Abstract- Shimla is the capital of hilly state of India, called Himachal Pradesh. A table top airport is existing there since long which was initially used only for small aircrafts for civil & defense purpose. The height of this airport is 1546m from mean sea level and its latitude & longitude are 31.08410 N and 77.06620 E respectively. Due to increase of traffic, the airport boundaries had to be extended in all the sides for which hill slopes had to be stabilized. Mostly the soil all around was poorly graded sand with gravels. Therefore combination of various ground improvement solutions had to be adopted. Self drilling anchors, gabion walls, anti-slide piles had to be adopted besides shotcreting on slopes. One of the significant solutions successfully adopted was Chloris bioremediation by growing of Bermuda (Cynondon dactylon) grass along slopes. The seeds of Bermuda were mixed with seeds of gayana and Medicago. The grass was grown on jute mesh which was held in position through wooden/steel pegs and seeding was done before monsoon. This deep rooted grass has properties that it can be grown on high altitudes also. Limit Equilibrium approach was used for stability analyses of slopes. For some areas, back analyses was also conducted to obtain soil parameters. Analyses were conducted for virgin slopes and slopes after treatment. Construction work has been completed as per the suggested design.
Keywords: Bioremediation, Slope Stabilization, Table top airport, ground Improvement.
Shimla is one of the best tourist destinations in India. Being capital of Himachal state, it is well connected with air routes. But because it is a hilly state, there was hardly any space available there on plain grounds. Therefore airport was constructed there on hill top only about 25 years back. This airport was hitherto used for 1-2 VIP flights per day or for defense purpose, that too for small aircrafts. But due to increase in traffic, govt. of India is increasing its size on all the four sides so that even Boeings planes can ply on this airport. The new design includes construction of increased size of runway, RESA (Runway End Safety Area), waiting lounges, public amenities etc. Since the existing airport has valley on all sides, the big size retaining walls, stabilization of existing hill slopes etc. had to be done. The present paper discusses the important ground improvement measures adopted at this site. Very effective bioremediation measures suiting to altitude and climatic conditions of site had also been adopted at this site.
2. Geotechnical Parameters of Site
The detailed geotechnical investigations had been done at site. The soil parameters are summarised in Table-1 below:
Since the strata was dominated by gravels, it was decided to reinforce the existing slopes by way of soil anchors. At the same time gabion walls have been provided with reinforced backfill (Fig.1) as per Mittal (2014). Proper surface drains were also provided on both sides of runway to drain off water as quick as possible. Self drilling anchors (SDA) were provided (Fig. 2) to stabilise the existing slope passing through gabion walls. To stabilise the backfill soil the geogrids and geocell reinforcement were used (Figs. 3 & 4). The designs were done by FEM analysis. The typical design analyses are given in Figs. 5 and 6 for gabion wall and soil anchors respectively. The Fig. 6 illustrates antislide pile also.
3. Climate Chart of Airport Site
The climate chart of study area is given as follows (Table 2).
4. Bioremediation Measures
At this site, for hill slope stabilization seeding of grass and herb mixture, were also adopted as discussed in following paras.
4.1 Dry Seeding
Seeds (15-30 g/m2) and organic fertilizer (120 g/m2) are scattered by hand or machine where soil stabilization need be done. It can be applied on flat slopes with rough surfaces. On Steeper slopes where it is necessary to cover the soil quickly, a cover crop seeding is used. Cover crop such as vetch, rye and clover are excellent plant for erosion control. Special mixture of seeds of Chloris gyana, Medicago sativa, Cynadon dactylon and Roemer fescue are spread in a mixture of 12 gm/m2 and covered with soil. On steep slopes which have a smooth surface, hydroseeding is also used. Seed (20-30 gm/m2), Organic fertilizer (100 gm/m2), mulch (e.g. cellulose, straw 80 gm/m2) and Alga product as glue (100 gm/m2) are mixed in a special barrel with water and pumped out on to the slopes (2 liter/m2). On very steep slopes it is also advisable to fasten a jute mesh on the slopes because it fixes the hydroseeds. On very steep slopes and on failure edges a stable jute mesh covers the straw. This is jute mesh straw seeding method. The life span of jute mesh depends on the climate and the weather but is normally 2-3 years. For areas where this lifetime is too short, it is better to use the coconut mesh which usually lasts for about four years. On area where rock fall could happen or rock could break out of the soil, it is useful to nail an iron mesh into the ground covering the straw layer. This method is called wire mesh straw seeding.
4.2 Hydro Seeding
It is a planting process that uses slurry of seed and mulch. This was also adopted in some area around new airport site.
5. Characteristics of grasses used for hill slopes stabilisation
5.1 Chloris gayana
It is a species of grass commonly known as Rhode grass. It can grow in many types of habitat. It is also cultivated in some area as a palatable grass for animals and a ground cover to reduce erosion and quickly revegetate denuded soil. It is tolerant of moderately saline and alkaline soil and irrigation. C. gayana is able to deal with soil erosion on slope field by holding top soil. It can also be mixed with legumes such as Alfa alfa, cowpea which also improve soil nutrients. Root length density has a great influence on soil stability. C. gayana roots are able to extract water at a depth of 4.25 meter. The grass has good draught tolerance. Its seeds germinate quickly (1-7 days) depending upon temperature and often achieve full ground cover within three months of sowing. C. gayana grass survives on little rainfall, can grow on low pH soils. It is too good when it comes to covering base soil. The fact that C. gayana can grow quickly means to protect the soil from eroding. This grass was used in combination with Cyandon dactylon (Burmuda grass) and Medicago sativa within the gabion material also. All these grass were selected as per climatic chart (Table-2) of site. This had grown very fast as clearly evident from Fig.7. The same grass was sown alongside hill slopes after putting the Jute and Polymer meshes. Within one monsoon months, this grass also sprouted at very fast pace (Fig. 8).
