Grouting to reduce seepage at Neckartal Dam, Namibia
Authors: R. Greyling1, E. Lillie1, D. Mouton1, P. Segatto2
Conference: ICOLD Annual Meeting
Date: September 29-October 3, 2024
1 Knight Piésold, Pretoria, South Africa
2 Rostruct, Johannesburg, South Africa
ABSTRACT
Neckartal Dam is an 80m high RCC curved gravity dam in Namibia that spilt for the first time in January 2021. As the dam filled to full supply level (FSL), usual increases in seepage were observed, but all measured seepage rates were within stipulated acceptable limits for a large concrete dam.
Seepage from the dam is monitored at eight v-notch gauges in the dam galleries. During the course of the year, measured seepage at the gauges remained relatively constant or had even slightly decreased until May 2021, when seepage measurement from the upper left bank gallery v-notch gauge steadily increased to a rate that was no longer considered acceptable.
High seepage was observed predominately from a single drainage hole in the left bank, where the dam wall contacts a steep left bank foundation.
Volumetric changes occur within an RCC dam during the hydration and temperature dissipation cycle. Temperature rises due to cementitious hydration and will peak within the first couple of weeks following placement. However, once developed, it takes several years to dissipate to a steady state seasonal equilibrium temperature.
It is important for the safety of the dam that major seepage paths be sealed to avoid potential erosion of the foundation at the position of the open joint and/or flooding of the dam galleries, which may result in damage to electrical equipment for operating the hydromechanical equipment and the instrumentation for monitoring the behaviour of the dam.
In this paper, the authors describe the evaluations made and the approach followed to successfully reduce the seepage observed to an acceptable rate by undertaking sequenced precision drilling and grouting of a specific target area of the dam foundation post-construction. The grouting operation was particularly difficult, considering the high water pressures experienced from the full dam and several seepage paths through the foundation. The use of thick, stable cementitious grout, combined with systematic drilling, water pressure testing and grouting, was successful in reducing flow significantly.
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