Guidelines for the Design and Construction of Geosynthetic Reinforced Embankments on Soft Subsoils

IRC 113: Scope - Key Specifications & Tables

The scope covers design and construction of reinforced soil structures on soft soils, focusing on geosynthetics (geogrids, geotextiles), fill placement, and embankment stability.

Key Specifications:

• Reinforcement Materials:

  • Geogrids: Planar polymeric networks, UV stabilized (ASTM D4355), chemically inert (pH 4-9, up to 11 with reduction factors).
  • Geotextiles: Woven polyester/polypropylene, UV resistant, dimensionally stable.
  • Manufacturers must provide certified properties (Tables 5, 6, 7).

• Fill Placement (Clause 9.6):

  • Fill layers: max 200 mm thickness.
  • Use well-graded sand with specified angle of internal friction.
  • Fill spread longitudinally along reinforcement.
  • Avoid tracked vehicles on unprotected reinforcement.
  • Use lightweight equipment; inverted U-shaped fill placement recommended.

Tables Summary:

Design Strength Reduction (Section 3.7):

Long-term design strength = Ultimate tensile strength × Reduction Factors (for creep, installation damage, chemical/biological degradation, etc.)

Fill Placement Diagram (Inverted U Construction):

flowchart LR
    A[Centerline] --> B[Fill placement symmetrically outward]
    B --> C[Reinforcement layers covered with sand]
    C --> D[Lightweight dozers/graders used]
    D --> E[Avoid tracked vehicles on unprotected layers]

References:

• IRC:SP:59-2002 (Geotextiles in Pavements)
• BS 8006-2010 (Reinforced Soils)
• ASTM D4355 (UV Stabilization Test)

For detailed design,

Key Specifications & Tables for Location of Soft Subsoils in India (IRC 113)

1. Geotechnical Properties of Soft Clays (Table 1)

2. Classification of Soft Soils Based on Shear Strength (Table 2)

3. Key Parameters Affecting Embankment Stability

• Height of embankment (H)
• Base width (B)
• Depth of

Design Considerations in IRC 113: Key Formulas, Tables, and Specifications

1. Strength Requirements (Clause 5.1 & 3.7)

• Design Tensile Strength of Reinforcement:

[ T = \frac{T_{ult}}{RF_{CR} \times RF_{ID} \times RF_{w} \times RF_{CH} \times f_s} ]

Where:

• (T) = Long-term tensile strength (kN/m)
• (T_{ult}) = Ultimate tensile strength (short-term) (kN/m)
• (RF_{CR}) = Reduction factor for creep
• (RF_{ID}) = Reduction factor for installation damage
• (RF_{w}) = Reduction factor for weathering
• (RF_{CH}) = Reduction factor for chemical/environmental effects
• (f_s) = Factor for data extrapolation

Table 3: Typical Reduction Factors

2. Factors of Safety at End of Construction (Table 4)

IRC 113 - Key Formulas & Specifications for Numerical Examples

1. Design Tensile Strength of Reinforcement (Clause 3.7)

• Tensile strength is based on the characteristic strength of the reinforcement.
• Use the design tensile strength ( f_{td} = \frac{f_{yk}}{\gamma_m} ), where:
  • ( f_{yk} ) = characteristic tensile strength of reinforcement
  • ( \gamma_m ) = partial safety factor (typically 1.25 to 1.5)

2. Stability Factors (Clause 5.14)

• For embankment design on soft soil: [ N = 5.14 \quad \text{for } 52 \quad \text{(Eq. 3)} ] [ N = 4.14 + 0.5 \times 5 \quad \text{for } 22 \quad \text{(Eq. 4)} ]
• Where:
  • ( B ) = Width of embankment bottom (m)
  • ( D ) = Depth of soft soil (m)

3. Construction Stage Factors (Clause 1.25)

• Partial safety factors for stages:
  • 1.25 at the end of construction of a stage
  • 1.5 at the end of waiting period for the stage

4. Lateral Sliding (Clause 3.3)

• Ensure factor of safety against lateral sliding > 1.5 (typical)
• Check shear resistance of soil-reinforcement interface

Summary Table for Design Factors

flowchart LR
    A[Soft Soil Embankment Design]

Material Requirements per IRC 113

1. Design Tensile Strength of Reinforcement (Clause 3.7)

The long-term tensile strength ( T ) is calculated as:

