gharpure vol4 346 (2)

8/10/2019 Gharpure Vol4 346 (2)

1/7

1

INTRODUCTION

Constructing an airport in mountainous terrains ofSikkim, a northern state of India nestled in Himalayanranges is a challenge taken up by Airports Authority ofIndia. A table land required for the Airport is beingcarved out from mountainous terrain by cutting the hillside and filling the same material on valley side, in aland of 200 acres, to accommodate runway strip of size1820 x 150m and other infrastructure for the airport.The runway strip has north south orientation with dueconsideration to topography and requirement of Air-

port operation. The hill slopes from west (uphill side)to east (downhill side). To create a table land for theAirport, project handled an earthwork of 6.5 millioncubic meter. Retention of fill on valley side within theairport boundary called for construction of retainingstructures with height varying from 30 to 74m over alength of 1480m. On hill side the cut slopes have aheight extending upto 100m.

Apart from project being in the mountainous terrainof young Himalaya, there were several other challeng-es to be tackled. Sikkim is one of the worlds highest

rainfall area and receives annual precipitation ofaround 4000mm. The project site received 3900 mmrainfall in the year 2011 while it was under construc-tion. This poses another challenge of very short work

window for construction besides requirements for con-tinuously tackling drainage issues.

Figure 1: Site photo before construction

Providing proper drainage system to take care ofthe surface and sub surface run off with due consid-eration to the domestic requirement of water by theinhabitants downstream of airport was a big chal-lenge.

Implementation of appropriate erosion controlmeasures on the bear slopes after cutting was alsoessential considering the heavy rainfall in the region.

Composite soil reinforcement system for retention of very high andsteep fills A case study

A.D.Gharpure

Maccaferri Environmental Solutions Private Limited, Pune, India

S.KumarAirports Authority of India, Pakyong, Sikkim, India

M. ScottoOfficine Maccaferri S.p.A., Italy

ABSTRACT: Airports Authority of India planned for the construction of an airport at Pakyong, which is atown in the mountainous range of the Himalayas located in Sikkim, India. For runway construction of thisairport huge cutting of earth and its filling on the valley side was done to get a level platform. To retain andstabilize this fill of height varying from 30m to 74m, Composite soil reinforcement system is employed. Thisflexible and draining type of retaining structure is said to be the tallest reinforced soil structure in the world,which is debatable. This paper presents the construction and design principles of this system, details of an ef-fective system used for drainage of the mountain water (surface & sub-surface) and bioengineering techniquesadopted to give an eco friendly scenic beauty along the sides of runway. The utility of these techniques formaking eco-friendly retaining structures in the areas which are prone to severe landslides, earthquake andvery high rainfall is also highlighted.

5th European Geosynthetics Congress. Valencia 2012 Proceedings Vol 5. Topic: SOIL IMPROVEMENT AND REINFORCEMENT

www.eurogeo5.org Pag 346

8/10/2019 Gharpure Vol4 346 (2)

2/7

Figure 2: Layout Pakyong Airport

Figure 3: L-Section along Runway centre line

2 SELECTION OF APPROPRIATE SYSTEM

The site is prone to high seismicity and lies inseismic zone IV. Thus system selected for construc-tion of retaining structures had to be highly resilientto withstand the earth quake forces, high pore water

pressure, very steep slope and substantial sub-surface water flows. Additionally the structure hadto blend with the scenic natural beauty of the regionand should not adversely impact the environmentand local habitat.

Conventional masonry and RCC retaining walls

were ruled out due to the prevailing extreme siteconditions as described above. A flexible retainingwall with a free draining facia was an ideal choicefor this high terrain at very steep slope due to its in-herent ability to accommodate settlements and vibra-tions due to dynamic forces. The free draining naturehelped dissipation of pore water pressure in a veryefficient and effective way.

The foundation depth extent can be minimized inthe case of such structures and it can be placed evenover filled up soils to a good extent, if the specificconstruction procedures are followed. Vegetationcan take its roots easily through the facia, and astime passes roots will enhance the stability of thestructure and provide better erosion control. The

possibility of greening the facia using natural mate-

rials was one of the key factors to select the systemamong the different alternatives.

Figure 4: Site Photograph showing the runway direction

Airports Authority was specific to verify the ex-istence of successful case studies of similar nature.

Few good case studies of age more than 10 years inIndia and across the world installed in similar situa-tions of height ranging from 3 m to 40 meter gavethe confidence to the authority to adopt Compositereinforced soil slopes and Gabion gravity walls asthe most suitable retention and stabilization methodsfor hill slope and valley slope of Pakyong Airport.

