Shideh Dashti

Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil

Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soils relative density and thickness, and the buildings weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.

Centrifuge Testing to Evaluate and Mitigate Liquefaction-Induced Building Settlement Mechanisms

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settle- ment of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity i.e., the shaking intensity rate. In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly.

Mobile Phones as Seismologic Sensors: Automating Data Extraction for the iShake System

There are a variety of approaches to seismic sensing, which range from collecting sparse measurements with high-fidelity seismic stations to non-quantitative, post-earthquake surveys. The sparse nature of the high-fidelity stations and the inaccuracy of the surveys create the need for a high-density, semi-quantitative approach to seismic sensing. To fill this void, the UC Berkeley iShake project designed a mobile client-backend server architecture that uses sensor-equipped mobile devices to measure earthquake ground shaking. iShake provides the general public with a service to more easily contribute more quantitatively significant data to earthquake research by automating the data collection and reporting mechanisms via the iShake mobile application. The devices act as distributed sensors that enable measurements to be taken and transmitted with a cellular network connection. Shaking table testing was used to assess the quality of the measurements obtained from the iPhones and iPods on a benchmark of 150 ground motions. Once triggered by a shaking event, the devices transmit sensor data to a backend server for further processing. After a seismic event is verified by high-fidelity stations, filtering algorithms are used to detect falling phones, as well as device-specific responses to the event. A method was developed to determine the absolute orientation of a device to estimate the direction of first motion of a seismic event. A “virtual earthquake” pilot test was conducted on the UC Berkeley campus to verify the operation of the iShake system. By designing and fully implementing a system architecture, developing signal processing techniques unique to mobile sensing, and conducting shaking table tests to confirm the validity of the sensing platform, the iShake project serves as foundational work for further studies in seismic sensing on mobile devices.

Numerical Simulation of Building Response on Liquefiable Sand

AbstractThe effective design of earthquake-resistant structures and liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. In this paper, the results from centrifuge experiments of structures with shallow foundations on liquefiable sand were used to evaluate the predictive capabilities of a state-of-the-practice numerical tool. Fully-coupled numerical simulations with the UBCSAND model implemented in FLAC-2D captured building settlements measured in these experiments reasonably well for one scaled input motion, mostly within factors of 0.7 and 1.8. The soil model captured the overall contribution of deviatoric displacement mechanisms and localized volumetric strains during partially drained cyclic loading. The primary limitation of the model became evident for slower rates of earthquake energy buildup, when the extent of soil softening and building displacement was overestimated by up to a factor of 4. The observations from recent case hi...

Liquefaction-induced building movements

Liquefaction or cyclic softening from earthquake shaking have caused significant damage of buildings with shallow foundations. In recent earthquakes, buildings have punched into, tilted excessively, and slid laterally on liquefied/softened ground. The state-of-the-practice still largely involves estimating building settlement using empirical procedures developed to calculate post-liquefaction, one-dimensional, consolidation settlement in the “free-field” away from buildings. Performance-based earthquake engineering requires improved procedures, because these free-field analyses cannot possibly capture shear-induced and localized volumetric-induced deformations in the soil underneath shallow foundations. Recent physical and numerical modeling has provided useful insights into this problem. Centrifuge tests revealed that much of the building movement occurs during earthquake strong shaking, and its rate is dependent on the shaking intensity rate. Additionally, shear strains due to shaking-induced ratcheting of the buildings into the softened soil and volumetric strains due to localized drainage in response to high transient hydraulic gradients are important effects that are not captured in current procedures. Nonlinear effective stress analyses can capture the soil and building responses reasonably well and provide valuable insights. Based on these studies, recommendations for estimating liquefaction-induced movements of buildings with shallow foundations are made.

Numerical and Centrifuge Modeling of Seismic Soil–Foundation–Structure Interaction on Liquefiable Ground

AbstractThe effective mitigation of the liquefaction hazard requires an improved understanding of the consequences of liquefaction in terms of ground shaking, permanent displacement, and building performance. In this paper, results from centrifuge experiments of a shallow-founded structure on liquefiable sand are used to evaluate the predictive capabilities of a state-of-the-art numerical tool. Solid-fluid, fully-coupled 3D nonlinear numerical simulations were performed using the PDMY02 soil model implemented in a software modeling domain. The numerical model captured excess pore pressures and accelerations well in the free-field, but largely underestimated volumetric settlements due to loss of water during shaking. This was associated with the drastic increase in soil hydraulic conductivity when approaching liquefaction, which was not taken into account numerically, as well as the underestimation of soil volumetric compressibility. The contribution of volumetric strains to total building settlement was, ...

