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.     

The mechanical behaviour of synthetic sandstone with varying brittle cement content

The purpose of this work was to investigate the influence of cement on the mechanical behaviour of granular rocks. Following the technique described in den Brok et al. [den Brok, S. W. J., David, C. and Bernabé, Y., Preparation of synthetic sandstones with variable cementation for studying the physical properties of granular rocks. C. R. Acad. Sci., 1997, 325, 487–492], two blocks of synthetic sandstones with different cement content were prepared for mechanical testing under hydrostatic and triaxial conditions. The results of the mechanical tests show that the behaviour of the synthetic rocks compares well with that of natural sandstones. Increasing the amount of cement from 3 to 5% in volume had important consequences on the mechanical properties: the critical pressure, strength and elastic moduli were significantly increased and the brittle-to-ductile transition was shifted towards higher pressures. We compared our results to the models of Zhang et al. [Zhang, J., Wong, T. -F. and Davis, D. M., Micromechanics of pressure-induced grain crushing in porous rocks. J. Geophys. Res., 1990, 95, 341–352] and Wong et al. [Wong, T. -F., David, C. and Zhu, W., The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation. J. Geophys. Res., 1997, 102, 3009–3025]. We conclude that Zhang et al.'s microstructural parameter φD (i.e. the product of porosity φ by grain size D) appeared to be a scaling parameter for both the failure envelopes and the critical pressure as defined in these models. Intuitively, the contact length L is expected to play a crucial role in the mechanical properties of granular materials. Accordingly, we made a statistical analysis of this microstructural parameter in our synthetic materials and in a suite of natural sandstones. A positive correlation with Young's modulus and a negative correlation with porosity were found. This last result gives a physical background for the use of parameter (φD) in theoretical models. We want to emphasize that working on synthetic sandstones allows for a better control of the structural parameters (grain size, sorting, cement content, etc.) which appear to be so important for the mechanical properties of granular rocks.     

Effect of temperature and pressure on the thermal conductivity of sandstone

Effective thermal conductivity (ETC) of dry sandstone was measured over a temperature range from 275 to 523 K and at pressures up to 400 MPa with a guarded parallel-plate apparatus. The estimated uncertainty of the ETC measurements is 2%. The porosity of the sample was 13%. A rapid increase of ETC was found for dry sandstone at low pressures between 0.1 and 100 MPa along various isotherms. At high-pressure range (P>100 MPa) a weak linear dependence of the ETC with pressure was observed. The pressure effect is negligibly small after first 80–100 MPa where bridging of microcracks or improvement of grain contacts takes place. We interpreted the measured ETC data using a various theoretical and semi-empirical models in order to check their accuracy and predictive capability. The effect of structure (size, shape, and distribution of the pores), porosity, and mineralogical composition on temperature and pressure dependences of the ETC of sandstone was discussed. To estimate the effect of temperature and pressure on the ETC of sandstone the pressure, β_P, and temperature, β_T, coefficients of ETC were calculated from the measured values of ETC. The measured values of the ETC were also used to calculate the values of the isothermal compressibility, χ_T, and thermal expansion coefficient, α. The equation of state of sandstone was developed using the measured ETC data.     

A simple model for stress-induced anisotropic softening of weak sandstones

Weak sandstones possess deformational behavior different from hard rocks; these phenomena include shear dilation and softening of the deformational moduli. It has been found previously that under hydrostatic loading, the bulk modulus increases as confining pressure arises; and that under shear loading, the weak sandstone may transform from its original isotropy to a stress-induced anisotropic material, and the deformational modulus can accordingly be softened as well. These phenomena contribute to the increase of crown settlements during tunnel excavations, and account for several cases of tunnel squeezing. Consequently, a model capable of simulating major deformational characteristics of weak sandstones is needed for engineering purposes. A simple yet innovative constitutive model is accordingly proposed. This proposed model is characterized during the simulation as having: (1) non-linear volumetric deformation under hydrostatic loading; (2) significant shear dilation prior to the failure state; (3) isotropic stiffening of deformational moduli under hydrostatic loading; and (4) anisotropic softening of deformational moduli under shearing condition.The proposed model was formulated based on the linear elastic model, and it accounts for the variations of moduli E and G through different loading conditions. It was found that the proposed model is able to closely simulate the actual deformational characteristics of weak sandstones. In addition, the proposed model only needs six material parameters, and all these parameters can be easily obtained from experiments. This model was then incorporated into a finite element program and was used to analyze a squeezing tunnel case.     

