Shear strength response of reinforced pond ash

Reinforced pond ash is a composite material, which can be used as an alternative construction material in the field of geotechnical engineering. To study the shear strength response of reinforced pond ash, a series of unconsolidated undrained (UU) triaxial test has been conducted on both unreinforced and reinforced pond ash. In the present investigation the effects of confining pressure (σ₃), number of geotextile layers (N), and types of geotextiles on shear strength response of pond ash are studied. The results demonstrate that normal stress at failure (σ_1f) increases with increase in confining pressure. The rate of increase of normal stress at failure (σ_1f) is maximum for three layers of reinforcement, while the corresponding percentage increase in σ_1f is around (103%), when the number of geotextile layers increases from two layers to three layers of reinforcement. With increase in confining pressure the increment in normal stress at failure, Δσ increases and attains a peak value at a certain confining pressure (threshold value) after that Δσ becomes more or less constant. The threshold value of confining pressure depends on N, dry unit weight (γ_d) of pond ash, type of geotextile, and also type of pond ash.     

Simulation of water-bloom in a eutrophic lake—II. Reassessment of buoyancy,gas vacuole and Turgor pressure 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.     

Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions

This paper investigates the impact of clay and moisture contents on the shear behavior of compacted earth taking into account loading-unloading cycles. Fine sand was added to a natural soil, thereby obtaining three different soils with clay contents of 35%, 26%, and 17%, respectively. A series of triaxial tests was conducted on samples previously equilibrated at three different values of relative humidity (RH). The evolution of failure strength f_c, Young's modulus E, and residual strain ε_res was investigated according to the clay content and the RH, the last two parameters being measured during the loading-unloading cycles. Firstly, the relative humidity at which the samples were fabricated and conditioned was seen to have a strong impact on the mechanical characteristics of the earthen material. An increase in RH led to a decrease in both failure strength f_c and Young’s modulus E, and an increase in plastic strain. The tendencies were found to depend on the clay content of the samples. Secondly, with an increasing stress level, a progressive decrease in Young’s modulus and an increase in residual strain ε_res (after a loading-unloading cycle) appeared. Thirdly, within the range of the investigated clay contents, both failure strength f_c and residual strain ε_res increased with an increasing clay content at constant values of RH and confining pressure, the rate of this increase being a function of the RH. Young’s modulus E was relatively insensitive to changes in the clay content, its variation being less than 20% for all cases. Finally, based on a particular definition of Bishop's effective stress, involving a specific functional form χ(s), the failure states of all the samples were observed to lie approximately on a unique failure line crossing the origin in the (p′-q) plane regardless of the matric suction and confining pressure.     

Effective stress law for the permeability of a limestone

The effective stress law for the permeability of a limestone is studied experimentally by performing constant-head permeability tests in a triaxial cell with different conditions of confining pressure σ and pore pressure p_f. Test results show that a pore pressure increase and a confining pressure decrease both result in an increase of the permeability, and that the effect of the pore pressure change on the variation of the permeability is more important than the effect of a change of the confining pressure. A power law is proposed for the variation of the permeability with the effective stress (σ′=σ–n_kp_f). The permeability-effective stress coefficient n_k increases linearly with the differential pressure and is greater than unity as soon as the differential pressure exceeds few bars. The test results are well reproduced using the proposed permeability-effective stress law. A conceptual pore-shell model based on a detailed observation of the microstructure of the studied limestone is proposed. This model is able to explain the experimental observations on the effect of the total stress and of the pore pressure on the permeability of the limestone. Effective stress coefficients for the stress-dependent permeability which are greater than one are obtained. It is shown that the controlling factor is the ratio of the different bulk moduli of the various constituents of the rock. This ratio is studied experimentally by performing microhardness tests.     

Effect of cracking,corrosion and repair on the frequency response of RC beams

This paper presents a laboratory investigation to study the effect of cracking, corrosion and repair on the frequency response of simply-supported reinforced concrete beams. The beams were subjected to four-point bending at 70% of their theoretical ultimate loads. Corrosion was then induced using an electro-chemical acceleration technique. The beams were then repaired to restore their load-carrying and deflection capacities. Based on response time histories, the fundamental frequency characteristics of the beams were deduced using Fourier transform (FT) and Hilbert-Huang transform (HHT). Experimental results revealed significant changes in the normalized frequency, R_f, with decrease in R_f associated with load-induced cracking, an increase in R_f associated with moderate reinforcement corrosion and a subsequent decrease in R_f associated with repair. The results of this study support the use of frequency response, obtained with an appropriate signal processing technique such as HHT, to infer the health of monitored structures, taking into account environmental conditions, likelihood of corrosion occurrence, properties of repair material and interfacial bonding.     