Seismic Performance of Shallow Founded Structures on Liquefiable Ground: Validation of Numerical Simulations Using Centrifuge Experiments

AbstractThe results of fully coupled, three-dimensional (3D), nonlinear finite-element analyses of structures founded on liquefiable soils are compared with centrifuge experiments. The goal is to provide insight into the numerical model’s capabilities in predicting the key engineering demand parameters that control building performance on softened ground for a range of structures, soil profiles, and ground motions. Experimental and numerical observations will also guide future analyses and mitigation decisions. The numerical model captured excess pore pressures and accelerations, the dominant displacement mechanisms under the foundation, and therefore building’s settlement, tilt, and interstory drift. Both experimental and numerical results showed that increasing the structure’s contact pressure and height/width (H/B) ratio generally reduces net excess pore pressure ratios in soil but amplifies the structure’s tilting tendencies and total drift. The settlement response of a structure with a greater pressu...

Seismic Performance of Underground Reservoir Structures: Insight from Centrifuge Modeling on the Influence of Structure Stiffness

AbstractThe available simplified analytical methods for the seismic design of underground structures either assume yielding or rigid-unyielding conditions. Underground reservoir structures do not fall into either of these categories. In this paper, we present the results of three centrifuge experiments that investigate the seismic response of stiff-unyielding buried structures in medium dense, dry sand and the influence of structure stiffness and earthquake motion properties on their performance. The structure to far-field spectral ratios were observed to amplify with increased structural flexibility and decreased soil-confining pressure at the predominant frequency of the base motion. Lateral earth pressures and racking displacements for a range of structural stiffnesses were compared with procedures commonly used in design. Pre-earthquake measured lateral earth pressures compared well with expected at-rest pressures. However, none of the commonly used procedures adequately captured the structural loadin...

Dynamic Calibration of Tactile Sensors for Measurement of Soil Pressures in Centrifuge

Tactile pressure sensors are flexible, thin sheets containing a matrix of sensors, which are used to measure earth pressures in geotechnical applications. Although more successful in static and 1-g shaking table tests, available tactile sensors do not capture the full amplitude content of dynamic signals in centrifuge experiments. This is due to under-sampling and the sensor’s frequency-dependent response. A minimum sampling rate of 3000 samples per second is recommended in centrifuge testing to avoid under-sampling and capture frequencies up to 300 Hz in model scale. A new dynamic calibration methodology is proposed to characterize the sensor’s frequency-dependent response by evaluating how it attenuates pressure at higher frequencies. Sinusoidal loads are applied to the sensor at different frequencies, and the applied pressure is simultaneously recorded by a reference load cell and a tactile sensor. A transfer function is then calculated by dividing the Fourier pressure amplitude of the load cell by that of the tactile sensor at a given frequency. To dynamically calibrate tactile sensors, this transfer function may be used as an amplitude correction function under general loading. Through a series of blind dynamic tests, the proposed frequency-dependent, dynamic calibration methodology is shown to reduce the peak residuals between the tactile and reference sensor recordings from approximately 0.55 to 0.15 at frequencies below 300 Hz.

Evaluating the Reliability of Phones as Seismic Monitoring Instruments

Emergency responders must “see” the effects of an earthquake clearly and rapidly for effective response. This paper presents a novel use of cell phone and information technology to measure ground motion intensity parameters. The phone sensor is an imperfect device and has a limited operational range. Thus, shake table tests were performed to evaluate their reliability as seismic monitoring instruments. Representative handheld devices, either rigidly connected to the table or free to move, measured shaking intensity parameters well. Bias in 5%-damped spectral accelerations measured by phones was less than 0.05 and 0.2 [log(g)] during one-dimensional (1-D) and three-dimensional (3-D) shaking in frequencies ranging from 1 Hz to 10 Hz. They did tend to overestimate the Arias Intensity, but this error declined for stronger motions with larger signal-to-noise ratios. With these ubiquitous measurement devices, a more accurate and rapid portrayal of the damage distribution during an earthquake can be provided.