Guidelines for Use in Prediction of Subsidence and Associated Parameters in Coal Mines Having Nearly Horizontal Single Seam Workings

IS 15026: Scope - Key Specifications & References

• Scope: IS 15026 covers design and construction aspects related to rock bolting and tunnel supports, referencing related Indian Standards for materials and methods.

Key References (Table 2)

Important Rounding Rule

• Final test/analysis values must be rounded per IS 2:1960, retaining the same significant digits as specified.

Perforated Bolts (Clause 11.2.3 & Table 5)

• Bolts inserted inside a cement-filled perforated tube, forcing mortar to bond bolt and rock.
• Diameter relation:

• Bolts must be straight within ±1 mm.
• Pull-out tests on 5% of bolts to verify capacity.

flowchart LR
    A[Bore Hole] --> B[Insert Perforated Sleeve filled with Cement Mortar]
    B --> C[Push Bolt inside Sleeve]
    C --> D[Mortar Oozes through Perforations]
    D --> E[Forms Homogeneous Bond: Bolt + Sleeve + Rock]

This scope ensures proper material selection, testing, and installation for reliable rock bolting in tunnels.

IS 15026: Key Definitions & Tables for Tunnelling Ground Conditions

Key Definitions (Clause 6.1)

• Radial Tunnel Closure (La): The inward displacement of tunnel walls.
• Tunnel Radius (a): Radius of the tunnel cross-section.
• Normalized Tunnel Closure (u_la): Percentage ratio of radial closure to tunnel radius.

Classification of Ground Conditions for Tunnelling (Table 1)

Important Notes

• Squeezing is ruled out if in-situ max tangential strain < critical strain = (UCS / Elastic modulus).
• UCS = Unconfined Compressive Strength of rock.

Summary Formula

[ u_{la} = \frac{L_a}{a} \times 100% \quad \text{(Normalized Tunnel Closure)} ]

Where:

• (L_a) = radial tunnel closure (mm)
• (a) = tunnel radius (mm)

This classification guides excavation and support system selection for tunnels as per IS 15026.

IS 15026 Part 2 focuses on rock mass quality for predicting support pressure in underground openings. Although the code lacks explicit formulae, key concepts from rock mass characterization and behavior include:

Key Parameters & Classification Systems:

• Rock Quality Designation (RQD): Percentage of intact core pieces >10 cm.
• Rock Mass Rating (RMR): Combines RQD, uniaxial compressive strength, spacing of discontinuities, condition of discontinuities, groundwater, and orientation.
• Q-System:
  [ Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF} ] where:
  • (J_n) = joint set number
  • (J_r) = joint roughness number
  • (J_a) = joint alteration number
  • (J_w) = joint water reduction factor
  • (SRF) = stress reduction factor

Support Pressure Prediction:

• Empirical relationships link rock mass quality to support pressure.
• Use RMR or Q values to select support types and estimate loads.

Typical Tables (from IS & guidelines):

Summary:

• Use RQD, RMR, Q-system for rock mass classification.
• Apply empirical formulas for support pressure based on these indices.
• Refer to IS 15026 Part 2 and tunneling guidelines for detailed procedures.

flowchart TD
    A[Rock Mass] --> B[RQD]
    A --> C[Joint Set Number (Jn)]
    A --> D[Joint Roughness (Jr)]
    A --> E[Joint Alteration (Ja)]
    A --> F[Water Factor (Jw)]
    A --> G[Stress Reduction Factor (SRF)]
    B & C & D & E & F & G --> H[Calculate Q]
    H --> I[Estimate Support Pressure]
    I --> J[Design Support System]
``

IS 15026: Prediction of Subsidence Parameters (Rock Mass Quality & Support Pressure)

Key Points from Clause 6.1 & Table 1 (Ground Classification)

Important Formulas

• Normalized Tunnel Closure
  [ u_{la} = \frac{\Delta a}{a} \times 100% ] where:

  • ( \Delta a ) = radial tunnel closure
  • ( a ) = tunnel radius

• Critical Strain for Squeezing
  [ \varepsilon_{crit} = \frac{UCS}{E} ] where:

  • ( UCS ) = Uniaxial Compressive Strength of rock
  • ( E ) = Modulus of Elasticity of rock

Summary

• Use Table 1 to classify ground conditions for tunnelling.
• Check normalized tunnel closure ( u

Key Points from IS 15026 on Tunnel Instrumentation & Monitoring

1. Instrumentation Location & Density (Clause 5.1)

• Instrument tunnels especially in squeezing ground conditions.
• Due to low survival (~30%), instrument multiple sections to ensure reliable data.
• Continue post-monitoring until support stabilizes.

