Shrinkage and desaturation properties during desiccation of reconstituted cohesive soil

A vacuum evaporation method, proposed by the authors to reduce the water content more quickly than by air drying, was applied to six saturated reconstituted cohesive soil samples to investigate shrinkage and desaturation properties during desiccation. The test conditions were a vacuum pressure of p_v=?93.9 to ?97.5 kPa, a consolidation pressure of σ_v=68.6–392 kPa, an initial water content of w₀=0.59–0.92 w_L, and an initial surface area of the specimen of A_s0=20–205 cm², where w_L is the liquid limit. The results obtained for these restricted conditions are as follows. The vacuum evaporation of pore water from the soil occurs at a vacuum pressure higher than about ?93 kPa (|p_v|>93 kPa), but the evaporation process is very slow. The minimum void ratio, e_min, at the no-shrinkage phase of the soil subjected to the vacuum pressure, becomes a constant value. The relations e_min≈1.15 e_s and w_s≈87(e_min/G_s) are obtained, where e_s is the void ratio corresponding to the shrinkage limit, w_s, and G_s is the specific gravity of the soil particles. Using the vacuum evaporation method, the continuous relations for w?e, w?V/V₀, and w?S_r are more easily and more rapidly obtainable than with the conventional method by air drying. These three relations were formulated using two parameters, namely, an experimental parameter that is simply obtained using vacuum evaporation tests and a parameter that can be assumed and determined easily. The three formulated relations show a good agreement with the experimentally obtained results. Furthermore, if the basic physical parameter, w_s, has already been obtained, then the three relations can be estimated roughly without the performance of any tests.     

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.     

Effect of precipitation on the geological development of badlands in arid regions

The main objective of this paper was to study the effect of precipitation on the geological development of badlands in arid regions. Field observations and laboratory investigations were carried out to characterize the surface soils of the Avonlea badlands in southern Saskatchewan, Canada, which is currently experiencing a rapid development in infrastructure. Three distinct surface sediments were identified: an upper slope of cemented sandstone, a mid-slope on weathered mudrock and a relatively flat basal pediment. Each of these soils reacts differently to rainfall events. Precipitation results in grain size reduction which, together with the consistency limits, explains the water sensitivity of the soils; the fines increased by 16% in the cemented sandstone, 78% in the weathered mudrock and 33% in the basal pediment. Erosion susceptibility was highest in the basal pediment (2% clays) followed by cemented sandstone (cementation by 6% active clays) and then by weathered mudrock (cohesion due to the 40% clay).     

Study on swelling properties of an expansive soil deposit in Saskatchewan, Canada

The main objective of this paper was to study the swelling properties of Regina clay. The deposit comprises a topsoil (surface to 0.3 m depth), an expansive clay (0.3–8 m), and a bottom till (8–9 m). High liquid limit (70 ± 15 %) and plastic limit (33 ± 4 %) indicated high water retention and adsorption capacity for the clay. Irrespective of the cover type (vegetation and cracked road), the field water content in summer closely matched the plastic limit. The clay was characterized by medium-to-high swelling that was best predicted by SP = 0.16 (I _p)^1.188. The soil had 51 % clay minerals including smectite (32 %), illite (7 %), kaolinite (5 %), and chlorite (3 %). With a CEC of 40 cmol(+)/kg, Ca²⁺ and Mg²⁺ were found to be the main exchangeable cations: the specific surface area was 50 m²/g. The SP and P _s for a 1.2 m deep sample measured 12 % and 260 kPa, respectively, and the estimated surface heave of 180 mm gradually diminished to 3.6 m depth. These values matched well with consistency-based correlations.     