3 COMPOSITE REINFORCED SOIL SLOPES ONVALLEY SIDE

3.1 System details:

To retain the fill within the Airport boundary onvalley side, retaining structures of height varyingfrom 30 to 74 m and length 1480 m approximately,was required to be constructed. Composite rein-forced soil system has been adopted for constructionof vertical retaining structures and stabilized steepslopes. The system is based on principals of soil re-inforcement where in, tension elements i.e. highstrength polymeric grids are introduced in the soilmass as reinforcement to retain the soil vertically or

at steep slope by virtue of interaction between soiland reinforcing elements. To make optimum utiliza-tion of space available and minimize cost, rightcombination of vertical wall and steep slope has

been adopted at various chainages for constructionof retaining structure. Schematic arrangement show-ing combinations of vertical wall and steep slope isshown in Figure 5.

Facing elements comprise of Gabion units whichare used for vertical reinforced soil wall. The systemconsists of a Gabion box at the front with integrated

tail of double twist steel wire mesh coated with Zincand PVC; as shown in Figure 6. The tail acts as sec-ondary reinforcement in the system; which holds thefacia panels in position by the virtue of interactionwith frictional fill soil. For steep reinforced slopes,

Buildings



8/10/2019 Gharpure Vol4 346 (2)

3/7

facing comprises of a stiffened woven steel wiremesh panel specially engineered to promote vegeta-tion to grow on the facia. This engineered productunit also comprises of mechanically woven meshmade up of Zinc +PVC coated steel wire as shownin Figure 7. To keep the vegetated face in goodalignment and shape, a weld panel and steel tie rodsare introduced at the facing surface below the steel

woven mesh.

Figure 5: Schematic arrangement of composite soil reinforce-ment system with Gabion and Vegetated Facia (P. Di Pietro,

2002)

Figure 6: Schematic arrangement of soil reinforcement system

with Gabion Facia for Vertical walls (Maccaferri TDS, Ter-

ramesh System, 2010)

Figure 7: Schematic arrangement of soil reinforcement system

with Green Facia for steep slopes (Maccaferri TDS, Green Ter-

ramesh, 2010)

High strength geostrip which has been used as

main tensile element or primary reinforcement ismanufactured from high strength polymeric strips.Bundles of polyester yarns are co-extruded with pol-yethylene to form individual polymeric strips. Highstrength geostrip is formed by uni-axial arrangement

of these high strength strips connected to each otherat intermittent intervals by transverse polymericstrips of lower strength.

3.2 Design Aspects:

The design of the slopes and wall structures werecarried as per guidelines given in British Standard

(BS: 8006) for the static analysis and FHWA docu-ment (NHI-00-043, 2001) for the seismic analysis.Seismic Coefficients were calculated according toIndian Standard IS 1893(Part 1): 2000. Due to thevariations in topography & boundary conditions, thestructure height and slopes had to be varied fromwall chainage to chainage; detailed stability analyseswere carried out on representative sections of thecomposite reinforced soil structures, depending uponthe height and other key parameters. Cross section ofthe structure at a chainage having maximum retain-ing height (74m) is shown in Figure 8. All stability

checks were performed using software MacStARS,based on limit equilibrium approach. A typical out-put of MacStARS is presented in Figure 9.

Figure 8: Cross section of the structure at a chainage havingmaximum retaining height (74m)

Figure 9: A typical output of MacStARS software

W

SLOPE ANGLE

STEEL TIES

H

L

WRAPPED PORTION

WELDED PANEL UNIT

COIR MAT

SECONDARY REINFORCEMENT

High strength geostrip as pri-

mary reinforcement

Gabion unit as wall facing elementand integrated tail mesh as secondary reinforcement



8/10/2019 Gharpure Vol4 346 (2)

4/7

8/10/2019 Gharpure Vol4 346 (2)

5/7

cal people are using the water from the creeks calledJhoras which are running across the runway tomeet their day to day water requirement. To ensurethat maximum water is available to the habitantsdownstream for their basic needs (as there is no oth-er source of water), the drainage system was pro-

posed in such a manner that no water would be re-tained by Airport interference. 11 No. Jhoras

(Natural Streams) were identified within the siteboundary among which 9 were crossing the runwaystrip. The Jhoras crossing runways were being chan-nelized by 5 box culverts under the runway to down-stream side. This was achieved by providing seriesof catch water drains and stepped intercepting drainsalong with gabion cascades. Water from these cas-cades was finally conveyed outside the site boundary

by the RCC box culverts, which was made to followthe natural course (Refer Figure 12 and Figure 13).

Figure 11: Hill side during the installation of Erosion controlmat prior to the construction of Toe wall.

Figure12: Diagrammatic representation of Drainage networkProvided

Keeping in mind the fact that distributing naturalsystem has a big impact on the stability of hillslopes, the outflow of RCC box culverts have been

planned at the location of natural Jhora at airport

boundary. This would not only help locals sourcingthe water for daily use but also help in maintainingthe equilibrium of natural drainage system beyondthe airport boundary.

