BMC are installed in phases. The bottom part is installed in a process similar to a CMC technology. A specially designed displacement auger installed on a machine equipped with a high torque and static vertical thrust head displaces the soil horizontally towards the hole centerline. When the displacement auger reaches the required depth, the injection grout based on a concrete mixture is pumped under pressure to the hole. Pumped concrete flows through the auger tube. The concreting process is performed under a pressure which does not cause any damage to the hole walls and prevents from mixing the soil with the injection grout.
A BMC head is formed applying the SC technology in a point of construction of the CMC core. Using a specially designed vibratory probe, mounted on a hardware unit, and in three basic stages a gravel head of BMC columns is formed:
- A vibroprobe is driven into the ground to the required depth, usually from 1.0 m to 3.0 m; the insertion process is often compressed air- or water-compressed.
- Aggregate feeding – the space created in the first stage is now filled with aggregate,
- Aggregate compaction implemented gradually – usually every 0.5 m; this is how columns with a diameter from 40 – 120 cm are formed.
Application The BMC technology is a kind of supplement to the CMC technology, therefore it can be used analogically to the CMC applications in almost any soil conditions. The technology works well in soft loams and silts, anthropogenic soils (uncompacted fills, heaps) and in organic soils (peat, aggradate mud, gyttjas) with a moisture content above 100 % if deposited below the bottom level of the granular fill.
The application of the BMC usually results from the necessity to create a transition layer between the constructed structure and the subsoil reinforced with the columns as well as the force distribution between the two. The BMC technology can be applied to all kinds of structures such as enclosed buildings (foundation slabs), infrastructure (road and rail embankments) and special structures (wind turbine foundations). Typical loads transferred by the column are within the range from 250 up to 600 kN. The columns are located similarly to the CMCs and SCs, i.e. in a square or triangular grid with a side length ranging from 1.5 m up to 3.0 m. Most commonly the granular fill has the diameter ranging from 2 up to 4 times larger than the concrete column core.
- Powerful combination – the BMC combine advantages of the Stone Columns and the Controlled Modulus Columns. The soil is not over-stiffen and there is no risk of column buckling or bulging,
- Economy – the reduction in the thickness of the platform transmitting the load from the structure to the improved soil, optimization of the geosynthetic layer thickness and fast construction of the BMCs – these are only selected aspects that make this technology economical.
- Comprehensive improvement – the improvement of mechanical properties of the soil between the columns, and while forming the column by swelling / compacting the soil.
- Versatile application – the application to almost any soil type including compressible, organic (peat, aggradate mud, gyttjas) and man-made soils.
- Environmentally friendly – in the course of the BMC core formation the surface soil structure is undamaged and there is no excavated material. Hence, no need for removal of earth mass. The granular fill of the column may be formed from recycled material (concrete rubble).