Design criteria for plain concrete lining in water and power tunnels

Plain concrete lining is used in the construction of many power tunnels in rock masses having good and fair qualities under internal pressure (p). This type of concrete is susceptible to cracking and jointing as a result of the construction process. The average crack opening (2u) is estimated to be approximately equal to (1 + ν) t(f_t + p)/E, where E and ν are the modulus of deformation and Poisson's ratio of rock mass, respectively; and t and f_t are, respectively, the thickness and ultimate tensile strength of the concrete lining. In order to ensure that the crack will close over time, this opening (2u) should not be allowed to exceed a permissible limit. Case histories are included to obtain the permissible limits for crack opening and spacing.     

Heterogeneous photocatalytic removal of toluene from air on building materials enriched with TiO2

This paper reports the potential of heterogeneous photocatalysis as an advanced oxidation technology for removal of toluene from air using TiO₂ as a photocatalyst in building materials. First, the photocatalytic activity of two types of TiO₂ containing building materials, i.e. roofing tiles and corrugated sheets, has been investigated at ambient conditions (T=25.0 °C; relative humidity RH=47%; toluene inlet concentration [TOL]_in=17–35 ppbv). Toluene removal efficiencies up to 63% were observed at a gas residence time (τ) of 17 s. Second, the effect of RH (1–77%), [TOL]_in (23–465 ppmv) and τ (17–115 s) on toluene removal has been systematically investigated using TiO₂ containing roofing tiles as photocatalytic building materials. Results revealed lower toluene removal efficiencies at higher RH and [TOL]_in, whereas a positive effect was observed with increased τ. Under optimal conditions, toluene removal efficiencies up to 78±2% and elimination rates higher than 100 mg h⁻¹ m⁻² roofing tile were obtained. A decline in photocatalytic activity by a factor of 2 was observed after operation at gas residence times shorter than 69 s and [TOL]_in higher than 76 ppmv. Washing the building materials with deionized water, simulating rainfall, could partially (i.e. by a factor 1.3) regenerate the catalyst activity.     

Estimation of colloidal deposition from heterogeneous populations

This paper reports a study of size-heterogeneous colloid filtration in a new bed using different types of colloids under different conditions of flow and solution chemistry. Depth-wise variation of the particle-size-distributions f_i(x), and the total liquid-phase colloid concentration, C(x) are measured which are used to estimate the depth-wise variation of the liquid-phase concentration for each distinct section of the heterogeneous population, C_i(x). It is observed that log C_i(x) is linear with depth, for some systems, while it shows deviation from linearity, with the slope decreasing with depth, for others. Deposition-rates for these distinct sections of the heterogeneous population, k_i, are estimated from the slopes of the log C_i(x) data. These deposition-rates were then compared with predicted homogeneous-population deposition-rates from Colloid Filtration Theory (CFT), which shows agreement between the CFT-based-deposition-rates and heterogeneous-population-data based deposition-rates, for low flow velocities. At higher flow velocities a gap between the CFT-based and Data-based deposition-rates is observed. Deposition-rates from CFT are then used in a heterogeneous-colloid-filtration model, to examine if heterogeneous colloid deposition can be expressed as the sum of its parts. It is observed that, the sum-of-parts model provides a reasonable estimate of colloidal deposition from heterogeneous populations. Based on these results, it is possible to make predictions of colloidal deposition from complex heterogeneous suspensions. A new method for studying heterogeneous colloid filtration is also proposed.     

