A Centrifuge Model Test of the Ground Motion Response in a Moderately Stiff Soil

The ground motion response in a moderately stiff soil in seismic events has been traditionally studied based on the actual field records which, however, have yet to offer consistent results regarding the amplification effect of the ground motion. In the present study, a centrifuge model of the moderately stiff soil field is designed to study the amplification effect of the ground motion in response to seismic loads. Four El Centro waves of different strengths are used as the input wave at the base under a gravitational field of 75 g. Ground motion data at different depths are collected via a number of sensors to study the acceleration peak, time history, and response spectrum of the ground motion. The measured amplitude and energy of seismic waves are found to gradually increase from the bottom to the surface during the propagation of seismic waves, and the peak acceleration at the surface is significantly magnified. The response spectrum analysis shows that the acceleration response spectrum gradually moves to the high-frequency direction from the base to the surface and the value of the response spectrum decreases with the increase of the depth in the present study.

Site Response, Basin Amplification, and Earthquake Stress Drops in the Portland, Oregon Area

ABSTRACT Site response, sedimentary basin amplification, and earthquake stress drops for the Portland, Oregon area were determined using accelerometer recordings at 16 sites of 10 local earthquakes with MD 2.6–4.0. A nonlinear inversion was applied to calculate site response (0.5–10 Hz), corner frequencies, and seismic moments from the Fourier spectra of the earthquakes. Site amplifications at lower frequencies of 0.1–2.0 Hz were determined from Fourier spectra of four regional earthquakes with Mw 5.8–6.4. Amplifications were calculated relative to a stiff-soil site outside the Portland and Tualatin basins. Sites on artificial fill and Holocene alluvium show strong amplification peaks (factor of 5) around 1–2 Hz. Sites on the Portland Hills, consisting of thin soil over basalt, display spectral peaks at 4–5 Hz (factor of 4). Spectral peaks at both sites are similar to those predicted for vertically propagating S waves from VS profiles determined at these sites using a borehole and refraction microtremor analysis. The largest amplifications at 0.1–1 Hz were found at stiff-soil sites in the Tualatin basin, based on recordings of regional earthquakes. Amplifications of a factor of 10, at about 0.3 Hz, were observed for a site in the deeper portion of the Tualatin basin and a factor of 7 at 0.5–0.6 Hz for two adjacent sites closer to the border of that basin. Stiff-soil sites in the Portland basin exhibit amplifications of 2–3 at frequencies of about 0.3–0.8 Hz. The frequencies of the amplification peaks for the deep Tualatin basin site can be explained by S-wave resonance in the shallow sediments, but the observed amplification is underestimated. Earthquake stress drops determined from the inversion range from 3 to 11 MPa, with no overall dependence on seismic moment.

Soil flow mechanisms of full-flow penetrometers in layered clays through particle image velocimetry analysis in centrifuge test

This paper reports the soil flow mechanisms observed in centrifuge tests around full-flow (T-bar and ball) penetrometers in layered clays. The layered clay samples consisted of soft–stiff, stiff–soft, soft–stiff–soft, and stiff–soft–stiff soil profiles. Particle image velocimetry (PIV), also known as digital image correlation (DIC), allowed accurate resolution of the flow mechanism around the faces of the T-bar and half-ball penetrated adjacent to a transparent window. For the T-bar, overall, a full symmetrical rotational flow around the T-bar dominated the behavior. A novel “trapped cavity mechanism” was revealed in stiff clay layers, with the evolution of the trapped cavity being tracked. No soil plug was trapped at the base of the advancing T-bar regardless of penetration from stiff to soft layer or the reverse. For the ball, two key features of the soil flow mechanism were identified, including (i) a combination of vertical flow, cavity expansion type flow, and rotational flow for a fully embedded ball and (ii) a stiff soil plug trapped at the base of the ball advancing in a stiff–soft clay deposit. For both penetrometers, a squeezing mechanism mobilized as they approached a soft–stiff layer interface.

Parametric Analysis of Slope Stability for River Embankment

This paper has aimed to investigate the slope stability for various conditions like embankment geometry, water level and soil property. The analysis has been performed by using the XSTABL program for different slope heights, slope angles and flood conditions with a fixed soil cohesion value. Since the rapid drawdown is the worst case for a particular embankment therefore, the analysis has been further performed with different cohesion values. From this investigation it has been noticed that the increase of cohesion of soil can increase the stability to a great extent. All the analysises have been performed for twenty bore logs. It has been found that the underlying soil affects the stability of slope as the failure surface intersects the soil of this region. It has been also observed that the loose, liquefiable sandy soil decreases the stability while the stiff soil with sufficient cohesion value stabilizes the slope of embankment. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited.

