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1.
Anil Misra Lance A. Roberts Steven M. Levorson 《Geotechnical and Geological Engineering》2007,25(1):65-77
Load displacement analysis of drilled shafts can be accomplished by utilizing the “t-z” method, which models soil resistance
along the length and tip of the drilled shaft as a series of springs. For non-linear soil springs, the governing differential
equation that describes the soil-structure interaction may be discretized into a set of algebraic equations based upon finite
difference methods. This system of algebraic equations may be solved to determine the load–displacement behavior of the drilled
shaft when subjected to compression or pullout. By combining the finite difference method with Monte Carlo simulation techniques, a probabilistic load–displacement analysis can be conducted. The probabilistic analysis is advantageous
compared to standard factor of safety design because uncertainties with the shaft–soil interface and tip properties can be
independently quantified. This paper presents a reliability analysis of drilled shaft behavior by combining the finite difference
technique for analyzing non-linear load–displacement behavior with Monte Carlo simulation method. As a result we develop probabilistic relationships for drilled shaft design for both total stress (undrained)
and effective stress (drained) parameters. The results are presented in the form of factor of safety or resistance factors
suitable for serviceability design of drilled shafts. 相似文献
2.
H. Akgün 《Environmental Geology》1997,31(1-2):34-41
Design of plugs for abandonment of boreholes and shafts may be governed by the bond strength between the plug and host rock.
This paper presents the results of push-out tests on cement grout plugs in salt. Two types of expandable cement grouts have
been tested. The average interface shear strengths ranged from 2 to 12 MPa (290 to 1740 psi). Peak shear stresses at failure,
assuming an elastic stress distribution along the interface, were up to eight times higher. Standard deviations commonly reached
20%. Dissolution along the interface was observed, and may have been enhanced by clay inclusions in the salt. This dissolution
appears to have reduced bond strengths. Application of the results to the design of plugs for larger openings (for example,
shafts, drifts, or boreholes) is discussed.
Received: 6 March 1996 · Accepted: 16 July 1996 相似文献
3.
Drilled shafts are, typically, designed by considering the axial ultimate limit state. In this design methodology, the axial
displacement requirements are verified once the design is completed. As an alternative, drilled shafts may be designed by
considering the axial service limit state. Service limit state foundation design is more efficient when done using the load
and resistance factor design (LRFD) approach. Furthermore, reliability may be rationally incorporated into the design process
that utilizes the LRFD method. In this paper, we develop probabilistic approaches for axial service limit state analysis of
drilled shafts. The variability of shaft-soil interface properties is modeled by lognormal probability distribution functions.
The probability distributions are combined with a closed-form analytical relationship of axial load-displacement curves for
drilled shafts. The closed-form analytical relationship is derived based upon the “t–z” approach. This analytical relationship
is used with the Monte Carlo simulation method to obtain probabilistic load-displacement curves, which are analyzed to develop
methods for determining the probability of drilled shaft failure at the service limit state. The developed method may be utilized
to obtain resistance factors that can be applied to LRFD based service limit state design. 相似文献
4.
The use of drilled shafts to stabilize an unstable slope has been a widely accepted practice. There are two basic design and analysis issues involved: one is to determine the global factor of safety of the drilled shafts stabilized slope and the other one is to determine the design earth thrust on the drilled shafts for structural design of the shafts. In this paper, a limiting equilibrium method of slices based solution for calculating global factor of safety (FS) of a slope with the presence of a row of drilled shafts is developed. The arching mechanisms due to the presence of the drilled shafts on slope were taken into account by a load transfer factor. The method for calculating the net force applied to the drilled shaft from the soil mass was also developed. The interrelationships among the drilled shaft location on the slope, the load transfer factor, and the global FS of the slope/shaft system were derived utilizing the developed numerical closed‐form solution. An illustrative example is presented to elucidate the use of the solution in optimizing the location of the drilled shafts on slope to achieve the desired global factor of safety of the slope/shaft system. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
5.
Mark P. S. Krekeler Erin P. Argyilan Jill Lepp Lance E. Kearns 《Environmental Earth Sciences》2009,59(2):411-420
Constructed wetlands can be a low-cost, ecologically based technology to locally treat domestic wastewater prior to discharge.