Ageing of aluminium hydroxide flocs

The ageing processes of kaolinite suspensions flocculated with Al₂(SO₄)₃ under a variety of different but well-controlled flocculation conditions were studied by measuring their average diameter, diameter distribution, strength and electrophoretic mobility.Three periods could be distinguished in the ageing process. Initially shrinkage of the flocs is mainly due to cementation-aggregation. In the first 6–8 h, dependent on the flocculation conditions, the average diameter reduces from 500 to 220 μm. During the following 4 days a shrinkage to 180 μm occurs, mainly due to condensation-polymerization and crystallization. In the third period, the average diameter increases because of Oswald ripening of the crystals.During each period the change of the diameter as a function of age can mathematically be described by: d_f,i = d∞ + d′· t_i^+i−ω. With d_∞, d′ and ω constants dependent on the flocculation conditions.During ageing, the floc diameter distribution narrows. The shrinkage of the flocs during ageing is definitely not due to erosion of particles from the floc surface.The change of the electrophoretic mobility during ageing provides information on the thickness and homogeneity of the layer of insoluble hydroxides around the destabilized clay particles.The influence of a wide range of different flocculation process parameters on the floc ageing process is summarized, as is the influence of the ageing process on floc strength and floc density. The evolution of the effective floc density can be described by: .The constants K_p and a are functions of the ratio quantity of suspended solids vs coagulant dose.     

Measuring the supply response to housing allowances

Summary The annual cost of housing inputs,V _f=P _fQ_h, equals the sum of six components:Annual cost of service inputs,P ₈Q₈.Real return to land,r P ₁Q₁.Real return to improvements,rP _k(t)Q_k.Current maintenance cost,P _mM(A+t)Q_k.Current insurance cost,nP _k(t)Q_k.Depreciation, .This paper draws on research conducted by The Rand Corporation as part of the Housing Assistance Supply Experiment. That research was sponsored and funded by the Office of Policy Development and Research, U.S. Department of Housing and Urban Development, under Contract No. H-1789. Views and conclusions stated here are tentative and do not represent the official opinion of the sponsoring agency. Discussions with C. Lance Barnett and Ira S. Lowry strengthened the theoretical content of this analysis, and the work of Therman Britt and Richard Stanton contributed to the empirical results.     

A modified Activated Sludge Model to estimate solids production at low and high solids retention time

In this paper, a modified version of the IWA-ASM1 model capable of correctly simulating the production of solids over a wide range of solids retention time (SRT) is presented. The parameters of the modified model have been estimated by integrating the results of respirometric and titrimetric tests with those of studies conducted on pilot scale plants that treat industrial wastewaters of differing characteristics.On the basis of the experimental results and their subsequent processing, it appears that the production of solids may be satisfactorily estimated using the modified model in which fractions X_P and X_I are supposed to be hydrolysable with a first-order kinetic.In the cases that were examined, the constant of the aforementioned kinetics was estimated to be k_i = 0.012 d⁻¹ and k_i = 0.014 d⁻¹, for tannery and textile wastewater respectively.A reliable calibration of the parameter k_i was possible when data relative to the experiment conducted in the pilot plants for no less than 60 d and in conditions of complete solid retention was utilized.     

Use of discriminant analysis to predict arson-prone structures

Summary The technique of discriminant analysis is widely used for discriminating arson from nonarson structures. Since in this particular case only two groups are involved viz., the arson and the match groups, the computations required for obtaining a discriminant function are relatively simple. Because of this, it is possible to delete a variable by examining the value of a _i d _i where a _i is the coefficient for the i ^th variable in the discriminant function and d _i is the difference between the means for the arson and the match groups for the i ^th variable (i=1, 2, …, p). Thus, if a _i d _i <0 then we delete the i ^th variable from consideration because inclusion of such a varable, increases the probability of misclassification. A prior knowledge of a _i d _i is particularly useful when we have large numbers of variables to be considered. This will help to reduce the number of discriminant functions one needs to consider in order to arrive at the discriminant function which minimizes the misclassification probability. Frequently, in developing a discriminant function one strives to increase the probability of correct classification. However, this probability reflects two components, viz., the probability of correct classification of known arson structures and the probability of correct classification of known nonarson structures. Even if the overall probability of correct classification is high, it does not necessarily imply, as the illustrations of Boston and New York City indicate, that the resulting discriminant function is also efficient in the sense that more arson cases are correctly classified than the nonarson cases. To this end, a procedure has been suggested to compute the cost of the wrong decision for a discriminant function which can be used to compare the cost of two similar types of discriminant functions. Using the relative cost efficiency criterion, it has been shown that the discriminant function for Newark is more efficient than the discriminant function for either Boston or New York City. Reference: Shiledar Baxi, H., “Use of Discriminant Analysis to Predict Arson-Prone Structures,” Fire Technology, Vol. 20, No. 4, November 1984, p. 17     

