Cyclic resistance and liquefaction behavior of silt and sandy silt soils

The liquefaction behavior and cyclic resistance ratio (CRR) of reconstituted samples of non-plastic silt and sandy silts with 50% and 75% silt content are examined using constant-volume cyclic and monotonic ring shear tests along with bender element shear wave velocity (V_s) measurements. Liquefaction occurred at excess pore water pressure ratios (r_u) between 0.6 and 0.7 associated with cumulative cyclic shear strains (γ) of 4% to 7%, after which cyclic liquefaction ensued with very large shear strains and excess pore water pressure ratio (r_u>0.8). The cyclic ring shear tests demonstrate that cyclic resistance ratio of silt and sandy silts decreases with increasing void ratio, or with decreasing silt content at a certain void ratio. The results also show good agreement with those from cyclic direct simple shear tests on silts and sandy silts. A unique correlation is developed for estimating CRR of silts and sandy silts (with more than 50% silt content) from stress-normalized shear wave velocity measurements (V_s1) with negligible effect of silt content. The results indicate that the existing CRR–V_s1 correlations would underestimate the liquefaction resistance of silts and sandy silt soils.     

渤海海域黏性土剪切波速与抗剪强度统计关系的初步研究

剪切波速与地基土的抗剪强度、剪切模量和卓越周期等参数密切相关,是地震安全性评价中判定场地类别的一个主要指标和参数。鉴于海域工程中剪切波速往往难以直接由原位测得,而室内实验结果又常常与野外现场物探测试值存在较大差异,因此,如何通过其他途径有效获取满足工程需要的剪切波速参数在海域工程的地震安全性评价等方面具有迫切的实用需求。为此本文通过对渤海海域数十个石油平台项目中一系列饱和黏性土样品的剪切波速与抗剪强度实验数据的统计分析,尝试采用多种可能的函数来拟合确定二者之间的经验关系。结果表明:对于渤海海域黏性土剪切波速V_s与抗剪强度S_u之间的最佳统计经验关系为幂函数V_s=53.751S_u~(0.376)。此关系可为渤海海域工程中通过不排水抗剪强度估算剪切波速提供一种简便可行的实用性方法。     

A comparison of numerical simulations of eddy generation performed with a two- and a three-layer quasi-geostrophic model of oceanic mesoscale eddies

Abstract

The generation of eddies by a large-scale flow over mesoscale topography is studied with the help of two- and three-layer nonlinear quasi-geostrophic models of the open ocean. The equations are integrated forward in time with no eddies present initially. For a given time, the displacement of the interface between layers two and three (ζ) tends to a well-defined limit (function of the horizontal spatial coordinates) as ρ ₃ - ρ ₂ → 0 (ρ_r is the density of layer r). Even for values of α[= (ρ ₃ - ρ ₂)/(ρ ₂ - ρ ₁)] ^as small as 0.01 the potential energy due to ζ is not negligible and it can reach, in some cases, a considerable fraction of the total eddy energy.     

Estimation of sedimentary layer shear wave velocity using micro-tremor H/V ratio measurements for Bangalore city

For assessing earthquake hazard of metro cities, knowledge of soil amplification, thickness and properties of sedimentary layer are essential. In order to map the soil thickness using microtremor survey method, in Bangalore city, it is required to calibrate the relation between fundamental resonance frequency of the soil layer and its thickness for the region. For this purpose microtremor survey was carried out at 34 locations in the city where borehole log was available. The resonance frequency of the soil is evaluated from the microtremor recordings using the H/V ratio technique. A nonlinear regression relation between the thickness of sedimentary layer h (m), from the borehole logs, and the resonance frequency f_r (Hz), was derived as h=(58.3±8.8)f_r^{−(0.95)±0.1}. Using the model of shear wave velocity increasing with depth at these locations, the derived average shear wave velocity and the corresponding soil thickness were used, to get an empirical relation between V_S (m/s) and depth z(m), as V_s=(174±28)(1+z)^0.16±0.07. This relation also compares reasonably with the fit obtained between simulated V_S and depth from borehole logs for Bangalore city. The calibrated relations can be used at locations in Bangalore city where borehole logs are not available, for finding the thicknesses and shear wave velocities of the local soil layers at the survey locations.     

Acoustic reflection logging for open and cased holes

In the present work, the waveforms of reflected wave sonic log for open and cased boreholes are calculated. Calculations are performed for a borehole containing an acoustic multipole source (monopole, dipole, or quadrupole). A reflected wave is more efficiently excited at resonant frequencies. These frequencies for all source types are close to the frequencies of oscillations of a fluid column located in an absolutely rigid hollow cylinder. It is shown that the acoustic reverberation is controlled by the acoustic impedance of the rock Z = V_p ρ_s for fixed parameters of the borehole fluid, where V_p is the compressional wave velocity in the rock, and ρ_s is the rock density. This result is correct for all types of acoustic sources (monopole, dipole, or quadrupole). Methods of the waveform processing for determining parameters characterizing the reflected wave are discussed.     

