The sand composition plays a decisive role in the geotechnical behavior of sand-bentonite mixtures, as this study has shown. Effect of bentonite content and initial compaction conditions on the relationship between consolidation coefficient (cv) and pressure for sand-bentonite mixtures. Effect of bentonite content and initial compaction conditions on the coefficient of volume change (mv)-pressure relationship for different sand-bentonite mixtures.
CONCLUSIONS AND SCOPE FOR FUTURE WORK 191-196
LIST OF TABLES
Symbols Used
INTRODUCTION
- GENERAL
- MOTIVATION FOR THE STUDY From the available literature it is observed that
- ORGANIZATION OF THE THESIS
Dumbleton and West, 1966) to understand the behavior of sand-bentonite mixtures in terms of their application as a landfill liner material, fill material for nuclear waste disposal schemes. However, very few studies have been reported on the influence of sand on the engineering characteristics of sand-bentonite mixtures. This study aims to understand the influence of sand particle size on the hydromechanical behavior of a sand-bentonite mixture.
LITERATURE REVIEW
GENERAL
REVIEW ON BUFFER AND BACKFILL MATERIAL
Buffer materials are mixes that contain a higher proportion of bentonite, usually about 40 to 50% of the mix proportion, and are used to backfill the disposal pit. The buffer and fill require a very low hydraulic conductivity (less than 1x10-10 m/s) and therefore the design and development of buffer and fill materials, which fill the disposal facility, are important for the development of waste technology high level nuclear. annihilation. In order to design the specifications, such as dry density and bentonite content, of buffer and fill materials, we need to investigate the hydraulic properties through experiments and quantitatively evaluate the hydraulic conductivity of compacted sand-bentonite mixtures.
DESIGN CRITERIA FOR BACKFILL MATERIAL .1 Introduction
- Design criteria
Minimum limiting values of engineering properties as set out by SKB (2010) are hydraulic conductivity to be lower than 1x10-10 m/s; and swelling. An average installed dry density of 1240 kg/m3 was specified for backfill as the minimum desired value required to limit the expansion of buffer and ensure that the saturation density requirements of the buffer (1950 kg/m3) are met. While Japan Nuclear Cycle Development Institute (1999) required the hydraulic conductivity of fill material to be between 1x10-11 m/s and 1x10-12 m/s.
BENTONITE .1 Introduction
- Structure of Montmorillonite
- Swelling behaviour of Bentonite
- Inner-crystalline swelling
- Osmotic swelling
The polarity of the water molecule is an important factor in the internal crystalline swelling of clay. The driving force for the osmotic swelling is the large difference in concentration between the ions electrostatically held close to the clay surface and the ions in the pore water of the rock (Fig. 2.5.a). The distribution of the negative potential changes with the valence and radius of the ions in the double layer and with the electrolyte concentration in the pore water.
LITERATURE REVIEW ON BACKFILL MATERIAL
The value of this potential decreases with increasing distance from the surface and reaches zero in the pore water. The profile of the potential curves, and therefore the repulsion at a given distance, varies with the valence and radius of the counterions in the double layer and with the concentration of electrolytes in the pore water. The results of swelling pressure showed that it is highly dependent on the amount of bentonite present in the mixture and the initial dry density of the mixture.
DESIGN CRITERIA FOR LANDFILL LINERS
Another notable observation was that for mixtures with a bentonite content of less than 20%, when in contact with water, there was not enough bentonite to fill all the voids in these mixtures.
REVIEW ON LANDFILL LINERS
Laboratory studies have shown that this lower limit of hydraulic conductivity can be satisfactorily met with expanded clay materials such as bentonite, and saturated hydraulic conductivity of up to 5x10-12 m/s can be achieved under controlled conditions (Hoeks and Agelink, 1982 ). Some authors have encouraged engineers to use gravel soils for barriers since gravel strengthens the material, reduces shrinkage potential, and may even reduce hydraulic conductivity if the nearly impermeable gravel particles block flow paths (Shelley and Daniel, 1993). Hazardous substances from these wastes are chlorinated hydrocarbons, cyanides, organic substances such as PCBs and heavy metals (Matrecon, 1980).
LANDFILL COMPONENTS AND ITS FUNCTIONS
- Bentonite
- Geomembrane
- Geotextile
- Geosynthetic clay liner (GCL)
- Geonet
It is used to prevent the movement of soil and debris in the leachate collection system and to protect geomembrane from holes. These materials facilitate the movement of leachate, but trap particles to reduce clogging in the leachate collection system. It is used in landfill liners in place of sand or gravel for the leachate collection layer, as geonets transport leachate faster than sand and gravel.
TYPES OF LANDFILL LINER SYSTEMS .1 Single liner system
- Double liner system
The thickness of geomembrane used in landfill liner construction is regulated by environmental regulatory agencies. Composite liner systems are most effective at limiting municipal solid waste (MSW) leachate migration into the subsurface and are not suitable in hazardous waste disposal facilities. A double liner system consists of either two single liners or two compound liners or single and a compound liner.