The source of hydrogen sulfide in anoxic sediment

Putrefactive hydrogen sulfide production in the upper 4 cm of sediment in two small freshwater and eutrophic Southeast Michigan, U.S.A., lakes ranged from 0.13 to 1.51 with an average of 0.46 mg S l^?1 day^?1. Sulfate reductive production of hydrogen sulfide at the same sites ranged from 0.7 to 3.2 with an average of 1.54 mg S l^?1 day^?1. Putrefactive hydrogen sulfide production represented 5.1–53% (average of 18.3–27.6%) of the total hydrogen sulfide produced at the two lakes over an April–October study period. ³⁵S labeled substrates were used to estimate hydrogen sulfide production rates.Proteolytic bacteria averaged 2.2 × 10⁴ cells ml^?1 sediment whereas sulfate reducers averaged 4.8 × 10² cells ml^?1 sediment. Putrefactive hydrogen sulfide production correlated highly with numbers of proteolytic bacteria (r² = 0.84) but the correlation between sulfate reduction and sulfate reducing bacteria was low (r² = 0.13).Interstitial soluble inorganic sulfate, protein and organic carbon were not closely correlated with hydrogen sulfide production rates or bacteria enumeration results. Natural substrate concentrations (S_n) used to estimate hydrogen sulfide production were supported by kinetic bioassay results.     

Gas transport parameters of differently compacted granulated bentonite mixtures (GBMs) under air-dried conditions

Granulated bentonite mixtures (GBMs) have been regarded as effective buffer materials in the deep geological disposal of radioactive waste due to their operational advantages, such as ease of transportation and in-situ placement/backfilling. Many studies have been done to characterize the hydraulic and thermal properties of GBMs as well as their swelling properties. Only limited studies, however, have investigated their gas transport properties, even though these properties affect their compactness during in-situ placement/backfilling and subsequent gas diffusion and advection in the buffer zone. The aim of this study is to understand the gas transport parameters, i.e., air permeability (k_a) and gas diffusivity (D_p/D_o), of tested samples compacted at different dry densities (DDs) under air-dried conditions, linking them with the measured density distribution characteristics determined by microfocus X-ray computed tomography (MFXCT) analysis. Two types of GBMs were used in this study: 1) FE-GBM (prepared from National Standard® bentonite, Wyoming, USA): this material was used in the Full-scale Emplacement (FE) experiment at the Mont Terri rock laboratory, Switzerland) and 2) OK-GBM (prepared from a bentonite, originating from Japan, with the trade name of OK bentonite, Kunimine Industries). The tested samples were firstly packed in a 100-cm³ acrylic core with different DDs, ranging from loose to dense (1.09 to 1.75 g/cm³), and scanned by MFXCT. The weighting factors, w_f (fine fraction; lower density) and w_c (coarse fraction; higher density) (w_f + w_c = 1), were determined after the peak separation of the measured CT brightness histograms from the reconstructed three-dimensional multiplanar reconstruction (MPR) images of the MFXCT analysis. The measured k_a and D_p/D_o were highly dependent on the DDs, the k_a (?) values fitted well with a power law model, and the D_p/D_o (?) was predicted accurately by several previously proposed models. For both FE-GBM and OK-GBM, there were good linear relationships between the gas transport parameters and w_c × DD, implying that the weight of the coarse fraction controlled k_a and D_p/D_o. Moreover, the Kozeny-Carman model, incorporating the measured volumetric surfaces from the MFXCT analysis, was able to predict the k_a values well.     

A decade’s (2014–2024) perspective on cassava’s (Manihot esculenta Crantz) contribution to the global hydrogen cyanide load in the environment

In recent years, developing countries have increased their cassava (Manihot esculenta) production for food security. Cassava contains cyanogen glycosides, mainly as linamarin, which through bio-catalysis, i.e. enzyme hydrolysis, results in hydrogen cyanide (HCN). HCN is released into the environment through numerous ways with subsequent volatilisation. Thus, the HCN released during the period 2002–2013 was estimated between 0.025?×?10^?3 to 6.71 ppq (African), 0.012?×?10^?3 to 1.01 ppq (Asian) and 0.007?×?10^?3 to 0.920?×?10^?3 ppq (South American). Furthermore, a decade’s (2014–2024) projection of HCN volatilisation displays increases of 60.5% (Africa), 57.7% (Asia) and 50.5% (South America) when compared with the current production. Furthermore, gas released during cassava plants’ growth, i.e. HCN, NH₃, and NO₂, was quantified in healthy plants. Varying concentrations of HCN were released. These further indicated the presence of a pseudo-halogenic gas in the environment – a contributor to climate change.     

