Understanding the effect of a static driving shear stress on the liquefaction resistance of medium dense granular soils

The influence of a driving static shear stress on the liquefaction resistance of medium dense granular soils is investigated. A laboratory study of the behavior of five sands (percentage of silt varying between 2 and 42%, and D₅₀ varying between 0.55 and 0.09 mm) was undertaken. These five sands were sampled from the foundations of two dams in Quebec (Canada). Sixty cyclic direct simple shear (DSS) tests were performed with and without a driving static shear stress. Relative densities D_r varied between 60 and 70%, and static shear stress ratios =τ_st/σ′_vc varied between 0.2 and 0.6. The presence of static shear stress increased the cyclic strength of these soils. A relation between and K() factor giving the effect of preshearing on cyclic shear resistance is obtained. The mechanical competence is examined as a function of D₅₀ and percentage of fines. The observed dilatancy yielded rather cyclic mobility than liquefaction.     

Effect of hydrostatic pressure on the lattice parameters of Fe2SiO4 olivine up to 70 kbar

The pressure dependence of the three lattice parameters and unit cell volume of fayalite (Fe₂SiO₄ olivine) was determined by X-ray diffraction under hydrostatic pressures up to 70 kbar. In order to eliminate stress inhomogeneity within a composite material consisting of a specimen mixed with an internal-pressure standard, a liquid (1 : 1 mixture of ethanol and methanol) was used as a pressure-transmitting medium. The isothermal bulk modulus calculated on the basis of the second-order Birch-Murnaghan equation of state gives the values K₀ = 1.19 ± 0.10 Mbar and K₀′ = 7 ± 4, and if we assume K₀′ = 5: K₀ = 1.24 ± 0.02 Mbar. Three axes of fayalite were found to be compressible in the following order, b >c >a. Comparisons with the results obtained under non-hydrostatic compression are made.     

Silicate perovskite analogue ScAlO3: temperature dependence of elastic moduli

The elastic moduli of ScAlO₃ perovskite, a very close structural analogue for MgSiO₃ perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al₂O₃ measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO₃ perovskite, the value of (∂K_S/∂T)_P is −0.033 GPa K⁻¹ and (∂G/∂T)_P is −0.015 GPa K⁻¹. The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂K_S/∂T)_P| is typically about twice |(∂G/∂T)_P|. The value of (∂K_S/∂T)_P for ScAlO₃ is intermediate between those inferred less directly from V(P,T) studies of Fe-free and Fe- and Al-bearing MgSiO₃ perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P–V–T equation of state of (Mg,Fe)SiO₃ perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO₃ perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂G/∂T)|_P for ScAlO₃ is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO₃ perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO₃ perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679].

Combining the results from the previous studies and current measurements on ScAlO₃ perovskite, we extracted the parameters (q and γ₀) needed to fully specify its Mie–Grüneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K_S(T), unlike V(P,T) data, tightly constrain the value of q. It is concluded that ScAlO₃ has ‘normal’ γ₀ (1.3) and high q (3.6). The high value of q indicates that ScAlO₃ has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).     

Liquefaction in silty soils—screening and remediation issues

Current techniques for liquefaction screening, ground modification for liquefaction mitigation, and post-improvement verification rely on knowledge gained from extensive research on clean sands, field observations of liquefied ground, and judicial correlation of normalized penetration resistance [(N₁)₆₀,q_c1N] or shear wave velocity (v_s1) data with field liquefaction observations. Uncertainties prevail on the direct extrapolation of such techniques for silty soil sites. This paper examines laboratory data on liquefaction resistance, strength, and v_s1 of sands and silty soils using grain contact density as the basis. Effect of silt content on cyclic resistance, strength, m_v, and c_v is examined in this light. Rational insights on effects of silt content on the current screening techniques based on (N₁)₆₀, q_c1N, and v_s1 to silty soils are offered. Recent advances and modifications to the traditional densification, drainage, and permeation grouting techniques to make them viable for silty soils are discussed.     

Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as R_c/R_A (R_c=air corrected sample ³He/⁴He, R_A=air ³He/⁴He), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R_c/R_A<0.05, crustal helium domain). Further north, there is a resolvable ³He anomaly consistent with a mantle contribution (R/R_A>0.1, mantle helium domain). The highest values of 0.27–0.38 R_A occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50–100 km north of the Indus-Zangpo suture zone. There seems to be no association between the ³He anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the ³He anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the ³He anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet.     