REVIEW ON DIFFERENT CRITERIA USED IN DESIGNING LINERS Matthew and Jones (1999) observed that the important variables in the construction of
Soils to be compacted over a range of water content using three compaction efforts representing the high, medium and low effort expected in the field; An acceptable zone will be drawn that includes test results that meet or exceed the design criteria; The acceptable zone will then be adjusted to take into account other factors, for example shear strength considerations, shrinkage/swell criteria and local construction practices that may be relevant to a particular project.
REVIEW OF LITERATURES ON THE BEHAVIOUR OF SAND- BENTONITE MIXTURES
He also observed that the hydraulic conductivity decreases with the increase of the bentonite fraction in the mixture. From his study, Lambe (1964) observed a hydraulic conductivity ratio of up to 60 for a given void ratio for the soil samples compacted on the dry side of the optimum moisture content (OMC) and the wet side of the OMC, indicating the importance of soil structure (Lambe, 1954) on the hydraulic conductivity of the soil. The results showed that the hydraulic conductivity decreases with an increase in the bentonite content of the mixtures.
SUMMARY AND CRITICAL APPRAISAL OF LITERATURE REVIEW A review of literature regarding the sand types used in the making of liners, buffers and
Since bentonite is a relatively expensive material, most studies in the literature had a similar goal. From the data in Table 2.1 it can be seen that the proportion of sand used in the literature repeatedly consisted of a fine sand content of 30% to 30%. 70% and average sand content from 70% to 30% on a dry weight basis. It was agreed that the same ranges would be used in the current study when assessing the effect of the fine sand to medium sand ratio on sand-bentonite mixtures.
OBJECTIVES OF THE PRESENT STUDY
To investigate the influence of sand type on the consolidation properties of bentonite-sand mixtures made with two bentonites with varying bentonite-sand ratios. To understand the influence of initial compaction condition and sand particle size on the hydraulic and mechanical properties of bentonite-sand mixtures made with two bentonites with varying bentonite-sand ratios. To understand the influence of bentonite quality on the swelling, hydraulic and mechanical properties of different bentonite-sand mixtures containing sand of different particle sizes.
SIGNIFICANCE OF THE STUDY
MATERIALS AND METHODS
MATERIALS USED IN THE STUDY
TESTING METHODOLOGY .1 Bentonite
- Atterberg limits
- Free swelling test
- Cation exchange capacity (CEC)
- Specific surface area (SSA)
- Sand and sand-bentonite mixes .1 Sieve analysis
- Specific gravity
- Atterberg limits
- Standard proctor compaction test
- Consolidation test
- Determination of swelling potential and swelling pressure
- Determination of hydraulic conductivity and compressibility
- Shrinkage test
The cation exchange capacity (CEC) is the amount of exchangeable cations required to balance the negative charge on the surface of the clay particles. The specific surface area is the ratio between the surface area of a material and its mass, expressed in m2/g. The specific surface area of the bentonites was determined according to the method described by Cerato and Lutenegger (2002).
The consolidation test was performed on the sand-bentonite mixtures to evaluate the hydraulic conductivity and compressibility of the mixtures. Indirect determination of hydraulic conductivity from consolidation tests has several advantages over other permeability tests. By attributing all resistance to low hydraulic conductivity, Terzaghi's theory must inevitably lead to an underestimation of hydraulic conductivity.
In relation to determining the hydraulic conductivity of clay soil, the consolidation test has been widely used (Newland and Alely, 1960; Mesri and Olson, 1971; Budhu, 1991; Sivapullaiah et al., 2000). Δe is the change in void ratio of the sample due to swelling and eo is the initial void ratio before swelling. For each increase in pressure, the change in thickness of the soil sample was measured from the gage readings.
Once the effect of water content on fine sand bentonite mixtures and medium sand bentonite mixtures is understood, the second phase of the study begins, i.e.
RESULTS AND DISCUSSION
EFFECT OF SAND CONTENT AND PARTICLE SIZE ON THE BEHAVIOR OF SAND-BENTONITE MIXTURES
- ATTERBERG LIMITS
- COMPACTION CHARACTERISTICS
- CONSOLIDATION CHARACTERISTICS
- Time-Swelling characteristics of sand-bentonite mixes
- Swelling potential of sand-bentonite mixes
- Swelling pressure of sand-bentonite mixes
- Effect of bentonite content and initial compaction conditions on e-log k relationship for various sand-bentonite mixes
- Effect of bentonite content and initial compaction conditions on e-log P relationship for sand-bentonite mixes
- Effect of bentonite content and initial compaction conditions on Coefficient of consolidation (c v )-Pressure relationship for sand-bentonite mixes
- Effect of bentonite content and initial compaction conditions on time required for 90% consolidation (t 90 )-Pressure relationship of sand-bentonite mixes
- FIELD EMISSION SCANNING ELECTRON MICROSCOPE (FESEM) IMAGES OF COMPACTED SAND-BENTONITE MIXES
- SHRINKAGE CHARACTERISTICS
- Shrinkage behaviour of fine sand-bentonite-1 and medium sand-bentonite-1 mixes
- Shrinkage behaviour of fine sand-bentonite-2 and medium sand-bentonite-2 mixes
- UNCONFINED COMPRESSIVE CHARACTERISTICS
- Unconfined compressive strength (UCS) of fine sand-bentonite-1 and medium sand-bentonite-1 mixes
4.1.3.4.1 Effect of bentonite content and initial compaction conditions on the e-log k relationship for FS-B1 and MS-B1 mixtures. 4.1.3.5.1 Effect of bentonite content and initial compaction conditions on the e-log P relationship for FS-B1 and MS-B1 mixtures.
