Groundforce has supplied propping equipment to support a large basement excavation during construction of Dublin’s new shopping destination, Project Kells. The equipment includes the latest heavy duty hydraulic struts to be introduced into the company’s fleet.

Project Kells is a high profile retail and office development being built by John Sisk & Sons in the heart of Dublin. It occupies a curved corner close to Trinity College, and is bounded on two sides by roads and existing buildings. Tracks and overhead lines for Dublin’s Luas transit system run alongside the site.

Secant piled retaining wall

The new building will have six floors above ground and two basement levels, with the basement being excavated to a depth of 9.5m across the entire site. This basement has been created by installing a secant piled retaining wall around the perimeter, made up of 360 piles averaging 15m in depth.

The curved corner section of this secant piled wall is tied back under the Luas lines by permanent 15m long ground anchors installed at an angle of 45^o.

The rest of the piled wall is being supported during the basement excavation by 12 heavy duty hydraulic struts supplied by Groundforce, which range in length from 7m to 30m in length, and sit across the corners of the irregularly shaped basement.

The propping solution – designed to work in combination with the ground anchors – is made up of the company’s MP250, MP375, MP500 and MP750 models.

Largest capacity proprietary strutting system

The MP375 and MP750 are the most recent additions to Groundforce fleet, with the MP375 offering an intermediate size between two existing models, and the 1,220mm diameter MP750 being the largest capacity proprietary strutting system on the market, having a design resistance of 11,700kN. 

The temporary propping system was designed to loadings of 200kN/m - 225kN/m SLS, with four of the props fitted with load monitors that provide real time loading information, so the contractor can see that actual loads do not exceed design loads.

In addition to the loading requirements, the props also have to meet very tight stiffness and deflection criteria to ensure there is no risk to neighbouring buildings and services or to the Luas lines.

The props were installed in October 2020, after a large capping beam was cast on top of the secant piled retaining wall. Groundforce senior engineer (major projects) Adam Fletcher said: “This was a very interesting and challenging scheme to work on, with a solution involving proprietary kit and ground anchors working alongside each other.”

John Sisk & Sons contracts manager Eoin O’Meara said: “One reason we chose the Groundforce propping system to support the excavation was because the hydraulic component of the prop makes it easier for installation and removal.”

The 23,000m³ basement excavation is now underway, and the struts will be removed once the new permanent floor slabs have been cast.