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Madras Agric. J. 92 (4-6) : 341-343 April-June 2005 Research Notes

A low cost subsurface dyke using bentonite clay- a success story P.SUSEELA Asst. Professor, Water Management Research unit, Kerala Agrl. University, Vellanikkara

Compared to the national average, unit land of Kerala is receiving 2.5 times more rainfall. Due to the steep and undulating topography, rainwater is not much retained on the land in comparison with other states, there by obviating the advantages of high rainfall. Immediately on cessation of rains, subsurface water drains off along the steep slope resulting in poor retention of water in the catchment area. Only 5% of the runoff is believed to be converted into the ground water resources. In spite of heavy rainfall received in Kerala, the state experiences severe draught during summer months due to the inefficient utilization of rainwater (Basak, 1998). Considering the topographic features, high population density and non-availability of waste lands of the state, the most suitable method of water harvesting is the construction of check dams/subsurface dams (Shree Padre, 2002).

at 14 locations based on the hydrological and topographical conditions (Augson, 1997).

Subsurface dyke is an impermeable layer that is built below the ground surface with the intention of obstructing the natural subsurface flow of water, thereby augmenting the recharge of ground water and raising the water table (Thomas et al. 1998). Drinking water has become a scarce commodity along the Bharatapuzha and many wells have dried up in the villages along the course of river. Water scarcity in most of the places begins by March and in some places even as early as December. This scarcity lasts till the arrival of southwest monsoons in June. Hence, in order to reduce the water scarcity problem in Thrissur, Malappuram and Palakkad districts, check dams/subsurface dams were proposed to construct across Bharatapuzha

In order to study the suitability of the site for constructing the dam, four drill holes were taken upto the clay level. The average depth of clay level was found to be at 3m. Rocky formation was found at a depth of 6m at the right bank and 7m at the left bank. Hence an impervious cut-off could be provided at relatively shallow depth.

Selection of site In the present study, subsurface dyke was constructed at Thrangali Kadavu in Palakkad district across Bharatapuzha, considering the topography of the river and banks, soil properties, economy and benefits (Leu. Chenghua et al. 1995). Construction of subsurface dyke at Thrangali Kadavu is suitable, since it can avail water for irrigation (a lift irrigation scheme is situated approximately 105m upstream of the proposed weir line) and drinking purpose (a drinking water scheme is also situated 1 km upstream of subsurface dyke). Moreover, the width of river is comparatively less (300 m) at this site.

Construction details of subsurface dam Subsurface dam was constructed by using a new technology, which proved to be very cheap compared to the conventional concrete structures. Sand-cement-Bentonite grout was used to construct the impermeable layer for arresting the subsurface flow.

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Preparation of bentonite slurry Slurry tank was constructed near the trench for easy circulation of bentonite slurry into the trench. Slurry pit was lined with LDPE, 150 micron sheet and the sides were protected by sandbags Initially bentonite was mixed in small quantities in 200 litre barrels and then the slurry filled in the slurry pit. Water was added to get the bentonite slurry having a specific gravity of 1.11 (roughly 175 g bentonite is necessary to prepare 1 cumec slurry). Slurry was mixed thoroughly by using a mud circulation pump keeping both the suction and delivery pipe inside the slurry pit. Bentonite slurry was prepared 24 to 48 hours before the construction of the trench for proper mixing and expansion ofbentonite Construction of trench A trench having a width of 0.6m was constructed (over a stretch of 2m) using poclain across the river. The sides of the trench were protected by pumping the prepared slurry into the trench through the canvas pipe during the progress of excavation. The excess slurry will flow back to the slurry tank through the return channel. Preparation of grout mix One m3 of Sand-cement-Bentonite grout was prepared by using the materials in the proportion of Sand Cement Bentonite Water

: : : :

1500 kg/cumec 60 kg/cumec 68 kg/cumec 450 lit/cumec

Total volume of all the ingredients in the grout should be equal to 1000 lit or 1m3 V = 1500/2.8+60/3.2+68/3+450 = 1027 lit, say 1 m 3

(Sp.gr of sand = 2.8; Sp.gr of cement = 3.2; Sp.gr of bentonite =3) The excavated sand from the trench (trench made for constructing the subsurface dyke) was spread over an area of 7m x 7m. Then, small pieces of clay lumps and cement were spreaded over the sand layer and the roto tiller was then run to dry mix the material Bentonite slurry of adequate bentonite content was then spread over the mixed material and run roto tiller. The grout was prepared at the flowing consistency and was then fed by gravity into the tremmie. The grout was mixed in the trench by mixing device. A Sand - Cement - Bentonite wall having a length of 320 m [width of the river (300m) + anchorage of 10m to the right and left bank of the river], depth of 3m (stiff clay was present at an average depth of 3m) and a thickness of 0.6m was constructed. Cross sectional view of the subsurface dyke is given in fig.l. In order to protect the diaphram wall from scour due to degradation of river bed downstream, an apron structure consisting of soil-cement has been provided and tied to the diaphram. Soil cement concrete weir body was constructed by using 1600 kg of sand, 120 kg of cement, 120 kg of clay for 1 m3 soil cement concrete. At the bottom of the river, slurry wall was protected by providing gabbion work. Gabbion work was provided for a length of 3 m on the upstream side and 12 m on the downstream side. Gabbion work is the arrangement of layers of packed rocks in wellprotected GI net work. Both left and right bank of the river was also protected by providing gabbion work for a length of 25rn on u/ s and d/s upto a height of high flood level. Performance evaluation The performance of the subsurface dyke was studied by monitoring the water level in the adjoining wells at left and right bank of

A low cost subsurface dyke using bentonite clay- a success story

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Fig.1. Ross Selection of Sub-surface check dam across river Bharathapuzha

the river within a distance of 1 km upstream and downstream side of the dyke (totally 28 wells on the downstream side and 12 wells in the upstream side), the water source used for lift irrigation scheme and the well utilized for drinking water supply, before and after the construction of the dam. The water level in the adjoining wells was found to be increased during the summer months by about 2 feet. Before construction of the dyke, water in the well, constructed 1 km upstream of the subsurface dyke, which is used for drinking water supply was available only for about 2 hours pumping. But after the construction of the dyke, water is available for pumping continuously for 24 hours. The lift irrigation scheme (105 m upstream of the weir line) in the upstream of the dyke was also benefited by the construction of the dyke. The experience at Thrangali Kadavu indicated that subsurface dyke is an efficient barrier to arrest the subsurface flow of water and to conserve the ground water in Kerala with its unique topography having hills and valleys. The dyke could efficiently arrest the rate of depletion of drainable water in the catchment area due to the steep topography.

References Augson, G., George, Santhosh Joseph, Shiju Thomas and Simi, V.U. (1997). Study of Bharatapuzha and Design of Check dam. B. Tech project report. Govt. Engg. College Thrissur Basak, P. (1998). Water resources of Kerala myths and realities. Water scenario of Kerala. State committee on science and technology and Environment, Govt. of Kerala publication. pp: 1-6. Leu. Chenghua., Leu, C.H., Tesai, H. Y. and Chang, S.H. (1995). Site selection of check dams based on geographical information systems. Journal of Chinese soil and water conservation. 26 (1). pp : 49-55 Shree Padre. (2002). Rain Water Harvesting. Kairali Offset Press, Thrissur. Thomas, J., Joy, P.P. and Samuel, M. (1998). Subsurface dike for ground water conservation. Proceedings of tenth Kerala Science Congress. pp: 510-512.

(Received : February 2004; Revised : March 2005)