Guidelines for the design and construction of prestressed rock anchors

IS 10270: Scope & Key Specifications for Prestressed Rock Anchors

Scope (Summary)

• Applies to design and construction of prestressed rock anchors.
• Defines terms like prestressing force (Clause 2.15):

  Load initially applied to anchors to meet design load + expected losses.

• Testing (Clause 8.2):

  100% anchors stressed to 110% of design load during stressing to verify suitability.

Key Definitions & Units (Clause 2.0 & SI Units)

• 1 N = 1 kg·m/s²
• 1 Pa = 1 N/m²

Testing of Anchors (Clause 8.2)

Prestressing Force (Clause 2.15)

[ P = P_d + L ]

• (P) = Initial prestressing force
• (P_d) = Design load
• (L) = Allowance for losses (friction, relaxation, etc.)

flowchart LR
    A[Design Load (Pd)] --> B[Add Losses (L)]
    B --> C[Initial Prestressing Force (P)]
    C --> D[Apply to Anchor]
    D --> E[Test at 110% Pd]

Summary: IS 10270 ensures prestressed rock anchors are designed with adequate prestressing force and tested at 110% design load for safety and reliability.

IS 10270: Terminology and Definitions - Key Points

• Prestressing Force (Clause 2.15):
  The initial load applied to anchors to meet design load plus allowance for expected losses.

• Anchor Testing Strength (Clause 8.2):
  100% of anchors are stressed to 110% of design load during stressing to verify suitability.

• Units (SI) as per IS 10270:

  Quantity	Unit	Symbol	Definition
  Length	metre	m
  Force	newton	N	1 N = 1 kg·m/s²
  Pressure / Stress	pascal	Pa	1 Pa = 1 N/m²
  Energy	joule	J	1 J = 1 N·m
  Power	watt	W	1 W = 1 J/s
  Electric current	ampere	A
  Thermodynamic temp.	kelvin	K

• Reference Figures:
  Fig. 1 illustrates anchor components and terminology (not provided here).

Summary Table: Anchor Testing Load

flowchart LR
    A[Design Load (P)] --> B[Test Load = 1.10 × P]
    B --> C[Anchor Stressing]
    C --> D{Pass?}
    D -->|Yes| E[Anchor Approved]
    D -->|No| F[Anchor Rejected]

This ensures anchors meet strength and durability requirements per IS 10270.

IS 10270: Key Materials Specifications for Prestressed Rock Anchors

1. Prestressing Steel (Clause 4.2)

• Must conform to IS 2090-1962 (High tensile steel wires for prestressing).
• Typical properties include:
  • High tensile strength (~1860 MPa)
  • Good ductility and relaxation characteristics

2. Cement (Clause 6.1)

• Ordinary Portland Cement as per IS 269 (Third Revision).
• Supersulphated cement and other special cements may be used depending on environmental conditions.

3. Drilling & Hole Preparation (Clause 6.1)

• Temporary casing or bentonite mud circulation used to stabilize boreholes.
• Hole diameter depends on anchor capacity.
• Casing mandatory for inclined bores.

Design Considerations & Testing (Clauses 2.15 & 8.2)

Units & Symbols (SI Units)

Tentative Bond Strength Relation (Fig. 2)

flowchart LR
    A[Materials] --> B[Prestressing Steel (IS 2090)]
    A --> C[Cement (IS 269)]
    A --> D[Drilling & Hole Stabilization]
    B --> E[High Tensile Strength]
    C --> F[Ordinary/Supersulphated Cement]
    D --> G[Temporary Casing/Bentonite]

Summary: Use IS 2090 steel, IS 269 cement; stabilize boreholes with casing or bentonite; test anchors at 110% design load; bond strength depends on rock compressive strength.

IS 10270: Design Considerations for Prestressed Rock Anchors

Key Points from Clause 5 and Related Clauses:

• Prestressing Force (Clause 2.15):
  Initial prestressing force = Design load + Allowance for losses.

• Data Required for Design (Clause 3.1):

  • Service life (temporary/permanent)
  • Design load per anchor
  • Soil investigation including:
    • Borehole log & penetration depth (≥ 10 m into rock)
    • Undrained shear strength & bulk density (IS 2720 Part XI)
    • Shear strength & unit weight of rock (IS 1121 Part IV, IS 1122)
    • Sulphate & chloride content (IS 2720 Part XXII)
    • Permeability & water table data (IS 5529 Part II)

• Testing (Clause 8.2):
  Anchors stressed to 110% of design load during stressing to verify suitability.

Typical Formula for Prestressing Force:

[ P_i = P_d + L ]

Where:

• (P_i) = Initial prestressing force
• (P_d) = Design load per anchor
• (L) = Loss allowance (due to friction, relaxation, creep, etc.)

