Consolidation and swell characteristics of dispersive soils stabilized with lime and natural zeolite

2.1 Soil and additives

The soil samples used in this study were taken from Afyon province located in central Turkey. The soil samples of this study are classified in accordance with the Unified Soil Classification System (American Society for Testing and Materials, ASTM); the group symbols of soils are CH (high plasticity clay) and ML (low plasticity silt). To characterize the soil samples, some physico-chemical tests were performed. Physical properties including specific gravity, particle size distribution, determination of Atterberg limits, the compaction characteristics and hydrometer test results were obtained by ASTM methods [24]. Some of the geotechnical and chemical properties of the soil samples are given in Tables 1 and 2. Two samples were prepared for each test to verify reproducibility of results.

Lime (calcium hydroxide) and natural zeolite are used as stabilization agents to solve dispersivity problems of these soil samples. Lime used for the stabilization is obtained from a marked in Eskisehir, Turkey. The zeolite used as an additive in this study was obtained in the Manisa-Gordes region. In this study, the grounded zeolite was 40 μm in size, the same form as that used in industrial applications. The physical and chemical properties of the additives used in this study are shown in Table 3.

2.2 Preparation of samples

First, soil samples taken from fields were dried in the oven at 105°C and then ground and passed through a No. 4 sieve. Different amounts of the zeolite (0%, 1%, 3%, 6%, 10%, 15% and 20% by dry weight of the soil) and lime (2% by dry weight of the soil) were added to the prepared soil samples. After the addition of additives, soil samples were mixed in order to obtain a uniform mixture. Soil-additive mixtures were prepared for each soil sample by mixing in optimum water content, which was determined at the standard Proctor energy level (ASTM D 698). Consolidation, swell percentage, swell pressure, crumb and pinhole tests were performed on the prepared samples with the optimum water content.

2.3 Pinhole test

The pinhole test is widely considered to be one of the most reliable physical tests to determine dispersivity, because it simulates the actions of water draining through a pipe in soil. For performing the test, there is an important limitation about the soil, that it should have more than 12% minus 0.005 mm particles and with a plasticity index ≥8%. Acciardi [25] performed a detailed analysis on the hydraulic characteristics of pinhole test equipment for dispersive and non-dispersive soils.

The standard pinhole test has been introduced to estimate the dispersibility properties of compacted fine-grained soils. In the test, the cracks within the embankment are represented by a hole punched through an entire specimen and the laminar flow of pure water is provided within this hole under water heads of 50 mm, 180 mm and 380 mm. As a result of the test, flow rate is recorded and effluent turbidity is estimated. During the test, flow velocity is between 30 cm/s and 160 cm/s. For the test, two separate projects were commenced to define flow conditions during the test and redesign the pinhole test equipment in Turkey [26], [27]. The first project, which was started in 1998, introduced the results for determining the problems of dispersive soils. The second one, entitled as “Redesign of pinhole test equipment used for identification of dispersive soils” was started in 2005 and successfully completed in 2007. The Scientific and Technological Research Council of Turkey financially supported both of them. As a result of these research projects, some typical problems of test equipment have been defined and new equipment, which is called mechanized pinhole test equipment, was developed to use instead of the standard equipment of the pinhole test. In this system, the water forces and flow rates during the test are controlled by electronic equipment, and the obtained data can be stored on digital media [27]. In this study, pinhole tests were performed using mechanized pinhole test equipment and the evaluation of the results was performed according to the method proposed by Acciardi [25].

2.4 Crumb test

The crumb test is the simplest of the physical tests and indicates the tendency of the soil to deflocculate in the presences of distilled water. The crumb test is also known as a rapid field test for the determination of the dispersivity. The soil is sieved from 4.75 mm sieve and collected on a 2.36 mm sieve. A sample having 15 mm equal volume shape is put in a pot that is filled up with 250 ml distilled water. The reaction between the soil and water causes colloidal (<0.002 mm) particles to segregate and form a suspension. The classification is performed by recording observations at certain time intervals [28].

2.5 Swell potential test

Swell potential has been used to describe the ability of a soil to swell, in terms of volume change or the pressure required to prevent swelling. In this study, swell percentage and swell pressure tests were carried out on samples prepared with standard Proctor energy level (600 kJ/m³) and 2% lime and natural zeolite at different rates of additives (0%, 1%, 3%, 6%, 10%, 15% and 20%). A standard test ring having 70 mm diameter and 20 mm height was used in the swell percentage and swell pressure test. The swell percentage test was performed on the odometer devices. In experiments in which the swell percentage is determined, after the samples have been subjected to a load (7 kPa) and just after being water-drowned, swell changes were measured at different time intervals (0.5 min, 1 min, 2 min, 4 min, 8 min, 16 min, 32 min, 60 min, 120 min, 240 min, 360 min, …, 2880 min) using a digital deformation meter. Test results have been evaluated based on swell percentage-time and swell percentage-additive content change depending on the additive content.

