Vivek Singh, Founder, BuildStreet

Economization of Road Subgrade Material by Mechanical Stabilization

Subgrade makes the formation level of the road crust. The material specifications of subgrade material are therefore more demanding than the soil used in general fill or embankment. In some construction projects, it becomes difficult to get the required subgrade material at an economical lead. In such cases, the properties of locally available materials are required to be improved by stabilising them, either chemically or mechanically, in order to use them as subgrade material. One such case study where the available soil was stabilised mechanically and utilised as subgrade material is presented hereunder.

The Problem
In a city infrastructure project, the CBR value of locally available soil was not meeting the design requirement of 8%. Availability of required quality of subgrade soil was at a lead of 50 km from the project site.

Impact
  • Increase in the cost of subgrade construction due to haulage cost
  • Number of haulage trucks needed to be increased to keep pace with the project schedule
Visible Options
  • Lime Stabilisation
  • Cement Stabilisation
  • Using some stabilising agent
Limitations
Lime or Cement Stabilisation was not helping in reducing the cost. Getting approval for any stabilising agent was a lengthy process and would have taken a lot of time.

Opportunity
There were some stone quarries in the nearby areas whose overburden was a highly weathered rock. In its present stage, it was like gravely soil with nil PI value. The material was available at an attractive landed price since the supply was higher than the demand. An initial study was conducted in which this material was mixed with the soil available in the embankment Borrow Area and an improvement in CBR was observed.

Detailed Study and Lab Tests
Following tests were conducted for this study on Borrow Area Soil, Granular Material (Quarry Overburden Material) and their blends (80:20 & 90:10):
  • Index properties
  • Grain size analysis
  • CBR tests on blended material
The test results are tabulated below.

S. No. Material Gravel Content Sand Content Fines Content Plasticity/ Classification CBR
1 Granular (Quarry Overburden) Material 85% 13% 2% NP >100%
2 Borrow Area Soil 0% 5% 95% MI-ML around 5%
3 80% Soil+20% Granular 17% 9% 74% NP 27% (average)
4 90% Soil+10% Granular 8% 9% 83% NP 20% (average)
Methodology of Work
  1. Soil and Granular Material were dumped on the bed in three layers in the following sequence:
    1. Soil in loose thickness so as to attain a compacted thickness of 75mm
    2. Granular material in a loose thickness so as to attain a compacted thickness of 50mm
  2. All three layers were uniformly mixed with the help of a rotovator
  3. Required water added followed by grading to attain required level and camber
  4. Rolling pattern of “1 plain pass+1 pass with High Vibration+1 pass with Low Vibration+ 1 pass with High Vibration” was found good to attain greater than 97% compaction during the mock-up trial.
Cost Analysis
Cost analysis was done considering the following attributes:
  • Landed cost of overburden material to the stockyard
  • Landed cost of soil (from maximum 27 km average lead)
  • Cost of inter-carting of granular material from stock to the roadbed
  • Cost of in-situ mixing of soil and overburden material
The above cost was then compared with the cost of conventional subgrade construction from the borrow area at a 50-52 km average lead.

SL No. Description UOM Quantity Rate
(Rs Per Cum)
 Amount (Cr.)
1 Use of Borrow earth in Subgrade incl. (without granular material & 50 To 52 Km as average lead) L.cu.m. 10 523 52.3
2 Use of Soil (80%- With 27 Km as average Lead) and Granular Material (20%) Mix in Subgrade incl. (without granular material) L.cu.m. 10 494 49.4
Saving In Option 2 Over Option 1  Rs. (Cr) 2.9  
  % 5.9
Discussions
Over and above the lab tests, a close observation was done during the rainy season. Due to the very nature of the project the construction traffic was bound to ply over the prepared beds during the monsoon. The mechanically stabilised layer had clearly shown a very good improvement in performance when traffic was plying directly over it. This soil, when not stabilised, used to become weak when exposed to rainfall even after compaction and could not bear the load of even light vehicles.

The reason for the soil (un-stabilised) to behave (as mentioned above) was probably because it was uniformly graded silt (95-100% fines). As is evident from the test results, the blended material has got all three major grades of soil (Gravel, Sand, and Fines) which makes it a well-graded material that is more stable and easily compactable. The shear strength of well-graded soil is always better than uniformly graded soil and hence also the CBR.

The improvement in the most desired subgrade soil property, CBR, was clearly evident when the granular material is mixed with the fine-grained soil. A few points to be noted:
  • Less than 10% of granular material was not considered blending because of the mixing method chosen. Lesser proportion can be considered if:
    1. the mixing is done at the plant using a pug mill
    2. if it can be proved that uniform mixing can be done by site mixing method (s).
  • In spite of getting the desired CBR value at 10% quarry overburden, it was proposed to mix 20% so as to eliminate all possible errors and ensure the required CBR of subgrade in the absence of any data on process performance.
  • The CBR was conducted on the site mixed material on a regular basis i.e., samples were collected from the bed after mixing and grading and before compaction. This was done to measure the performance of the site process as well as for quality control. The CBR values of such samples were generally between 20 - 27% and the minimum CBR which was ever found was 15%.
The statistical analysis of the CBR values of field samples was conducted and the mix was re-designed to the proportion of 85:15:Soil:Granular. This enhanced the cost saving to 10.5%. The costing may change from situation to situation; hence, detailed working must be done before taking a decision.

Conclusion
The following were the enhancement in value for different stakeholders:
  • 80% of the locally available soil got utilised
  • Reduction of costlier material, the soil, by 20% and 15% has resulted in cost savings of 5.9% and 10.5%, respectively
  • A good process control not only ensures good quality but also ensures savings in cost
  • Resources saving in terms of reduction in the number of haulage trucks achieved
  • Faster construction because of lesser transportation and stacking of overburden
  • Lesser carbon footprints than using cement or lime
  • Ensured quality subgrade constructed by ensuring 15% min. CBR against the design requirement of 8%.
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