Impact and cyclic shaking on loose sand properties in laminar box using gap sensors

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 (V_s) 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.     

Lithology and hydrocarbon mapping from multicomponent seismic data

Elastic rock properties can be estimated from prestack seismic data using amplitude variation with offset analysis. P‐wave, S‐wave and density ‘reflectivities’, or contrasts, can be inverted from angle‐band stacks. The ‘reflectivities’ are then inverted to absolute acoustic impedance, shear impedance and density. These rock properties can be used to map reservoir parameters through all stages of field development and production. When P‐wave contrast is small, or gas clouds obscure reservoir zones, multicomponent ocean‐bottom recording of converted‐waves (P to S or Ps) data provides reliable mapping of reservoir boundaries. Angle‐band stacks of multicomponent P‐wave (Pz) and Ps data can also be inverted jointly. In this paper Aki‐Richards equations are used without simplifications to invert angle‐band stacks to ‘reflectivities’. This enables the use of reflection seismic data beyond 30° of incident angles compared to the conventional amplitude variation with offset analysis. It, in turn, provides better shear impedance and density estimates. An important input to amplitude variation with offset analysis is the V_s/V_p ratio. Conventional methods use a constant or a time‐varying V_s/V_p model. Here, a time‐ and space‐varying model is used during the computation of the ‘reflectivities’. The V_s/V_p model is generated using well log data and picked horizons. For multicomponent data applications, the latter model can also be generated from processing V_s/V_p models and available well data. Reservoir rock properties such as λρ, μρ, Poisson's ratio and bulk modulus can be computed from acoustic impedance, shear impedance and density for pore fill and lithology identification. λ and μ are the Lamé constants and ρ is density. These estimations can also be used for a more efficient log property mapping. V_p/V_s ratio or Poisson's ratio, λρ and weighted stacks, such as the one computed from λρ and λ/μ, are good gas/oil and oil/water contact indicators, i.e., pore fill indicators, while μρ mainly indicates lithology. μρ is also affected by pressure changes. Results from a multicomponent data set are used to illustrate mapping of gas, oil and water saturation and lithology in a Tertiary sand/shale setting. Whilst initial log crossplot analysis suggested that pore fill discrimination may be possible, the inversion was not successful in revealing fluid effects. However, rock properties computed from acoustic impedance, shear impedance and density estimates provided good lithology indicators; pore fill identification was less successful. Neural network analysis using computed rock properties provided good indication of sand/shale distribution away from the existing wells and complemented the results depicted from individual rock property inversions.     

Experimental and conceptual evidence about the limitations of shear wave velocity to predict liquefaction

Based on the liquefaction performance of sites with seismic activity, the normalized shear wave velocity, V_s1, has been proposed as a field parameter for liquefaction prediction. Because shear wave velocity, V_s, can be measured in the field with less effort and difficulty than other field tests, its use by practitioners is highly attractive. However, considering that its measurement is associated with small strain levels, of the order of 10⁻⁴–10⁻³%, V_s reflects the elastic stiffness of a granular material, hence, it is mainly affected by soil type, confining pressure and soil density, but it is insensitive to factors such as overconsolidation and pre-shaking, which have a strong influence on the liquefaction resistance. Therefore, without taking account of the important factors mentioned above, the correlation between shear wave velocity and liquefaction resistance is weak.In this paper, laboratory test results are presented in order to demonstrate the significant way in which OCR (overconsolidation ratio) affects both shear wave velocity and liquefaction resistance. While V_s is insensitive to OCR, the liquefaction resistance increases significantly with OCR. In addition, the experimental results also confirm that V_s correlates linearly with void ratio, regardless of the maximum and minimum void ratios, which means that V_s is unable to give information about the relative density. Therefore, if shear wave velocity is used to predict liquefaction potential, it is recommended that the limitations presented in this paper be taken into account.     

Constitutive model for soil amplification of ground shaking: Parameter calibration,comparisons, validation

A one-dimensional constitutive model, developed for the nonlinear ground response analysis of layered soil deposits, is calibrated and validated experimentally in this paper. The small number of parameters renders the model easily implementable, yet quite flexible in effectively reproducing almost any type of experimentally observed hysteretic soil behavior. In particular, the model generates realistic shear modulus and damping curves as functions of shear strain, as well as stress–strain hysteresis loops. The model is calibrated against three sets of widely-used published shear modulus and damping (G : γ and ξ : γ) curves and a library of parameter values is assembled to facilitate its use. The model, along with a developed explicit finite-difference code, NL-DYAS, for analyzing the wave propagation in layered hysteretic soil deposits, is tested against established constitutive models and numerical tools such as Cyclic1D [12] and SHAKE [42], and validated against experimental data from two centrifuge tests. Emphasis is given on the proper assessment of the V_s profile in the centrifuge tests, on the role of soil nonlinearity, and on comparisons of two inelastic codes (NL-DYAS and Cyclic1D) with equivalent linear (SHAKE) analysis.     

