Soil properties for Construction project

Difference Between Reinforced & Retained Zones

Comparison Table

Sr. No.	Parameter / Aspect	Reinforced Zone	Retained Zone
1	Plasticity Index (PI)	Less than 6	Up to 20
2	Cu (Uniformity Coefficient)	Greater than 2	Not specifically required
3	Angle of Internal Friction (Ø)	Not less than 30° (32° preferred)	More than 25°
4	Permissible % of Fines (Passing 75 micron)	Preferably less than 15% with restrictions	Relaxed if PI & Ø criteria are met
5	Effect of High PI & Fines	Stickiness, poor compaction, lower friction, weakens reinforcement	Tolerable if PI & Ø within limits
6	Fill Quantity Usage	Used within reinforced zone with geogrid/geotextile	Used outside reinforced zone behind retaining structures
7	Material Specification Uniformity	Same as retained fill for 2-lane/4-lane highways	Relaxed for 6-lane/4-lane with surcharge
8	Soil Type Preference	Well-graded granular with controlled fines & PI	Can use relaxed criteria if reinforced fill unavailable
9	Design Impact of Lower Friction Value	If Ø drops, grid strength needs to increase (uneconomical)	No direct impact mentioned
10	Compaction & Shear Strength Requirements	High MDD 1.8–2.2 g/cc, OMC 10–15%, φ 30–40°, c 10–20 kPa	Relaxed shear strength as long as φ > 25°
11	Permeability Requirement	Low permeability: 10-5 to 10-7 m/s	Not explicitly specified

Backfilling Soil Properties & Atterberg Limits

Physical Requirements for Backfilling Soil (IS Code Recommendations)

Property	Recommended Range
Grain Size Distribution	Sand: 60-80% Silt: 10-30% Clay: 5-15%
Liquid Limit (LL)	30-40
Plastic Limit (PL)	20-30
Plasticity Index (PI)	10-20
Maximum Dry Density (MDD)	1.8-2.2 g/cc
Optimum Moisture Content (OMC)	10-15%
Angle of Internal Friction (φ)	30-40°
Cohesion (c)	10-20 kPa
Coefficient of Permeability (k)	10⁻⁵ to 10⁻⁷ m/s
Minimum Dry Density	1.6 g/cc
Maximum Moisture Content	15%

Important Notes

• Values mentioned are typical recommendations; actual project values should be finalized through site-specific geotechnical investigations.
• Always consult a qualified Geotechnical Engineer for verifying suitability of backfill material and confirm parameters by lab and field tests.

Soil Index Properties (Atterberg Limits)

The Atterberg Limits are fundamental parameters used to classify fine-grained soils based on their consistency and plasticity at varying moisture contents. These limits define the critical transitional states a soil undergoes as its water content changes — from a solid, to a plastic, and eventually to a liquid state.

Originally introduced by Swedish soil scientist Albert Atterberg in 1911, these limits remain a cornerstone in modern geotechnical engineering for soil classification, behavior assessment, and design applications. 

Why Are Atterberg Limits Important?

Fine-grained soils exhibit significant changes in consistency as moisture content varies. These changes impact:

• Bearing capacity of soil
• Settlement behavior
• Slope stability
• Compaction characteristics
• Pavement and embankment performance

Four States of Fine-Grained Soils Based on Moisture Content:

As moisture decreases, soil transitions through:

1. Liquid State
2. Plastic State
3. Semi-Solid State
4. Solid State

Engineering Applications

• Foundation Design: High PI soils indicate swelling/shrinkage risk — requiring stabilization or special foundations.
• Pavement Subgrade: Soils with high PI deform easily — needing treatment or replacement.
• Earth Dam Embankments: Consistency limits assess crack susceptibility and seepage risk.
• Slope Stability: Wide plastic ranges may cause unpredictable failure under varying moisture conditions.

Atterberg Limits (Main Types)

Limit	Definition	Test Significance
Liquid Limit (LL)	Minimum moisture content at which soil changes from plastic to liquid state with small shear strength.	Indicates water content beyond which soil behaves as a viscous fluid.
Plastic Limit (PL)	Minimum moisture content at which soil can be rolled into 3mm threads without cracking.	Represents boundary between plastic and semi-solid state.
Shrinkage Limit (SL)	Maximum moisture content at which further water loss does not reduce soil volume.	Assesses volume change behavior and shrinkage potential.