Apparent stress scaling and statistical trends

Statistical relations between different earthquake parameters, such as M₀ (seismic moment), E_S (seismic energy), τ_a (apparent stress), A (rupture area), g (average slip acceleration), are investigated. For this purpose, a kinematic earthquake model representing averaged earthquake rupture process is formulated. The model implies a scaling relationship for τ_a as a function of three other parameters, related to kinematic (M₀), geometric (A) and material (g) source characteristics, which, according to the model, can change independently. This scaling relation is used to explain statistical trends that characterize different earthquake data sets (including micro-, small, moderate and large events) plotted in the log τ_a − log M₀ space, and to determine the area in this space, where typical earthquakes occur. The scaling relationship is interpreted in terms of the apparent stress minimum (i.e., the most uniform among the possible earthquake rupture patterns). It is concluded that, although the apparent stress increases on an average with increasing seismic moment, small and large earthquakes are essentially similar.     

Determination of groundwater flow-through regimes of shallow lakes and wetlands from numerical analysis of stable isotope and chloride tracer distribution patterns

A numerical model previously developed to systematically examine groundwater flow in vertical section near shallow surface water bodies such as lakes, wetlands and ponds is further developed to include simulation of the distribution patterns of hydrogeochemical and stable isotopic tracers in relation to the surface water body and the geometry of distribution patterns of the tracers in the groundwater release zone of the lake. Many different possible flow regimes are identified, however, in this paper attention is focused on flow-through water bodies, as these are the flow regimes observed in field validation. Two shallow lakes on the Swan Coastal Plain of south-west Western Australia are the subject of field studies where hydrogeochemical and stable isotopes are used to validate the flow-through groundwater flow regime predicted by the modelling confirming the validity of the approach. The flow regime transition diagrams introduced in earlier papers are extended to include consideration of the hydrogeochemical and stable isotopic indices Cl_L/Cl₊ and (1000+δ_L)/(1000+δ₊). These ratios are introduced as an additional two of nine non-dimensional ratios that are necessary to analyse the problem. The ratios represent the chloride and isotopic composition (Cl_L and δ_L), respectively, in the groundwater release zones of the lakes, relative to these parameters in the groundwater capture zone (Cl₊ and δ₊) for the lake. Field data from the case studies plotted on appropriately configured transition diagrams demonstrates the overall validity of the modelling approach and its underlying assumptions. It is concluded that isotopic and hydrogeochemical data are invaluable in interpreting the interaction between lakes or wetlands and regional aquifers as it is very difficult to make physical or hydraulic measurements in the field that allow an understanding of lake–aquifer interaction. The tools and concepts developed that are summarized in the presented transition diagrams are invaluable starting points for the consideration and analysis of other case-specific examples of groundwater–surface water interaction and will improve the scientific basis of decision-making concerning lake and wetland management and groundwater interaction by water resource and environmental managers.     

Behavior of stratified sand–silt–gravel composites under seismic liquefaction conditions

Laboratory seismic liquefaction studies have generally dealt with homogeneous soil conditions only, although stratified soils exist for various soil deposits. The main objective of this research project was to compare the behavior of stratified and homogeneous sand–silt–gravel composites during seismic liquefaction conditions for various silt and gravel contents. An experimental program was undertaken in which a total of eighty stress-controlled undrained cyclic triaxial tests were performed. Two methods of sample preparation were used for each soil type. These methods included moist tamping (representing homogeneous soil conditions) and sedimentation (representing layered soil conditions). The silt contents ranged from 0 to 50%, and soils with 10 and 30% gravel contents were tested. The confining pressure in all test series was 100 kPa. The results indicate that the liquefaction resistances of layered and uniform soils are not significantly different, despite the fact that the soil fabric produced by the two methods of sample preparation is totally different. This finding justifies applying the laboratory tests results to the field conditions for the range of variable studied.     

