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1.
This paper focuses on using high-frequency GAP-SENSORs (GSs), accelerometers, and load cells in a laminar shear box (LSB) filled with loose Toyoura sand to understand the effects of impact loads and cyclic shaking at 1-G on soil properties. The shear wave velocity at small strain (Vs) was calculated directly from first arrival reference using displacement time-history of two GSs under impact loading. Moreover, from first peak using the reduced deformation amplitude technique, damping ratio was calculated. In addition, shaking table tests were performed under harmonic loading with amplitude of acceleration inside the model ground varying from 0.02 g to 1 g. The frequencies of excitation varied from 1 Hz to 10 Hz. GSs and inside accelerometers were used to directly measure the outside lateral deformation and shear stress at different elevations of LSB, respectively. Results show that the shear modulus (G) and the damping ratio (D) behavior of model sand are generally consistent with the behavior presented by similar tests using only accelerometers. In addition, damping ratio increases as frequency loading increases. Characteristic changes in two shear stress components in shaking loading conditions were also investigated using high precision inside load cells.  相似文献   

2.
Shaking table tests were conducted on saturated clean Vietnam sand in the large biaxial laminar shear box (1880 mm×1880 mm×1520 mm) at the National Center for Research on Earthquake Engineering (NCREE), Taiwan. The settlement of sand specimens was measured and evaluated during and after each shaking test. Without liquefaction, the settlement of sand caused by shaking is very small. Significant volume changes occur only when there is liquefaction of sand. The volumetric strain of liquefied sand was calculated according to the measured settlement and the thickness of liquefied sand in the specimen. Relations between volumetric strain after liquefaction and the relative density of saturated clean sand were developed for various shaking durations and earthquake magnitudes. They are not affected by the shaking amplitude, frequency, and direction (one- or multidirectional shaking).  相似文献   

3.
This paper presents the dynamic soil–structure analysis of the main telescope T250 of the Observatorio Astrofísico de Javalambre (OAJ, Javalambre Astrophysical Observatory) on the Pico del Buitre. Vibration control has been of prime concern in the design, since astrophysical observations may be hindered by mechanical vibration of optical equipment due to wind loading. The telescope manufacturer therefore has imposed a minimal natural frequency of 10 Hz for the supporting telescope pier. Dynamic soil–structure interaction may significantly influence the lowest natural frequency of a massive construction as a telescope pier. The structure clamped at its base has a resonance frequency of 14.3 Hz. A coupled finite element–boundary element (FE–BE) model of the telescope pier that accounts for the dynamic interaction of the piled foundation and the soil predicts a resonance frequency of 11.2 Hz, demonstrating the significant effect of dynamic soil–structure interaction. It is further investigated to what extent the coupled FE–BE model can be simplified in order to reduce computation time. The assumption of a rigid pile cap allows us to account for dynamic soil–structure interaction in a simplified way. A coupled FE–BE analysis with a rigid pile cap predicts a resonance frequency of 11.7 Hz, demonstrating a minor effect of the pile cap flexibility on the resonance frequency of the telescope pier. The use of an analytical model for the pile group results in an overestimation of the dynamic soil stiffness. This error is due to the large difference between the actual geometry and the square pile cap model for which the parameters have been tuned.  相似文献   

4.
The kinematic bending of single piles in two-layer soil is explored to account for soil stiffness degradation and associated damping increase with increasing levels of shear strain, a fundamental aspect of soil behaviour which is not incorporated in current simplified seismic design methodologies for pile foundations.A parametric study of a vertical cylindrical pile embedded in a two-layer soil profile to vertically-propagating S waves, carried out in the time domain by a pertinent beam-on-dynamic-Winkler-foundation (BDWF) model, is reported. Strain effects are treated by means of the equivalent-linear procedure which provides soil stiffness and damping ratio as function of shear strain level. Whereas the approach still represents a crude representation of the actual soil behaviour to dynamic loading, it is more realistic than elementary solutions based on linear visco-elasticity adopted in earlier studies.The paper highlights that soil nonlinearity may have either a detrimental or a beneficial effect on kinematic pile bending depending on the circumstances. The predictive equations for kinematic pile bending in visco-elastic soil recently developed by the Authors are extended to encompass strain effects. Numerical examples and comparisons against experimental data from case histories and shaking table tests are presented.  相似文献   

