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.     

Determination of relevant parameters influencing gas permeability of mortars

This paper is devoted to the study of the influence of the microstructure of cementitious materials on their gas permeability. For that purpose, cement pastes with four water to cement ratios W/C and mortar specimens with two aggregate volume contents have been cast. On the one hand gas permeability tests with a low pressure device have been carried out on desaturated materials and on the other hand porosity features have been determined by mercury porosimetry measurements. Gas permeability depends upon the mixture parameters (W/C, aggregate volume content, aggregate size) and the moisture content. Correlation between gas permeability and pore size distribution has been discussed from Katz–Thompson equation too. Our results showed that permeability predicted from the threshold radius of the capillary porosity for cement pastes and mortars whatever the aggregate volume content corresponds to permeability measured after drying at relative humidity, RH equal to 90%. For preconditioning with lower RH, the measured permeability is influenced by microcracking generated by drying shrinkage.     

The effect of conditioning to a predetermined weight loss on the permeability of concrete

The durability properties of concrete can be assessed via gas permeability tests. The moisture content of the concrete plays a major role in its permeability to gas, and the conditioning stage of any gas permeability test aims to minimise and standardise the amount of moisture present in the concrete via oven drying, usually at a temperature of 105 °C. However, this is known in some circumstances to take a considerably longer period of time in comparison to the permeability testing itself. This study examines whether a relationship exists between the moisture content of a specimen and its permeability by investigating the permeability of concrete specimens dried by varying amounts. It was found that a simple relationship of the form y = ax^b could be established for both normal and high strength concrete subjected to pressure decay-time permeability testing.     

Permeability Characteristics of High-Quality Undisturbed Gravelly Soils Measured in Laboratory Tests

A series of permeability tests on both high-quality undisturbed samples and reconstituted samples was performed using a large-scale triaxial cell. Based on the test results and discussion, the following conclusions were noted.

• (1)The coefficient of permeability in the horizontal direction is larger than that in the vertical direction. However, its difference is between 10% and 70% and not so large.
• (2)The coefficient of permeability decreases with increasing confining stress. The effect of the confining stress can be understood as the effect of the void ratio. And the small change of the void ratio due to consolidation only leads to a small change of the coefficient of permeability.
• (3)There is no clear correlation between the physical properties and the coefficient of permeability.
• (4)The coefficient of permeability of gravelly soils is almost the same as that of sandy soils, even though the 50% diameter of gravelly soils is about ten to a hundred times that of sandy soils. This result implies that the large size particles of gravelly soils have no significant effect on permeability characteristics of gravelly soils.
• (5)A new definition for determination of D₁₀, D₂₀ and fines content only from those soil particles with a diameter smaller than 2 mm was introduced. A correlation similar to sandy soils can be seen between the coefficient of permeability and D₁₀, D₂₀, and fines content based on the new definitions.
• (6)The effect of the small size particles of the gravelly soils on the coefficient of permeability was found significant based on the test results using samples with a special blend of particle size and density reconstituted from undisturbed samples.
• (7)The coefficient of permeability of multi-layer gravelly soils, in the direction perpendicular to the sedimentation, was significantly affected by the lowest permeability. And the coefficient of permeability of the multi-layer in total agrees well with the theoretical estimation.
• (8)Although the data is limited, there was no significant difference of coefficient of permeability between undisturbed and reconstituted samples. This result accords with that reported for sandy soils by Hatanaka et al. (1997). This result also means that the coefficient of permeability of gravelly soil is not affected by the soil fabric. As a result, the in-situ coefficient of permeability of gravelly soils can be well estimated for practical purposes from the reconstituted samples with the same gradation properties.