Significant OH production under surface cleaning and air cleaning conditions: Impact on indoor air quality

We report measurements of hydroxyl (OH) and hydroperoxy (HO₂) radicals made by laser‐induced fluorescence spectroscopy in a computer classroom (i) in the absence of indoor activities (ii) during desk cleaning with a limonene‐containing cleaner (iii) during operation of a commercially available “air cleaning” device. In the unmanipulated environment, the one‐minute averaged OH concentration remained close to or below the limit of detection (6.5×10⁵ molecule cm^?3), whilst that of HO₂ was 1.3×10⁷ molecule cm^?3. These concentrations increased to ~4×10⁶ and 4×10⁸ molecule cm^?3, respectively during desk cleaning. During operation of the air cleaning device, OH and HO₂ concentrations reached ~2×10⁷ and ~6×10⁸ molecule cm^?3 respectively. The potential of these OH concentrations to initiate chemical processing is explored using a detailed chemical model for indoor air (the INDCM). The model can reproduce the measured OH and HO₂ concentrations to within 50% and often within a few % and demonstrates that the resulting secondary chemistry varies with the cleaning activity. Whilst terpene reaction products dominate the product composition following surface cleaning, those from aromatics and other VOCs are much more important during the use of the air cleaning device.     

Matrix versus fracture permeability in a regional sandstone aquifer (Wajid sandstone,SW Saudi Arabia)

Sandstones are often characterized as fractured aquifers. We present a case study of the Wajid sandstone, which forms a regional aquifer system in SW Saudi Arabia, where matrix, fracture, and large-scale hydraulic conductivities are coincident. The measurements deal with different scales and methods and are based on porosity and permeability measurements in the laboratory, as well as pumping tests in the field. Porosities of the sandstone samples in general are high and range between less than 5?% and more than 45?%. Gas permeabilities for strongly cemented samples are <?1 mD, whereas most samples range in between 500 and 5,000 mD. There is only a weak anisotropy with preference of the horizontal x-, y-directions. Hydraulic conductivities of the matrix samples (5.5?·?10^?6 m/s and 1.1?·?10^?5 m/s for the Upper and Lower Wajid sandstone, respectively) were in the same order of magnitude compared to hydraulic conductivities derived from pumping tests (8.3?·?10^?5 m/s and 2.2?·?10^?5 m/s for the Upper and Lower Wajid sandstone, respectively).     

Effect of Wind on Smoldering Combustion Limits of Moist Pine Needle Beds

We studied moist pine needle beds burning under the effect of wind, in order to determine the upper moisture limit for which there is fire propagation for different wind velocities. For this purpose we built a wind tunnel that allowed us to burn a 600 mm by 150 mm by 40 mm bed under wind velocities between 0.5 m/s and 5.0 m/s and controlled air temperature. Results show an increase in moisture limit from 54% to approximately 140%, for the velocity range indicated. Combustion at limiting conditions proceeds mainly by smoldering with some periods of flaming combustion. It was observed that, for conditions close to extinction, the smoldering front is not quenched at the surface. Additionally, it was also observed that a strong flow of hot gases exit from the fuel bed at the free surface. These two observations lead to the conclusion that the main heat sink is moisture evaporation and that heat losses to the surroundings is reduced by the blowing effect of the hot gases coming off the bed. A dimensional analysis suggests a correlation between moisture limit and wind velocity of the form M = A ? B/ \( {v} \) _w ² , where M is moisture limit for fire propagation, A and B are constants, and \( {v} \) _w is wind speed. Two dimensionless numbers helped to plot the smoldering temperature and fire propagation velocity in a more meaningful way. They are \( \Uppi_{1} = {{T_{sml} c_{p,g} } \mathord{\left/ {\vphantom {{T_{sml} c_{p,g} } {v_{w}^{2} }}} \right. \kern-0pt} {v_{w}^{2} }} \) and \( \Uppi_{2} = {{Mh_{fg} } \mathord{\left/ {\vphantom {{Mh_{fg} } {v_{w}^{2} }}} \right. \kern-0pt} {v_{w}^{2} }} \) , where T _sml is smoldering temperature, c _p,g is the gas specific heat, M is fuel moisture content and h _fg is the latent heat of water evaporation. A relatively high moisture limit at 5 m/s wind velocity is possible due to the relatively high air flow into the smoldering front and the efficient heat feedback produced in forward smoldering.     