Soil characterization of Tınaztepe region (İzmir/Turkey) using surface wave methods and nakamura (HVSR) technique

To determine the shear wave velocity structure and predominant period features of T?naztepe in ?zmir, Turkey, where new building sites have been planned, active–passive surface wave methods and single-station microtremor measurements are used, as well as surface acquisition techniques, including the multichannel analysis of surface waves (MASW), refraction microtremor (ReMi), and the spatial autocorrelation method (SPAC), to pinpoint shallow and deep shear wave velocity. For engineering bedrock (V _s > 760 m/s) conditions at a depth of 30 m, an average seismic shear wave velocity in the upper 30 m of soil (AV_s30) is not only accepted as an important parameter for defining ground behavior during earthquakes, but a primary parameter in the geotechnical analysis for areas to be classified by V _s30 according to the National Earthquake Hazards Reduction Program (NEHRP). It is also determined that Z1.0, which represents a depth to V _s = 1000 m/s, is used for ground motion prediction and changed from 0 to 54 m. The sediment–engineering bedrock structure for T?naztepe that was obtained shows engineering bedrock no deeper than 30 m. When compared, the depth of engineering bedrock and dominant period map and geology are generally compatible.     

Liquefaction characteristics of gap-graded gravelly soils in K0 condition

A series of undrained cyclic direct simple shear tests, which used a soil container with a membrane reinforced with stack rings to maintain the K₀ condition and integrated bender elements for shear wave velocity measurement, were performed to study the liquefaction characteristics of gap-graded gravelly soils with no fines content. The intergrain state concept was employed to categorize gap-graded sand–gravel mixtures as sand-like, gravel-like, and in-transition soils, which show different liquefaction characteristics. The testing results reveal that a linear relationship exists between the shear wave velocity and the minor fraction content for sand–gravel mixtures at a given skeleton void ratio of the major fraction particles. For gap-graded gravelly sand, the gravel content has a small effect on the liquefaction resistance, and the cyclic resistance ratio (CRR) of gap-graded gravelly sands can be evaluated using current techniques for sands with gravel content corrections. In addition, the results indicate that the current shear wave velocity (V_s) based correlation underestimates the liquefaction resistance for V_s values less than 160 m/s, and different correlations should be proposed for sand-like and gravel-like gravelly soils. Preliminary modifications to the correlations used in current evaluations of liquefaction resistance have thus been proposed.     

Density and undrained shear strength of bed sediment from ND-CPT

The density and the undrained shear strength (s _u) of bed sediments at either side of the Isahaya Bay dike in the Ariake Sea, Japan, were investigated using nuclear density cone penetration tests (ND-CPTs). The nuclear density cone penetrometer (ND-CP) was operated from a boat, conducting 71 ND-CPTs. Furthermore, 26 undisturbed samples were obtained for soil density and s _u measurements to calibrate the ND-CPT data. The results show that the density and s _u obtained from in situ with the ND-CPTs agree well with those from the laboratory tests on undisturbed samples, and the obtained density profiles show good repeatability. The vertical variation in density and s _u of the bed sediment can be obtained from the ND-CPT results. The relations between the density and s _u show that s _u increases with an increase in density, but that this relation is site specific. The values of s _u show an exponentially growing trend and the values of log s _u show a linearly increasing trend with density. The vertical distribution of the bed sediments can be described using the density values obtained with the ND-CPT.     

The maximum-slope method for the interpretation of electrical soundings over two horizontal layers

Using the Carpenter configuration, in which the distances between the successive electrodes of a four-electrode system are kept at r, s and r (as shown in Fig. 1), where r = ma and s = la, or the Wenner-Carpenter configuration, in which r = s, the corresponding formulae for the apparent resistivities ρ_A^α, ρ_A^β and ρ_A^γ for the α, β and γ configurations are used to compute maximum gradient values for various values of the reflection coefficient K. For any observed curve for these resistivity types, the maximum slope is determined. A set of standard curves is presented which, provided that K is known, then allows the thickness of the top layer to be determined.In the case that K is unknown, another set of standard curves is presented allowing determination of K provided that the maximum slopes of any two of the three resistivity curves are known.The method has been tested for both field and laboratory observations. Discrepancies between the observed and actual values of resistivity are noted and attributed partly to instrumental errors.     

