About one third of Keller's vibro concrete columns (VCC) contracts include CPT testing by Fugro. Despite the small additional investigation cost (for one day on site), the ability to offer a lump sum price, the speed of construction and often substantial savings in comparison to a conventional piling scheme, can make the CPT/VCC package a very attractive proposition.
Vibro concrete columns behave in a similar manner to end-bearing piles. The potential for supporting load can be enhanced by enlarging the toe of the VCC and, in granular soils, densifying the soils that support the column loads. The technique is most commonly used where soft soils are underlain by sand or gravel, as typically encountered in the Thames Valley west of London.
The technique can be used in conditions which are normally beyond the capability of conventional vibro stone columns. Required loads are normally in the range of 400kN to 750kN, and to be certain of generating adequate column capacity the depth of granular soils, their grading and density must be established. The thickness of sand or sand with gravel must also be proven to avoid overstressing of underlying clays or chalk.
CPT equipment can perform 12 to 15 test locations in a single day, with much less disturbance than conventional boreholes. Data is not only faster but considerably more repeatable than standard penetration tests which rely heavily on the driller's ability to perform them properly. Computer generated print-out results also permit the immediate instruction of intermediate test locations if an anomaly is found.