An equivalent plasticity theory for jointed rock masses

A non-representative volume element (NRVE) approach to equivalent rock mass properties shows that the form of the elastic–plastic constitutive equations is the same for homogeneous material elements and multiple-material elements, subsequently homogenized. Thus the average stress and strain increments in an arbitrary jointed rock mass volume are related by {dσ}=([C*_ep]){dε} where σ is effective stress. The equivalent elastic-plastic properties matrix [C*_ep] is the sum of an equivalent elastic moduli matrix [C^*] and a plastic ‘correction’ matrix [C*_p, as usual. However, there are no equivalent plastic potentials Y^* or yield functions, failure criteria F^* or strengths. The equivalent elastic-plastic properties are constructed from the elastic moduli and strengths of the rock mass joints, the intact rock between and strain influence functions that relate local to overall average strains. Numerical examples that simulate laboratory-like tests on jointed rock cubes illustrate the approach.     

Characterizing the deformation behavior of Tertiary sandstones

Tertiary sandstones possess deformational behavior different from hard rocks, especially the relatively larger amount of volumetric dilation during shearing. Such excess dilation contributes to the increase of crown settlement during tunnel excavation and accounts for several cases of tunnel squeezing within Tertiary sandstones. Therefore, the deformation behavior of Tertiary sandstones sampled from more than 13 formations was studied. To distinguish the volumetric deformation induced by hydrostatic stress or by shear stress as well as to decompose the elastic and the plastic components of strains, special experimental techniques, including pure shear tests and cycles of loading–unloading were applied.The experimental results reveal that the deformation of Tertiary sandstone exhibits the following characteristics: (1) significant amount of shear dilation, especially elastic shear dilation; (2) non-linear elastic and plastic deformation; (3) plastic deformation occurs prior to the failure state. Furthermore, features of plastic deformation were inferred from experimental results and, as a result, the geometry of plastic potential surface and the hardening rule were accordingly determined. A constitutive model, involving nonlinear elastic/plastic deformation and volumetric deformation induced by shear stress, is proposed. This proposed model simulates the deformational behavior for the shear-dilation-typed rocks reasonably well. Furthermore, tests on the versatility of the proposed model, including varying hydrostatic stress and stress paths, indicate that the proposed model is capable of predicting deformational behavior for various conditions.     

The genetic mechanism of a translational landslide

Translational landslides are common in China, particularly in the interbedded mudstone sandstone sequence of the Three Gorges reservoir area and the Sichuan basin. The paper discusses the genetic mechanism and limit equilibrium criterion for single and multi-translational landslides based on physical model tests and numerical analysis. It focuses particularly on the generation of hydrostatic pressure and uplift pressure under intense rainfall conditions and the corresponding deformation and failure processes. A model was constructed based on the actual conditions at the 17 million m³ Fengdian Kualiangdzi landslide and the 25 million m³ Tiantai village landslide. It revealed a clear relationship between the critical water head value and the dip angle of the sliding plane.      

Quantitative three-dimensional description of a rough surface and parameter evolution with shearing