5.
Multi-layered soil profiles, where one or more layers consist of loose liquefiable material, most commonly require pile foundations extending beyond the liquefiable layer to competent material. Under seismic loads, if the loose layer liquefies, then large localized plastic demands may be generated in the piles. To study this behavior and provide detailed data to validate numerical models, a 1-g shaking table experiment was conducted considering a single reinforced concrete pile embedded in a three-layer soil system. The model pile of 25 cm diameter was tested under increasing amplitude earthquake excitation in a sloped laminar soil box. The test specimen was designed at the lower bound of typical design to promote yielding, per ATC-32 (Applied Technology Council, 1996) [1]. The pile penetrated 7D (D=pile diameter) into a multi-layered soil configuration composed of a stiff uppermost crust overlying a saturated loose sand layer and a lower dense layer of sand. Plastic demands in the pile were characterized using curvature profiles coupled with back-calculation of the plastic hinge length and post-test physical observations. Results from this test quantify the post-yield behavior of the pile and serve as a complement to previously conducted centrifuge tests.  相似文献   

6.
Pile foundation as well as other underground structures could be seriously affected by soil liquefaction during strong earthquakes. Damages on pile foundation due to liquefaction can be reduced by implementation of some soil improvement method. Main objective of present study is developing of drain method that can improve the soil in order to mitigate the destructiveness of liquefaction on superstructure supported by pile foundation. Series of shaking table tests were conducted on 2×2 pile foundation and soil model was improved by drains. Configurations of drains around piles, intensity of shaking were one of the parameters that were changing during the tests in order to investigate the response of pile foundation in improved soil condition.Shaking table tests and performed On-site experiment showed the following effects of the new drain method. (1) When the intensity of earthquake motion is 200 gal or less, generation of excess pore water pressure is reduced and the pile bending moment is decreased, (2) when the intensity of earthquake motion is stronger (300 gal or more), drainage effect prevents disappearance of subgrade reaction, and (3) proposed new type of drain can control excess pore water pressure without clogging.  相似文献   

7.
In western India during the Bhuj earthquake (Mw 7.6) on January 26, 2001, the Anjar City at ~30 km southwest of Bhuj experienced three types of damage scenario: severely damaged, less damaged and non-damaged. Similar damage patterns were also observed for the 1819 (Mw 7.8) and the 1956 (Mw 6.0) earthquakes. Microtremor array measurements were conducted in and around the Anjar city to examine the strength of soil structures and damage pattern. Significant differences are observed in frequencies and amplitudes in horizontal-to-vertical spectral ratio (HVSR) using microtremor measurements. The severely- damaged site shows two peak amplitudes: 2.8 at 1.2 Hz; and 4.0 at 8.0 Hz. The less-damaged site also shows two amplitudes: 2.5 and 2.1 at 1.4 Hz; and 2.0 Hz, respectively. The non-damaged site, on the other hand, shows that the HVSR curves become almost flatter. Similar results for three types of damage scenario based on analyses of earthquake records are also observed for the study area. The microtremor array measurements has revealed shear wave velocity Vs≥400 m/s at 18 m depth in the non-damaged, at 40 m in the less-damaged and at 60 m depth in the severely-damaged sites. The site amplitudes and the Vs values show a good correlation with the soil characteristics and damage pattern, suggesting that strength of soil layers at varying depths is a dictating factor for the estimate of the earthquake risk evaluation of the area under study.  相似文献   

8.
This paper discusses the design, the installation, and the experimental and numerical evaluation of the effectiveness of a stiff wave barrier in the soil as a mitigation measure for railway induced vibrations. A full scale in situ experiment has been conducted at a site in El Realengo (Spain), where a barrier consisting of overlapping jet grout columns has been installed along a railway track. This barrier is stiff compared to the soil and has a depth of 7.5 m, a width of 1 m, and a length of 55 m. Geophysical tests have been performed prior to the installation of the barrier for the determination of the dynamic soil characteristics. Extensive measurements have been carried out before and after installation of the barrier, including free field vibrations during train passages, transfer functions between the track and the free field, and the track receptance. Measurements have also been performed at a reference section adjacent to the test section in order to verify the effect of changing train, track, and soil conditions over time. The in situ measurements show that the barrier is very effective: during train passages, a reduction of vibration levels by 5 dB is already obtained from 8 Hz upwards, while a peak reduction of about 12 dB is observed near 30 Hz immediately behind the barrier. The performance decreases further away from the jet grouting wall, but remains significant. The experimental results are also compared to numerical simulations based on a coupled finite element–boundary element methodology. A reasonable agreement between experiments and predictions is found, largely confirming the initially predicted reduction. This in situ test hence serves as a ‘proof of concept׳, demonstrating that stiff wave barriers are capable of significantly reducing vibration levels, provided that they are properly designed.  相似文献   

