The sand–clay mixtures can be used for hydraulic barriers on slopes, as barriers for nuclear waste repositories or thermohydraulic backfills for geothermal boreholes. Moreover, the unsaturated compacted clay–sand mixtures can be used in civil engineering works such as construction of roads, dams and other types of embankments. They can be used also in raw earth building material. The sand–clay mixture is used generally in compacted fills and in compressed earth bricks (CEB) that require a high dry unit weight, as clay particles can fill the voids between sand particles. Further, mixing sand and clay (bentonite) may lead to a lower swelling pressure than that for clay alone . Generally, the sand fraction is added to clayey soil to have a high shear strength or low compressibility or to reduce its swelling and the clay fraction is added to granular soil to reduce its permeability.
The behaviour of sand–clay mixture is governed by the granular phase when the sand matrix is predominant or by the cohesive phase when the clay matrix is predominant. Different studies were conducted to show the effect of sand or clay content on the behaviour of sand–clay mixtures. They generally showed that when sand was mixed with kaolin clay, the soil changed its behaviour from sand to clay. This clay content is enough to fill the voids of the granular portion at its maximum porosity. For example, Novais-Ferreira  described the existence of three zones of behaviour of the given sand–clay mixtures as a function of clay content: the first one, when the corresponding clay content is ≤28%, non-cohesive behaviour, where cohesion is negligible and the angle of friction is high (above 30°); the second one, when the clay content is between 28% and 41% (transition behaviour), where the soil is sensitive both to cohesion and the friction angle; the third one, when the clay content is ≥41%, cohesive behaviour, where cohesion is higher and the angle of friction is lower. However, Skempton  postulated for the clays that if the clay fraction is less than approximately 25%, the mixture behaves much like sand or silt rather than clay, but the residual strength is controlled almost entirely by the sliding friction of the clay minerals when the clay fraction is above 50%. Muir Wood and Kumar  demonstrated that the predominance of the mechanical behaviour of the clay matrix on the mechanical behaviour of the mixture occurs when the clay content is greater than 40% and the volume fraction of the granular constituent reaches about 0.45.
In recent years, many experimental tests and methods were conducted to study the hydro mechanical behaviour of sand–clay mixtures, such as the shear strength and the compression behaviour [4,5,6,7,8,9,10,11,12,13], the unconfined compressive strength (UCS) [14,15,16,17,18], the compaction characteristics [19,20,21,22] and the hydraulic properties [19, 23, 24]. In addition, the effect of suction and water content on the mechanical behaviour of sand–clay mixtures has also been investigated by many authors [13,14,15,16,17].
For shear strength, Vallejo and Mawby  showed that the shear strength is governed by the granular phase when the sand content is greater than 75% and by the cohesive phase when the sand content is lower than 40%. When the sand content is between 40% and 75%, the shear strength of the mixtures is partially controlled by the granular phase. Prakasha and Chandrasekaran  found that the inclusion of sand grains in a clay matrix leads to an increase in pore pressure resulting in a decrease in undrained shear strength, while Shafiee et al.  reported that the undrained shear strength increases with increasing sand content.
Likewise, Mun et al.  studied the impact of clay content on the undrained shear strength and on the compression curves of unsaturated sand–clay mixtures using the triaxial compression tests. The results indicated that sand–clay mixtures have lower undrained shear strength than sand or clay, but there is an intermediate rate of increase in shear strength with increasing strain rate, and the suction in the clay matrix has an important effect on the preconsolidation stress, which decreases as the percentage of clay increases. Pakbaz and Moqaddam  conducted consolidated-drained direct shear tests to study the effect of increasing clay content and the effect of gradation of sand on the behaviour of overconsolidated sand–clay mixtures and on their shear strength properties. The result of tests indicated that at a particular sand gradation, with an increase in clay content, the shear strength decreased. Also, at a constant clay content, the shear strength decreased with a decrease in sand grains’ size. Cabalar and Mustapha  also studied the effect of particle gradation of sands with distinct shapes (rounded and angular) on the variation of the liquid limit (LL) and on the shear strength of clay mixed with different percentages of these sands. The results of the conducted tests indicated a decrease of undrained shear strength with an increase in the amount of sand. The use of rounded sands in a clay matrix leads to the development of higher undrained shear strength values, on which gradation of the sands has no effect.
