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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 ADJUSTED, NO MORE THAN TWO LINES, please translate Ivo Herle

1, Marco Hentschel

1, Jimmy Wehr

2 and Jan Boháč

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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.

RÉSUMÉ - Start here the abstract in French language, maximum 5 lines. For abstracts and text, use Arial 12 pt, with single line spacing. This abstract will be translated by the organising committee, if the authors do not to provide their own translation. please… translate…

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 vibro-replacement”. 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.