Review of a new shear-strength criterion for rock joints

Barton, N., 1973. Review of a new shear-strength criterion for rock joints. Eng. Geol., 7: 287–332.

The surface roughness of rock joints depends on their mode of origin, and on the mineralogy of the rock. Amongst the roughest joints will be those that formed in intrusive rocks in a tensile brittle manner, and amongst the smoothest the planar cleavage surface in slates. The range of friction angles exhibited by this spectrum will vary from about 75° or 80° down to 20° or 25°, the maximum values being very dependent on the normal stress, due to the strongly curved nature of the peak strength envelopes for rough unfilled joints.

Direct shear tests performed on model tension fractures have provided a very realistic picture of the behaviour of unfilled joints at the roughest end of the joint spectrum. The peak shear strength of rough—undulating joints such as tension surfaces can now be predicted with acceptable accuracy from a knowledge of only one parameter, namely the effective joint wall compressive strength or JCS value. For an unweathered joint this will be simply the unconfined compression strength of the unweathered rock. However in most cases joint walls will be weathered to some degree. Methods of estimating the strength of the weathered rock are discussed. The predicted values of shear strength compare favourably with experimental results reported in the literature, both for weathered and unweathered rough joints.

The shear strength of unfilled joints of intermediate roughness presents a problem since at present there is insufficient detailed reporting of test results. In an effort to remedy this situation, a simple roughness classification method has been devised which has a sliding scale of roughness. The curvature of the proposed strength envelopes reduces as the roughness coefficient reduces, and also varies with the strength of the weathered joint wall or unweathered rock, whichever is relevant. Values of the Coulomb parameters c and Φ fitted to the curves between the commonly used normal stress range of 5–20 kg/cm² appear to agree quite closely with experimental results.

The presence of water is found in practice to reduce the shear strength of rough unfilled joints but hardly to affect the strength of planar surfaces. This surprising experimental result is also predicted by the proposed criterion for peak strength. The shear strength depends on the compressive strength which is itself reduced by the presence of water. The sliding scale of roughness incorporates a reduced contribution from the compressive strength as the joint roughness reduces. Based on the same model, it is possible to draw an interesting analogy between the effects of weathering, saturation, time to failure, and scale, on the shear strength of non-planar joints. Increasing these parameters causes a reduction in the compressive strength of the rock, and hence a reduction in the peak shear strength. Rough—undulating joints are most affected and smooth—nearly planar joints least of all.     

Potentiometric Determination of Silver Thiolate Formation Constants Using a Ag2S Electrode

Formation constants for silver thiolates were obtained by titration of the ligand in a constant temperature, ionic strength and pH medium and measuring the potential change at a Ag2S electrode. A non-linear equation was derived from which the first and second silver formation constants, 1 and 2, and the sulfide group acid dissociation constant, Ka, were determined. An overall estimate of the uncertainty in the derived parameters was obtained using a Monte Carlo approach. The procedure was compared to a previous work on AgHS°. Log 1, log 2 and - log Ka results were obtained for cysteine (11.9 ± 0.5, 15.2 ± 0.4, 7.8 ± 0.1), glutathione (12.3 ± 0.3, 14.3 ± 0.8, 8.8 ± 0.3) and 3-mercaptopropanoic acid (12.0± 0.4, 14.0 ± 0.4, 10.5 ± 0.3) at 20 °C and 0.01 m ionic strength.     

A multi-proxy approach to assessing isolation basin stratigraphy from the Lofoten Islands, Norway

This study takes a comprehensive approach to characterizing the isolation sequence of Heimerdalsvatnet, a coastal lake in the Lofoten Islands, northern Norway. We use established methods and explore new techniques to assess changes in marine influence. Bathymetric and sub-bottom profiles were acquired to examine basin-wide sedimentation and a 5.8 m sediment core spanning the last 7800 cal yr BP was analyzed. We measured magnetic susceptibility, bulk organic matter properties, molecular biomarkers, diatom assemblages, and elemental profiles acquired by scanning X-ray fluorescence. These characteristics of the sediment reflect detailed changes in salinity and water column conditions as the lake was progressively isolated. Three distinct litho/chemo-stratigraphic units represent a restricted marine phase (7800-6500 cal yr BP), a transitional phase characterized by intermittent marine influence (6500-4900 cal yr BP), and complete isolation and freshwater sedimentation (4900 cal yr BP to present). Although there are uncertainties in the estimate of the threshold elevation of the lake, the timing of these phases generally corresponds with previous interpretations of the local relative sea-level history. This record captures sea-level regression following the Tapes transgression and supports the interpretation of a subsequent sea-level stillstand, dated in Heimerdalsvatnet from 6500 to 4900 cal yr BP.     

