Grouting Issues for Rehabilitation of Ukai Spillway Block under DRIP

Seepage is a complex problem in dams. Seepage results not just in loss of water, it increases pore pressure in the body of the dam. The loss of cementitious material from the masonry over the years results in reduced integrity of the dam structure and can become a dam safety problem. A systematic and scientific approach is needed to address the issue of seepage in masonry dams. Grouting is the universally accepted method of addressing the problem which not only helps in reducing the quantum of seepage but also impregnates the masonry with cementitious material making up for its loss over the years. However, proper investigations using the state-of-the-art methods is a pre-requisite in drawing an efficient grouting plan. Grouting methodology, drilling pattern, grouting equipment, grout material, admixtures to be used, grout pressure to be applied, tests to be conducted for the drill holes before and after actual grouting are matters of careful design and execution.

This paper deals with a case study of Ukai dam in Gujarat state of India wherein grouting an overflow block successfully addressed its seepage problem. It describes the systematic approach adopted before actual grouting in terms of investigations carried out to assess the need for grouting, exploration of various grout materials, suitability of grout equipment, methodology of actual grouting, pre and post grout tests carried out and the efficacy of the grouting achieved.

Ukai is a Multipurpose project on the second largest westward draining interstate river Tapi in Gujarat which is considered to be the lifeline of south Gujarat. The dam is more than 50 years old and consists of 868 m long uncoursed rubble stone masonry gravity dam and 4058 m long earthen dam of zoned filled embankment. The Masonry dam has a maximum height of 80.7 m above its deepest foundation in the NOF. The spillway block 9 showed excessive seepage of about 400 LPM measured at the foundation/drainage gallery. Therefore, it was decided to tackle this block first. There were 5 continuous leakage/seepage spouts which were found to be interconnected in the dye tests conducted. The reservoir could not be depleted for operational reasons and treatment had to be given at a high reservoir level. The core recovery from the drilled holes was very low. In the dye test the dye traveled within 8-11 minutes from the Bore holes 1 to 7, into nearby Vertical Porous Drain (VPD-23) at stage 15-18 m. At stage 18-21 m the dye test indicated connectivity of the grout holes with the downstream ogee surface. The high discharge through the bore holes was possibly due to the passage of reservoir water with suction points on the upstream face. Use of thick colgrout mix also did not work. Polyurethane (PU) grout with quick setting admixture followed by cementitious grout ultimately reduced the discharge in the VPD from 400 LPM to 120 LPM. With this the grouting could continue in the planned manner which is now being extended to other blocks of the spillway to render desired safety to Ukai dam.