Design a soil-nailing system that reinforces a cut slope or excavation face with grouted steel nails. Adjust the nail length, drill diameter, bond stress and spacing to see the pullout resistance, design tensile force and factor of safety against pullout for one nail update in real time.
Parameters
Nail length L
m
Total length of the steel nail drilled into the ground
Drill diameter d
mm
Outer diameter of the grouted nail (the friction surface)
Bond (side friction) stress q_s
kPa
Bond strength per unit area at the grout-soil interface
Nail spacing S
m
Vertical and horizontal grid spacing of the nails on the face
Wall height H
m
Height of the cut or excavation face being reinforced
Results
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Pullout resistance per nail (kN)
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Effective bond length (m)
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Tributary area per nail (m²)
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Design tensile force per nail (kN)
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Factor of safety vs pullout
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Pullout safety verdict
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Soil nail wall cross-section — tension animation
The excavated face is covered with shotcrete, and nails are drilled in a grid at a slight downward angle. The dashed line is the assumed failure surface; behind it is each nail's effective bond length (resistant zone). The pulsing colour shows the tension carried by the nails.
Pullout resistance T_pullout and pullout factor of safety FOS. d: drill diameter, L_bond: effective bond length, q_s: grout-soil bond stress. Only the nail length behind the assumed failure surface, anchored in the resistant zone, provides pullout resistance.
Effective bond length L_bond (taken as 60% of the nail length L) and the design tensile force per nail T_demand. Ka: active earth pressure coefficient (= 0.33), gamma: soil unit weight (= 18 kN/m³), H: wall height, S: nail spacing. 0.5*Ka*gamma*H is the average earth pressure over the wall height and S squared is the wall area carried by one nail.
What is Soil Nailing?
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I have never heard of "soil nailing". I have seen steep cut slopes with lots of steel bars sticking out of them — is that what this is?
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That is exactly it. When a steep cut has to be made into a slope — for a road, a building basement, or a railway — the freshly exposed face would, left alone, tend to slide. Soil nailing is one of the most economical ways to hold it. As the excavation is taken down in stages, a regular grid of holes is drilled into the exposed face, slightly downward, a steel reinforcing bar is inserted in each, and the hole is filled with cement grout. A thin sprayed-concrete (shotcrete) facing then ties the nail heads together. The result is a block of ground that has been internally reinforced.
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But why does just sticking bars in stop it from sliding? They are only rods.
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The mechanism is subtle and worth understanding. The soil behind the face naturally wants to slide along some curved or planar failure surface. The nails cross that surface, and the part of each nail behind the surface — anchored in the stable, resistant ground — grips the soil by friction along the grout-soil interface. As the soil mass begins to move outward, it stretches the nails, mobilising tension in them, and that tension pins the moving wedge back to the stationary ground. Soil nailing therefore creates a reinforced, self-stable soil block, much as the rebar in reinforced concrete works with the concrete.
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So they hold by being stretched. Can the nails ever simply pull out?
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Yes — that is the "pullout" failure this tool checks. If the bond length is too short or the grout-soil friction too weak, the nail simply slides out of the ground. The pullout resistance is the friction acting over the cylindrical surface of the embedded grouted length, so the formula is T_pullout = pi*d*L_bond*q_s. A larger drill diameter, a longer bond length, and a stronger bond stress all increase the resistance. Try shortening the nail length on the left and you will see the pullout resistance drop and the factor of safety fall.
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There is an "effective bond length" — so it is not the full nail length?
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That is the key point. The part of the nail in front of the failure surface moves together with the sliding soil mass, so it does nothing for pullout. Only the part anchored in the resistant zone behind the surface generates pullout resistance — that is the effective bond length. This tool assumes, as a safe estimate, that 60% of the nail length is the effective bond length. In real design the failure surface is located properly and the bond length is evaluated carefully, but that fraction is enough to build intuition.
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If the factor of safety is too low, what should I change?
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There are several levers. Making the nails longer increases the effective bond length and the pullout resistance. A larger drill diameter widens the friction surface and does the same. And tightening the spacing helps a lot — when the spacing is reduced, the wall area each nail carries shrinks with S squared, so the design tensile force per nail drops sharply. Look at the "FOS vs spacing" chart below: closing up the spacing raises the factor of safety steeply. In practice you combine nail length, drill diameter, spacing and grout quality so there is a margin against both pullout and bar rupture.
Frequently Asked Questions
The pullout resistance of one soil nail is the side friction acting on the grout-soil interface: T_pullout = pi*d*L_bond*q_s, where d is the drill diameter, L_bond is the effective bond length anchored in the stable resistant zone behind the failure surface, and q_s is the grout-soil bond stress. This tool takes the effective bond length as 60% of the nail length and computes the friction over that cylindrical surface as the pullout resistance.
The part of the nail in front of the assumed failure surface (on the sliding-wedge side) moves together with the soil, so it contributes nothing to pullout. Only the part anchored in the stable resistant zone behind the failure surface generates pullout resistance; this is the effective bond length. As a safe estimate this tool assumes the effective bond length equals 60% of the nail length. In practice the failure surface is located separately and the bond length is evaluated rigorously.
