Volume Change Behaviour of Expansive Clay-Sand Blends
Volume Change Behaviour of Expansive Clay-Sand Blends
B.R. Phanikumar
Professor, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail:
C. Amshumalini
Research Student, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail:
R. Karthika
Research Student, Department of Civil Engineering, VIT University, Vellore–632 014, India.
E-mail:
ABSTRACT: Expansive soils are highly problematic because they undergo detrimental volumetric changes corresponding changes in moisture regime. They swell when they absorb water and shrink when water evaporates from them. As a result, civil engineering infrastructure is severely cracked. Of various innovative foundation techniques, chemical stabilization and blending expansive clay with non-expansive material have also been found to have met with success. This paper presents swell-consolidation behaviour of artificially prepared sand-expansive clay mixtures, wherein fine sand was used in the blends at different contents. The fines content in the expansive clay was also varied. Swell potential, swelling pressure, compression index and co-efficient of consolidation were studied.
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Volume Change Behaviour of Expansive Clay-Sand Blends
1. INTRODUCTION
1.1 Problems with Expansive Soils
Expansive soils pose the problem of swelling on absorption of water during monsoon and shrinkage on evaporation of water in summer (Chen 1988, McKeen 1988, Nelson & Miller 1992, Kenneth 1993). As a result of the swell-shrink behaviour of expansive soils, lightly loaded structures such as foundations, pavements, canal beds, and linings and residential buildings founded in them are severely damaged (Chen 1988). The annual cost of damage to civil engineering structures is estimated at $1000 million in the USA, £150 million in the UK, and many billions of pounds worldwide (Gourley et al. 1993).
1.2 Innovative Foundation Techniques
Several innovative foundation techniques have been suggested for counteracting the problem of heave and shrinkage of expansive soils, which include sand cushion technique (Satyanarayana 1966), under-reamed piles (Sharma et al., nd granular pile-anchors (Phanikumar 1997, 1978) a Phanikumar et al. 2004). Belled piers, lime-slurry pressure injection and geomembranes as moisture barriers are also among the innovative techniques in practice (Chen 1988). Apart from these techniques, stabilization of expansive soils with various additives including fly ash, lime, cement and calcium chloride (Sankar 1989, Desai and Oza 1997, Hunter 1988, Phanikumar & Sharma 2004) has also met with considerable success.
This paper presents results of laboratory tests performed on artificially prepared expansive soil-sand mixtures. Swell potential, swelling pressure and compression characteristics like compression index, compressibility and volume compressibility were determined.
2. EXPERIMENTAL INVESTIGATION
2.1 Test Materials
Remoulded expansive clay and fine sand was the test materials used in the investigation. The soil was collected from Amalapuram, Andhra Pradesh. It is a highly swelling soil with a Free Swell Index (FSI) of 180%. It was a CH soil as per USCS classification. Table 1 shows the index properties of the expansive soil.
Table 1: Index Properties of Expansive Soil
Specific gravity / 2.72Liquid limit (%)
Plastic limit
Plasticity index / 100
27
73
Gravel (%) (>6.20–4.75 mm) / 0
Sand (%) (4.75–0.075 mm) / 7
Silt (%) (0.075–0.002 mm) / 24
Clay (%) (<0.002 mm) / 69
Free Swell Index (FSI) / 250
USCS classification / CH
Fine sand, used as the blend material, had the particle size between 0.075 mm and 0.425 mm.
2.2 Tests Conducted and Variables Used
1-D swell-consolidation tests were conducted on the sand-clay blends by free swell method. Swell potential, swelling pressure, compression index and coefficient of consolidation were determined.
The dry unit weight of the expansive soil was kept constant at 12 kN/m3. Oven-dry soil passing 425 µm and 75 µm sieve was used for the preparation of the test specimens, which meant that the initial water content of the specimens was 0%. Oven-dry samples were used in the testing in order to obtain measurable swell potential and swelling pressure. Fine sand content was varied as 0%, 10%, 20% and 30% by dry weight of the expansive soil.
2.3 Preparation of Test Specimens and Test Procedure
Oven-dry soil passing 425 µm sieve and 75 µm sieve was weighed corresponding to the prefixed dry unit weight of 12 kN/m3 based on the volume of the consolidometer ring (dia = 60 mm and height = 20 mm). Fine sand was also weighed according to its content in the blend and mixed with the expansive soil thoroughly. The blend was statically compacted into the consolidometer ring in four layers, each layer having a thickness of 5 mm. A thin layer of silicone grease was applied to the inner wall of the consolidometer ring to minimize friction between the swelling soil and the consolidometer ring. The compacted blend specimen was sandwiched between two filter papers and porous discs.
