Civil Engineering December 2021 | Vol 29 No 11

26 December 2021 Civil Engineering T he Zimbali Lakes Resort is a multi- generational living development situ- ated on the North Coast of Durban. Earthworks needed to be conducted before the construction of the facilities, and the initial construction required high engineered fills designed to reach a height of up to 15 m. The excessive load, projected differential settlement, and the high water table necessitated control measures. There are often design constraints affiliated with construction of embank- ments taking place on the East Coast of South Africa, in particular Durban and surrounding areas, due to the soil charac- teristics of the area. The in-situ material that is found is often characterised by weak strength and high compressibility. Extensive site considerations and careful design measures must be carried out to ensure that premature bearing capacity failure is mitigated. Traditional methods of specialised geotechnical work are commonly used for soil improvement of embankments. However, there are a handful of projects that have tried and tested the technology of using high strength geogrids. The per- formance of an embankment is improved by using high strength geosynthetic ge- ogrids which can absorb the high imposed load and spread or dissipate these loads downwards into the ground. GEOGRID SOLUTION In the case of Zimbali Lakes Resort, a vast amount of earthworks needed to be carried out on soils that have low shear strength, high compressibility and time-related settle- ment issues. Reinforcement was required to increase the shear strength of the soil before it could support the self-weight of the fill. The in situ cut banks beneath the fills were to be reduced to a maximum of 1:2 to reduce the differential settlement effects propagating from the road surface. Further to this, to reduce the water table, a rockfill heel using 800 kN/m (UTS) geogrids was created with varying lengths of grids for varying heights of fill. A woven geotextile was used as a separation and filtration layer, while the high strength geogrid was used to provide the tensile strength improvement. The moisture within the fill may cause time-related settlement problems. The adjudged outcome being that, as the pore water dissipates, the embankment settles. The rockfill toe with a layer of geogrid on the upper and lower bound assists with the separation of material and accelerates the consolidation of the fill. The high strength properties of the geogrid provide much needed stability to the embankment by limiting differential settlement and base sliding and protect the embankment against internal and global failures. Using high strength geogrids for the basal improvement of the embankment required a detailed feasibility analysis and systematic design approach with accurate computations. The design was carried out according to the guidelines prescribed in the BS 8006-1:2010 code of practice for strengthened/reinforced soils and other fills. During construction the performance of the geogrid is most critical as the low permeability of the soil does not allow for consolidation to occur in the normal time frame of construction. It is therefore important that the short-term shearing strength improvement provided by the geogrid is, at all times, greater than or equal to the applied loads until consolida- tion is reached. INCREASING UPTAKE The use of high strength geogrids is being cultivated in the South African construc- tion industry as engineers develop a greater understanding and trust in the benefits that they provide. The number of projects using high strength geogrids is increasing and the sustainability aspect, providing a 120-year design life, is encouraging. The carbon footprint of geosynthetics is also much lower than that of traditional methods, thus providing an environmentally friendly solution.  SANRAL SA I CE 2021 Nat i onal Awards Category: Techn i cal Excellence FINALIST Technical Excellence category (Sponsored by Maccaferri SA) KEY PLAYERS Client: Zimbali Lakes Consultant: SMEC South Africa Contractor: Fountain Civil Engineering The use of high strength geogrids at Zimbali Lakes Resort Placing the geogrids with correct orientation