9.
Various pre-seismic and co-seismic effects have been reported in the literature in the solid Earth, hydrosphere, atmosphere, electric/magnetic field and in the ionosphere. Some of the effects observed above the surface, particularly some of the pre-seismic effects, are still a matter of debate. Here we analyze the co-seismic effects of a relatively weak earthquake of 28 October 2008, which was a part of an earthquake swarm in the westernmost region of the Czech Republic. Special attention is paid to unique measurements of infrasonic phenomena. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Infrasonic oscillations (～1–12 Hz) in the epicentre region appear to be excited essentially by the vertical seismic oscillations. The observed oscillations are real epicentral infrasound not caused by seismic shaking of the instruments or by meteorological phenomena. Seismo-infrasonic oscillations observed 155 km apart from the epicentre were excited in situ by seismic waves. No earthquake-related infrasonic effects have been observed in the ionosphere. Necessity to make vibration tests of instruments is pointed out in order to be sure that observed effects are not effects of mechanical shaking of the instrument.  相似文献   

10.
On 6 April 2009 a Mw=6.1 earthquake produced severe destruction and damage over the historic center of L’Aquila City (central Italy), in which the accelerometer stations AQK and AQU recorded a large amount of near-fault ground motion data. This paper analyzes the recorded ground motions and compares the observed peak accelerations and the horizontal to vertical response spectral ratios with those revealed from numerical simulations. The finite element method is considered herein to perform dynamic modeling on the soil profile underlying the seismic station AQU. The subsurface model, which is based on the reviewed surveys that were carried out in previous studies, consists of 200–400 m of Quaternary sediments overlying a Meso-Cenozoic carbonate bedrock. The Martin-Finn-Seed's pore-water pressure model is used in the simulations. The horizontal to vertical response spectral ratio that is observed during the weak seismic events shows three predominant frequencies at about 14 Hz, 3 Hz and 0.6 Hz, which may be related to the computed seismic motion amplification occurring at the shallow colluvium, at the top and base of the fluvial-lacustrine sequence, respectively. During the 2009 L’Aquila main shock the predominant frequency of 14 Hz shifts to lower values probably due to a peculiar wave-field incidence angle. The predominant frequency of 3 Hz shifts to lower values when the earthquake magnitude increases, which may be associated to the progressive softening of soil due to the excess pore-water pressure generation that reaches a maximum value of about 350 kPa in the top of fluvial-lacustrine sequence. The computed vertical peak acceleration underestimates the experimental value and the horizontal to vertical peak acceleration ratio that is observed at station AQU decreases when the earthquake magnitude increases, which reveals amplification of the vertical component of ground motion probably due to near-source effects.  相似文献   

11.
In Ottawa, Canada, unusually high amplification ratios have recently been measured in clayey silts (called ‘Leda Clays’) at low levels of earthquake-induced ground shaking. However, the contribution of seismic Q, or material damping (ξ=1/2Q), to the overall ground motion at soft soil sites across the city is not well understood. This research investigates attenuation measurements in soft soils (Vs<250 m/s) for ongoing seismic hazard evaluation in the Ottawa area. The work focuses on in situ measurements of damping in two deep boreholes drilled into Leda Clay. To investigate the possibility of frequency-dependent dynamic properties of these materials at low strains, a new approach to the spectral ratio technique has been developed for the measurement of Qs in the field using a mono-frequency vibratory source (generating signals between 10 and 100 Hz), and two identical downhole 3-component geophones. Monofrequency signals also allowed for the measurement of dispersion (variation of velocity with frequency). Analysis of the data show that dynamic properties are, for the most part, independent of frequency in the homogenous silty soils, yielding negligible variation in shear wave velocity (<2 m/s) across the frequency test band, and small strain Qs's ranging from 170 to 200 (damping of 0.25–0.30%) over soil thickness intervals ranging from 10 to 60 m. At intervals within 20 m of the ground surface, laminated silt and clay beds of elevated porosity are found to have slight influence on the frequency dependence of damping for frequencies greater than 70 Hz (damping increase to 0.6%).  相似文献   