2. Tunnel Closure Limits in Squeezing Ground

• Vertical closure < 4% of tunnel height
• Horizontal closure < 4% of tunnel width

3. Seismic Area Considerations (Clause 1.25)

• Effect zone extends ± B (span of tunnel opening) from faults/thrusts.
• Design support pressure = 1.25 × ultimate support pressure (per IS 13365 Part 2).

Summary Table: Tunnel Closure Limits

Monitoring Recommendations

• Use instruments like extensometers, convergence meters, piezometers.
• Monitor deformations, stresses, pore pressures continuously.
• Data guides support system adjustments.

flowchart LR
    A[Start Tunnel Excavation] --> B{Squeezing Ground?}
    B -- Yes --> C[Install Multiple Instruments]
    C --> D[Monitor Deformations & Pressures]
    D --> E{Support Stabilized?}
    E -- No --> D
    E -- Yes --> F[Continue Construction]
    B -- No --> F

This ensures safety and adapts support design dynamically.

IS 15026: Support Systems and Components - Key Points

1. Support Spacing & Layout (Clause 12.8)

• Spacing depends on rock conditions, load, and method of attack.
• Supports should be spaced to ensure stability during excavation.
• Layout must consider ease of installation and safety.

2. Suitable Support Types (Clause 12.5.1)

• All support types in Clause 10.3 apply for full face rock excavation.
• Refer Table 2 (IS 15026) for detailed guidelines on support types vs. rock conditions.

3. Design for Speed of Erection (Clause 12.8.5)

• Minimize number of members.
• Simplify joints; reduce bolts.
• Fabricate with adequate bolt/wrench clearance; avoid close fits.

4. Components Design (Clause 12.7)

• Follow IS 4880 (Part 6) for design of tunnel support components (e.g., steel sets, bolts, plates).

Example: Support Spacing Formula (Conceptual)

[ S = \frac{K \times R_c}{L} ] Where:

• ( S ) = spacing between supports
• ( K ) = factor based on rock quality
• ( R_c ) = rock strength
• ( L ) = length of unsupported span

Summary Table (Conceptual)

flowchart LR
    A[Rock Excavation] --> B[Determine Rock Quality]
    B --> C[Select Support Type (Clause 10.3)]
    C --> D[Refer Table 2 for Spacing]
    D --> E[Design Components (IS 4880 Part 6)]
    E --> F[Fabricate & Install with Speed Considerations]

References:

IS 15026: Steel Fibre Reinforced Shotcrete (SFRS) - Key Points

Shotcrete Ingredients (Clause 7.7.1 & Table 3)

• Cement: 446-558 kg/m³ (6.35 mm agg.), >445 kg/m³ (10 mm agg.)
• Blended Sand (6.35 mm max): 1483-1679 kg/m³ (6.35 mm agg.), 697-880 kg/m³ (10 mm agg.)
• Aggregate (10 mm): 700-875 kg/m³ (only for 10 mm agg. mix)
• Steel Fibres: 39-157 kg/m³ (6.35 mm agg.), 39-150 kg/m³ (10 mm agg.)
• Water/Cement Ratio: 0.40-0.45 by weight
• Additives: Micro silica fumes (8-15% cement mass), accelerators (2-5% cement mass), superplasticizers (3-6 l/m³), hydration and curing agents.

Fibre Content (Clause 7.4)

• Steel fibres: 0.5% to 2% volume (1.5% to 6% by weight)

• 2% fibre content causes mix and spraying difficulties.

