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2/ Seismic cloaking through buried vertical concrete inclusions | Year 3 of the project

29 September 2016 ( maj : 5 October 2016 ), by Administrateur

By buried vertical concrete inclusions, we refer to the rigid inclusions, also called CMC, constructed by Menard as a ground improvement solution (Plomteux et al, 2003; Masse et al, 2009; Plomteux and Ciortan, 2010). Developed by Menard initially in France to support structures such as warehouses, industrial buildings, medium weight housings, roads, railways, embankments, and storage tanks, the Controlled Modulus Columns (CMC) are predominantly used for sites with soft cohesive soils, loose sand, chalk, organic soil and peat. The current practice is to install columns with diameter between 280 and 500 mm.

The principle of the Controlled Modulus Columns Soil Improvement system is to form a composite material made of the grout column and surrounding soft soil. The system uses a displacement auger powered by equipment with very large torque capacity and very high downward thrust, which displaces the soil laterally with minimal spoil and vibration (Fig. 1a). The auger is screwed into the soil to the required depth, which increases the density of the surrounding soil and thus increases its load bearing capacity. When the required depth or a pre-defined drilling criteria (usually rotational torque) is reached, a highly workable grout-cement mixture is pumped through the centre 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 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.

Figure 1.
(a) Construction process for Controlled Modulus Columns; (b) Composite column/surrounding soil material (left) which act as a ground improvement with equivalent deformation modulus parameters (right).

Once the columns are completed, and except in the case of shallow footings and isolated loadings, a load transfer and distribution layer (the transition layer) is installed at the top of the CMC columns. This layer, of a total thickness of 0.4 to 2 m, is generally made of well compacted granular material. The entire process is vibration free and creates minimal spoil, which provides for cleaner project sites.
In usual cases, the determination of the dimensions, spacing, and material of the CMC is based upon the development of an optimal combination of support from the columns and from the soil mass. The objective is to limit settlements for the project within the allowable range, and therefore to obtain the requested value for the equivalent deformation modulus of the improved soil. This ground behaves indeed as a composite material CMC/surrounding ground the equivalent deformation modulus of which can be controlled by the various inner parameters of the design, namely (Fig. 1b):

  • the mesh of the network,
  • the diameter of the CMC,
  • the potential anchorage of the columns in a competent substratum,
  • the thickness and the modulus of the transition layer,
  • the modulus of the grout that is used.
    The modulus of the various layers of soil (Young’s modulus) is also a key input data.

Up to now, CMC have rarely been used as a para-seismic soil treatment technique, even if they can be used in seismic regions. In such case, the focus is more on justifying that their integrity will not be affected by an earthquake rather than on using them effectively to combat the effect of an earthquake on the above structure. Indeed, they are primarily designed to increase the bearing capacity of a given soil and reduce potential settlements under the structure. When designed to withstand the effect of a seism, the shear modulus of the soil, a function of the wave velocity in the corresponding soil layer, is used in the design. Several aspects are then taken into consideration in the traditional approach:

  • the torsor of the seismic forces applied to the soil by the above structure,
  • the effect of the horizontal soil displacement (kinematic effect and inertial effect) on the columns,
  • the effect of the vertical displacement (directly on the inclusions) is usually negligible compared to the horizontal displacement and is usually ignored.

To go beyond this passive approach (CMC are designed to take the seismic load rather than act on it), Menard undertook in 2012 several preliminary full scale experiments with Institut Fresnel, which led to several publications (Brule et al, 2014, 2015). This exploration work was based at the time on a regular network of holes in the ground, something that cannot have a direct realistic application in the field of ground improvement.

With the deployment of a dense seismic network on a CMC field as proposed in WP1.2 (see Fig. 2), the objective would be to carry out similar experiments on a large network of CMC and evaluate and verify the impact of this regular network on the characteristics of the seismic surface waves. The ultimate goal would be to define or confirm the key parameters controlling the dissipation of the horizontal waves by the network of CMC, the inclusions now acting as an active anti-seismic barrier.

Figure 2.
Example of experimental configurations with buried concrete colomns as proposed by Menard with controlled modulus columns.