     

Influence of mineral admixtures on compressive strength,gas permeability and carbonation of high performance concrete

Compressive strength, gas permeability and carbonation of high performance concrete (HPC) with fly ash (FA) or ground granulated blast furnace slag (GGBFS) were experimentally investigated and the relationships among them were analyzed. Test results showed that influences of FA with replacement up to 60% on these properties investigated are significantly affected by water–binder (w/b) ratios. However, unlike FA, influences of GGBFS on HPC are little affected by w/b ratios, similar changing trends could be observed for both w/b ratios selected. Moreover, HPC with GGBFS shows much better performance than that with FA at the same w/b ratio. In general, replacing FA/GGBFS with cement could not benefit the properties investigated, especially at the higher w/b ratio selected and relationship between compressive strength and gas permeability of HPC greatly depends on w/b ratios and mineral admixture types. Carbonation is obviously related to gas permeability for both HPC with FA/GGBFS.     

Swelling properties and coefficient of permeability of friction-reducing polymer for pull-out of temporary sheet piles

Harmful cracks and settlements occur after the pull-out removal of temporary sheet piles. This is because a large amount of soil is discharged from the ground due to the significant skin friction at the steel surface. In order to prevent these cracks and settlement problems, a friction-reducing polymer was developed. After a sheet pile coated with the polymer is installed into the ground; the polymer absorbs ground water and transforms to a swollen gel layer which separates the soil and the sheet piles. In this investigation, a newly designed column type test apparatus was used to simulate ground conditions. The swelling test was carried out, followed by a continuous permeability test. Based on the obtained test results, the swelling properties and coefficient of permeability of the friction-reducing polymer in the ground were determined. The main conclusions are as follows. The values of the water-absorbing ratio, Ra_max, decrease with increasing earth pressure, p′, independent of the pore water pressure, u. Equations to determine the values of Ra_max are proposed as functions of p′ only. The maximum swelling pressure, at which the friction-reducing polymer cannot absorb water and swell, is p′_max?=?560?kPa. Even at a depth of approximately 100?m, the polymer absorbs ground water, and the swollen gel layer forms. The approximate range of coefficient of permeability, k, of the swollen gel layer is 10^?13–10^?11 m/s; these are extremely small values. The swollen gel can be considered an impermeable material. Depth distributions of the thickness and the coefficient of permeability of the swollen gel layer are illustrated in practical charts, based on the test results.     

Influence of clogging substances on pore characteristics and permeability of geotextile envelopes of subsurface drainage pipes in arid areas

This study investigates the influence of clogging substances on pore characteristics and permeability of geotextile envelopes that were used for 3, 7 and 15 years in irrigated farmlands in Xinjiang region, which is arid and suffers from the soil salinity problem. Results show that the macropores (above 125 μm) of envelopes are evidently clogged, whereas the smaller pores less than 100 μm are still unblocked after operation. The permeability coefficients of geotextile envelopes after serving for 3 and 15 years are smaller than the minimum required permeability coefficients after clogging. The main chemical components of clogging substances in the geotextile envelope are silicon dioxide and calcium carbonate. Calcium carbonate content of the geotextile envelope is consistent with calcium carbonate content of soil. Chemical clogging susceptibility increases with the operation time of the subsurface drainage pipes. The ratio of O₉₀ size of envelope material over d₉₀ of soils (O₉₀/d₉₀) and saturation index (SI) can be used to assess the susceptibility of physical and chemical clogging respectively. This study provides a preliminary reference for estimating the clogging susceptibility of geotextile envelopes in arid areas.     

Permeability prediction in geotextile envelope after chemical clogging: a coupled model

This study develops a coupled model of chemical clogging and permeability coefficient of geotextile envelope. Based on the distribution characteristics of crystal precipitates on geotextile envelope and their influence on the permeability coefficient, a permeability coefficient model of an actual geotextile envelope that considers the overlapping effect is developed. Then, the densification effects of geosynthetic fiber hypothesis and the filter cake effect hypothesis are proposed to simulate the processes of increasing fiber diameter after crystal precipitation and the accumulation of crystal precipitates on the surface of geotextile envelope. The crystal precipitation module and permeability coefficient module are coupled, and their experimental values are used to confirm the availability of the model. Results indicate the satisfactory performance of the model. In addition, the parameter sensitivity analysis and trend prediction show that the saturation index SI and solution flow rate V are the main factors that affect the chemical clogging and permeability of geotextile envelope. When the solution conditions are not considered, the sensitivity of geotextile envelope parameter d_f increased with the amount of precipitation in crystal precipitation. When the pores of the geotextile envelope are completely clogged, the permeability coefficient of the geotextile envelope will drop sharply, then decline slowly.     