• Plasticity Index (PI = LL - PL): Plasticity Index (PI) is the difference between the Liquid Limit (LL) and the Plastic Limit (PL). Range of water content over which soil behaves plastically. Higher PI indicates greater swell potential.

PI = LL - PL

Soils with a higher PI tend to exhibit more plastic behavior and are prone to significant volume changes with fluctuations in moisture content, affecting foundation and pavement designs.

Soil Classification Based on Plasticity Index:

Soil Type	Degree of Plasticity	Liquid Limit (LL)	Plastic Limit (PL)	Plasticity Index (PI)	Plasticity Class (PI)
Sand	Nil	20	20	0	0
Silt	Low	25	20	5	< 7
Silty Clay	Medium	40	25	15	7–17
Clay	High	70	40	30	> 17

Recommended Properties for Retained Zone

Parameter	Recommended Value
Angle of Internal Friction (Ø)	> 25°
Plasticity Index (PI)	< 20

Soil Classification with Plasticity Chart (IS 1498)

Classification is done using the following empirical lines on the plasticity chart:

• A-Line: PI = 0.73(LL - 20)
• U-Line: PI = 0.90(LL - 8)

Soil groups commonly classified are:

• CL: Low plasticity clay
• CH: High plasticity clay
• ML: Low plasticity silt
• MH: High plasticity silt
• CL-ML: Clayey silt

IS 2720 Series (Methods of Tests for Soils)

The IS 2720 series prescribes standardized laboratory and field test procedures to determine various physical, chemical, and engineering properties of soils, essential for geotechnical investigations in civil engineering projects.

Soil Sample Preparation & Basic Properties

Part	Test Name	Technical Purpose
IS 2720 (Part 1): 1983	Preparation of Dry Soil Samples	Standardizes soil sample preparation for testing.
IS 2720 (Part 2): 1973	Moisture Content	Determines soil water content, critical for compaction and strength.
IS 2720 (Part 3/Sec 1 & 2): 1980	Specific Gravity (Fine and Coarse Soils)	Measures density of soil solids relative to water.

Soil Classification & Plasticity Characteristics

Part	Test Name	Technical Purpose
IS 2720 (Part 4): 1985	Grain Size Analysis	Determines particle size distribution for soil classification.
IS 2720 (Part 5): 1985	Atterberg Limits	Determines soil consistency and plasticity behavior.
IS 2720 (Part 6): 1972	Shrinkage Factors	Measures soil volume change upon drying.

Compaction & Strength Characteristics

Part	Test Name	Technical Purpose
IS 2720 (Part 7 & 8)	Water Content-Dry Density Relationship	Establishes optimum moisture and maximum dry density for compaction.
IS 2720 (Part 9): 1971	Plasticity Index	Classifies soils based on plasticity.

Shear Strength Determination

Part	Test Name	Technical Purpose
IS 2720 (Parts 10-13)	Shear Tests (Direct, Unconfined, Triaxial)	Determines soil shear strength parameters.

Other Strength & Behavior Tests

Part	Test Name	Technical Purpose
IS 2720 (Part 14-17)	Density, CBR, Permeability Tests	Assesses compaction, bearing capacity, and seepage properties.

Field Density Determination

Part	Test Name	Technical Purpose
IS 2720 (Part 18 & 19)	Sand Replacement & Core Cutter	Measures in-situ soil density.

Chemical & Miscellaneous Properties

Part	Test Name	Technical Purpose
IS 2720 (Parts 20-27)	Various Chemical and pH Tests	Determines chemical content, corrosivity, and other chemical properties of soils.

For detailed project-specific applications, always consult a certified geotechnical engineer and refer to the latest version of the respective IS code part.

Expansive Soil Management

As per IS 9451:1994 Guidelines

What are Expansive Soils?

Expansive soils, such as Black Cotton Soils, contain minerals like montmorillonite which swell when wet and shrink when dry. These volumetric changes pose serious risks to structures like buildings, roads, and pavements due to uneven movements.

Identification of Expansive Soils

• Plasticity Index (PI): Greater than 20%
• Liquid Limit (LL): Greater than 50%

Problems Associated with Expansive Soils

Sr. No.	Problem	Reason
1	Differential Settlement	Uneven swelling and shrinkage beneath different structural parts.
2	Foundation Uplift (Heave)	Soil expansion when wet lifts structures unevenly.
3	Structural Cracks	Seasonal soil movement causes structural distortion and cracking.
4	Seasonal Stability Issues	Variation in soil strength and volume with seasons.