The choice of a general criterion to determine the shear strength of rough rock joints is a topic that has been investigated for many years. The major problem is how to measure and then to express the roughness with a number (e.g., joint roughness coefficient) or a mathematical expression in order to introduce the morphology of the joint into a shear strength criterion. In the present research a large number of surfaces have been digitised and reconstructed using a triangulation algorithm. This approach results in a discretisation of the joint surface into a finite number of triangles, whose geometric orientations have been calculated. Furthermore, during shear tests it was observed that the common characteristic among all the contact areas is that they are located in the steepest zones facing the shear direction. Based on this observations and using the triangulated surface data, it is possible to describe the variation of the potential contact area versus the apparent dip angle with the expression A_θ^*=A₀[(θ_max^*−θ^*)/θ_max^*]^C, where A₀ is the maximum possible contact area, θ_max^* is the maximum apparent dip angle in the shear direction, and C is a “roughness” parameter, calculated using a best-fit regression function, which characterises the distribution of the apparent dip angles over the surface. The close agreement between analytical curves and measured data therefore suggests the possibility of defining the influence of roughness on shear strength by the simple knowledge of A₀, C and θ_max^*. Based on the samples studied here, the values of these parameters capture the evolution of the surface during shearing. Moreover, they tend to be characteristic for specific rock types, indicating that it might be possible to determine ranges for each rock type based on laboratory measurements on representative samples.     

Time-dependent deformation behaviors of weak sandstones

Time-dependent deformation behavior of rocks has a significant impact on the stability of rock slopes or underground constructions. This paper presents systematic experimental data regarding time-dependent deformation of a typical weak sandstone, known as the Mushan sandstone. The observed deformations are further separated to distinguish elastic and viscoplastic behaviors of the weak sandstone through the use of multi-staged loading–creep-unloading–reloading tests. The stress path is designed to be a purely hydrostatic loading followed by a pure shear, so that the deformations induced by these two types of stresses can be distinguished.For elastic behavior, although the nonlinear stress–strain relations vary according to the applied hydrostatic stress, these stress–strain relations can be normalized by the applied hydrostatic stress or the bulk modulus and converted into a single consistent stress–strain curve. Inelastic behavior is then obtained by subtracting the elastic deformation from the total deformation. As a result, the characteristics of the viscoplastic deformation are that: (1) the direction of the viscoplastic flow is time independent, and (2) it has a similar direction to the conventionally defined plastic flow. As such, the viscoplastic potential has a similar shape to the plastic potential, but the size of the former changes with time, while the latter has a size that is time independent. Meanwhile, through the calculation of irreversible work, direct evidence of orthogonality between the yield surface and the plastic flow, as well as the viscoplastic flow, is observed. Thus, it is reasonable to assert that the yield surface, the plastic potential, and the viscoplastic potential all have the same geometry. Consequently, the associated flow rules are applicable to modeling the time-dependent deformational behavior of weak sandstones.     

Frictional effects on the volumetric strain of sandstone

The contributions of frictional slip on the nonlinear, hysteretic deformation of sandstone in the reversible regime (i.e., prior to the onset of permanent deformation) for uniaxial strain compression are investigated through an analysis of a Hertz–Mindlin face-centered cubic sphere pack model and laboratory stress–strain tests on Berea sandstone. The analysis demonstrates that the dynamic¹ moduli are path-independent functions of the strain. The analysis also reveals that for uniaxial strain consolidation it is possible to decompose the volumetric strain into a path-independent contribution from nonlinear grain contact deformation and a path-dependent contribution from frictional (slip) compaction. Laboratory stress–strain measurements on Berea sandstone support these findings and, in addition, reveal that frictional compaction accounts for a significant portion of the volumetric strain of Berea sandstone.     

The effects of depressed pH on flagfish reproduction, growth and survival

Breeding communities of flagfish, Jordanella floridae, were exposed to northern Ontario lake water (hardness 28 mg l⁻¹ CaCo₃) adjusted to depressed pH levels of 6.0, 5.5, 5.0 and 4.5. Control water (pH 6.8) received no acid treatment. Egg production, egg fertility and fry growth was impaired (P < 0.05) at all exposure levels. Flagfish fry survival was reduced (P < 0.05) at pH 5.5 and 5.0 and no fry survived at pH 4.5. Variability of hatching in all treatments precluded any identifiable hatching response to depressed pH. Reduction in the reproductive processes monitored indicated the following order of sensitivity: egg production > fry survival > fry growth > egg fertility.Results of this study coincide with reproductive investigations on brook trout and fathead minnows indicating the “no effect” level of pH depression for successful reproduction to be pH 6.5.     

