Introduction
Groundforce Shorco has supplied its modular hydraulic propping system to provide temporary structural support for a basement excavation in a quiet residential street in south London.
Project
Contractor IC&T Projects has commenced work on site, starting from the existing gravel ground level. Initial works include preparing the site for party wall underpinning and constructing the piling crush mat before commencing work on the basement excavation which is supported by a contiguous piled retaining wall with an insitu-cast capping beam.
Challenge
The basement excavation covers the full width of the site, meaning that it abuts existing residential properties on both sides. These two houses will remain fully occupied throughout the duration of the project.
A major concern is to minimise disturbance to local residents and especially to avoid any structural impact on the two houses adjacent to the site. Therefore, all site activities have been planned for minimal noise, vibration and duration.
IC&T contracts manager Sergio Lobato, who is himself a qualified structural engineer, initially planned to brace the perimeter of the excavation with a fabricated steel framework bearing on the concrete capping beam. However, after researching alternative methods he concluded that a proprietary modular system, like Groundforce Shorco’s, would be quicker and easier to install.
A more fundamental challenge was that whatever temporary works solution was chosen, it would have to remain in place until after the ground floor slab had been cast. With the Groundforce props bearing directly onto the capping beam, not only would assembly of the slab’s shuttering be extremely difficult but the props would end up underneath the slab, making extraction and removal virtually impossible.
Solution
In order to avoid trapping the equipment beneath the ground floor slab, IC&T’s Sergio Lobato proposed a modification in which the props would be installed above the capping beam, leaving space beneath in which the slab formwork could be assembled.
In order to do this, IC&T cast 1.25m-high upstands – or ‘blisters’ – into the capping beam with steel end-bearing plates to which the Groundforce Shorco props could be attached.
The excavation is roughly rectangular and measures approximately 30m long x 15m wide. Groundforce Shorco supplied five of its modular hydraulic props: one MP150 spanning 13m across the full width of the excavation at its mid-point, and four smaller MP50s, ranging in length from 5.4m to 8.5m, installed as knee-braces across the corners of the excavation.
Accuracy and precision were crucial to the success of this project. Calculations carried out by specialist geotechnical engineer Pile Designs were used by Groundforce Shorco to refine its own design criteria. Detailed calculations were carried out to allow for pre-loading, deflection, thermal loadings and other factors affecting the capacity and stability of the solution.
The calculations also had to take into account the rotational forces imposed upon the concrete piles as a result of the props being located above ground level.
Groundforce delivered the MP150 and MP50 props in sub-assemblies which were bolted together on-site by IC&T’s team. By locating the props above the level of the capping-beam, installation and removal of the equipment was achieved quickly and with minimal difficulty.