Utilization of Low CBR Soil for Flexible Pavements for Low Volume Roads with Robosand Stabilization

Abstract

Many rural areas in developing countries like India lack adequate and affordable access to transport infrastructure and services. The problem is more persistent in low volume and low CBR value sub-grades. The objective of this project is to evaluate the required flexible pavement for very low volume traffic roads with very low CBR values with practically very low traffic of not more than 30-40 vehicles mostly comprising passenger cars where the level of serviceability is very high. The IRC recommended pavement composition is for unpaved gravel roads in rural areas with low volume traffic. There are various disadvantages associated with construction of the road using gravel like dust generation, and gravel loss over a period of time due to passage of vehicles and inaccessibility during rains and the quality of serviceability is unsatisfactory. Problems with sub grade having low CBR values are stability and large deformations or settlements. A project has been taken up where the CBR of sub grade soil is less, the soil is improved by mechanical stabilization with an additive of robosand. The emphasis of the project is ‘Utilization of low CBR soil for flexible pavements for low volume roads with robosand stabilization’. The addition of robosand has improved the sub soil condition in achieving higher CBR value. Locally available soils mixed with crushed robosand serve as effective reinforcement in soft soils for different sub-grade resulting in technically better sub-grade as well cost economy in savings of aggregate material and also reducing carbon foot print.

Introduction

Overview:
Rural road connectivity is crucial for promoting economic growth, agricultural productivity, and poverty reduction. In India, the Pradhan Mantri Gram Sadak Yojana (PMGSY) aims to connect rural habitations through durable, all-weather roads. Many such roads are gravel-based and require strong subgrade foundations to withstand traffic loads.

Study Focus:

This study, conducted in Sulthanpur, Medak District, Telangana, explores improving the strength of subgrade soils—particularly low CBR black cotton soil—using robosand (a byproduct of stone crushing).

Objectives:

1. Enhance subgrade strength in low CBR conditions using robosand.

2. Evaluate and compare soil performance after stabilization.

3. Identify 20% robosand as the optimum mix for improved engineering properties.

Methodology & Key Tests:

• Soil Classification: The native soil is fine-grained silty sand with clay content, having low natural strength (CBR = 7.02%).

• Compaction Tests: Conducted to find Maximum Dry Density (MDD) and Optimum Moisture Content (OMC).

  • MDD improves from 1.691 kN/m³ (0% robosand) to 1.727 kN/m³ (20%).

  • OMC decreases from 17.105% to 13.924%, enhancing compaction efficiency.

• CBR Tests: Measure subgrade strength improvements.

  • CBR increases from 7.02% (0%) to 12.5% (20%).

  • Pavement thickness can be reduced from 55 cm to 20 cm, resulting in significant cost savings.

• UCS Tests: Measure unconfined compressive strength.

  • UCS increases with robosand content, peaking at 20%, beyond which strength declines.

Key Findings:

• 20% robosand is optimal for improving both compaction and strength.

• Excessive robosand (over 20%) leads to reduced bonding and mechanical strength due to poor particle interaction.

• Stabilized soil reduces material usage and construction costs by enabling thinner pavement designs.

• Robosand proves to be a viable, cost-effective stabilizer for rural road construction.

Conclusion

From the experimental works carried out, following conclusions can be made regarding the aspect of strength improvement of soil due to mixing of robosand. From the results and discussions of Standard Proctor Test on admixture soil, the maximum dry unit weight (MDU) is observed to be maximum at 20% robosand. MDU values starts to decrease for higher percentage of robosand(above 20% respectively). It is also observed that, OMC values show just opposite trend of MDU values for admixture soil. From the results of CBR Test, addition of robosand in percentages increases the CBR value from 7.02 (Natural soil) to 9.83 (for 5% robosand), 10.61(for 10% robosand), 11.82 (for 15% robosand), and 12.50 (for 20% robosand). Beyond the limit of 20% robosand, CBR values start showing decreasing pattern. It means that the thickness of sub-base can be reduced to about 30 cm and this directly leads to saving in pavement construction cost. To determine the Unconsolidated, Undrained strength of a low CBR soil the Unconfined Compressive Strength test was carried out for all the 5%, 10%, 15% and 20% fraction mixtures of robosand and the strength is ranging between 140.233 KPa to 149.06 KPa. The free swell index FSI was determined and found to be ranging between 86.33% to 72.33% for the fractions of robosand mixtures between 10% to 20%. Similarly, the Optimum Moisture Content and Maximum Dry Density test were carried out to determine the degree of compaction and the results varied between 18% - 16.5% and 1.58-1.64 for the fractions of robosand mixtures between 10% to 20%.

References

[1] Pradeep Muley, Research Scholar, IIT Roorkee, and P. K. Jain, Professor, MANIT Bhopal, “Betterment and Pridiction of CBR of Robosand Mixed Poor Soils” Proceedings of Indian Geotechnical Conference December 22-24,2013, Roorkee [2] Swapan Kumar Bagui “Pavement Design for Rural Low Volume Roads Using Cement and Lime Treatment Base” Jordan Journal of Civil Engineering, Volume 6, No. 3, 2012 [3] “Quality Assurance Handbook for Rural Roads EQUIPMENT AND TEST PROCEDURES” Ministry of Rural Development Government of India [4] V.K. Sinha, H.N. Singh & Saurav Shekhar Paper No. 535 “Rutting In Flexible Pavements – A Case Study” [5] Raja J and G.L. Siva Kumar Babu published in VOL.41 NO.6 “Bamboo as Subgrade Reinforcement for Low Volume Roads on Soft Soils” JUNE 2013 INDIAN HIGHWAYS [6] IS : 2720 -1977 , Indian Standard methods of test for soils (All parts)

Copyright

Copyright © 2025 Mrs. Soniya Grace, Mr. Depavath Jagan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.