Stone Columns

Abstract


Gravel columns method consist in build a column series, which will be placed uniformly, they have different geometries such as, square, triangle, hexagonal, etc. With this technic it'll reinforce the soft soil.

This treatment has developed as extention of the classical vibrocompactation applied in this case to cohesive soils.

Vibrocompactación or vibroflotación is abettering technique useful for granular soils. It is about to a process that achieves the densification of the material for bettering, by means of the introduction of a vibrator in soil until the wished treatment depth. The vibrator penetration of the, it gets due to the weight, vibration energy, and by the action of a flow of air or water according to the vibrator type.

The attempt to employ thevibrocompactación for bettering no granulars soils, has given place to the gravel columns technique. The incapacity of cohesive soils to densify by means of the vibrator introduction provokes to the the formation of vertical perforations. The bettering technique with gravel columns consist precisely in fill these perforations in ascendant direction, with introduced and compacted gravel, by thevibrator. As result, it forms a resitant material column in every of the points, that is a gravel column.




































Adaptation of soils for it's treatment by vibrocompactación, (Glover, 1982).


The A zone, corresponds to soils with vibrocompactables granulometries; the granulometries of coarse particles next to the right limit itcorresponds to big blocks that could impede the correct penetration of the vibrator to the terrain.

The B zone defines the granulometries most adequate for it's densification by means of vibrocompactación. It treats the clean sands with content of fine particles, less than 10%.

In the C zone, the presence of a important fraction of silty material, that makes the vibration generates a cilindrichollow around the vibrator. This impedes the transmision of vibration to the soil of around. In this case the compactation only is posible by means of the material addition for refill the hollows.

For lower granulometries, placed to the left of the C zone, The most suitable solution is the gravel columns. Fine fraction, is limited to a maximum grain size of 0.06mm. in a 15%.


ObjectivesThe gravel columns are employed as treatment to soft soils, with this method, we'll get the following improvements.

1.- Settlements reduction.
2.- Consolidation process accelerated.
3.- Increase of Bearing capacity.
4.- Increase of stability against displacements.
5.- Reduction of soil liquefaction.






Constructive methods.

As to the methods of construction of gravel columns,we can observe two types of techniques:

Vibrosustitución (wet way).
Vibrodesplazamiento (dry way).

The phases of both methods are very similar. the difference between them is mainly in the quantity of compressed air or water pressure, as help method for the penetration of the vibrator.


Vibrosustitución or wet way.


Scheme of wet way construction (Raju et al., 2004).

Theconstruction of the gravel columns by means of wet way, it follows this sequence:

1.- Penetration of the vibrator: Once placed the vibrator in the right point to treat, this penetrates in terrain thanks to the combinated effect of the self weight, energy of vibration and the action of water pressure it comes from the vibrator´s tip.

2.- Enlarge the perforation: The flow of water pressure, makes aflow towards the exterior the walls of the drill that removes and drag the existing fine particles, so provoke an elongation of the initial perforation.

3.- Sustitution: Once reached the wished depth, it'll proceed to the perforation with gravel, the gravel contribution is carried out in layers of 50cm.. Which must be introduced and compacted, by the up and down movements of the vibrator....