Velocities,elastic moduli and weathering-age relations for pacific layer 2 basalts

Compressional (V_p) and shear (V_s) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V_pandV_s show wide ranges (3.70to6.38km/sec forV_pand1.77to3.40km/sec forV_sat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of ?1.89and?1.35km/sec per100my forV_pandV_s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.     

Small-strain dynamic characterization of clayey soil from the Texcoco Lake,Mexico

The area occupied by the former Texcoco Lake was part of a system of lakes inside the Basin of Mexico. The subsoil there has been studied in the past but there is still a need for more and more thorough investigations into the dynamic properties of its highly compressible clays. This paper describes the results of an experimental laboratory research in which a triaxial cell fitted with bender elements was used to measure shear wave velocities (V_s) in clay specimens from the former Texcoco Lake. Soil specimens were subjected to isotropic loading–unloading cycles and values of V_s were determined during the saturation stage and after each stress increment or decrement. Our results show that irrespective of the testing method, shear waves velocities differ in no more than 7–15%.     

砂-粉混合料剪切波速的弯曲元及共振柱试验对比试验研究

为探讨测试方法、试验条件以及级配特征对砂-粉混合料剪切波速的影响,对具有不同细粒含量FC,相对密度Dr以及初始有效围压$\sigma_{3 \mathrm{c}}^{\prime}$的砂-粉混合料进行弯曲元和共振柱试验。结果表明：当Dr =35%或50%时,剪切波速Vs随FC的增大先减小后增大;当Dr =60%时,Vs随FC的增大而减小;弯曲元试验测得的Vs明显大于共振柱试验测得的Vs,随着FC的增大,弯曲元试验与共振试验得到的Vs差值逐渐减小,而当FC＞20%时,两种试验得到的Vs基本相同。在考虑Vs弥散性之后,不同FC的混合料弯曲元与共振柱试验得到的Vs结果具有较好的一致性。基于Hardin模型建立的砂-粉混合料Vs预测方法具有较好的预测效果。     

Monofrequency in situ damping measurements in Ottawa area soft soils

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 (V_s<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 Q_s 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 Q_s'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%).     

Vp‐Vs relationship and amplitude variation with offset modelling of glauconitic greensand‡

The relationship between V_p and V_s may be used to predict V_s where only V_p is known. V_p/V_s is also used to identify pore fluids from seismic data and amplitude variation with offset analysis. Theoretical, physical, as well as statistical empirical V_p‐V_s relationships have been proposed for reservoir characterization when shear‐wave data are not available. In published work, the focus is primarily on the V_p‐V_s relationship of quartzitic sandstone. In order to broaden the picture we present V_p‐V_s relationships of greensand composed of quartz and glauconite by using data from the Paleocene greensand Nini oil field in the North Sea. A V_p‐V_s relationship derived from modelling is compared with empirical V_p‐V_s regressions from laboratory data as well as from log data. The accuracy of V_s prediction is quantified in terms of root‐mean‐square error. We find that the V_p‐V_s relationship derived from modelling works well for greensand shear‐wave velocity prediction. We model the seismic response of glauconitic greensand by using laboratory data from the Nini field. Our studies here reveal that brine‐saturated glauconitic greensand can have a similar seismic response to that from oil‐saturated quartzitic sandstone and that oil‐saturated strongly cemented greensand can have a similar amplitude variation with offset response to that from brine‐saturated weakly cemented greensand.     

Shear wave velocity-based evaluation and design of stone column improved ground for liquefaction mitigation

The evaluation and design of stone column improvement ground for liquefaction mitigation is a challenging issue for the state of practice. In this paper, a shear wave velocity-based approach is proposed based on the well-defined correlations of liquefaction resistance (CRR)-shear wave velocity (V _s)-void ratio (e) of sandy soils, and the values of parameters in this approach are recommended for preliminary design purpose when site specific values are not available. The detailed procedures of pre- and post-improvement liquefaction evaluations and stone column design are given. According to this approach, the required level of ground improvement will be met once the target V _s of soil is raised high enough (i.e., no less than the critical velocity) to resist the given earthquake loading according to the CRR-V _s relationship, and then this requirement is transferred to the control of target void ratio (i.e., the critical e) according to the V _s-e relationship. As this approach relies on the densification of the surrounding soil instead of the whole improved ground and is conservative by nature, specific considerations of the densification mechanism and effect are given, and the effects of drainage and reinforcement of stone columns are also discussed. A case study of a thermal power plant in Indonesia is introduced, where the effectiveness of stone column improved ground was evaluated by the proposed V _s-based method and compared with the SPT-based evaluation. This improved ground performed well and experienced no liquefaction during subsequent strong earthquakes.