单桩复合地基上块体振动特性分析

本文利用轴对称有限元-无限元耦合模型和现场试验,系统地分析了搅拌桩、碎石桩的长度、直径和波速及其桩身缺陷对单桩复合地基上圆形块体竖向振动速度导纳特性的影响,结果表明块体的低频速度导纳幅值比共振频率更能稳健地反映出复合地基的性状变化。本文研究对进一步完善复合地基动测理论与技术具有参考价值。     

Cycloid crack sequences on Europa: Relationship to stress history and constraints on growth mechanics based on cusp angles

The original model developed to explain cycloidal cracks on Europa interprets cycloids as tensile fractures that grow in a curved path in response to the constantly rotating diurnal tidal stress field. Cusps form when a new cycloid crack segment propagates at an angle to the first in response to a rotation of the principal tidal stress orientation during a period of no crack growth. A recent revised model states that a cycloid cusp forms through the creation of a secondary fracture called a tailcrack at the tip of an existing cycloid segment during shearing motion induced by the rotating tidal stress field. As the tailcrack propagates away from the cusp, it becomes the next cycloid segment in the chain. The qualitative tailcrack model uniquely accounts for the normal and shear stresses that mechanically must resolve onto the tip of an existing cycloid segment at the instant of cusp formation. In this work, we provide a quantitative framework and test of the hitherto purely conceptual tailcrack model. We first present a relative age sequence inferred from geologic mapping of multiply cross-cutting cycloids in Europa's trailing hemisphere and place this into the context of the global stress history. The age sequence requires a cumulative minimum of 630° of shell reorientation due to nonsynchronous rotation to account for the observed range of orientations of cycloids of different ages. We determined the back-rotated longitudes of formation of two cycloid chain examples and used mathematical modeling of europan tidal stresses to show that the tailcrack model for cusp formation is not only viable, but places constraints on the overall development of a cycloid chain by controlling the timing of cusp development within Europa's orbit. For all cusps analyzed, the exact ratio of resolved shear to normal stress required to form the cusp angles by a process of tailcracking, as governed by the principles of linear elastic fracture mechanics, is produced at the tip of a shearing cycloid segment during Europa's orbit. Cusp formation occurs after the point in the orbit at which the maximum tensile principal tidal stress occurs, implying that tensile tidal stresses are not directly responsible for cusp development. Instead, cusps develop when a tailcrack forms at the tip of a cycloid segment in response to the highly perturbed stress field induced during concomitant opening and shearing at the tip of the cycloid segment.     

Stability of ice on Mars and the water vapor diurnal cycle: Experimental study of the sublimation of ice through a fine-grained basaltic regolith

In order to advance our understanding of the long-term stability of subsurface ice, the diurnal martian water cycle, and implications for liquid water, we determined diffusion coefficients and adsorption kinetics for the water vapor produced by the sublimation of ice buried beneath various layers of fine-grained (<63, 63-125, and 125-250 μm) basaltic powder under simulated martian conditions. Sublimation rates at shallower depths, <10 mm, were determined to be affected by mass transfer through the atmosphere in addition to the basalt layer. For greater depths, the measured diffusion coefficients for water vapor moving through basalt grains were 1.56±0.53×¹⁰⁻⁴, 2.05±0.82×¹⁰⁻⁴, and for the <63, 63-125, and 125-250 μm basaltic layers, respectively. Through the Brunauer, Emmett and Teller (BET) isotherm, which assumes multiple molecular layers of adsorbed water, we determined the adsorption constants of 52.6±8.3 at 270 K for <63 μm, 39.0±6.4 at 267 K for 63-125 μm, and 54.3±9.3 at 266 K for 125-250 μm, resulting in surface areas of 2.6±0.1×¹⁰⁴, 1.7±0.3×¹⁰⁴, , respectively. These results suggest that while diffusion is too rapid to explain the purported diurnal cycle in water content of the atmosphere, adsorption is efficient and rapid, and does provide an effective mechanism to explain such a cycle. The present diffusion data suggest that very thin, <50 pr μm, shallow, 10 mm, ice deposits would last for >10 h at ∼224 K, just above the freezing point of saturated CaCl₂. Temperatures can remain above ∼224 K over most of the planet, which means that water, even as saturated brine, will sublimate before the freezing point is reached and liquid could be formed. On the other hand, 1 m ice layers below 1 m of fine-grained basaltic regolith at 235 K and 10 Pa of atmospheric water could last 600 to 1300 years. At deeper depths and lower temperatures, ice could last since the last major obliquity change 400,000 years ago.     

Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system

High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ¹⁸O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values (δ¹⁸O = 6.0 ± 0.6‰ (2σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ¹⁸O and an apparent bimodal distribution with modes of 6.6 ± 0.4‰ and 7.3 ± 0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ¹⁸O with zircon growth has been observed. The δ¹⁸O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ¹⁸O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ¹⁸O values (5.3 ± 0.6‰ (2σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth.

