role of density on the behaviour of vibrated stone columns in soft soils

ROLE OF DENSITY ON THE BEHAVIOUR OF VIBRATED STONE COLUMNS
IN SOFT SOILS
TITLE OF THE PAPER IN FRENCH, ARIAL 12 BOLD CAPITALS, ITALICS. LEFT
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Ivo Herle 1, Marco Hentschel 1, Jimmy Wehr 2 and Jan Boháč 3
1
Institute of Geotechnical Engineering, Technische Universität Dresden, 01062 Dresden,
Germany;
2
Keller Holding GmbH, Offenbach, Germany;
3
Department of Engineering Geology, Faculty of Science, Charles University, Albertov 6, 128 4
Praha 2, Czech Republic
ABSTRACT - Vibro-replacement using vibrated stone columns (described e.g. by the norm EN
14731) is an often applied method for the improvement of soft soils. Usually, only simple
models are used for the representation of the column behaviour and soil density is not directly
taken into account. In situ investigations show that the density of the vibrated stone columns is
not constant and may depend on several factors. Dynamic sounding is not suitable for the
determination of the relative density in this case. Laboratory model tests underline the role of
density and of load transfer with respect to the bearing capacity of the vibrated stone columns.
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1. Introduction
The role of density on the behaviour of granular soils is well known. Higher density is
accompanied by higher stiffness and strength. During the installation of vibrated stone columns
it is therefore desirable to achieve the maximum possible compaction.
However, there is no routine direct control of the density of vibrated stone columns.
Compaction success is examined by indirect methods like energy consumption of the vibrator.
This is definitely useful for the production but there is no reliable correlation between such
control quantities and the actual soil density. Mathematical models of the system performance
cannot provide satisfactory answers due to the complex mechanical behaviour of gravel and
surrounding soils. In reality, non-linear hysteretic stress-strain response is coupled with
dynamic and pore water effects. On the contrary, only simple models are usually considered in
calculations.
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2 In situ tests
Measurement of the gravel density in columns produced by the vibro-replacement can be
achieved either directly (i.e. obtaining volume and mass of a part of the column) or indirectly (by
sounding methods). At three different sites (two in Germany and one in the Czech Republic),
dynamic penetration according to the German standard DIN 4094-3 was combined with the
replacement method according to DIN 18125-2 (see Figure 1). Work safety regulations limited
the depth of the direct investigation to the maximum of 2 m.
Figure 1. Direct measurement of density by replacement method
In order to evaluate the density from dynamic penetration, one needs to use a
correlation between the number of blows N10 and the density index ID. An example of such an
evaluation for the site Kerspleben (close to Erfurt, Germany) is given in Figure 2. The results
suggested ID about 0.60 which was subsequently checked by the field test.
Direct determination of the soil density yields another picture. The values of the
measured dry density increase with depth and lie between 1.8 and 2.0 g/cm3, see Figure 3.
These seem to be high values but in order to determine the relative density one needs to know
the density limits. The latter depend mainly on granulometric properties of the gravel, i.e. grain
size distribution, grain shape and grain surface (roughness).
Laboratory tests on the gravel material taken from the gravel columns pointed out to a
decrease of mean grain size d50 with depth (Figure 4) and an increase of non-uniformity
coefficient U=d60/d10 with depth (Figure 5). These are certainly signs of grain crushing produced
by the vibrator operation. A marked grain abrasion could be also recognized by an image
analysis of grains before and after compaction. It can be expected that finer grains pass from
the upper layers downwards. In this way, lower parts of the gravel columns can be expected to
become even relatively denser than the upper parts.
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Figure 2. Dynamic penetration in a gravel column and correlation of the number of blows N10
with index of relative density ID.
Figure 3. Increase of dry density with depth (Tiefe).
An experimental determination of the maximum and minimum void ratios emax and emin,
respectively, is inevitable for the assessment of the relative density. Figure 6 depicts the
measured development of these values. It may be noticed that the limit void ratios decrease
with depth, mainly as a result of increasing non-uniformity. In situ void ratios corresponding to
dry densities pd in Figure 3 are shown as well. They mimic the values of emin, thus proving the
maximum compaction was achieved in the gravel columns under investigation. The field values
of e can be further compared with diagramms published by Youd (1973), summarizing the limit
void ratios for different sands (Figure 7). Also in the latter case the in situ void ratios are close
to emin values.
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Figure 4. Decrease of mean grain size d50 with depth (Tiefe).
Figure 5. Increase of non-uniformity coefficient U with depth (Tiefe).
Concluding in situ tests, it has been documented by direct measurements that void ratio
of stone columns after densification with vibrator reaches the values close to emin. The same
result follows from the comparison with diagram by Youd (1973). Indirect methods like dynamic
penetration are unreliable in this respect.
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Figure 6. Decrease of limit void ratios and of in situ void ratio with depth.
Figure 7. In situ void ratios (points) compared with values of emin after Youd (1973).
3. Model tests
Is the role of density in gravel columns really of a crucial importance? In order to
clarify this question, several model test were performed in the laboratory. Only one half of
the column was considered at different boundary and loading conditions. The surrounding
soil was modeled by Soiltron, a soft mixture of sand and plastic grains (Laudahn 2004). The
displacement field was registered by image processing with PIV (Figure 8). A pronounced
difference in the load transfer between dense and loose columns was observed (Figure 9),
underlining the importance of suitable equipment like a depth vibrator. Details were
described by Hentschel (2005).
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Figure 8. Experimental setup and displacement magnitudes measured with particle image
velocimetry.
Figure 9. Load-displacement curves for a loose (Versuch 01) and a dense (Versuch 02) sand
column.
4. References
Hentschel, M. (2005). “Density and bearing capacity of the gravel columns produced by vibroreplacement”. Diploma-Thesis, Inst. of Geotechnical Engineering, TU Dresden, (in German).
Laudahn, A. (2004). “An approach to 1-g modelling in geotechnical engineering with Soiltron.”
PhD-Thesis, Inst. of Geotechnical Engineering and Tunnelling, University of Innsbruck
Youd, T.L. (1973). “Factors controlling maximum and minimum densities of sand.” Evaluation of
relative density and its role in geotechnical projects involving cohesionless soils, ASTM STP
523, 98-112.
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