Shaft resistance of a pile embedded in rock

A rational calculation procedure is proposed for establishing the shaft resistance of a pile embedded in rock, based upon the Hoek and Brown failure model. The state of the art of the calculation of the pile shaft resistance is analysed. Nearly all the recommendations that have appeared in the technical literature, for calculating the ultimate shear strength of a shaft embedded in rock (τ_ult) propose that τ_ult=ασ_c^k(σ_c;τ_ulten MN/m²) where the coefficient α, considered as a constant dimensional value, ranges from 0.1 to 0.8, if the unconfined compressive strength (σ_c) is expressed in MN/m². In most cases, the exponent k is 0.5.A comparison is made between the results yielded and the different empirical theories that have been put forward with respect to this shaft resistance. It can generally be stated that the results obtained with this theory are reasonable for long and deeply socketed piles (high confining pressures) but the results are on the safe side in some cases where short piles (low confining pressures) are involved.This paper is a continuation of the works developed by the same authors with piles working at the tip, socketed in rock.     

Trace metals: Cd, Cu, Pb and Zn in gelatinous macroplankton from the Northwestern Mediterranean

Gelatinous macroplankton organisms were collected in May 1984 in Villefranche-sur-Mer Bay and analysed for cadmium, copper, lead and zinc. Analyses were carried out by polarography for Cd, Cu and Pb and by flame atomic absorption for Zn. Phosphorus was also measured in the samples as a biomass parameter due to difficulties inherent in measuring dry weight of gelatinous organisms. The samples belong to the Tunicates, the Cnidarians (Hydromedusae, Siphonophores and Scyphomedusae), the Ctenophores and the Molluscs. Crustaceans living on some Tunicates were also sampled.As regards cadmium, copper and lead, mean concentrations did not show significant differences among the phyla studied: especially for Tunicates with mean values of 0.1 ng Cd μg P⁻¹, 2.0 ng Cu μg P⁻¹ and 0.9 ng Pb μg P⁻¹ and for Cnidarians with mean values of 0.5 ng Cd μg P⁻¹, 2.0 ng Cu μg P⁻¹ and 0.9 ng Pb μg P⁻¹ and for Cnidarians with mean values of 0.5 ng Cd μg P⁻¹, 2.0 ng Cu μg P⁻¹, 1.0 ng Pb μg P⁻¹. On the other hand, mean zinc concentrations were significantly lower in Tunicates (7.9 ng Zn μg P⁻¹) than in Cnidarians (36.8 ng Zn μg P⁻¹).Zinc seems to be preferentially concentrated in organisms which are rich in collagen, constituting the mesoglea, such as the Cnidarians, the Ctenophore and the gelatinous Mollusc studied, rather than in organisms rich in tunicin such as the Tunicates.     

Acute toxicity and behavioral responses of coho salmon (Oncorhynchus kisutch) and shiner perch (Cymatogaster aggregata) to chlorine in heated sea-water

The acute toxicity and behavioral response to chlorinated and heated sea-water was determined for coho salmon smolts and 1–3 month old shiner perch. LC₅₀'s were determined for 7.5, 15, 30 and 60 min exposure times; 13, 16 and 20°C (Δt = 0, 3, 7°C) temperatures and total residual oxidant (TRO) concentrations ranging from 0.077 to 1.035 mg l⁻¹. The mean 60 min LC₅₀ for shiner perch was significantly reduced (P ≤ 0.05) from 308 μg l⁻¹ TRO at 13°C to 230 μg l⁻¹ TRO at 20°C. The 60 min LC₅₀ for coho salmon decreased from 208 μg l⁻¹ TRO at 13°C to 130 μg l⁻¹ at 20°C. The LC₅₀'s for coho salmon in chlorinated sea-water averaged 55% of those for shiner perch. The relationship between TRO concentration, exposure time, and percent survival in chlorinated sea-water at 13°C is presented for both species.A significant (P ≤ 0.01) avoidance threshold for coho salmon occurred at 2 μg l⁻¹ TRO and was reinforced with increasing temperature. A significant (P ≤ 0.01) avoidance threshold for shiner perch occurred at 175 μg l⁻¹ TRO, while a significant preference (P ≤ 0.05 or 0.01) response at 16°C and 20°C occurred at 10, 25, 50 and 100 μg l⁻¹ TRO. The ecological implications of the toxicity tests and the behavioral responses are discussed.     

