Remedial Measures for Lowering Ground Water Table at Pantnagar Airport
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RemedialIGC 2009 Measures, Guntur, INDIAfor Lowering Ground Water Table at Pantnagar Airport REMEDIAL MEASURES FOR LOWERING GROUND WATER TABLE AT PANTNAGAR AIRPORT U.K. Guru Vittal Scientist ‘F’, GTE Division, Central Road Research Institute, New Delhi–110 020, India. E-mail:[email protected] P.S. Prasad Scientist ‘C’, GTE Division, Central Road Research Institute, New Delhi–110 020, India. E-mail: [email protected] Sudhir Mathur Scientist ‘G’ & Head, GTE Division, Central Road Research Institute, New Delhi–110 020, India. E-mail: [email protected] ABSTRACT: Waterlogged areas are considered to be those where the level of sub-soil water table or standing water is such that, for prolonged periods the subgrade immediately below the pavement is well within the capillary fringe of the water table. As a result of migration of water by capillarity, soil immediately below the pavement gets saturated and this leads to a gradual loss of bearing capacity, which may eventually lead to pavement failure. Different techniques like providing sub-surface drains, introducing capillary cut-off, raising embankment height, etc are adopted when such conditions are encountered in the field. Pantnagar airport in Uttarakhand state is situated close to industrial layouts at Rudrapur and is a gateway to Kumaon region, especially hill resort of Nainital. Airports Authority of India (AAI) has taken up several works to improve the existing infrastructure at Pantnagar airport. Construction of perimeter road all along the airport boundary wall and extension of runway are among them. The entire area of airport is on plain ground sloping gently from East to West and also from North to South. Being located near the Himalayan foot hills, the airport experiences heavy rainfall. The problem of waterlogging is experienced at different locations in Pantnagar airport area. The waterlogging in the area is caused by relatively shallow water table. This paper provides details of field inspection, laboratory test results and remedial measures suggested for lowering ground water table for runway extension, taxi-track and perimeter road in waterlogged areas. 1. INTRODUCTION Central Road Research Institute (CRRI) recently undertook studies to design measures to overcome the problem of water- The problem of waterlogging wherein the ground water table logging at Civil Airport at Pantnagar in Uttarakhand state. exists at a shallow depth below the ground (many times at Salient findings of the CRRI study are discussed in this paper. the ground level itself) is encountered in several parts of our country. Where flooding for continuously long periods also takes place side by side along with waterlogging, the 2. GEOTECHNICAL INVESTIGATIONS progressive deformation of the subgrade and pavement is The Pantnagar Civil Airport is located near G.B. Pant accentuated by ingress of water from the top of the wearing University of Agriculture & Technology at Pantnagar which surface of a pavement comprising usually of a thin bituminous is at a distance of about 247 km from Delhi. The ground layer. The inadequate waterproofness of the surface is impaired elevation above MSL at the airport is about 233 m. Airport is further by stripping of the bituminous binder due to prolonged situated on a plain terrain but the south western part of the contact with water. Infiltration of rain water through the airport is at a lower elevation than the rest of the airport. shoulders is another factor which aggravates the situation. If Hence the waterlogged areas in the airport perimeter road salts like sulphates or carbonates are present in sub-soil, they (Figs. 1 and 2) are located near South-Western side of the are carried in solution by capillary water, which may lead to airport. The length of the perimeter road in the waterlogged failure of concrete pavements. However salts do not affect area is about 600 m. High water table problem is encountered WBM or bituminous construction directly, provided the filler at several parts of the runway and taxi-track especially near material used is inert and free from injurious constituents. South-Western part. The airport area experiences heavy Even then, water being the main enemy of road pavements, rainfall, the maximum precipitation in a day can be as high as premature failure of road pavements in waterlogged areas may 80 to 90 mm and exceptionally very high rainfall up to 200 mm take place unless suitable remedial measures are applied. to 225 mm in 24 hours have been recorded some times. 