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Chapter 1
Generalities
1.1 Introduction
The design and construction of the structures on soft soil deposits have been, historically, a challenge for the geotechnical engineers so that serviceability and limit state conditions as well as cost and time schedule are properly addressed. In order to meet these requirements, a great variety of construction methods is available. These may include, for instance, soft soil replacement, basal embankment reinforcement, use of lightweight fill material, prefabricated vertical drains and surcharge or vacuum to accelerate settlements, stage construction, use of granular stone columns, or different alternatives of soil cement mixture such as deep mixing or pile embankments with basal reinforcement (Almeida and Marques, 2013).
Among all available construction methods, the soft ground treatment with stone columns is one of the most widely adopted for reducing settlement and improving stability and load capacity (Poorooshasb and Meyerhof, 1997; Greenwood, 1970). However, when stone columns are installed in extremely soft soils, they may not provide significant load capacity owing to low lateral soil confinement. McKenna et al. (1975) reported cases in which the stone column was not restrained by the surrounding soft clay, which led to excessive bulging and soft clay squeezing into the voids of the stone aggregates, reducing the bearing capacity of the stone column as well as its drainage capacity.
The use of traditional stone columns is usually limited to the values of soft soil und-rained strength Su around 15 kPa (EBGEO, 2011), thus confining the stone column with a high-tensile-stiffness geosynthetic encasement (Raithel et al., 2002; Alexiew et al., 2005; Di Prisco et al., 2006; Murugesan and Rajagopal, 2006) can overcome this difficulty. There are also limited reports (Wehr, 2006; Yee and Raju, 2007) that the soft soils with Su values lower than 15 kPa have been treated with traditional stone columns.
Ghionna and Jamiolkowski (1981) and Van Impe and Silence (1986) were probably the first to recognize that columns could be encased by geotextiles. They produced an analytical design technique to assess the required geotextile tensile strength, thus an ultimate limit state (ULS) analysis. Details on this technique were also provided by Kempfert et al. (1997). In addition, Raithel and Kempfert (2000) produced an overall design calculation method for analyzing the service limit state; say assessing column and soft soil settlements based on the geotextile radial tensile stiffness. An update, including use on recent projects in Europe, was provided by Raithel et al. (2005); Alexiew et al. (2005); Alexiew and Thomson (2014); andAlexiew and Raithel (2015) and in South America by De Mello et al. (2008).
1.2 General principles
The general scheme of the bearing system with geosynthetic encased columns (GECs), developed in Germany in the mid-1990s, is depicted in Figure 1.1. The encasement used for the columns commonly consists of a woven geotextile with high-tensile modulus and low creep coefficient, which results in favorable drainage characteristics of the granular column and low strains in the geotextile. The column filling material can be sand or gravel; the latter, however, provides higher overall stiffness of the encased column, but has to be compatible with the geosynthetic material used to prevent its damage. In addition, the use of granular spoilage is possible in some cases. The geosynthetic encasement also controls the column diameter, minimizes material losses, increases overall column stiffness, and avoids granular column contamination, thus preserving the drainage features.
Figure 1.1 (a) Scheme of GEC (Murugesan and Rajagopal, 2006); (b) outline of an embankment on soft soil over GECS (Alexiew and Thomson, 2014)
Due to the higher stiffness of the GECs, load concentration (arching) occurs, thus reducing the vertical stresses on the soft foundation soils. The vertical load on a GEC generates also horizontal radial normal stresses outwards and radial widening of the column. This consequently results in counter-pressure from the surrounding soft soils and a confining resistance from the encasement, the latter being the key difference from “conventional” stone columns. The mobilized confining ring tensile force Tmob in the encasement depends on its tensile stiffness (modulus J) and hoop strain. The tensile force Tmoband the corresponding radial strain (elongation) control the radial behavior and consequently the vertical performance of the GEC in terms of settlement and bearing capacity. The less the GEC compresses, the higher is the embankment load supported by the GEC and the smaller is the vertical stress taken by the soft soils in between GECs.
