that pore water pressure distribution in embedded structures produces an apparent rate-dependent behavior. This opinion was examined precisely and quantitatively by analyzing a full-scale model test to find that the pore pressure theory cannot account for the observed behavior. Then the authors conducted a new type of triaxial tests in which the effective stress was made extremely low, similar to the situation in fully liquefied sand, by free falling of a test device in a vertical shaft, thus making the gravity be zero, and a rate-dependent nature of liquefied sand was observed. By using the observed behavior of liquefied sand, a viscous model was developed. This viscous model was made use of to evaluate the performance of different mitigation measures that were proposed for river levees and other embankments subjected to liquefaction problems