Advantages (Clause 14.4.1)

• Effective in weak rock masses.
• Requires less thickness than conventional shotcrete.
• No welded mesh needed.
• Reduced rebound with proper grading and spraying.

Typical Mix Table (kg/m³)

flowchart LR
    A[Steel Fibre Reinforced Shotcrete Mix] --> B[Cement]
    A --> C[Blended Sand]
    A --> D[Coarse Aggregate]

Rock Bolting and Anchoring per IS 15026

Key Specifications & Tables

1. Perforated Bolts (Clause 11.2.3)

• Insert perforated metal tube filled with cement mortar into borehole.
• Push plain/ribbed bolt inside, forcing mortar through perforations, bonding bolt, tube & rock.
• Table 5: Diameter Relation

• Bolts/anchors straightness tolerance: ±1 mm.
• Pull-out tests on 5% of bolts to verify capacity (P_bolt).

2. Wedge and Slot Bolt (Clause 11.2.1)

• Mild steel rod, threaded one end, split other end (~125 mm).
• Insert wedge (20 mm sq, 150 mm long) into split, drive into hole.
• Expands split end to grip rock.
• Use 10 mm thick plate (200x200 mm) + tapered washer + nut.
• Bolt diameter: 25 or 30 mm.
• Not suitable for soft rocks.

3. Installation and Spacing (Clause 11.3.5)

• Prestress rock by bolting immediately after blasting.
• Bolt spacing < 0.5 × bolt length.
• Bolt space (t) shown in Fig. 9 (not included here).

Important Notes:

• Always prestress bolts before load develops.
• Pull-out capacity test essential for quality control.
• Choose bolt type based on rock strength (wedge & slot not for soft rock).

flowchart LR
    A[Borehole Drilled] --> B[Insert Perfo Sleeve filled with Mortar]
    B --> C[Insert Bolt inside Sleeve]
    C --> D[Mortar oozes through perforations]
    D --> E[Bonding of Bolt, Sleeve & Rock]

This summary provides essential IS 15026 guidance for rock bolting techniques.

IS 15026: Grouting Procedures and Pressures

Key Specifications (Clauses 13.3 & 13.4)

• Backfill Grouting Pressure:

  • Normal: 2 kg/cm² (0.2 MPa) (13.3.1.1.7)
  • Maximum recommended: 5 kg/cm² (0.5 MPa) (13.4.1)

• Consolidation Grouting Pressure:

  • Normal max: 7 kg/cm² (0.7 MPa)
  • Special cases (with adequate cover & stable joints): up to 20 kg/cm² (2.0 MPa) (13.4.1)
  • Should be applied in one or more stages with increasing pressures depending on rock formation & grout intake (13.3.1.3.7)

• Rock Cover Consideration:

  • Adequate cover = more than 3 times tunnel diameter
  • If cover is adequate, pressure governed mainly by design & rock characteristics.

Summary Table of Grouting Pressures

Practical Notes

• Avoid applying high pressures close to concrete lining to prevent damage.
• Increase consolidation grouting pressure gradually in stages.
• Ensure rock joints will not open under applied pressure.
• Rock cover must be checked before deciding max pressure.

flowchart TD
    A[Start Grouting] --> B{Type of Grouting?}
    B -->|Backfill| C[Apply 2 kg/cm² initially]
    C --> D{Pressure OK?}
    D -->|Yes| E[Max 5 kg/cm² allowed]
    B -->|Consolidation| F[Apply staged pressures]
    F --> G{Rock cover > 3x diameter?}
    G -->|Yes| H

IS 15026: Design of Reinforced Rock Arches — Key Formulas and Specifications

1. Capacity of Reinforced Rock Arch (Clause 11.4.1)

[ P_{bolt} = B \times F ]

Where:

• (P_{bolt}) = tension capacity of bolt/anchor (t)
• (B) = bolt/anchor spacing (m)
• (F) = load per bolt capacity (t)

Minimum uniaxial compressive strength of reinforced rock mass:
[ \sigma_{cm} \geq 9 \text{ MPa} ]

2. Effective Thickness of Reinforced Arch (Clause 11.4.1)

• For bolts: [ l' = l - \frac{FAL}{2} - \frac{S}{2} + S ]