The swell pressure test was performed using direct methods. Potential volume change (PVC) equipment was used to determine the swell pressure. There is no standard procedure for the PVC meter test, so we used the method proposed by Lambe [29]. The PVC meter test involves determining the pressure arising from the inhibited swell deformation that develops after saturating the compacted soil sample with water. A proving ring handle was placed above the sample, which was compacted and placed in the system. The sample was soaked in water, the swell pressure was measured at a series of time intervals (0.5 min, 1 min, 2 min, 4 min, 8 min, 16 min, 32 min, 60 min, 120 min, 240 min, 360 min, …, 2880 min) using digital deformation meters connected to the data logger and values were periodically read from the proving ring, converted to loads in known units using a calibration curve or by multiplying them by the proving ring factor and recorded. The swell pressure was calculated by dividing the load by the cross-sectional area of the specimen.

2.6 Consolidation test

One-dimensional consolidation tests were performed to evaluate the influence of a stabilization agent on consolidation behavior of dispersive soil. In these experiments, different amounts of zeolite (0%, 1%, 3%, 6%, 10%, 15% and 20% by dry weight of the soil) and lime (2% by dry weight of the soil) were mixed with soils in dry condition. Then, water was added to the sample. This quantity of water was equal to the optimum water content which was determined by the standard Proctor test. Then, samples were compacted in a consolidation mold at maximum dry density. After preparation of the samples, the standard odometer test procedures were applied [24]. Considering swell pressure values of samples pressure stages 100 kPa, 200 kPa, 400 kPa, 800 kPa, 400 kPa and 50 kPa were chosen. To minimize evaporation or loss of fluid during the consolidation experiment, a plastic cover was placed over the odometer cell.

3.1 Pinhole test results

The results of the pinhole tests performed on samples with different additive contents are presented in Figure 1. According to the pinhole test results, it can be observed that Samples 1 and 2 show dispersive (D1) properties. Sample 1 exhibited intermediate-dispersive soil (ND3) behaviors with additive percentages of 2% lime, 2% lime+1% zeolite and 2% lime+3% zeolite, respectively, and it is classified as non-dispersive soil (ND1) with 2% lime+6% zeolite additive level. In Sample 2, it demonstrated features of a non-dispersive soil (ND1) at a level of 2% lime additive. The changes of dispersive properties with the additive percentages of the soil samples are given in Table 4. In both samples, it is observed that dispersive properties were improved depending on the additive content. The level of additive content for improvement in dispersivity characteristics of the samples was different. The reason for these results is that the value of the exchangeable sodium percentage of Sample 1 is higher than that of Sample 2 (Table 2). Therefore, improvement of the dispersive feature of Sample 1 occurred at a higher level of zeolite additive. This shows that zeolite additive can be used to improve dispersive soil with lime.

Figure 1 :

Pinhole test results of the soil samples mixed with different additive contents.

3.2 Crumb test results

The results of the crumb test conducted for two separate samples in each additive proportion are provided in Figure 2. The effect of the additive content on the dispersivity of the samples can be clearly seen in Figure 2. According to the results of the crumb test, Sample 1 is classified as non-dispersive soil (K1) with 2% lime+6% zeolite additive level. Sample 2 is classified as non-dispersive soil (K1) with 2% lime additive level. These results are in agreement with the pinhole test results.

Figure 2:

Crumb test results of the soil samples mixed with different additive contents.

3.3 Swell potential test results

Test results showing the swell percentage and the swell pressure in different additives were obtained as shown in Figure 3. The swell percentage of Sample 1 having high plasticity was obtained as 7.61% in a non-additive state. It was observed that the swell percentage reduction occurred depending on the increased amount of additives in the sample; swell percentage was 2.03% at a ratio of 2% lime+1% zeolite additive and 2.27% at a ratio of 2% lime+6% zeolite additive level. It was observed that there was a significant decline in the swell percentage at a rate of 2% lime additive and no significant change in the levels of increasing zeolite. This is due to zeolite being a pozzolan material and lack of sufficient lime content to react.

Figure 3:

Swell percentage and swell pressure vs. time plots for samples mixed with different additive contents.

Due to Sample 2 having low plasticity, swell percentage was measured as 0.84% in non-additive condition. While the swell percentage was 0.41% at a rate of 2% lime additives, it was obtained as 0.63% at a rate of 2% lime+20% zeolite additive level. The changes of swell percentage depending on increased amounts of additive in both samples are presented in Figure 4.

Figure 4:

Relationship between additive content and swell percentage of the soil samples.

