⚡ electrical engineering

Shear Strength Enhancement and Optimum Dosage Prediction of Fly Ash–Stabilized Cohesive and Cohesionless Soils

This study demonstrates that incorporating 12% fly ash optimally enhances the shear strength of both cohesive and cohesionless soils through distinct mechanisms of densification and pozzolanic cementation, while a proposed quadratic regression model effectively predicts this optimum dosage for sustainable geotechnical applications.

Original authors: MD. MOIN Akon, Khan MD Mohaiminul Islam Sho, Asif Alam Chowdhury, Swarnali Ahmed

Published 2026-07-06

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: MD. MOIN Akon, Khan MD Mohaiminul Islam Sho, Asif Alam Chowdhury, Swarnali Ahmed

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a sturdy house, but the ground beneath you is either too loose and sandy (like a beach) or too sticky and muddy (like wet clay). In both cases, the ground is too weak to hold up heavy structures. Traditionally, engineers fix this by mixing in cement or lime, but these materials are expensive and leave a big carbon footprint on the environment.

This paper explores a clever, eco-friendly alternative: using Fly Ash.

What is Fly Ash?

Think of Fly Ash as the "dust" left over after burning coal to make electricity. In Bangladesh, a power plant called Barapukuria creates about 52,000 tons of this dust every year. Usually, this dust is dumped into large ponds, which takes up land and can be bad for the environment. The researchers asked: Can we turn this waste problem into a solution for weak soil?

The Experiment: Mixing the "Recipe"

The team took two very different types of soil from Bangladesh:

Sandy Soil: Like dry beach sand. It doesn't stick together; it just slides apart.
Clay Soil: Like wet mud. It sticks together but is soft and squishy.

They mixed these soils with Fly Ash in three different amounts: 8.5%, 12%, and 15%. They then waited 28 days (letting the mixture "rest" or cure) and tested how strong the soil became by trying to shear (cut) it or crush it.

The Results: Finding the "Goldilocks" Zone

The researchers discovered that adding Fly Ash isn't a simple "more is better" situation. It's more like baking a cake: if you add too little flour, the cake falls apart; if you add too much, it becomes a dense, heavy brick. You need the perfect amount.

1. For the Sandy Soil (The "Loose Sand" Problem)

The Goal: Make the loose grains lock together tighter.
The Result: At 12% Fly Ash, the sand became 27% stronger.
The Analogy: Imagine a box of marbles (sand grains). If you just shake the box, they roll around loosely. If you sprinkle in a little bit of fine sand (Fly Ash), it fills the tiny gaps between the marbles, packing them tighter so they can't move as easily. This is called "void filling."
What happened at 15%? It got slightly weaker. Why? Because there was too much fine dust. It started acting like a lubricant, separating the sand grains instead of helping them lock together.

2. For the Clay Soil (The "Sticky Mud" Problem)

The Goal: Make the sticky mud harder and less squishy.
The Result: At 12% Fly Ash, the clay became 34% stronger.
The Analogy: Think of clay as a wet sponge. Adding Fly Ash is like adding a special ingredient that helps the sponge dry out and harden slightly, making it stiffer. This happens because of a chemical reaction (called pozzolanic reaction) where the ash and the soil bond together.
The Catch: At 15%, the results were messy. Some samples were very strong, others were weak. The researchers realized that at this high amount, it became very hard to mix the ash evenly into the sticky clay. It was like trying to mix chocolate chips into thick dough by hand—you end up with pockets of too much chocolate and pockets of none.

The Mathematical "Crystal Ball"

The researchers used a math formula (a quadratic regression model) to predict the perfect amount.

They tested 8.5%, 12%, and 15%.
The math suggested that the absolute peak strength might actually happen somewhere between 13% and 14%, a tiny bit higher than their best tested amount of 12%.
Important Note: The paper warns that this math is just a rough guess based on limited data. It's like drawing a smooth curve through just four dots; you need more dots to be sure the curve is right.

The Big Takeaway

This study proves that 12% Fly Ash is the "sweet spot" for making both sandy and clayey soils in Bangladesh stronger.

For Sand: It works by physically packing the grains tighter.
For Clay: It works by slightly hardening the bonds between particles.

By using this waste product, engineers can build stronger roads and foundations without using expensive cement, while also cleaning up the massive piles of fly ash currently sitting in ponds.

What the Paper Does Not Say

It does not claim this will work for every type of soil in the world, only the specific sand and clay they tested.
It does not say this soil is ready for immediate use in real-world construction without further testing. The study was done in a lab, and the authors admit they need to test it in the field and for longer periods to be sure it lasts.
They did not look at the microscopic level (using microscopes) to see exactly how the particles bonded; they only measured the strength and guessed the mechanism based on what they saw.

In short: Fly Ash is a useful, free, and eco-friendly tool to strengthen weak ground, but you have to mix in exactly the right amount (around 12%) to get the best results.