• For mesh reinforced shotcrete: [ l' = l - \frac{FAL}{2} - \frac{S}{4} + S ]

Where:

• (l) = length of bolt/anchor (m)
• (FAL) = fixed anchor length (usually (= 100 \times) diameter of anchor bars)
• (S) = size of opening (m)

3. Design Philosophy (Clause 14.2.3 & 14.1.1)

• Use trial and error to balance ultimate pressure and design capacity.
• For support pressure > 5 kg/cm², use steel ribs embedded in shotcrete.
• Effective width (l') of reinforced arch is critical.
• Seepage pressure (u) considered in design, especially for water-charged rock masses.

4. Seepage Pressure and Grouting (Clause 14.2.5)

• Seepage pressure may equal internal water pressure.
• Worst case: tunnel empty, seepage acts on lining.
• Grouting recommended where thick shotcrete is provided to resist high grouting pressure.

Summary Table

Seismic Effects & Design Adjustments (IS 15026 Highlights):

• Seismic Increase in Support Pressure:
  For tunnels near faults/thrusts in seismic zones, increase ultimate support pressure by ~25% to account for accumulated strain (Clause 14.2.1).

  • If distance from fault > 2B (B = tunnel width), seismic effects are negligible.

• Ground Condition Classification (Clause 6.1, Table 1):
  Ground conditions affect seismic design and support selection:

  Ground Type	Description	Seismic Design Note
  Competent self-supporting	Massive rock, no support needed	Minimal seismic impact
  Incompetent non-squeezing	Jointed rock, support needed	Supports must accommodate seismic loads
  Squeezing (ua/a = 1-3%, 3-5%, >5%)	Plastic deformation into tunnel, time-dependent	Design for deformation and seismic loading
  Rock burst	Violent failure under high overstress	Special seismic-resistant support required

• Key Formula for Squeezing Check:
  [ \text{Critical strain} = \frac{\text{UCS}}{E} ] where UCS = uniaxial compressive strength, E = modulus of elasticity of rock.

• Design Adjustment Summary:

  • Increase support pressures by 25% near faults.
  • Select support type based on ground condition and seismicity.
  • Ensure supports can accommodate plastic strains and dynamic loads.

flowchart TD
    A[Tunnel Near Fault?] -->|Yes| B[Increase Support Pressure by 25%]
    A -->|No, Distance > 2B| C[Negligible Seismic Effect]
    B --> D[Select Support Based on Ground Type]
    C --> D
    D --> E{Ground Condition}
    E -->|Competent| F[Minimal Support]
    E -->|Incompetent| G[Support for Stability]
    E -->|Squeezing| H[Design for Plastic Deformation]
    E -->|Rock Burst| I[Special Seismic Resistant Support]

References: IS 15026 Clause 14.2.1, Clause

IS 15026: Special Support Accessories – Crown Bars & Truss Panels

1. Crown Bars (Clause 12.7.5 & 12.7.5.1)

• Purpose: Temporary roof support during tunnel excavation before rib sets are erected.
• Construction:
  • Built-up double channels (Fig. 15), H-beams, or square timber beams.
  • Positioned parallel to tunnel axis.
  • Supported either:
    • On outer flanges of ribs (Fig. 16A), or
    • Suspended by hangers attached to ribs (Fig. 16B).
• Functions:
  • Support roof immediately after ventilation.
  • Support roof/roof ribs over bench shot, supplementing wall plates.

2. Truss Panels (Clause 12.7.6)

• Not detailed explicitly in provided context but generally:
  • Used as longitudinal bracing between ribs/posts.
  • Increase buckling resistance about minor axis.
  • Prevent displacement during blasting.

3. Longitudinal Bracing (Clause 12.7.7.1)

• Rods and collar braces (Fig. 18) commonly used.
• Bracing unnecessary if lagging attached firmly to webs bridges ribs/posts.

