Characterisation of Soil Water Characteristics Curve and Its Use in Dewatering
CHARACTERISATION OF SOIL WATER CHARACTERISTICS CURVE AND ITS USE IN DEWATERING
K. Premalatha
Assistant Professor, Division of Soil Mechanics and Foundation Engineering, Anna University Chennai, Chennai–600025, India. E-mail:
Pa. Suriya
Postgraduate Student, Division of Soil Mechanics and Foundation Engineering, Anna University Chennai, Chennai–600025, India. E-mail: ,
ABSTRACT: In the field of geotechnical engineering in many places the soil is assumed to be fully saturated but in reality its not true. In field most of the soils are in partially saturated or in unsaturated state. The determination of the Soil Water Characteristics Curve (SWCC) of these soils is important, because this curve is the primary constitutive relationship for the prediction of the engineering behaviour of these unsaturated soils. To find the influence of the stress state two different soils clay and sand was selected for this laboratory investigation. The basic properties of soils were determined based on relevant IS codes. SWCC of these soils are determined using the pressure plate extractor. The influence of sand in the SWCC and dewatering potential is also observed. Tests were also carried to find out the stress state. The stress state has a remarkable influence in the compressibility of soils. As the percentage of clay increases the influence of stress state is increased. The coefficient of compressibility Cc values of clay and Clay-sand mix, after application of suction of 100 kPa is lesser than consolidation without application of suction. The residual water content is higher than that of consolidation after suction.
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Characterisation of Soil Water Characteristics Curve and Its Use in Dewatering
1. INTRODUCTION
The geotechnical engineering problems involving unsaturated soils are classified into three general phenomena. They are flow phenomena, stress phenomena and deformation phenomena. The majority of practical engineering problem generally involves all the three phenomena concurrently and in coupled fashion. SWCC describes the constitutive relationship between soil suction and soil water content.
The general shape of SWCC for various soils reflects the dominating influence of material properties including pore size distribution, grain size distribution, density, organic material content, clay content and mineralogy on the pore water retaining behaviour. Understanding the general behaviour of SWCC and its relationship to the physical properties of soil that it describes is one of important critical component of unsaturated soil mechanics. Several methods are available for obtaining the SWCC for a particular soil. SWCC may be determined directly or indirectly in the laboratory. Direct methods include pressure plate, bunchner funnel, tensiometer and pressure membranes. These methods measure the pore water pressure in the soil or impose a known air pressure to soil and allow the water content to attains equilibrium with the imposed air pressure. Among these methods conventional Pressure plate extractor is the most common method and is used in this study.
Vacuum Consolidation Methods (VCM) are used worldwide to improve the stability soft clay. Recent literature reports more number of construction case of VCM of its control over upheaval of the ground and lateral flow when the fill is constructed. Preconsolidation by vacuum consist in applying vacuum pressure on a deposit by pumping water from a grid of vertical and horizontal drains, decreasing pore pressure inside the deposit. Literature reveals that the estimation of drainage discharge and pore water pressure in the field are comparable with field observation. But there is no correspondence between the estimated settlement and observed settlement. The settlements are estimated based on Cc values of clay achieving the zero excess pore water pressure. Application of suction changes the stress state of the soil. This change in stress state changes the compressibility of soil. The objective of this study is to find the influence of stress state in the compression index of clay sand mix.
2. MATERIALS
The Clay and sand soil samples were collected from local sites, and tested for basic properties. The basic properties of clay and sand are listed as Table 1 and Table 2.
Table 1: Index Properties of Clay
Gravel (%) / -Sand (%) / 7
Silt (%) / 19
Clay (%) / 74
Liquid limit (%) / 82
Plastic limit (%) / 24
Plasticity index (%) / 53
OMC (%) / 27.3
Dry density(g/cm3) / 1.434
Activity / 0.72
B.I.S Classification / CH
Table 2: Gradation Characteristics of Sand
Gravel(%) / 1Coarse Sand (%) / 6
Medium Sand (%) / 64
Fine Sand (%) / 29
D10 / 0.23
D30 / 0.40
D60 / 0.49
Cu / 2.13
Cc / 1.419
BIS Classification / SP
3. DETERMINATION OF SWCC
Using pressure plate extractor, the drying soil water characteristics curve was obtained in the laboratory for all the soil samples (Fig. 1). This apparatus works in the principle of axis translation technique.