Bewertung nichtlinearer Nachweiskonzepte für Verbundstützen anhand des Kriteriums der Versagenswahrscheinlichkeit

Evaluation of non‐linear structural analysis for composite columns based on criterion of probability of failure. Four methods to proof the ultimate limit state of composite columns are presented. Non‐linear structural analysis is used and the main focus is laid upon the structural reliability. The criterion for the structural reliability is the probability of failure P_f. The results of all four methods are compared relatively to each other and to the reliability index β=4.7 given in EC 1 and DIN 1055‐100. At the end a discussion of the characteristic features of all four methods including their advantages and disadvantages is given.     

Erosion d’une vase de l’estuaire de la Loire sous l’action du courant

This research work deals with an experimental study on the erosion phenomenon of a mud under the action of a water current. It is observed from research works carried out by Partheniades (J Hydraul Div ASCE 91(HY1):105–139, 1965), Migniot (La Houille Blanche 1&2:11–29, 95–111, 1989), Ockenden and Delo (GeoMar Lett 11:138–142, 1991), Aberle et al. (Mar Geol 207:83–93, 2004), among others, that mud erosion process by an hydrodynamic action depends mainly on sediment properties. Based on a literature study, this critical stress is assumed to be proportional either to the effective cohesion (Eq. 2) or to the yield stress (Eq. 4) of the sediment bed. Six erosion test series have been performed at six different concentrations of a mud from the Loire estuary. Some properties of the tested sediments are: a solid particles density ρ _s = 2,550 kg m^?3, a liquidity and plasticity limit at 140 and 70% of the water content, respectively, a mean size of the dispersed mineral fraction determined by laser techniques of 10 μm, and a volatile matters content of 11.86% by total dry weight burned at 550°C. As the rheological behaviour is difficult to describe, the sediment strength is characterized by only a parameter, namely, the yield stress τ _y . It is measured with a coaxial cylinder Brookfield LVT viscosimeter following a defined procedure (Hosseini in Liaison entre la rigidité initiale et la cohésion non drainée dans les vases molles—Relation avec la dynamique sédimentaire. Thèse, Université de Nantes, 167 p, 1999). For each studied concentration, three successive erosion tests are carried out, and for every erosion test, 15 successive measurements of τ _y are made. The mean values and the standard deviations of τ _y are shown in Table 1 as a function of the bed sediment concentration C. A confined flume has been conceived and built to characterize the erosion rates. With this device, a current-induced shear stress is generated above an homogeneous deposited sediment (Fig. 1). The bed shear stress τ _o is calculated from the measured mean velocity V by Eq. 5. The friction coefficient c _f involved in Eq. 5 has been evaluated from measurements of the hydraulic pressure loss. Finally, the validity of Eq. 5 has been confirmed by five calibration tests on the incipient of the movement of sands for which results are compared with Shields diagram in Fig. 2. The observed erosion mechanisms affecting cohesive sediments depend mainly on the value of the yield stress. For a fluid mud (τ _y less than 3 N m^?2), the bed shear stress produces at first a wavy motion on the bed surface with a progressive undulation. When the shear stress increases, resuspension produces a diluted sediment cloud which is entrained and dispersed by the flow. For a plastic mud (τ _y greater than 3 N m^?2), erosion occurs by a wrenching of aggregates which are transported near the bottom. Initially, the eroded aggregates measure several millimetres in size; but, once transported, aggregates break into very rigid and compact aggregates of maximum size close to 1 mm in all the cases. During erosion tests, erosion volumetric rates E _v have been evaluated under steady-state bed shear stress τ _o from the observed variation in time of suspended sediment concentration. A generalized erosion is assumed for E _v ≥ E _vo = 3 × 10^?7 m s^?1 (that is the equivalent of 1 mm sediment layer eroded per hour). The generalized erosion occurs above a critical bed shear stress τ _oe which is linked to τ _y and mud density through Eq. 7. A phenomenological law defined by Eq. 8 is proposed to calculate the erosion rate as a function of yield stress and hydrodynamic shear stress at the bottom. For τ _o < τ _oe , a small erosion is observed, which is described by a formulation given by Cerco et al. (Water quality model of Florida Bay. U.S. Army Engineer Research and Development Center, ERDC/EL TR?00-10, Vicksburg, USA, 260 p, 2000). Above τ _oe , a small increase in τ _o produces an important erosion rate rise which is described by a formulation suggested by Mehta and Partheniades (Resuspension of deposited cohesive sediment beds. In: Proceedings of the 18th coastal engineering conference, Cape Town, South Africa, 2:1569–1588, 1982). Figure 3 shows as a function of τ _o the observed values of E _v (discrete symbols) as well as the proposed model (in solid lines) for the six concentrations. For the theoretically maximum value of E _v , the hydrodynamic shear stress is very high with regard to the sediment cohesion; and then, the water flux produces an entrainment of underlying fluid mud layers. The asymptotic law obtained from the model of Kranenburg and Winterwerp (1997) plotted in dashed lines in Fig. 3, is expressed by E _v = 0.3 u _* . A sediment trap inserted in the experimental system allows a sample of eroded mud aggregates to be obtained. It is observed that the maximum value of the diameter D _M of the eroded aggregates depends on the density and yield stress of the initially deposited mud according to Eq. 10. In the same way, the density of the aggregates issued from plastic mud erosion is measured following an original experimental method and procedure (Table 3). The erosion of plastic muds with a concentration from 310 to 420 kg m^?3 produces aggregates with a concentration close to 400 kg m^?3 and yield stress a little greater than 100 N m^?2.     

