Civil Engineering June 2022 | Vol 30 No 5

34 June 2022 Civil Engineering pits, dynamic cone penetrometer testing, seismic refraction surveys and labora- tory testing (that included large scale rockfill testing). Additional geotechnical investigations, as part of the GIBB design study, sought to optimise the CFRD plinth alignment through the use of subsurface geophysical investigations to better delineate the highly variable weathered rock profile. Several quarry sites were investigated in and around the dam basin and ultimately two rockfill quarries were selected which had sufficient quantities of unweathered rock for rockfill and concrete aggregates. The quantity of available materials was verified by performing core drilling. Ngwadini Dam is underlain by Umkhomazi Gneiss and associated soils (mixture of alluvial and clayey colluvial sands thinning out to residual clayey gravelly sands at the valley top slopes). The dam site geology comprises completely weathered granitic-gneiss (Zone A) underlain by highly weathered granitic-gneiss (Zone B) which is under- lain by better quality highly weathered granitic-gneiss rock (Zone C) that overlies moderately weathered granitic-gneiss (Zone D) which is underlain by mod- erately weathered to slightly weathered granitic-gneiss rock (Zone E). Where possible, the CFRD plinth and other dam structures will be founded on Zone E material, but where very deep excavations become uneconomical for the plinth, the plinth will be founded on Zone D material with a lengthening of the plinth structure to reduce the hydraulic gradient across it. DETAILS OF CONCRETE-FACED ROCKFILL DAM DESIGN The design of a CFRD is influenced by the grading, compaction, strength, stiffness and durability of rockfill material that effects the amount of embankment settle- ment during and after construction. The shape of the river valley and this settlement will influence material deformation and the potential for developing stresses in the concrete face. For Ngwadini Dam the expected rockfill deformation modulus is in the order of 40 MPa to 50 MPa, but this will be confirmed by constructing a trial embankment early on during construction. Rockfill placed in the various zones of the embankment will be free-draining and will be compacted to acceptable densities that will result in settlement rates that can be accommodated by compressible sealed joints in the concrete face slab. The requirement for low rockfill compress- ibility decreases towards the downstream side while rockfill permeability increases. The grading of the various material zones should prevent the movement of fines between the zones and the friction angle of the rockfill material is not constant and is influenced by the confining pressure. Zoning of the Ngwadini Dam embankment follows the current inter- national best practice for CFRDs. The zoning configuration for the core zones (Zones 3B and 3C) were optimised during design by considering the following: Q Q Cost (Zone 3C cheaper than Zone 3B) Q Q Settlement Q Q Slope stability. Figure 11 Final dam zoning arrangement Figure 10 Plinth founding conditions Geotechnical zone contact Seismic refraction line E (2 000 m/s) Seismic refraction line F (2 000 m/s) Legend Zone RMR UCS (MPa) > 40 > 30 > 20 > 10 Soil Weathering grade W3 / W2 W4 W4 W4 W5 50 12 5 2 N/A Geotechnical zones