In terms of UCS, Anuchit  studied the effect of sand content for different applied suctions and concluded that the UCS increased with increasing sand content for all the ranges of applied suction. For the effect of suction on UCS of clayey soils with different sand contents, the author found that the UCS increased with increasing matric suction for the matric suction range less than 50 kPa, but it decreased with increasing matric suction for the suction range greater than 50 kPa, whatever was the sand content in the mixture. Khan et al.  studied the compressive strength of compacted natural clay of high plasticity mixed with 20% and 40% of sand. The authors found that the compressive strength decreased with an increase in sand content because of increased material heterogeneity and loss of sand grains from the sides during shearing tests. The authors also showed that the compressive strength increased with decreasing water content of the material and for each sand–clay mixture, the compressive strength increased with an increase in density of the material. Sun et al.  carried out series of one-dimensional compression tests on the unsaturated compacted sand–bentonite mixture using suction-controlled oedometer. The results indicated that the yield stress increases and the compression index decreases with an increase of suction.
For compacted sand–clay mixture, some of the researchers investigated the effect of the percentages of sand and clay on the soil structure and the compaction properties (optimum water content and dry density values). For example, Kenney et al.  observed that there may be an optimal clay content that would lead to an increase in the dry density of a compacted sand–bentonite mixture. However, Howell et al.  concluded that the addition of clay may lead to an increase or decrease in the optimal water content and maximum dry density under the standard Proctor compaction effort according to the type of processed clay soil (water sorptivity and swelling potential), curing period and the mixing procedure. Cabalar and Mustafa  carried out California bearing ratio (CBR), unconfined compression strength (UCS) and compaction tests on various contents of sand and clay mixture. The results showed a decrease in UCS and an increase in CBR values with an increase in sand content.
The mixture with an appropriate sand content is generally used to valorise the fine raw earth to an eco-building material like compressed earth block (CEB) and to improve its hydromechanical behaviour defined by the UCS and the strain modulus. These parameters are measured generally when the CEB is made at a water content near saturation to reach the maximum density. However, the CEB dries before being implemented in the building. Therefore, the main objective of this paper is to study the behaviour of the compacted clay–sand mixtures on the drying–-wetting paths, and then to investigate the influence of suction, water content and the added sand content on UCS and on the strain modulus of the compacted clay–sand mixture. To reach these purposes, series of unconfined compression tests at different water contents are performed on the kaolinite–sand mixtures compacted in standard Proctor properties. Furthermore, to link the water content to suction, the soil–water characteristic curves (SWCCs) of the compacted materials are highlighted by investigation of the effect of added sand percentage on the drying–wetting paths.
The clayey soils used in manufacturing of the building materials, dams and road embankments need to be amended generally with sand to meet the standards in strength and to reach the recommended behaviour. In this study, a commercial kaolinite (Sibelco, Hostun, France) noted 100K (100% kaolinite) is taken as a reference clayey soil. Two mixtures are prepared using this kaolinite and a French building commercial sand 0/4 provided by ULTIBAT and referenced ULTIBAT-91. These mixtures are noted as 75K25S (75% of kaolinite and 25% of sand) and 50K50S (50% of kaolinite and 50% of sand), respectively.
Many researchers have worked on the kaolinite 100K, both in terms of characterisation and mechanical behaviour [24,25,26,27,28,29]. Therefore, in this study, the lab experiments were focused on the soil properties of the mixture kaolinite–sand, such as the grain size distribution analysis, Atterberg limits and compaction. In accordance with the NF EN ISO 17892-4  standard, the grain size distribution curves of the kaolinite 100K, sand and the mixtures 75K25S and 50K50S are shown in Fig. 1. The grain size disribution characteristic parameters are summarised in Table 1. The used sand is well-graded with a uniformity coefficient
Geotechnical properties of the tested soils.
|Kaolinite 100K (Kheirbek-Saoud 1994) ||0.44||11.67||100||58||40||20||20||17.2||1.7|
Standard Proctor tests were conducted in accordance with Afnor NF P 94 093  standard. The curves highlighted the relationship between water content and dry density curves of the kaolinite 100K, and the mixtures are presented in Fig. 4. The Proctor optimum water contents and the maximum dry densities (
The optimum water contents and maximum dry densities of the soils are presented in Table 1. The kaolinite 100K showed a maximum dry density of 1.7 g/cm3 at an optimum water content
Fig. 4 shows that this increase in density due to the increase of sand content needs lower water content (
Comparison between measured and correlated
LL: liquid limit
To determine the hydromechanical properties of sand–kaolinite mixtures, two different types of tests were conducted. The first one concerned the UCS test carried out on samples prepared in standard Proctor conditions (
The UCS tests were carried out according to the standard NF EN ISO 17892-7 . The Kaolinite 100K and the mixtures 75K25S and 50K50S were first air dried and then wetted to the standard optimum water content
From the stress–strain curve ( The point corresponding to 50% of the maximum strength of the material is located on the stress–strain curve. Then the secant line through the origin of the axes and through this point is drawn. The slope of this secant line represents the large strain secant modulus called
The point corresponding to 50% of the maximum strength of the material is located on the stress–strain curve.
