**5. Results and Discussion**

#### *5.1. Stabilization of High Plasticity (CH) Clay*

The results of the CH clay stabilization using fly ash and binders are given in following subsections. Engineering properties of stabilized mixtures were compared with the properties of untreated soil. Mechanical properties of tested materials without binders are shown in Table 5.

For mixtures with lime, a significant increase in shear strength, compressibility parameters (constrained modulus Mv) and CBR was obtained after one day and therefore no further testing was performed after 28 days.

#### 5.1.1. Unconfined Compressive Strength (UCS)

Increased soil strength is the main indicator of successful soil stabilization [14,16,18,23,31]. The results of UCS tests are presented in Figure 3. For mixtures with fly ash without a binder, the effects of stabilization were negligible because of low UCS of used fly ashes. With the addition of lime or cement, the pozzolanic reaction started and there was significant strength gain over time. Strength gain was more pronounced with the addition of lime.

The addition of PolybondTM in the minimum recommended amount led to an increase of UCS after one day, but results indicate that there was insensitivity of UCS to the elapsed time. Since the PolybondTM stabilization mechanism is primarily based on the reduction of bound water, the observed trend was expected.

**Unconfined Compressive Strength UCS [kPa]**

**Figure 3.** Unconfined compressive strength (UCS) of different mixtures with CH clay.

5.1.2. Shear Strength Parameters in Terms of Effective Stresses

Shear strength parameters affect the safety of any geotechnical structure. They are essential for earth structures design, calculation of bearing capacity and earth pressures, stability analysis of natural slopes, cuts and fills [63,64]. Effective shear strength parameters were determined using a direct shear test and they are presented in Figure 4.

**Figure 4.** Shear strength parameters of different mixtures with CH clay.

Obtained results show that the friction angle does not substantially change with the addition of fly ash and PolybondTM. With the addition of cement, there was a mild increase of the friction angle, but a significant increase was noted with the addition of lime after only one day. The cohesion significantly increased with time for all tested mixtures. For mixtures with fly ash without binders, a slow pozzolanic reaction occurred due to the presence of reactive CaO. After the addition of cement or lime, a more pronounced pozzolanic reaction occurred as well as the creation of cement joints. The effects of treating CH clay with PolybondTM were particularly expressed in terms of soil cohesion—the increase of cohesion was evident.

#### 5.1.3. Compressibility Parameters

In order to calculate the consolidation settlement of soils, compressibility parameters are required. The constrained modulus from a one-dimensional compression (oedometer) test is a commonly used parameter to determine the settlement of a tested material. The vertical effective stress level 100–200 kPa was selected to display the results. A similar trend was observed for other stress levels. Constrained moduli were increased for all mixtures (Figure 5). Stabilization effects were greater with the addition of cement or lime.

**Figure 5.** Compressibility parameters of different mixtures with CH clay.

#### 5.1.4. California Bearing Ratio (CBR)

The California bearing ratio (CBR) is a parameter that describes the strength of roads subgrade. It is used for the determination of pavement thickness and its component layers [65,66]. Clays generally have low CBR values (<5), which make them inappropriate for road subgrade construction. Obtained results showed significant CBR gain. In the case of mixtures with fly ash and PolybondTM, there was a mild, but important increase of strength, which made the tested soil usable for road construction. Test results (Figure 6) were in line with [18,19,22,23,31]. It is obvious that used binders had a significant influence on CBR gain.

**Figure 6.** The California bearing ratio of different mixtures with CH clay.

#### 5.1.5. Swell Potential

The volumetric change of soil causes movements in structures and imposes additional loads to structures [67,68]. According to [16], fly ash replaces some of the volume held by clay particles and acts as a mechanical stabilizer.

By addition of fly ash, the swell potential of all tested mixtures was entirely eliminated, which was somewhat expected considering the medium degree of expansivity of tested CH clay [48]. On the other hand, the addition of PolybondTM, reduced the swelling deformation to about 1%.

#### *5.2. Fly Ash and Ash-Slag Mixtures as a Material for Embankment*

The engineering properties of ash and ash-slag mixture were discussed below. As in the case of high plasticity clay, tests were performed on the samples with and without binders and the results were compared. For all tested materials similar trends were observed and therefore test results for fly ashes and ash-slag mixtures would be considered together. Mechanical properties of tested materials without binders are given in Table 6.

#### 5.2.1. Unconfined Compressive Strength (UCS)

The results of UCS tests are presented in Figure 7. Samples without a binder had very low UCS (Table 6). The pozzolanic reaction started with the addition of binders and water and constant strength gain over time could be observed. The substantial increase was recorded for fly ash samples. The strength gain was more pronounced with lime addition.

**Figure 7.** UCS of waste materials with binders.

#### 5.2.2. Effect of Frost

The frost resistance of fly ash and ash-slag mixtures treated with binders was tested by measuring the UCS reduction. Samples aged 28 days were exposed to repeated freezing and thawing (15 cycles) and the UCS was determined. The frost resistance index is represented by the relation between the UCS of the sample after 15 freezing/thawing cycles and UCS of the reference sample (28 days old). According to standard SRPS U.B1.050:1970 [58], the mixture is frost resistant if the index is greater than 80%.