江苏射阳地区地震尾波Q_c值特征

选取2009—2018年江苏省射阳地震台200 km范围内M_L≥2.0地震波形资料,基于Sato单次散射模型,计算该区域S波尾波Q_c值,并探求其时间变化特征。采用相同方法计算盐城、灌云地震台地震波形资料尾波Q_c值,并将3个地震台的计算结果进行对比。结果表明:射阳地区是江苏省尾波Q_0低值区域,Q_c值随频率的变化关系为Q_c(f)=33.7±3.4f ~(1.16±0.04);在2016年10月20日射阳M_(L )4.8地震前,射阳地区尾波Q_c值有增大现象,Q_c值变化特征表现为上升—下降—上升—发震;盐城、射阳、灌云地震台所记录地震的尾波Q_c值在震前均出现明显的高值异常变化。     

The equation of state of CaSiO3 perovskite to 108 GPa at 300 K

Pressure–volume measurements have been performed for CaSiO₃ perovskite to 108 GPa at 300 K using NaCl and argon pressure-transmitting media, and energy dispersive X-ray diffraction (EDXD) in a diamond-anvil cell (DAC). By determining a parameter that is the product of the elastic anisotropy, S, and the uniaxial stress component, t, for each data point, we define the stress condition of the sample. For different points at the same pressure in a temperature-quenched sample, the St value can differ by as much as a factor of 5, indicating heterogeneity in the stress condition. This may be responsible for the large scatter of earlier P–V measurements in the DAC which in general used a large diameter X-ray beam. Also, the St value provides insight into the elastic anisotropy, S, of CaSiO₃ perovskite and platinum. The sign of S (positive) for CaSiO₃ perovskite agrees with first principles calculations but the magnitude may be inconsistent. A new compression curve at 300 K was obtained for CaSiO₃ perovskite by using those data points which represent the most nearly hydrostatic conditions. It is observed that the data points with high St values yield larger volumes than the points with small St values at a given pressure. By selecting the data points having low St values (St≤0.005), combining with lower pressure large volume press (LVP) measurements and fitting to third order Birch–Murnaghan equation of state (EOS), we find that CaSiO₃ perovskite is more compressible (V₀=45.58±0.05 ų, K_T0=236±4 GPa, and K_T0′=3.9±0.2 GPa) than suggested by previous studies. The density and bulk modulus of CaSiO₃ perovskite at lower mantle pressures and 300 K are 1–3% greater and 5–15% smaller, respectively, than found in previous studies. This study demonstrates that defining the stress state of the sample is crucial to obtain an accurate 300 K compression curve for unquenchable high-pressure phases.     

易县地震台分量钻孔应变观测数据分析

选取易县地震台YRY-4型分量式钻孔应变仪2012—2020年观测数据,进行Venedikov调和分析,并对2015—2020年四分量数据S₁、S₂、S₃、S₄进行自洽检验,发现应变观测数据连续性、可靠性较高。选取该台2018—2019年钻孔应变原始数据S₁、S₂、S₃、S₄,利用面应变S₁ + S₃、S₂ + S₄相关系数,分析河北省3次M_L≥4.0地震前数据异常变化,并利用剪应变S₁ - S₃、S₂ - S₄的大小,分析受抽水干扰的NE分量主应力方位角,判断干扰源方位,结果发现：①面应变与剪应变在地震发生前均会出现异常变化,且剪应变异常变化幅度较大,应与震中相对于观测井的位置有关;②对于受抽水干扰的钻孔应变NE分量,利用剪应变固体潮数据推算主应力方位角,可得干扰源在钻孔应变观测室NW80.96°方位,与实际位置存在一定偏差。     

Numerical modeling for wave–seabed–pipe interaction in a non-homogeneous porous seabed

An evaluation of the wave-induced pore pressures and effective stresses has been recognized by marine geotechnical engineers as an important factor in the design of marine pipelines. Most previous investigations for such a problem have considered the pipeline as a rigid material. Thus, the internal stresses within the pipeline have not been examined in the wave–seabed–pipe interaction problem. In this paper, we consider the pipeline itself to be an elastic material, and link the analysis of the pipeline with the wave–seabed interaction problem. Based on the numerical model presented, the effects of pipe geometry and variable soil characteristics on the wave-induced pore pressure and internal stresses will be discussed in detail. It is found that the internal normal stresses in the angular direction (σ_pθ) and shear stress (τ_p) within the pipe are much larger than the amplitude of wave pressure at the surface of the seabed.     

Strain localisation in bimineralic rocks: Experimental deformation of synthetic calcite–anhydrite aggregates

Deformation of synthetic calcite–anhydrite aggregates to large shear strains (up to γ = 12.4 at 600 °C, 300 MPa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10^− 3 s^− 1) resulted in the first experimental observation of strain localisation from initially homogeneous rocks. In contrast to experiments on pure calcite and anhydrite, which deformed homogeneously to large strains (γ ≥ 5), all experiments on calcite–anhydrite mixtures resulted in heterogeneous deformation at γ > 1 and the formation of narrow localised bands in the microstructures at γ > 4. In these bands, the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align, thereby forming microstructural layering similar to planar fabrics in natural mylonites. A switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and/or grain boundary sliding occurs simultaneously with strain localisation. It is concluded that deformation-induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite–anhydrite mixtures. The study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly-mineralic mylonites.     