12.
An experimental development of a computer controlled photoelectric ocular system applied for the LaCoste and Romberg G949 gravimeter made the continuous observation of time variation of gravity possible. The system was operated for half a year in the Sopronbánfalva Geodynamical Observatory to test its capabilities. The primary aim of this development was to provide an alternative and self-manageable solution instead of the standard electronic (Capacitive Position Indicator) reading of this type of gravimeter and use it for the monitoring of Earth tide. It, however, turned out that this system is sensitive enough to observe the effect of variable seismic noise (microseisms) due to the changes of ocean weather in the North Atlantic and North Sea regions at microGal level (1 μGal = 10−8 m/s2). Up to now not much attention was paid to its influence on the quality and accuracy of gravity observations because of the large distance (>1000 km) between the observation place (generally the Carpathian–Pannonian basin) and the locations (centres of storm zones of the northern hydrosphere) of triggering events. Based on an elementary harmonic surface deformation model the noise level of gravity observations was compared to the spectral characteristics of seismic time series recorded at the same time in the observatory. Although the sampling rate of gravity records was 120 s the daily variation of gravity noise level showed significant correlation with the variation of spectral amplitude distribution of the analysed high pass filtered (cut-off frequency = 0.005 Hz) seismograms up to 10 Hz. Also available daily maps of ocean weather parameters were used to support both the correlation analysis and the parameterization of the triggering events of microseisms for further statistical investigations. These maps, which were processed by standard image processing algorithms, provide numerical data about geometrical (distance and azimuth of the storm centres relative to the observation point) and physical (mass of swelling water) quantities. The information can be applied for characterizing the state of ocean weather at a given day which may help the prediction of its influence on gravity measurements in the future. Probably it is the first attempt to analyse quantitatively the effect of ocean weather on gravity observations in this specific area of the Carpathian–Pannonian region.  相似文献   

13.
We present a new, impulsive, horizontal shear source capable of performing long shot profiles in a time-efficient and repeatable manner. The new shear source is ground-coupled by eight 1/2″ (1.27 cm) × 2″ (5.08 cm) steel spikes. Blank shotshells (12-gauge) used as energy sources can be either mechanically or electrically detonated. Electrical fuses have a start time repeatability of < 50 μs. This source can be operated by a single individual, and takes only ~ 10 s between shots as opposed to ~ 30 s for six stacked hammer blows. To ensure complete safety, the shotshell holder is surrounded by a protective 6″ (15.24 cm)-thick barrel, a push-and-twist-locked breach, and a safety pin.We conducted field tests at the 17th Street Canal levee breach site in New Orleans, Louisiana (30.017° N 90.121° W) and at an instrumented test borehole at Millsaps College in Jackson, Mississippi (32.325° N 93.182° W) to compare our new source and a traditional hammer impact source. The new shear source produces a broader-band of frequencies (30–100 Hz cf. 30–60 Hz). Signal generated by the new shear source has signal-to-noise ratios equivalent to ~ 3 stacked hammer blows to the hammer impact source. Ideal source signals must be broadband in frequency, have a high SNR, be consistent, and have precise start times; all traits of the new shear source.  相似文献   

14.
In this paper, distribution of the dynamic soil properties in a region comprising the majority of the Izmir Karşıyaka municipality borders is investigated. A number of geotechnical and geological parameters of the region, which involve the results of formerly and recently drilled borings as well as the results of the geotechnical tests performed on undisturbed specimens are considered. Moreover, collected data, and the variation of dynamic soil properties in the region is demonstrated utilizing a Geographical Information System (GIS) software, in which a square grid of 250 m cell size is determined. The variation of the average shear wave velocities in the upper 30 m of the region as well as the soil classes among NEHRP are thematically investigated to determine the variation of soil properties in the region, which may be triggered by high water table at alluvial areas. Simulated ground motions are created using a scenario earthquake on active İzmir fault of 6.9 magnitude, having an average distance of 10 km to the region under investigation. As a consequence, these motions are utilized to map the distribution of maximum spectral amplifications, maximum surface spectral accelerations as well as the ground shaking maps, for a better understanding of the risk in the region.  相似文献   