The consolidometer was arranged in position and loading hanger was placed upon the specimen. A seating load of 5 kPa was applied on the specimen. Swelling was allowed in the blend by free swell method, wherein the sample was inundated with water. Heave was monitored with a dial gauge corresponding to time intervals of 0, ½, 1, 2, 5, 10, 20, 30, 60 and 120 minutes and thereafter every 24 hours from the time of inundation. When equilibrium heave was reached by the specimen, compressive load increments were applied on the specimen to bring the swollen sample to its initial void ratio. Equilibrium heave was understood to have reached when there was no further movement in the dial gauge. The sample was consolidated under load increments of 10 kPa, 20 kPa, 40 kPa, 80 kPa, 160 kPa and 320 kPa. At the end of consolidation, the sample was removed and water content determined.
3. DISCUSSION OF TEST RESULTS
Figure 1 shows the e-log p curves of the expansive clay (425 µm)-sand blends for varying sand content (0%, 10%, 20% and 30%). Similar e-log p curves were obtained in the case of 75 µm fraction also, but they are not being presented here.
As the initial dry unit weight of the specimens was kept constant at 12 kN/m3, the initial void ratio (eo) of all the specimens was the same at 1.21. Though all the specimens had an initial surcharge pressure of 5 kPa, the final or equilibrium void ratio at the end of the process of swelling was different for different specimens having varying sand content. The equilibrium void ratio was the highest for the sample having 0% sand content or the unblended sample. However, the equilibrium void ratio decreased with increase in sand content, indicating that swelling or volume increase decreased with increasing sand content. The amount of fine sand used in different samples replaced the particles of swelling clay and thus reduced expansion or volume increase of clay-sand blends. Hence, the equilibrium void ratio of sample having 20% sand content starting with the same initial void ratio was the lowest. Thus the e-log p curves got shifted to lower positions as the fine sand content in the blend increased. This means that the amount of heave ∆H or swell potential ((∆H/H) ´ 100) undergone by the specimen reduced with increase in sand content. Swell potential written in the form of the following equation was determined according to the definition given by Jennings in 1963.
It is written as,
S = ((∆H/H) ´ 100) (1)
Figure 2 shows the variation of swell potential with sand content. Swell potential decreased significantly with increase in sand content.
Fig. 1: e-log p Curves
Fig. 2: Variation of Swell Potential with Sand Content
As the sand content in the blends was decreased from 0% to 30%, the swell potential decreased by 71% and 50% respectively in blends containing 425 µm and 75 µm fractions. This suggests that physical alteration of expansive clay soils blending them with non-swelling sandy materials significantly reduces their swelling nature.
Swelling pressure (ps) is defined as the pressure required to bring back a completely swollen soil specimen to its initial void ratio (eo). This can be obtained from the e-log p curves as the pressure corresponding to the initial void ratio. Swelling pressure of clay sand blends was determined from the e-log p curves shown in Figure 1 by drawing a horizontal line from the point corresponding to the initial void ratio. Swelling pressure clay-sand blends having a sand content of 0%, 10%, 20% and 30% were found to be respectively equal to 230 kPa, 150 kPa, 110 kPa and 75 kPa when the fines content in the blend was passing through 425 µm sieve.
Figure 3 shows the variation of swelling pressure with sand content for fines passing both 425 µm and 75 µm sieves. Significant reduction in swelling pressure with increase in sand content was observed in the specimens of both clay fractions. Swell potential decreased significantly with increase in sand content. As the sand content in the blends decreased from 0% to 30%, swell potential decreased by 71% and 50% respectively in blends containing 425 µm fraction and 75 µm fractions.
Fig. 3: Effect of Sand Content on Swelling Pressure
Figure 4 shows the influence of sand content on coefficient of volume compressibility of the blends decreased. Coefficient of volume compressibility (mv) also decreased moderately up to a sand content of 20%, but decreased steeply as the sand content was increased from 20% to 30%. Data for 425 µm fraction alone are available.
The slope of the linear portion of the e-log p curve is called the compression index (Cc). This indicates the amount of compression or reduction in void ratio under a given stress range. Figure 5 shows the variation of Cc with sand content in the blends. Data for 425 µm fraction alone are available.
Fig. 4: Influence of Sand Content (%) on Coefficient of Volume Compressibility, mv
Fig. 5: Variation of Cc with Sand Content (%)
Cc decreased significantly with increase in sand content, indicating that the compressibility of the blends also decreased significantly with increase in sand content. As the sand content increased from 0% to 30%, Cc decreased by 55%. This shows that blending expansive clays with fine sand improves their compressibility characteristics apart from reducing their swelling nature.
4. CONCLUSIONS
Swell potential (S%) decreased significantly with increase in sand content. As the sand content in the blends was decreased from 0% to 30%, swell potential decreased by 71% for 425 µm fraction. Swelling pressure also decreased as sand content in the blends increased. Similar observations were made in the case of 75 µm fraction also. Coefficient of volume compressibility (mv) also decreased with increasing sand content. Coefficient of volume compressibility reduced by 30% as the sand content was increased from 0% to 30% in the case of 425 µm fraction. Compression index (Cc) too decreased significantly with increase in sand content, indicating that the compressibility of the blends decreased significantly with increase in sand content. As the sand content increased from 0% to 30%, Cc decreased by 55%.
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Volume Change Behaviour of Expansive Clay-Sand Blends
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