The 2011 Off the Pacific Coast of Tohoku Earthquake which occurred on March 11 caused serious damage to the infrastructural facilities in the eastern area of Japan due to the strong motion of the earthquake and the subsequent tsunami. This paper includes a brief outline of the earthquake damage to various facilities in relation to geotechnical engineering, and the main geotechnical engineering problems which have emerged after this disaster, including ground subsidence caused by crustal movement, the consolidation of soft clay and liquefaction, liquefaction induced damage to river levees and tailing dams, the geotechnical damage related to Tsunami impact, and disaster waste management.
This paper focuses on the cyclic resistance assessment of two non-plastic silty sands that likely experienced severe liquefaction during the 2011 Off the Pacific Cost of Tohoku Earthquake (Japan). Several samples were retrieved from fill and alluvial deposits at a site investigation in Chiba. Besides a conventional triple-tube sampler (TB), a cutting-edge sampling technique, namely “Gel-Push sampler” (GP), was adopted to obtain high quality undisturbed samples. In the laboratory, a series of cyclic triaxial tests were conducted on several specimens recovered by both GP and TB samplers. Preliminary results of liquefaction resistance and shear wave velocity compared to field PS logging measurements revealed that that the sampling technique and quality of samples highly affected the evaluation of liquefaction resistance of the investigated sands. Therefore, to address this issue, a classification of sample quality for undisturbed samples was provided, based on change in density and shear velocity/dynamic shear modulus measured in the field and in the laboratory.
During the recent devastating earthquakes in Christchurch, many residential houses were damaged due to widespread liquefaction of the ground. In-situ testing is widely used as a convenient method for evaluating liquefaction potential of soils. Cone penetration test (CPT) and standard penetration test (SPT) are the two popular in situ tests which are widely used in New Zealand for site characterization. The Screw Driving Sounding (SDS) method is a relatively new operating system developed in Japan consisting of a machine that drills a rod into the ground by applying torque at seven steps of axial loading. This machine can continuously measure the required torque, load, speed of penetration and rod friction during the test, and therefore can give a clear overview of the soil profile along the depth of penetration. In this paper, based on a number of SDS tests conducted in Christchurch, a correlation was developed between tip resistance of CPT test and SDS parameters for layers consisting of different fines contents. Moreover, using the obtained correlation, a chart was proposed which relates the cyclic resistance ratio to the appropriate SDS parameter. Using the proposed chart, liquefaction potential of soil can be estimated directly using SDS data. As SDS method is simpler, faster and more economical test than CPT and SPT, it can be a reliable alternative in-situ test for soil characterization, especially in residential house constructions.
It is commonly understood in Japan that “drainage”, “compaction” and “solidification” are principles for ground improvement. Many ground improvement techniques have been developed to meet the demands of the times i.e. “economic efficiency”, “reliability (improvement effect)” and “environmental-friendliness”. However, some drawbacks of the methods have been eliminated, like the implementation system developed recently under the restricted condition to improve the limited space around the existing structures. This paper describes the relationship between the conventional and the innovative ground improvement methods followed by their aims of development in Japan. Furthermore, technical aspect of each innovative ground improvement method is outlined, such as “sand-injection type sand compaction pile method” and “twin-flow cement slurry jet mixing method”. Finally, the effectiveness of ground improvement technologies evaluated on their application examples near existing structure for the purpose of seismic reinforcement against natural hazard is reported.
In New Zealand, rainfall-induced slope failures cause millions of dollars’ worth of damage annually. As a means of mitigating the associated risk, a site-specific early warning system was developed based on field instrumentation, laboratory tests and finite element modeling. The selected site is a cut slope adjacent SH-1, one of Auckland’s busiest state highways. The site consists of Northland Allochthon residual soil, known for its susceptibility to rainfall-induced landslides. In fact, a landslide occurred at the site in the winter of 2008 following a period of prolonged rainfall. Firstly, a variety of laboratory tests were undertaken on soils taken from the slope to better understand their shear strength and hydraulic characteristics. Next, a cross section of the slope about 45m from the 2008 landslide site was instrumented with 13 volumetric water content sensors installed at different depths, together with a tipping bucket rain gauge to monitor rainfall events. With the rainfall and volumetric water content readings monitored since 2010, the soil properties and boundary conditions of the monitored slope cross-section were calibrated using a transient seepage analysis program (SEEP/W). After successful validation, the matric suction/pore-water pressure profile was coupled with a limit equilibrium analyses (SLOPE/W) to simulate the 2008 landslide. For this purpose, an artificial neural network (ANN) was trained to predict the factor of safety (FoS), which successfully validated the model using the 2008 rainfall record, obtaining FoS=1.0 at the exact time the 2008 landslide occurred. Finally, the ANN-based methodology was extended to predict the FoS at the monitored site in the future, using rainfall forecasts. This can serve as basis of an early warning system as a means to mitigate the risk of rainfall induced landslides.
Earthquake induced landslides were one of major causes for a large number of deaths and huge economic loss in the Loess Plateau. In order to evaluate the influencing area of seismic landslides, we presented a method of predicting sliding distance of seismic landslides in Loess Plateau by means of fuzzy information model based on 93 cases data of seismic loess landslides. Firstly, the types, characteristics, and influencing factors of seismic landslides in Loess Plateau were studied based on field investigation, back analysis and laboratory tests. Secondly, Morgenstern-Price method was used for calculating the seismic stability of loess slopes. The relationships between safe factor and influencing factors were established, which include seismic intensity, relative altitude, slope angle, unit weight, internal cohesion, internal friction angle. Furthermore, the factors of influencing sliding distance of landslides were figured out. Thirdly, a fuzzy information matrix was established based on 93 loess seismic landslides caused by South Tianshui 8.0 earthquake in 1654, Tong Wei 7.5 earthquake in 1718, Haiyuan 8.5 earthquake in 1920 and Gulang 8.0 earthquake in 1927. And then a fuzzy information model was developed for calculating sliding distance and influencing area of seismic loess landslides. Fourthly, the influencing areas of 22 potential landslides in Tianshui city were respectively predicted. The predicted results shown that the method presented in the paper is more accurate for seismic loess landslides, comparing with other methods.
To investigate the wave characteristics generated by subaerial landside in Three Gorges Reservoir, China, a large laboratory physical experiment was conducted. Specially, the channel of Baishuihe landslide in the reservoir was selected as prototype and reproduced with scale of 1:200 under the similarity conditions. A series of test with different collocation of parameters were performed. Empirical equations were derived by the means of dimensionless analysis and multiple nonlinear regression. The proposed equations were then used to quickly predict the wave features (maximum wave amplitude, runup, and wave attenuation) of Gongjiafang landslide in the Wu Gorge. Numerical simulation of waves generated by Gongjiafang landslide was carried out using DualSphysics which is a SPH method based application. Comparison between the wave features derived from the numerical simulation and the prediction equations was looked insight. Results show better agreements of both methods either in maximum wave amplitude and runup. In Addition, wave characteristics (wave length, velocity and period) were well analyzed using the wave gauges’ data in the numerical simulation. It can be concluded that the modeling work using SPH method should be well adopted in investigating the problems of landslide generated waves in detail, while the equations derived from the experiment enable a quick basis in the prediction of the main wave features. Combination of both methods will be an excellent way in analyzing the practical case and thus provides significant means for hazard assessment and prevention.