Results for ash-slag mixture TENT A with 7% of lime were missing because the samples were damaged during testing. Tests were not performed for fly ash KOS FA. The results are shown in Figure 7. The obtained frost resistance indices were within: 75–86% for TENT B, 69–83% for TENT A and 43–82% for KOS AB. Absolute values of UCS after freezing/thawing cycles classify mixtures as stiff to hard [64] and despite some lower indices values, the mixtures could be considered as frost resistant. The low index value for KOS AB with 2% of cement might be due to damage of samples during testing.

#### 5.2.3. Shear Strength Parameters in Terms of Effective Stresses

Considering fly ash and ash-slag mixtures as a fill material for embankments, the strength of compacted material is of major importance to geotechnical engineers. High shear strength ensures higher bearing capacity and slope stability. Shear strength parameters are given in Figures 8 and 9.

**Figure 8.** Shear strength parameters of fly ash samples with binders.

**Figure 9.** Shear strength parameters of ash-slag samples with binders (some test results were omitted because obtained values were too high).

Compared to the strength of the compacted sand (as traditional fill material) [37], all tested mixtures without binders had high values of friction angle (ϕ' = 31–35◦). According to USA Navy [69] the friction angle for compacted sandy soils typically range from 31◦ to 45◦. Test results show an increase of shearing resistance for all samples over time and with the increase of binder amount. Similar results were obtained for both binders. Obtained friction angles after 28 days were within the range 39◦ to 45◦, which made tested materials generally comparable to the traditional compacted sandy soils.

Considering cohesion as an apparent shear strength parameter that captures the effects of intermolecular forces, soil tension or cementation, class F fly ash and ash-slag mixtures exhibit no cohesive characteristic in the saturated state [44]. In this case, cohesion is a consequence of the approximation of the non-linear failure envelope with a linear one. The failure envelope obtained from the strength test is a curved line for mostly granular materials, but solving the majority of soil mechanics problems, it is sufficient to approximate the shear stresses as a linear function of the normal stresses. The magnitude of cohesion is thus defined by the intersect segment on the shear stress axis. After adding binders and with the addition of water, the pozzolanic reaction occurred as well as the formation of bonds between the soil particles. The increase in cohesion was evident due to the cementation process, but there was no clear trend over time and with an increase in the % of the binder. For fly ash samples, the substantial increase over time had recorded for KOS FA and for TENT B with cement addition, while with the addition of lime there was no further increase of cohesion after 7 days. For ash-slag mixtures, there was a scattering of cohesion results, probably due to the inhomogeneity of the samples and due to the method used for strength determination. In the direct shear test, the orientation of the failure plane was predetermined as being near the middle of the sample height. Better results might be obtained in the triaxial device where the orientation of the failure plane is governed by the soil structure.

For fly ash samples KOS FA with higher % of binders (5% cement and 6.8% lime), no further testing was carried out after 28 days, because there was a significant increase in tested parameters for a smaller binder amount. Some test results for ash-slag mixtures were omitted because the obtained values were too high.

### 5.2.4. Compressibility Parameters

Compression of compacted fly ash or ash-slag mixture in wide embankments can be considered as one dimensional [37]. Thus, constrained moduli from oedometer (one-dimensional consolidation) tests were obtained and results are shown in Figure 10. According to Kim et al. [37] and Carrier [70] relevant constrained moduli should be calculated for vertical stresses ranging from zero to 200 kPa, a range of stress levels typically expected in highway embankments.

**Constrained modulus Mv [kPa] (100-200 kPa)**

**Constrained modulus Mv [kPa] (100-200 kPa)**

**TENT B+Cem 2% TENT B+Cem 4% TENT B+Lime 5% TENT B+Lime 7% KOS FA+Cem 2.5% KOS FA+Cem 5% KOS FA+Lime 4.8% KOS FA+Lime 6.8%**

**Figure 10.** Compressibility parameters of waste materials with binders.

Ash-slag samples without binders exhibited slightly greater compressibility than fly ash samples. With the addition of binders constrained moduli of fly ash TENT B and ash-slag mixtures increased with time and with the percent of the binder. For fly ash samples KOS FA the stabilization effects were negligible.

Additionally, a comparison was made with sand compressibility given in [70]. Figure 11 shows typical moduli values for sand compacted at relative compaction (RC) of 85% and 99% and moduli (after 28 days) obtained from research as a function of vertical effective stresses. Constrained moduli are shown for the midpoint of the stress interval for which they are calculated. For the same compaction levels, fly ash samples TENT B with binders and ash-slag mixtures TENT A with lime are significantly

less compressible than sand, while most of the other values of moduli lie near the upper limit of the sand moduli range.

**Figure 11.** Constrained moduli of waste materials and sands.

Effective vertical stress [kPa]
