Study Notes on Excavation and Timbering

UNIT IV - EXCAVATION AND TIMBERING

TIMBERING

  • Timbering refers to the temporary supports to the sides of excavations.

  • It is also sometimes called planking or strutting.

  • Purposes of timbering:

    • Protect the operatives while working in the excavation.

    • Act as a retaining wall to keep the excavation open.

  • The type and amount of timbering required depends on:

    • The depth of the excavation.

    • The nature of the subsoil.

  • Considerations:

    • Soils may not require timbering for short periods.

    • Weather conditions, depth, type of soil, and duration of operations need assessment.

    • Each excavation must be evaluated separately.

SUITABLE TIMBERS FOR TIMBERING

  • Types of suggested timbers include:

    • Scotts Pine

    • Baltic Redwood

    • Baltic Whitewood

    • Douglas Fir

    • Larch

    • Hemlock

MEMBERS USED IN TIMBERING

  • Polling Board:

    • A flat wooden plank in direct contact with the soil, positioned vertically on the trench side.

  • Sheeting:

    • The process of holding a polling board or planks together or using a sheet instead of a plank.

  • Wales (or Walling):

    • A component attached to the trench wall; supports the polling board; perpendicular to the polling board and parallel to the trench wall.

  • Strut:

    • A piece of wood used to support sheeting/walling, maintains distance between wales and trench mainline.

TIMBERING MEMBERS (Diagrams)

  • Members illustrated include:

    • Vertical sheeting

    • Horizontal sheeting

    • Strut

    • Wale

    • Wall plate

    • Cleat

    • Needle

    • Various types of braces and rakers

BRACING

  • Bracing:

    • Diagonal support between walls to strengthen the timbering framework.

  • Runners:

    • Special type of shoe made of iron placed at the bottom of the polling board in soft or hard soil conditions.

    • Runners aid stabilization and support the polling boards, which are lowered deeper during excavation.

METHODS OF TIMBERING

  • There are FIVE main methods:

    • Stay Bracing

    • Box Sheeting

    • Vertical Sheeting

    • Runners

    • Sheet Piling

1. STAY BRACING METHOD

  • Used when:

    • Soil is moderately firm.

    • Excavation depth does not exceed 2m.

  • Method details:

    • Polling boards placed on trench sides, held by 1 or 2 rows of struts.

    • Polling boards spaced 3 to 4m apart, extending to full excavation depth.

  • Specifications:

    • Width of polling board: 250 mm

    • Thickness of polling board: 40 mm to 50 mm

    • Size of strut:

    • 100 mm x 100 mm (for depth ≤ 2 m)

    • 200 mm x 200 mm (for depth > 2 m)

2. BOX SHEETING METHOD

  • Utilized when:

    • Soil is loose.

    • Depth of excavation does not exceed 4m.

  • Description:

    • A box-like structure using sheeting, wales, struts, and bracing.

3. VERTICAL SHEETING METHOD

  • Applicable when:

    • Soil is loose and depth of excavation does not exceed 10m.

  • Similar to Box Sheeting:

    • Called double-stage timbering, involves vertical sheets paired with horizontal wales, struts, and bracing.

  • Additional details:

    • Offset is generally at depths of 3 to 4m, with width between 30cm and 60cm.

    • Suitable for laying gutter and water pipes at increased depths.

4. RUNNERS METHOD

  • For use in:

    • Very loose soil conditions.

  • Description:

    • Runners are long, thick wooden planks with iron shoes attached at the bottom.

    • Driven into the ground approximately 30cm from the trench bottom.

    • Supported by wales and struts.

5. SHEET PILING METHOD

  • Suitable for:

    • Large area excavations.

    • Loose and soft surrounding soil.

    • Situations where water is present during excavation.

    • When excavation depth exceeds 10m.

    • Greater trench width conditions.

  • Types of sheet piles:

    1. Wooden Sheet Pile: Used up to a depth of 10 m.

    2. Steel Sheet Pile: Used up to a depth of 30 m; comes in various shapes and sizes.

PRECAUTIONS DURING TIMBERING

  • 1. Slip of Earth:

    • Soil erosion can occur; monitor trench surroundings closely. If cracks appear, cease excavation immediately and relocate workers to safety.

  • 2. Testing of Struts:

    • Regular checks on strut positioning and rigidity are essential.

  • 3. Sagging of Wales:

    • As soil pressure increases, wales may bend; inspect and reinforce when needed.

  • 4. Movement of Timbering Members:

    • Periodic inspection is crucial for all timbering components to ensure they hold firm in the changing soil pressure conditions, particularly during rain.

  • 5. Removal of Timbering:

    • Safely remove boards and supports in stages post-excavation, filling in layers of soil (20cm to 30cm) to prevent settling.

  • 6. Safety Helmets:

    • Essential for worker safety to prevent injuries from falling debris.

  • 7. Ladders:

    • Place ladders at intervals (30m in hard soils, 15m in loose soils) for emergency escape.

  • 8. Traffic Control:

    • Movement of machinery along the trench edge should be supervised by experienced personnel.

  • 9. Proper Tools:

    • Ensure workers have the appropriate tools for emergencies.

  • 10. Supervisors:

    • Must oversee workers in trenches to ensure safety protocols are followed.

  • 11. Protection of Adjacent Properties:

    • Provide temporary support if excavation threatens nearby building foundations.

  • 12. Shelter:

    • Arrange resting places away from the trenches for workers.

  • 13. Fencing and Lights:

    • Erect wire fencing around trenches and use red lights at edges to maintain visibility.

  • 14. Design and Arrangements of Timbering Members:

    • Calculate members' sizes based on expected loads considering soil type and excavation depth for effective timbering support.