Uplift plates & anchors
Shallow and deep breakout capacity for bearing plates and helices, plus grouted ground-anchor bond capacity.
Tension members — tie-downs, guyed-tower foundations, transmission anchors — resist uplift in one of two fundamentally different ways. A bearing plate or helix lifts a wedge of soil and resists pull-out by breakout; a grouted ground anchor resists by side bond along the grout–soil interface. The uplift tool covers both, and this page explains each mechanism and every input it uses. For tension on a drilled shaft with a belled base, see the belled-uplift method instead.
Two mechanisms
The two modes in the tool are not variations on one formula — they engage the ground in opposite ways, and the right choice follows from the hardware:
- Plate / helix — breakout. A bearing plate, helix, or anchor pad is pulled upward against the soil above it. Resistance comes from lifting a cone or column of soil: its weight (in sand) or the shear it takes to tear the failure surface (in clay). Capacity is a breakout factor times the soil resistance times the plate area.
- Grouted anchor — side bond. A grouted bond zone transfers load to the soil through shear stress along its cylindrical surface. Resistance is the grout–soil bond stress acting over the interface area; nothing is lifted wholesale.
Pick the tab that matches the element. The plate path needs the soil profile that builds the breakout cone; the anchor path needs only the bond geometry and the interface bond stress.
Plates & helices
A plate or helix resists uplift by breakout. The capacity is a dimensionless breakout factor times a soil resistance times the bearing area. The two soil types use different factors and different resistances:
In sand the resistance is the effective overburden γ·L and the frictional breakout factor B_q grows with the friction angle φ. In clay the resistance is the undrained strength Cu and the cohesive factor B_c rises with embedment but is capped at 9 — the deep-bearing limit. Both then multiply the plate area A.
The width or diameter of the bearing plate or helix.
Why it matters. Together with the embedment it sets the L/D ratio that decides shallow vs deep breakout behaviour. Default 0.6 m.
The bearing area of the plate or helix that the breakout pressure acts on.
Why it matters. Capacity scales directly with it — the breakout factor × γ·L (sand) or Cu (clay) gives a pressure, and area turns that into a force. Default 0.283 m² (a 0.6 m circular plate).
The depth of the plate below the ground surface.
Why it matters. A deeper plate mobilizes a larger soil cone and raises both γ·L and the breakout factor — up to the critical depth, beyond which the failure becomes local and added depth contributes through side resistance instead. Default 3 m.
Whether the soil is cohesionless (sand) or cohesive (clay).
Why it matters. It selects the whole breakout model: sand uses the frictional factor B_q(φ) on γ·L, clay uses the cohesive factor B_c (capped at 9) on Cu. The parameters below adapt to the choice.
The unit weight of the soil above the plate.
Why it matters. It sets the weight of the soil cone resisting pull-out and the overburden γ·L that drives the sand breakout pressure. Use buoyant unit weight below the water table. Default 18 kN/m³.
The drained angle of internal friction of the sand.
Why it matters. It controls the critical depth and the frictional breakout factor B_q — a few degrees materially change capacity. Used only for cohesionless soil. Default 32°.
The undrained shear strength of the clay.
Why it matters. The cohesive breakout capacity is roughly B_c·Cu·A, so it scales the plate resistance directly. Used only for cohesive soil. Default 50 kPa.
Grouted ground anchors
A grouted anchor carries no breakout cone. Load transfers to the soil as shear along the cylindrical surface of the bonded zone, so the capacity is simply the interface area times the bond stress:
A larger grout diameter D and a longer bonded length L_b both add interface area and carry more uplift — but only up to practical efficiency limits, because bond stress is not perfectly uniform over a long bond. The unit bond stress Ca comes from the soil type and the installation method.
The diameter of the grouted bond zone.
Why it matters. Capacity Qu = π·D·L_b·Ca is proportional to the grout–soil interface area, so diameter scales it directly. Default 0.15 m.
The length of the bonded (grouted) zone that transfers load to the soil.
Why it matters. It is the load-transfer length — a longer bond carries more uplift, up to the practical efficiency limit where added length stops contributing proportionally. Default 8 m.
The grout-to-soil bond (adhesion) stress — the ultimate unit side resistance at the interface.
Why it matters. It is the stress that side bond mobilizes, set by the soil type and installation method (pressure grouting raises it). It scales the whole anchor capacity. Default 150 kPa.
Shallow vs deep breakout
For a plate, how the soil fails depends on how deep it is buried relative to its width. There is a critical depth L_cr — a function of the friction angle φ in sand — that separates two regimes:
- Shallow mode (L ≤ L_cr): the full soil cone reaches the surface. The entire wedge above the plate is lifted, and the plate breakout term carries the whole capacity.
- Deep mode (L > L_cr): the soil cone no longer daylights. Failure is local around the plate, the breakout factor is capped, and the plate term is supplemented by side resistance mobilized over the length (L − L_cr) above the local failure zone.
Why the mode matters
Reading the results
The headline figure is the ultimate uplift Qu in kN. For a plate the tool also reports the breakdown that produced it; for a grouted anchor it reports the single side-bond capacity.
Plate / helix results
- Critical depth L_cr — the embedment beyond which breakout becomes local (deep mode). Above it the cone daylights; below it does not.
- Breakout factor — the dimensionless coefficient B_q (sand) or B_c (clay) that multiplies γ·L or Cu to give the breakout pressure.
- Plate capacity — the breakout resistance of the plate itself, the dominant term in shallow mode.
- Side resistance — the shaft side resistance over (L − L_cr) in deep mode, which adds to the plate breakout when the plate is deep.
- Mode badge — shallow or deep, telling you which failure regime governs.
Grouted-anchor result
For an anchor the tool reports the ultimate uplift only — the side-bond capacity π·D·L_b·Ca. There is no breakout cone, critical depth, or mode to report. Divide the ultimate value by an appropriate factor of safety to get the allowable tension load, and confirm your inputs follow the project units and conventions.