Dynamic testing of free field response in stratified granular deposits

https://doi.org/10.1016/j.soildyn.2016.02.014Get rights and content

Highlights

  • •

    Experimental results concerning the dynamics of stratified granular deposits.

  • •

    Modal testing techniques for natural frequency measurement on the shaking table.

  • •

    Method to evaluate the equivalent fundamental periods of stratified deposits.

  • •

    Shear strain-stress behaviour analysis in relation to a wide set of seismic inputs.

  • •

    Influence of granular material properties on dynamic response of layered soils.

Abstract

The dynamic free field response of two stratified deposits with different stiffness ratios between the top and the bottom layer was analysed by shaking table testing. The granular deposits were contained in a laminar shear box and subjected to a wide set of dynamic inputs with different frequency content. Two exploratory modal testing techniques were employed to measure the natural frequency of the individual layers and the results were employed in the calculation of the fundamental period of the overall stratified profile by an extended variant of the Madera procedure [1]. The dynamic response was investigated in relation to the frequency content of the dynamic excitation, the granular material properties and the stiffness characteristics of the enclosing container. The measured dynamic stiffness for the mono-layered and the bi-layered sand deposits compare well with previous empirical curves for sands increasing the confidence in the shaking table and shear stack testing as tools of dynamic investigation of granular media.

Introduction

The shearing stress–strain behaviour of soils is key to understanding how sites and buildings respond to earthquakes. The effect of local soil conditions on the observed magnitude and patterns of seismic damage to buildings has been studied extensively in the last four decades [2], [3], [4], [5], [6], [7]. The shearing behaviour of soils, notably the shear modulus and damping ratio were found to be the properties that govern dynamic soil-structure interaction at all strain levels. The measurement of these properties has been the central objective in numerous laboratory and field studies [8], [9], [10], [11], [12], [13]. Among the various tools employed in the dynamic analysis of granular media, shaking table testing is an important one, due to its capability of reproducing a wide set of real and artificial seismic inputs with relevance for the free field response. A large number of shaking table studies [14], [15], [16], [17], [18] employ flexible container boxes (‘shear stacks’) designed to replicate the free field response of a soil in plane strain conditions. Their role is to shear the soil via vertically propagating shear waves produced by the accelerating shaking table. In a large scale shear stack a large volume of soil can be tested, therefore the results may be more representative of the prototype field conditions. The boundary effects in a large shear stack are smaller than in table-top shear devices and the volume of soil situated in the central part of the container reproduces better the free field conditions for a given wavelength. It is also known that the design of a shear stack can be tuned to operate over wide strain ranges with granular materials of different stiffness value [19]. This, in particular, makes the shear stack useful for studying the large-strain dynamic moduli under seismic excitation. Several laminar shear box designs have been reported for both uniaxial and biaxial loading [20], [21], [22], [23], [24], [25], [26]. While there is a large body of literature on tests on homogeneous soils, experimental data on stratified deposits is scarce. This paper presents an experimental programme of dynamic testing carried out on three deposits of dry granular material at Bristol University. An homogeneous deposit of sand and two bi-layered deposits of sand and rubber granules were tested in a uniaxial shear stack. The shear stack was assembled rigidly on the platform of a shaking table. Pulse tests and random white noise tests were carried out to measure the natural frequency of the individual layers. The fundamental periods of the stratified deposits were evaluated and compared to an analytical solution from the literature [1]. The influences brought by the particulate material characteristics and the dynamic input parameters on the free field response were analysed. Aspects such as container-deposit coupling and deposit mode shapes were interpreted in relation to the deposit stiffness and the applied dynamic inputs.

Section snippets

Shear stack

The shear stack employed in this study was a medium-sized uniaxial laminar container of length 1.2 m, width 0.5 m and height 0.8 m. The shear stack was built at Bristol University in 1993 [20] and has been since subjected to several design improvements directed in particular towards lowering its stiffness [27]. Previous investigations of the shear stack have shown that its fundamental frequency when empty is about 6 Hz. The stack is active in one direction only, with its eight aluminium rings

Modal testing

Modal testing techniques that measure the shear wave velocity of the materials in ‘as-pluviated’ state in the shear stack can reveal a more realistic value of stiffness than the laboratory techniques employing small material samples confined in simplified boundary conditions (e.g. resonant column tests). Modal techniques employed ‘in-situ’ are believed to be less disturbing for the granular texture, therefore having more chance of capturing the small strain stiffness of the deposits.

