Excavation Shoring

Shoring Excavation 

Excavation Shoring becomes necessary when the unsupported vertical excavation exceeds 4 feet and if the horizontal distance required for sloping the excavation per OSHA regulations exceeds the distance to property boundaries, existing structures or if the sloped excavation would present access constraints during construction.

There are many excavation shoring techniques and primarily include the following:

  1. Soil Nails and Shotcrete
  2. Anchored/Tieback Drilled Shaft/Beam and Lagging
  3. Cantilever Drilled Shaft/Beam and Lagging
  4. Tangent/Secant Pile Walls
  5. Reticulated Micropile Walls

Permanent or Temporary Excavation Shoring?

If planned for in advance, excavation shoring systems can serve not only as temporary excavation support, but also as the permanent foundation wall.  Often times design changes in the building foundation system are required to utilize the temporary excavation shoring system as the permanent foundation wall. 

If not used as the permanent foundation wall, the shoring system can be design as a permanent earth retention system to reduce the lateral load on the below grade structural wall which allows the below grade wall to reduce in thickness. Anchor elements of a permanent shoring system must account for corrosion loss and be protected from corrosion. 

Selection of Appropriate Excavation Shoring System

The most appropriate excavation shoring technique and corresponding cost depends upon many variables including:

  • Project size and location and available access to mobilize equipment.
  • Proximity of shoring system alignment to property boundaries.
    • If shoring elements extend beyond property line and easement from affected property owner is required.
  • Proximity of shoring system to existing structures.
    • Existing structure foundation type and depth relative to elevation of bottom of excavation. Significant surcharge loads may apply, and an underpinning system may be required.
    • Clearance between planned construction and existing structures.
    • Existence of horizontal projections from existing structures or other overhead obstructions that limit equipment clearance required for shoring system installation.
  • Performance criteria
    • What are minimum and/or maximum criteria for performance of the system considered, i.e. max. or min. movement, deflection, settlement.
    • What is minimum required design life of system?
    • Are neighboring property owners cooperative or litigious?
  • Presence of overhead utilities
    • Shoring system equipment/clearance from overhead utilities
  • Presence of underground utilities
    • Density of underground utilities and available ‘windows’ for shoring system components.
    • Alignment of utilities and utility material type and presence & extent of bedding material.
      • Existing utility trench parallel near shoring alignment presents risk of instability of temporary cut face during construction.
    • Ground conditions
      • Soil cohesive to allow temporary stable cut face for shotcrete application.
      • Groundwater – is groundwater above, or below final depth of excavation.
      • Depth to rock and variability of depth to rock
    • Local building authority allowable practices
    • Temporary construction surcharges
      • Proximity of concrete pump trucks
      • Location of mobile crane
      • Any unusual heave crane picks and location
    • Building components deeper than planned building foundation
      • Grease traps
      • Storm water detention vaults
      • Elevator shafts
      • Required over-excavation below bottom of planned footing
    • Planned structure waterproofing
      • Waterproofing material can be applied atop shoring system?
    • Excavation Shoring System is temporary or permanent?
      • Shoring System serves dual purpose as temporary excavation shoring system and permanent foundation wall?

Excavation Shoring System Descriptions

Soil nails & shotcrete are generally the most economical excavation shoring system.  Soil nails are steel bars installed and grouted in a drill hole placed at regular intervals 5 ft to 6 ft horizontally and the same vertically.  The system can conform to complex geometries and is appropriate in a variety of ground conditions provided the soil has adequate cohesion to maintain a 5 ft vertical unsupported face long enough for shotcrete to be applied.  Soil nails are first drilled and shotcrete applied in 5 ft vertical lifts or less along the entire perimeter of the excavation.  Another 5 ft lift of soil is removed, soil nails installed, then shotcrete is applied and the process repeats until the planned depth of excavation is reached.

Drilled Shaft/Beam and Lagging Walls utilize either concrete drilled shafts or steel soldier piles or ‘H’ piles drilled and installed on 8 to 10 ft centers around the perimeter of the planned excavation.  The excavation proceeds in 5 ft lifts as wood or shotcrete lagging is installed atop the drilled shafts or soldier beams.  If the excavation height is much higher than 12-14 ft Anchors or tiebacks are installed through the soldier beam into the soil behind and then post-tensioned to provide lateral load counter to lateral earth loads thereby reducing the net horizontal deflection.

Tangent Pile Walls are constructed as drilled shafts spaced such that there is little to no clear space between drilled shafts.  Secant Pile Walls are constructed as drilled shafts with overlapped spacing, are expensive and are generally only considered if the excavation shoring system will also serve as the exterior wall or shell of the planned structure.  

Reticulated Minipile Walls are expensive and are only considered when access constraints only allow smaller equipment and anchors are not feasible or allowed due to geometry or because an easement for the excavation shoring system onto the neighboring property is not attainable.

Contact GeoCraft Builders Today

GeoCraft Builders has provided excavation shoring for projects throughout the Front Range and in remote, steep mountain terrain. Let us put our experience and expertise to work in order to help determine the safest and most economical excavation shoring system for your project.

 

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