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Evaluation of Liquefaction Potential of Soil by Down Borehole Method

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Evaluation of Liquefaction Potential of Soil by Down Borehole Method Missouri University of Science and Technology Scholars' Mine International Conferences on Recent Advances 2010 - Fifth International Conference on Recent in Geotechnical Earthquake Engineering and Advances in Geotechnical Earthquake Soil Dynamics Engineering and Soil Dynamics 26 May 2010, 4:45 pm - 6:45 pm Evaluation of Liquefaction Potential of Soil by Down Borehole Method Sumit Ghose Bengal Engineering and Science University, India Ambarish Ghosh Bengal Engineering and Science University, India Follow this and additional works at: https://scholarsmine.mst.edu/icrageesd Part of the Geotechnical Engineering Commons Recommended Citation Ghose, Sumit and Ghosh, Ambarish, "Evaluation of Liquefaction Potential of Soil by Down Borehole Method" (2010). International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. 15. https://scholarsmine.mst.edu/icrageesd/05icrageesd/session01/15 This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. EVALUATION OF LIQUEFACTION POTENTIAL OF SOIL BY DOWN BOREHOLE METHOD Sumit Ghose Ambarish Ghosh Ex-Post Graduate Student, Department of Civil Engineering Professor, Department of Civil Engineering Bengal Engineering and Science University Bengal Engineering and Science University Howrah-711 103 Howrah-711 103 INDIA INDIA ABSTRACT The procedure used in this research work for evaluation of liquefaction potential due to earthquake is by correlating the cyclic stress ratio(CSR) and the cyclic resistance ratio(CRR) obtained from standard penetration test and shear wave velocity. Shear wave velocity in the different layers of soils have been determined by the use of down borehole instrument. P- and S-wave velocity data of near- surface soils (upper 21 m) are analyzed and correlated to depth and sedimentological properties. The results show that the S-wave velocity is much more sensitive to changes in lithology and mechanical properties than the P-wave velocity, which is characterized by a narrow range of values. The data shows that Vs is better correlated with silt content than with clay content for the sediments of the area investigated. While Vs has increased with increasing clay content it decreases with increasing silt content. Model curves for earthquake of various magnitudes have been developed after detailed study and analysis of the enormous data. This model curves serve the purpose of demarcating the zones of liquefaction and non-liquefaction. Using these model curves and assuming an earthquake of a particular magnitude for which the model curves have been plotted, the liquefaction potential of soil if subjected to earthquake of that particular magnitude can be evaluated. INTRODUCTION Liquefaction is one of the most interesting, complex and saturated cohesionless soil under undrained loading controversial topics in the sphere of geotechnical earthquake conditions. When loose sand is subjected to seismically induce engineering. This natural phenomenon came into prominence vibratory motion, it tends to decrease in volume. If it is and attracted the attention of geotechnical engineers in a three saturated and drainage is impeded, some of the interparticle month period in 1964 when the Good Friday earthquake (Mw stress is transferred to the water. The transferred load causes a =9.2) in Alaska was followed by Niigata earthquake (Ms =7.5) rise in the pore water pressure; generally, the higher the in Japan. In both cases large scale destruction were caused by intensity of vibration, the greater the potential for increase in liquefaction induced failures. In 30 years since the occurrence pore water pressure. As the pore pressure approaches the of these fatal earthquakes, liquefaction has been studied confining pressure on the soil, shear resistance is lost. As a extensively by many researchers around the globe. Different consequence a structure situated on this soil may tilt and settle, terminologies, procedures and methods of analysis have been resulting in differential motions which may cause severe proposed and a prevailing approach has been slow to emerge. damage. Then f = (- u) tan ( 1 ) The Mechanism of Liquefaction where f is the shearing resistance or strength of the soil, is The phenomenon of liquefaction involves soil deformation the total normal stress, u is the pore water pressure within the caused by monotonic, transient or repeated disturbances of soil and is the effective angle of internal friction. Paper No. 1.36a 1 Liquefaction Potential: Methods of Evaluation Type of Sensors Used Geophones are available with different sensitive axes, usually The most common procedure around the world for evaluating horizontal or vertical. In the experiment performed in the field, liquefaction potential is “simplified procedure” developed a horizontally oriented geo phone was used. Horizontal originally by Seed and Idriss (1971) using blow counts from geophones are often mistakenly called “shear phones” because the Standard Penetration Test correlated with a parameter they are commonly used for shearwave surveys, but shear called the cyclic stress ratio which represents the cyclic waves can be oriented in any direction depending on the loading on the soil. Since 1971, this procedure has been polarization of the source. The geo phone is oriented in the modified and updated. Parallel procedures based on Cone same axis as the particle motion. It will be quite sensitive to Penetration Test, Shear wave velocity measurements, and the shear waves, and relatively insensitive to any compressive Becker Penetration Test was introduced and has been revised waves. and updated. During the past two decades several procedures based on Vs have been developed for predicting liquefaction. Nature of the Shear Wave These procedures were developed from laboratory studies, analytical studies, penetration – Vs correlations or field performance data and in-situ Vs measurements.The most recent addition to these methods is the use of shear wave velocity by the down or cross borehole method, seismic cone penetrometer test (SCPT), suspension logger, and Spectral Analysis of Surface Waves (SASW). GENERATING AND MEASURING SHEAR WAVES Fig. 1. Zero phase wavelet with strong first arrival Elastic waves are generated in the ground by using an energy The record from the experiment conducted resembles an source, and these elastic waves are detected at single or ordinary seismic record and an illustrative example from the multiple locations by vibration sensors called geophones. The experiment conducted is shown in Fig. 1. signals thus collected are displayed on a seismograph. In addition to the shear waves, there are P waves with different refraction paths, reflections, surface waves and various This is a classic zero-phase wavelet with a strong first arrival converted waves, but the two major wave types of interest followed by larger excursions which die down after a few from the view point of liquefaction potential evaluation are the cycles. In a properly done survey, a good shear wave record compression (P-waves) and the shear waves (S-waves). A will be less complex than refraction data, because mode major problem encountered is that the shear waves travel conversions are not considered and because the survey slower than P-waves and thus will be embedded in the geometry is chosen to minimize multiple arrivals. To confirm complex wave train some where after the first arrival of the that we really have a shear wave, another record is taken by wave series. In a normal refraction survey, identification of the hitting the other end of the plank. It should look like the one in P-waves is simple since they arrive first in the record. After Fig. 2. the “first arrivals”, many other waves will be buried in the later part of the seismic record. Hence a solution is necessary for obtaining a pure shear wave without any adulteration from any other wave type. The answer is to use a seismic energy source that generates mostly shear waves, and use of vibration sensors sensitive to shear waves. One extremely effective and popular mechanism to generate a clean shear wave is to simply use a wooden plank weighted down with a vehicle (to prevent sliding of the plank when struck with a hammer while generating the wave). By hitting the end of the plank with a Fig. 2 . Wavelet by hitting other end of the plank hammer, a shearing stress is applied to the ground. The shear wave propagates in the direction perpendicular to the plank The first break is in the opposite direction, which is the towards the geophone. confirmation that the arrival is most likely a shear wave. Superimposing the two single waves a combined shearwave showing the opposite polarities is obtained as shown in Fig. 3. Paper No. 1.36a 2 hammer combination, and since the waves travel nearly vertically, there are fewer ambiguities about the path. The main disadvantage of downhole surveys is that attenuation and natural filtering by the earth rounds off the seismic arrivals so that it can be difficult to pick
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