International Common Names of Chloris gayana grass are as follows:
a) English: Rhode grass
b) French : Herbe de Rhodes
c) German : Rhodes gras
d) Spanish : Heirba de Rhodes
5.2 Cyandon dactylon (Burmuda Grass)
It is the fast growing grass that spread by seeds and stolons and rapidly colonizes new areas and grows forming mats. It is perennial grass with underground rhizomes and on the ground runners. It spreads horizontally and bears nodes and internodes of about 10 cm length. Each node roots in the soil and produce short culms (tillers) up to 25 cm height. The rhizome are mainly in the top 10 cm of the soil but may penetrate to a depth of 35 cm. C. dactylon is tolerant of extremely high temperature but susceptible to hard and prolonged frost.
International Common Names of Cyandon dactylon are as follows:
a) English: Bahama grass; couch grass; devil grass; dog's tooth grass; quick grass; star grass.
b) Spanish: grama Bermuda; grama común; grama de Espana; gramilla; pasto bermuda; zacate de gallina.
c) French: chiendent; chiendent dactyle; gros chiendent; herbes-des-Bermudes; pied de poule
d) Portuguese: capim-coastcross; capim-da-bermuda; capim-de-burro; grama; grama-bermuda; grama-seda; mate-me-embora
5.3. Medicago sativa
Commonly known as alfa alfa, it is recommended for seeding mined soil to reduce erosion and works as a soil conditioner. It is used as part of erosion control projects because alfa alfa has deep roots that will grow vigorously in compacted soil. There is general concern about introducing species because it is nitrogen fixing species and can be a way to falicitate gyana, Cyanadon and Reimer that have higher soil nutrient requirement. Alfa alfa is well adopted to a wide range of climate and edaphic conditions, but it does best in deep loamy soil with porous sub soils. Alfa alfa can tolerate drought and is known as good producer in dry year. Alfa alfa is generally considered a cool season species, although succulent growth can take place through the summer. Growth begins early in the spring. Many strains are adopted to different climate plains as in hilly regions, up to an altitude of about 2400 m, it can withstand high temperature of 39-41⁰ C as well as rather Low temperature. Alfa alfa is a perennial herb. It can reach 24-35 inches (60-90 cm). It has deep top root 10-17 feet (3-5 m) in sandy soil and 23-30 feet (7-9 m) under favourable conditions. Many older plant roots up to 63 feet (19 m) deep. The roots form nodules in association with Rhizobium bacteria which fix atmospheric nitrogen.
The annual cycle of the weed starts with bud reactivation by the end of winter. There is no innate dormancy and buds can activate if adequate temperature and humidity is available by any times of the year. Base temp. for bud sprouting ranges from 7.7⁰ C to 10⁰ C. New rhizomes are generated when temperature exceeds 15-20⁰ C. Extreme temp. were lethal to bud survival but from 0-30⁰ C the rate of sprouting increases with increasing temp. Its growth is favoured by medium to heavy, moist, well drained soil and is used an anti erosion cover on bunds and embankments.
a) English: Lucerne
b) Spanish: Luzerne, Luzerne cultivee
c) French: alfalfa, mielga
d) Portuguese: alfafa, Luzerna
Since there were some issues of acquiring of land near the terminal point of runway, RESA structure had to be provided to make the aircraft to fully halt situation in case the pilot is unable to apply full brakes. In RESA area, the ground is generally left soft and marshy so that wheels of aircraft come to standing halt state, should the aircraft enters in that area. The advantage of RESA in such types of topographical conditions is that in less space, the aircraft can come to halt in case emergency. However at the edges of RESA, a very effective hill slope stabilization had been done to cater the loads of aircraft. The general layout of new airport is shown in Fig. 9.
All the ground improvement measures adopted at airport sites have worked extremely well. The bio-remediation measures adopted at the site have also been quite successful as its growth is regularly being monitored. Most of the work had been completed in April 2020 and till now, no deformations have been reported back despite of passing of 2 monsoons. The present case study is a good example of taming the mountains with effective ground improvement measures including bio-remediation measures for construction of important structures like airport structures.
Authors are grateful to Ms. Nehul Tyagi and Ms. Hemlata Negi for compilation of data and analysis of slopes for preparation of this paper.
 Mittal, Satyendra & Shukla, J.P. (2008), "Soil Testing for Engineers", Khanna Publishers, Delhi – 110006.
 Mittal, Satyendra (2014), "An Introduction to Ground Improvement Engineering", SIPL publications house, New Delhi.
 Satyendra Mittal, "Pile foundation design & construction", CBS Publishers & distributors, N. Delhi.
 Florineth, Florin & Gerstgraser, Christoph "Soil bioengineering measures for hill and slope stabilization works with plants", www.fao.org/3/x0622e0s.htm
Dr. Satyendra Mittal, Professor, Department of Civil Engineering, Indian Institute of Technology, Roorkee- 247667 (India)
Dr. Sangeeta Singh, Professor, SSDPG College Roorkee- 247667 (India)
Aditya Jain, Mg. Director, HMBS Textiles (P) Ltd, New Delhi
Aman Midha, Design Engineer, HMBS Textiles (P) Ltd, New Delhi