The unbalanced thrust at a bend, tee, dead end, or reducer — and the concrete bearing area needed to hold it against the soil. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: A thrust block only works if it bears against undisturbed soil at a value a geotechnical engineer will stand behind. The safe-bearing presets here are textbook starting points; saturated, disturbed, or organic soil can carry almost nothing, and in that case the answer is restrained joints, not a bigger block. Size for the test pressure, which is higher than working pressure. This covers horizontal bearing thrust; vertical (uplift) bends need weight blocks or restraint.
Results
Pipe cross-sectional area
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Unbalanced thrust
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Safe bearing (design)
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Required bearing area
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Suggested block face
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How the Numbers Are Derived
Thrust and bearing area
Internal pressure pushes on the pipe's cross-sectional area, A = π/4 · OD². The unbalanced thrust depends on the fitting: a bend is T = 2 · P · A · sin(θ/2), a dead end, cap, or tee branch is T = P · A, and a reducer is T = P · (A₁ − A₂). With P in psi and A in square inches, T comes out directly in pounds. The block's bearing area is Ab = T · SF / Sb, where Sb is the soil's safe bearing value in psf and SF the safety factor — giving Ab in square feet.
Field notes
The thrust acts along the bisector of the bend, outward. The block goes on the outside of the bend, poured against undisturbed trench wall — never against backfill, which hasn't the bearing to resist anything.
Size for test pressure, not working pressure. The joint sees its highest load during the hydrostatic test, and that's what has to be restrained.
Keep concrete off the joint and bolts. The fitting must stay accessible and repairable; wrap or form so the block bears on the fitting body, not the mechanical joint.
In poor soil — high water table, organics, disturbed fill — the safe bearing value approaches zero and blocks get absurdly large. That's the signal to switch to restrained joints, which carry the thrust in the pipe wall over a calculated length instead of into the soil face.
Vertical bends that thrust upward need a weight (gravity) block or restraint, not a bearing block — this tool sizes the horizontal bearing case.
References
DIPRA — Thrust Restraint Design for Ductile Iron Pipe.
AWWA M41 (ductile iron) and Uni-Bell PVC design guidance — thrust blocking and restraint.