Influence of bacteria on the compressive strength, water absorption and rapid chloride permeability of fly ash concrete

This paper presents the results of an experimental investigation carried out to evaluate the influence of Sporoscarcina pasteurii bacteria on the compressive strength and rapid chloride permeability of concrete made without and with fly ash. Cement was replaced with three percentages (10, 20 and 30) with fly ash by weight. Three different cell concentration (0, 10³,10⁵,10⁷ cells/ml) of bacteria were used in making the concrete mixes. Tests were performed for compressive strength, water absorption and rapid chloride permeability at the age of 28 days. Test results indicated that inclusion of S. pasteurii in fly ash concrete enhanced the compressive strength, reduced the porosity and permeability of fly ash concrete. Maximum increase (22%) in compressive strength and four-times reduction in water absorption was observed with 10⁵ cells/ml of bacteria. This improvement in compressive strength was due to deposition on the bacteria cell surfaces within the pores.Calcite deposition in concrete observed nearly eight times reduction in chloride permeability of fly ash concrete. The present work highlights the influence of bacteria on the properties of concrete made with supplementing cementing material such as like fly ash. Usage of bacteria like S. pasteurii improves strength and durability and strength of fly ash concrete through self-healing effect.     

Water absorption,permeability, and resistance to chloride-ion penetration of lightweight aggregate concrete

This paper presents an experimental study to evaluate effect of cumulative lightweight aggregate (LWA) content (including lightweight sand) in concrete [water/cement ratio (w/c) = 0.38] on its water absorption, water permeability, and resistance to chloride-ion penetration. Rapid chloride penetrability test (ASTM C 1202), rapid migration test (NT Build 492), and salt ponding test (AASHTO T 259) were conducted to evaluate the concrete resistance to chloride-ion penetration. The results were compared with those of a cement paste and a control normal weight aggregate concrete (NWAC) with the same w/c and a NWAC (w/c = 0.54) with 28-day compressive strength similar to some of the lightweight aggregate concrete (LWAC). Results indicate that although the total charge passed, migration coefficient, and diffusion coefficient of the LWAC were not significantly different from those of NWAC with the same w/c of 0.38, resistance of the LWAC to chloride penetration decreased with increase in the cumulative LWA content in the concretes. The water penetration depth under pressure and water sorptivity showed, in general, similar trends. The LWAC with only coarse LWA had similar water sorptivity, water permeability coefficient, and resistance to chloride-ion penetration compared to NWAC with similar w/c. The LWAC had lower water sorptivity, water permeability and higher resistance to chloride-ion penetration than the NWAC with similar 28-day strength but higher w/c. Both the NWAC and LWAC had lower sorptivity and higher resistance to chloride-ion penetration than the cement paste with similar w/c.     

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p_e, montmorillonite void ratio e_m and critical sand content α_s are introduced to determine the e_m-p_e relationship and finally the k-e_m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α_s, the e_m-p_e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α_s, the e_m-p_e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p_esα as the intersection. In addition, the e_m-k relationship also shows the linear trend when α ≤ α_s, and the slope of the line increases with the increase of the salt solution concentration. When α > α_s, the k-e_m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.     

Experimental Study on the Measurement of S-p Relations of LNAPL in a Porous Medium

A simple and effective device for measuring saturation-capillary pressure relation (S-p relation) in LNAPL-water system is designed with electrical conductivity probes, hydrophilic tensiometers and hydrophobic tensiometers. With this device, the S-p relation in LNAPL-water system is characterized. The S-p relation in air-LNAPL system is obtained by a scaling factor method. At a given saturation of wetting phase, it is found that the descending sequence of capillary pressures is in the order of air-water, LNAPL-water and air-LNAPL systems. The descending sequence of entry pressure and displacement pressure is also in the order of air-water, LNAPL-water and air-LNAPL systems. Furthermore, the relative permeability of air is larger than the LNAPL in their corresponding aqueous two phase systems, and the relative permeability of the LNAPL is slightly larger than water in their corresponding gaseous two phase systems.     