When test results were evaluated in terms of swell pressure, swell pressure in Sample 1’s non-additive state value is calculated as 61.48 kPa. Swell pressure values fell to the lowest level at 2% lime+3% zeolite rate (21.1 kPa) and showed an increasing trend with increasing amount of zeolite. At 2% lime+20% zeolite additive rate, it reached a value of 35.9 kPa. While swell pressure is determined as 11.28 kPa for Sample 2 in non-additive condition, a reduction in the level of 2% lime+6% zeolite additive (4 kPa), and a slight increase after this level was observed with increasing amount of zeolite (Figure 5).

Figure 5:

Relationship between additive content and swell pressure of the soil samples.

According to these results, it is observed that dispersive soils in particular with high plasticity may have high swell potential. The more dispersed the soil system is, the greater will be the swelling caused by the concentration gradients existing at a clay-water interface. This situation is in agreement with the literature [19], [30]. The structure of the soil and the osmotic influences set up at the surface of clay, due to the differences in the concentration gradients between the pore and eroding fluid, produce swelling of the clay surface. This swelling would reduce the interparticle bonding forces and thus would be a significant factor in the erosion of cohesive soils by water. However, there is low swell potential in dispersive soil with low plasticity. In summary, the swell feature of dispersive soils can be said to be dependent on its plasticity and the swell properties of the dispersive soil can be improved using lime and zeolite additives.

3.4 Consolidation test results

One aspect of this experimental study focuses attention on the influence of additives on the settlement activity of stabilized dispersive soil. For this purpose, a series of consolidation experiments was performed on stabilized samples with different zeolite and fixed lime content (2% by dry weight of the soil), which was compacted in a consolidation mold with optimum water content and maximum dry density. A comparison between unstabilized and stabilized samples consolidation characteristics was carried out to observe the effect of lime and zeolite additives on untreated samples. The results of conventional and modified compression curves are given in Figure 6. Modified compression curves (e/e₀ vs. log p) are used in this study for the analysis of compressibility characteristics of stabilized soils with lime and zeolite additive. Since e/e₀ is a function of change in void ratio, it would not change the shape of the graph, but rearrange the curve in such a way that the relationship between the stiffness of sample can be clearly seen [31].

Figure 6:

The conventional and modified compression curves for soil samples mixed with different additive contents.

The variations in compression index (Cc) and swell index (Cs) by the amount of additives are presented in Figure 7. When Figure 7 is examined, it can be seen that there is an improvement in the compressibility properties of both samples as the amount of additive increases. The Cc and Cs values for Sample 1 in non-additive condition were obtained as 0.123 and 0.073, respectively. The lowest compression and swell index values were obtained as 0.05 and 0.015, respectively, with a 2% lime+3% zeolite additive level. There was a slight increase in the index values of increased levels of zeolite additives. For Sample 2, however, having a lower index value, the Cc and Cs values in non-additive condition were obtained as 0.093 and 0.031, respectively. There was a reduction in these values with the additive content, especially, at 2% lime+3% zeolite additive level; the Cc and Cs values were obtained as 0.033 and 0.011, respectively.

Figure 7:

Relationship between compression index (Cc) and swell index (Cs) of the soil samples depending on additive contents.

Coefficient of volume change (m_v) values exhibited similar behavior to changes in the index value (Figure 8). The coefficient of volume change for stress range of 200–400 kPa of Sample 1 in non-additive condition was obtained as 0.143 m²/MN. This value was 0.027 m²/MN at 2% lime+3% zeolite additive level, which means that almost 81% of settlement was prevented. There was no significant change in m_v values with an increase in additive content. While the coefficient of volume change for stress range of 200–400 kPa of Sample 2 in non-additive condition was obtained as 0.047 m²/MN, this value was 0.023 m²/MN at 2% lime+3% zeolite additive level, respectively. For Sample 2, a 51% reduction in the settlement will occur. This situation is in agreement with the literature. Ouhadi et al. [32] stated that consolidation settlement will be reduced by 93% by the addition of 6% lime on soft clay. The pozzolanic reaction occurring between lime and the soil was stated as the cause of this situation.

Figure 8:

Variations of coefficient of volume change (m_v) vs. different additive content.

When results were evaluated as a whole, lime and zeolite additives had a positive effect on compressibility characteristics of dispersive soil. The smallest m_v value was obtained especially in 2% lime+3% zeolite additive rate. A notable decrease in the increased amount of zeolite was not observed. In the literature, Ouhadi and Goodarzi [10] expressed that the dispersibility features and the plasticity index of dispersive soil decrease and soil compressibility increases by the addition of aluminum. This is because of the formation of flocculate structure due to the addition of aluminum by ion exchange. However, it is indicated in this study that the compressibility decreased as a result of pozzolanic reaction occurring between the soil, lime and zeolite.