1.3 Applications (Alexiew and Thomson, 2014)
GEC may be applied in soft soil deposits with values of undrained strength Su lower than 30 kN m-2, being better suited for Su values lower than 15 kPa. However, it is also possible to apply GEC in the case of Su values as low as 5 kPa. The main range of interest of GEC are soft soils with constrained modulus Eoed between 0.5 and 3.0 MPa and their thickness from 8 to 30 m. The minimum recommended embankment height above the GECs is 1.5 m.
The range of settlement values compensated during the construction stage in GEC applications is usually 0.1–0.5 m. Moreover, as the GECs work as “mega-drains,” primary consolidation and settlements occur relatively quickly in comparison with prefabricated vertical drains. Post-construction settlements in GEC applications are usually quite small, and differential settlements are inexistent when the embankment height is above the critical height.
GECs are quite suitable in the case of foundations sensitive to lateral thrust such as piles in the vicinity of high embankments, stock piles, or other directly founded loads. GECs have also been applied in seismic areas for keeping the integrity of granular columns under “shearing” seismic impact. A further application of GECs is for existing railway embankments being upgraded for higher speed trains, as GECs increase their dynamic stability (Alexiew et al., 2015).
1.4 Execution methods
Encased columns can be installed with or without lateral displacement of the soft soil, thus two different methods are generally available with regards to the GEC construction technology.
The first technique, as shown in Figure 1.2, is the displacement method, in which a closed-tip steel pipe is driven down into the soft soil followed by the insertion of the circular woven geotextile and sand or gravel fill in the sequence. The tip opens when the pipe is pulled upwards under optimized vibration designed to compact the column.
The displacement method is commonly used for very soft soils (e.g., Su < 15 kN m–²). GECs executed with the displacement of soft soil usually have a diameter of approximately 0.80 m, and the diameter of the geotextile is ideally equal to the inner tube diameter (Alex-iew et al., 2003). The column spacing ranges typically between 1.5 and 2.5 m and the tensile stiffness modulus (J) of the geotextile generally varies between 1500 and 6500 kN m–1.
The second construction technique is the replacement method with excavation of the soft soil inside the pipe. With the replacement method, an open steel shaft is driven deep into the bearing layer and the soil within the shaft is removed out by augering, as illustrated in Figure 1.3. The replacement method is preferred for soils with relatively higher penetration resistance, or when vibration effects on nearby buildings and road installation have to be minimized.
Figure 1.2 Displacement method for GEC installation (Alexiew et al., 2005, after Huesker)
Figure 1.3 Replacement method for GEC execution (Gniel and Bouazza, 2010)
1.5 Material properties selection
1.5.1 Soft soil
The design of GECs-supported embankments requires high-quality soft soil parameters to be obtained in a well-specified and controlled site investigation program. This includes in situ and laboratory tests performed on good quality undisturbed soft soil samples. Among the soft soil parameters, the compressibility characteristics, the stress history, and the strength properties are those that mainly control the settlement and stability of the embankment over soft soil deposit.
The Standard Penetration Test (SPT) is the dominant in situ test for preliminary soil investigation, but very often, it is complemented by other in situ and laboratory tests. The Vane Shear Test (VST) is usually employed to determine the in situ undrained strength and clay sensitivity. The Piezocone Test (CPTu), with pore pressure measurements, is particularly effective for soft clays, as it allows the estimation of both strength and consolidation characteristics, which are key properties of such soft soils (Lunne et al., 1997; Schnaid, 2005; Robertson and Cabal, 2015). In addition, the CPTu provides the soil stratigraphy, as well as an estimation of the stress history.
Table 1.1 summarizes the tests usually performed and the soft soil parameters obtained from each test. The parameters shown in Table 1.1 are defined in the list of symbols.
Table 1.1 Recommended laboratory and in situ tests and geotechnical design parameters (Almeida and Marques, 2013)
1.5.2 Granular column material
The geotechnical properties of the granular column fill material can be determined by common laboratory tests such as direct shear or triaxial tests, thus allowing to obtain the parameters to be used in either numerical or analytical calculations. These tests, however, are performed very often.
For GEC executed with stone columns, the gravel should be clean, preferably crushed stone, ...