Then the secant line through the origin of the axes and through this point is drawn.
The slope of this secant line represents the large strain secant modulus called
SWCC is a representation of the fundamental behaviour of soil matric suction with moisture content property. SWCC is generally completed with the variation of void ratio corresponding to the water content and suction curves. In the present research, to measure the water retention behaviour of the soil in a wide range of suction, different controlled and measured suction techniques are used to determine the drying–wetting paths of samples compacted at the standard optimum state. These methods are summarised as follows:
Tensiometric plates are made of a low-porosity sintered glass (or ceramic) filter set in a glass funnel and saturated with de-aired water. The plates play the role of a semi-permeable separation. They are in contact with a reservoir and a measurement column also filled with de-aired water. The specimens are placed on the filter and a negative pore water pressure (or suction) with respect to the atmospheric pressure is imposed in the column by the difference in height between the specimen and the free end of the column.
In the osmotic method, the specimen is placed in contact with a solution of large-sized molecules of polyethylene glycol (PEG 6000) through a semi-permeable membrane with pores smaller than 5 nm. The membrane allows only the passage of water. Due to a difference in the concentration of the solution across the membrane, water flows from lower concentrations to higher ones. This results in the application of matrix suction to the soil, which increases with the concentration of PEG. When equilibrium is reached, the hydration potential of PEG is equal to that of the soil. The relationship between the applied suction and PEG concentrations is deduced from a parabolic relation proposed by Delage et al.  as follows:
The salt solutions technique consists in enclosing the samples in desiccators containing different salt solutions (KNCS, K2SO4, NaNO2, NaCl, CuSO4) to control the relative humidity of the atmosphere, and therefore the suction in the specimens. This allows humidity exchanges between the atmosphere and soil until equilibrium. Total suction applied is related to the imposed relative humidity by Kelvin law (Eq. 4) as follows:
The initial suction of compacted samples corresponding to the optimum water content is measured using the filter paper method according to the standard ASTM D5298-16 . The value of matric suction is derived from the water content of calibrated filter paper referenced Whatman N°42, which is protected on both sides by two ordinary filter papers and placed in contact with the soil specimens during compaction test. Afterwards, the filter paper is enclosed with soil specimen in an airtight container until moisture equilibrium is established. The filter paper (Whatman N°42) is then extracted and its water content is measured immediately to avoid evaporation.
For all the used techniques, once the moisture equilibrium state is reached, the water content and the total volume of the specimen are measured by immersion in a non-wetting oil (commercial Kerdane), followed by drying in an oven at 105°C for 24 h, allowing the void ratio (e) and degree of saturation (Sr) of the material to be calculated. The obtained parameters (w, e, Sr) are related to the imposed and measured suctions.
The drying–wetting path tests were carried out on samples compacted at the standard optimum water content to the corresponding maximum dry density. The preparation technique of the compacted samples is explained in section 3.1. Each sample was cat into small cubes of about 2cm3 to be tested.
The hydromechanical behaviour of the material can be studied by the drying–wetting path tests. Fig. 7 summarises all the results of drying–wetting tests performed on samples compacted in SPO. These results are represented in five corresponding diagrams: a) void ratio versus water content; b) void ratio versus suction; c) degree of saturation versus water content; d) degree of saturation versus suction and e) water content versus suction. The values of the initial suction (
Initial conditions of drying–wetting test.
LL: liquid limit
The relationships between the void ratio (
On the wetting path, when the water content is greater than the optimum water content values (
The same general behaviours of the changes in void ratio, water content and degree of saturation versus suction or water content of the studied soils were observed by Serbah et al.  and Fleureau et al.  when they carried out an investigation to relate the drying–wetting paths of soils and their LLs.
From the wetting path of 14 clayey soils compacted under the standard Proctor optimum conditions, Fleureau et al.  showed a good linearity in the plane [log (s), e] and [log(s), w] and defined the slopes of the planes (
In the plane of suction versus normalised water content [log (s), w/Dms] (Fig. 10), the wetting paths of studied mixtures were located inside the single spindle of points, as suggested by the study of Fleureau et al. .
The relationship between the deviatoric stress q and the axial strain ɛ1 at standard water content
The increase and then a decrease in UCS with an increase in the added sand content, as observed in Fig. 11b, can be explained as follows.
