Deep compaction (vibrocompaction – vibroreplacement - vibrodisplacement)

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Description

Vibrocompaction

The vibrocompaction technique, also known as vibroflotation, is suitable for compacting thick layers of loose granular deposits (gravels, sands). The maximum depth of compaction is typically limited by the lifting equipment. Depths up to 70 metres have been achieved (Moseley and Kirsch, 2004; www.vibroflotation-ng.com).

Deep compaction is normally achieved according to the following steps (Figure 1.):

1. A probe is penetrated to the desired depth under its own weight with minimal vibration and with the assistance of high pressure water jet from the tip of the probe, progressively displacing the soil beneath it (Figures 2 and 3).

   

   

    

2. At the desired depth the vibroprobe is activated, oscillating laterally and transferring vibrations horizontally into the surrounding soil, compacting it. The area of influence depends on several factors, mainly the mechanical characteristics of the vibroprobe, the target relative density to be achieved, the nature of the soil and groundwater levels. Guidance on what can be achieved with standard equipment is provided by Elias et al. (2001). A spacing of 3 m is typically adopted when using standard equipment in favourable conditions .

3. The vibroprobe is slowly raised towards the surface while vibrating. The overlaying soil will gradually sink in, as the lowermost material is densified (Figure 4). Additional sand is usually dropped into the hole to ensure full compaction of the area.

   

    

Deep compaction due to vibration may also be achieved penetrating a hollow steel tube into the soil, a method referred to as Terra-Probe; the steel tube is vibrated down to the desired depth and then drawn up again while the hollow steel tube is vibrating; this procedure is repeated several times to get the required degree of compaction. In this technique the vibrator is mounted on the top of the stell tube and imparts vertical, rather than horizontal vibration, resulting in a much smaller area of influence. A spacing of 1.5 m is typically adopted when using the Terra-Probe equipment in favourable conditions; the greater quantities are compensated in part by the greater speed compared to vibrofloatation.

Notwithstanding the addition of material at each treatment point during compaction, in both cases settlement is normally induced by vibration, which can be compensated either by overfilling with clean granular soil prior to compaction or by conventional filling and compaction at the end of treatment, if required.

According, for example to Bergado et al. (1999) and Mc Carthy (2007), clay and silty content should be less than 15 to 20% for the method to be effective. Higher contents of silt and clay will limit the ability of water to drain away rapidly and may result in the sides of the hole not “collapsing” promptly onto the probe, reducing the effectivness of energy transfer from the probe to the surrounding soil, thus limiting the compaction process. Gravel content should be less than 20%; higher gravel contents may limit the ability of the probe to penetrate the soil to be compacted, thus limiting the maximum depth of treatment to, say, 10 m depth.

Great caution is necessary when performing deep compaction near existing services or structures, to limit settlements, and below the groundwater level, to limit the resulting excess pore pressures not to trigger local or general instability.

Provided vibrocompaction is carried out properly and with the appropriate spacing between treatment points, the treated soil may be considered as a continuous medium with improved and more homogeneous mechanical characteristics; in particular, as a consequence of the increase of density, both stiffness and strength are increased.

 

Stone columns

Stone colums consist of underground colums of crushed rock or gravel, installed by techniques similar to those adopted and described above for vibrocompaction. Stone columns are adopted where vibrocompaction ceases to be effective, e.g. where silt and clay content is higher than 15 to 20%. Two different methods can be adopted to form stone columns, e,g, vibroreplacement and vibrodisplacement. In both cases, stone columns reinforce all the layers crossed, including un-compactable layers. Depending on the nature of the soil and the particulars of the technique used, their installation may also result in the compaction of the original soil between columns.

 

Vibroreplacement can be carried out using either a wet process (water jet) or a dry process (air jet); normally the wet method is more effective. Besides the vibroprobe and the supporting crane, which are essentially the same used for vibrocompaction, the spread of equipment includes a compressor and a wheel loader (Figure 6).

The vibroprobe, which is the only specialist equipment, can be easily transported by container and assembled on site (Figure 7), while the rest of the equipment can be hired locally. The technique consist of the following steps (Figure 5):

1. As in the case of vibrocompaction, the probe is lowered to the desired depth. In silty soils the fines are washed to the surface by water circulation (Figure 8); the washings need to be collected and disposed of in a controlled manner.