Foundation Treatment Methods

A. Foundation Design Techniques

• Deep Foundations (Piles, Piers): Transfer load to deeper, stable soil strata.
• Raft Foundations: Mat-type foundations distributing loads evenly across a large area.
• Isolated / Strip Footings with Cushion Layers: Use of sand or murrum cushion to absorb soil movements.
• Stiffened Mat or Beam-Slab Foundations: Provide added rigidity to resist soil-induced movements.

B. Soil Stabilization Methods

• Chemical Stabilization: Use of lime or cement to reduce soil plasticity and improve strength.
• Moisture Control Measures: Installation of drains, waterproof membranes, and maintaining consistent moisture levels.
• Compacted Sand Cushion Method: Replace expansive soil with a well-compacted sand layer to interrupt capillary water rise.
• Pre-wetting Method: Saturating soil in advance to reduce future swelling potential.
• Soil Replacement: Removing expansive soil and replacing it with non-expansive granular material.

Construction Recommendations

• Maintain uniform moisture levels around structures throughout the year.
• Avoid planting large trees near foundations.
• Provide flexible joints in structures to accommodate minor settlements or movements.
• Ensure effective site drainage to prevent waterlogging near structures.

Special Considerations for Roads and Pavements

• Granular Sub-Base (GSB) Layers: Reduce risk of reflective cracking in pavement surfaces.
• Lime-Treated Subgrade: Stabilizes subgrade soils and improves load-bearing capacity.
• Drainage Ditches and Cross-Drainage Structures: Manage surface and subsurface water efficiently.

IS 9451:1994 provides essential procedures for identifying, evaluating, and mitigating expansive soil problems to ensure the long-term stability and durability of civil engineering structures.

Technical Explanation: Permeability in Earth Dams

What is Permeability?

Permeability is the ability of soil to transmit water. In earth dams, the core must be impermeable to prevent seepage which can lead to internal erosion and structural failure.

Requirements for Impermeable Core Materials:

• Fines Content: Minimum 15% passing Sieve No. 200 (75 microns) for reduced permeability.
• Plasticity Index (PI): Between 10 and 20 for workability and resistance to cracking.
• Internal Erosion Resistance: Fine-grained, cohesive materials resist washout under seepage forces.

GC Soils for Mixed-Core Dams:

GC (Gravelly Clay) materials are preferred as they:

• Provide sufficient fines and suitable PI.
• Offer good shear strength and stability.
• Resist internal erosion effectively.

Effect of Fines on Permeability and Plasticity:

Adding fines reduces permeability but excessively high fines raise PI beyond acceptable limits, reducing workability and compaction quality. Lab tests on 16 samples confirmed an optimum fines range (20-40%) for core materials.

Summary Table:

Parameter	Ideal Range / Condition	Importance
% Passing Sieve No. 200 (75μm)	≥ 15% (20–40% in GC soils)	Reduces permeability
Plasticity Index (PI)	10–20	Maintains flexibility without excessive plasticity
Internal Erosion Resistance	Must be high	Prevents piping and washout
Shear Resistance (GC soils)	Considerable	Enhances dam stability

Conclusion:

An effective earth dam core material balances fines content, PI, shear strength, and internal erosion resistance to ensure long-term safety and durability of the structure.

Tests for Subgrade & GSB Material

1. Atterberg Limits (IS: 2720 Part 5)

Purpose: Determine Liquid Limit, Plastic Limit, and Plasticity Index for classifying fine-grained soils.

Requirement: PI ≤ 25% for subgrade soil (MoRTH Clause 903.2.1)

2. Standard Proctor Compaction Test (IS: 2720 Part 7)

Purpose: Determine Optimum Moisture Content (OMC) and Maximum Dry Density (MDD).

3. California Bearing Ratio (CBR) Test (IS: 2720 Part 16)

Purpose: Evaluate soil/subgrade/GSB strength under soaked conditions.

• Subgrade: CBR ≥ 10%
• GSB: CBR ≥ 30%

4. Free Swell Index Test (IS: 2720 Part 40)

Purpose: Assess swelling potential of soil in water.

Initial Free Swell: 8%, Reduced to 4% after treatment with TerraSil compound.

Key Laboratory Tests for GSB Material

5. Gradation (IS: 2720 Part 4)

Purpose: Ensure proper particle size distribution. As per MoRTH Table 400-1.