162 Remedial Measures for Lowering Ground Water Table at Pantnagar Airport Table 1: Test Results on Sub-Soil Samples from Different Boreholes Bore Depth Soil NMC LL Gravel Sand Silt Clay hole PI (m) type (%) (%) (%) (%) (%) (%) No. 0.65 MH– 43 51 22 0 15 68 17 BH OH 1 1.20 SW 24 26 NP 1 61 30 8 2.60 SP 17 25 NP 13 80 7 0 0.35 SM 14 25 NP 0 54 41 5 BH 1.80 SM 21 26 8 0 41 52 7 2 2.30 ML 27 29 9 0 13 78 9 Fig. 1: Waterlogging in Perimeter Road Alignment 3.65 SP 24 28 NP 19 70 11 0 0.30 ML 14 25 NP 0 49 43 8 BH 1.20 SW 5 – – 44 52 4 0 3 1.70 SP 11 29 NP 22 75 3 0 2.60 ML 30 26 9 0 41 51 8 From the sub-soil investigations, it was seen that sub-soil in the western edge of the airport where waterlogging was seen has clayey silt of about 1.0 m thickness below which sandy soil is there. About 17 per cent of clay particles are present in the top 1.0 m layer of sub-soil at this location. The sub-soil in the taxi-track as well as runway extension portion (BH 2 and BH 3 respectively) comprises of sandy soil of different types like SW, SP, SM upto about 2.5 to 3.0 m depth. Silty soil of low plasticity (ML) was encountered at a depth of 2.6 m in the runway extension portion. Pebbles in the size range of gravel particles were found to be mixed with sandy sub-soil up to an Fig. 2: Shallow Water Table by the Side of Perimeter Road extent of 45 per cent at BH 3 location. The water table was at a shallow depth of about 1.6 m at BH 2 and BH 3 locations while Before undertaking design of any geotechnical measures, proper it was at the ground level in BH 1 location. Except for the area understanding of the sub-soil conditions is necessary. With where waterlogging is seen, the liquid limit of sub-soil samples this view sub-soil investigations were carried out in the airport were less than 30 per cent and most of these samples were area and disturbed as well as undisturbed soil samples were non-plastic. Due to presence of clay, the top layer of sub-soil in collected. For sub-soil investigations, boreholes of 150 mm BH 1, has liquid limit of 51 per cent. This soil also has higher diameter were drilled using hand auger and extension rods. A value of plasticity index equal to 22. total of three boreholes were made in different areas of the airport. Bore hole 1 (BH 1) was made in the south western The results of the field and laboratory tests on sub-soil indicate portion (waterlogged perimeter road) while Bore hole 2 (BH that waterlogging in the airport occurs mainly due to low 2) was made near Taxi track. Bore Hole 3 (BH 3) was made ground elevation and subsequent water collection in the area. in the proposed runway extension portion. Disturbed samples Eventhough sandy sub-soil is seen, lack of drainage measures were collected from the cutting edge of the auger at different lead to waterlogging. Hence remedial measures like raising depths. Seamless flush jointed casing of 150 mm internal the height of the embankment and providing capillary cut-off diameter was used to prevent any caving of boreholes and it was layer or sub-surface drainage measures would be required. inserted simultaneously during the advancement of boring The details of remedial measures suggested (location wise) operations whenever required. The undisturbed samples were are given in the next section. collected from the boreholes with the help of a thin walled sampler, as per IS: 2132–1986, ‘Code of practice for thin 3. REMEDIAL MEASURES PROPOSED walled tube sampling of soils’. Laboratory tests were carried out as per relevant IS standards on undisturbed and disturbed soil 3.1 Remedial Measures for Perimeter Road samples for identification and classification purposes. Summarised results of the tests carried out on sub-soil samples About 600 m length of the perimeter road in the South-Western are shown in Table 1. side of the airport has been affected by waterlogging and 163 Remedial Measures for Lowering Ground Water Table at Pantnagar Airport slushy sub-soil conditions. Two alternative remedial measures 3.1.2 Lowering the Sub-Soil Water Table by Trench have been suggested for perimeter road. Details of these two Drain (Alternative II) alternatives are given below: As an alternative to raising the height of the embankment and providing capillary cut-off layer, depressing the water table 3.1.1 Raising the Height of the Perimeter Road Embankment (Alternative I) by providing suitable sub-surface drainage measures can be taken up. Satisfactory results can be achieved by providing deep The first alternative would be raising the height of the buried drains which are known as ‘Trench Drains (geotextile embankment of the perimeter road in the area affected by encapsulated aggregate sub-surface drain)’ by the side of the waterlogging, laying the capillary cut-off sand layer and road pavement for lowering the water-table.