Risk analysis of fractured rock mass underground structures

Recent accidents in underground structures have raised the risk awareness of the geotechnical engineering community. Geotechnical design is subject to significant uncertainties in load and strength parameters as well as in engineering models. However, engineering models which objectively address such uncertainties in design are still scarce. This paper presents an objective framework for the quantification of the risks involved in underground structures excavated in fractured rock masses, where structural failures may occur due to block falls. The framework considers the structure as a distributed system, where falling block probabilities are integrated over the main structural dimension. Random block size and geometry, arising from random joint orientation, are taken into account, as well as uncertainties in joint strength and geometrical parameters. A cost function is used to quantify failure consequences in terms of the block size. The framework is demonstrated in an application to a case study involving a real structure: the Paulo Afonso IV power station cavern. Results of the case study show that the studied cavern presents high reliability and very low risk. The framework proposed herein is shown to be a practical tool for the risk evaluation of underground structures constructed in rock masses, such as caverns and tunnels.     

Holocene evolution of the Triftje- and the Oberseegletscher (Swiss Alps) constrained with <Superscript>10</Superscript>Be exposure and radiocarbon dating

To develop a more precise understanding of Alpine glacier fluctuations during the Holocene, the glacier forefields of the Triftjegletscher and the Oberseegletscher east of Zermatt in the Valais Alps, Switzerland, were investigated. A multidisciplinary approach of detailed geological and geomorphological field mapping combined with ¹⁰Be exposure and radiocarbon dating was applied. A total of twelve samples of boulders and bedrock were taken from both Little Ice Age (LIA) landforms, as documented by the Dufour map published in 1862, and from landforms outside of the LIA. The resulting ¹⁰Be ages range between 12590 ± 350 a and 420 ± 170 a. A piece of wood found embedded in the Little Ice Age moraine gave radiocarbon ages that range between 293 cal years BP up to modern (356–63 cal years before 2013). Based on these results, four tentative steps of the Holocene evolution could be distinguished. An early Holocene stage, which documents the decay of the Egesen stadial glaciers when the first parts of the study area became ice free. This was followed by a phase with no evidence of glacier advance. Then in the late Holocene, the glaciers advanced (at least) twice. An advance around 1200 a, as shown by several moraine ages, coincides with the Göschenen II cold phase. A more extensive readvance occurred during the LIA as shown on the historical maps and underpinned by one ¹⁰Be exposure age and the radiocarbon age. This later advance destroyed or overprinted the earlier landforms in most parts of the area.     

Effects of a Beach Nourishment Project in Jamaica Bay,New York,on Horseshoe Crab (<Emphasis Type="Italic">Limulus polyphemus</Emphasis>) Spawning Activity and Egg Deposition

Restoration of horseshoe crab spawning habitats through beach nourishment may be considered as a potential strategy to enhance reproductive success in areas where estuarine beaches have been lost to coastal erosion and development. The US Army Corps of Engineers performed a beach nourishment project at Plumb Beach (Jamaica Bay, Brooklyn, NY) in 2012 to stabilize the shoreline. While the addition of sand was done to protect infrastructure, it created an opportunity to examine the responses of American horseshoe crabs (Limulus polyphemus) to beach nourishment using a BACI (before-after control impact) design. During Spring 2012, before beach nourishment, horseshoe crabs made minimal use of the highly degraded western section of Plumb Beach in comparison to a nearby reference site, as quantified by numbers of spawning adults at high tide and densities of horseshoe crab eggs in core samples. In the first post-nourishment field season (Spring 2013), there was no detectable increase in horseshoe crab spawning activity on the newly restored beach. In 2014 and 2015, the density of spawning females began to increase at the nourished beach, although their numbers and especially the density of horseshoe crab eggs remained much lower than at the reference site. Three years after beach nourishment, differences in sediments texture (mean grain diameter, percent gravel, sorting, skewness, and hardness) were still evident between the nourishment and reference sites. Our results suggest that (1) at this site, beach nourishment appeared to bring about only slow increases in horseshoe crab spawning density after several seasons and (2) subtle differences in beach geomorphology over relatively short distances can be detected by horseshoe crabs and may underlie their selection of specific nesting sites.     

Migmatite-like textures in anthracite: Further evidence for low-grade metamorphic melting and resolidification in high-rank coals

Previous studies demonstrated that melting, initiated by supercritical fluids in the 375–400 °C range, occurred as part of anthracite metamorphism in the Appalachian Basin. Based on the known behavior of vitrinite at high temperatures and, to a lesser extent, at high pressures, it was determined that the duration of the heating, melting, and resolidification event was about 1 h. In the current study, featureless vitrinite within banded maceral assemblages demonstratesthe intimate association of melted and resolidified vitrinite with anthracite-rank macerals. By analogy with metamorphosed inorganic rocks, such associations represent diadysites and embrechites, i.e., cross-cutting and layered migmatites, respectively. Even though the temperature of formation of the anthracite structures is several hundred °C lower than that seen in metamorphosed inorganic rocks, anthracites are metamorphic rocks and the nomenclature for metamorphic rocks may be appropriate for coal.     