A sequential response surface method and its application in the reliability analysis of aircraft structural systems

The reliability analysis of aircraft structural systems is difficult to evaluate due to the complexity of the g-function g(x), the probability density function ƒ(x) and the dimension m of the problem. In many cases, the g-function can be very complex and the number of evaluation of g(x) may dominate the computation cost. Response Surface Methods may alleviate the problem by giving a simple approximation, g′(x), to the true g-function, which can then be used instead of g(x) for the reliability analysis.In this paper, a new Sequential Response Surface Method together with Monte Carlo Importance Sampling (MCIS) is suggested. Based on the method, a reliability analysis program RSM for aircraft structural systems is developed.Several examples are presented to illustrate the method.     

Compaction of flocculated material

The device used in the experiment consists of a flat-bottomed graduated cylinder and a coaxial plunger. A suspension flocculated with chemicals is sedimented after being mechanically worked within the graduate, and the supernatant water is removed with a pipette. The plunger is thrust into the sludge at a constant speed. The sludge is not only compressed but also flows into the annular gap between the plunger and the graduate, resulting in liberation of water. The liberated water is accumulated on the sludge in the annular gap. The “sludge bulkiness” β is used to describe the volumetric proportion of sludge and solids in it. The sludge bulkiness values before and after the “plunger test” are denoted as β_i and β_f, respectively. The values of β_i and β_j have been explored as a function of the time of the mechanical working. As a result, there is a definite time lag between the maximum value of β_i and the minimum value of β_f. The minimum value of β_j is obtained when the sludge consists of “pelleted flocs”.     