[ T = \alpha_T \times RF_{CR} \times RF_{ID} \times RF_w \times RF_{CH} \times f_s \times T_{ult} ]

• ( T_{ult} ): Ultimate tensile strength (kN/m)
• ( \alpha_T ): Short-term ultimate tensile strength factor
• ( RF_{CR} ): Reduction factor for creep
• ( RF_{ID} ): Reduction factor for installation damage
• ( RF_w ): Reduction factor for weathering
• ( RF_{CH} ): Reduction factor for chemical/environmental effects
• ( f_s ): Extrapolation factor

Durability factor: ( RFD = RF_{CH} \times RF_w )

2. Typical Reduction Factors (Table 3)

Use only certified values (Annexure 2).

3. Factors of Safety at End of Construction (Table 4)

IRC 113: Subsoil Investigation and Testing - Key Points

1. Borehole Spacing & Depth

• One borehole per 100 m length of embankment.
• Borehole depth: full depth of soft soil layer.

2. Shear Strength Testing

• Use in-situ vane shear test or undisturbed sample testing for unconfined compressive strength.
• Static Cone Penetration Test (SCPT) may be used after site-specific correlation with vane shear.
• Standard Penetration Test (SPT) is not reliable for soft cohesive soils.

3. Compressibility Tests

• Determine:
  • Coefficient of consolidation, ( C_v )
  • Compression index, ( C_c )
  • Liquid limit, plastic limit, natural moisture content, void ratio.

4. Stage Construction

• Shear strength increase must be field-verified before next fill stage.
• Refer HRB SR No. 13 for shear strength increase formulae.
• Laboratory consolidation tests under staged loads recommended.

5. Instrumentation & Monitoring

• Monitor:
  • Shear strength increase (vane shear/lab tests)
  • Pore water pressure (piezometers)
  • Settlements (gauges at various depths)
  • Lateral displacement (inclinometers if needed)
• Refer IRC:75 (1979) and HRB SR:14 (1995) for instrumentation details.

Table: Soft Soil Classification by Undrained Shear Strength (kPa)

Recommended Tests

IRC 113: Instrumentation and Monitoring for Embankments on Soft Ground

Key Parameters to Monitor (Clause 7)

• Shear Strength Increase: Monitor via vane shear tests or lab tests on undisturbed samples.
• Pore Water Pressure: Use piezometers installed in soft subsoil layers.
• Settlement: Settlement gauges at embankment centerline, shoulders, and at various depths.
• Lateral Displacement: Inclinometers near embankment toe if large lateral movements expected.

Instrumentation Program

• Develop a comprehensive plan including:
  • Location of instruments
  • Frequency of monitoring
  • Data collection & interpretation by trained personnel
• Refer IRC:75 (1979) and HRB SR:14 (1995) for detailed instrumentation guidelines.

Typical Instrumentation Setup

graph LR
A[Surface Settlement Gauges] --> B[Soft Soil Layers]
C[Piezometers] --> B
D[Inclinometers] --> E[Embankment Toe]
B --> F[Shear Strength Tests]

Additional Notes

• Embankments on soft soils have low initial safety factors; monitoring ensures safety as soil strength improves.
• Stage construction requires verifying strength gain before subsequent fill placement.
• Instrumentation is essential for early detection of potential failures.

This ensures safe, controlled construction and long-term stability of embankments on soft subsoils.

IRC 113: Drainage Requirements (Clause 5.3)

• Drainage is critical to prevent water accumulation behind reinforcement layers, which can reduce soil strength and cause failure.
• Provide adequate drainage layers (e.g., well-graded sand) behind reinforcements to ensure free water flow.
• Use filter materials to prevent soil particles from clogging drainage.
• If groundwater is encountered, dewatering arrangements must be made during filling (Clause 9.6).
• Fill should be placed in 200 mm layers, spread longitudinally to reinforcement.
• Avoid tracked vehicles on unprotected reinforcement to prevent damage.
• Design should consider hydraulic gradients to avoid uplift or piping failures.
• Use prefabricated vertical drains or other methods to accelerate consolidation in soft soils.