Alkalinity changes in the Sargasso Sea: geochemical evidence of calcification?

Strong seasonal patterns in upper ocean total carbon dioxide (TCO₂), alkalinity (TA) and calculated pCO₂ were observed in a time series of water column measurements collected at the US Joint Global Ocean Flux Study (JGOFS) BATS site (31 °50′N, 64 °10′W) in the Sargasso Sea. TA distribution was a conservative function of salinity. However, in February 1992, a non-conservative decrease in TA was observed, with maximum depletion of 25–30 μmoles kg⁻¹ occuring in the surface layer and at the depth of the chlorophyll maximum (˜ 80–100 m). Mixed-layer TCO₂ also decreased, while surface pCO₂ increased by 25–30 μatm. We suggest these changes in carbon dioxide species resulted from open-ocean calcification by carbonate-secreting organisms rather than physical processes. Coccolithophore calcification is the most likely cause of this event although calcification by foraminifera or pteropods cannot be ruled out. Due to the transient increase in surface pCO₂, the net annual transfer of CO₂ into the ocean at BATS was reduced. These observations demonstrate the potential importance of open-ocean calcification and biological community structure in the biogeochemical cycling of carbon.     

Calculation of beach change under interacting cross-shore and longshore processes

This paper presents a mathematical approach and numerical model that simulates beach and dune change in response to cross-shore processes of dune growth by wind and dune erosion by storms, and by gradients in longshore sand transport that will alter shoreline position. Sub-aerial transport processes are represented, whereas sub-aqueous transport is neglected. The system is tightly coupled morphologically, with the berm playing a central role. For example, the potential for sand to be transported to the dune by wind depends on berm width, and sand lost in erosion of the dune during storms can widen the berm. Morphologic equilibrium considerations are introduced to improve reliability of predictions and stability of the non-linear model. An analytical solution is given under simplification to illustrate properties of the model. Sensitivity tests with the numerical solution of the coupled equations demonstrate model performance, with one test exploring beach and dune response to potential increase in storm-wave height with global warming. Finally, the numerical model is applied to examine the consequences of groin shortening at Westhampton Beach, Long Island, New York, as an alternative for providing a sand supply to the down-drift beach. Results indicate that the sand will be released over several decades as the shoreline and dune move landward in adjustment to the new equilibrium condition with the shortened groins.     

The role of decarbonization and structure in the Callie gold deposit, Tanami Region of northern Australia

The Callie deposit is the largest (6.0 Moz Au) of several gold deposits in the Dead Bullock Soak goldfield of the Northern Territory’s Tanami Region, 550 km northwest of Alice Springs. The Callie ore lies within corridors, up to 180 m wide, of sheeted en echelon quartz veins where they intersect the 500-m-wide hinge of an ESE-plunging F₁ anticlinorium. The host rocks are the Blake beds, of the Paleoproterozoic Dead Bullock Formation, which consist of a > 350-m-thick sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones and fine sandstones. The rocks are Fe-rich and dominated by assemblages of chlorite and biotite, both of which are of hydrothermal and metamorphic origin. A fundamental characteristic of the hydrothermal alteration is the removal of graphite, a process which is associated with bleaching and the development of bedding-parallel bands of coarse biotite augen. Gold is found only in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals. Auriferous quartz veins differ from barren quartz veins by the presence of ilmenite, apatite, xenotime, and gold and the absence of sulfide minerals. The assemblage of gold–ilmenite–apatite–xenotime indicates a linked genesis and mobility of Ti, P, and Y in the mineralizing fluids. Geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks with high FeO/(FeO+Fe₂O₃) and low C/(C+CO₂) ratios (> 0.8 and < 0.2, respectively). This association can be explained by reactions that convert C from reduced graphitic host rocks into CO₂ and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite. These reactions would increase the CO₂ content of the fluid, facilitating the transport of Ti, P, and Y from the host rocks into the veins. Both CO₂ and CH₄ produced by reaction of H₂O with graphite, effervesced under the lower confining pressures in the veins. This would have partitioned H₂S into the vapor phase, destabilizing Au–bisulfide complexes; the loss of CO₂ and H₂S from the aqueous phase caused precipitation of gold, ilmenite, apatite, and xenotime. It is proposed that this process was the main control on gold precipitation. Oxidization of iron in the very reduced wall rocks, resulting in reduction of the fluid, provided a second mechanism of gold precipitation in previously decarbonized rocks, contributing to the high grades in some samples. Although sulfide minerals, especially arsenopyrite, did form during the hydrothermal event, host rock sulfidation reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals. Sulfide minerals likely formed by different mechanisms from those associated with gold deposition. Both the fold architecture and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into the hinges of the anticlines. Within the anticlines, a reactive cap of fine-grained, graphitic, reduced Fe-rich turbidites above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite, and the associated effervescence of CO₂ from the fluid caused the precipitation of gold. Exploration for similar deposits should focus on the intersection of east–west shear zones with folds and Fe-rich graphitic host rocks.     

