Static and dynamic rocking stiffness of shallow footings on sand: centrifuge modelling

Abstract

Small-strain foundation response has mostly been studied analytically, with limited experimental verification against 1g physical model tests. This paper revisits the problem of small-strain foundation response, conducting a series of centrifuge model tests, aiming to eliminate the limitations of 1g testing. A centrifuge modelling technique is developed, combining static pushover and dynamic impulse testing for similar systems. To allow for derivation of meaningful insights, a novel procedure for in-flight measurement of the distribution of shear modulus with depth is also developed. The latter combines spectral analysis of surface waves (SASW) measurement of the shear modulus G0 at the surface, and estimation of the distribution of the shear modulus G with depth using acceleration measurements in shaking tests. A novel centrifuge tube–actuator is developed and employed to discharge spherical projectiles against single-degree-of-freedom models lying on shallow foundations on sand. This allows generating dynamic impulse excitation, which is used to measure the small-strain dynamic rocking stiffness. The developed actuator is versatile, and was also used for in-flight SASW testing. The centrifuge model tests are shown to confirm the widely used and well-known formulas. This good agreement can also be seen as a confirmation of the validity of the developed experimental techniques.

Performance of geotechnical seismic isolation system using rubber‐soil mixtures in centrifuge testing

“…The SASW method is based on the dispersion of Rayleigh wave, of which the velocity is reasonably close to that of shear wave for the range of Poisson's ratio expected in this study. This method was adopted by Murillo et al 55 and Taeseri et al 56 to assess shear wave velocity and shear modulus of soils near the surface in centrifuge model.…”

Physical Model of Shallow Foundation under Dynamic Loads on Sands

“…The physical models of SDOF systems are commonly excited by shaking tables or actuators, fixed or hinged, to the model [8][9][10][11][12]17]. The actuator applies impact, harmonic, or random dynamic loads.…”

“…Previous research used multiple displacement measurement systems for the SDOF models, as follows: miniature uniaxial accelerometers [9], potentiometers [10,12], vibration meters [41], wire and laser transducers [11], particle image velocimetry-PIV [17], high-speed cameras [8], and image processing [10]. The authors of this work developed a procedure to measure displacements based on computer vision algorithms, following the trend shown in the latest research.…”

“…The use of SDOF models to study soil behavior includes different approaches: Ref. [8] studied small-strain foundation response based on a series of centrifuge model tests; an SDOF model with a rocking mode of vibration was used to develop the investigation studying the seismic performance of shallow and embedded foundations on leveled and inclined soil.…”

“…The author also argued, based on evidence, that "soil-foundation plastic yielding under seismic excitation is unavoidable, and even in times desirable". Other authors proposed alternative analyses to understand the soil behavior whenever dynamic loads are applied using SDOF systems with rocking vibration mode [7][8][9][10][11][12][13]. This paper considers the same scope and tries to give more insights into the overturning process triggered by earthquakes and unbalanced rotating and reciprocating parts of machines, which produce transient and steady-state dynamic loads.…”