Key Specifications for Drainage Layer:

Typical Drainage Design Formula:

[ q = k \cdot i \cdot A ]

• (q) = discharge (m³/s)
• (k) = permeability of drainage layer (m/s)
• (i) = hydraulic gradient
• (A) = cross-sectional area of drainage layer (m²)

flowchart LR
    SoilLayer --> DrainageLayer[Drainage Layer (Sand)]
    DrainageLayer --> FilterLayer[Filter Layer]
    FilterLayer --> FreeFlow[Free Water Flow]
    FreeFlow --> Outlet[Drain Outlet]

Summary: Ensure well-graded sand drainage layers with proper thickness and filter criteria, provide dewatering if groundwater is present, and protect reinforcement from damage during filling.

IRC 113: Construction Aspects Summary

1. Site Preparation & Reinforcement Handling (Clauses 9.1 - 9.5)

• Site Preparation: Clear and level site; remove unsuitable material.
• Reinforcement Storage: Store geogrids/geotextiles in dry, shaded areas to avoid UV degradation.
• Placing Reinforcement: Place in longitudinal direction; avoid wrinkles.
• Jointing: Overlap or mechanical joining as per manufacturer specs.
• End Anchorages: Proper anchorage to prevent pullout.

2. Fill Considerations (Clause 9.6)

• Fill Material: Use well-graded sand with specified angle of internal friction.
• Layer Thickness: Place fill in 200 mm layers.
• Placement Direction: Spread fill longitudinally along reinforcement.
• Traffic Restrictions: No tracked vehicles on unprotected reinforcement.
• Fill Placement Techniques:
  • Inverted U (Fig. 12) for less severe conditions.
  • U shaped (Fig. 13) for others.
• Equipment: Use lightweight dozers/graders.

3. Geogrid Specifications

• Must be UV stabilized (ASTM D4355), chemically inert (pH 4-9, up to 11 with reduction).
• ISO 9001 or CE certified manufacturing.

4. Geotextile Specifications

• Made from polyester/polypropylene yarns.
• UV and chemical resistant.
• Dimensionally stable.
• Certified properties to be submitted by manufacturer.

5. Key Formula: Long Term Design Strength

[ \text{Long Term Strength} = \text{UTS} \times RF_1 \times RF_2 \times RF_3 \times RF_4 ] Where RF = Reduction Factors for creep, installation damage,

Embankment Widening - Key Formulas & Specifications (IRC 113)

1. Embankment Geometry & Load

• Slope: 3H:1V
• Side slope length: ( L_a = 9,m )
• Base width: ( B = \text{Crest width} + 2L_a = 10 + 2 \times 9 = 28,m )
• Traffic load: ( q = 20,kN/m^2 )

2. Bearing Capacity

• Undrained shear strength of clay, ( c_u = 10.9,kPa )
• Bearing capacity factor, ( N_c = 6.94 ) (from formula ( N_c = 4.14 + 0.5 \times D/B ))
• Bearing capacity:
  [ q_{ult} = c_u \times N_c = 10.9 \times 6.94 = 75.65,kPa ]
• Stress below embankment:
  [ P = \gamma \times H + q = 18 \times 3 + 20 = 74,kPa ]
• Factor of Safety (FoS):
  [ FoS = \frac{q_{ult}}{P} = \frac{75.65}{74} \approx 1.02 \quad (<1.3 \text{ unsafe}) ]

3. Rotational Stability

• FoS from Bishop’s method without reinforcement = 1.21 (<1.3, unsafe)
• Recommendation: Use basal geogrid reinforcement to improve stability.

4. Fill & Construction Specifications (Clause 9.6)

• Fill in 200 mm layers only.
• Use well-graded sand with appropriate friction angle to cover reinforcement.
• Place fill longitudinally along reinforcement; avoid tracked vehicles on unprotected reinforcement.
• Use lightweight dozers/graders for spreading.
• Use inverted U or U-shaped fill placement for soft soils (see Fig.12 & 13 in IRC).

5. Geogrid Specifications (Tables 5-7)

• Material: UV

Reinforcement Jointing (IRC 113 - Clause 9.4)

• Overlap Length: Minimum 300 mm or as specified by Engineer.
• Orientation: No joints/seams along the principal strength direction of basal reinforcement.
• Anchorage: Overlaps must have sufficient anchorage length to carry design loads.

Design Tensile Strength of Reinforcement (Clause 3.7)

The long-term tensile strength ( T ) is:

[ T = \frac{T_{ult}}{RF_{CR} \times RF_{ID} \times RF_{w} \times RF_{CH} \times f_s} ]

Where:

Note: (RF_{CH} \times RF_{w} = RFD) (Reduction factor for durability).