Summary Table: Soil & Rock Parameters for Design

Units & Symbols (SI Units)

IS 10270: Fixed and Free Length of Anchors (Clause 5.2)

Key Definitions:

• Fixed Length (Bond Length): Length of anchor bonded to rock/grout, resisting pullout by bond and shear.
• Free Length: Length not bonded, free to elongate under load.

Fixed Length Design (Clause 5.2.1):

• Determined by allowable bond stress between:
  • Steel and grout
  • Grout and rock
  • Shear strength of rock
• Use:
  • Rock's Unconfined Compressive Strength (UCS) vs bond stress correlation (Fig. 2)
  • Typical rock data (Table 1)
  • Site experience (rock roughness, weathering, shear strength)

Free Length Design (Clause 5.2.2):

• Based on pullout capacity check (Fig. 3)
• Safety Factors:
  • Permanent anchors: ≥ 1.5
  • Temporary anchors: ≥ 1.25
• Minimum desirable free length: 5 m (if pullout criterion demands)

Summary Table (Conceptual):

Pullout Capacity Check Formula (Conceptual):

[ \text{Pullout Capacity} = \tau_b \times A_b ]

Where:

• (\tau_b) = allowable bond stress (from rock/grout data)
• (A_b) = bonded surface area = (\pi \times d \times L_{fixed})

Ensure:

[ \frac{\text{Pullout Capacity}}{\text{Applied Load}} \geq \text{Safety Factor} ]

flowchart LR
    A[Anchor] --> B[Free Length (Elongates)]
    A --> C[Fixed Length (Bond

IS 10270: Key Points on Grouting Procedures

1. Waterproofing & Consolidation Grouting (Clause 6.3 & 2.7)

• After drilling rock, perform a water test.
• If excessive water loss, pressure grout the hole (ref: IS 6066-1971).
• Redrill and repeat until satisfactory Lugeon value is achieved.
• Consolidation grouting strengthens and waterproofs rock around the hole before cable insertion.

2. Fixed Length Grouting (Clauses 2.5 & 6.6)

• Grout injected under pressure in the fixed length (anchorage zone).
• Use thick grout mix (water-cement ratio ~0.5) for primary grouting.
• Ensure free length remains ungrouted for elongation.
• Alternatively, place grout at hole bottom before lowering anchor.

3. Allowable Bond Stress for Rock Types (Table 1, Clause 5.2.1)

4. Factor of Safety for Pullout (Fig.3 Concept)

• Load transferred by bond, ( P ) (tonnes)
• Ensure:
  [ P \leq \text{Weight of cone} ] [ 2P < \text{Weight of shaded area} ]

Summary Diagram: Grouting Process Flow

flowchart TD
    A[Drill Hole] --> B[Water

Corrosion Protection for Prestressed Anchors (IS 10270)

Key Methods (Clause 7.0 - 7.2)

1. Fixed Length Protection:

   • Neat Cement Grouting: Repeated pressure grouting to achieve permeability < 3 lugeons, preventing water ingress.
   • Epoxy Treatment (IS 8230-1976):
     • Pretreat wires by sand/grit blasting and cleaning with thinner.
     • Apply 3 coats of epoxy:
       • 1st coat → dry 2-3 hrs
       • 2nd coat → dry 24 hrs
       • 3rd coat → apply while tacky + sprinkle quartz sand for bonding.
   • Extra High Tensile Wires: Added to reduce stress and corrosion; limited by anchor hole diameter.

2. Free Length Protection (Clause 7.1):

   • Use galvanized iron pipe (IS 1239 Part I - 1979) or PVC pipe enclosing wires.
   • Fill annular space with neat cement grout.
   • Paint wires with epoxy-based paint after cleaning and priming.

Specifications Summary:

Notes:

• Sheathing (Clause 2.10): Enclosure of prestressing steel for corrosion protection.
• Extra wires reduce stress corrosion by lowering individual wire stress.

flowchart TD
    A[Prestressed Anchor] --> B[Fixed Length]
    A --> C[Free Length]
    B --> D[Neat Cement Grouting]
    B --> E[Epoxy Treatment]
    B --> F[Extra High Tensile Wires]
   

Testing of Anchors as per IS 10270: Clause 8.2

• Full Load Testing:
  All anchors are stressed to 110% of the design load during stressing to verify strength and suitability.

• Purpose:
  To ensure anchors can safely carry the design load with a 10% margin, confirming reliability.

Key Specifications:

Additional Notes:

• Soil and rock properties must be investigated before design (Clause 3.1), including shear strength, density, permeability, and chemical content.
• Testing ensures anchors meet performance under actual service conditions.