Self-healing of circular and slit defects in GCLs upon hydration from silty sand under applied stress

The self-healing of a GCL with artificial defects (circular holes and rectangular slits, both with and without the carrier geotextile preserved below the holes) upon hydration on a Godfrey silty sand (GSS) subgrade with w_fdn = 5, 10 and 16% under 2–100 kPa is examined. Circular holes with the carrier geotextile missing below holes with diameters up to 25.4 mm self-healed on the w_fdn = 5% and 10% GSS but not on 16% GSS, while none self-healed when carrier geotextile was preserved below the holes. When DI water was introduced to the surface under 100 kPa, circular holes with diameter up to 38.1 mm self-healed. Neither the single 15 mm-wide slit nor double 15 mm-wide parallel slits with 20 mm-wide strip of undamaged GCL between them resting on w_fdn = 10% GSS under 20 kPa fully self-healed. The introduction of simulated synthetic landfill leachate (SSL) to the GCL surface under 70 kPa did not result in self-healing. Post-hydration k tests found that GCL without a carrier geotextile below a hole up to 25.4 mm in diameter would not have a significant adverse effect on the hydraulic conductivity compared with an intact GCL provided the permeant was tap water rather than SSL.     

Effect of ammonium on the hydraulic conductivity of geosynthetic clay liners

Hydraulic conductivity and swell index tests were conducted on a conventional geosynthetic clay liner (GCL) containing sodium-bentonite (Na-B) using 5, 50, 100, 500, and 1000 mM ammonium acetate (NH₄OAc) solutions to investigate how NH₄⁺ accumulation in leachates in bioreactor and recirculation landfills may affect GCLs. Control tests were conducted with deionized (DI) water. Swell index of the Na-B was 27.7 mL/2 g in 5 mM NH₄⁺ solution and decreased to 5.0 mL/2 g in 1000 mM NH₄⁺ solution, whereas the swell index of Na-B in DI water was 28.0 mL/2 g. Hydraulic conductivity of the Na-B GCL to 5, 50, and 100 mM NH₄⁺ was low, ranging from 1.6–5.9 × 10^?11 m/s, which is comparable to the hydraulic conductivity to DI water (2.1 × 10^?11 m/s). Hydraulic conductivities of the Na-B GCL permeated with 500 and 1000 mM NH₄⁺ solutions were much higher (e.g., 1.6–5.2 × 10^?6 m/s) due to suppression of osmotic swelling. NH₄⁺ replaced native Na⁺, K⁺, Ca²⁺, and Mg²⁺ in the exchange complex of the Na-B during permeation with all NH₄⁺ solutions, with the NH₄⁺ fraction in the exchange complex increasing from 0.24 to 0.83 as the NH₄⁺ concentration increased from 5 to 1000 mM. A Na-B GCL specimen permeated with 1000 mM NH₄⁺ solution to chemical equilibrium was subsequently permeated with DI water. Permeation with the NH₄⁺ converted the Na-B to “NH₄-bentonite” with more than 80% of the exchange complex occupied by NH₄⁺. Hydraulic conductivity of this GCL specimen decreased from 5.9 × 10^?6 m/s to 2.9 × 10^?11 m/s during permeation with DI water, indicating that “NH₄-bentonite” can swell and have low hydraulic conductivity, and that the impact of more concentrated NH₄⁺ solutions on swelling and hydraulic conductivity is reversible.     