Fig. 1: A 15 Bar Ceramic Plate Extractor
3.1 SWCC of Clay
Soil samples at liquid limit water content were placed in the appropriate rings, levelled properly with care and covered with waxed papers. The soil samples were allowed to stand for saturation for at least 24 hours. They were removed after 24 hours, excess water from the ceramic plate was removed with pipette. The ceramic plate with the sample rings were mounted in the extractor and the outflow tube was connected with ‘O’ rings. The lid was mounted properly and the clamps were screwed down. The outflow tube was connected to the tip of the burette, fixed on a stand below the extractor cell. The pressure in the extractor was turned on with a pressure unit and maintained at 1 bar. After one hour, water from the pressure plate cell started flowing into the burette. The water level in the burette or collector and the pressure cell were noted for every hour. The pressure is adjusted to maintain a constant value of 1 bar. After 24 hours the water level in the burette stops rising. The same pressure was then maintained for 6 more hours and it was ensured that there was no further rise of burette water level. This showed that equilibrium had been attained.
After covering the outflow tube with a tight polythene cover, the pressure in the pressure plate cell was released fully. The soil samples were taken out and weighed immediately. The soil samples were replaced again in the cell for continuation of the experiment at different suction values 3, 5, 8, 10, 12 and 14 bars. After the last run (14 bars) the samples were taken out for drying in the oven. The dry weight of the samples and moisture content and different pressure stages were computed. The pressure maintained for different levels of equilibrium were the suction values at the corresponding levels of water content. The relationship between water content and suction obtained from the above procedure is shown in Figure 2.
Fig. 2: Soil Water Characteristic Curve for Clay
The SWCC of the clay and its parameters are determined from Figure 2 are listed in Table 3. The residual water content and suction of the clay are 12% and 1000 kPa recpectively. The slope of desaturation zone is the indicator of dewatering potential and is 0.376 in water content versus suction plot.
Table 3: SWCC Parameter for Clay
Description / ψi (kPa) / wr (%) / ψr (kPa) / CW / SrClay / 30 / 12 / 1000 / 0.376 / 14.95
3.2 SWCC of Sands
Though the SWCC of pure sand has limited application, tests were carried out at possible dry density and degree of saturation. The soil water characteristics curves of tested sandy soils are presented in the Figure 3. The test was conducted at lower degree of saturation (80% to 90%), the boundary effect zone did not exist. There is only one slope in the desaturation zone. The residual water content was in the range of 1 to 7%. The influence of density was very minimum for sandy soils.
Sand at a initial dry density of 1.5 g/cm3 attained the residual saturation at 60 kPa for a water content of 7%. Increase in the initial dry density by 0.1 g/cm3 decreased the residual water content to 3% for a suction value of 70 kPa. This sand is influenced predominantly by the initial dry density and is used for clay sand mix
Fig. 3: SWCC of Sand 1 at Different Density
3.3 SWCC of Clay-Sand Mix
To understand the influence of increase in percentage of sand in clay 3, different clay sand mixes were proportioned and the SWCC at initial dry density of 1.5g/cm3 was obtained and used for the interpretation. Figure 4 shows the SWCC of Clay 3 at density of 1.5g/cm3. Figure 5 shows the SWCC of 1C:1S mix. Addition of sand decreased the air entry value, residual water content and residual suction. Soil water characteristics curve parameters for the clay sand mix is given in the Table 4.
Fig. 4: SWCC of Clay 3 at Density of 1.5 g/cm3
Fig. 5: SWCC of 50% Clay-50% Sand Mix at Density of
1.5 g/cm3
Table 4: SWCC Parameters for Clay-Sand Mix
Description / ψi (kPa) / wr (%) / ψr (kPa)Clay / 20 / 12 / 1000
3C : 1S / 15 / 10 / 1000
1C : 1S / 13 / 6 / 1000
1C : 3S / 12 / 3 / 900
Sand / - / 7 / 180
Initial dry density of the sample is also influencing the shape of the SWCC. There is change in shape of desaturation zone due to the addition of sand. The change in the slope is predominant for mix of 1C:1S and 1C:3S. The range of breakage of the desaturation zone is given in Table 5. For 1C:3S, this variation is very minimal. The dewatering potential increases with increase in percentage of sand. This potential is constant for clay and 1C:3S mix in the desaturation zone. Addition of sand varies this potential in the desaturation zone.