Ground motion modeling for multiple-input structural analysis

Coherency and apparent velocity are two major functions which control the simulation of strong ground motion at multiple stations. In this paper, after a brief review of a method of generating time series inputs for any given set of n surface locations having space coordinates x_i and y_i (i = 1, 2, …, n), compatible with the observed wave propagation properties, we present some of the results obtained in a study of coherency and apparent velocity of several earthquakes recorded at Smart-1 array. Based on data collected in the inner ring of Smart-1 array (stations spaced from 100m to 400m), we studied: (i) the loss of coherency function γ_ij(d_ij^L, d_ij^T, f) for pairs of records obtained at stations i, j, as a continuous function of frequency f, and projected separation distances d_ij^L and d_ij^T in the longitudinal direction of preferential wave propagation and in the transverse direction, respectively; (ii) the apparent velocity V_app as a function of frequency f. In face of the regularity exhibited by the coherency function, an heuristic model with several parameters was assumed and parameter values are estimated for each earthquake using least square methods. No such regularity was observed in relation to apparent velocities. Preliminary conclusions of dependence of those two functions upon the geologic and seismotectonic characteristics of the Smart-1 environment is analysed and a set of recommendations for engineering applications is briefly discussed. The paper concludes with the presentation of an example of simulation of ground motion using the above mentioned results.     

Financial effects of establishing a public office or public enterprise

This paper deals with the effects on municipal budgets of establishing public offices or enterprises. The methodology described can assist with locational decisions and quantifies employment, income, and financial effects with the help of a macroeconomic model. A measure for changes in the cash flow position of a town is also introduced Two examples — the establishment of a district heating firm and a concert hall — demonstrate different financial effects arising from the contrasting characteristics of each enterprise and the nearby towns most affected. Whereas the district heating firm increases the municipal cash flow, a reduction results from the establishment of the hall.Notation a ₁₂,a ₂₁ Grouped parameters indicating the effects of income variation in the hinterland on the settlement town's income and vice versa - a _1,2 Grouped parameters relating the variation of autonomous expenditure to total income of both regions - b ₁,b ₂ Grouped parameters which reflect the consequences of settlement measures for the settlement town and its hinterland - s _E ⁿ The relation of officials' salaries to total expenditure on the public office - n _E ^H1 The relation between additional lower rank employment for the inhabitants of the settlement town to the change in lower paid positions - n _v ^H The relation between employees and value added - dB _E ¹ Number of lower paid employees working at the public office - dB _A ¹ Lower paid employees who get jobs as a consequence of settlement actions - dN ^H1 Total employment effect - dN _E ^H1 The direct employment effect - dN _p ^H1 The indirect employment effect - c ₁,c ₂ Grouped parameters showing the effects of private and public debt on income in region 1 and region 2 - e _1,2 Grouped parameters for the demonstration of department movements due to total income variations - g Direct effects of autonomous expenditure on debt - dS _E Salaries for top officials - dS _E ^m Salaries for higher officials - dE Autonomous expenditure required for the actual establishment of the public office and for the necessary settlement measures - Y ^H1 The income effects - dY _E ⁿ Income of higher level officials employed at the public office and those who obtain jobs as a consequence of settlement actions - dY _E1 Income of lower paid employees working at the public office and those who obtain jobs as a consequence of settlement actions     

Trace metal analysis by anodic stripping voltammetry: effect of sorption by natural and model organic compounds

A variety of model organic compounds representative of the types of organic substances occurring in natural waters were found to sorb onto hanging mercury drop electrodes in the anodic stripping voltammetric analysis of trace metals. Sorption effects on the ASV diagnostic parameters E_p and i_p (peak current and peak potential) were pH dependent, with greater effect under acidic conditions in some cases, and were significant at organic concentrations in the ppm range. Compounds tested included gelatin and alkaline phosphatase as representatives of soluble proteins. Triton X100 (a nonionic surfactant), and a variety of polysaccharides, including agar, alginic acid and starch. Effects of sorption included depressed i_p values, shifts in E_p to more positive values, and broader peaks. Implications of these effects on the interpretation of ASV measurements in terms of analysis and speciation of trace metals in natural waters are discussed.     