Internal waves in a rotating stratified fluid in an arbitrary gravitational field

Abstract

Small amplitude oscillations of a uniformly rotating, density stratified, Boussinesq, non-dissipative fluid are examined. A mathematical model is constructed to describe timedependent motions which are small deviations from an initial state that is motionless with respect to the rotating frame of reference. The basic stable density distribution is allowed to be an arbitrary prescribed function of the gravitational potential. The problem is considered for a wide class of gravitational fields. General properties of the eigenvalues and eigenfunctions of square integrable oscillations are demonstrated, and a bound is obtained for the magnitude of the frequencies. The modal solutions are classified as to type. The eigenfunctions for the pressure field are shown to satisfy a second-order partial differential equation of mixed type, and the equation is obtained for the critical surfaces which delineate the elliptic and hyperbolic regions. The nature of the problem is examined in detail for certain specific gravitational fields, e.g., a radially symmetric field. Where appropriate, results are compared with those of other investigations of waves in a rotating fluid of spherical configuration and the novel aspects of the present treatment are emphasized. Explicit modal solutions are obtained in the specific example of a fluid contained in a rigid cylinder, stratified in the presence of vertical gravity, with the buoyancy frequency N being an arbitrary prescribed function of depth.     

Lateglacial and early Holocene surface exposure ages of glacial boulders in the Taiwanese high mountain range

Glacial landforms and sediments mapped in three presently unglaciated mountain massifs, the Nanhuta Shan, the Hsueh Shan and the Yushan, support the concept of repeated, multi-stage glaciations in the Taiwanese high mountain range during the late Pleistocene. New results from surface exposure dating using in situ produced cosmogenic ¹⁰Be measured in samples taken from erratic and moraine boulders in Nanhuta Shan at altitudes between 3100 and 3500 m are presented here. The results confirm independent and previously reported Optically Stimulated Luminescence (OSL) ages and ¹⁰Be exposure ages from glacial deposits in the same area and suggest a Lateglacial and early Holocene glaciation, the so called Nanhuta glacier advance with two substages at about 12–15 ka and 9.5 ka BP. The respective equilibrium line altitudes (ELA) were calculated at 3340 m and 3440 m with corresponding ELA depressions of 610 ± 100 m and 510 ± 100 m relative to the present day (theoretical) ELA, which is estimated to be at about 3950 ± 100 m in Taiwan. Large-scale erosional landforms indicate a much wider glacier extent during an earlier stage, which is not dated in Nanhuta Shan so far. Luminescence dating from near Hsueh Shan suggests an age of marine isotope stage (MIS) 4 for this stage.     

Volcanic ash hazard climatology for an eruption of Hekla Volcano,Iceland

Ash produced by a volcanic eruption on Iceland can be hazardous for both the transatlantic flight paths and European airports and airspace. In order to begin to quantify the risk to aircraft, this study explored the probability of ash from a short explosive eruption of Hekla Volcano (63.98°N, 19.7°W) reaching European airspace. Transport, dispersion and deposition of the ash cloud from a three hour ‘explosive’ eruption with an initial plume height of 12 km was simulated using the Met Office's Numerical Atmospheric-dispersion Modelling Environment, NAME, the model used operationally by the London Volcanic Ash Advisory Centre. Eruptions were simulated over a six year period, from 2003 until 2008, and ash clouds were tracked for four days following each eruption.Results showed that a rapid spread of volcanic ash is possible, with all countries in Europe facing the possibility of an airborne ash concentration exceeding International Civil Aviation Organization (ICAO) limits within 24 h of an eruption. An additional high impact, low probability event which could occur is the southward spread of the ash cloud which would block transatlantic flights approaching and leaving Europe. Probabilities of significant concentrations of ash are highest to the east of Iceland, with probabilities exceeding 20% in most countries north of 50°N. Deposition probabilities were highest at Scottish and Scandinavian airports. There is some seasonal variability in the probabilities; ash is more likely to reach southern Europe in winter when the mean winds across the continent are northerly. Ash concentrations usually remain higher for longer during summer when the mean wind speeds are lower.     

Mookaite石——西澳大利亚的装饰石

Mookaite石是一种吸引人的多色装饰石,产于西澳大利亚Kennedy山脉西缘Gascoyne河以北的少数地方。从岩石学上,Mookaite石由Windalia放射虫岩层的硅化放射虫岩构成,该岩层为Carnarvon大盆地沉积序列的一部分。硅化作用形成瓷状岩后,Fe染色导致其具有引人瞩目的黄色、红色、紫色和棕色图形。瓷状岩细粒,坚硬,无定向性,抛光性极好,在世界范围内广受欢迎。     

Phosphorus and iron cycling in deep saprolite,Luquillo Mountains,Puerto Rico

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ⁵⁶Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ⁵⁶Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10^? 14 mol m^? 2 s^? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.     

Stream geochemistry, chemical weathering and CO2 consumption potential of andesitic terrains, Dominica, Lesser Antilles

Recent studies of chemical weathering of andesitic-dacitic material on high-standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO₂ consumption yet reported. However, the paucity of stream gauge data in many of these terrains has limited determination of chemical weathering product fluxes. In July 2006 and March 2008, stream water samples were collected and manual stream gauging was performed in watersheds throughout the volcanic island of Dominica in the Lesser Antilles. Distinct wet and dry season solute concentrations reveal the importance of seasonal variations on the weathering signal. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO₃:Na and Mg:Na ratios suggest crystallinity of the parent material may also play an important role in determining weathering fluxes. From total dissolved solids concentrations and mean annual discharge calculations we calculate chemical weathering yields of (6-106 t km⁻² a⁻¹), which are similar to those previously determined for basalt terrains. Silicate fluxes (3.1-55.4 t km⁻² a⁻¹) and associated CO₂ consumption (190-1575 × 10³ mol km⁻² a⁻¹) determined from our study are among the highest determined to date. The calculated chemical fluxes from our study confirm the weathering potential of andesitic-dacitic terrains and that additional studies of these terrains are warranted.