Table 5: Details of Desaturation Zone in Clay-Sand Mix
Desaturation zone / 1C : 3S / 1C : 1S / 3C : 1S1. Suction range Water content Slope / 20–800
32–3
0.181 / 13–100
35–30
0.056 / 15–150
40–35
0.05
2. Suction range Water content Slope / -- / 100–500
30–20
0.143 / 150–700
35–26
0.134
3. Suction range Water content Slope / -- / 500–1000
20–6
0.465 / 700–1000
26–12
0.90
4. INFLUENCE OF STRESS STATE IN THE SWCC
To study the stress state influence in SWCC, two methods of testing were discussed in the literature. First method is varying the air pressure ua and effective stress σ’ and second method is consolidating the soil for a particular surcharge and to determine the SWCC. In this study, the second method of stress state influence in SWCC was studied. Soil sample of 19 mm of height and 60 mm diameter was prepared at density of 1.5 g/cm3. The prepared sample was consolidated in the conventional one dimensional odeometer consolidation apparatus upto a pressure of 1600 kPa. This consolidated sample was placed in the pressure plate extractor and relation between suction and water content was observed. This test is also conducted for the 1C: 1S mix. Also to understand the influence of suction in one dimensional consolidation test results, soil sample of above size were also placed in the pressure plate extractor, 100 kPa suction was applied in increments. This sample was taken out from the pressure plate extractor and placed in the conventional one dimensional consolidation ring and subjected to the consoli-dation pressure of 100 kPa initially and further increments was given at the interval of 24 hours upto 1600 kPa.
Suction versus water content and void ratio versus log P obtained from the above procedure are shown in Figure 6 and Figure 7. The water content at every stage of the tests are listed in the Table 6 and Table 7.
Fig. 6: SWCC of Clay and 1C:1S Mix before and after Consolidation
Fig. 7: Void Ratio versus log P of Clay and Clay-Sand (1:1) Mix before and after Suction
Table 6: Water Content Details of Suction Test after Consolidation
Water content, (%) / Clay / 1:1clay-sand mixInitial water content / 46 / 42.24
After Suction / 26.22 / 22.5
After consolidation / 21.49 / 14.9
Table 7: Water Content Details of Suction Test before Consolidation
Water content, (%) / Clay / 1:1 clay-sand mixInitial water content / 46 / 42.24
After consolidation / 30 / 17
After suction / 12.9 / 5.32
The water content at the end of the test higher than that of consolidation after suction. Application of the suction, prior to the loading increased the residual water content. The effect of increase of percentage in sand is reflected in the residual water content.
Increase in percentage of sand decreased the water content. Application of suction prior to consolidation changed the stress state of the clay sample and the clay becomes partially saturated. Because of the change in stress state the compressibility of clay decreases. The increase in percentage of sand decreases the reduction in the compressibility. 50% addition of sand to the clay reduced the Cc value of clay 0.27 from 0.31 at its saturated condition. The influence of addition of 50% of sand in this stress state variation is not significant and reduction in the compressibility is 0.038. But for clay, the reduction is 0.178. The stress state increases, as the percentage of clay increases and decrease with coarser fraction. The above results also indicate the advantage of consolidation of clay before vacuum dewatering.
5. CONCLUSIONS
From the test results following conclusions are drawn.
For predominantly medium size sand, 0.1 g of increase in initial dry density decreased residual water content by 4%. The slope of desaturation zone i.e., the dewatering potential is 0.376 in water content versus suction plot.
As the percentage of sand increases, then the SWCC parameters such as air entry value, residual water content and degree of saturation decreases. The slope of the desaturation zone is not constant. Variation in the slope was observed for 1C:1S and 3C:1S mix. The stress state has a remarkable influence in the compressibility of soils. The influence of stress state increases, as the percentage of clay increases with decrease in coarser fraction. The coefficient of compressi-bility Cc values of clay and Clay-sand mix, after application of suction of 100 kPa is lesser than consolidation without application of suction. The residual water content is higher than that of consolidation after suction. Consideration of this decrease in the compressibility to evaluate the settlement of soils in Vacuum consolidation will give correspondence between the estimated and observed settlement.