Controlled Modulus Columns (CMC)

The controlled modulus columns (CMC), or known as rigid columns is very
efficient ground improvement technique through reinforcing, used in very soft soils with very low resistance and very low consistency. In these soils, rigid columns are necessary to provide lateral resistance upon loading.

The principle of the Rigid Inclusions Soil Improvement system is to form a composite – grout column/surrounding soft soil – material. The system uses a displacement auger powered by equipment with very large torque capacity and very high downward thrust, which displace the soil laterally with virtually no spoil or vibration. The CMC diameter usually ranges from 300mm to 600mm drilled using auger advanced into the soil to a required depth and as such it increases the density of the surrounding soil while increasing its load bearing capacity at the same time. When the required depth or a preset drilling criteria (usually rotational torque) is reached, a highly workable grout-cement mixture is pumped through the center of the hollow auger.

The grout mixture then flows under low pressure out of the auger base as it is retracting to obtain a 100% grout high-capacity column that can be used in close vicinity of sensitive structures and that generates virtually no above ground spoils. The grout is injected under low pressure, typically less than 10 bars (145 psi) and no soil mixing takes place during the pressure grouting. To ensure that the soil above the auger remains compacted, the top of the auger is equipped with reverse direction flights. The result is a pile shaft type column that is effectively bonded to the surrounding soil. Near ground surface, the reverse flights also effectively limit generation of spoils, reducing the need for removal.

Columns can be tested by jacking against the base of the installation machine
and thus no large testing frame is required. For quality control, an onboard computerized monitoring device record permanently date and time of drilling start, concreting start and completion, total length of the column, verticality, drilling energy, drilling vertical speed, drilling rotational speed, downthrust applied to the auger, torque applied to the auger, total volume of grout placed in the RI column, column profile, pumping pressure, speed of extraction and output- flow of grout.

This monitoring device also provides the operator with control based upon: – the minimum pressure to observe before starting the withdrawal of the auger to ensure a good filling of the drilling tools and of the subgrade of the excavation, – the maximum speed of the withdrawal or the minimal grout pressure to respect during extraction to ensure the continuity of the column at the selected diameter. Once the columns are finished, except in the case of shallow footing foundation support and isolated loadings, a load transfer and distribution layer (the transition layer) are installed at the top of the RI columns.

This layer, of a tota thickness of 0.4 to 2 m , is made of well compacted granular material.