Typical Reduction Factors (Table 3)

Key Specifications for Jointing:

• Overlap length must be clearly detailed in design drawings.
• Avoid joints along the main tensile direction.
• Overlaps must be anchored to carry full design load.
• Use certified reduction factors for design strength calculation.

flowchart

IRC 113: Case Studies - Key Points & Specifications

The IRC 113 includes 12 case studies illustrating practical applications of geosynthetics in road projects, e.g., Southern Transport Development Project, Seethawaka Industrial Park, Sri Lanka.

Relevant Clauses & Tables:

• Clause 1.25: Specifies construction stage waiting periods (1.25 and 1.5 times specified durations).
• Clause 3.7: Design tensile strength of reinforcement.
• Clause 4.1 & Table 10: Specifications for non-woven geotextiles used as separation layers.

Important Table - Properties of Non-Woven Geotextile (Table 10):

Key Formulas (from IRC & general practice):

• Design Tensile Strength of Reinforcement, ( T_d = \frac{T_u}{F_s} )

  • ( T_u ) = ultimate tensile strength from tests
  • ( F_s ) = factor of safety (typically 1.5 to 2.0)

• Lateral Sliding Stability: [ FS = \frac{Resisting Forces}{Driving Forces} \geq 1.5 ]

• Rotational Stability: [ FS = \frac{Sum\ of\ Moments\ resisting\ rotation}{Sum\ of\ Moments\ causing\ rotation} \geq 1.5 ]

Summary:

• Case studies demonstrate real-world application of geosynthetics.
• Use tested geotextile properties from Table 10 for design.
• Follow tensile strength and stability factors as per clauses.
• Refer to IRC:SP:59-2002 for detailed geotextile functions.

flowchart LR
    A[Project Requirement] --> B[Select Geotextile Type

IRC 113: Key References, Formulas & Tables Summary

1. Design Tensile Strength of Reinforcement (Clause 3.7)

Long-term tensile strength ( T ) is calculated as:

[ T = \frac{T_{ult}}{RF_{CR} \times RF_{ID} \times RF_{w} \times RF_{CH} \times f_s} ]

• ( T_{ult} ): Ultimate tensile strength (short-term)
• ( RF_{CR} ): Reduction factor for creep
• ( RF_{ID} ): Reduction factor for installation damage
• ( RF_{w} ): Reduction factor for weathering
• ( RF_{CH} ): Reduction factor for chemical effects
• ( f_s ): Data extrapolation factor

Typical reduction factors (Table 3):

2. Specifications of Geosynthetics

• Geogrids: Must be UV stabilized (ASTM D4355), chemically inert (pH 4-9, up to 11 with reduction factors), dimensionally stable.
• Geotextiles: Polyester or polypropylene, UV resistant, dimensionally stable.

Certified Properties to be submitted (Tables 5, 6 & 7):

3. Factors of Safety (Table 4)

| Failure Mode | Basal

Key BOQ Items & Specifications from IRC 113 for Reinforced Soil Works

1. Reinforcement Materials (Unit: Sq.M. or as specified)

• Bonded Geogrids, Extruded Geogrids, Woven/Knitted Geogrids, Woven Geotextiles, Geocomposites, Geocells
• Supply & laying details include tensile strength, cell height, seam strength, carbon black content, and expanded cell size.
• Example:
  • Woven Geotextile: tensile strength (project-specific) kN/m, max opening size (mm), roll length & width (m).

2. Separation Layer (Unit: Sq.M.)

• Non-woven geotextile with specified:
  • Grab tensile strength (kN/m)
  • Trapezoidal tear strength (kN/m)
  • Puncture strength (kN/m)
  • Apparent opening size (mm)
  • Permittivity (sec⁻¹)

3. Drainage Layer (Unit: Cum)

• Supply, laying, and compaction of gravel or well-graded sand as per drainage specs.

Important Tables Summary

Fill Placement Guidelines (Clause 9.6)

• Fill in 200 mm layers, longitudinal spreading over reinforcement.
• Use well-graded sand with specified friction angle.
• Avoid tracked vehicles on unprotected reinforcement.
• Use inverted U construction for fill placement over soft soils.

Design Strength Reduction Factors (Refer Section 3.7)

• Apply reduction factors to manufacturer’s UTS to get long-term design strength for 60 &