Lateral confinement as a means for enhancing load bearing and energy absorption in axially compressed tubes

The energy absorption and load-bearing capacity of single and concentric multilayer tubes under axial compression are studied with an eye toward optimization per structural mass or volume available for deformation. The specimens are laterally confined on their inner and outer surfaces by rigid walls to stabilize the deformation, but the effect of confinement diminishes as the number of layers is increased. The diameter of the tubes and the tightness of the confinement or relative density ρ are systematically varied. The confinement tends to reduce the wavelength of the characteristic buckle, which leads to additional energy dissipation modes over the classical tube, notably axial compression and friction. Simple analytic relations are developed to assess the mechanical performance as a function of ρ and other system parameters. When considering mean stress, crush energy and stroke or densification strain on the basis of minimizing mass and volume simultaneously, ρ≈0.5 seem to be a viable choice, resulting in considerable enhancement of the performance over common cellular structures, for which ρ is typically <0.1.     

Improved rotatory-flow technique applied to Cod (Gadus morrhua L.)

An improved version of rotatory-flow apparatus is described, allowing tests of fitness with fish of different size at constant temperature. Cod (Gadus morrhua, L.) was used as experimental animal, and its treatment and behaviour during different phases of the test are described in some detail. Each test results in determination of the “critical rev min⁻¹” at which the fish is just brought to rotate with the water. The mean of a series of 10 critical rev min⁻¹ with the same fish, multiplied by the interior circumference of the rotational tube used, gives the “critical peripheral velocity” of the specimen. Critical peripheral velocities of a sample of fish, plotted against the standard length of the fish, arrange themselves along a straight line which intersects the vertical axis near the origin. Divided by the corresponding standard lengths the critical peripheral velocities give “reaction quotients”, the mean of which describe the reaction of the whole sample of fish.Optimal experimental conditions have been found by investigating the effects of systematic variation in streaming velocity, relationship between rotational tube diameter and fin-breadth, temperature, and effects of repetition of test-series on successive days etc. on the results of testing.     

The influence of grain size and porosity on crack initiation stress and critical flaw length in dolomites

The influence of rock texture on crack initiation stress (σ_i) and critical flaw length (L_i) is studied by a series of triaxial tests performed on monomineralic dolomites. The critical flaw length, as predicted by analytical models, is shown to be larger than the measured mean grain size (d_m) by two–three orders of magnitude. This discrepancy is explained by rock texture variations, which influence the fracture propagation mode and consequently fracture initiation stress. The quantification of rock texture is accomplished using porosity. Fracture initiation stress is shown to be inversely related to both porosity and mean grain size. When porosity is low, the sensitivity of σ_i to mean grain size is high. This effect is reduced with higher porosity values. A model for initial flaw length is developed by a synthesis of Griffith initiation criteria with our empirical model for fracture initiation stress. Initial flaw length is found to be directly proportional to the elastic modulus, mean grain size and porosity of the rock. When porosity and mean grain size decrease simultaneously, the initial flaw length rapidly decreases and approaches the mean grain size value. Therefore, the classical assumption that grain size scales initial flaw size is shown to be valid only in the very restricted case of low porosity-low grain size rocks. In such textures, where void space is minimal, available crystal faces function as truly initial flaws, and variations in mean grain size influence crack initiation stress significantly. In more porous textures, however, the initial flaw length is shown to be up to two orders of magnitude higher than the mean grain size in the rock, depending upon the porosity and mean grain size values. In such textures crack initiation stress is much less sensitive to variations in mean grain size, indicating that the role of individual grains is less significant.     

