Instantly compute the pull-out capacity of a ground anchor — essential for retaining walls, earth-retention, dam foundations and transmission towers that have to be "pulled down and held" against uplift. From the borehole diameter, bond length and bond strength, plus the safety factor and design load, you can read the utilisation T/T_allow at a glance.
Parameters
Borehole diameter d_h
m
Diameter of the drilled hole. Typical anchors use 90-150 mm.
Bond length L_b
m
Length of the section where the grout is bonded to the ground and transfers load.
Bond strength tau_b
kPa
Average ground-grout bond strength. Roughly 600-1200 kPa for weathered rock.
Design load T
kN
Design tensile force acting on a single anchor.
Safety factor F_s
Typically 2.5-3.0 for permanent anchors, 1.5-2.0 for temporary.
Results
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Bond perimeter pi*d_h (m)
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Ultimate capacity T_ult (kN)
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Allowable capacity T_allow (kN)
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Design load T (kN)
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Utilisation T/T_allow (%)
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Pull-out capacity verdict
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Ground anchor side view — bond-stress distribution animation
A borehole is drilled into the ground behind the retaining wall and an anchor is inserted. Over the bond length L_b, the grout and the surrounding ground resist by shear (bond) stress. The arrows show the bond stress on the borehole wall (pointing toward the surface = resistance against pull-out).
Bond-length sensitivity — T_ult is proportional to L_b
Safety-factor sensitivity — utilisation T/T_allow is proportional to F_s
The ultimate pull-out capacity T_ult is the borehole perimeter (pi*d_h) times the bond length L_b times the bond strength tau_b — the uniform-bond value over the side surface of the bond zone. The allowable capacity T_allow follows by dividing by the safety factor F_s.
Above 100% utilisation the design load T exceeds the allowable capacity (NG). 80-100% is a tight-margin zone, and below 50% is the over-designed zone.
Note: in reality the bond stress concentrates near the head (ground-surface side) of the bond zone and decreases toward the bottom. The uniform-bond assumption is a first-cut estimate for early design; on critical structures, tau_b must be corrected with site pull-out tests.
What is the pull-out capacity of a ground anchor?
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I keep seeing "ground anchor" in civil-engineering news. What is it, really? Is it different from a normal anchor bolt?
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Good question. A ground anchor is a device that "uses the ground itself as a reaction body to drag a structure into the earth and hold it down". Unlike an anchor bolt driven into concrete, you drill 10-20 m into the ground, fix a high-strength steel strand in the bottom few metres with grout (non-shrink mortar), and then jack the strand at the surface to put a permanent tensile force into the structure. In short, it is a construction method that "plants a giant screw into the ground and bolts the structure to the Earth".
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I see! But how can it carry that much tension underground? It looks like the grout is just "stuck" to the soil…
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That is exactly the crux: everything is decided by the bond (shear) strength tau_b at the ground-grout interface. When you pull the anchor, shear stress builds up along the side of the grout cylinder. As long as that stress stays below the bond strength of the ground, it does not pull out. So the pull-out capacity is simply "perimeter x bond length x average bond strength" — that is, T_ult = pi*d_h*L_b*tau_b. Larger diameter, longer bond, harder ground = larger capacity. Exactly as intuition says.
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Then if I just set the bond length to 20 m or so, I can keep adding capacity, right?
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In theory, yes. But in reality the bond stress is not uniform along the whole bond length. When the anchor is loaded, stress concentrates in the first few metres near the head (ground-surface side) and tapers toward zero at the bottom. So if you push the bond length past about 10 m the returns drop off. In practice the rule is "set the bond length at 6-10 m as the baseline; if that is still not enough, make the diameter larger or add more anchors". The uniform-bond formula in this tool is conservative and easy to handle, which is why it survives as the go-to early-design tool in the industry.
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Is the safety factor of 2 also a margin for the "not really uniform" part?
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Roughly, the safety factor absorbs three uncertainties: (1) the bond stress is not actually uniform, (2) ground scatter — tau_b can move +/-30% even in the same layer, and (3) long-term creep and groundwater effects. The Japanese Anchor Association uses F_s = 2.5-3.0 for permanent anchors (50+ years of service) and F_s = 1.5-2.0 for temporary anchors (only a few years during construction). Even more important, every production anchor is acceptance-tested at 1.25 times the design load. The reliability comes from this two-layer approach: desk calculation plus a site test on every anchor.
Frequently Asked Questions
The most basic design equation assumes that the side surface of the grout cylinder in the bond zone resists with a uniform bond stress, giving T_ult = pi*d_h*L_b*tau_b. Here d_h is the borehole diameter, L_b is the bond length (the part of the grout that is in intimate contact with the ground) and tau_b is the average bond strength between ground and grout (kPa). The ultimate capacity is divided by the safety factor F_s (typically 2.0 to 3.0) to give the allowable capacity T_allow = T_ult / F_s, which is then compared with the design load T. This tool runs that calculation in real time for every parameter.
No — in reality it is not uniform. When an anchor is loaded, strain concentrates near the head (ground-surface side) of the bond zone and the bond stress is largest there. It decays toward the bottom, and on long anchors with a bond length over 10 m the lower end can carry almost no stress. Even so, the uniform-bond assumption survives in standard design because (1) it is simple and conservative, and (2) the average tau_b can be confirmed by site pull-out tests. On critical structures, the stress distribution is checked separately by analysis or in-situ tests.
It varies strongly with ground type. Typical guide values: 600-1200 kPa for weathered or soft rock, about 1000-2500 kPa for hard rock, 300-800 kPa for dense sand or sandy gravel, and 100-300 kPa for silt or clayey soil. In soils it also depends on overburden pressure (depth). Use these guide values for first-cut design, but in the later design stage always confirm tau_b with site borings, in-situ tests and pull-out tests. Ordering dozens of permanent anchors based on a single boring is extremely risky.
