51. Construction of Deep Cutt-Off Grout Curtain in the Fossil Valley: A Case Study

Parbati Hydroelectric Project-Stage-II is located in the lesser Himalayan zones, in the Kullu District of Himachal Pradesh state. This site is ideal site for dam construction. But a thick fluvio-glacial material, which has inferred as Fossil Valley, occupies the right bank of the dam. The valley may become a conduit of water seepage from the reservoir, due to the permeable natue of material. The permeability of the overburden ranges between 1 x 10^–5 and 6 x 10^–7 m/s and the permeability of bedrock between 0 and 6 UL. Consequently, an extensive cut-off curtain grouting, within the fossil valley material, need to be carried out almost along the dam axis. The construction of a cut-off grout curtain is done by permeation grouting, using ‘tube-a-manchetts’ (TAM i.e. sleeved grouting pipes) through the alluvial deposit. The minimum width of grout curtain is 1750 mm. Here, sacrificial system of grouting is adopted, that means TAM pipe used for grouting will remain embedded in ground for future use also. The grouting holes are drilled by cased roto-percussion system, with down the hole hammer, utilizing foam as drilling fluid. The new-patented SYMMETRIX SYSTEM is adopted for drilling. The drill hole pattern and unique procedure of Symmetrix drilling is discussed in this paper. The maximum depth for drilling is about 101 m. It is very difficult to maintain verticality of the drill hole on such depths. For this purpose a tunnel is excavated at a depth of 50 m. The drilling is done in two parts, upper part is called upper curtain (from elevation 2200 m to 2150 m i.e. 50 m max. depth) and below tunnel is called lower curtain (from elevation 2145 to 2100 i.e. 45 m max. depth). The work is divided into a number of activities viz. locating drill holes, drilling, sleeve grouting, first phase grouting and second phase grouting, which are discussed in detail in this paper. After the number of trials drill hole pattern is decided as grid of four drill holes in square with additional hole at centre. Hence the grouting is done in three rows namely upstream row, down stream row and central row. To create the impervious seepage barrier in the path of water, grouting is done in two stages. Infirst stage of grouting, fine cement slurry is injected through all three rows; however in the second stage grouting is done only for central row with ultra fine cement or certain chemicals like sodium silicate. So the central portion of grout curtain becomes more impervious. The field test requiement of such material, grout mix design etc. is also discussed in the paper. The grout pumps are operated by sophisticated computer operated system, which will result in not only improving the speed of execution but saving in resources also. Due to this computer system of grouting, one can get the exact quantity of material consumed per day, which will be useful for various purposes like store, billing etc. The borehole deviation is very important factor in this project. If the borehole deviation of drilled hole is not within the permissible limit or if it intersects to another hole, it needs to scarifies such holes, which will result in loss of contractor's resources. One more software, named Bore Trak is used for measuring the vertical deviation of drilled hole. With this software one can find the vertical as well plannimetric profile of drilled holes. With the help of this profile the drilling angle of next hole is changed in such a way that the new hole will not intersect the previous hole. Further difficulties during the execution and the probable remedial measures are also discussed in the paper. It is useful for practicing engineers and contractors working with drilling, grouting and allied works.