Research using O-cell Technology in Spain
In the North of Spain, overlooking the Bay of Biscay, lies the beautiful Basque city of San Sebastian-DonostÃa. A haven for sun seekers and food lovers alike, this city was the location of a research project undertaken by the University of Madrid with the help of O-cell Technology.
This project was situated on the site of the construction of a new bridge and the underlying Flysch rock made this location the idea site for the research project of the University of Madrid assisted by the Rodeo Kronsa Group who provided the piling expertise. Pile design in rock has always been difficult. Many papers have been written around the world and many differing specifications for design are in use. More often than not, these pile designs have been over-conservative. In order to further the research, Professors Olalla and Serrano have been investigating the behaviour of the shaft friction of piles in rock and have published two articles in the IJRMMS (2004, vol 39(1) and 2006, vol 43 (5)). These articles describe a new theory based on other properties of the rock, similar to the Hoek and Brown (1980) findings. To check these and other empirical theories, further investigation using in situ testing to determine ultimate capacities was required. A secondary objective was to determine the pile end bearing resistance. Two further papers had been written on this topic in IJRMMS (2002, vol 39 (7) Parts I and II).
In order to achieve useable data for this research, controlled loading of a fixed shaft length in the rock was required. The simplest way to achieve a precise length is by use of a multi-level test arrangement. Two levels of O cell are placed so that the mid-section is a know length and therefore the skin friction area is readily quantifiable.
The pile bore was made 17m long and one O cell level was set very close to the toe to measure the end bearing properties. The second upper level was placed exactly 2.5 metres above this level. A zero shear sleeve section was inserted into the top section of the pile to give a second 2.5 metre section between the upper O-cell level and the zero shear zone.
The pile was loaded in two stages, the initial stage loading the toe of the pile using the upper section of the pile as resistance. Once the toe of the pile has moved downwards, the bottom O-cell level provided zero resistance to downward movement when the upper level of O-cell is loaded in the second stage.
The objectives of the test were to mobilise the end bearing to close to failure and fully mobilise either segment of 2.5 metres pile in the Flysch rock. Optimism was shared by all that pile had been fitted with sufficient loading capacity to achieve the objectives. The maximum mobilised reaction from the bi directional test was 32.5 MN, far in excess of the value predicted before testing commenced.
Post test analysis of the behaviour using Cemsolve allowed a prediction of the ultimate end bearing at in excess of 20 MN (twice the scheduled test capacity). The predicted ultimate values of the combined rock socket above and below the upper O-cell would be of the order of 40 MN.
Further research will continue on this subject but the testing has proved that sometimes even the most pessimistic of design parameters taken still results in very conservative values for pile design in rock.
Fugro Loadtest are delighted that both the University of Madrid and Rodeo Kronsa Group allowed us to participate in this research project.