Nanosize radon short-lived decay products in the air of the Postojna Cave

At two points in the Postojna Cave, short-term monitoring in summer and in winter of air concentrations of radon and radon decay products, equilibrium factor, unattached fraction of radon decay products (f_un), barometric pressure, relative air humidity in the cave and air temperature in the cave and outdoor has been carried out, with the emphasis on f_un. Dose conversion factors, calculated on the basis of f_un values obtained (ranging from 0.09 to 0.65) exceed 5 mSv WLM^− 1, by a factor of 11.5-14.0 in summer and of 3.0-4.0 in winter for mouth breathing, and 3.1-3.5 in summer and 1.5-1.7 in winter for nasal breathing.     

Swell–compression characteristics of a fiber–reinforced expansive soil

This study presents results of an experimental program with respect to fiber's capacity of mitigating the swelling behavior of an expansive soil. Two types of tape–shaped fibers, i.e. fiber A (width f_w = 2.5 mm) and fiber B (f_w = 7 mm), were used as the reinforcements. The fibers were included at three contents, i.e. f_c = 0.5%, 1% and 1.5%, each having two lengths or aspect ratios (f_AR = 15/2.5 and 30/2.5 for fiber A, and f_AR = 15/7 and 30/7 for fiber B). For a given fiber type (constant f_w), improvement in swelling potential/pressure was observed to be a direct function of f_c and f_l (fiber length) or f_AR, with the former taking on a more pronounced role. In addition, for a given f_c and f_l, the wider fiber (lower f_AR) was more efficient in restricting swelling. The compression characteristics were cross–checked with the swelling properties to arrive the optimum stabilization scenarios. For both fiber types, f_c = 0.5% suggested an optimal case. However, where compressional deformations are not a primary concern, higher inclusions up to 1% could also be an acceptable choice.     

Fatigue performance of reinforced concrete beams strengthened with CFRP sheets

The paper presents the results of an experimental and analytical study involving the static and accelerated fatigue testing performance of nine reinforced concrete beams externally strengthened with different number and configuration of CFRP sheets. The beams were tested for the stress ranges of 0.25f_y–0.35f_y, 0.45f_y–0.65f_y, 0.65f_y–0.90f_y, and 0.45f_y–0.90f_y. After validating a nonlinear finite element analysis (NLFEA) with experimental test results, NLFEA was extended to provide a better understanding of the effect of fatigue stress range, number of CFRP layers, and the CFRP contact area with concrete on the performance of the reinforced concrete (RC) beams. The stress ranges have a significant effect on the permanent deflection at mid-span especially for the stress range of 0.45f_y–0.90f_y. Cyclic fatigue loading produced a time-dependent redistribution of the stresses that lead to a sudden drop in concrete stresses and a mild increase in steel and CFRP sheet stresses as fatigue life was exhausted. In addition, the authors recommend fatigue design considerations for calculating the reduction in the stiffness and ultimate load capacity due to fatigue loading.     

Reliability analysis of net cross-section resistance with accidental eccentricity of holes

The aim of the research was to determine reliability function of net cross-section resistance in relation to accidental eccentricity of nominally centric holes. The response surface method was used by taking into account the relevant Eurocode design provisions and reliability requirements. Within the response surface method, the central composite design method and the least square method were used with the employment of Monte Carlo simulations. The probable distributed variables such as strength f _y, breadth b, thickness t, diameter d ₀ and eccentricity e were determined by the central composite design method. 280 different numerical simulations were set up with varying variables. A log-normal distribution for strength (f _y) and a normal distribution for geometrical variables (b, t, d ₀,e) were employed by taking into account the coefficients of variations: V _fy=0.07, V _b,=0.005, V _l=0.05 and V _d0=0.005. In order to determine the influence of eccentricity on partial safety factor, several normal distributions with different variation factors were applied in the analysis. The influence of the edge distance of the hole e ₂ over d ₀ ratio on the partial safety factor was determined by varying mean values of variable b. For comparison, two types of steel were used: structural steel S235 and high strength steel S690. Numerical simulations of the net cross-section resistance F _u were performed with ABAQUS 6.7. The response surface for the net cross-section resistance was determined by introducing a quadratic approximation function and by applying the least square method. The partial safely factor was then (statistically) obtained by means of robust Monte Carlo simulations on the calculated response surface.     

