Laboratory Investigations on Black Cotton Soil Stabilized With Inorganic Additives and Marginal Materials

Abstract

Many roads constructed over the Black Cotton (BC) soil in Chikmagalur district, Karnataka, India, face many problems due to the seasonal variation of the moisture in the subgrade and observed swelling and shrinkage. Therefore, in the present investigation, BC soil obtained from the Chikmagalur district is mixed with different marginal materials, viz, class F fly ash, limestone powder, construction demolition waste (CDW), coconut, and arecanut fibers. These marginal materials used are not capable of improving the strength properties of soil. Therefore, two types of binders, viz, ordinary Portland cement (43 grade) and alkali solution (NaOH solution of 8 molar concentration mixed with Na2SiO3 solution to obtain SS/SH (Na2SiO3 solution/NaOH solution) ratios of 0.5, 1.0, and 1.5), were used to improve the strength (viz, Unconfined Compressive Strength (UCS), California Bearing Ratio (CBR), flexural strength) and durability properties of the soil. When durability tests were conducted, the BC soil treated with 3, 6, 8, 10, 12, and 14% cement could not resist the soil loss under Wetting-Drying (WD) and Freezing-Thawing (FT) cycles. If stabilized materials are to be used in pavement, as per IRC 37: 2018, WD and FT durability tests are mandatory. FT test is essential for cold regions like Arunachal Pradesh, Jammu & Kashmir, Ladhak, etc. However, to study the behavior in adverse conditions, both tests were conducted. The soil properties were further im- proved by adding class F fly ash and cement. The cement dosages of 3, 4, 5, 6, and 7%, along with fly ash dosages of 0 to 32%, improved soil strength but could not control the soil loss within 14% when the durability tests were conducted. The BC soil stabilized again with 8, 10, 12, and 14% cement and fly ash dosage from 10 to 42%. The BC soil stabilized with 8% cement, and a high fly ash dosage of 42% failed in the WD test. At (cement+fly ash) dosages of (10+30), (10+35), (10+40), (12+30), (12+34), (12+38), (14+25), (14+30), and (14+36)% the BC soil exhibited soil loss of <14% after 12 WD and FT cycles. When the UCS test was conducted, there was an improvement in UCS at standard Proctor compaction. The mixes that passed durability tests (both WD and FT) were also evaluated at modified Proctor density and found improvement in UCS values. The stabilized BC soil exhibited higher resistance to weathering actions under FT cycles compared to WD cycles. The soil stabilized with higher cement content iv (14%) and fly ash dosage (>30%) exhibited a maximum retained UCS after subjecting to durability tests. At a higher dosage of fly ash (>30%), the mix exhibited low plasticity with uniform distribution of cement cluster formations based on the Scanning Electron Microscopy (SEM) images and led to significant volume stability with improved soaked California Bearing Ratio (CBR(soaked)). The above mixes with high-volume stability are preferred for pavements. The mixes that passed durability tests were further blended with 0.50% coconut fibers to study the further improvement in strength properties. The inclusion of fibers slightly reduced the density of stabilized soil mixes and thereby a marginal decrease in UCS values; however, the flexural strength of specimens increased. All the mixes exhibited significant improvement in retained UCS after durability tests. Due to the densest compact soil mix, the resistance to penetration has improved and exhibited higher CBR values. The BC soil is further strengthened by adding an alkali solution. Initially, the BC soil was treated with class F fly ash (<50%) and activated using the alkali solution. There is a marginal improvement in strength due to less dissolution of aluminosilicate materials at a lower SS/SH ratio. The strength gain is more at a high SS/SH ratio of 1.5 due to increased dissolution. To improve the strength further, 5% limestone powder was added with fly ash. However, there is no enhancement in strength. Further to en- hance the strength properties, arecanut fibers were added, the stabilized soil resulted in a marginal decrease of UCS values, with improvement in flexural strength. Again when the soil was replaced with CDW (<50%), there was a significant improvement in UCS and CBR(unsoaked) values for all alkali-activated mixes. At the same time, the CBR(soaked) values are in the range of 5–8% for various mixes. All alkali-activated specimens failed during the durability test due to mineral constituent leaching from the set soil. The alkali solution could not retain bonding due to the high moisture affinity of BC soil present in the mix. SEM images showed formations of cemented intercluster. Hydration products formed resulted in strength improvement, as observed from X-Ray Diffraction (XRD) patterns. As per IRC SP-72: 2015, the only durability passed soil mixes can be used as a modified soil layer or as an improved subgrade. The critical strain values obtained by considering the stabilized soil as subgrade, Cement-Treated Sub-Base (CTSB) for high-volume pavements were within limits as per the IRC 37: 2018. However, these mixes are generally not preferred as a Cement-Treated Base (CTB) due to the complications involved in mixing, compaction, low interface friction, etc., requiring complete quality control.