Wetland systems implemented in the resort village of Akumal in the state of Quintana Roo, in the Yucatan Peninsula of Mexico
have been considered moderately successful over approximately 10 years of operation. Yet the wide-scale implementation of
constructed wetlands has not occurred in the region nor throughout Mexico due largely to the cost of the crushed aggregate
used in these systems. Variation in the geotechnical and mineralogical properties of 29 samples of local calcareous beach
sands collected from the tourist areas of Akumal and Tulum in the Yucatan, Mexico were analyzed and compared to those of crushed
aggregate currently used in constructed wetland systems in Akumal. Powder X-ray diffraction on beach sand samples from Akumal
and Tulum indicates that the proportion of aragonite varies from 24 to 84%, calcite varies from 7 to 38% and Mg-calcite varied
from 8 to 57% compared to crushed aggregate which consists of 0–25% aragonite and 75–100% calcite. Compared to crushed aggregate
beach sands are unimodal and have better uniformity coefficients, making them a better geotechnical choice than crushed sands
with respect to these properties. Grain-size of the beach sands is much smaller compared to that of the crushed aggregate
currently used in the constructed wetland systems in Akumal which should enhance the efficiency of the filtration of organic
matter and bacteria. The average hydraulic conductivity measurements of 0.010 and 0.016 cm/s for beach sands from Akumal and
Tulum, respectively are comparable or less than the average of 0.016 cm/s for crushed aggregate currently used in engineered
wetland systems. Yet a combination of high bulk porosity, intragranular pores, and the presence of Mg-calcite observed in
beach sands may likely result in higher dissolution rates versus the crushed aggregate. Preliminary assessment suggests that
the use of local sand may be a technical, economic, and perceptual enhancement to the systems. Still a careful and thorough
analysis of the potential environmental and aesthetic impacts of mining beach sands is necessary. 相似文献
6.
Rossano Piazza Ana Carolina Ruiz-Fernández Mauro Frignani Roberta Zangrando Luca Giorgio Bellucci Ivo Moret Federico Páez-Osuna 《Environmental Geology》2008,54(7):1537-1545
Concentrations of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were measured in 13 surficial
sediment samples collected at three lacustrine locations in the surroundings of Mexico City and four coastal areas of the
States of Sinaloa, Sonora, Oaxaca and Veracruz. Total PCB concentrations span the interval 3.18–621 ng g−1. The highest values (63.7–621 ng g−1) were found in Mexico City, which is a highly anthropogenically impacted area, whereas low concentrations (3.18–12.9 ng g−1) were characteristic of seven places, some of them subject to intense hydrodynamics. In these latter cases, values increase
by 18–73 times if normalised against the fine fraction (silt plus clay) content in sediment. Two samples from Mexico City
exceed the ERM (Effect Range Median) guidelines and are likely to cause adverse effects. Samples contain only lower chlorinated
PCBs (mainly 2-, 3- and 4-CB), thus suggesting that the most used PCB commercial mixture was Aroclor 1242. The homologue composition
of the sample taken close to the nuclear power plant of Laguna Verde is identical to this commercial mixture. PAHs in the
same samples have relatively low concentrations (14.9–287 ng g−1), well below ERL (Effect Range Low) guidelines. The composition of PAH mixtures accounts for the influence of both petrogenic
and pyrolitic sources, with these latter prevailing at some places in Mexico City. 相似文献
7.
