The geosynthetics have dramatically transformed the practice of geotechnical engineering field around the world. These products have made it possible to introduce novel concepts to ground improvement. Heretofore impossible design and applications are made possible with the geosynthetics. The uniqueness of construction with geosynthetic products are, (i) Technical Superiority with variety of engineered products (ii) faster construction (iii) Forming flexible structural system that performs exceptionally well during seismic hazards. (iv) Better control on the quality of products. The major potential usage for the Geosynthetics is in reinforcing soils and ground improvement. This paper explains one of such developments in the Geosynthetics Engineering for ground improvement.

The rapid growth of infrastructural developments has lead to such a stage that, many a times constructions have to be taken up on weak grounds which were rejected in the past due to poor soil bearing capacities. Engineers constantly strive for improving such soils by innovative methods. Especially for the construction of road or railway embankments and other flexible structures on soft soils, conventional solutions like replacing the unsuitable soils or bypassing them with deep foundations like piles may prove to be expensive and time consuming. In such cases it is customary to use stone columns (otherwise called as granular piles) which are nothing but vertical columnar elements formed below the ground level (usually up to a competent stratum) with compacted and uncemented stone fragments or gravels. The stone aggregates (generally of size between 20 mm and 75 mm) are compacted within the vertical hole formed in the ground by a vibrating probe or by ramming. Such columns may have diameters usually in the ranges of 600 mm to 1 m and length up to a maximum of 15 to 20 m. They are commonly arranged in square or triangular grid pattern in plan with centre to centre spacing of 1.5 to 3.5 m. The presence of the column creates a composite material of lower compressibility and higher shear strength than the original soil. The stone columns also promote the drainage of pore water there by accelerating the consolidation of the clay soil.

When the stone column is subjected to vertical loading, it undergoes significant vertical compression caused by the lateral bulging of the aggregates predominantly in the top portion of the column (generally within a depth of about four times the diameter of the stone column). As the column bulges the granular material presses into the surrounding soft soil and transfers stress to the soil. This bulge, in turn, increases the lateral stress within the clay (inducing a passive pressure condition in clay) which provides additional confinement for the stones. The passive pressure from the surrounding clay makes the column to resist the vertical load on the stone column. Hence the load carrying capacity of the stone column largely depends on the strength of the surrounding clay. In soft clay soils, the bulging of stone columns will be more leading to larger surface settlements rendering the efficacy of the stone columns to very low. This is a major limitation of stone column technique especially in very soft soils.

The other limitations and the problems encountered in installing stone columns in soft clays and extreme soft soils like marine clay are (i) Loss of Stones: The stones charged in to the column may squeeze out of the column due to low confinement from the surrounding soft clay due to which the formation of stone column itself may be doubtful (ii) Contamination of Stone Aggregate: The surrounding soft clay may intrude into the stone aggregate contaminating the stone aggregate which eventually reduces the frictional properties of the aggregate and impedes the drainage function of the column. (iii) Limited Bearing Capacity: As the stone columns are dependent on the surrounding clay soil, the load carrying capacity of the stone column can not be improved more than 25 times the strength of the soft clay. Hence, it may not be possible to design economical spacing for stone columns in case of very heavy loads. In such situations the performance of stone column itself may need to be improved by suitable means.

Geosynthetic Encased Stone Columns: One of the methods to improve the performance of the stone columns installed in weak soils is wrapping the individual stone column by a suitable geosynthetic (geogrid or geotextile) in a tubular form (Figure 1). This encasement helps the column in several ways to improve the overall performance of the stone column because of the composite action of the materials. The behaviour of these encased stone columns will be somewhere between that of ordinary stone columns and rigid concrete piles. Some of the benefits of geosynthetic encasement can be listed as follows.

• The encasement imparts additional lateral confinement which helps in increasing the load capacity of the stone column by many folds.
• The lateral squeezing of stones in to surrounding soft clays is prevented and hence loss of stones while installation is minimised. More over the stone aggregates can be compacted to a higher degree of compaction.
• When the stone columns are encased in geotextiles, it promotes the vertical drainage function of the stone column by acting as a good filter to prevent fines from mixing with the stone aggregate.
• As the encasement prevents contamination of aggregates by surrounding clay the frictional properties of the aggregates and the drainage functions of the column are preserved.
• The confinement offered by the encasement also enhances the shear resistance of the stone column when it is subjected to lateral soil movements.