Effects of temperature and chemical formulation on the acute toxicity of pentachlorophenol to Simocephalus vetulus (Schoedler, 1858) (Crustacea: Cladocera)

Abstract

The influence of temperature on the acute toxicity of a technical formulation (86%) and pure formulation (99%) of pentachlorophenol (PCP) to less than 24‐h‐old Simocephalus vetulus neonates was determined with 48‐h static toxicity tests. The technical grade PCP was significantly more toxic to S. vetulus than the pure PCP (P < 0.05). Sensitivity of S. vetulus to technical PCP also significantly increased with temperature (P < 0.05), but a significant temperature effect was not found with the pure PCP. The mean 48‐h LC₅₀ values for neonates exposed to technical PCP were 140 and 199 μg l^?1 at 22°C and 16°C, respectively, and for those exposed to pure PCP were 262 and 304 μg l^?1, respectively.     

Ocean acidification and photic‐zone anoxia at the Toarcian Oceanic Anoxic Event: Insights from the Adriatic Carbonate Platform

Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms has important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event (ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of ?2.2‰ reflects the massive injection of CO₂ into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO₂ overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO₂ in the ocean‐atmosphere system from pre‐event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.     

Sodium remote from Io

We find that faint sodium emission originating in the middle Jupiter magnetosphere has two distinct kinematical components. The “normal” signature of atoms on bound orbits with large apojoves seems always to be present, and we suggest these atoms are an extension of the bright, near-Io sodium cloud. The “fast” signature, with speeds up to at least 100 km sec^?1, is seen only occasionally, and we suggest it is due to an interaction of the near-Io sodium cloud with the corotating, heavy-ion plasma. Both elastic and charge-exchange collisions seem consistent with the observed kinematical and temporal signatures. Elastic collisions seem marginally more capable of producing the high observed sodium atom speeds. We predict observable occurences of the fast component in the hours following passage of the Io sodium cloud through the plasma centrifugal symmetry surface if Io is at a favorable orbital longitude. Between 10 and 20 R_J we find an atomic sodium density ～10^?2 cm^?3. If the photoionization lifetime applies, an Io source of at least 10²⁶ sodium atoms sec^? is required to maintain this remote sodium population.     

Human development is linked to multiple water body impairments along the California coast

To elucidate relationships between land cover and water quality along the central California coast, we collected monthly samples from 14 coastal waterway outlets representing various degrees of human development. Sites were distributed between three salinity categories, freshwater, estuarine, and marine, to better understand land cover-water quality relationships across a range of coastal aquatic ecosystems. Samples were analyzed for fecal indicator bacteria (FIB), dissolved nutrients, stable nitrogen isotopes in particulate organic matter, and chlorophylla (chla). Sediment samples from 11 sites were analyzed for the concentration of the anthropogenic organic contaminant perfluorooctane sulfonate and its precursors (ΣPFOS). While the data indicated impairment by nutrient, microbial, and organic contaminants at both agricultural and urban sites, the percentage of agricultural land cover was the most robust indicator of impairment, showing significant correlations (p<0.05) to FIB, nutrient, chla, and ΣPFOS levels. FIB densities were strongly influenced by salinity and were highest at sites dominated by agriculture and urbanization. Nutrient levels and chla correlated to both agricultural and urban land use metrics as well. Positive correlations among FIB, nutrients, chla, and ΣPFOS suggest a synergy between microbial, nutrient, and organic pollution. The results emphasize the importance of land management in protecting coastal water bodies and human health, and identify nutrient, microbial, and organic pollution as prevalent problems in coastal California water bodies.