Effects of chemical precipitation on the permeability of geotextile envelopes for subsurface drainage systems in arid areas

This paper focuses on the effects of chemical precipitation on the permeability of geotextile envelopes for a subsurface drainage system in arid areas by conducting precipitation experiments of the geotextile in static or flowing solution. The results show that the precipitation process is not significantly promoted or inhibited by the network structure of geotextile. The precipitates in the form of rhombus wrap around the fiber surface. The number of geotextile pores with the smaller diameters decreases significantly after precipitation experiments. As the increase of the area density of precipitates (ΔR), the variation of the pore area (ΔS) and the variation of permeability coefficient (ΔK) of the geotextile decrease rapidly at first and then slowly. The ΔK and ΔS VS ΔR data were best fitted with logarithmic trend line. This study provides a preliminary reference for quantifying the chemical clogging process of geotextile envelopes in arid areas.     

Experimental study on the clogging effect of dredged fill surrounding the PVD under vacuum preloading

Clogging effect surrounding prefabricated vertical drains (PVDs) is a typical problem when vacuum preloading is applied to a dredged fill foundation. A large-scale model test was designed to clarify the cause and mechanism of the clogging effect, and the basic physical and mechanical parameters of the soil in the clogging zone were tracked during the test. The results demonstrated that a clogging zone was formed around the PVD in the early stage of improvement with conventional vacuum preloading, and the boundary of the clogging zone was approximately 0.2–0.4 of the boundary radius. The clogging zone surrounding the PVD was formed because of the overall movement of the soil toward the PVD under the high vacuum pressure gradient, rather than fine particle migration. The soil in the clogging zone exhibited permeability anisotropy and equivalent ‘smear’ effect. The permeability ratio (k_h/k_v) was less than 1, and the ratio of horizontal permeability coefficients at the test distances of 45 cm and 10 cm were 9.6 at a depth of 20 cm and 8.9 at a depth of 80 cm. An analysis of the microstructure of the soil in the clogging zone demonstrated that the clay particles tended to be vertically oriented. The re-orientation of the clay particles reduced the horizontal permeability coefficient and led to the permeability anisotropy of the soil in the clogging zone. Thus, decrease in the horizontal permeability coefficient and equivalent ‘smear’ effect of the soil in the clogging zone affect the consolidation of dredged fill, which leads to the clogging effect. The permeability anisotropy also slightly affects consolidation.     

Comparison of Klinkenberg-corrected gas permeability and water permeability in sedimentary rocks

We measured intrinsic permeability of sedimentary rocks from the western foothills of Taiwan by using nitrogen gas and distilled water as pore fluids in effective-pressure cycling tests at room temperature. The observed difference in gas and water permeabilities was analyzed in view of the Klinkenberg effect. This effect is due to the slip flow of gases at pore walls, which enhances gas flow when pore sizes are very small. Our experimental results showed that (1) gas permeability was larger than water permeability by several times to one order of magnitude, (2) gas permeability increased with increasing pore pressure, and (3) water permeability increased slightly as the pore-pressure gradient across the specimen increased. Results (1) and (2) can be explained quantitatively by an empirical power law in relation to the Klinkenberg constant b that is applicable in low permeable range. This correlation enables us to estimate water permeability from gas permeability. The Klinkenberg effect is important when permeability is lower than 10^?18 m² and at low pore-pressure differentials, and correction for the effect is essential to estimate water permeability from gas permeability measurement data. A simplified Bingham flow model for water can partially explain the trend of result (3), although non-Darcy flow behavior or inertial forces of water–rock interaction are needed to account for the observed deviation from Darcy's law.     

