Modulus Columns BMC
Bi – Modulus Columns BMC technology complements CMC (Controlled Modulus Columns) technology, and gravel columns SC (Stone Columns), combining the advantages of both these technologies. The first projects using this method took place in the nineties in France and in the USA, and since then it has been more often used around the world.
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.