A generalized three-dimensional failure criterion for rock masses

The smooth convex generalized failure function, which represents 1/6 part of envelope in the deviatoric plane, is proposed. The proposed function relies on four shape parameters (Ls, a, b and c), in which two parameters (a and b) are dependent on the others. The parameter L s is called extension ratio. The proposed failure function could be incorporated with any two-dimensional (2D) failure criteria to make it a three-dimensional (3D) version. In this paper, a mathematical formulation for incorporation of Hoek-Brown failure criterion with the proposed function is presented. The Hoek-Brown failure criterion is the most suited 2D failure criterion for geomaterials. Two types of analyses for best-fitting solution of published true tri-axial test data were made by considering (1) constant extension ratio and (2) variable extension ratio. The shape and strength parameters for different types of rocks have been determined by best-fitting the published true tri-axial test data for both the analyses. It is observed from the best-fitting solution by considering uniform extension ratio (Ls) that shape constants have a correlation with Hoek-Brown strength parameters. Thus, only two parameters (σc and m) are needed for representing the 3D failure criterion for intact rock. The statistical expression between shape and Hoek-Brown strength parameters is given. In the second analysis, when considering varying extension ratio, another parameter fis introduced. The modified extension ratio is related to f and extension ratio. The results at minimum mean misfit for all the nine rocks indicate that the range of f varies from 0.7 to 1.0. It is found that mean misfit by considering varying extension ratio is lower than that in the first analysis. But it requires three parameters. A statistical expression between f and Hoek-Brown strength parameters has been established. Though coefficient of correlation is not reasonable, we may eliminate it as an extra parameter. At the end of the paper, a methodology has also been given for its application to isotropic jointed rock mass, so that it can be implemented in a numerical code for stability analysis of jointed rock mass structures.     

Observations on FORM in a simple geomechanics example

This technical note studies the First Order Reliability Method (FORM) applied to a plane strain Mohr-Coulomb drained (c′, tan ?′) element test. The influences of distribution types and linear correlation between random variables are studied. The approximation of assuming a “first order” limit state function is assessed by comparison with direct integration of the probability distribution function in the failure region. The results indicate that FORM overestimates p_f when random variables are lognormally distributed and underestimate p_f when random variables are normally distributed.     

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.     

Stiffness parameters of municipal solid waste

This paper presents results of the deformability parameters of municipal solid waste (MSW) determined through laboratory (oedometer and triaxial tests) and in situ tests [standard penetration tests (SPT) and cone penetration tests (CPT)], as well as by a monitoring system including topographic control marks, open-tube piezometers, earth and pore pressure cells and inclinometer tubes with magnetic settlement plates and spiders. The studied landfill is located in a textile industry area, and was landfilled with some direct waste (10 %) and mostly pre-treated waste (90 %, from composting and triage). The influence of: the waste age, construction phase of the landfill and confining stress on the results of stiffness modulus, lateral waste pressure coefficient at-rest (K ₀) and Poisson’s ratio (ν) are presented, as well as an attempt to establish correlations to estimate MSW stiffness modulus from SPT and CPT tests. The results indicate the increases of K ₀ and ν with the average effective confining stresses during the first 2 years of the post-construction phase, reaching maximum values of 0.7 for K ₀ and 0.4 for ν, and then decreasing gradually until stabilizing at values of around 0.23–0.48 for K ₀ and around 0.19–0.32 for ν. The stiffness modulus values determined by different methods are consistent with each other and corroborate the fact that typical values for MSW are low. These values are between 0.4 and 2 MPa for a mean confinement stress of about 50 kPa, and tend to increase linearly with the increase of the confinement stress. The correlations established with SPT and CPT tests are of the same type as usually proposed for granular soils; however, the constant values are significantly lower than the lowest values reported for normally consolidated sandy soils.     

Effect of shearing history on stress wave velocities of sands observed in triaxial compression tests

For the accurate design of geotechnical structures subjected to static and dynamic loadings, precise estimation of elastic wave velocities and hence, small strain stiffness of soil is essential. However, the interpretation of elastic wave velocities propagating in deformed/sheared soil has not been comprehensively explored. In this research, shear (V_s) and compression wave velocity (V_p) measurements have been undertaken on three kinds of uniformly graded sands during drained triaxial compression by keeping minor principal stress constant. Planar piezoelectric transducers have been used to overcome the limitations associated with the more commonly used bender elements, such as distortion of transducers during specimen shearing. This technical note reveals that the increase of major principal stress in the wave propagation direction has a more significant influence on V_p in comparison to V_s. The axial strain (ε_a) at which peak V_s is noted is comparable to the ε_a at which specimen dilation or phase transformation takes place. The V_s values show a substantial drop after phase transformation, although there is an increase in the mean stress level. However, V_p increases even after specimen dilation takes place, and it is the major principal stress that dictates its evolution during triaxial compression. Moreover, for a given material and initial stress level, elastic wave velocities of specimens prepared at different initial densities approach one another during shearing and later merge at a large ε_a.     