6. Atterberg Limits for GSB (IS: 2720 Part 5)

LL ≤ 25%, PI ≤ 6%

7. Modified Proctor Test (IS: 2720 Part 8)

MDD typically ≥ 1.8 g/cm³

8. CBR for GSB (IS: 2720 Part 16)

CBR ≥ 30% (soaked)

9. Aggregate Impact Value / Los Angeles Abrasion (IS: 2386 Part 4)

AIV ≤ 40%, LAA ≤ 40%

10. Water Absorption (IS: 2386 Part 3)

≤ 2%

11. Field Density Test (IS: 2720 Part 28/29)

Minimum 97% of MDD (MoRTH Clause 903.2.2)

Material and Layer Specifications

• Material: Natural aggregates per IS:383
• Fines passing 0.075 mm sieve: ≤ 10%
• Layer Thickness: 100–200 mm compacted

Summary of Key Test Requirements

Parameter	Requirement
Gradation	As per MoRTH Table 400-1
LL (Liquid Limit)	≤ 25%
PI (Plasticity Index)	≤ 6%
CBR (Subgrade)	≥ 10%
CBR (GSB)	≥ 30%
AIV / LAA	≤ 40%
Water Absorption	≤ 2%
Field Compaction	≥ 97% MDD

This document summarizes all essential testing procedures and criteria for subgrade and GSB materials as per MoRTH, IRC, and relevant IS Codes.

Soil Tests (IS 2720 Series)

Test	Purpose	Method	Use/Remarks
Moisture Content (IS 2720 Part 2)	Determine water content in soil	Oven dry at 105-110°C for 24 hrs	Affects compaction & strength
Grain Size Analysis (IS 2720 Part 4)	Classify soil type (sand, silt, clay)	Sieve (coarse), Hydrometer (fine)	Foundation design, permeability
Atterberg Limits (IS 2720 Part 5)	Measure LL, PL, PI	Casagrande device (LL), Thread rolling (PL)	Identify expansive soil (PI > 20)
Proctor Compaction (IS 2720 Part 7/8)	Find OMC & MDD	Compact in layers using rammer	Ensure 95%-98% MDD in field
SPT (IS 2131)	Assess soil density/strength	Drive sampler with 63.5 kg hammer	SBC estimation (SBC ≈ 10N for sand)
UCS (IS 2720 Part 10)	Measure cohesive soil strength	Axial load till failure	Clay foundation design (UCS/2 = cohesion)
Direct Shear (IS 2720 Part 11/13)	Determine shear parameters (c, φ)	Shear soil in box under normal load	Slope, retaining wall, foundation stability
Permeability (IS 2720 Part 17)	Measure water flow through soil	Constant head (sands), Falling head (clays)	Drainage & seepage control design
Field Density (IS 2720 Part 29/33)	Check in-situ soil density	Core cutter (fine soils), Sand replacement (coarse soils)	Verify compaction in field
CBR (IS 2720 Part 16)	Subgrade strength for pavements	Measure penetration resistance	Highway subgrade: CBR ≥ 8%

Highway Tests

1️⃣ Tests on Aggregates

Test	Purpose	Standard
Aggregate Impact Value	Assess toughness to impact	< 30% for base & surface layers (IRC:SP:11)
Los Angeles Abrasion	Resistance to abrasion	< 40% base, < 30% wearing courses
Flakiness & Elongation Index	Assess shape for compaction & stability	Combined < 35%
Water Absorption	Porosity and durability check	< 2%
Specific Gravity	Density for mix design	2.6-2.9

2️⃣ Tests on Bitumen

Test	Purpose	Standard
Penetration Test	Measure hardness/softness	30-100 (VG-30: 50-70)
Softening Point	Temperature susceptibility	> 47°C (VG-30)
Ductility	Elasticity & adhesion	> 75 cm at 27°C
Viscosity	Workability during mixing	As per IS 73, IRC:SP:11
Specific Gravity	Mix design calculation	0.97-1.02

3️⃣ Tests on Subgrade & Embankment Soil

Test	Purpose	Standard
CBR	Subgrade strength	> 8% (IRC:37)
Proctor Compaction	OMC & MDD	95-98% MDD in field
Atterberg Limits	Plasticity assessment	PI < 6%
Grain Size Analysis	Gradation check	As per IRC:37, IRC:SP:20

4️⃣ Tests on Bituminous Mixes

Test	Purpose	Standard
Marshall Stability	Stability and flow check	Stability > 9kN, Flow: 2-4mm (IRC:37)
Bitumen Extraction	Bitumen content verification	4.5-6% for BC
Density & Void Analysis	Compaction & air void content	Air voids 3-5%, VMA as per IRC:37