Laboratory Investigation on Electro-Osmosis Properties of Singapore Marine Clay

In order to be able to design the electro-osmosis scheme, two major properties of clay, electro-osmosis permeability and coefficient of consolidation, need to be known. These properties can be measured in the laboratory by running the electro-osmosis consolidation test. This type of test is carried out with the help of modified triaxial apparatus in which the soil sample has an electric current applied, and its volumetric change accurately measured. It was found that the electro-osmosis permeability (k_e) of Singapore marine clay ranges between 10^–8 and 10^–9 m/v-s, whereas the electro-osmosis coefficient of consolidation (c_ve) varied between 0.17 and 2.81 m²/yr for tests under an electric potential gradient between 2 to 12 volts; c_ve increased with voltage applied. Physical and compressibility properties changes were noted after the electro-osmosis process. Gain in effective stress due to electro-osmosis was significant and magnitude of effective stress gain was directly proportional to the voltage applied. However vertical displacement was found to be smaller and significant magnitude of volume change was contributed from lateral deformation. This paper describes the test apparatus, and procedures and discusses the results from electro-osmosis tests, carried out on Singapore Marine Clay.     

Proposal of a New Concept for the Vertical Drain Design Considering the Effects of Intermediate Permeable Layers and Smear

For more than 60 years, vertical drains have been employed to promote more rapid consolidation of relatively thick deposits of fine-grained soils. Although the design of vertical drains is based on Barron's or Hansbo's theory considering the clay layer to be homogeneous, some field data from the improved grounds using the vertical drains have sometimes shown that the advantage of modifying the commonly used equations due to the presence of permeable laminations in the clay. So there is a necessity of thorough study in the effects of intermediate permeable layers to have an optimum design, which will save time and money. This paper discusses the effects of the presence of intermediate permeable layers and smear using the consolidation equations developed by considering the balance equations regarding the movement of water in a small element and non-linear relationships in order to take the change of permeability and effective stress due to the change in void ratio into account. Finite difference analysis has been done for the combined effect of vertical and radial consolidation and the results are given with two defined parameters, such as; K=[k_s/k_c]•[H_s/H_c] and α₉₈=t_98(2D)/t_98(radial). A new design methodology and a sediment classification considering both permeability and thickness of different layers are proposed here with a set of graphs and a design example.     

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (V_S1) and perpendicular (V_S2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.     

Design and measurement of negative permeability metamaterials made from conductor-coated high-index dielectric inclusions

We investigate metal-dielectric inclusions that provide highly resonant permeability. These inclusions are made of high-index slabs with conductor plating on some faces. Microwave measurements are performed on several types of gratings made from these patterns. Broad band experiments are carried using a coaxial cell. Negative permeability levels lower than −20 have been observed in the GHz range, along with loss levels as low as 0.09 at μ′ = −1. We show that among the different theoretical approaches that are adequate to describe these materials, the field summation approach provides a very straightforward way to derive the effective permeability. The simple analytical expression of the permeability is in excellent agreement with the experiments. These inclusions are shown to be attractive both from the technological and the intrinsic performance point of view for metamaterial manufacturing. The effects of inclusion imperfections and finite thickness are discussed.     

Enhanced backscattering of electromagnetic waves from randomly rough gratings on negative magnetic metamaterials

We investigate the scattering formalism of the Green's theorem including a discontinuity in the magnetic permeability μ (isotropic and homogeneous). For this purpose we make use of the continuity or saltus conditions for the EM field across the interface between media having different ? and μ. Numerical analysis has been made for linearly polarized light and randomly rough media with negative μ on the phenomenon of backscattering enhancement. We obtain enhanced backscattering peaks for both polarizations (s and p) in the case of rough surfaces with large correlation length. On the other hand, in the case of subwavelength roughness, enhanced backscattering is also observed, but mediated by s-polarized surface-plasmon polaritons (negative μ) similar to that mediated by p-polarized surface-plasmon polaritons for conventional metallic media (negative ?).