IRC 36 (2010) outlines best practices for building earth embankments and subgrades tailored for road projects across India. It offers detailed guidance on selecting materials, conducting soil investigations, designing embankments, executing compaction procedures, and implementing quality assurance to achieve robust and lasting road foundations. This code is vital for engineers, contractors, and planners engaged in highway and infrastructure earthwork.
Overview
IRC 36 (2010) outlines best practices for building earth embankments and subgrades tailored for road projects across India. It offers detailed guidance on selecting materials, conducting soil investigations, designing embankments, executing compaction procedures, and implementing quality assurance to achieve robust and lasting road foundations. This code is vital for engineers, contractors, and planners engaged in highway and infrastructure earthwork.
Audience
Contents
Structure
This section defines the range of IRC 36, covering specifications for earth embankment construction and associated earthworks in highway projects. It highlights essential parameters such as soil classification per IS 1498, sieve analysis sizes, plasticity and shrinkage limits, free swelling index, maximum dry density, and optimal moisture content. Compaction and density criteria are emphasized, referencing Clause 8.3.9 and 4.4.6, with Tables 3 and 4 detailing minimum relative compaction percentages and dry unit weights required for different embankment and subgrade materials. Ensuring these parameters assures stability and performance of road foundations.
This part provides essential definitions related to soil compaction and embankment construction, supported by compaction and density specifications found in Tables 3 and 4. Clause 4.4.6 mandates use of materials conforming to these compaction percentages. Relative compaction is defined as a percentage of laboratory maximum dry density (IS 2720 Parts 7 or 8) required for various elements: subgrade and shoulders require at least 97%, embankments need 95%, and treated expansive clays must meet 95%. Minimum dry unit weights vary with road classification and embankment height, ensuring stability.
Guidance for material selection includes compaction and density requirements alongside soil classification. Subgrade and earthen shoulders must achieve a minimum relative compaction of 97%, embankments at least 95%, and expansive clays are prohibited for subgrade and allowed for embankments only after treatment with 95% compaction. Density standards vary with embankment height and flooding exposure, with minimum dry unit weights specified for National/State Highways and rural roads. Soil classification follows IS 1498, considering plasticity and CBR values to evaluate material suitability.
This segment details requirements for soil surveys and laboratory assessments, including grain size distribution, plasticity limits, organic content, and classification using symbols like ML, CL, CH. Soils like low plasticity inorganic clays show unit dry weights from 1.44 to 2.08 g/cm³ and CBR values up to 15%. Highly organic soils such as peat are unsuitable for subgrade. Laboratory tests must cover sieve analysis, Atterberg limits, shrinkage and swelling indices, maximum dry density, optimum moisture content, IS 1498 classification, and CBR values. Material choices for embankments and subgrades depend on these evaluations.
This chapter focuses on key structural properties, including a minimum dry density requirement of 16.5 kN/m³ for embankment materials, except lightweight fills like cinder or fly ash. Construction involves soil surveys, accurate setting out of limits, and controlled compaction to meet density and stability targets. Proper drainage and allowance for settlement are crucial to maintain embankment integrity. Layered construction with rigorous compaction and quality control is mandatory as outlined in various clauses.
Procedures for delineating construction limits include establishing road alignment on-site with instruments such as theodolites and total stations. Centreline pegs are placed at consistent intervals, with additional pegs marking horizontal curve beginnings, ends, and tangent intersections. Sketches illustrate control peg placement to firmly define embankment boundaries. Mechanically stabilized steeper slopes are permissible under specified conditions, ensuring safety within the defined construction zone.
Initial activities encompass setting out construction boundaries and preparing the site. Control peg layouts guide earthwork positioning, while soil investigations validate design assumptions. These preliminary steps secure proper alignment and dimensional accuracy. Although detailed formulas or tables for these operations are not included here, the code references them within Section 7.
Guidelines for borrow pit use include excavation of test pits to depths not exceeding anticipated borrow depths by more than 150 mm, with representative sampling. Borrow pits must be sited to avoid infringing a 1 vertical to 4 horizontal slope from the embankment edge, preserving foundation stability. Only approved materials are permitted, reserving the highest quality for subgrade layers. Riverbank borrow pits require a minimum 15 m setback from the bank toe, factoring flood magnitude and complying with specific clauses.
Construction methods involve spreading soil in thin layers (up to 150 mm loose thickness) and compacting thoroughly with suitable machinery such as power rollers or mechanical tampers. Equipment selection depends on soil type, with vibratory rollers preferred for rock till and coarse soils, and sheep foot rollers optimal for clayey soils. Detailed equipment recommendations and compaction procedures are found in the referenced HAB Special Report No. 3.
For roads built on flood banks, specifications include maintaining a cover between 0.75 m and 1.25 m over the saturation line, which slopes at 1 vertical to 6 horizontal from the high flood mark. Rear berms may be incorporated to ensure this coverage. Soil properties may affect saturation line slope adjustments. Repairs after water damage require removal and recompaction of affected soil layers using mechanical equipment, with density testing mandated. Narrow cuts must be widened to accommodate compaction machinery.
