Specifications for lime stabilised base course

```html

Specifications for Lime-Stabilized Base Course in Black Cotton Soil Areas

Introduction

Black cotton (BC) soil, also known as expansive soil, poses significant challenges in road construction due to its high swelling and shrinkage potential when exposed to moisture changes. Traditional methods, such as using rubble or boulder soling (150-250 mm thick) overlaid with water-bound macadam (WBM), often lead to wavy surfaces and distress because of poor subgrade support and soil intrusion into voids during wet seasons.

The California Bearing Ratio (CBR) of untreated BC soil under saturated conditions is typically around 4%. For traffic intensities of 400-1500 tons per day, a total pavement thickness of about 270-380 mm is required based on CBR design methods (per IRC:37 guidelines). Lime stabilization enhances soil properties, increasing CBR to 20% or more, reducing required thickness and costs.

Research from Central Road Research Institute (CRRI) and state labs shows that 4-5% slaked lime mixed with BC soil and compacted at optimum moisture content (OMC) of 20-25% yields a minimum CBR of 20% under saturated conditions. This allows a 225-250 mm lime-stabilized base to support a 150 mm hard crust wearing surface.

Benefits of Lime Stabilization

Lime stabilization is a cost-effective, sustainable method widely used in India for expansive soils. Key benefits include:

• Improved Strength: Increases CBR from 4% to 20-30% with 4-6% lime, enhancing load-bearing capacity (IRC:51-1992).
• Reduced Swelling: Decreases plasticity index (PI) by 20-50%, free swell by up to 65%, and swelling pressure by 76% (studies on Indian BC soils).
• Enhanced Durability: Forms cementitious compounds (calcium silicate/aluminate hydrates) that bind soil particles, reducing volume changes and improving resistance to freeze-thaw cycles.
• Cost Savings: Reduces pavement thickness by 20-30%; e.g., a study showed 28% cost reduction in flexible pavement design using lime-fly ash mix.
• Environmental Advantages: Utilizes locally available soil, reduces need for imported aggregates, and lowers carbon footprint compared to cement stabilization.

Drawbacks: Requires 7-28 day curing; effectiveness depends on soil mineralogy (high clay content needs more lime). Potential for carbonation in long-term if not properly mixed.

Current practices often combine lime (3-8%) with fly ash (10-20%) for better pozzolanic reactions, achieving CBR up to 40% (IRC:88-2010). Nano-materials like organosilane can further enhance water repellency.

Recommended Design and Thickness

For economical design, use a two-layer base course:

• Lower layer (125 mm compacted): BC soil + 2% lime (CBR ≈ 8-9% saturated).
• Upper layer (125 mm compacted): BC soil + 5% lime (CBR ≥ 20% saturated).

Total base: 250 mm, topped with 150 mm WBM wearing course and two-coat asphalt surface dressing.

Data from Studies:

Lime Content (%)	CBR (Saturated, %)	UCS (kPa)	Source
0 (Untreated)	2-4	80-100	CRRI, IRC
2-3	8-12	150-200	Studies on Indian BC soil
4-6	20-30	200-300	IRC:51-1992, Recent research
7-9 (with fly ash)	30-40	>300	IRC:88-2010

Optimum lime: 4-7% based on pH test (target pH 12.4) or UCS/CBR maximization.

Construction Specifications

Follow IRC:49-1973 for soil pulverization and IRC:51-1992 for mixing.

1. Embankment Preparation: Minimum 450-600 mm height using available soil, compacted in 200-225 mm layers.
2. Lower Layer: Spread 150 mm loose BC soil over full embankment width.
3. Lime Addition: Spray 2% freshly slaked lime (by weight), water profusely, and allow reaction for 3 days.
4. Mixing: Thoroughly mix using phavras (manual) or machinery at 20-25% moisture (OMC).
5. Compaction: Roll with 8-10 ton roller from edges to center, achieving density ~1.72 g/cm³ and 1:48 camber.
6. Upper Layer: Repeat with 150 mm loose BC soil + 5% lime.
7. Curing: 7 days with wet sand/straw or sprinkling (avoid ponding).
8. Wearing Course: 150 mm WBM (two 75 mm compacted layers) with blinding and wet rolling.
9. Surface Dressing: Two coats of asphalt; if budget-constrained, use from repair grants.

Quality Control: Ensure soil PI >10% and >25% passing 75µm sieve for effective lime reaction (IRC guidelines). Test CBR post-curing.

Modern Updates and Research Insights

Recent studies (2020-2024) confirm lime's efficacy:

• Combining with fly ash (1:4 ratio) achieves CBR >8% and UCS 224 kPa, reducing costs by 28% (Springer study, 2024).
• Nano-additives (e.g., organosilane) make soil water-repellent, maintaining CBR even in soaked conditions.
• Environmental: Reduces aggregate mining; lime-fly ash is sustainable per IRC:SP-72-2015.

Case: In Uttar Pradesh (UPPWD), lime-stabilized BC soil sub-bases extend pavement life by 20-30% in high-rainfall areas.

Always consult latest IRC codes (e.g., IRC:37-2018 for design) and site-specific testing for optimal lime dosage.