Demand for plunge columns inserted with great accuracy within piles appears to be on the increase. In June Ground Engineering reported on a project where columns were installed with a verticality of only 1 in 200, and this has now been bettered by Bachy Soletanche which has just completed a project involving plunge columns placed with a verticality of 1 in 400.
The ability to achieve such accuracy is giving structural engineers increased confidence to specify top-down basement construction in which the key vertical structural elements below ground floor are installed by the foundation contractor as part of the foundation work. Although it complicates the foundation package, the technique offers significant potential to save the client both time and cost.
Such accuracy is all the more impressive at the early stages of a project, when sites are particularly cramped and congested, and may well be undergoing major muckshifting operations. Despite these constraints, piling contractors are clearly up to the challenge of providing innovative solutions to meet the exacting demands of the structural engineer.
Bachy Soletanche's contract at 100 Wood Street in central London was for a new office building designed by Foster & Partners. The two storey basement is to be topped by a 10 storey, 18,500m2 office building constructed with structural steel and lightweight concrete, clad in glass and Portland stone. Developer is Helical Bar, and main contractor HBG Construction. Structural engineering is by the Waterman Partnership.
The Wood Street project was originally tendered and priced on the basis of forming piles concreted up to 5.5m below ground level and left with permanent liners up to surface. The original design was to fix columns within these permanent liners. Together with HGB Construction and Waterman, Bachy Soletanche designed a plunged column alternative which offered considerable savings.
The basement is, instead, being constructed with 48 columns set within rotary bored piles to an accuracy of 6mm in plan, and to a vertical tolerance of 1:400. The task is made more demanding in that 13 different column sizes have had to be accommodated, ranging from 200mm to 500mm square.
Key components in the Bachy Soletanche column placing system are a plunge frame and circular 'top hat'. The top hat, which was specially developed for the Wood Street contract, is a circular guide frame which is fixed and levelled at the top of the pile casing. It holds the plunge frame and can move it a short distance in any direction in a perfectly horizontal plane.
The plunge frame has two sets of rollers, which are adjusted to column size, so that the column is held perfectly in line with the frame. Once the column is positioned within the frame it is lowered into the top hat, and the arrangement surveyed and adjusted so that the column and frame are positioned centrally over the centre point of the column. The column is checked for verticality using an inclinometer attached to its side, and the verticality is fine tuned using scissor jacks at the bottom of the frame.
Finally, the plan location of the now vertical column is checked and adjusted using four jacks on the levelled and stabilised top hat. Adjustment of the top hat moves the plunge frame and column
in plan location, without affecting the column's verticality. Posit- ioning takes typically 30 to 40 minutes.
Bachy Soletanche site agent Steve Regen explains the approach is a development of the company's already tested plunging frame. While the system previously allowed columns to be positioned to very high vertical tolerances, plan position could not be guaranteed better than within 15mm. The real innovation is the top hat, which enables columns to be located to within just 5mm
of their intended locations.
Incredibly, this is greater accuracy than the rolling tolerance of the columns, which is 12mm in section and 1 in 250 for verticality. For this reason every column is surveyed on site and adjustments made to the roller settings in the plunge frame to accommodate variations in the rolled columns. Paradoxically, this need to survey and set the plunge frame rollers uniquely for each column has meant that dealing with the 13 different column sizes on site has not caused a major headache.
Columns are plunged into rotary bored piles of between 1,050mm to 1,800mm in diameter and up to 41m deep, formed conventionally using standard rotary drilling equipment. The piles are required to have a verticality of 1 in 75 although generally much better is achieved without special measures, reports Regen.
Before production work, Bachy Soletanche used static load testing to confirm the load capacity. Piles were loaded to two and a half times working load, followed by loading to failure which as predicted occurred at three times working load. By confirming the design assumptions, static load testing allowed the safety factor to be reduced from 3.0 to 2.0.
Work on site was geared to producing one pile/plunged column a day. Typically the day starts by removing the plunge frame and top hat from the previous evening's installation. Activities throughout the shift continue with boring and positioning of the next column, culminating in the late afternoon with the column in position and its base embedded in the freshly poured pile.
The frame is left overnight, giving sufficient time for the concrete to cure. This ensures the column is adequately fixed within the pile, allowing the frame to be removed at the start of work the following day.
Another notable aspect of Bachy Soletanche's pounds750,000 contract has been pre-drilling through a 1950s concrete pad foundation left from the previous building at the site for 30 of
the column locations. This was achieved using tungsten tipped core barrels and high torque rigs. The company has also installed 20 other load bearing piles and king posts to provide temporary retaining function at the site.