JG Afrika’s investigation and design provided a cost-effective means of improving the safety of Ludeke Dam, a Category III dam that is owned by the Department of Water & Sanitation and managed by uMngeni-uThukela Water. This practical solution also enabled urgent repairs to be undertaken to the dam without disrupting water supply to surrounding communities. At present, about 266 000 people rely on this water source which is supplied via the Mbizana Bulk Water Supply Scheme.
Holding more than 14,5million m3 of water, Ludeke Dam is 240m in length and almost 40m high. It features an earth-core dam with a concrete-lined side channel and spillway.
However, following its first filling in 2014, two safety-related issues were observed and over time, defects became apparent that compromised the safety of the dam. These were due to vertical movement of the joints of the gravity key wall on the left flank. In addition, seepage occurs on the right flank, although not considered a safety problem following an extensive investigation by JG Afrika.
Jan Norris, a JG Afrika Director and Executive Board Member, led the team of engineers who undertook the inquiry into the causes of degradation and supervised the rehabilitation works. Colin Scott served as the Resident Engineer on site.
“We reviewed all as-built records, as well as design and construction reports, before undertaking a topographic survey and inspection of all the pressure-relief wells and discharge manholes. Our geotechnical investigation included a geophysical electrical resistivity and electro-magnetic survey, and we drilled boreholes between the toe drains and pressure relief wells and through the crest. We also dug trail pits; undertook auger borings; and excavated downstream along the right flank. Furthermore, a cone-penetration test with pore water pressure measurement was undertaken through the crest of the embankment,” Norris says.
An assessment of the construction reports revealed that difficulties were encountered when grouting the right flank. This is considering the very highly fractured and jointed rock mass encountered from below 8m. Most of the cut-off had quaternary grout holes whereas the right flank only advanced to the primary stage a 6m centres. Norris says that during construction, design modifications were made. These include widening the base of the cut-off to account for additional seepage and deepening the toe drain to manage seepage as opposed to preventing it. In addition, 12m-deep, 400mm-diameter pressure relief wells were installed beyond the downstream toe at between 8m and 12m spacings.
JG Afrika determined that the total seepage at the right flank is about 3l/s. Most of the seepage occurs around the hillside on the right flank and some through the embankment The water is clear with no sign of fines or boiling and geological contact ascertained. A strategy has since been developed to manage the seepage consisting of sub-soil drains.
Based on the outcome of the investigation, JG Afrika explored three design options to solve the problems encountered on the left flank. The first entailed demolishing the existing key wall and constructing a new jointless reinforced concrete key wall on a piled foundation. Another solution that was explored entailed providing new vertical joint seals between all the wall panels and stabilising the key wall foundations using grout injection. The third option involved providing new vertical joint seals between all the wall panels that could accommodate movement of 100mm.
A belts-and-braces approach was adopted to repair the damaged joints of the concrete gravity walls. The existing seal was replaced with polyurethane and hydrophilic joint components providing a flexible yet robust seal. Considering that the front face is partially located against rockfill and rip-rap, a torch-applied membrane located beneath stainless steel plates, designed to slide over each other to accommodate future movement, provides extra protection.
JG Afrika also made recommendations to the Department of Water & Sanitation. “We advised that the client ensure there is adequate geotechnical information on the in-situ geology of the embankment abutments, which needs to be referenced when undertaking seepage analysis. Furthermore, we recommended that gravity walls must not be founded on the clay core of embankment dams in future,” Norris concludes.