2. The probe is then lifted up a short distance (e.g. 0.5 m) and a backfill of stone is introduced in the hole from the top (Figure 9). The added material is then repenetrated by the vibroprobe, which compacts it and pushes it against the surrounding soils, ensuring good contact and energy transfer between the probe and the surrounding soil, increasing the width of the stone columns.

3. The procedure described at point 2 is repeated until the stone column reaches the surface.

 

Vibrodisplacement is performed dry (air jet) according to the following steps (Figure 10):

1. As in the case of vibrocompaction and vibroreplacement, the probe is lowered to the desired depth. Contrary to vibroreplacement, the use of air jets only precludes the washing out of fines and all the soil is displaced laterally. However, this results in a greater resistance to penetration and “preloosening” may be required, especially if local dense layers exist above the layers to treated. This can be carried out by inserting a continuous flight auger and retrieving it by counterrotation without soil removal (Figure 13).

2. The probe is then lifted up a short distance (e.g. 0.5 m) and gravel loaded in an airlock chamber is delivered to the bottom through the vibroprobe or a separate pipe (Figures 11 and 12). The grading must be carefully controlled to avoid blockage of the delivery pipe. The added material is then repenetrated by the vibroprobe, which compacts it and pushes it against the surrounding soils compenetrating or displacing it, ensuring good contact and energy transfer between the probe and the surrounding soil, increasing the width of the stone columns and inducing further densification/compaction of the soil between columns.

3. The probe is gradually lifted in stages, continuously adding and compacting coarse material as described at point 2. More material will be added where soil is weaker.

   

   

   

   

    

The methods should not be used in saturated soft sensitive clays as the vibration and pressures from the stone columns on the surrounding soil may exceed its strength and destabilize the slope (Ground Improvement Solutions, 2010) 

Great caution is necessary when performing deep compaction near existing services or structures, to limit settlements and horizontal displacements, and below the groundwater level, to limit the resulting excess pore pressures not to trigger local or general instability. The use of compressed air may also have undesirable side effects in certain circumstances.

Stone columns increase stability through all soil layers because of higher shear strength of the coarse fill material; their installation may also improve the mechanical characteristics of the soil between columns, especially if the vibrodisplacement method is used; in certain conditions they may also improve drainage, provided a suitable outfall exists or is provided.

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Design methods

In general all the methods described above are applicable in saturated relatively coarse grained materials (gravels, sands, sandy silts) susceptible to liquefaction related phenomena induced by monotonic or cyclic (vibration, earthquake, waves, etc.) stress changes.

Stone columns may be used also to improve the composite shear strength of a deposit, but this may be better achieved by other methods unless it is also possible and necessary to mobilize their potential drainage effect.

 

Vibrocompaction

The degree of compaction to be reached by vibrocompaction should be determined in terms of achieving a significant reduction in the susceptibility of the soil to develop of excess pore pressures under monotonic or cyclic loading. This presupposes a detailed understanding of the triggering mechanisms. The following general considerations apply:

• Under static loading, the density of the material must be sufficient to preclude the occurrence of stress states located on or above the Collapse /Instability Surface (Sladen et al., 1985; Ishihara, 1993; Lade, 1992; Lade, 1993) or, more in general  within the Instability Zone (Lade and Pradel, 1990; Leong et al., 2000; Chu et al., 2003), where flow-type instability could be triggered. Examples of how the density of the materials affects the position of the Instability Zone and hence the stability of the slope are presented and discussed in the Deliverable 1.1 of the SAFELAND Project., where the terminology used here is also explained in detail

The evaluation of the potential for flow-type instability in sandy materials can be made on the basis of simplified procedures which use the results of SPT and/or CPT tests (see for example Ishihara, 1993; Fear and Robertson, 1995; Cubrinowski and Ishihara, 2000; Olson and Stark, 2003a). Alternatively, a comprehensive program of laboratory tests on both “undisturbed” and reconstituted samples should be carried out to determine the Steady State Line and the position of the in situ state of the material referred to this line (see for example Been & Jefferies, 1985; Boulanger, 2003). In fact, it has been recognized that flow-type instability may occur only where penetration resistances are lower than appropriately defined threshold values and/or the initial states are located slightly below the Steady State Line.

