Identification of dynamic soil properties through shaking table tests on a large saturated sand specimen in a laminar shear box

doi:10.1016/j.soildyn.2016.01.007

Soil Dynamics and Earthquake Engineering

Volume 83, April 2016, Pages 59-68

https://doi.org/10.1016/j.soildyn.2016.01.007 Get rights and content

Highlights

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How the boundary effect affects the identified soil properties is evaluated.
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The soil-filled laminar box system is regarded as a two-layer system.
•
The dynamic properties of box and soil are identified through three methods.
•
The boundary effect results in underestimating shear wave velocity.
•
The large specimen is not uniform in contrast to the assumed uniform condition.

Abstract

Many laminar shear boxes have recently been developed into sliding-frame containers that can reproduce 1D ground-response boundary conditions. The measured responses of such large specimens can be utilized to back-calculate soil properties. This study investigates how the boundary effect in large specimens affects the identified soil properties through shaking table tests on a soil-filled large laminar box conducted at the National Center for Research on Earthquake Engineering in Taiwan. The tested soil-box system is unique because only 80% of the container is filled with soil. This system can be regarded as a two-layer system: an empty top and soil-filled bottom. The dynamic properties of this two-layer system are identified through various approaches, including theoretical solutions of wave propagation, free vibration, and nonparametric stress–strain analyzes. Therefore, the coupling effect of the box and soil can be evaluated. Results show that, compared with the two-layer system considering the influence of the box, the conventional approach with a single-layer system slightly underestimates shear wave velocity but obtains the same damping ratio of the soil layer. In addition, the identified modulus reduction and damping curves in the two-layer system are consistent with those obtained in a laboratory test on a small specimen. Furthermore, based on detailed acceleration measurements along different depths of soil, a piecewise profile of shear wave velocity is built. The identified shear wave velocity increases with depth, which is not uniform and differs from the constant velocity typically assumed for the specimen.

Introduction

Soil in a level ground of infinite extent under earthquake shaking is usually modeled as a soil element undergoing a simple shear loading condition. Small soil specimens are generally tested in laboratories (e.g., using triaxial compression apparatuses and simple shear devices) under regular or irregular dynamic loads to study soil behavior, such as stress–strain relationship and liquefaction. Stress conditions and deformations in soil elements in these test types are significantly affected by boundary conditions because of the size of the specimens. Moreover, loading conditions generally do not reflect real-field situations because of the limitations of loading devices.

An increasing number of downhole arrays are currently available for measuring motions on ground surface and within a soil profile. These arrays provide valuable data in understanding in situ soil behavior under earthquake shaking. Different approaches, including nonparametric stress–strain analysis [1], [2] and parametric [3], [4] inverse analysis schemes, have been developed to identify dynamic soil behavior through downhole measurement. However, learning soil behavior from field measurements is an inherently inverse problem that can be challenging to solve. The extraction of soil properties also depends largely on the availability and spacing of measurements in a downhole array. Furthermore, the uncertainty of in situ conditions is uncontrollable.

As an intermediate condition between small-specimen laboratory tests and less controllable downhole measurements, large soil specimens are placed on shaking tables [5], [6] or centrifuges [7]; thus, soil behavior under realistic seismic loading conditions can be observed and analyzed. Several laminar shear boxes (e.g., [6], [13]) have recently been developed into sliding-frame containers to reproduce 1D ground-response boundary conditions. Dietz and Wood [6] evaluated dynamic soil properties by shaking table test on a large soil-filled laminar box. Three different excitation motions: random [9], pulse [10] and sinusoidal [11] were employed to evaluate the strain-dependent shear stiffness and damping. Afacan et al. [7] constructed centrifuge models in a laminar container to study the site response of soft-clay deposits over a wide strain range. Dense sensor arrays were used for the back-calculation of modulus-reduction and damping values. Mercado et al. [4] identified shear wave velocities and their reduction against strain through an optimization analysis of the centrifuge test of a laminar container. Experimental data from large specimens complemented the laboratory geotechnical investigation technique. However, the boundary effect can also be an issue that affects how soil properties are identified, as in the case of small specimens. Lee et al. [12] investigated the boundary effects of a laminar container on the seismic response acquired from accelerometers and pore pressure transducers at various depths and distances from the end walls. The results of the analysis revealed minimal boundary effects on the seismic responses, which confirmed the finding of the previous study [7], [13] that measurements on the container agree with that within the specimen. Therefore, a laminar container may be used effectively to simulate 1D shear wave propagation in shaking table tests and the measurements on the container are commonly adopted for analysis. Nevertheless, although boundaries do not affect responses and measurements, they can still influence the identified properties. Such an issue has not yet been discussed.

In this study, the boundary effect on how dynamic properties are identified is investigated through a series of shaking table tests on a large soil-filled laminar box. The tests are conducted at the National Center for Research on Earthquake Engineering (NCREE) in Taiwan [8]. Unlike the common test condition that the container is fully filled with soil, only 80% of the container is filled with soil in these tests. Therefore, not only the shear wave propagation characteristics of sand can be identified, but also the coupling effect of the box and soil can at the same time be evaluated. The coupling effect of the box and soil is assessed by regarding the box and soil as a two-layer system with an empty top and soil-filled bottom. The shear wave velocity (Vs), damping ratio (D), and their variation against strain are identified through various approaches, including theoretical solutions of wave propagation, free vibrations, and nonparametric stress–strain method. The boundary effect (i.e., the laminar box) on the identified value is discussed.