Bestimmung thermischer Eigenschaften der Gesteine des Unteren und Mittleren Buntsandsteins

For accurate planning of vertical borehole heat exchanger systems, knowledge of thermo-physical ground parameters is critical. This study reports laboratory-measured thermal conductivity and diffusivity values of Mesozoic sandstones (Lower and Middle Buntsandstein) from four wells. The measurements were made on drill core using an optical scanning method. The mean thermal conductivities of the sandstones range between 2.6?±?0.3 W?/?(m?·?K) and 3.1?±?0.4 W?/?(m?·?K) for dry conditions and between 3.6?±?0.3 W?/?(m?·?K) and 4.1?±?0.6 W?/?(m?·?K) after saturation with water. The mean thermal diffusivity values range between (1.6?±?0.2)?·?10??6 m²?/?s for dry and (2.0?±?0.6)?·?10??6 m²?/?s for water-saturated sandstones. Thermal properties are closely related to the petrography and lithostratigraphy of the sandstones. Additionally, three temperature correction methods were applied for the purpose of evaluating the comparative accuracy and the correction schemes with respect to local in-situ conditions. The results show that the temperature corrections proposed by Somerton (Thermal properties on temperature-related behavior of rock/fluid systems, Elsevier, New York, S 257, 1992) and Sass et al. (J Geophys Res, 97:5017–5030, 1992) are most suited for the respective sandstone data set.     

The effect of nutrition on the response of field populations of the calanoid copepod Acartia tonsa to copper

Continuous flow toxicity tests were conducted on field populations of adult Acartia tonsa collected from Narragansett Bay. Potential algal food at the collection site was estimated from ATP and chlorophyll analysis. There was positive correlation (P < 0.01) between chlorophyll a and ATP. Integration of these data with quantitative zooplankton analysis established that A. tonsa had a significant grazing effect on algal biomass (P < 0.05). The natural population dynamics of A. tonsa during this period were assessed from microscopic analysis of tow material.The sensitivity of A. tonsa to total copper ranged from 9.0 to 78.0 μg l⁻¹ for 72 h LC50's. Results show an inverse correlation (P < 0.05) between the log LC50 and adult A. tonsa density at the time of collection. The relationship between A. tonsa sensitivity to copper and the food ration (algal density/A. tonsa density) has been described by a quadratic function which has a correlation coefficient of 0.78. This indicates that the log LC50 increases with increasing food ration up to a point and then remains constant. A comparison of confidence intervals about the LC50's indicates greater variability in the response of field animals compared to previous studies with cultured populations. This data suggests that field populations of A. tonsa can be expected to exhibit a wide range of sensitivity to other pollutants. Sensitivity is strongly correlated with population density and food ration.     

Influence de sediments argileux incorpores au milieu d'insemination sur le succes de la fecondation chez la truite arc-en-ciel (Salmo Gairdneri)

The aim of this paper is to determine whether clay sediments suspended in water can prevent trout eggs from being fertilized. Kaolinite-rich clays (granulometric fraction: <2μm) (Fig. 1) were suspended in an artificial insemination diluent in doses ranging between 0 and 20 g l⁻¹. The eggs were exposed for 1, 10 or 20 min (experiment A) or inseminated (experiment B) in the diluent-sediment mixture. In experiment (C), the eggs were exposed to this mixture at three different temperatures (10, 15, 20°C). After insemination, the eggs were incubated for 10 days at 10°C and the percentage of eyed-eggs was used as an approximation of the fertility rate. The presence of clay sediments in the medium in which artificial insemination was carried out did not affect fertilization rate after the ovules had been exposed during 1 min to clay suspensions, at any of the temperatures used (8°C: Fig. 3; 10–15 or 20°C: Fig. 5) or at any of the sperm dilution rates (10⁻², 10⁻³, 10⁻⁴) (Fig. 3). On the contrary, there was a significant decline (P < 0.01) in the fertilization rate after the eggs had been exposed for 10 min at 8°C to doses of sediment exceeding 1.2 g l⁻¹ (Fig. 2). The fertilization rate also decreased significantly (P < 0.05) when the dose of sediment in the medium increased after 20 min at 20°C and 40 min at 15°C (Fig. 4). The 15 and 20°C temperatures were unfavourable for the eggs anyway. It is probable that fertility decreased due to micropyle clogging when the eggs were exposed longer than 10 min to the sediments. It is concluded that presence of sediments in the medium in which the gametes meet does not prevent fertilization.