15.
An improved rheology model, inspired from explicit experiments is conceived to represent rate-dependent cyclic shear behavior of high damping rubber bearings at subzero and room temperatures. Total stress has been decomposed into nonlinear rate independent elasto-plastic stress, nonlinear elastic stress and nonlinear visco-elasto-plastic overstress branches. To represent nonlinear viscosity behavior, ‘overstress branch’ has been generalized by putting linear elastic spring in parallel to nonlinear elasto-plastic model, placed in series with nonlinear dashpot. Constitutive relations for model elements have been designated for respective fundamental phenomenon observed in constant strain rate experiments. An optimum calculation approach is developed to determine a unique set of overstress parameters capable not only of representing constant strain rate cyclic tests but also sinusoidal tests with variable input strain rates. Essential abilities of the proposed model and adequacy of estimated parameters have been confirmed by comparing numerical simulation results with experiments conducted at −30 °C, −10 °C and 23 °C.  相似文献   

16.
Microzonation studies using ambient noise measurements have become very popular in the last years for local soil characteristics determination. For defining the capabilities of the ambient noise based methods, a physical understanding of the nature of noise is essential. In this way, many studies have shown that the noise wavefield composition is heterogeneous, comprising both body and surface waves with fundamental and upper modes.In this work, a recently developed method of time-frequency polarization analysis, based on the stationary wavelet packet transform, is used to study how the waves’ polarization influences in the application of the HVSR method. For this purpose, each recorded signal is first decomposed into a set of sub-signals, according to their reciprocal ellipticity. After that, the H/V analysis is carried out on the raw recorded signal, as well as on the obtained sub-signals, evaluating the contribution of each sub-signal (i.e., each reciprocal ellipticity range) to the expected H/V result.For the study, single station seismic noise was recorded at 11 sites around the province of Alicante (southeastern Spain). The obtained results illustrate that high frequency peaks, above 1–2 Hz, are clearly represented by just using the sub-signals associated to the ellipticity range of [0–0.1], whereas low frequency peaks, down 0.5 Hz, need a wider ellipticity interval for being properly retrieved, i.e., [0–0.4] or even [0–0.6]. Therefore, linearly polarized waves’ contribution, range [0–0.1], is enough for retrieving the high frequency H/V peaks, but not for obtaining the complete H/V curve, especially the low frequency peaks. In these cases, the elliptical polarized Rayleigh waves’ contribution cannot be misestimated in HVSR computation.  相似文献   

17.
The objective of this study was to explore the slope position and land use change effects on the variability in magnetic susceptibility and 137Cs inventory as the soil redistribution indicators in a hilly semiarid calcareous area in Iran. The selected study area is located in a hilly region with pasture and cultivation land use of Fereydunshahr, Isfahan Province in west-central Iran. In the two mentioned dominant ecosystems, four slope positions including summit, shoulder, backslope and footslope were identified and in each land use and slope position, three cores were selected to collect 72 soil samples from three depths (0–10, 10–20, 20–30 cm) in an area of 15 × 15 cm. Additional 28 soil samples were collected from the reference site for soil loss and deposition calculations by using the Cs-137 measurement. The results of the study with the use of the Cs-137 technique showed that the average soil loss in the pasture land (46.4 t ha− 1 yr− 1) was significantly (p < 0.05) lower than the average soil loss in the cultivated land (80.4 t ha− 1 yr− 1). The highest soil loss in both land uses was obtained in the shoulder position, 60.1 and 84.4 t ha− 1 yr− 1, respectively, for the pasture and cultivated lands. Moreover, the highest rates of soil deposition was observed in a footslope position in both land uses and they were 34 and 32.4 t ha− 1 yr− 1 for the pasture and cultivated lands, respectively. Magnetic susceptibility was significantly (p < 0.05) greater in pasture (χlf = 41.51 × 10− 8 m3/kg) than in the cultivated land (χlf = 34.90 × 10− 8 m3/kg). The pasture land with a lower soil loss rate, indicated significantly higher magnetic susceptibility in all landform positions as compared to that in the cultivated land. The results of the correlation analysis showed that among the studied soil physico-chemical properties, χlf (r = 0.83, p < 0.01) in the pasture land had the highest correlation with the Cs-137 inventory. Throughout the non-linear regression analysis, χlf was introduced for relating soil parameters and the cesium inventory explained 68% and 79% of the total variability of 137Cs in the pasture and cultivated lands, respectively. The results implied that the variability in the magnetic susceptibility within the hillslope is consistent with the variation of the Cs-inventory; and the results thus demonstrate the slope and land use effects on soil redistribution.  相似文献   

