The Design of Vibro Replacement





The Design of Vibro Replacement

Heinz J. Priebe

Vibro Replacement is an accepted method for subsoil improvement, at which large-sized columns

of coarse backfill material are installed in the soil by means of special depth vibrators. The

performance of this composite system consisting

of stone columns and soil, is not determinable

by simple investigation methods like soundings, and therefore, such methods are

not suitable for design purposes. However,

theoretically, the efficiency of Vibro Replacement

can be reliably evaluated. The method elaborated on a theoretical basis and described

in this contribution, is easy to survey

and adaptable to different conditions due to the

separate consideration of significant parameters.

Practically, it comprises design criteria for

all frequently occurring applications.


Introduction


Vibro

replacement is part of the deep vibratory compaction techniques whereby loose or soft

soil is improved for building purposes by means of special depth vibrators. These techniques

as well as the equipment required is comprehensively described elsewhere [1]. Contrary

to vibro compaction which densifies noncohesive

soil by the aid of vibrations and improves

it thereby directly, vibro replacement

improves non compactible cohesive soil by the

installation of load bearing columns of well

compacted, coarse grained backfill material. The

question to what extent the density of

compactible soil will be improved by vibro compaction,

depends not only on the parameters of the soil being difficult to determine, but also

on the procedure adopted and the equipment provided. However, the difficulty of a reliable

prognosis is balanced by the fact that

the improvement achieved can be determined easily

by soundings. With

vibro replacement the conditions are more or less revers. Considerable efforts only like

large-scale load tests can prove the benefit of stone columns. However, a reliable conclusion

can be drawn about the degree of improvement which results from the existence of

the stone columns only without any densification of the soil between. This is possible because

the essential parameters attributable to

the geometry of the layout and the backfill material

can be determined fairly good. In such a prognosis the properties of the soil, the equipment

and the procedure play an indirect role only and that is mainly in the estimation of

the column diameter. Basically,

the design method described herewith was developed some twenty years ago and published

already [3]. However, in the meantime it came to several adaptions, extensions and

supplements which justify a new and comprehensive description of the method. Nevertheless,

the derivation of the formulae is renounced with reference to literature.


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