**3. Geosynthetics**

*Slope Engineering*

of 1480 m (**Figure 5**).

*Cost comparison of different options.*

**Table 1.**

Gabions (**Figure 6**).

**Figure 5.**

*Sikkim airport - aerial view (after [7]).*

*Slope face completely covered with vegetation few months after installation (after [7]).*

infrastructures over 200-acre area. Retention of fill on the valley side needed construction of retaining structures with heights varying from 30 to 74 m over a length

**Construction Method Construction cost \$M Property Cost \$M Total Cost \$ M** Reinf. Concrete Wall 1.52 Nil 1.52 Reinforced Earth Wall 1.42 Nil 1.42 1.25H:1 V Rockfill 0.93 0.30 1.23 1H:1 V Reinf. Slope 0.48 Nil 0.48 2H:1 V Earth Fill 0.30 0.90 1.20

On hill side, the cut slopes have a height extending up to 100 m. To retain and stabilize this fill of varying height, a composite soil reinforcement system was employed [7]. To make optimum utilization of space available and minimize cost, combination of vertical wall and steep slope has been adopted for construction of retaining structure. Facing elements for the reinforced soil wall comprise of

**16**

**Figure 6.**

Geosynthetics are mostly planar products manufactured from polymeric material and used with geomaterials such as soil and rock, as integral part of manmade project or system for better performance, economy, better quality control, rapid installation, cost competitiveness, lower carbon footprint, requirement of smaller parcel of land for embankments, etc.

Geotextiles and Geogrids are used normally for reinforcing embankments or natural slopes either to obtain higher factor of safety or for construction with steep slopes. Allowable Geotextile/Geogrid strength is arrived at using several factors to account for degradation, creep, installation damage, etc. The allowable tensile strength, Tall, is

$$\mathbf{T\_{all}} = \mathbf{T\_{ult}} \;/\; \mathbf{R\_{FID}} \* \mathbf{R\_{FCR}} \* \mathbf{R} \mathbf{F\_{CBD}} \tag{1}$$

Where Tall and Tult - allowable and ultimate tensile strengths respectively, RFID, RFCR and RFCBD - reduction factors (all >1.0) for installation, creep and chemical and biological damage respectively. The combined or overall reduction factor is about 2.0 for design.
