ENVIRONMENTAL ENGINEERING
Geogrids in civil engineering applications
Geogrid installation phases at Zimbali Lakes
Soil, from a mechanical interaction point of view, suppor ts compression, but much lower tension stresses. For this reason, when tensional resistance is required, the use of geogrids is essential. By Samantha Naidoo, Pr Tech Eng,* and Blaise Jacob, BSc (Eng)**
T
he uses of geosynthetics in civil engineering are relatively well defined according to their functions. They are employed, for example, to reinforce retaining walls, as well as sub-bases or subsoils below roads or structures. The following two case studies provide practical examples of optimum interventions that achieved the best engineered result.
CASE HISTORY 1 – ZIMBALI LAKES Geogrid used: MacGrid™ WG MacGrid WG is a geogrid for soil reinforcement made from high-molecular-weight, high-tenacity polyester multifilament yarns. The yarns are woven on tension in the specified machine direction and finished with a polymeric coating. MacGrid WG geogrids are engineered to be mechanically and chemically durable, and resistant to biological degradation.
Problem Zimbali Lakes – an elite, multigenerational living space – required the completion of various earthworks phases during its development. The initial construction required high fills to be constructed. The in-situ soft rock can be defined as clayey sand, while the inferred material classification is G10+, with a very low inferred shear strength of ɸ = 21 degrees.
Solution The high engineered fills were initially designed to reach a height of up to 15 m. They therefore required a high-strength geogrid with an ultimate tensile strength of 800 kN/m to improve the shear strength and global stability of the embankment. 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 geogrids was created with varying lengths of grids for different heights of fill. A woven geotextile was used as a separation and filtration layer, while the WG 80 specified was used to provide the strength required. 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 provide muchneeded stability to the embankment by limiting
IMIESA July 2021
Design The design was carried out according to the guidelines prescribed in BS 8006-1:2010 – Code of practice for strengthened/reinforced soils and other fills. The design is based on limit state design principles where ultimate and serviceability limit state considerations are accounted for.
Soil parameters considered in the design of the embankment A geotechnical investigation was performed to understand the current properties of the in-situ
TABLE 1 Mechanical properties of geogrid
Mechanical properties Tensile strength (MD) Strain at max strength (MD) Tensile strength (CMD) Strain at max strength (CMD) Demand analysis 2015 (millions per annum)
FIGURE 1 Zimbali Lakes: cross section of embankment with 22 m rockfill toe
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differential settlement and base sliding, and protect the embankment against internal and global failures.
Unit kN/m % kN/m % 31.3
Geogrid 805 10 105 12 20.56
Test method EN ISO 10319 EN ISO 10319 EN ISO 10319 EN ISO 10319 12.96
FIGURE 2 Zimbali Lakes: cross section of embankment with 16 m rockfill toe