Agustín Gómez-Alvarez Diana Meza-Figueroa Arturo I. Villalba-Atondo Jesús Leobardo Valenzuela-García Jorge Ramírez-Hernández Javier Almendariz-Tapia 《Environmental Geology》2009,57(7):1469-1479
The San Pedro River (SPR) is located in northern Sonora (Mexico) and southeastern Arizona (USA). SPR is a transboundary river
that develops along the Sonora (Mexico) and Arizona (USA) border, and is considered the main source of water for a variety
of users (human settlements, agriculture, livestock, and industry). The SPR originates in the historic Cananea mining area,
which hosts some of the most important copper mineralizations in Mexico. Acid mine drainage derived from mine tailings is
currently reaching a tributary of the SPR near Cananea City, resulting in the contamination of the SPR with heavy metals and
sulfates in water and sediments. This study documents the accumulation and distribution of heavy metals in surface water along
a segment of the SPR from 1993 to 2005. Total concentrations of Cd, Cu, Fe, Mn, Pb, and Zn in surface waters are above maximum
permissible levels in sampling sites near mine tailing deposits. Nevertheless, a significant decrease in the Fe and SO4
2− in surface water (SO4
2−: 7,180–460.39 mg/L; Fe: 1,600–9.51 mg/L) as well as a gradual decrease in the heavy and transition metal content were observed
during the period from 1994 to 2005. Approximately 2.3 km downstream of the mine tailings, the heavy metal content of the
water drops quickly following an increase in pH values due to the discharging of wastewater into the river. The attenuation
of the heavy metal content in surface waters is related to stream sediment precipitation (accompanied by metal coprecipitation
and sorption) and water dilution. Determining the heavy metal concentration led to the conclusion that the Cananea mining
area and the San Pedro River are ecosystems that are impacted by the mining industry and by untreated wastewater discharges
arising from the city of Cananea (Sonora, Mexico). 相似文献
8.
9.
The Upper San Pedro River Basin in Mexico and the United States is an important riparian corridor that is coming under increasing
pressure from growing populations and the associated increase in groundwater withdrawal. Several studies have produced three-dimensional
maps of the basin fill sediments in the US portion of the basin but little work has been done in the Mexican portion of the
basin. Here, the results of a ground-based transient electromagnetic (TEM) survey in the Upper San Pedro Basin, Mexico are
presented. These basin fill sediments are characterized by a 10–40 m deep unsaturated surficial zone which is composed primarily
of sands and gravels. In the central portion of the basin this unsaturated zone is usually underlain by a shallow clay layer
20–50 m thick. Beneath this may be more clay, as is usually the case near the San Pedro River, or interbedded sand, silt,
and clay to a depth of 200–250 m. As you move away from the river, the upper clay layer disappears and the amount of sand
in the sediments increases. At 1–2 km away from the river, sands can occupy up to 50% of the upper 200–250 m of the sediment
fill. Below this, clays are always present except where bedrock highs are observed. This lower clay layer begins at a depth
of about 200 m in the central portion of the basin (250 m or more at distances greater than 1–2 km from the river) and extends
to the bottom of most profiles to depths of 400 m. While the depth of the top of this lower clay layer is probably accurate,
its thickness observed in the models may be overestimated due to the relatively low magnetic moment of the TEM system used
in this study. The inversion routine used for interpretation is based on a one-dimensional geologic model. This is a layer
based model that is isotropic in both the x and y directions. Several survey soundings did not meet this requirement which invalidates the inversion process and the resulting
interpretation at these locations. The results from these locations were rejected. 相似文献
10.
U. Vollenweider 《Rock Mechanics and Rock Engineering》1984,17(4):197-214
Summary The 3150 m long Beckenried viaduct traverses an unstable creeping, slope, the surface of the sound rock lying at depths from 10 to 60 m. Altogether 44 of the total 58 piers of the bridge founded on the sound rock had to be protected by shafts against the creeping soil and loosened rock layers. Based on geological explorations and previous monitoring of the slope displacements the clearance between the rectangular concrete piers and the elliptical shafts was specified to be 1.5 m in the dip direction of the slope and up to ±1.0 m in lateral direction. In order to avoid failure of the shafts due to the unusually large slope movements likely to occur during the service life of the viaduct, an entirely new design concept was applied. It involves flexible shafts consisting of four main parts, namely a rigid shaft collar, articulated ring elements, a rigid trapezoidal cylinder and basal displacement rings resting on the pier footing. Fundamentally, this design permits both: sliding of the shaft as a whole along particular slip surfaces in the loosened rock at depth but also differential displacements due to the creep of the soil strata above it. The sliding at the base contributes to 50% and more to the total surface movements. The shafts also serve as a permanent drainage system for the highly water sensitive slope material. The paper describes the criteria upon which the novel design of the individual shafts were made and also particular constructional details. After a construction period of only four years the bridge opened to traffic in December 1980. Observations made on the completed structure indicate a behaviour of the slope and the shafts which is well in accordance with the predictions. 相似文献
11.