Table 3: Monthly rainfall (mm) data for Monsoon period

(http://www.imd.gov.in/section/hydro/distrainfall/webrain/sikk

im/sikkim.txt)

Year May June July August September

2006 418.8 461.9 468.2 466.1 401.7

2007 281.4 455.4 656.3 377.2 476.0

2008 177.4 611.5 529.0 541.5 348.9

2009 335.4 355.4 408.6 454.1 180.1

2010 272.7 504.6 601.0 493.8 375.8

Figure13: Culvert outlet with stepped gabions as energy dissi-

paters

7 CONSTRUCTION

The high strength geostrips which act as primaryreinforcement are laid out horizontally on the pre-pared ground. The facia element is installed and fillmaterial is placed and compacted in layers. Once

backfill reaches the top of the facia element, subse-quent layer of geostrip with facia units are placed ontop of the earlier unit, as per design requireement.The primary reinforcement, high strength geostripswas placed at more than 1m vertical spacing. Thisnot only enabled rapid construction, as the contrac-tor did not have to stop so often to install geostrips,

but also reduced the quantity of geostrips required.

Gabion facia units are filled with graded rockforming a dry-stone wall appearance. Slope unitsare designed to get suitably vegetated. Biodegrada-

ble coir matting is factory fitted behind the frontwoven wire mesh face of the unit. Top soil is then

placed behind this coir and in front of the compactedbackfill to provide the substrate for vegetation estab-lishment on the face of the reinforced slope.

The contractor has an onsite laboratory to carryout a range of soil testing; including ensuring 98%modified Proctor density for the structural fill soil.

Placement and compaction of the fill layers is car-ried out in the morning. A substantial local work-force was used for much of the stone filling in thegabion wall unit as well as topsoil and seed place-ment within the vegetated slope unit. The local



8/10/2019 Gharpure Vol4 346 (2)

6/7

workforce, are predominantly terrace farmers, oper-ating in 40 day cycle after which they need to returnhome to work their fields. Skilled labours were usedfor the installation and filling of Gabion facia forvertical walls. Scaffoldings were used during instal-lation and filling of Gabions which gave better fin-ishing to the wall face.

Figure14: Wall under Construction- Gabion facia Installation

Figure15: Wall under Construction- Installation of slope units

Figure16: Wall under Construction- High Strength GeostripsInstallation in layers.

8 PRESENT STATUS OF THE PROJECT

The whole structure is nearing its completion andthe construction has been particularly challengingfor various reasons; Progress is limited by heavy rainfall, particularly

during the spring and summer monsoon period. Plant had to be dismantled during transportation

to enable it to pass over infrastructure with vehi-cle weight limitsConstruction of embankments of these heights re-

quires significant skill and a critical component isthe placement and compaction of the fill. Moisturecontent must be within a specified range, and the fillmaterial has to be in a suitable condition with thismoisture content.

Figure 17: Vegetated slope facia completely covered with veg-etation few months after installation

The robustness of the design was proven when thestructure was exposed to not only extreme rainfall intwo consecutive years, which has disrupted progressof construction, but also to a critically seismic eventof magnitude 6.8 in September 2011. This earth-quake significantly damaged other infrastructure inthe surrounding area as well as triggered many land-slides in Sikkim, but the composite reinforced soilstructure remained unaffected.

Figure18: Present Status at site



8/10/2019 Gharpure Vol4 346 (2)

7/7

9 SUMMARY AND CONCLUSIONS

In order to retain the slopes on the sides of therunway of the proposed upcoming Sikkim Airport,Composite reinforced soil system and Gabion wallswhich are flexible and free draining had been adopt-ed over conventional rigid structures such as RCCwalls.

By employing these systems, it has been possible tobuild environmental friendly, stable and cost effec-tive soil stabilizing structures for Sikkim Airport,mainly due to the following steps taken:

1. Balancing of cutting and filling.2. Selection of environmental friendly systems for

construction of retaining structures and slope pro-tection works. This has not only helped to reducethe carbon foot print but has also helped to buildstructures which are perfectly blending with thescenic beauty of the land.

3. The drainage system has been planned keeping inmind not only the project requirement but alsomeeting the domestic requirement of water by in-habitants downstream of airport.

4. The structure withstood extreme rainfall whichdisrupted the construction progress and a criticalseismic event of magnitude 6.8 in September2011.

10 REFERENCES

AASHTO 1997 Interims, Bridge DesignSpecification, Section 5

BS: 8002 1994 Earth Retaining Structures

BS: 8006 2010 Code of practice for

strengthened / reinforced soils and other

fills

IS 1893 - 2000 Criteria for earthquake re-

sistant design of structures

FHWA NHI-00-043 (2001) MechanicallyStabilised Earth Walls And Reinforced Soil

Slopes Design and Construction Guidlines Maccaferri Technical Data Sheet, 2010 Ter-

ramesh System

Maccaferri Technical Data Sheet, 2010

Green Terramesh

P. Di Pietro (2002)- Design and construction

of soil reinforced structures using composite

reinforcementsystems: modern and cost ef-

fective alternatives for high walls and slopes,

Proceedings of the Seventh International

Conference on Geosynthetics, 7-ICG



gharpure vol4 346 (2)

Documents