Strength characteristics of dispersive soil stabilized with fly ASH

Abstract

Dispersive soil is problematic soil that erodes or dislodges easily in the presence of flowing water or even stagnant water. The soil is highly susceptible to erosion due to the presence of a high percentage of exchangeable sodium ions in the soil particles, and that is why it is called sodic soil. This study aims to determine the effectiveness of fly ash in stabilizing dispersive soil. Fly ash is considered an industrial waste that is harmful to the environment and so requires proper disposal. In this study, the potential industrial waste fly ash (Type F) has been used to reduce the dispersivity of sodic or dispersive soil.

At first, to determine the degree of dispersivity crumb test, double hydrometer test, pinhole test and chemical tests are carried out. Compacted samples are prepared using three fly ash contents (3%, 6%, and 9% by weight). Stress-strain behavior of stabilized compacted samples is assessed by conducting unconfined compression tests on samples cured for different periods, i.e., 0, 7, 14 and 21 days. The microfabric arrangement before and after adding various contents of fly ash with dispersive soils is observed by scanning electron microscopy (SEM).

It is observed from the crumb test that dispersivity of soil is reduced with the addition of various percentages of fly ash contents. The dispersivity of soil reduces from Grade 4 to Grade 1, which is nondispersive. Compaction tests are carried out on both treated and untreated soil samples following the Standard Proctor specification. It is observed that due to the progressive addition of fly ash with the dispersive soil, the maximum dry density increases a little, and on the other hand, the optimum moisture content (OMC) decreases with the increasing fly ash. Addition of fly ash with 0%, 3%, 6% and 9% decreases the OMC to 19.9%, 19.8%, 18.4% and 14.5%, respectively.

The double hydrometer test reveals that the flocculation process has been improved with the addition of fly ash which acts as a binding agent. Additions of 3%, 6% and 9 % fly ash reduce the value of dispersion by 25%, 29% and 33%, respectively, from untreated soil. It is observed from the Pinhole test that the degree of soil dispersivity decreases significantly from the state ND4 to ND1with the increment of fly ash contents.

Unconfined compression test exhibits for soil sample prepared with 3% fly ash that failure strain of treated soil sample is reduced by 4% than that of untreated soil. Along with reducing the degree of dispersiveness of soil, fly ash content increases the overall unconfined compressive strength by 325 kPa in 21 days of curing of remolded soil samples which is 2.5 times of 0-day sample. It is also observed that with the increase of curing age, the water content of the soil sample reduces, which makes the sample brittle. Hence, a higher percentage of water content is required for proper hydration and pozzolanic reaction. SEM images of the treated sample show that fly ash reacts with the soil particles and forms calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels and improves compactness, cementation, and bonding among the soil particles. The microstructure of treated soil with fly ash reveals that the voids in between the soil particles are reduced. Hence, it provides resistance and improves the strength characteristics of dispersive soil.