Dégradation des eaux de rejets issues d’une raffinerie d’huile de coton par plasma d’arc électrique rampant à pression atmosphérique

Wastewater samples from liquid effluents of cotton oil refinery in Far-Northern Cameroon were exposed to a gliding arc discharge for various exposure times in a cold plasma reactor. The analysed results showed high levels of organic pollution parameters (TOC = 4635 mg/L, COD = 6943, 98 mg/L) before treatment. More than 56% TOC and 75% COD abatement were obtained after 30 min treatment in batch conditions with a laboratory reactor. The resultant pollution abatement is attributed both to strong oxidizing effects of °OH and NO° radicals formed in the plasma and their derivatives (H₂O₂, ONO₂H and NO₃H). The conductivity and TDS increase linearly and the pH, TOC and COD decrease with increasing exposure time to the discharge. The oxidation obeys a pseudo first-order kinetics law (kinetic rate: k_1DCO = 0, 0381 min^?1 and k_1COT = 0, 0245 min^?1).     

Influence of pore water chemistry on GCL self-healing with hydration from silica sand

The self-healing of a GCL with a circular hole is examined in experiments where the GCL, overlain by geomembrane, is hydrated from a silica sand subgrade (SSS) having three different pore water chemistries. Factors considered included: hole size, subgrade initial moisture content w_fdn, GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for w_fdn = 16% than for w_fdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I < 0.1 mM). However, only the 14.3 mm-diameter hole fully self-healed and only when w_fdn = 16%. In contrast, when the GCL is hydrated from SSS with pore water having an ionic strength, I, of 20 and 30 mM, the self-healing for w_fdn = 5% is better than for w_fdn = 10%, which is better than for w_fdn = 16%, although none of the holes self-healed. When a ~0.5 m hydraulic head was applied above the GCL under σ_v = 20–100 kPa, a 38.1 mm-diameter hole self-healed with water having I < 0.1 mM, a 25.4 mm-diameter hole self-healed with pore water with I = 20 mM and 30 mM, but none self-healed with simulated synthetic landfill leachate (SSL). Post-hydration hydraulic conductivity (k) tests with SSL suggest that a hole up to 14.3 mm-diameter would not pose a significant adverse impact on the k compared to an intact GCL; however, this is not the case for the larger holes tested.     

Assessment of gaseous emissions and radiative forcing in Indian forest fires

This paper reports a study of the gaseous emissions from Indian forest fires from 2005 to 2016 and their potential impact on radiative forcing. Initially, forest burned area is quantified using MODIS-MCD45A1 data. Results showed that annual burned area of the study period ranges from 8439 km² to 25,442 km² and the maximum forest area is burned during February, March, and April in any year. Gaseous emissions are estimated using emission factors, the mass of fuel available for combustion, combustion factor, and burned area. CO₂, CO, and CH₄ are the major emissions during forest fires with an annual average of 105 × 10⁶ tonnes, 6 × 10⁶ tonnes, and 3.25 × 10⁵ tonnes, respectively. The average radiative forcing (RF) for CO₂, CH_4, and N₂O is estimated as 1.8 Wm^?2, 0.49 Wm^?2, and 0.177 Wm^?2, respectively. An important finding in the present study is the recurrence of forest fires during the pre-monsoon season.     

An Approach for Assessment of Compaction Curves of Fine Grained Soils at Various Energies Using a One Point Test

Compaction curves of soils are essential for establishing practical and reliable criteria for an effective control of field compaction. This paper deals with the development of a practical method of assessing laboratory compaction curves of fine-grained soils. It is found that for a given fine-grained soil compacted at a particular compaction energy, the relationships between water content (w) and degree of saturation (S) are represented by power function, which are w=A_dS^B_d and w=A_wS^B_w for the dry and the wet sides of optimum, respectively (where A_d, A_w, B_d and B_w are constant). The B_d and B_w values and optimum degree of saturation (ODS) are mainly dependent upon soil type irrespective of compaction energy. The A_d and A_w values decrease with the logarithm of compaction energy and the decrease rates are practically the same for any compacted fine-grained soil. This leads to a simple and rational method to assess the compaction curve wherein the compaction energy varies over a wide range using a one point test (a single test). Assuming that fine-grained soils compacted under standard Proctor energy behave in agreement with Ohio's curves, the modified Ohio's curves for the other three compaction energy levels (296.3, 1346.6 and 2693.3 kJ/m³) are developed based on the proposed method. These curves can be used to assess the entire compaction curves at the required compaction energy based on a single set data of dry unit weight and water content.     

