Smear zone identification and soil properties around stone columns constructed in-flight in centrifuge model tests

Abstract

Stone columns are primarily used for the purpose of ground improvement in fine grained soils in order to reduce settlements and the risk of bearing failure. They are also designed to improve the drainage conditions in the ground and to accelerate the consolidation processes within the clay. However, smear and disturbing effects caused during the construction of stone columns result in degradation of consolidation performance in comparison with the theoretically ideal conditions. Model stone columns are constructed in-flight under 50 times gravity in centrifuge tests and the soil micro-structure in the vicinity of these columns is investigated by applying different methods, including environmental scanning electron microscopy and mercury intrusion porosimetry. The results of these tests confirm that smear and disturbance occur owing to stone column installation and the region influenced can be divided into three sections: a penetration zone (1) where the sand particles are squeezed through the clay; a smear zone (2) where the soil particles have experienced a significant reorientation; and a densification zone (3) where the structure of the clay does not appear to change, but compaction of the clay is measurable. The extremes of the disturbed zone around model stone columns are determined to extend to about 2 . 5 times the column radius.

Numerical analyses of stone column installation in Bothkennar clay

“…However, the densification is especially important in the area closer than 2 column radii (Zone 3a). Weber et al (2010) measured a similar value of 2.5 column radii for the densification area. They fitted the data points of porosity and density using a hyperbolic function; a similar function may be here proposed to fit the void ratio: …”

“…For example, the increase of pore pressures and horizontal stresses, and the remoulding of the surrounding soil have been measured in the field (Watts et al 2000, Watts et al 2001, Kirsch 2004, Gäb et al 2007, Castro & Sagaseta 2012. There have also been attempts to investigate these effects through physical modelling of the process by means of centrifuge testing (Lee et al 2004, Weber et al 2010, but the soils used are reconstituted and hence, not fully representative of natural clays.…”

Consequences on water retention properties of double-porosity features in a compacted silt

“…The pressure was increased stepwise to 400 MPa using pressure transmission oil. The intruded volumes were corrected for compressibility of mercury [35].…”

Influence of stone column installation on settlement reduction

“…However, the densification is especially important in the area closer than 2 column radii (Zone 3a). Weber et al [16] measured a similar value of 2.5 column radii for the densification area. They fitted the data points of porosity and density using a hyperbolic function; a similar function may be here proposed to fit the void ratio:…”

“…the increase of pore pressures and horizontal stresses, and the remoulding of the surrounding soil has been measured in the field [10][11][12][13][14]. There have also been attempts to investigate these effects through physical modelling of the process by means of centrifuge testing [15,16], but the soils used are reconstituted and hence not fully representative of natural clays.…”