Summary Diagram:

flowchart LR
    A[Design Load] --> B[Test Load = 1.1 × Design Load]
    B --> C{Anchor Testing}
    C -->|Pass| D[Anchor Approved]
    C -->|Fail| E[Review & Redesign]

Reference: IS 10270:1982, Clause 8.2 (Testing of Anchors)

IS 10270: Installation and Construction Practices - Key Points

1. Testing of Anchors (Clause 8.2)

• 100% of anchors must be stressed to 10% over design load during stressing to verify suitability.

2. Data Required for Design & Construction (Clause 3.1)

• Service life: Temporary or permanent.
• Design load per anchor.
• Soil investigation including:
  • Borehole logs (minimum 10 m rock penetration).
  • Undrained shear strength & bulk density (IS 2720 Part XI).
  • Shear strength & unit weight of rock (IS 1121 Part IV, IS 1122).
  • Sulphate & chloride content (IS 2720 Part XXII).
  • Permeability & water table data (IS 5529 Part II).

3. Units & Symbols (SI Units)

Summary Table for Anchor Testing Load

flowchart TD
    A[Start: Soil Investigation] --> B[Borehole Logging]
    B --> C[Determine Soil & Rock Properties]
    C --> D[Design Anchor Load]
    D --> E[Installation of Anchors]
    E --> F[Stress Testing: 1.1 × Design Load]
    F --> G{Pass?}
    G -- Yes --> H[Anchor Accepted]
    G -- No --> I[Remedial Action]

Note: Follow related IS codes for soil/rock testing referenced in IS 10270 for detailed parameters.

IS 10270: Safety & Quality Control Key Points

1. Safety Testing of Anchors (Clause 8.2)

• 100% anchors are stressed to 110% of design load during stressing to verify suitability.
• This ensures anchors can safely carry loads beyond design values.

2. Design Data Requirements (Clause 3.1)

• Service life: Temporary or permanent.
• Design load per anchor.
• Soil investigation including:
  • Borehole logs with soil/rock types and minimum 10 m core drilling in rock (IS:1982-1979).
  • Undrained shear strength & bulk density at depths (IS:2720 Part XI-1971).
  • Shear strength & unit weight of rock (IS:1121 Part IV-1974, IS:1122-1974).
  • Sulphate & chloride content in soil & groundwater (IS:2720 Part XXII-1977).
  • Permeability of rock and fissures (IS:5529 Part II-1973).

3. Units & Definitions (SI Units)

Summary formula for anchor stressing:

[ \text{Test Load} = 1.10 \times \text{Design Load} ]

flowchart LR
    A[Design Load] --> B[Test Load = 1.1 x Design Load]
    B --> C[Anchor Stressing]
    C --> D{Pass?}
    D -->|Yes| E[Accept Anchor]
    D -->|No| F[Reject/Repair Anchor]

References: IS 10270 clauses 3.1, 8.2; IS 1982, IS 2720, IS 1121, IS 5529 for soil and rock testing.

IS 10270: Guidelines for Design & Construction of Prestressed Rock Anchors - Applications

Key Specifications & Applications:

• Prestressing Force (Clause 2.15):
  Initial load applied to anchors to cover design load + expected losses.

• Testing of Anchors (Clause 8.2):
  100% anchors stressed to 110% of design load during stressing to verify suitability.

• Applications:
  Prestressed rock anchors are used for:

  • Stabilizing slopes and retaining walls
  • Supporting foundations in rock strata
  • Load transfer in rock masses for bridges, dams, tunnels

Important Data & Standards Referenced (Clause 3.1):

• IS 1892-1979 (Foundation code)
• IS 2720 (Part 22)-1972 (Permeability tests in bedrocks)
• IS 5529 (Part 2)-1985 (Code of practice for permeability tests)

Units & Definitions (SI Units):

Summary Formula for Anchor Load Testing:

[ \text{Test Load} = 1.10 \times \text{Design Load} ]

flowchart LR
    A[Design Load] --> B[Test Load = 1.1 × Design Load]
    B --> C[Anchor Stressing]
    C --> D[Check Suitability]

This ensures anchors are reliable under slightly higher than design loads, confirming safety and performance.

IS 10270: References and Related Standards Summary

Key Related IS Standards:

Important Specifications:

• Anchor Testing (Clause 8.2):

  • 100% of anchors are stressed to 110% of design load during testing to verify suitability.

• Terminology (Clause 2.0):

  • Definitions per Fig. 1 (not shown here) clarify terms related to prestressed rock anchors.

• Units (SI Units):

Amendment Highlights:

• Substitutions in referenced IS codes to latest versions for accuracy and compliance.
• Terminology changes from 'Free' to 'Fixed' in Clause 5.2.2 for anchorage conditions.

Summary Diagram of Anchor Testing Load

graph LR
A[Design Load] --> B[Apply 110% Load]
B --> C{Anchor Suitability}
C -->|Pass| D[Anchor Approved]
C -->|Fail| E[Anchor Rejected]

Use these references and specifications for design, testing, and construction of prestressed rock anchors as per IS 10270.