Vulnerability assessment of irregular steel structures under traveling fires

This paper investigates the response of tall setback irregular steel moment resisting structures under traveling fires. A seven‐story steel regular structure (RS) is first designed for gravity and seismic loads and then it is fireproofed for a fire resistance rating (FRR) of 120 min based on the ISO834 fire. Some architectural changes are then imposed on the RS to make it a setback irregular structure (SBS). Based on the traveling fire methodology, both structures are then subjected to fire curves ranging from 1% to 100% for the fire load density (q_f) of 570 MJ/m². As in the SBS, the distribution of gravity loads is not uniform; the structural fire analyses are performed twice: when the fires start from left to right and then vice versa. The results show the FRR of the RS is minimized under the fire size of 22% with the collapse time of 87.0 min. In the SBS, when the fires start from left to right and then from right to left, the FRR is minimized under the fire size of 20% with the collapse times of 75.0 and 79.0 min, respectively. This shows that the SBS is more vulnerable toward traveling fires than the RS. In order to monitor the role of q_f in the FRRs of the structures, q_f is then reduced, and the required analyses are repeated. This process continues to where no collapse is observed under all the fire sizes. The results indicate that the RS would remain stable if the q_f is decreased to 480 MJ/m², implying that a 16% increase should be considered to the required FRR. As for the SBS, the q_f should be decreased to 440 MJ/m² to make it stable, meaning that a 25% increase should be considered to the required FRR.     

Simplified P–M interaction curve for square steel tube filled with high-strength concrete

The purpose of this study is to propose simplified strength equations that can be conveniently used to establish a P–M interaction curve of square concrete filled tubes (CFTs) with concrete strength of up to 100 MPa. The method presented in the author's previous study [Choi Y-H, Foutch DA, LaFave JM. New approach to AISC P–M interaction curve for concrete filled tube beam-columns. Eng Struct 2006;28(11):1586–98] was used as a basic unified formula for pure steel members and CFT ones, and a parametric study was performed to determine the contribution of the concrete, which were expressed by two variables: a normalized maximum moment, α, and the axial load ratio at the maximum moment, β. The two variables were formulated with respect to tube width-to-thickness ratio (b/t) and relative concrete compressive strength to yield strength of the steel tube (f′_c/F_y). Then, the proposed method were compared to experimental data found in literatures, which showed greatly improved results in terms of accuracy and amount of computation, when compared to the current AISC design methods.     

Simulation of water-bloom in a eutrophic lake—III. Modeling the vertical migration and growth of Microcystis aeruginosa

The rate of vertical migration (rising or sinking) of Microcystis aeruginosa colonies in water was represented by Stokes' law. The density, p_p of the colony, estimated conversely from observations on vertical migration rate by using Stokes' law, was shown as a function of gas vacuoles' fraction, V_f in algal cells.Referring to the experimental studies by previous workers on factors that the affect the value of V_f, gas vacuoles in the cells were assumed to collapse instantly, V_f decreasing to V_feq once V_f values exceed those of V_feq. The latter values of V_feq were defined from a cumulative and normal distribution of gas vacuoles that withstand the turgor pressure. P, Incidentally, the regeneration rate of gas vacuoles in the cells that were subjected to sonication, yielding V_f = 0 (non-vacuolate) was constant regardless of the post-sonication environment of light and/or dark.Taking for granted that there exist upper and lower limits of turgor pressures for a given algal cell, an equation on the rate of change in turgor presure of the cell was derived. Presentation of these rate equations is a prerequisite for modeling and simulating emergence and/or disappearance of the waterbloom in still waters of eutrophic lakes and/or ponds.     

