Remedial Measures for 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:
P.S. Prasad
Scientist ‘C’, GTE Division, Central Road Research Institute, New Delhi–110 020, India.
E-mail:
Sudhir Mathur
Scientist ‘G’ & Head, GTE Division, Central Road Research Institute, New Delhi–110 020, India.
E-mail:
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.
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Remedial Measures for Lowering Ground Water Table at Pantnagar Airport
1. INTRODUCTION
The problem of waterlogging wherein the ground water table exists at a shallow depth below the ground (many times at 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 progressive deformation of the subgrade and pavement is accentuated by ingress of water from the top of the wearing surface of a pavement comprising usually of a thin bituminous layer. The inadequate waterproofness of the surface is impaired further by stripping of the bituminous binder due to prolonged contact with water. Infiltration of rain water through the shoulders is another factor which aggravates the situation. If salts like sulphates or carbonates are present in sub-soil, they are carried in solution by capillary water, which may lead to failure of concrete pavements. However salts do not affect WBM or bituminous construction directly, provided the filler material used is inert and free from injurious constituents. Even then, water being the main enemy of road pavements, premature failure of road pavements in waterlogged areas may take place unless suitable remedial measures are applied. Central Road Research Institute (CRRI) recently undertook studies to design measures to overcome the problem of water-
logging at Civil Airport at Pantnagar in Uttarakhand state. Salient findings of the CRRI study are discussed in this paper.
2. GEOTECHNICAL INVESTIGATIONS
The Pantnagar Civil Airport is located near G.B. Pant University of Agriculture & Technology at Pantnagar which is at a distance of about 247 km from Delhi. The ground elevation above MSL at the airport is about 233 m. Airport is situated on a plain terrain but the south western part of the airport is at a lower elevation than the rest of the airport. Hence the waterlogged areas in the airport perimeter road (Figs. 1 and 2) are located near South-Western side of the airport. The length of the perimeter road in the waterlogged area is about 600 m. High water table problem is encountered at several parts of the runway and taxi-track especially near South-Western part. The airport area experiences heavy rainfall, the maximum precipitation in a day can be as high as 80 to 90 mm and exceptionally very high rainfall up to 200 mm to 225 mm in 24 hours have been recorded some times.
Fig. 1: Waterlogging in Perimeter Road Alignment
Fig. 2: Shallow Water Table by the Side of Perimeter Road
Before undertaking design of any geotechnical measures, proper understanding of the sub-soil conditions is necessary. With this view sub-soil investigations were carried out in the airport area and disturbed as well as undisturbed soil samples were collected. For sub-soil investigations, boreholes of 150 mm diameter were drilled using hand auger and extension rods. A total of three boreholes were made in different areas of the airport. Bore hole 1 (BH 1) was made in the south western portion (waterlogged perimeter road) while Bore hole 2 (BH 2) was made near Taxi track. Bore Hole 3 (BH 3) was made in the proposed runway extension portion. Disturbed samples were collected from the cutting edge of the auger at different depths. Seamless flush jointed casing of 150 mm internal diameter was used to prevent any caving of boreholes and it was inserted simultaneously during the advancement of boring operations whenever required. The undisturbed samples were collected from the boreholes with the help of a thin walled sampler, as per IS: 2132–1986, ‘Code of practice for thin walled tube sampling of soils’. Laboratory tests were carried out as per relevant IS standards on undisturbed and disturbed soil samples for identification and classification purposes. Summarised results of the tests carried out on sub-soil samples are shown in Table 1.
Table 1: Test Results on Sub-Soil Samples
from Different Boreholes
(%) / LL
(%) / PI / Gravel (%) / Sand (%) / Silt (%) / Clay (%)
BH 1 / 0.65 / MH–OH / 43 / 51 / 22 / 0 / 15 / 68 / 17
1.20 / SW / 24 / 26 / NP / 1 / 61 / 30 / 8
2.60 / SP / 17 / 25 / NP / 13 / 80 / 7 / 0
BH 2 / 0.35 / SM / 14 / 25 / NP / 0 / 54 / 41 / 5
1.80 / SM / 21 / 26 / 8 / 0 / 41 / 52 / 7
2.30 / ML / 27 / 29 / 9 / 0 / 13 / 78 / 9
3.65 / SP / 24 / 28 / NP / 19 / 70 / 11 / 0
BH 3 / 0.30 / ML / 14 / 25 / NP / 0 / 49 / 43 / 8
1.20 / SW / 5 / – / – / 44 / 52 / 4 / 0
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 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 it was at the ground level in BH 1 location. Except for the area where waterlogging is seen, the liquid limit of sub-soil samples were less than 30 per cent and most of these samples were non-plastic. Due to presence of clay, the top layer of sub-soil in BH 1, has liquid limit of 51 per cent. This soil also has higher value of plasticity index equal to 22.