INFLUENCE OF FINE AND MEDIUM SAND COMPOSITION ON THE BEHAVIOR OF SAND-BENTONITE MIXTURES
- ATTERBERG LIMITS OF FINE SAND-MEDIUM SAND-BENTONITE MIXES
- COMPACTION CHARACTERISTICS OF FINE SAND-MEDIUM SAND- BENTONITE MIXES
The difference between the highest and lowest values observed for any proportion of B2 ranged from 2 to 10%. For both FS-MS-B1 and FS-MS-B2 mixtures, although the differences were small, the observed values indicated that the liquid limit of the soil mixtures was in the following order;. The plastic limit of soil is defined as the water content at which the shear strength increases by 100 times the liquid limit (Haigh et al., 2013).
For a given bentonite content, the fine sand, medium sand, and bentonite mixtures had different plasticity limits for each fraction of fine sand and medium sand, and no particular trend regarding the effect of sand fraction could be identified. Sridharan and Prakash (1998) stated that the shrinkage limit of soil is a physical phenomenon, unlike other Atterberg limits, and depends primarily on the particle size distribution. Tables 4.9 and 4.10 show that for any given bentonite content there is a unique FS-MS ratio that results in the lowest shrinkage limit.
When comparing FS-MS-B1 and FS-MS-B2 with FS-B1, MS-B1, FS-B2 and MS-B2 mixtures, increasing the sand proportion in the mixture is not necessarily reflected as an increased shrinkage limit that the idea that the shrinkage limit is a relative packaging phenomenon as proposed by Sridharan and Prakash (1998). All fine sand-medium sand-bentonite mixtures were compacted with standard compaction efforts in accordance with ASTM D 698 (2012). The compaction characteristics exhibited by FS-MS-B1 and FS-MS-B2 mixtures are shown in Table 4.11 through Table 4.14.
The optimum moisture content (OMC) values exhibited by the FS-MS-B1 mixtures show that, for any given bentonite proportion, the variation in OMC values for all different fine sand-medium sand proportions was less than 3 % and increasing fine sand content in these mixes had very little influence on OMC, indicating that the change in OMC is primarily a function of bentonite type and mix content.
B1 Mix
- CONSOLIDATION CHARACTERISTICS OF FINE SAND-MEDIUM SAND-BENTONITE MIXES
- Swelling potential of fine sand-medium sand-bentonite mixes .1 Swelling potential of FS-MS-B1 mixtures
- Swelling pressure of fine sand-medium sand-bentonite mixes .1 Swelling pressure of FS-MS-B1 mixtures
- Effect of sand proportioning on e–log k for various fine sand-medium sand- bentonite mixes
- Effect of sand proportioning on e-log P relationship for various fine sand- medium sand-bentonite mixes
- Effect of sand proportioning on Coefficient of consolidation (c v )-Pressure relationship for various fine sand-medium sand-bentonite mixes
- Effect of sand proportioning on Coefficient of volume change (m v )-Pressure relationship of fine sand-medium sand-bentonite mixes
The swelling characteristics exhibited by FS-MS-B1 samples compacted at OMC-MDD are shown in Table 4.15. The swelling characteristics exhibited by FS-MS-B2 samples compacted at OMC-MDD are shown in Table 4.16. The hydraulic properties exhibited by FS-MS-B1 mixtures compacted at OMC-MDD are as shown in Fig.
The hydraulic properties exhibited by FS-MS-B2 mixtures compacted at OMC-MDD are shown in Fig. The hydraulic conductivity of FS-MS-B2 mixtures was found to decrease as the bentonite content increased, as was the case for FS-MS-B1 mixtures. For any given bentonite content and FS-MS ratio, the hydraulic conductivity of FS-MS-B2 mixtures was relatively lower than that of FS-MS-B1 mixtures.
The void ratio–pressure relationship exhibited by FS-MS-B1 mixtures statically compacted by OMC-MDD is represented in Fig. The void ratio–pressure relationship exhibited by FS-MS-B2 mixtures statically compacted by OMC-MDD is represented in Fig. Coefficient of consolidation (cv) pressure ratio exhibited by FS-MS-B1 mixes compacted by OMC-MDD is shown in Fig.
Coefficient of Consolidation (cv)-Pressure ratio exhibited by FS-MS-B2 mixes compacted by OMC-MDD is shown in Fig.