The Basin of Mexico is a closed basin of lacustrine character, with an average elevation of 2200 m above sea level. The watershed covers a vast extension in five states. Mexico City and its metropolitan area are located within this basin. The aquifer system is the main source of water supply for more than 20 million people. Water consumption is about 60 m3/s. The aquifer supplies about 43 m3/s from around 1000 wells at 70–200 m depth. Pumping policies have generated subsidence and degradation of the ground water quality in the Basin of Mexico The lacustrian clay layers play an important role in the local hydrogeology, protecting the aquifer from pollution, but the transition and piedmont areas are highly vulnerable to surface pollutants. 相似文献
12.
The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary
eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive
vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous
in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF)
is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The
crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc
worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites,
andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO,
and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of
incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline
rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle
wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements.
Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for
those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low
CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust.
In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type
alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the
hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from
behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial
variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities
that are related to previous extraction of alkaline to sub-alkaline basaltic melts.
Received: 23 June 1998 / Accepted: 23 December 1998 相似文献
13.
L. W. Daesslé L. G. Mendoza-Espinosa V. F. Camacho-Ibar W. Rozier O. Morton L. Van Dorst K. C. Lugo-Ibarra A. L. Quintanilla-Montoya A. Rodríguez-Pinal 《Environmental Geology》2006,51(1):151-159
The Guadalupe Valley aquifer is the only water source for one of the most important wine industries in Mexico, and also the main public water supply for the nearby city of Ensenada. This groundwater is monitored for major ion, N-NO3, P-PO4, Fe, As, Se, Mo, Cd, Cu, Pb, Zn and Sb concentrations, as well as TDS, pH, dissolved oxygen and temperature. High concentrations of N-NO3 (26 mg l−1), Se (70 μg l−1), Mo (18 μg l−1) and Cu (4.3 μg l−1) suggest that groundwater is being polluted by the use of fertilizers only in the western section of the aquifer, known as El Porvenir graben. Unlike the sites located near the main recharge area to the East of the aquifer, the water in El Porvenir graben has low tritium concentrations (<1.9 TU), indicating a pre-modern age, and thus longer water residence time. No significant variations in water quality (generally <10%) were detected throughout 2001–2002 in the aquifer, suggesting that reduced rainfall and recharge during this dry period did not significantly affect water quality. However, the wells nearest to the main recharge area in the Eastern aquifer show a slight but constant increase in TDS with time, probably as a result of the high (∼200 L S−1) uninterrupted extraction of water at this specific recharge site. Relatively high As concentrations for the aquifer (10.5 μg l−1) are only found near the northern limit of the basin associated with a geological fault. 相似文献
14.
H. Z. Abidin R. J. Davies M. A. Kusuma H. Andreas T. Deguchi 《Environmental Geology》2009,57(4):833-844
Global positioning system (GPS) and satellite-based InSAR (Interferometric Synthetic Aperture Radar) measurements of the subsidence
and uplift of a populated area of Sidoarjo, East Java are due to the eruption of the Lusi mud volcano (2006–present). These
data are the first direct quantitative measurements of deformation due to the growth of a mud volcano edifice. The GPS data
were recorded over periods of a few hours to several months and show that between June 2006 and September 2007, the earth’s
surface has been subsiding at rates of 0.1–4 cm/day. Maximum rates of subsidence occurred in an area 300–400 m to the northwest
of the main mud volcano vent. Horizontal displacements were 0.03–0.9 cm/day and were also towards this area. In general uplifts
of up to 0.09 cm/day were recorded in areas outside of the edifice. Changes in elevation measured using satellite imagery
(InSAR technique) provide regional datasets of subsidence and uplift. They confirm that during the first year a roughly circular
area was undergoing sag-like subsidence centered to the northwest of the main vent and that uplift was occurring 3–4 months
after the initiation of the eruption due to the movement Watukosek fault system. Subsidence occurred due to the weight of
mud and man-made dams and the collapse of the overburden due to removal of mud from the subsurface. Assuming constant rates
of subsidence of 4 cm/day, then in the centre of the edifice there would be up to 44 m of subsidence in 3 years, and up to
16 m in 10 years. The mud volcano is now in a self-organizing state with new fluid conduits forming as a result of the collapse.