Installation of Geosynthetic Encased Stone Column: The geosynthetic encased stone columns can generally be installed by displacement method. Replacement method of installation may not be possible in soft to very soft  soils as the soil may collapse while boring. Moreover the displacement method has the benefit of compressing the surrounding soil while the casing pipe is driven. Two ways of installation of geosynthetic encased stone columns are possible as explained in the following sections.

(i) Wrapping the Casing Pipe with the Geosynthetic: In this method the casing pipe is first wrapped around with the geosynthetic and the bottom of the geosynthetic encasement is fitted with an anchor plate. The plate will hold the encasement intact while the casing pipe is pulled out. The stone fill is charged through the casing pipe and when the casing pipe is pulled out the geosynthetic encases the stone fill. The stone fill inside the column is compacted by tamping. The construction sequence is shown schematically in Figure 2(a). This method is suitable for both geogrid and geotextile encasements.

(ii) Geosynthetic Encasement inside the Casing Pipe: In this method the casing pipe with a flap open at the bottom is lowered in to the ground till it reaches the firm stratum. Then a sack of the geosynthetic is lowered into the casing pipe. The required quantity of aggregate is filled in the sack and the full column is formed inside the casing pipe. After this the casing pipe is retrieved slowly with vibration. This vibration causes the compaction of the aggregate inside the geosynthetic. This method of installation is most suitable for soft geosynthetics (such as geotextiles and polyester geogrids, etc.) Figure 2(b) shows the sequence of installation of this method.

Performance of the Geosynthetic Encased Stone Columns: As geosynthetic encasement of stone column is a recent technique, as such, they are yet to be tried in the real field to get their advantages. The authors have performed extensive laboratory model studies and numerical analysis to investigate the benefits of encasing the stone columns with different types of geosynthetics. Figure 3(a) shows a typical group of Geosynthetic Encased Stone Columns (referred as ESC) installed in a clay bed prepared in a laboratory testing tank. Fig. 3(b) shows one of the load tests being carried out on the stone columns. The investigations mainly brought out the improved performance of the ESCs in contrast to the ordinary stone column without encasement (referred as OSC). Figure 4 shows one of the results from the laboratory studies. From the graph it can be stated that the ESCs can bear a pressure of about 3 to 5 times as that of the OSCs for the given settlement on the column. The stiffness of the ESCs was found to be more and the ESCs were found to act like semi-rigid piles. For same pressure loading and settlement criteria, with the ESCs either the diameter of the stone columns can be made lesser or the spacing can be increased, resulting in cost saving on the stones and time consumption.

All other aspects of the investigation have certainly proved the system to be most efficient and advantageous in several ways (For more details the readers may refer to the publications mentioned in the references). One of the major hurdles which holds back this technique is the practical installation difficulties. But these difficulties can be overcome by devising suitable installation system as explained in  this article.

Conclusions: One innovative use of geosynthetics is in enhancing the performance of the conventional stone columns used as ground improvement for flexible structures. The geosynthetics wrapping the individual stone columns enhances the performance of the stone columns in many ways especially when they are installed in extreme soft soils like marine clays. This paper investigated and highlighted the benefits of such systems and suggested some installation techniques that can be adopted in the field.

References:

• Murugesan, S. and Rajagopal, K. (2006) Geosynthetic encased stone columns: Numerical evaluation. Geotextiles and Geomembranes, 24(6), 349-358.
• Murugesan, S. and Rajagopal, K. (2007) Model tests on vertical load capacity of geosynthetic encased stone columns. Geosynthetics International. 16(6), 346 354.
• Murugesan, S. and Rajagopal, K. (2009) Shear Load Tests on Stone Columns With and Without Geosynthetic Encasement. Geotechnical
  Testing Journal, ASTM. 32(1) Paper ID GTJ101219).

S. Murugesan Assistant General Manager, Geosynthetics Division,Garware-Wall Ropes Ltd., Pune-411019.

K. Rajagopal,Professor and Head, Department of Civil Engineering,IIT Madras, Chennai - 600036.