A quick, simple and cost effective foundation substructure over a range of soft or weak soil.
Installation Procedure
The ‘Vibroflo’ tube is accurately located over the column position. Concrete is then pumped through an internal tube. This fills the protective bottom chamber which is constructed to be the diameter of the Vibro Concrete Column (V.C.C) to be formed (normally 450mm).
Using top vibration, the ‘Vibroflo’ tube is then penetrated into the ground to the required depth whilst continuously pumping concrete. This ensures there is no contamination of the V.C.C.
At this founding level, the granular sand or gravel layer is further compacted by surging the machine. Concrete continues to extrude from the protective ‘skirt’ and an enlarged bulb of concrete is formed at the base. When the designed degree of improvement is achieved the ‘Vibroflo’ tube is then lifted gradually to the surface as the concrete continues to be pumped through the tube.
The rate of this withdrawal is carefully controlled by monitoring the concrete pressure using electronic transducers positioned within the lower full tube extrusion chamber. Producing, a very high quality V.C.C. (with top to bottom total integrity).
If required, a concrete mushroom shaped expanded head can be formed at the surface to facilitate the transfer of area loadings. Steel starter bars or reinforcement can then be inserted in the wet concrete.
Advantages
The construction and installation of VCC’s is a very quiet process with minimal vibration, which dependant upon soil conditions, can permit installation relatively close to existing structures (up to 1.5m in some instances)
Generally no spoil is generated due to the displacement system which is an advantage, especially on contaminated sites
Column lengths are normally shorter compared to conventional piling in equivalent soils
The installation technique is a fast operation with typical production rates in the order of 200 to 300 linear metres per daily shift (dependent upon soil conditions).
The VCC technique permits a quick, simple and cost effective foundation sub-structure over a range of soft/weak soils. In the past, the usual recourse in such soils would have been to adopt a more expensive piled sub-structure
Ground conditions appropriate for Vibro Concrete Columns
The VCCs technique is most appropriate when there is a limited thickness of particularly soft/weak soils. It is applicable for soil strata with a shear strength of greater than 15kPa, up to around 60kPa, under specific conditions and can be utilised for soils with shear strengths in the range of 8kPa to 15kPa - providing these are generally not thicker than 1.0m.
Ideally the soil to be penetrated should be of constant strength or increasing strength with depth, to eliminate the risk of ‘necking’.
Applications of Vibro Concrete Columns
The most common application to date has been for embankment constructions where the use of VCCs can significantly reduce the overall time for embankment construction time, and therefore reduce contract periods.
Light to medium loaded structures such as industrial units, offices and low rise housing. They are principally designed to take vertical axial loads and can only accommodate low horizontal loads or tensile forces.
To support foundation loads and slabs, but slabs must be sufficiently reinforced to prevent ‘punching’ of the concrete columns through the slab.
As the concrete columns are effectively piles - foundations or ground beams should be designed to span between them.