It is split into three stages: (1) suitability tests on trial anchors, (2) verification tests on a few production anchors before full installation, and (3) acceptance tests on every production anchor. Suitability tests load the anchor in steps to 1.5-2.0 times the design value and record displacement and creep to verify that the bond length is sufficient. Acceptance tests typically load to about 1.25 times the design load and check that the elongation and the displacement during the hold period meet the creep criterion. Running the acceptance test on every anchor is the defining feature of the method, ensuring the quality of each individual anchor.
Real-World Applications
Stabilising cut and embankment slopes: On road and railway cut slopes, ground anchors are deployed in large numbers to suppress sliding along discontinuities in the rock. The surface is covered with shotcrete and anchor plates while the interior is "stitched" against the weight of the slope. On cut sections of the New Tomei Expressway or the Chuo Expressway, it is not unusual to see several hundred anchors per kilometre. Design loads of 400-1200 kN per anchor and bond lengths of 6-12 m are the standard range.
Earth-retention walls and self-standing retaining walls: For deep excavations in urban areas (subway stations, underground car parks, basement floors of buildings), ground anchors are attached to diaphragm walls or sheet piles so that the reaction against the backfill earth pressure is taken from inside the ground. Unlike strutted excavations there is no temporary support inside the excavation space, which is a huge advantage for heavy-machinery operation and for the construction of the main structure. The method was widely adopted on station works of the Ginza Line and Oedo Line in Tokyo.
Uplift prevention of concrete dams and weirs: The base of a gravity dam or weir is subjected to buoyancy and overturning moments from the upstream water pressure. By driving ground anchors into the foundation rock and "tying" the dam body to the rock, stability can be secured without increasing the concrete section. It is also the method that was massively added to existing dams during the seismic-retrofit reviews after the 1995 Great Hanshin Earthquake.
Transmission towers and wind-turbine foundations: Tall towers are subjected to large overturning moments from wind, and an uplift force is generated on one side of the foundation. If the foundation sits on rock or stiff ground, extending ground anchors from the footing and fixing them in the ground is more economical than deep pile foundations. The same approach is now applied to the foundations of large wind-turbine towers, which are rapidly increasing in number.
Common Misconceptions and Pitfalls
The biggest pitfall is "deciding the bond strength tau_b only from literature values and skipping site tests". tau_b depends not only on the ground type but also on the condition of the borehole wall (whether slime remains), the grout injection pressure and the curing conditions. It is not unusual for it to vary by +/-30-50% even within the same stratum. Even if the nominal safety factor is 2, a tau_b that is overestimated by a factor of 2 leaves you with essentially no real margin. "Skipping the site pull-out test because it costs too much" ends up being far more expensive. Especially for permanent anchors, always measure tau_b with several test anchors first and only then finalise the design value.
Next, "believing that capacity grows linearly with bond length". The formula T_ult = pi*d_h*L_b*tau_b is linear in L_b, but only under the assumption that the average bond strength tau_b is uniform along the whole length. In reality the bond stress concentrates near the head (ground-surface side), so doubling the bond length from 6 m to 12 m only raises the pull-out capacity by about a factor of 1.5. In many cases it is more effective to make the diameter larger. In design practice it is recommended to correct the length-efficiency factor using empirical formulas (Bustamante & Doix etc.) based on site test data.
Finally, "confusing the anchor capacity with the tensile strength of the steel strand". What this tool computes is the pull-out capacity governed by the "ground-grout bond", not the tensile strength of the steel strand itself (which is in the thousands of kN). In real design four failure modes must all be checked: (1) tensile rupture of the steel strand, (2) bond between grout and steel, (3) ground-grout bond (this tool), and (4) bearing of the structure (bearing plate, retaining wall). The smallest of these governs. In most cases (3) governs, but on soft ground or with short anchors (1) or (4) can take over, so all four must be checked.
How to Use
Enter the anchor hole diameter (mm) — typical range 150–300 mm for soil anchors in retaining wall applications.
Input the bond length (m) — the embedded anchor length in stable soil, usually 4–8 m depending on soil profile and required capacity.
Specify the soil-grout interface shear strength (kPa) — typically 50–150 kPa for clay, 80–200 kPa for sand, determined from site investigation.
Enter your design load (kN) — the factored tensile load the anchor must sustain.
Review the bond perimeter, ultimate pull-out capacity, allowable capacity (with safety factor ~1.5), utilisation ratio, and pass/fail verdict.
Worked Example
Consider a permanent ground anchor for a 12 m deep diaphragm wall in stiff clay. Hole diameter = 200 mm, bond length = 6 m in competent clay, interface shear strength = 120 kPa, design load = 500 kN. Bond perimeter = π × 0.2 = 0.628 m. Ultimate capacity = 0.628 × 6 × 120 = 451.9 kN. With safety factor 1.5, allowable = 301 kN. Since design load (500 kN) exceeds allowable capacity (301 kN), the verdict is FAIL — either increase bond length to ~10 m or enlarge hole diameter to 280 mm.
Practical Notes
Shear strength values must come from pull-out tests, triaxial testing, or empirical correlations with SPT N-values; do not assume; underestimation risks anchor slippage in permanent works.
For temporary anchors (6–12 months), safety factor 1.3 is acceptable; for permanent anchors in dams and critical slopes, use 1.5–2.0.
Bond length effectiveness reduces in weathered rock or loose sand; verify with borehole logs and consider sacrificial anchor testing before full production installation.
Tension cracks above the bond zone can reduce effective shear transfer; always keep anchor below groundwater table if possible to maximize interface friction.