Technical Summary: Shear Strength Enhancement and Optimum Dosage Prediction of Fly Ash–Stabilized Cohesive and Cohesionless Soils

Problem Statement
The study addresses the dual challenge of managing industrial waste and improving geotechnical performance in Bangladesh. Fly ash, a by-product of coal-fired power plants, presents significant environmental disposal issues, with the Barapukuria Thermal Power Plant alone generating approximately 52,000 metric tons annually. Concurrently, local soils often exhibit poor engineering properties, such as low shear strength and high compressibility. While conventional stabilizers like cement and lime are effective, they carry high costs and significant carbon footprints. The research seeks to determine if Class F fly ash from the Barapukuria plant can serve as a sustainable, low-cost alternative to stabilize both cohesive (clay) and cohesionless (sand) soils, specifically identifying the optimum dosage for maximum strength gain.

Methodology
The research employed a comparative experimental approach using two distinct soil types collected from Bangladesh:

Cohesionless Soil: Poorly graded sand (SP) from Gabtoli, Dhaka.
Cohesive Soil: Clay of intermediate plasticity (CI) from Matidali, Bogra.
Stabilizer: Class F fly ash (92.9% SiO₂ + Al₂O₃ + Fe₂O₃; 0.56% CaO) from the Barapukuria plant.

Soil samples were mixed with fly ash at dosages of 0% (control), 8.5%, 12%, and 15% by dry weight. Specimens were compacted at Optimum Moisture Content (OMC) and cured for 28 days at 25 ± 2°C.

Sandy Soil: Evaluated using the Direct Shear Test (ASTM D3080) at three normal stress levels to determine the internal friction angle ( $\phi$ ).
Clay Soil: Evaluated using the Unconfined Compression Test (ASTM D2166) to determine undrained shear strength ( $C_u$ ).

The study utilized quadratic regression models to analyze the non-linear relationship between fly ash dosage and shear strength parameters.

Key Results
The study observed a distinct non-linear "rise–peak–decline" pattern in strength improvement for both soil types, indicating an optimum dosage rather than a linear relationship.

Cohesionless (Sandy) Soil:
- 8.5% Dosage: No significant change in shear strength; the friction angle remained at 29°, identical to the untreated soil.
- 12% Dosage: Peak performance was achieved. The friction angle increased from 29° to 37°, representing a 27.6% improvement.
- 15% Dosage: A slight decline occurred, with the friction angle dropping to 36°.
- Mechanism: The improvement at 12% is attributed to physical mechanisms: void filling, denser particle packing, and increased surface roughness/interlocking. Excess fines at 15% began to disrupt direct grain-to-grain contact.
Cohesive (Clay) Soil:
- 8.5% Dosage: Undrained shear strength ( $C_u$ ) increased from 79.63 kN/m² to 97.60 kN/m² (~22.6% improvement).
- 12% Dosage: Peak performance was achieved with an average $C_u$ of 106.36 kN/m², a 33.6% improvement over the untreated soil. Specimens showed consistent "Stiff" consistency.
- 15% Dosage: Results showed significant scatter (Coefficient of Variation ≈ 39%), with one specimen dropping below the untreated strength and another showing a massive increase. The average ($94.24$ kN/m²) was lower than the 12% dosage.
- Mechanism: Improvements are attributed to reduced plasticity, better compaction, and limited early pozzolanic activity. However, the authors note that the very low CaO content (0.56%) of the Class F fly ash limits extensive chemical cementation under ambient curing. The high variability at 15% was attributed to difficulties in achieving uniform mixing of fine particles in the clay matrix.

Key Contributions

Comparative Analysis: The study provides a direct experimental comparison of fly ash stabilization across both cohesive and cohesionless soils under identical laboratory conditions, a gap identified in existing literature.
Optimum Dosage Identification: It establishes 12% as the optimum fly ash dosage for maximizing shear strength in both soil types under the tested conditions.
Predictive Modeling: The authors developed quadratic regression models to describe the non-linear strength-dosage relationship. Differentiation of these models suggests a theoretical peak strength at approximately 14.1% for sand and 14.4% for clay, suggesting future testing in the 13–14% range could yield marginally higher results.
Local Context: The research characterizes the specific Class F fly ash from Bangladesh's only coal-fired power plant, providing data relevant to local geotechnical practice.

Significance and Claims
The paper claims that utilizing Barapukuria fly ash offers a practical framework for sustainable geotechnical engineering in Bangladesh. By diverting fly ash from ash ponds, the method addresses environmental disposal challenges while simultaneously improving the bearing capacity and reducing deformation susceptibility of subgrades and embankments.

The authors modestly frame their findings as a "preliminary design tool" and an "empirical trend-fitting" exercise rather than a validated universal standard. They explicitly acknowledge limitations, including the lack of microstructural analysis (SEM/XRD) to directly verify mechanisms, the use of only two specimens per condition (leading to statistical uncertainty at higher dosages), and the absence of long-term durability or field-scale validation. Consequently, the study concludes that while 12% fly ash is highly effective for immediate strength gain, future research must incorporate microstructural characterization, larger specimen sets, and long-term field testing to fully validate the approach for engineering design.