• Under seismically induced cyclic loading, the density of the material should be sufficient to limit the development of excess pore water pressures; considering the short duration of seismic motion, reference may be made to “fully” undrained conditions. The verifications may be carried out as follows:

• Step 1. Evaluation by the “simplified” method originally developed by H.B. Seed and coworkers of the susceptibility to triggering of seismic liquefaction, taking into account the effect of static shear stress by the coefficient  K_a (see for example Idriss and Boulanger, 2008), to determine Factors of Safety against liquefaction F_L at different depths. Evaluation of the seismically induced excess pore water pressures as indicated, for example, by Seed et al. (1976), Ishihara and Nagase (1980), Finn (1981), Marcuson et al. (1990), Idriss and Boulanger (2008).

• Step 2. Evaluation of slope stability using limit equilibrium methods and an equivalent pseudo-static action to model the earthquake loads. The analyses must be carried out in undrained conditions in terms of effective stresses (UES) and/or in terms of total stresses (UTS). The UES conditions will be considered in layers where the analyses of liquefaction potential have given safety factors everywhere higher than 1; the amount of excess pore pressures to be considered in calculation will be determined from step 1. The UTS conditions will be considered in layers where the liquefaction potential analyses have given safety factors equal to or less than 1; the undrained shear resistances to be considered in these layers may be determined according to the recommendations given by Olson and Stark (2002), Olson and Stark (2003a), Olson and Stark (2003b) and Mesri (2007).

• 2D or 3D numerical dynamic analyses should be carried out as a final check, and in any case where it is necessary to estimate the seismically induced displacements. These analyses should be carried out in the time domain in undrained conditions using advanced costitutive models (see for example Manzari and Dafalias, 1997; Li and Dafalias, 2000; Li, 2002) capable of replicating the monotonic and cyclic soil behaviour measured in laboratory tests on “undisturbed” and reconstituted samples.

• For under water slopes and for wave induced cyclic loading (see for example Madsen, 1978; Okusa, 1985; Magda et al., 1994; Sassa and Sekiguchi, 1999; Sassa and Sekiguchi, 2001; Sassa et al., 2001), the density of the material should be sufficient to limit the development of excess pore water pressures. Considering the typical frequency of waves and the duration of storms, the development of excess pore pressures occurs under conditions of partial drainage, requiring 2D or 3D numerical dynamic analyses carried out in the time domain in conditions of coupled consolidation using advanced costitutive models as described above for the earthquake case.

Tests should be carried out after  the treatment to verify that the required density has been reached.

 

Stone columns

Stone columns are inclusions of highly compacted stone or gravel with excellent mechanical characteristics and high permeability which act both as reinforcement and as drainage elements which favour the dissipation of excess pore pressures.

Verifying the effectiveness of stone columns is much more complex compared to vibrocompaction, since it involves the behavior of a dishomogeneous and discontinuous medium, thus necessarily requiring some gross simplifications.

The simplified methods currently available are based on limt equilibrium methods and on the following assumptions:

• Stone columns are sufficiently free draining to be immune from excess pore pressures; they can be modelled in terms of drained strength parameters  under all loading/environmental conditions.

• The surrounding soil can be modelled in terms of the least of its drained and its undrained strength; the latter may be evaluated on the basis of empirical correlations as proposed, for example, by Olson and Stark (2002), Olson and Stark (2003a), Olson and Stark (2003b) and Mesri (2007) at pre-liquefaction and post-liquefaction conditions.

The improvement of the natural soil due to the installation of stone columns is normally ignored unless proven and quantified by appropriate full scale field tests

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• Barker R.F. (1991) “Developments in biotechnical stabilization in Britain and the Commonwealth” Proceedings of Workshop on Biotechnical Stabilization, University of Michigan, Ann Arbor, 83-123.
• Been K., Jefferies M.G. (1985). “A state parameter for sands”. Géotechnique 35, n° 2.
• Bergado D.T., Anderson L.R., Miura N., Balasubramaniam A.S. (1996). ”Soft Ground Improvement in Lowland and Other Environment”. ASCE Press, ASCE, New York.
• Boulanger R.W. (2003). “Relating Ka to relative state parameter index”. Journal of Geotechnical and Geoenviroonmental  Engineering., ASCE, 129(8)770-773.
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• Ground Improvement Solutions (2010) http://www.groundimprovement.ch/GroundImprovementSolutions/Techniques.html
• Idriss, I.M., Boulanger, R.W. (2008). “Soil liquefaction during earthquakes”. MNO-12, Earthquake Engineering Research Institute, Oakland, CA, USA.
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• Sassa S., Sekiguchi H., Miyamoto J. (2001) “Analysis of progressive liquefaction as a moving-boundart problem” Géotechnique 51, n° 10.
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