A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem: Reconstructed drip water and relationship to local precipitation

The response of a climate proxy against measured temperature, rainfall and atmospheric circulation patterns at sub-annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record. Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records. An actively growing speleothem sampled from New St. Michaels Cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite. Stable isotope analysis of samples micromilled in 100 μm steps at the equivalent of bi-monthly intervals reveals fabric-correlated annual cycles in carbon isotopes, oxygen isotopes and trace elements responding to seasonal changes in cave microclimate, hydrology and ventilation patterns. Calcite δ¹³C values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave ‘winter’ conditions of highest pCO₂, whereas the dark microsparitic calcite, characterised by elevated δ¹³C and δ¹⁸O values grows under low ‘summer’ pCO₂ conditions. Drip water δ¹³C_DIC reaches a minimum in March–April, at which time the attenuated δ¹⁸O signal becomes most representative of winter precipitation. An age model based on cycle counting and the position of the ¹⁴C bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the GNIP and instrumental climate record for Gibraltar. The δ¹⁸O characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the δ¹⁸O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence (r² = 0.47) with the δ¹⁸O of rainfall falling each year between October and March and on a decadal scale the δ¹⁸O of reconstructed winter drip water mirrors secular change in mean winter temperatures.     

汶川地震砾性土液化场地特征解析

通过成都平原砾性土场地勘察测试,研究汶川地震中大量砾性土液化场地的基本特性,找出一般规律,对砾性土场地液化发生主客观原因提出解释,并修正以往若干认识偏差.分析表明:汶川地震液化砾性土层粒径范围宽,含砾量5%~85%甚至更大,同时其实测剪切波速140~270 m·s~(-1),修正剪切波速160~314 m·s~(-1),都远超历史记录;液化砾性土场地1/2集中在Ⅷ度区内,表明如砂土层液化一样,砾性土场地大规模液化需要较强地震动触发,但超过触发强度后液化规模增长均有限;成都平原浅表地层二元基本结构是汶川地震中出现大量砾性土场地的客观条件之一,该结构可使饱和砾性土层处于封闭状态,构成了砾性土液化的基本条件;虽然液化砾性土层剪切波速很高,但实际上大多松散状态,是此次地震大量砾性土场地发生液化的客观条件之二;地震中地表(井中)喷出物与地下实际液化土类大相径庭,且液化层埋深大多小于6.0 m,以往以地表喷出物反推地下液化层土性类型的做法不再成立;认为砾性土层波速大、透水性好而不会液化的传统认识也不再成立,但砾性土层液化条件与砂土层液化条件不同,前者要求更高.     

Response spectra for differential motion of columns paper II: Out-of-plane response

It is shown that the common response spectrum method for synchronous ground motion can be extended to make it applicable for earthquake response analyses of extended structures experiencing differential out-of-plane ground motion. A relative displacement spectrum for design of first-story columns SDC (T, T_T, ζ, ζ_T, τ, δ) is defined. In addition to the natural period of the out-of-plane response, T, and the corresponding fraction of critical damping, ζ, this spectrum also depends on the fundamental period of torsional vibrations, T_T, and the corresponding fraction of critical damping, ζ_T, on the “travel time,” τ (of the waves in the soil over a distance of about one-half the length of the structure), and on a dimensionless factor δ, describing the relative response of the first floor. The new spectrum, SDC, can be estimated by using the empirical scaling equations for relative displacement spectra, SD, and for peak ground velocity, v_max. For recorded strong-motion acceleration, and for symmetric buildings, the new spectrum can be computed from Duhamel's integrals of two uncoupled equations for dynamics equilibrium describing translation and rotation of a two-degree-of-freedom system. This representation is accurate when the energy of the strong-motion is carried by waves in the ground the wavelengths of which are one order of magnitude or more longer than the characteristic length of the structure.     

Late Pleistocene to Holocene composite speleothem O and C chronologies from South Island, New Zealand—did a global Younger Dryas really exist?