An erratum to this article can be found at 相似文献
15.
The natural (electrical) potential (NP) method – also known as self-potential, spontaneous potential and streaming potential
(SP) – has been used to locate areas of groundwater flow in karst terrane. NP is the naturally occurring voltage at the ground
surface resulting from ambient electrical currents within the earth. The measurement of NP can be used to characterize groundwater
flow in karst terrane because electrical potential gradients are generated by the horizontal flow of water along fractures
or conduits and the vertical infiltration of water into fractures or shafts. NP data from a site on the Mitchell Plain of
southern Indiana, USA, revealed that NP data can be decomposed into three components: topographic effect, residual NP and
noise. At this site, NP was inversely proportional to elevation, but the correlation varied with time. The topographic correction
factor varied from –2.5 to –1.2 mV/m (NP change per unit elevation increase), with an average linear correlation coefficient
(R) of 0.95. Because the site slopes toward an adjacent creek that is the local groundwater discharge zone, one possible explanation
for this effect is a streaming-potential mechanism generated by groundwater movement toward the creek. The residual NP data
revealed three negative anomalies at the survey area. Two of them coincide with sinkholes. A part of the third anomaly is
coincident with a small valley, and concentrated infiltration does occur at this elevation in other valleys at the site, as
evidenced by the existence of sinkholes. However, the dispersed, low-magnitude nature of the third anomaly does not prove
the existence of concentrated groundwater recharge activity.
Received: 18 March 1998 · Accepted: 27 April 1998 相似文献
16.
滑坡-涌浪灾害威胁沿河两岸居民生产生活安全和航道安全。当前尚缺乏同步提供流固两相运动矢量的相关物理试验分析系统,以深刻分析滑坡-涌浪产生机制。文章提出了基于流固两相识别的粒子图像测速(PIV)技术和试验实现方法。利用2560×1024像素的工业相机,该PIV技术可实现在3 m×1.5 m视窗下最小1.17 mm的空间分辨率和0.01 s内最小0.117 m/s的观测速度。同时,提出了与该系统方法有关的误差来源和克服相关问题的解决方法。利用相关硬件设施示范性构建了滑坡-涌浪两相运动观测平台,并编制了专门的解算软件。对三维柱体颗粒崩塌、二维柱体颗粒崩塌及其涌浪和水下崩塌-涌浪进行了展示性试验,取得了良好效果。该系统可以揭示广泛的岩土体及水体运动全过程,具有很好的应用前景;将为滑坡-涌浪及相关动力学领域研究提供强有力的研究工具。 相似文献
17.
The increase of pile resistance with time is referred to as ‘set-up’. This behaviour of driven piles has been widely discussed in many studies by researchers. Meanwhile, there has been little, if any, information regarding this aspect for drilled shafts. Performing a bearing capacity test for a shaft over time, however, requires higher costs and more complicated rigs compared to a driven pile. A database including results from five Osterberg cell-tested drilled shafts conducted at two different stages is considered, from which the set-up effect is statistically analysed. The reliability-based analysis technique using Monte Carlo simulation (MCS) is used to develop separate resistance factors to account for different degrees of uncertainties associated with the predicted reference resistance and the predicted set-up resistance in the framework of the load and resistance factor design (LRFD) method. By incorporating set-up into design, shaft length or number of shafts can be reduced and economical design of drilled shafts can be achieved. 相似文献
18.
Two‐dimensional slope stability analysis for a slope with a row of drilled shafts needs a mechanism to take into account the three‐dimensional effect of the soil arching due to the spaced drilled shafts on slope. To gain a better understanding of the arching mechanisms in a slope with evenly spaced drilled shafts socketed into a stable stratum (or a rock layer), the three‐dimensional finite element modelling technique was used for a comprehensive parametric study, where the nonlinear and plastic nature of the soil and the elastic behavior of the drilled shafts as well as the interface frictions were modelled. Various factors were varied in the parametric study to include (1) the rigidity of the drilled shafts as influenced by its diameter, modulus of elasticity, and total length; (2) shafts spacing and location on the slope; (3) the material properties of rock and the socket length of shaft; and (4) the soil movement and strength parameters. Evidences of soil arching and reduction in the stresses and displacements through the load transfer mechanisms due to the presence of the drilled shafts were elucidated through the finite element method (FEM) simulation results. Design charts based on regression analysis of FEM simulation results were created to obtain a numerical value of the load transfer factor for the arching mechanism provided by the drilled shafts on the slope. Observations of the arching behavior learned from the FEM simulations provide an insight into the behavior of drilled shafts stabilized slope. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
19.