The results of the field and laboratory tests on sub-soil indicate that waterlogging in the airport occurs mainly due to low ground elevation and subsequent water collection in the area. Eventhough sandy sub-soil is seen, lack of drainage measures lead to waterlogging. Hence remedial measures like raising the height of the embankment and providing capillary cut-off layer or sub-surface drainage measures would be required. The details of remedial measures suggested (location wise) are given in the next section.
3. REMEDIAL MEASURES PROPOSED
3.1 Remedial Measures for Perimeter Road
About 600 m length of the perimeter road in the South-Western side of the airport has been affected by waterlogging and slushy sub-soil conditions. Two alternative remedial measures have been suggested for perimeter road. Details of these two alternatives are given below:
3.1.1 Raising the Height of the Perimeter Road
Embankment (Alternative I)
The first alternative would be raising the height of the embankment of the perimeter road in the area affected by waterlogging, laying the capillary cut-off sand layer and geotextile separator in the embankment and providing adequate number of cross drainage works to drain off standing water. Before constructing the road embankment, dewatering needs to be taken up by suitably diverting the standing water to natural drains and constructing a temporary bund and pumping out excess water from the area where road work is to be taken up. After dewatering the proposed road alignment area, slush/muck should be removed for a depth of about 0.5 to 0.75 m using machinery and labour so that embankment construction becomes feasible. Gravely soil with maximum dry density (as per modified proctor test) of 1.75 gm/cc or higher can be used for embankment construction. The embankment should be constructed for a height of atleast 30 cm above the existing ground level and a geotextile separation layer should be provided in the embankment as shown in Figure 3. The geotextile layer would act as separator between fill soil and capillary cut-off sand layer which would be placed above the geotextile. The geotextile would also prevent clogging of sand layer due to migration of soil particles from below. Either a woven or a non woven variety of geotextile can be used. The geotextile should have grab tensile strength of either 1100 N (for woven type) or 700 N (for non woven type) and its AOS should be 0.43 mm. After laying the geotextile layer, capillary cut off layer of coarse sand (having not more than 6 per cent passing 150 micron sieve) is to be placed and compacted. Capillary cut-off sand layer thickness should be about 0.3 m. The embankment was suggested to be constructed for a further height of about 60 cm above capillary cut off layer. It was further suggested that wearing course of the perimeter road in water submerged area be constructed using interlocking cement concrete block pavement.
3.1.2 Lowering the Sub-Soil Water Table by Trench
Drain (Alternative II)
As an alternative to raising the height of the embankment and providing capillary cut-off layer, depressing the water table by providing suitable sub-surface drainage measures can be taken up. Satisfactory results can be achieved by providing deep buried drains which are known as ‘Trench Drains (geotextile encapsulated aggregate sub-surface drain)’ by the side of the road pavement for lowering the water-table. These sub-surface drains are required to be constructed for a distance of atleast 15 m beyond the waterlogged area also all along perimeter road. Keeping in view, the ground elevations and direction of water flow, trench Drains are required to be constructed on one side of the perimeter road adjacent to the surface drain. The dimensions of the proposed sub surface drains are shown in Figure 4.
For construction of sub surface drains, trench excavation shall be first undertaken as per plans. A non woven type of geotextile having grab tensile strength of 1100 N, Apparent Opening Size (AOS) of 0.25 mm and permittivity of 0.20 m per sec should be used. The geotextile shall be placed without any wrinkles or folds and without any voids spaces between the geotextile and the ground surface. The trench shall then be filled with good quality, clean river pebbles/aggregates. The aggregates shall have a maximum size of 37.5 mm and percentage passing 150 micron sieve shall not be more than 10. After compacting the aggregates carefully by hand tamping or by using small plate compactor, the aggregates shall be enclosed in the geotextile. Good earth shall then be placed and compacted over the geotextile covered drain. The sub-surface drain adjacent to perimeter road should be taken outside the boundary wall at the lowest elevation point across the perimeter road and connected to natural stream or other storm water drain so that water from sub-surface drain flows out in an unhindered manner to the stream. The construction of sub-surface drain would lead to lowering of water table in waterlogged area. Subsequently, the road embankment for perimeter road in the waterlogged area can be constructed after removing the muck and slush for a depth of about 0.5 to 0.75 m from the road alignment area. The wearing course of the road pavement can be constructed using interconnecting concrete block pavement.