Measurements of Unsaturated Hydraulic Conductivity Functions of Two Fine-Grained Materials

Water unsaturated hydraulic conductivity (k_w) functions of two remolded fine-grained materials were measured over a wide range of degrees of saturation (S_r) with two methods. The instantaneous profile method (IPM) was used for S_r>50%. An original vapor equilibration method (also known as the vapor equilibrium technique, VET) was used for S_r<50%. Both materials compacted at the standard Proctor optimum water content and maximum density, have saturated hydraulic conductivities (k_sat)<10^-7 m/s. The VET couples the total soil suction (s) control from desiccators with saturated salt solutions with water mass measurements from a digital laboratory balance. The k_w measurements of the two techniques are consistent and complementary. The effect of hysteresis on the k_w functions at higher s values was also investigated. The experimental results suggest that the hysteretic effect on the k_w-S_r and k_w-s relationships cannot be neglected, and that the measured k_w are significantly dependent on the initial S_r. The VET tests on the specimens that were initially dried give the lowest values of k_w and the tests on the specimens that were initially saturated give the highest values of k_w. The relative hydraulic conductivity values are very small (k_rw<2×10^-5) in this saturation range (S_r<50%) for the tested materials.     

Atmospheric contamination of archaeological monuments in the Agra region (India)

An analysis of water-soluble samples collected from marble and sandstone of monuments for different ions have been done. The combustion, manufacturing and other polluting operations existing within Agra area have been investigated. The measurements of flue gases amounting to 3.63 × 10⁹ S.C.F. indicate atmospheric contamination and deterioration of archaeological monuments of Agra. It has been found that the principal sources of air contamination are the 325 iron foundries and 3 railway shunting yards located within 0.3 to 3.0 Km. of the main monuments. The topographical and micrometeorological conditions of the city have tended to favour and aggravate the concentration of effluents in the surrounding air of the monuments. The annual average existing level of SO₂ ranges from 16 to 20 micrograms/m³. The seasonal distribution of SO₂ and suspended particulate matter in the air at Taj Mahal, Red Fort and Sikandra have been discussed and illustrated. It has been observed that there is substantial sulphur dioxide contamination existing at Agra. The maximum concentration of SO₄^2? and NO₃^? amounting 0.46 and 0.38 respectively by weight percentage found existing at Red Fort cause efflorescences of sandstone.     

Investigation of block foundations resting on soil–rock and rock–rock media under coupled vibrations

In the present study,the dynamic response of block foundations of different equivalent radius to mass(R_o/m) ratios under coupled vibrations is investigated for various homogeneous and layered systems.The frequency-dependent stiffness and damping of foundation resting on homogeneous soils and rocks are determined using the half-space theory.The dynamic response characteristics of foundation resting on the layered system considering rock-rock combination are evaluated using finite element program with transmitting boundaries.Frequencies versus amplitude responses of block foundation are obtained for both translational and rotational motion.A new methodology is proposed for determination of dynamic response of block foundations resting on soil-rock and weathered rock-rock system in the form of equations and graphs.The variations of dimensionless natural frequency and dimensionless resonant amplitude with shear wave velocity ratio are investigated for different thicknesses of top soil/weathered rock layer.The dynamic behaviors of block foundations are also analyzed for different rock-rock systems by considering sandstone,shale and limestone underlain by basalt.The variations of stiffness,damping and amplitudes of block foundations with frequency are shown in this study for various rock—rock combinations.In the analysis,two resonant peaks are observed at two different frequencies for both translational and rotational motion.It is observed that the dimensionless resonant amplitudes decrease and natural frequencies increase with increase in shear wave velocity ratio.Finally,the parametric study is performed for block foundations with dimensions of 4 m × 3 m × 2 m and 8m×5m×2m by using generalized graphs.The variations of natural frequency and peak displacement amplitude are also studied for different top layer thicknesses and eccentric moments.     

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.