The δ¹⁸O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9 ± 0.5‰ (2σ), AD1: 11.7 ± 0.6‰ (2σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ¹⁸O melt.

High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration.     

Petrological inferences on the evolution of magmas erupted in the Andagua Valley,Peru (Central Volcanic Zone)

Major and trace element and Sr and Nd isotope data is presented from the Andagua valley scoria cone and lava field (15°32′ S 72°19′ W), Southern Peru in the northernmost part of the Central Volcanic Zone (CVZ). The rocks are all quite evolved in composition (SiO₂ = 55–64 wt.%) and classify as benmoreites, latites and few mugearites and trachytes. Samples are characterized by high Na₂O (4.2–5.2 wt.%), Sr (600–1300 ppm), Ba (800–1600 ppm). The main difference between the benmoreites and latites is in the Na₂O content that reach the highest so far reported from CVZ for these SiO₂ concentrations. The rocks are generally nearly aphyric but latites and trachytes are more porphyritic. Amphibole microphenocrysts generally are only present in latites and trachytes. The difference between benmoreite and latite samples is reflected in lower P₂O₅ and Zr content of the latite samples documenting the existence of two compositional different parental magma types. The investigated volcanic activity spans the Pleistocene to Recent with the historic activity concentrated in the area just south of Andagua. Combined relative stratigraphy, petrography and geochemistry define volcanic units and demonstrate that rocks from Chilcayoc Grande, Chilcayoc Chico 2, Jenchana, Sucna 1 and Chilcayoc Chico 1 represent the most recent volcanic activity. The main trend samples, each form a co-magmatic group resulting in sub-parallel trends in many variations diagrams. It is furthermore shown that these trends point towards calculated mixing lines relating the individual units through a binary mixing process, thus indicating a two stage evolution. In the case of Jenchana, Sucna 1 and Chilcayoc Chico 1, the samples define positive correlation trends in the Sr vs. Rb diagram that can be extrapolated back towards origo indicating nearly perfect incompatibility of Sr and Rb. This together with generally high Sr/Y (50–105) and low Y content (< 16 ppm) suggest lack of plagioclase fractionation and residual garnet in the source and is taken as evidence for relatively high pressure (lower crustal) origin of the mixing event. The amphibole bearing samples form individual co-magmatic groups that cannot be related to each other. This means that the amphibole bearing samples originates from different magmas. The lavas of the Ninamama group are comparable in age to the main trend samples but different in petrography and composition, why the two compositional different magmas must have existed within a small confined area within a limited time span.     

An isobaric–isenthalpic magma mixing model for the Hasan Dagi volcano,Central Anatolia,Turkey

Calc-alkaline rocks of the Hasan Dagi volcano (Central Anatolia, Turkey) are products of arc volcanism triggered by continental collision. Volcanic rocks of the Hasan Dagi range in composition from basalt to rhyolite but are dominated by andesite and dacite. Considering only the mass transfer part of the process leads to an incomplete picture of magma chamber processes. The exclusion of simultaneous calculations of heat and mass transfer between mixing magmas, however, has prevented petrologists from gaining new insights into the magma mixing process. Thus, we report our experimental results in conjunction with modeling with MELTS to test the ideas concerning the petrogenesis of Hasan Dagi volcanic rocks and quantitatively model the relevant petrogenetic processes. Our results demonstrate that the chemical diversity of Hasan Dagi volcano is inconsistent with the closed-system crystallization and differentiation. Thus, (1) our experimental modeling, (2) the agreement between the liquid line of descent defined by the natural rock data and the MELTS calculations, and (3) the agreement between the mineralogy of the rocks and calculated mineralogy corroborate the conclusion that the isobaric–isenthalpic magma mixing of basalt and rhyolite is the major controlling process in the petrogenesis of the Hasan Dagi magmas.     

Variability of the subtropical shelf front off eastern South America: Winter 2003 and summer 2004