18.
40Ar / 39Ar incremental heating ages for twenty one grains of cryptomelane, collected at 0, 42, 45, and 60 m depths in the Cachoeira Mine weathering profile, Minas Gerais, permit calculating long-term (10 Ma time scale) weathering rate (saprolitization rate) in SE Brazil. Pure well-crystallized cryptomelane grains with high K contents (3–5 wt.%) yield reliable geochronological results. The 40Ar / 39Ar plateau ages obtained decrease from the top to the bottom of the profile (12.7 ± 0.1 to 7.6 ± 0.1 Ma at surface; 7.6  ± 0.2 to 6.1 ± 0.2 Ma at 42 m; and 7.1 ± 0.2 to 5.9 ± 0.1 Ma at 45 m; 6.6 ± 0.1 to 5.2 ± 0.1 Ma at 60 m), yielding a weathering front propagation rate of 8.9 ± 1.1 m/m.y. From the geochronological results and the mineral transformations implicit by the current mineralogy in the weathering profiles, it is possible to calculate the saprolitization rate for the Cachoeira Mine lithologies and for adjacent weathering profiles developed on granodiorites and schists. The measured weathering front propagation rate yields a saprolitization rate of 24.9 ± 3.1 t/km2/yr. This average long-term (> 10 Ma) saprolitization rate is consistent with mass balance calculations results for present saprolitization rates in weathering watersheds. These results are also consistent with long-term saprolitization rates estimated by combining cosmogenic isotope denudation rates with mass balance calculations.  相似文献   

19.
The response of massive caisson foundations to combined vertical (N), horizontal (Q) and moment (M) loading is investigated parametrically by a series of three-dimensional finite element analyses. The study considers foundations in cohesive soil, with due consideration to the caisson-soil contact interface conditions. The ultimate limit states are presented by failure envelopes in dimensionless and normalized forms and the effects of the embedment ratio, vertical load and interface friction on the bearing capacity are studied in detail. Particular emphasis is given on the physical and geometrical interpretation of the kinematic mechanisms that accompany failure, with respect to the loading ratio M/Q. Exploiting the numerical results, analytical expressions are derived for the capacities under pure horizontal, moment and vertical loading, for certain conditions. For the case of fully bonded interface conditions, comparison is given with upper bound limit equilibrium solutions based on Brinch Hansen theory for the ultimate lateral soil reaction. A generalized closed-form expression for the failure envelope in M–Q–N space is then proposed and validated for all cases examined. It is shown that the incremental displacement vector of the caisson at failure follows an associated flow rule, with respect to the envelope, irrespective of: (a) the caisson geometry, and (b) the interface conditions. A simplified geometrical explanation and physical interpretation of the associativity in M-Q load space is also provided. Finally, the derived failure envelope is validated against low (0.67 Hz) and high frequency (5 Hz) dynamic loading tests and the role of radiation damping on the response of the caisson at near failure conditions is unraveled.  相似文献   

20.
Base isolation is an effective way to reduce earthquake energy transfer from ground to structure, but existing seismic isolation systems are not very suitable for rural buildings for some reasons. A new steel–asphalt composite layer for the seismic base isolation of housing units is present in this paper. Its dynamic characteristics and isolation effect are studied by shake table tests of two full-scale specimens. Different earthquake waves with different peak ground accelerations (PGA, from 0.1 g to 0.4 g) are input. Test results show that the isolation layer could efficiently reduce the input acceleration. Moreover, as the PGA increase, the isolation layer shows good function of the displacement limit.  相似文献   

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