Section snippets

Laminar shear box test

The laminar shear box developed at NCREE is composed of 15 layers of sliding frames, as shown schematically in Fig. 1 [13]. The size of the soil specimen in the box is 1880 mm×1880 mm×1300 mm. These 15 layers of frames (80 mm each) are separately supported on the surrounding rigid steel walls, one above the other, with a vertical gap of 20 mm between adjacent layers. Therefore, each layer of frames can move independently at different depths and directions, thereby mimicking 1D ground-response

Analysis approach

Three methods are collectively utilized to identify the dynamic properties of Vietnam sand (Vs or D) in a large shear box. The influence of very small excess pore water pressure generated in the tests on the soil properties is ignored. Each method is described below.

Vs and damping

The Vs and damping of Layers 1 and 2 identified by the wave propagation and free-vibration methods are summarized in Tables 3 and 4. Given that Vs₂ and ξ₂ are the properties that combine the frame and soil, obtaining the Vs and ξ of soil requires further calculation. Assuming that the frame and soil system act as a composite material during seismic shearing, the V_s of soil (V_s,soil) can be back-calculated based on the shear force equilibrium and strain compatibility of the composite material: $ρ_{}$

Conclusions

This study investigates the boundary effect on how soil properties are identified by conducting shaking table tests on a soil-filled large laminar box. Because only 80% of the container is filled with soil, the system is regarded as a two-layer system with an empty top and soil-filled bottom. The dynamic properties of the soil, box, and soil with box, such as damping and Vs, are successfully identified through wave propagation and free-vibration methods.

The identified Vs values of the frame are

Acknowledgments

The authors would like to thank the National Center for Research on Earthquake Engineering, National Applied Research Laboratories of Taiwan for providing the shaking table test data and the associated test reports.

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Evaluation of dynamic soil-structure interaction effects in buildings with underground stories using 1 g physical experimentation in a transparent shear laminar box
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This article presents a study of the Dynamic Soil-Structure Interaction (DSSI) using physical reduced–scale models of a building under different configurations of above-ground and underground stories. Each model was tested in a laminar shear box with a transparent front side. This characteristic of the device makes it possible to visualize the soil particles during the test and acquire data from the soil-structure interaction in a direct and spatially continuous way. A shaking table, an array of 3 high–speed and high–resolution cameras and a tactile dynamic pressure sensor were used. Ricker wavelets, with a wide range of frequencies and amplitudes, were used to evaluate the transfer functions in the system and the seismic response of model. Kinematic interaction effects were evaluated using the Digital Image Correlation (DIC) technique. It is verified that the experimental system allows a detailed tracking of the dynamic interaction between the building and the surrounding soil. The results show the direct relationship between the frequency content of the input wavelet and the dynamic pressure distribution acting on the underground levels, and the increase of the building’s first vibrating period. In addition, it was found that the distribution and magnitude of the lateral soil thrust is dominated by the superstructure vibration, and not the ground movement. This relationship becomes more important as the slenderness of the building increases, i.e., as the rocking mode of vibration becomes more significant. Additionally, in the case of buildings with basements, our results suggest that the combination of the increased confinement, the interaction of the foundation in terms of motion compatibility and the vibrations transferred from the superstructure to the ground produces an invariable reduction in effective base motion, suggesting that neglecting the effects of DSSI would be a conservative assumption.
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Citation Excerpt :
Considering the surrounding soil had the constraint and energy-dissipation effect on the tested soil, a laminar container was manufactured and used to simulate the boundary conditions of the tested soil. The laminar container had a special configuration, which could simulate the shear deformation of soil and meet the boundary requirements of soil infinite domain [34–38]. In this study, as shown in Fig. 2, a laminar container with the size of 3.2 m × 2.3 m × 1.54 m was manufactured, which consisted of rectangular steel frames, steel plates with grooves, steel balls, guide pulleys, inhibiting device and steel plates.
Dynamic interaction between soil and structure group (SSGI) has received increasing attention due to higher requirements for urban seismic hazard prevention in recent years, especially in densely built areas. However, investigations on SSGI effects are still limited so far. To develop a better understanding on the SSGI effect, shaking table tests were conducted on a series of soil-structure and soil-structure group systems, where the influence of structure height, the number of structures, the composition of structure group and seismic records was considered. Besides, a three-dimensional numerical model was proposed to account for the SSGI effect and was verified through experimental results, based on which parametric analyses were conducted. It is found from the test results that as compared with the soil-structure interaction (SSI), the SSGI effect on accelerations at the top of the superstructures ranged from −40% to 27%. In general, the SSGI effect is significant for soft soils, and increases with larger number of structures and smaller structure spacing. In addition, the SSGI can change the response spectra of ground acceleration and significantly alter the maximum Fourier amplitude of acceleration responses of structures.

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Identification of dynamic soil properties through shaking table tests on a large saturated sand specimen in a laminar shear box

Highlights

Abstract

Introduction

Section snippets

Laminar shear box test

Analysis approach

Vs and damping

Conclusions

Acknowledgments

Comput Geotech

Soil Dyn Earthq Eng

Soil Dyn Earthq Eng

Soil Dyn Earthq Eng

Soil Dyn Earthq Eng

Soils Found

Soil Dyn Earthq Eng

Lotung downhole array II: evaluation of soil nonlinear properties

J Geotech Eng

Learning of dynamic soil behavior from downhole arrays

J Geotech Geoenviron Eng