Roger Bullivant Limited (RB) were approached by The Baxi Group to assist in providing a foundation solution for their annual Christmas Tree Installation at their yard and office facility in Bamber Bridge, Preston. The formwork for their initial concrete base foundation had twisted during installation, and as a result they sought an alternative solution to provide a sturdy base for their proposed 25ft Norwegian Spruce.
The RB solution comprised of a single 273mm bottom-steel tube pile, driven to a depth of 6m into the local firm clayey Glacial Till strata, infilled with concrete, into which a smaller 178mm diameter full length “heavy-wall” tube of 12mm thickness was inserted. The top 0.5m of the inner tube would be left hollow so as to form a socket into which the Christmas tree trunk could be inserted.
This technique was chosen for a number of reasons: the first being the ability to install such a pile installation in confined access within a tight window of possession; the installation itself was carried and completed in the space of a morning. The other benefit for choosing a driven pile foundation solution was that during construction the method of installation doubles-up as its own design verification; whilst the pile was designed statically, the installation records provide a level of confidence, in real-time, that the pile will perform adequately under loading.
Due to the lack of information available, and the timescales involved, RB engineers had to derive the anticipated loads for the project to ensure the foundation could support the tree. They estimated the weight of the tree in both its natural and decorated states from research and correspondence with Christmas tree suppliers.
In modelling the tree as a minor structure, following design methodology and guidance
outlined by Highways England, RB engineers found a novel way to model the tree problem, and as a result were satisfied that the solution generated for this scheme was able to provide enough support for the tree against seasonal wind activity over the Christmas period. The introduction of the internal “heavy-wall” tube provides substantial lateral resistance, and the tube itself acts as a socket for the placement of the tree trunk, which can also allow for the tree to be bolted, or wedged, into position for improved stability.
The site area was then backfilled with stone, and was suitably compacted to allow for additional support around the tree. The RB solution, in its final state, is a dedicated tree base that can be re-used for future Christmas tree installations.