Oxygen and carbon data from eight stalagmites from northwest South Island are combined to produce composite records of δ¹⁸O and δ¹³C from 23.4 ka to the present. The chronology is anchored by 43 thermal ionization mass spectrometry (TIMS) uranium series ages. Delta ¹⁸O values are interpreted as having a first order positive relationship to temperature, but also to be influenced by precipitation in a complex manner. Delta ¹³C is interpreted as responding negatively to increases in atmospheric CO₂ concentration, biological activity and precipitation amount.

Six climatic phases are recognized. After adjustment of 1.2‰ for the ice volume effect, the δ¹⁸O record between 23 and 18 ka varies around −3.72‰ compared to the Holocene average of −3.17‰. Late-glacial warming commenced between 18.2 and 17.8 ka and accelerated after 16.7 ka, culminating in a positive excursion between 14.70 and 13.53 ka. This was followed by a significant negative excursion between 13.53 and 11.14 ka of up to 0.55‰ depth that overlapped the Antarctic Cold Reversal (ACR) and spanned the Younger Dryas (YD). Positive δ¹⁸O excursions at 11.14 ka and 6.91–6.47 ka represent the warmest parts of the Holocene. The mid-Holocene from 6 to 2 ka was marked by negative excursions that coincide with increased glacial activity in the South Island. A short positive excursion from 0.71 to 0.57 ka was slightly later than the Medieval Warm Period of Europe.

Delta ¹³C values were high until 17.79 ka after which there was an abrupt decrease to 17.19 ka followed by a steady decline to a minimum at 10.97 ka. Then followed a general increase, suggesting a drying trend, to 3.23 ka followed by a further general decline. The abrupt decrease in δ-values after 17.79 ka probably corresponds to an increase in atmospheric CO₂ concentration, biological activity and wetness at the end of the Last Glaciation, but the reversal identified in the δ¹⁸O record from 13.53 to 11.14 ka was not reflected in δ¹³C changes. The lowest δ¹³C values coincided with the early Holocene climatic suboptimum when conditions were relatively wet as well as mild.

Major trends in the δ¹⁸O_c record are similar to the Northern Hemisphere, but second order detail is often distinctly different. Consequently, at the millennial scale, a more convincing case can be made for asymmetric climatic response between the two hemispheres rather than synchronicity.     

Using SPIRAL (Single Pollen Isotope Ratio AnaLysis) to estimate C3- and C4-grass abundance in the paleorecord

C₃ and C₄ grasses differ greatly in their responses to environmental controls and influences on biogeochemical processes (e.g. water, carbon, and nutrient cycling). Difficulties in distinguishing between these two functional groups of grasses have hindered paleoecological studies of grass-dominated ecosystems. Stable carbon isotopic analysis of individual grains of grass pollen using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer holds promise for improving C₃ and C₄ grass reconstructions. This technique, SPIRAL (Single Pollen Isotope Ratio AnaLysis), has only been evaluated using pollen of known C₃ and C₄ grasses. To test the ability of SPIRAL to reproduce the abundance of C₃ and C₄ grasses on the landscape, we measured δ¹³C values of > 1500 individual grains of grass pollen isolated from the surface sediments of ten lakes in areas that span a large gradient of C₃- and C₄-grass abundance, as determined from vegetation surveys. Results indicate a strong positive correlation between the δ¹³C-based estimates of % C₄-grass pollen and the abundance of C₄ grasses on the landscape. The % C₄-grass pollen slightly underestimates the actual abundance of C₄ grasses at sites with high proportions of C₄ grasses, which can be corrected using regression analysis. Comparison of the % C₄-grass pollen with C/N and δ¹³C measurements of bulk organic matter illustrates the distinct advantages of grass-pollen δ¹³C as a proxy for distinguishing C₃ and C₄ shifts within the grass family. Thus SPIRAL promises to advance our understanding of grassland ecology and evolution.     

砂砾土动力特性的室内试验研究进展

简要回顾了国内外有关砂砾土动剪切模量和阻尼比的试验研究进展,总结了围压、固结比、颗粒形状、含砾量、颗粒破碎等因素对砂砾土动剪切模量、阻尼比及归一化动剪切模量比与阻尼比曲线特征的影响规律,发现砂砾土归一化动剪切模量比和阻尼比的曲线形状主要受围压大小的影响,其他因素的影响不明显;总结了砂砾土抗液化强度、振动孔压发展特征、液化判别方法方面的研究进展,含砾量、试样扰动、膜贯效应是影响砂砾土抗液化强度的主要因素;指出了砂砾土动力特性研究中存在的问题及今后应予以更深入关注的研究方向。