Temporary Ground Anchorages Heathrow Airport | Vibro | Keller UK

Temporary Ground Anchorages Heathrow Airport

Project
The key feature of ‘Safety by Design’ at T2B project was to eliminate the need for heavy wailing and propping in the main excavation area, thus reducing ‘working at height’ and heavy lifting and allowing ease of access for bulk excavation. The design, supply and installation of ground anchorages to temporarily support the diaphragm retaining walls during basement tunnel excavation and construction was required.

Solution
Ground anchorage entry level were located near the top of the diaphragm wall with installed lengths between 38m & 46m at 35 degrees below the horizontal, and axial  working loads of 700-900kN per anchor. The ability to generate these large forces in London Clay utilises Keller’s SMBA system, with multiple anchor units being installed into a single bore to achieve efficient force transfer into the ground.

Construction

  • Careful planning of the logistics before work commenced onsite was the key to the successful completion of this project. Working “airside” meant that all personnel had to be CRB checked have a full day induction, hand scan & safety talks all before full passes were issued & allowed onsite. All plant had to be rigorously checked & serviced before being allowed airside.
  • The segmental case and auger drilling system provided a flexible solution to deal with the variable ground conditions.  The use of temporary casing to stabilise the overburden enabled clean dry open hole drilling within the founding clay strata.
  • 7 unit SBMA tendons were utilised, with total fixed lengths of 27.0 – 29.0m required to achieve the design loads. Conventional multistrand fixed lengths are restricted to a maximum of 10.0 -12.0m and have a less efficient load transfer mechanism
  • The SBMA temporary anchors were fabricated to Keller’s specification. Installation of the site assembled tendons (up to 48m length) required a crane with 30m under-hook height carefully inserting into the pre grouted bore making sure the anchor tendon wasn’t damaged in the process.
  • Installation and testing of 2 no. Preliminary Trial Anchorages verified Keller’s geotechnical design with ultimate loads of 1974kN in Stiff London Clay : Factor of Safety demonstrated = 2.5
  • All 231 ground anchors were subject to testing up to 125% of their safe working load which have proven all of the anchors are performing as designed. The temporary ground anchors will be distressed in 2012 once the final basement roof slab is constructed

Sector :Infrastructure

Client: BAA

Engineer:Mott MacDonald

Principal Contractor:  Balfour Beatty Ground Engineering

Key to  Success: : A dedicated team commited to quality & safety. Onsite logistical & planning liaison ensured maximum productivity to suit evolving conditions and targets.

Keller’s SBMA system allowed economical design of temporary ground anchorages to cater for the high axial working loads required.

Programme: Phased to suit Main Contractors requirement : Average rig output 85m/shift

Project Value:  £1.2 Million

Form of Contract:  NEC3

 

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