In pulse

Concluding remarks

Shaking table test results of granular materials sheared in plane strain conditions in a large uniaxial shear stack were reported. The shear stress–strain behaviour of three configurations (one witness mono-layered deposit and two bi-layered deposits) was analysed under a set of seismic inputs with different frequency content. Two modal testing techniques (i.e. pulse testing and random white noise testing) were successful in measuring the fundamental periods of the individual layers. The

Acknowledgements

The financial support from ReLUIS (Rete di Laboratori Universitari Ingegneria Seismica) for conducting this research is gratefully acknowledged. The authors would like to thank Eleonora Iannacce and Domenico Lombardi for their contribution to the experimental work and to Professor David Muir Wood for his advice on granular material testing.

References (33)

  • S. Bhattacharya

    A Review of methods for pile design in seismically liquefiable soils

  • F.E. Richart et al.

    Vibrations of soils and foundations

    (1970)
  • H.B. Seed et al.

    Soil moduli and damping factors for dynamic response analysis

    (1970)
  • B.O. Hardin et al.

    Shear modulus and damping in soils

    J Soils Mech Found Div

    (1972)
  • M. Vucetic et al.

    Effect of soil plasticity on cyclic response

    J Geotech Eng

    (1991)
  • M.B. Darendelli

    Development of a new family of normalized modulus reduction and material damping curves

    (2001)

    • Cited by (11)

    • Calibration of a large shaking table equipment for testing geotechnical physical models under plane strain conditions

      2023, Soil Dynamics and Earthquake Engineering
      Citation Excerpt :

      Typically, seismic tests on scaled models are performed either under normal gravity conditions (1g), using shaking tables, or under an augmented gravity field, in dynamic centrifuge. Although the similitude requirements the between the model and the corresponding real prototype cannot be properly satisfied in terms of stress and strain under normal 1g conditions, it has to be recognised that shaking table tests have provided valuable insights into many geotechnical problems such as seismic response and identification of mechanical properties of soil deposits in free-field conditions (e.g. Ref. [1]; Dihoru et al. [2]; [3], liquefaction (e.g. Refs. [4–8], shallow (e.g. Refs. [9,10] and piled foundations (e.g. Refs. [11–15], piled bridge abutment systems [8,16–18], rigid (e.g. Refs. [19–21], flexible (e.g. Refs. [22–25] and earth-reinforced (e.g. Refs. [26–29] retaining walls, tunnels (e.g. Refs. [30,31], slopes and embankments (e.g. Refs. [32,33], soil-structure interaction (e.g. Refs. [34–39] and vibration mitigation strategies [12,40–43]. Recently, large-scale shear devices have been constructed to overcome some limitations of the more conventional small model containers and allow better reproducing the prototype field conditions; moreover, in a large soil container the boundary effects have a minor influence on the model response and the volume of soil situated in the central part of the container is able to mimic with reasonable approximation the prototype conditions.

    • A large-scale shaking table model test for acceleration and deformation response of geosynthetic encased stone column composite ground

      2021, Geotextiles and Geomembranes

    • On the variation of mechanical properties of saturated sand during liquefaction observed in shaking table tests

      2020, Soil Dynamics and Earthquake Engineering
      Citation Excerpt :

      Sinusoidal excitation tests were performed using centrifuge physical model [15] to investigate the behavior of saturated sand under cyclic shearing, and the idea of [11] was used to show the decrease of shear modulus and the increase of damping during the tests. 1-g shaking table tests were conducted on the one-layer and two-layer sand physical models using a flexible-boundary shear box [16] to investigate the influence of strain level and spectral content of earthquakes on the seismic behavior of sand. A higher stiffness of soil was observed under a relatively higher-frequency input motion, as well as the degradation of sand with respect to the rise of shear strain.

    • Shaking table tests on geosynthetic encased columns in soft clay

      2018, Geotextiles and Geomembranes
      Citation Excerpt :

      Since the virgin soil beds were tested with pulse loading, an understanding of overall small strain shear modulus of the models was tried to be developed. A similar approach to predict the resonant frequency and the small strain shear modulus of the soil contained in a laminar box is reported by Dihoru et al. (2016). Fig. 13 (a) and (b) illustrate the responses of the accelerometers under pulse loading for OSC-s and GEC-J1000-S, respectively.

    • Dynamic responses of calcareous foundation reinforced by microbially induced calcite precipitation

      2023, Yantu Gongcheng Xuebao/Chinese Journal of Geotechnical Engineering

    • Development and performance test of a stiffness-variable multidirectional laminar shear model container

      2022, Yantu Lixue/Rock and Soil Mechanics
    View all citing articles on Scopus
    View full text
    © 2016 Elsevier Ltd. All rights reserved.