Woven Geotextiles and the Mohr–Coulomb Failure Envelope: Rethinking Soil–Textile Interaction

In geotechnical engineering, the Mohr–Coulomb failure envelope is the cornerstone of understanding shear strength. It governs how soils behave under load, and by extension, how engineered reinforcements—like woven geotextiles—alter that behaviour.

Yet, despite the widespread use of woven geotextiles in slope stabilization, embankments, and retaining walls, the connection between textile properties and the Mohr–Coulomb parameters (cohesion c and friction angle φ), is often oversimplified in design practice.

This article examines the deeper interplay between woven geotextiles and the Mohr–Coulomb framework, giving seasoned professionals new insight into design optimisation and field performance.

1. Understanding the Mohr–Coulomb Failure Envelope

The Mohr–Coulomb criterion defines the shear strength (τ) of soil as: 

                                 τ = c + σ′ · tan(φ)

Where:

• c = soil cohesion (kPa) 
• σ’ = effective normal stress (kPa) 
• φ = internal friction angle (°) 

This linear relationship is plotted in stress space, with cohesion representing the y-intercept and friction angle determining the slope. It’s a simple yet robust way to model soil failure.

For design engineers, this equation predicts slip plane initiation and load capacity—critical in embankments, road subgrades, and reinforced soil structures.

2. Where Woven Geotextiles Enter the Equation

Woven geotextiles, particularly those made from tape x tape and tape x multifilament constructions, interact with soils by:

• Increasing apparent cohesion (c) through interlocking and particle restraint. 
• Modifying friction angle (φ) via interface friction between soil particles and the geotextile surface. 
• Providing tensile resistance across potential slip planes, effectively raising the overall shear resistance of the composite system. 

These effects are most pronounced in granular soils, but fine-grained soils also benefit from reduced deformation and delayed failure onset.

3. Interface Friction: The Overlooked Design Variable

While lab tests often characterise soil shear strength in isolation, field performance depends heavily on soil–geotextile interface friction.

Key factors influencing this include:

• Yarn surface texture – multifilament yarns often provide higher micro-interlocking than smooth tapes. 
• Weave tightness – higher ends and picks per inch reduce slip potential. 
• Polymer modulus – stiffer yarns resist shear displacement more effectively. 

Ignoring interface friction can lead to underestimating reinforcement performance in design calculations.

4. Shifting the Failure Envelope

When a woven geotextile is properly integrated into a soil mass, the Mohr–Coulomb envelope shifts upward and/or rotates:

• Upward shift → increase in apparent cohesion. 
• Rotation with increased slope → increase in effective friction angle. 

This directly translates to:

• Greater factor of safety in slope designs. 
• Reduced reinforcement length requirements in retaining structures. 
• Increased load-bearing capacity in subgrade reinforcement. 

5. Practical Implications for Engineers

For geotechnical designers, this means:

• Laboratory testing of soil–geotextile composites is far more reliable than soil-only tests. 
• ASTM D5321 (direct shear) and ASTM D6706 (pullout) tests provide the data needed to adjust c and φ in design. 
• The geotextile’s weave structure, yarn type, and surface treatment must be chosen in relation to the site’s soil type, not just strength class. 

Macfil Global’s Perspective

At Macfil Global Pvt Ltd, our woven geotextiles are engineered for predictable, test-verified shifts in the Mohr–Coulomb envelope. By tailoring weave geometry, yarn modulus, and polymer type, we optimise soil–textile interaction parameters—giving engineers the confidence to design beyond generic safety margins.

We encourage project teams to demand composite shear strength data from suppliers, ensuring reinforcement performance is quantified, not assumed.