Geogrids in civil engineering applications

from IMIESA July 2021

Admixture accelerators for cold conditions

Geogrid installation phases at Zimbali Lakes

Soil, from a mechanical interaction point of view, supports 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)**

The 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 differential settlement and base sliding, and protect the embankment against internal and global failures.

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)

Unit Geogrid Test method

kN/m 805 EN ISO 10319 % 10 EN ISO 10319 kN/m 105 EN ISO 10319 % 12 EN ISO 10319

Demand analysis 2015 (millions per annum) 31.3 20.56 12.96

FIGURE 1 Zimbali Lakes: cross section of embankment with 22 m rockfill toe FIGURE 2 Zimbali Lakes: cross section of embankment with 16 m rockfill toe

TABLE 2 Factors of safety achieved

Method name FOS achieved 16 m rockfill toe

Simplified Bishop 1.320

Spencer GLE/Morgenstern-Price 1.319 1.318

Strain at max strength (CMD)

% Demand analysis 2015 (millions per annum) 31.3

FOS achieved 22 m rockfill toe

1.354 1.353 1.352 12 20.56

material. The unit weight, friction angle and cohesion were important parameters required for the design and calculations undertaken.

The high-strength geogrid mechanical properties can be seen in Table 1.

Design analysis

The Mohr-Coulomb failure criterion was considered in the analysis. Fundamentally, the Mohr-Coulomb failure criterion represents the linear envelope that is derived from correlation between the shear strength of a material versus the applied normal stress. Factors of safety (FOS) required were above a minimum of 1.3, which was achieved (as shown in Table 2).

CASE HISTORY 2 – M4 DURBAN

Geogrid used: ParaGrid 100 and ParaGrid 150 ParaGrid geogrids are planar structures consisting of a biaxial array of composite geosynthetic strips. The strips comprise a core of high-tenacity polyester tendons encased in a polyethylene sheath.