ANALISIS DAYA DUKUNG FONDASI DANGKAL PADA TANAH BERLAPIS DENGAN MENGGUNAKAN PROGRAM BERBASIS ELEMEN HINGGA

Generally, foundation can be classified into shallow foundation and deep foundation. There are some methods to analyze the bearing capacity of foundation such as Meyerhof method and finite element method. The purpose of this study was to compare the bearing capacity value generated by these methods. There are some factors that affect the bearing capacity of foundation, therefore the author wants to analyze some factors such as upper layer soil thickness, soil consistency and foundation footing. The analysis results show that the bearing capacity of shallow foundation generated by finite element method experiences the biggest difference up to 28% compared to Meyerhof method. Bearing capacity of soft-stiff soil has the largest increase of up to 50% compared to soft-medium soil. Meanwhile at the medium-stiff soil has the largest increase up to 161% compared to soft-medium soil. The value of bearing capacity differs according to the upper layer soil thickness too. At 2.5 meters thick, the value of bearing capacity decrease up to 19% compared to 2 meters thick and decrease up to 160% compared to 1,5 meters thick. The bearing capacity value also varies depends on the footing type. Square footing was increased 0 – 17% compared to rectangular footing. Fondasi secara umum terbagi atas fondasi dangkal dan dalam. Ada beberapa metode untuk menganalisa daya dukung fondasi seperti metode Meyerhof dan metode elemen hingga. Dalam penulisan ini penulis ingin membandingkan nilai daya dukung pada metode tersebut. Ada beberbagai faktor yang mempengaruhi nilai daya dukung fondasi, oleh karena itu penulis ingin menganalisa beberapa faktor yang mempengaruhi daya dukung fondasi seperti tebal lapisan atas, konsistensi tanah dan bentuk fondasi terhadap daya dukungnya. Dari hasil perbandingan, kapasitas daya dukung dengan menggunakan metode elemen hingga memperoleh hasil lebih besar hingga 28% jika dibandingkan dengan metode Meyerhof. Kapasitas daya dukung pada metode elemen hingga memiliki hasil perbedaan terbesar. Kapasitas daya dukung pada tanah lunak-kaku mengalami kenaikan terbesar hingga 50% dibandingkan pada tanah lunak-sedang. Sementara itu kapasitas daya dukung pada tanah sedang-kaku mengalami kenaikan terbesar hingga 161%. Kapasitas daya dukung juga berbeda tiap tebal lapisan atas yang ditinjau. Pada tebal lapisan atas sebesar 2,5 meter, kapasitas daya dukungnya mengalami penurunan terbesar hingga 19% dibandingkan pada tebal lapisan atas sebesar 2 meter dan penurunan hingga 160% dibandingkan pada tebal lapisan atas sebesar 1,5 meter. Bentuk fondasi juga mempengaruhi kapasitas daya dukung. Kapasitas daya dukung pada fondasi berbentuk bujur sangkar mengalami kenaikan 0 – 17% dibandingkan dengan fondasi berbentuk persegi panjang.

Response of Coastal Structures against Tsunami Forces and Its Variation When Impact Load is Applied on Exterior and Interior Columns under Different Soil Conditions

The catastrophic tsunamis generated during Tohoku, Japan (2011), Indonesia (2004) and Alaska (1964) earthquakes caused severe damage to life and coastal structures and warned the coastal community on prepared and constructing safe structures against such events. Since most of the tsunamis are triggered by undersea earthquakes , it is necessary to establish analytical methods for obtaining the variation of response parameters when tsunami waves hit exterior and interior columns when the similar structure rests on different soils. In this paper, a G+9 storey coastal structure is chosen for the analysis and the response parameters like bending moments, shear forces, time periods, displacements and base shears are worked out considering with and without the effect of Soil Structure Interaction (SSI). From the results it is observed that the multi storied structure is more vulnerable when it rests on loose soil compared to when it rests on stiff soil or hard rock and when it is fixed at the base.

VS30 Seismic Microzoning Based on a Geomorphology Map: Experimental Case Study of Chiang Mai, Chiang Rai, and Lamphun, Thailand

Thailand is not known to be an earthquake-prone country; however, in 2014, an unexpected moderate earthquake caused severe damage to infrastructure and resulted in public panic. This event caught public attention and raised awareness of national seismic disaster management. However, the expertise and primary data required for implementation of seismic disaster management are insufficient, including data on soil character which are used in amplification analyses for further ground motion prediction evaluations. Therefore, in this study, soil characterization was performed to understand the seismic responses of soil rigidity. The final output is presented in a seismic microzoning map. A geomorphology map was selected as the base map for the analysis. The geomorphology units were assigned with a time-averaged shear wave velocity of 30 m (VS30), which was collected by the spatial autocorrelation (SPAC) method of microtremor array measurements. The VS30 values were obtained from the phase velocity of the Rayleigh wave corresponding to a 40 m wavelength (C(40)). From the point feature, the VS30 values were transformed into polygonal features based on the geomorphological characteristics. Additionally, the automated geomorphology classification was explored in this study. Then, the seismic microzones were compared with the locations of major damage from the 2014 records for validation. The results from this study include geomorphological classification and seismic microzoning. The results suggest that the geomorphology units obtained from a pixel-based classification can be recommended for use in seismic microzoning. For seismic microzoning, the results show mainly stiff soil and soft rocks in the study area, and these geomorphological units have relatively high amplifications. The results of this study provide a valuable base map for further disaster management analyses.