The epithermal Shila-Paula Au–Ag district is characterized by numerous veins hosted in Tertiary volcanic rocks of the Western Cordillera (southern Peru). Field studies of the ore bodies reveal a systematic association of a main E–W vein with secondary N55–60°W veins—two directions that are also reflected by the orientation of fluid-inclusion planes in quartz crystals of the host rock. In areas where this pattern is not recognized, such as the Apacheta sector, vein emplacement seems to have been guided by regional N40°E and N40°W fractures. Two main vein-filling stages are identified. stage 1 is a quartz–adularia–pyrite–galena–sphalerite–chalcopyrite–electrum–Mn silicate–carbonate assemblage that fills the main E–W veins. stage 2, which contains most of the precious-metal mineralization, is divided into pre-bonanza and bonanza substages. The pre-bonanza substage consists of a quartz–adularia–carbonate assemblage that is observed within the secondary N45–60°W veins, in veinlets that cut the stage 1 assemblage, and in final open-space fillings. The two latter structures are finally filled by the bonanza substage characterized by a Fe-poor sphalerite–chalcopyrite–pyrite–galena–tennantite–tetrahedrite–polybasite–pearceite–electrum assemblage. The ore in the main veins is systematically brecciated, whereas the ore in the secondary veins and geodes is characteristic of open-space crystallization. Microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15.5 wt% NaCl equivalent and homogenization temperatures bracketed between 200 and 330°C. Secondary CO2-, N2- and H2S-bearing fluid inclusions are also identified. The age of vein emplacement, based on 40Ar/39Ar ages obtained on adularia of different veins, is estimated at around 11 Ma, with some overlap between adularia of stage 1 (11.4±0.4 Ma) and of stage 2 (10.8±0.3 Ma). A three-phase tectonic model has been constructed to explain the vein formation. Phase 1 corresponds to the assumed development of E–W sinistral shear zones and associated N60°W cleavages under the effects of a NE–SW shortening direction that is recognized at Andean scale. These structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation. Phase 2 is a reactivation of earlier structures under a NW–SE shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce N50°E-striking S2-cleavage planes filled by the stage 2 pre-bonanza minerals. Phase 3 coincides with the bonanza ore emplacement in the secondary N45–60°W veins and also in open-space in the core of the main E–W veins. Our combined tectonic, textural, mineralogical, fluid-inclusion, and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation. 相似文献
20.
M. Mazari-Hiriart C. Hernández-Eugenio F. Rojo-Callejas R. Lozano-Santacruz 《Environmental Geology》2000,39(6):595-602
A surficial clay aquitard extends through the urban area where Mexico City is located. It has been assumed to function as
a protective layer to the underlying aquifer that provides 42 m3/s out of 63 m3/s of water used by 18 million inhabitants. To provide such protection, the aquitard must be impermeable to water flow and,
ideally, have a significant capacity to sorb contaminants. The latter aspect was addressed, studying the vertical variability
of sorption of perchloroethylene (PCE), a widely used organic compound considered to pose health risks in groundwater. Batch
sorption tests were used and the clay-rich strata in the depth interval from 8 to 75 m were studied. The results suggest that
sorption depends mainly on the fraction of organic carbon (foc) present in the clayey materials. The sorption data were fit to the linear and Freundlich models; many strata could be fit
well by either model, while some strata were distinctly non-linear. The linear isotherms showed a mean value of 32.8 ml/g,
and the Freundlich isotherm 96.6 ml/g, confirming that the clay-rich media have significant sorption capacity for PCE. From
the environmental perspective the clay-rich materials are serving as protection to the groundwater system.
Received: 2 November 1998 · Accepted: 15 February 1999 相似文献