Each nail carries a square of wall area equal to the spacing S, so its tributary area is S squared. The average earth pressure over the wall height H is estimated with the active earth pressure coefficient Ka and the soil unit weight gamma as 0.5*Ka*gamma*H, and the design tensile force T_demand is that pressure times the tributary area. The pullout factor of safety is the pullout resistance divided by this demand, and is generally targeted at 2 or above.
For a single nail two failure modes are checked. One is pullout - the nail sliding out of the ground because the bond length is too short or the grout-soil friction too weak - which this tool covers. The other is tensile rupture of the steel bar itself. For the wall as a whole, global sliding along a failure surface and local failure of the shotcrete facing must also be checked. Each nail is designed with a margin against both pullout and rupture.
Real-World Applications
Cut slopes for roads and railways: When a mountain road or railway is built, slopes are cut at steep angles to limit the land take. Reinforcing the exposed cut face with soil nailing is faster and cheaper than building a new retaining wall. Because the excavation is taken down in stages and nails are added at each lift, no large temporary staging structure is needed. The method is also widely used to stabilise and repair existing slopes prone to failure.
Building basement excavation (shoring): When a basement is built in a city, a near-vertical cut is needed right up to the property line. A soil-nail-plus-shotcrete shoring wall does not block the excavation with struts or anchors, so the dig and the structural works proceed efficiently. The lighter the equipment footprint must be on a tight site, the more this nimbleness pays off.
Strengthening existing walls and natural slopes: Nails are installed behind an existing retaining wall that has cracked or tilted with age, or into a natural slope destabilised by rainfall, to recover the factor of safety against global sliding. An estimate like this tool gives a first read on "how many kN of pullout resistance per nail is needed", guiding the choice of nail length, count and layout.
Pre-study for geotechnical design: Before running a detailed limit-equilibrium or finite-element analysis, a simple equilibrium calculation like this tool gives a first read on the pullout factor of safety per nail. If the estimate is far short, the layout can be revised before investing in mesh and soil parameters. Conversely, if a detailed analysis differs from this estimate by an order of magnitude, it is a sanity check that points to a mistake in the soil parameters or the assumed failure surface.
Common Misconceptions and Pitfalls
The biggest pitfall is computing pullout resistance from the full nail length. Only the effective bond length, anchored in the resistant zone behind the assumed failure surface, contributes to pullout. The part in front of the failure surface is one with the moving soil wedge, so however long it is, it does not add to the pullout resistance. Misjudging the position of the failure surface and overestimating the bond length gives a nail that is safe on paper but easy to pull out in reality. This tool conservatively assumes the effective bond length is 60% of the full length, but in real design the failure surface must be located separately and the bond length evaluated rigorously.
Next, assuming the bond stress q_s is a single fixed number. q_s is the bond strength at the grout-soil interface and varies strongly with soil type (sand, gravel, cohesive soil, weathered rock), groundwater level, installation method (gravity grouting versus pressure grouting) and how much the drilling disturbs the hole. Even on one site, q_s can scatter by several times between layers. The design value should not be a catalogue figure copied across; it should be confirmed by on-site pullout tests. Remember that if groundwater or rainfall loosens the soil, q_s falls and the pullout resistance falls with it.
Finally, "if pullout is OK the design is done" is not true. The safety of a soil nail wall is not decided by the pullout of a single nail alone. Tensile rupture of the steel bar itself, global sliding of the whole wall along a failure surface, and local failure of the shotcrete facing (punching at the nail head) — all of these failure modes must be satisfied before the wall can be called safe. This tool only checks the single mode of pullout. In real design the stability of each lift during staged excavation, and long-term corrosion and creep, must always be checked as well.
How to Use
Set nail length (0.5–8 m) and drill diameter (25–40 mm) using sliders or numeric inputs.
Enter bond strength (50–200 kPa for sand/silt, 100–300 kPa for clay) and horizontal nail spacing (0.75–2.5 m).
Simulator calculates pullout resistance, effective bond length, tributary area, and factor of safety; verify FSpu ≥ 1.3 before design acceptance.
Worked Example
For a 6 m deep cut in medium clay: nail length = 6 m, drill diameter = 32 mm, bond strength = 180 kPa (grouted), spacing = 1.5 m horizontal × 1.5 m vertical. Effective bond length = 5.4 m (0.9 × 6 m, excluding unstable zone), pullout resistance = π × 0.032 × 5.4 × 180 = 97.2 kN per nail. Tributary area = 2.25 m². With design load 60 kN, FSpu = 97.2 / 60 = 1.62 (acceptable > 1.3).
Practical Notes
Bond strength reduces in saturated silts; apply 0.7–0.8 reduction factor if phreatic surface ≤ 2 m below cut face.
Unbonded stem (top 0.5–1.0 m) does not contribute to pullout; simulator auto-excludes this zone.
Verify pullout against soil friction angle and cohesion; sandy soils typically yield lower bond (50–100 kPa) than stiff clay (150–250 kPa).
Increase nail length or reduce spacing if FSpu falls below 1.3 to maintain stability during excavation stages.