Control Modulus Column (CMC) or also known as Kolom Grout Modular (KGM) or Displacement Grout Column (DGC) or generally called rigid inclusion.
The concept behind the use of CMC is not the same as the concept of pile foundations. In the concept of rigid inclusions, the loads sustained by the soft soil is reduced (usually between 60 and 90%) in order to reduce the global and differential settlements. Nevertheless, the loads are not directly transmitted to depth. Here, the soft soil plays a role, and supports part of the load whereas in the pile foundation concept the soft soil is just bypassed (or used for skin friction consideration).
When we work with rigid inclusions, a Load Transfer Platform (LTP) is often used with a thickness generally ranging between 40 and 80 cm. In general, the load transfer platform may consist of single or multiple layers of geosynthetics (often geogrids) placed horizontally in a layer of well compacted granular material (often crushed stones or gravels).
Type of load transfer in the different usual foundation concepts
In comparison with other commonly utilized foundation concepts, the load transfer platform allows the transfer of the structural loads to the head of the CMC by means of an arching effect developing in the granular layer. This effect is caused by the differential settlement arising between the soft soil and the heads of the rigid inclusions at the base of the load transfer platform, which also results in the emergence of a negative skin friction along the rigid inclusions at shallow depth. This negative skin friction is a governing factor of the load transfer in the concept of rigid inclusions. Finally, it is to note that the installation of a load transfer platform also allows the decrease of the bending moments and the shear stresses in the foundation slab of the structure to be supported.
The process of load sharing mechanism in CMC is illustrated in Figure below. Since the ratio of stiffness between CMC and the soil is between 1:1,000 to 1:10,000 it is necessary to consider the vertical deformation separately for the CMC and the soil. The deformation of a point inside the CMC at a given initial depth is different from an adjacent point at the same depth in the soil. In other words, there exists a different field of deformation between the CMC and the surrounding soil as explained below:
- Stage 1: Due to the transfer of imposed stress to the soil (ssoil) through the load distribution layer (sand blanket), vertical deformation (settlement) of the soil (dsoil) occurs due to consolidation.
- Stage 2: As a result of consolidation settlement, stress is transferred from the surrounding soil to the CMC. The deformation at the same given depth (except at neutral plane) in the soil (dsoil) is different from the CMC (dCMC) due to different stiffness (ECMC > Esoil) and that dsoil > dCMC, negative skin friction is developed in the CMC.
- Stage 3: At greater depth, the point deformation dCMC > dsoil resulting in a stress transfer from the CMC back to the competent soil. This induces positive skin friction and base resistance.
- Stage 4: Overall, an equilibrium state of load distribution is achieved where the tip resistance, friction resistance and soil resistance is equals to the total load.
Design concept for CMC
Figure below shows the locations of the neutral plane where point deformation of CMC and soil is the same. At this location, the CMC column carries the maximum stress.
Graphs of vertical displacement, shear stresses and vertical stresses
CMCs are installed using a hollow stem equipped with a displacement auger coupled with a high torque coming from a high capacity pull-down CMC installation rig. The displacement auger consists of three components:
- The bottom part of the auger with its constant flight volume will evacuate the spoil upward during penetration.
- The middle part of the auger is the displacement part which has the same diameter as the auger and the hole. It prevents the spoil from reaching to the surface. It displaces it laterally, thus providing compaction to the soil.
- The upper part of the auger has flight in the opposite direction compared to the lower part. As a result, it brings any spoil caused by potential collapse of the hole downward to the displacement section. Hence, it improves the efficiency and the overall quality and continuity of the CMC inclusion.
The CMC rig has a high torque and strong downward thrust. The auger is advanced into the ground while rotating. No grout is injected at this stage. When the required depth is reached, the grout is pumped through the hollow stem with low pressure sufficient to prevent “caving-in” due to lateral pressure of the surrounding soil. The auger is extracted while the rotation is maintained in the same direction as during the penetration stage in
order to prevent loss of grout along the shaft of the hole and along the Kelly bar.
The cement grout used for CMC inclusion has a compressive strength of 10 – 20 MPa and a slump of 20cm for pumpability. During the installation process, the following information is monitored and recorded by the on-board computer system :
- Speed of rotation and advancement of auger.
- Torque, downward-thrust and augering energy during penetration.
- Pressure and volume of injected grout.
With the above information, the computer computes the diameter of CMC inclusion against the length as it is being installed. The displacement method used has no spoil during installation and no surface vibration. It does not require any water jetting or compressed air injection for penetration as in the case of installing stone columns. Hence, this method is environmental friendly and it is most appropriate for urban environment type