Stress wave transmission and frequency-domain responses of gap-graded cohesionless soils

Gap-graded cohesionless soils are extensively utilized in numerous man-made geotechnical structures such as earthen embankments, man-made fills and used for the prevention of seepage in dams and tailings of mines. However, there is a lack of research available on investigating the transitional behaviour of frequency responses of gap-graded cohesionless soils. Therefore, the present research explores the stress wave transmission in gap-graded silica sand mixtures (having median particle size ratio of 6.4) using disk-shaped piezoelectric transducers (DTs). By employing DTs, shear (V_s) and compression (V_p) wave velocities are measured using more planar waves and eliminate the possibility of fine particle segregation commonly encountered during insertion of bender elements. The experimental results indicate that at an equivalent void ratio, V_s and V_p decrease initially with the increase in fine silica sand content (F_s); however, once a transitional value (F_s,thr) is attained, V_s and V_p start to rise and move towards the values for fine silica sand. Such transitional behaviours of elastic wave velocities and frequency-domain responses are dependent on the state of packing, i.e. denser specimens achieve this at a lower F_s than the looser equivalents. It is observed that for a given F_s, there exists a linear relationship between the maximum transmitted frequency in the packing, i.e. low-pass frequency (f_lp) and the V_s. Furthermore, a novel approach has been described by which the type of gap-graded mixture, i.e. underfilled (internally unstable) or overfilled (stable), can be assessed by experimentalists from the slope of f_lp–V_sresponses. The advantage of adopting f_lp to categorize the gap-graded mixtures is that the same is more sensitive to F_s in comparison to V_s or V_p, and thus a more robust analysis can be achieved.     

Determination of Fire Hose Friction Loss Characteristics

Friction loss characteristics of fire hose have changed as a result of evolving hose manufacturing technology. Published friction loss characteristics may be overly conservative. While conservatism in fire protection is generally good, in this case it may lead to excessively high pump discharge pressures as the operator applies general rules-of-thumb. The overall objective of this research project was to develop friction loss characteristics for hose currently used by the fire service. A total of 86 tests were performed by three fire service organizations on 82 fire hose samples spanning 1–5 inches in diameter. Recorded hose dimensions, pressure, flow and friction loss data were used to calculate the friction factors. Three friction factors were calculated: C, the factor now used in published data; and, C_D and f. The traditional C factor combines hose diameter and roughness into a single constant. The C_D and f factors use the measured diameter to calculate a friction factor closely associated with hose interior roughness. The data indicate that most C factors calculated for the tested hose fall below the currently published values. The C_D and f factors provide more insight into friction loss characteristics, since the effects of actual inside diameter are considered. Overall, the friction loss characteristics observed for individual tested hose sections are a factor of the inside diameter and roughness. Inside diameter alone was not a predictor of the magnitude of the friction loss across all samples. A fairly large degree of variability was observed in the data.     

Effect of aspect ratio and volume fraction of steel fiber on the mechanical properties of SFRC

In this study, effects of aspect ratio (l/d) and volume fraction (V_f) of steel fiber on the compressive strength, split tensile strength, flexural strength and ultrasonic pulse velocity of steel fiber reinforced concrete (SFRC) were investigated. For this purpose, hooked-end bundled steel fibers with three different l/d ratios of 45, 65 and 80 were used. Three different fiber volumes were added to concrete mixes at 0.5%, 1.0% and 1.5% by volume of concrete. Ten different concrete mixes were prepared. After 28 days of curing, compressive, split and flexural strength as well as ultrasonic pulse velocity were determined. It was found that, inclusion of steel fibers significantly affect the split tensile and flexural strength of concrete accordance with l/d ratio and V_f. Besides, mathematical expressions were developed to estimate the compressive, flexural and split tensile strength of SFRCs regarding l/d ratio and V_f of steel fibers.     

Assessing the moisture-content-dependent parameters of stabilised earth materials using the cyclic-response admittance method

The cyclic-response admittance method is discussed along with its suitability for assessing the moisture-content-dependent thermal properties of porous fabrics under idealised dynamic conditions. The effects of insulation positioning in composite stabilised earth wall fabrics are analysed. The moisture dependent non-steady-state parameters of Y, f, ? and ω are presented for a range of stabilised earth materials in relation to their degree of saturation, S_r. Data to allow direct comparisons between specific cross-sectional composite wall fabrics using both conventional materials and stabilised earths are given. The effects of moisture-content distribution within stabilised earth wall fabrics is analysed in terms of the overall effect on the wall's non-steady-state parameters. The cyclic-response admittance method is modified to incorporate the moisture dependent non-steady-state parameters and the effects on admittance, decrement and time lag are discussed.