Typical Crown Bar Arrangement (Simplified Diagram):

graph LR
    Roof -->|Load| CrownBar
    CrownBar -->|Rest on| RibFlange
    CrownBar -->|Or hang from| RibHanger
    RibFlange --> Rib
    RibHanger --> Rib

Summary Table:

For detailed dimensions, refer to IS 15026 Fig. 15, 16A, 16B, and 18.

IS 15026: Quality Control and Testing of Supports — Key Points

IS 15026 references several IS codes for materials and construction practices relevant to tunnel supports, including IS 432, IS 456, IS 800, IS 4880, IS 5878, and others.

Key Specifications for Quality Control & Testing of Supports:

• Material Standards:

  • Use reinforcement as per IS 432 (Part 1 & 2) for steel bars and wires.
  • Concrete quality as per IS 456:2000.
  • Steel structures as per IS 800:1984.

• Support Types & Application:

  • Supports (rock bolts, steel ribs, shotcrete) must comply with detailed guidelines in Clause 12.5.1 and Table 2 (not provided here).
  • Suitable for full face excavation when bridge action period is adequate.

• Spacing & Layout:

  • Determined by rock mass quality and excavation method (Clause 12.8).
  • Rock mass classification per IS 13365 (Part 2):1992 guides support pressure prediction.

Typical Testing & QC Measures:

• Material Testing:

  • Tensile strength, elongation for steel bars (IS 432).
  • Compressive strength for concrete (IS 456).
  • Shotcrete quality per IS 9012.

• Support Testing:

  • Load tests on rock bolts and anchors.
  • Inspection of steel lining per IS 5878 (Part 6).

Example: Support Spacing Determination (Conceptual)

flowchart LR
    A[Rock Mass Quality] --> B[Classification (IS 13365)]
    B --> C[Support Pressure Prediction]
    C --> D[Select Support Type (IS 15026 Clause 12.5.1)]
    D --> E[Determine Spacing & Layout (Clause 12.8)]
    E --> F[Implement & Test Supports]

Summary:
Use referenced IS codes for materials and construction. Follow Clause 12.5.1 for support types and Table 2 for guidelines. Determine spacing/layout based on rock quality (IS 13365) and excavation method (Clause 12.8). Conduct material and load tests per relevant IS standards.

IS 15026 - Clause 14.2: Semi-Empirical Design Approach for Shotcrete & Reinforced Rock Arch

Key Points & Formulas:

• Design by Trial & Error:
  Select parameters so that ultimate pressure = design capacity of shotcrete and rock arch.

• Steel Ribs Usage:
  If support pressure > 5 kg/cm², embed steel ribs in shotcrete.
  Spacing of ribs is estimated to satisfy:
  [ \text{Equation 4.0 (not explicitly given in the excerpt)} ]

• Water Charged Rock Mass:
  Design tables neglect seepage pressure; use:
  [ \text{Equation 6.0 (not explicitly given)} ]
  to determine grouting extent. Thick shotcrete is necessary to withstand grouting pressure.

Related IS Codes for Reference:

Summary Diagram of Design Approach

flowchart TD
    A[Select Design Parameters] --> B{Is Support Pressure > 5 kg/cm²?}
    B -- Yes --> C[Use Steel Ribs Embedded in Shotcrete]
    B -- No --> D[Shotcrete Alone]
    C --> E[Estimate Steel Rib Spacing (Eq.4.0)]
    D --> F[Calculate Capacity by Trial & Error]
    E --> F
    F --> G{Water Charged Rock Mass?}
    G -- Yes --> H[Use Eq.6.0 to Determine Grouting Extent]
    G -- No --> I[Finalize Design]

Note: For exact formulas (Eq.4.0, Eq.6.0) refer to full IS 15026 text or related IS standards on tunnel supports and shotcreting.

IS 15026: Special Requirements & Recommendations Summary

Key Points on Special Requirements (Clause 14.2.4)

• In very poor rock conditions (thick shear zones, rock burst, highly squeezing), normal assumptions fail.
• Special specifications and flexible support systems must be applied.
• Monitoring and instrumentation are essential for safety and design validation.

Ground Conditions & Support Systems (from Table 2, Clause 6.1)

Important Specifications

• Shotcrete thickness: Increase by 30% for swelling ground.
• Tunnel shape: