Excavation Support Systems

Fundamentals of Building Construction, Materials & Methods

Learning Objectives

  • List the steps involved in earthwork and excavation.

  • Know the parameters influencing the selection of excavation support systems.

  • Know the types of excavation support and general earth-retaining concepts.

  • Visually identify and describe means and methods for different excavation support systems.

  • Understand differences between the excavation support systems.

Earthwork and Excavation Steps

  • Earthwork is a critical initial phase of virtually every construction project.

    1. Grubbing and Clearing:

    • This step involves removing vegetation, roots, and debris from the construction site.

    1. Topsoil Management:

    • The top layer of organic-rich soil is scraped off the pad site and stored for later use, allowing for proper foundation laying in subsequent steps.

    1. Excavation Process:

    • Excavation is essential for various components such as basements, shallow footings, utilities, or to address unstable or undesirable soil conditions.

Selection of Excavation Support

  • The choice of excavation support depends on the following key parameters:

    • Soil Type: Determines the most appropriate excavation method and support.

    • Depth of Excavation: Deeper excavations require more robust support mechanisms.

    • Type of Construction: The nature of the building being constructed may dictate specific excavation techniques.

    • Proximity of Surrounding Roads/Buildings: Nearby structures may restrict certain excavation methodologies.

    • Presence of Groundwater: Water levels can impact excavation stability and support needs.

Excavation Support Types

  • Sloped or Benched Excavation:

    • More cost-effective than sheeted excavation but necessitates sufficient space away from property lines and adjacent structures.

  • Sheeted Excavation:

    • Utilized in confined spaces wherein soil surrounding the excavation must be contained by a support system.

Sloped or Benched Excavation

  • The geometry of excavation selection is influenced by the soil's natural Angle of Repose, which is the steepest angle at which a sloping surface of loose material remains stable. 

    • Soil Type Stability Classification:

    • Most Stable: Clay, silty clay, and cemented sands can support steeper angles of benching.

    • Moderately Stable: Angular gravels, silt, loamy silt, and moist cohesive soils require sloped benches.

    • Least Stable: Granular soils (gravel, sand, loamy sand), submerged soil, or soils with water seepage can only have sloped benches; benching is not permissible.

Sheeted Excavation

  • Definition:

    • Sheeting refers to the solid panel systems used to create walls that hold back soil.

  • Shoring System:

    • A structural support system that actively resists loads to prevent collapse during the excavation process.

  • Methods for Lateral Support of Sheeted Excavation:

    • Crosslot Bracing: A system that provides support across the width of the excavation.

    • Rakers: Supports that are angled from the excavation wall to the ground.

    • Tiebacks: Anchors that provide tension support to keep the sheeting in place.

Bracing Example: Installation of Tiebacks or Ground Anchors

  1. Drilling:

    • A hole is drilled into stable rock or soil.

  2. Insertion and Grouting:

    • Steel prestressing tendons or rods are inserted into the hole and grouted to secure.

  3. Post-Grouting Tensioning:

    • Once the grout has hardened, the steel tendons or rods are put under tension using a hydraulic jack and anchored to a waler or steel plate.

Excavation Shoring Types

  • Soldier Beams and Lagging:

    • Construction example includes Berliner wall, which consists of steel H-pile soldiers with wood lagging and raker bracing.

  • Steel Sheet Piling:

    • These piles can be hammered or vibrated into place, with placement methods dependent on soil conditions.

    • Sheet piles are usually made from steel or fiber-reinforced polymers (plastic).

  • Secant Piles (Von Der Wand Pile Wall):

    • A type of wall that consists of intersecting piles creating stability and reducing water inflow.

Ground/Earth Improvement Techniques

  • Soil Mixing:

    • This technique creates columns of strengthened soil using a chemical binder (such as Portland cement and water) prior to excavation. The mixed soil remains in place post-excavation.

  • Excavation Shoring Type Example:

    • Mixed soil support incorporating soldier beams, walers, and tie-backs.

  • Rammed Aggregate Piers:

    • Composed of crushed rock which is compacted in lifts, densifying the surrounding soil. Piers can be up to 36 inches in diameter and reach depths of 30 feet.

  • Shotcrete:

    • Application example of shotcrete on mixed soil wall for stabilization during underground basement excavation.