Applicability of light absorbance as a measure of organic carbon in humic lake water

The results of statistical analysis showed that turbidity measurements at 436, 530 and 620 nm and COD data of unfiltered samples from Tjeukemeer. The Netherlands, between 1977 and 1980 were yearly significantly (P < 0.005) linearly correlated. However, the correlation coefficients and the slopes of these correlations varied irregularly.Between 1970 and 1980, the light absorbances at 250 (E₂₅₀) and 365 nm (E₃₆₅) were yearly significantly (P < 0.0005) linearly correlated with the COD of filtered samples. Apart from between 1970 and 1973, possibly due to sand-suction, this correlation of E₂₅₀ did not change from year to year and was independent of sampling stations. Therefore, E₂₅₀ is a suitable estimate of the concentration of dissolved organic carbon in eutrophic and humic Tjeukemeer water. The correlation found is almost identical to those reported for oligotrophic and humic Finnish lakes and Black Sea water.     

Biogeochemical cycling of arsenic in coastal salinized aquifers: Evidence from sulfur isotope study

Arsenic (As) contamination of groundwater, accompanied by critical salinization, occurs in the southwestern coastal area of Taiwan. Statistical analyses and geochemical calculations indicate that a possible source of aqueous arsenic is the reductive dissolution of As-bearing iron oxyhydroxides. There are few reports of the influence of sulfate-sulfide redox cycling on arsenic mobility in brackish groundwater. We evaluated the contribution of sulfate reduction and sulfide re-oxidation on As enrichment using δ³⁴S_[SO4] and δ¹⁸O_[SO4] sulfur isotopic analyses of groundwater. Fifty-three groundwater samples were divided into groups of high-As content and salinized (Type A), low-As and non-salinized (Type B), and high-As and non-salinized (Type C) groundwaters, based on hydro-geochemical analysis. The relatively high enrichment of ³⁴S_[SO4] and ¹⁸O_[SO4] present in Type A, caused by microbial-mediated reduction of sulfate, and high ¹⁸O enrichment factor (ε_[SO4-H2O]), suggests that sulfur disproportionation is an important process during the reductive dissolution of As-containing iron oxyhydroxides. Limited co-precipitation of ion-sulfide increased the rate of As liberation under anaerobic conditions. In contrast to this, Type B and Type C groundwater samples showed high δ¹⁸O_[SO4] and low δ³⁴S_[SO4] values under mildly reducing conditions. Base on ¹⁸O mass balance calculations, the oxide sources of sulfate are from infiltrated atmospheric O₂, caused by additional recharge of dissolved oxygen and sulfide re-oxidation. The anthropogenic influence of extensive pumping also promotes atmospheric oxygen entry into aquifers, altering redox conditions, and increasing the rate of As release into groundwater.     

Experimental observations of the mechanics of borehole failure in porous sandstone

To investigate borehole failure under hydrostatic stress in sandstone of varying porosity, experiments were conducted on three sandstone varieties with 2–5 mm bores. Tennessee, Darley Dale, and Penrith sandstone-types were selected to represent most of the spectrum of sandstone porosity variations (respectively, 0.07, 0.12 and 0.28). Confining pressures ranged up to 500 MPa, and pore fluid volumometry was used to detect bore failure. Failed samples were studied microstructurally using optical and scanning electron microscopy.Each rock type failed with the development of oriented breakout features normally seen to develop around boreholes loaded under non-hydrostatic stress, and in Tennessee sandstone at least, these developed in a consistent orientation with respect to an external reference frame (dip-direction of foreset beds). All of the rock types showed strength and/or elastic anisotropy, thus the formation of oriented breakouts under hydrostatic loading is attributed to the effects of anisotropy.Two modes of breakout development were observed. In Darley Dale and Penrith sandstones, a combination of intergranular shear and extensional fracture produced broad and shallow features with breakout width unaltered during growth. In Tennessee sandstone a purely extensional mode of intragranular fracture was observed, leading to deeper breakout features.The samples tested showed higher failure pressures for smaller bore sizes. When failure pressure is normalized with respect to grain crushing pressure P^*_i, and bore diameter with the product of porosity with grain size, all data lie on a single master curve, with bore failure pressure approximately 0.15 P^*_i in the regime of bore size-independent behaviour.