Basal reinforcement

Geosynthetics with high tensile strength and stiffness such as geogrids, woven tapes/strips are used in reinforcement applications. Examples are reinforced soil walls, reinforced steep slopes, slope repair by reinforced soil, basal reinforcement at the base of embankments on soft ground or embankments over piled foundations, as illustrated in Figure 5.6. 

Figure 13.21 Illustration of the application of a basal reinforcement—drainage geocomposite.

The following sections provide a basic overview on modeling, design, and the corresponding requirements for geosynthetic reinforcement, with an emphasis on basal-reinforced embankments, geotextile-encased columns (GECs), and pded embankments. 

Basal reinforcement mainly (1) increases the stability of the embankment by mobilizing the force in the reinforcement, (2) confines the fill material and forms a flexible foundation (ductile failure in contrast to rapid failure of unreinforced embankment), (3) handles infiltration and separation of the subgrade into fill material, and (4) makes the settlement of embankments more uniform. 

For cases in which the available construction time is limited or serviceabiUty criteria are high, methods other than basal-reinforced embankments need to be chosen. Most commonly, ground improvement using stone or gravel columns is chosen. However if the foundation soil is extremely soft, the use of conventional stone columns can lead to unacceptable deformation or even failure. This is because the soft soil... 

Nature of arching between adjacent pile caps and determination of unarched vertical stress acting on basal reinforcement... 

Resulting deflected shape of basal reinforcement and method used to determine generated tensile loads... 

Method by which unarched vertical stress over plan area of basal reinforcement is transferred by tensile loads to adjacent pile caps... 

The use of geotextiles to reinforce soft foundation soils has progressed from the earliest applications some 40 years ago to the current status of widespread use and success. This chapter presents three major application areas (1) basal reinforcement, (2) geotextile-encased granular columns, and (3) geosynthetic piled foundations. In addition to the descriptions, elements of design, and information on performance, a specific project example was given for each topic. Further advances can and should be made, but we beheve that the current state of the practice in the selected applications is properly reflected here. 

Lawson, C.R., 2012. Role of modelhng in the development of design methods for basal reinforced piled embankments. Eurofuge 2012, Delft, The Netherlands. April 23-24, 2012. Delft University of Technology and Deltares. 

Basal reinforcement beneath breakwaters and containment dykes... 

Figure 20.19 Basal reinforcement technique applied to breakwaters and containment dykes on soft foundations.

One technique to improve the stability of breakwaters and containment dykes con-stmcted on soft foundations is to use geotextile reinforcement across the base of the stmcture (Fig. 20.19). This basal reinforcement technique has been used successfully for many years in the onshore construction of embankments on soft foundation soils, and is discussed in detail in chapter Geotextiles used in reinforcing foundations. Because of its cost—benefits, this technique has also been used in marine engineering structures over the past 20 years. 

Figure 20.20 Design limit modes for basal reinforced breakwaters and containment dykes on soft foundations, (a) HydrauUc stabiUty, (b) rotational stabiUty, (c) strain in reinforcement and (d) settlement.

Ameratunga et al. (2006) describe the design and construction of a basal-reinforced rubble seawall (containment dyke) over soft marine clay for the expansion of the Port of Brisbane, Australia. Fig. 20.21 shows a typical cross section of the basal-reinforced rabble seawall. The seawall was constructed to contain the reclamation fill for the port expansion. The thickness of the soft foundation varied from 8 to 30 m in some locations it had an undrained shear strength as low as 5 kPa at the foundation surface which was approximately 3.5 m below the average water depth. The rabble seawall varied in height to 7.5 m, and an analysis showed that the soft foundation could not support this... 

Figure 20.21 T3fpical cross section of basal-reinforced rabble seawall.

Figure 20.22 Installation of basal reinforcement with dredged sand fill placed on top. Courtesy TenCate Geosynthetics.

The reinforced soil technique, such as basal-reinforced breakwater constmction, is an example of a land-based technique employed in a marine engineering environment to provide a cost-effective solution to improve the stability of stmctures constmcted on soft marine foundation soils. The presence of soft marine foundation soils close to coastlines in bays, harbours, estuaries and deltas makes for difficult conditions on which to constmct stable marine stmctures. Reinforced soil is one technique along with other associated foundation consolidation and strengthening technologies that is expected to be used more often in these locations. 

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