Geofabrics pullout capacity & care to take while Tensile Strength of Geofabric ≠ Pull-Out Resistance

SanjayShukla
SanjayShukla Posts: 12
in GeoStudio

I am discussing a Landfill site with Geofab as a part of capping liner system where overlaying soil depth is limited and require slope stability assessment and to check any contribution from Geofab for FOS ?!?!?

1️⃣ Tensile Strength of Geofabric ≠ Pull-Out Resistance

If the geofabric has:

  • Ultimate tensile strength = 21 kN/m

This value is already per metre width.
So whether the roll is 6 m or 8 m wide doesn’t change the design strength per metre strip.

For a 6 m roll:

  • Total ultimate capacity across full width = 6 × 21 = 126 kN
    But in slope stability or anchorage calculations, we always work per metre width, so:

Design tensile capacity = 21 kN/m (before reduction factors)

Pull-out failure occurs when interface resistance + anchorage resistance is less than the tensile force mobilised in the geofabric.

2️⃣ What Resists Pull-Out?

Pull-out resistance comes from:

✔ 1. Friction + Adhesion (Interface Shear Resistance)

Between:

  • Geofabric
  • Underlying soil
  • Overlying soil

✔ 2. Normal Stress from Overburden

σv=γ×H

For example:

  • γ = 18 kN/m³
  • H = 0.3 m

σv=18×0.3=5.4 kPa

3️⃣ Interface Shear Resistance

For Example:

  • Cohesion (c) = 5 kPa
  • δ = 11°

Shear resistance per interface:

τ=c+σvtan⁡δ

=5+(5.4×tan⁡11∘)

tan⁡11∘≈0.194

τ=5+(5.4×0.194)

=5+1.05

= 6.05 kPa

Since you have top and bottom interface, multiply by 2:

2×6.05=12.1 kPa

This means:

Pull-out resistance from soil friction = 12 kN per metre length of embedment

Because:

kPa × 1 m width = kN/m

So yes — 12 kN/m of embedment length.

4️⃣ Anchor Trench Contribution

If a 700 × 700 trench gives about: ≈ 8 kN/m resistance

Then total pull-out resistance per metre strip:

12+8=20 kN/m12 + 8 = 20 \text{ kN/m}12+8=20 kN/m

and this is already very close to the 21 kN/m tensile capacity of the geofabric.

That means:

⚠ You are at the limit.

After applying reduction factors (creep, installation damage, durability), allowable tensile strength will be much lower — maybe 10–14 kN/m.

So in reality:

Pull-out capacity may control before tensile rupture.

5️⃣ Why “100 kN/m Pull-Out” on YouTube is Misleading

People often confuse:

  • Ultimate tensile strength of heavy geogrids (not geofabrics)
  • Long embedment lengths (3–6 m)
  • High overburden (2–5 m soil cover)
  • Granular backfill with φ = 35–40°

For example:

If:

  • H = 3 m
  • γ = 20 kN/m³
  • φ = 35°
  • δ ≈ 30°
  • C=0

σv=60 kPa

τ=60tan⁡30∘≈35kPa

With 3 m embedment:

2×35×3=210kN/m

Now you see where big numbers come from.

But for:

  • 0.3 m cover
  • φ = 11°
  • Low cohesion

There is no way you reach 100 kN/m.

6️⃣ Important Point You Mentioned (Very Good Observation)

If soil is:

  • Dry
  • Non-cohesive
  • Loose sand

Then:

c≈0

So resistance becomes:

τ=σvtan⁡δ

With your values:

=5.4×0.194=1.05kPa= 5.4 \times 0.194= 1.05 kPa=5.4×0.194=1.05kPa

Double interface:

≈2.1kPa≈ 2.1 kPa≈2.1kPa

That is extremely small.

So yes:

In dry sandy soils with small cover thickness, pull-out resistance is very limited.

Anchor trench becomes critical.

7️⃣ Key Engineering Takeaway

For example:

✔ 300 mm cover
✔ γ = 18 kN/m³
✔ δ = 11°
✔ c = 5 kPa

Pull-out capacity ≈ 20 kN/m

Which is consistent and realistic.

The “100 kN/m” values only apply to:

  • Deep embedment
  • High normal stress
  • High friction angle
  • Geogrids in reinforced soil walls

Not for shallow geofabric anchorage.

So please be careful in selecting pull-out resistance of Geofabrics (Textile)

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