Special provisions allow mechanically stabilized slopes steeper than standard recommendations. Compaction and density criteria remain stringent, with embankments up to 3 m requiring minimum dry unit weights of 15.2 kN/m³ for major roads and 14.4 kN/m³ for rural roads, increasing for taller embankments or flood-prone areas. Subgrades and shoulders demand even higher densities to guarantee structural integrity under challenging conditions.
Earthen shoulders must be built using the same material as the top 500 mm of the subgrade embankment, compacted to match density standards. Their construction should proceed concurrently with pavement layers. In waterlogged or marshy zones, a coarse sand blanket per IRC 34 is required beneath shoulders. Minimum dry density for these shoulders and subgrade is specified to ensure durability.
Fly ash, encompassing pond, bottom, or mound ash but excluding fine fly ash alone, may be used as embankment fill. It must come from proximate thermal power stations and be covered with suitable soil layers meeting compaction and density requirements. Particle size and compaction parameters per IS 2720 must be documented and approved. Compaction should be continuous and thorough, achieving minimum dry densities. Earth shoulders over fly ash embankments follow the same compaction criteria.
Compaction control mandates adherence to density requirements listed in Tables 3 and 4. Field density is measured via sand replacement or nuclear densometer methods in accordance with IS 2720 standards. If target densities are not met, the layer must be loosened, moisture adjusted, and re-compacted. Quality assurance tests include particle size analysis, plasticity and shrinkage limits, free swelling index, Proctor compaction tests, soil classification, and CBR determination, ensuring material and compaction adequacy.
Materials for embankments and subgrades must fulfill compaction and density criteria to be accepted. Subgrade and shoulders require a minimum 97% relative compaction; embankments, 95%; expansive clays require treatment and compaction of at least 95%. Minimum dry unit weights vary by embankment height and road classification to ensure sufficient strength and stability in the constructed roadbeds.
Frequently Asked
IRC 36 Clause 4.4.1 specifies that suitable embankment and subgrade materials include soil, moorum, gravel, reclaimed pavement debris, fly ash, pond ash, or their combinations, provided they are free from organic debris such as logs, roots, and rubbish. Unsuitable materials encompass those from swamps, marshes, or bogs; peat and highly organic soils classified as OL, OH, or Peat under IS 1498; materials prone to spontaneous combustion; frozen soils; clays with liquid limits above 70 and plasticity indices over 45; soils containing leachable salts; and expansive clays with a free swelling index exceeding 50%. For subgrades, restrictions are stricter, disallowing clays with liquid limits over 50 and plasticity indices above 25, as well as expansive soils, to maintain foundation stability.
Per IRC 36 Clauses 8.3.3, 8.3.4, and 8.3.6, soil moisture content must be assessed before excavation. If the soil is too dry, water should be added either at the borrow pit through flooding or irrigation before excavation, or after spreading on the embankment by sprinkling with hoses or water browsers. Following moisture adjustment, the soil must be uniformly wetted using equipment such as graders or rotary mixers. Clods should be broken down to a maximum size of 75 mm for general layers and 50 mm for the top 500 mm. If mechanical processing is unavailable, clods must be reduced to less than 50 mm at the borrow site itself.
IRC 36 advises placing soil in layers not exceeding 150 mm loose thickness, then thoroughly compacting with power rollers or mechanical tampers if heavy machinery is impractical (Clause 11.2). Acceptance criteria require relative compaction of at least 95% of maximum laboratory dry density for embankments and 97% for subgrades and shoulders, based on IS 2720 Parts 7 or 8 (Clause 4.4.6). Expansive clays are prohibited for subgrade and require treatment for embankments with the same compaction standard. Minimum dry unit weights vary by embankment height and flooding conditions, e.g., a minimum of 15.2 kN/m³ for embankments up to 3 m on major roads. Density testing should be conducted at least once per 1000 m², averaging results from 5-10 tests.
IRC 36 defines flood bank and river levee embankments as man-made barriers protecting adjacent lands from flooding (Clause 10.1). Roads may be constructed atop these embankments when designed with sufficient width and ensuring the phreatic line stays within the embankment body. Design flood frequencies are set at 25 years for agricultural zones and 50 years for urban or critical infrastructure (Clause 10.2). Embankments may be homogeneous or zoned with impervious cores, featuring side slopes of 1 vertical to 2 or 3 horizontal to maintain stability. Erosion protection such as rip-rap, gabion toe walls, and geotextiles per IRC:SP:59 is required on river sides. The rear toe must have a cover between 0.75 m and 1.25 m above the saturation line sloping 1 vertical in 6 horizontal, with berms as needed.
IRC 36 permits the use of fly ash—defined as pond ash, bottom ash, or mound ash, excluding fine fly ash alone—in embankment fill (Clause 16.1). The fly ash should originate from nearby thermal power plants and be covered with a suitable soil layer conforming to compaction and density requirements outlined in Tables 3 and 4. Particle size distribution and compaction characteristics per IS 2720 Part 8 must be submitted and approved. Compaction should be done immediately after spreading using static, pneumatic, or vibratory rollers, compacting fly ash and soil cover layers simultaneously to at least 95% maximum dry density. Earth shoulders must be compacted to the same standard as subgrade layers.
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