Hydrographic data collected during surveys carried out in austral winter 2003 and summer 2004 are used to analyze the distributions of temperature (T) and salinity (S) over the continental shelf and slope of eastern South America between 27°S and 39°S. The water mass structure and the characteristics of the transition between subantarctic and subtropical shelf water (STSW), referred to as the subtropical shelf front (STSF), as revealed by the vertical structure of temperature and salinity are discussed. During both surveys, the front intensifies downward and extends southwestward from the near coastal zone at 33°S to the shelf break at 36°S. In austral winter subantarctic shelf water (SASW), derived from the northern Patagonia shelf, forms a vertically coherent cold wedge of low salinity waters that locally separate the outer shelf STSW from the fresher inner shelf Plata Plume Water (PPW) derived from the Río de la Plata. Winter T–S diagrams and cross-shelf T and S distributions indicate that mixtures of PPW and tropical water only occur beyond the northernmost extent of pure SASW, and form STSW and an inverted thermocline characteristic of this region. In summer 2004, dilution of Tropical water (TW) occurs at two distinct levels: a warm near surface layer, associated to PPW–TW mixtures, similar to but significantly warmer than winter STSW, and a colder (T∼16 °C) salinity minimum layer at 40–50 m depth, created by SASW–STSW mixtures across the STSF. In winter, the salinity distribution controls the density structure creating a cross-shore density gradient, which prevents isopycnal mixing across the STSF. Temperature stratification in summer induces a sharp pycnocline providing cross-shelf isopycnal connections across the STSF. Cooling and freshening of the upper layer observed at stations collected along the western edge of the Brazil Current suggest offshore export of shelf waters. Low T and S filaments, evident along the shelf break in the winter data, suggest that submesoscale eddies may enhance the property exchange across the shelf break. These observations suggest that as the subsurface shelf waters converge at the STSF, they flow southward along the front and are expelled offshore, primarily along the front axis.     

Internal waves,baroclinic energy fluxes and mixing at the European shelf edge

The energy flux in internal waves generated at the Celtic Sea shelf break was estimated by (i) applying perturbation theory to a week-long dataset from a mooring at 200 m depth, and (ii) using a 2D non-hydrostatic circulation model over the shelf break. The dataset consisted of high resolution time-series of currents and vertical stratification together with two 25-h sets of vertical profiles of the dissipation of turbulent kinetic energy. The observations indicated an average energy flux of 139 W m⁻¹, travelling along the shelf break towards the northwest. The average energy flux across the shelf break at the mooring was only 8 W m⁻¹. However, the waves propagating onshelf transported up to 200 W m⁻¹, but they were only present 51% of the time. A comparison between the divergence of the baroclinic energy flux and observed dissipation within the seasonal thermocline at the mooring showed that the dissipation was at least one order of magnitude larger. Results from a 2D model along a transect perpendicular to the shelf break showed a time-averaged onshelf energy flux of 153–425 W m⁻¹, depending on the magnitude of the barotropic forcing. A divergence zone of the energy flux was found a few kilometre offshore of the location of the observations in the model results, and fluxes on the order of several kW m⁻¹ were present in the deep waters further offshelf from the divergence zone. The modelled fluxes exhibited qualitative agreements with the phase and hourly onshelf magnitudes of the observed energy fluxes. Both the observations and the model results show an intermittent onshelf energy flux of 100–200 W m⁻¹, but these waves could only propagate ∼20–30 km onshore before dissipating. This conclusion was supported by a 25-h dataset sampled some 180 km onto the shelf, where a weak wave energy flux was found going towards the shelf break. We therefore conclude that shelf break generated internal waves are unlikely to be the main source of energy for mixing on the inner part of the shelf.     

Faroe Shelf Water

Faroe Shelf Water (FSW) is the water mass that occupies the shallow parts of the Faroe Shelf, surrounding the Faroe Islands (62°N, 7°W). Intensive tidal mixing induces a high degree of homogeneity and the circulation system allows a partial isolation from surrounding waters. This water mass, therefore, supports a unique ecosystem of great importance for commercial fish stocks and studies have shown a clear dependence of the ecosystem on the physical processes that maintain this system and control the exchange between the FSW and the off-shelf waters. In order to identify and quantify these processes, a large observational dataset has been analysed and related to alternative theories. From this analysis, the extent and properties of the FSW have been quantified and the degree of stratification explained in terms of the Simpson–Hunter theory. The residual clockwise circulation system, which is responsible for the partial isolation from off-shelf waters, is shown to be mainly generated by tidal rectification. The typical exchange rate of water between the FSW and the off-shelf regimes has been determined by the use of simple models based on the heat and the salt budgets but the actual exchange rate is found to vary considerably in time and space. These results support earlier suggestions that this exchange is the main limiting factor for the phytoplankton spring bloom on the Faroe Shelf and that variations in exchange rate are responsible for the large inter-annual variation in spring bloom timing and intensity. The observations indicate that the on-shelf/off-shelf exchange intensity is not symmetrically distributed around the shelf, but rather concentrated around the narrow southern tip of the Faroe Shelf, where off-shelf waters during intensive exchange events may be imported all the way to the shore.     

新型固化剂ESC在搅拌桩中的应用研究

通过新型岩土固化剂（ESC）搅拌桩体与水泥土搅拌桩体的对比试验以及ESC固化土作用机理的论述,说明了搅拌桩具有较高的侧摩阻力,同时证明了用ESC固化剂替代水泥能使搅拌桩固结土的强度大幅度得到提高,从而解决了搅拌桩因桩体强度低而存在的侧摩阻力难以发挥、单桩承载力低的问题。