Passive nails¶
Passive nails (rock bolts, soil nails) reinforce the wedge by mobilising bar resistance against shear and axial load as the wedge tries to move. They follow NTC 2018 §6.7 with the GeoStru SoilNail Variant A formulation (consistent with the FHWA-NHI-09-035 reference).
Capacity of the single nail¶
The nail has 3 axial limits + 1 shear limit:
| Limit | Symbol | Formula |
|---|---|---|
| Bar yielding | Ta1 | η · fy · π·(D−4)² / 4 |
| Bar ↔ grout bond | Ta2 | β · √fck · π · (D−4) · Lmm · FS⁻¹ |
| Hole pull-out (in rock) | Ta3 | π · Dhole · Lmm · τult · FS⁻¹ with τult = 0.1·σc |
| Hole pull-out (in soil) | Ta3 | (π·D·c' + 2·Dhole·Kα·σv·tan φ) · L · FS⁻¹ |
| Bar shear (Tresca) | Ta4 | 0.5 · fy · Anet |
Axial capacity: \(T_{max} = \min(T_{a1}, T_{a2}, T_{a3})\). Ta4 enters separately in the N-V interaction.
For nails under NTC §6.7 (Variant A) neither ξ nor γRa,t is applied: the design capacity equals Tmax (optionally divided by an extra local FS if required by the specification).
Clouterre N-V interaction¶
A passive nail crossing the sliding plane works axially and in shear simultaneously. Clouterre 1993 (FHWA-NHI-09-035) uses a linear envelope:
The software picks the corner of this envelope that maximises the stabilising contribution along the failure plane:
The optimum lies on one of the two axes:
- Axial mode: T = Tmax, V = 0
- Shear mode: T = 0, V = Vmax
For typical geometries (α + Δ ≈ 30°–60°) and moderate friction angle, axial mode wins. Shear mode wins for very flat planes (α + Δ → 0°).
Input fields¶
| Field | Notes |
|---|---|
| φ bar [mm] | bar diameter |
| L effective [m] | length crossing the failure plane and anchoring into intact rock |
| φ hole [mm] | borehole diameter (= bar diameter for self-drilling bars) |
| z avg [m] | average bulb depth from ground surface |
| α ref. [°] | reference inclination of the nail (for Kα Caquot in soil) |
| fy [MPa] | bar yield strength (typical 391 B450C, 826 SRS, 950 HD) |
| η · work rate [%] | steel work rate (default 70% per GeoStru desktop; 100% for full ULS yield) |
| fck grout [MPa] | cubic compressive strength of cement grout (typical 25–35 MPa) |
| β · steel↔grout bond [-] | 0.5 plain bars · 0.7–1.0 ribbed bars · 1.0 self-drilling |
| in rock / in soil toggle | switches between σc rock and (γ, φ, c) soil parameters |
| σc rock [MPa] | rock compressive strength (15–100 MPa typical) |
In the right panel "Design resistances · nails / anchors" you see the complete breakdown + NTC R.2/R.3/R.4/R.5 checks.
Typical use¶
- Surface stabilisation of slopes: nails 3–6 m long crossing the failure plane shallowly
- Tunnel portals: forepoling with 6–9 m self-drilling nails
- Cut faces: pattern nailing 2.5×2.5 m spacing, φ32 bars