The behaviour of the pure kaolinite 100K is essentially governed by the effective cohesion (the apparent cohesion) of the material noted ‘C’, which is given by Mohr–Coulomb criterion, and it is important for this high plastic material. The strength of the material is, therefore, mainly due to cohesion and not friction between the clay particles. When this kaolinite is mixed with 25% of sand, this added sandy granular fraction generates a granular friction between the particles, and with the remaining high percentage of kaolinite (75%), cohesion does not decrease significantly with the addition of 25% of sand. Consequently, the behaviour of the mixture is that of a sandy clayey soil. Therefore, the strength is due to both the effective cohesion of the kaolinite and the friction generated by the added sand fraction. Hence, an increase in UCS is observed. When the percentage of the added sand becomes important (50%), cohesion of the mixture decreases significantly and the observed behaviour is that of a clayey sand, in other words, dominated by the sand fraction, whose strength is essentially due to the capillary cohesion generated by the water meniscus between the grains. Because the UCS due to capillary cohesion is much lower than the UCS due to effective cohesion of the kaolinite, this may explain the drop in strength observed for the mixture 50K50S.
The behaviour of the pure kaolinite 100K is essentially governed by the effective cohesion (the apparent cohesion) of the material noted ‘C’, which is given by Mohr–Coulomb criterion, and it is important for this high plastic material. The strength of the material is, therefore, mainly due to cohesion and not friction between the clay particles.
When this kaolinite is mixed with 25% of sand, this added sandy granular fraction generates a granular friction between the particles, and with the remaining high percentage of kaolinite (75%), cohesion does not decrease significantly with the addition of 25% of sand. Consequently, the behaviour of the mixture is that of a sandy clayey soil. Therefore, the strength is due to both the effective cohesion of the kaolinite and the friction generated by the added sand fraction. Hence, an increase in UCS is observed.
When the percentage of the added sand becomes important (50%), cohesion of the mixture decreases significantly and the observed behaviour is that of a clayey sand, in other words, dominated by the sand fraction, whose strength is essentially due to the capillary cohesion generated by the water meniscus between the grains. Because the UCS due to capillary cohesion is much lower than the UCS due to effective cohesion of the kaolinite, this may explain the drop in strength observed for the mixture 50K50S.
To study the effect of the water content and suction on UCS and
Cylindrical samples were placed under a bell jar in a horizontal position to allow slow evaporation and avoid gravity flow. The average time varied from 1 to 3 weeks, depending on the target water content. Once the target water content was reached, the samples were wrapped in cellophane film and placed in an airtight bag for 48 h in a horizontal position to homogenise the water content within the specimen. After these steps, the samples were subjected to mechanical loading (UCS test).
It should be noted that during this drying phase, no significant shrinkage of the specimens was observed. In fact, the initial state of compaction ‘standard Proctor optimum’ was very close to the shrinkage limit, as shown in Fig. 7a and b.
Fig. 13a and b shows the variation of UCS versus water content and suction, respectively, for kaolinite 100K and the mixtures 75K25S and 50K50S. It can be observed that the UCS increased with decreasing water content and increasing suction, irrespective of the added sand content. It can be also deduced that the UCS increases when the kaolinite is mixed with 25% of sand and decreases when it mixed with 50% of sand, irrespective of the water content or suction of the samples. The 50K50S curve is located between 100K and 75K25S curves, according to Fig. 13a and b.
In fact, a decrease in water content increases the suction, thus increasing the capillary cohesion, which induces an increase in material strength.
The variations of
The change in the location of the curves of 100K and 50K50S when interpreted with suction concludes that when the water content varies, the main parameter to describe the mechanical behaviour of unsaturated soil is suction or the negative pressure and not the water content, which is a physical parameter. It is also true for UCS curves; when UCS is interpreted with suction (Fig. 14b), the gap between the curves of 100K and 50K50S observed with the water content in Fig. 14a is significantly reduced, from 0.3 MPa for a given water content to 0.1 MPa for a given suction.
Fig. 15 presents the variation of
In this study, the effect of suction and granular fraction content on the mechanical properties of the unsaturated fine material was studied. The unconfined axial compressive loading tests were performed on different clay–sand mixtures prepared under standard Proctor conditions (
The compaction tests reveal that an increase in sand content contributes to a decrease in the void ratio, and hence to an increase in the density of the mixture and decrease in the corresponding optimum water content. The drying–wetting curves show that the shrinkage limit decreases with the increase in percentage of the added sand, and consequently increases with the LL of mixtures.
The main results of the added SP effect at a given state (water content, density and suction) highlight the following:
When clay is mixed with 25% of sand, the UCS and When clay is mixed with 50% of sand, the UCS and
When clay is mixed with 25% of sand, the UCS and
When clay is mixed with 50% of sand, the UCS and
Geotechnical properties of the tested soils.
|Kaolinite 100K (Kheirbek-Saoud 1994) [
Comparison between measured and correlated γdmax and wSPO.
Initial conditions of drying–wetting test.
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