Calculate required embedment for a laterally loaded post or caisson using the module’s constrained or nonconstrained soil-pressure model. The calculation combines the applied lateral loads into a resultant force and height, then iterates to find embedment depth.
Quick start
- Enter a member description and above-grade post height in feet.
- Enter the post/caisson diameter in inches, passive-pressure increase with depth in pcf, and maximum passive pressure in psf.
- Select Constrained or Not constrained to match the actual support condition at grade.
- Enter the point and distributed lateral loads and their heights above grade.
- Run the calculation. Review required embedment, allowable soil pressure, the load diagram, and detailed calculations.
Load entry and resultant
A point load uses force in pounds and a height above grade. A distributed load uses intensity in lb/ft over a height starting at grade. A partial distributed load uses intensity in lb/ft and bottom and top heights above grade.
Load extents must lie within the entered post height. For a partial load, the bottom must be below the top. Complete every field in a used row and enter at least one lateral load.
The calculation sums lateral force P and moment M about grade and uses h = M/P as the equivalent load height. Distributed loads act at the centroid of their loaded length. Review these intermediate values in the report.
Soil pressure and iteration
The passive-pressure input is the pressure increase per foot of depth; the maximum input caps the pressure used in the calculation. These are separate inputs with different units.
For Not constrained, the implemented pressure is S1 = min[passive × min(d, 12)/3, maximum passive]. It calculates A = 2.34P/(S1b) and d = 0.5A[1 + √(1 + 4.36h/A)]. For Constrained, S3 = min(passive × d, maximum passive), and d = √[4.25Ph/(S3b)]. The calculation converts diameter b from inches to feet.
Because pressure depends on the unknown depth, the program starts with max(1 ft, h/2) and updates the trial depth using the average of trial and calculated depths. The default convergence tolerance is 0.5 inch, with a maximum of 200 iterations. The detailed output records the trial depths, pressure limits, substitutions, and convergence.
Results and scope
The result is required embedment in feet, together with the soil pressure used. Use the calculation report to review the load resultant and the selected constrained/nonconstrained method. The software does not establish that a physical restraint exists at grade; select the condition to match the project detail.
This module calculates lateral-load embedment. It does not design caisson reinforcement, check post bending strength, or calculate axial bearing, settlement, or uplift capacity. Soil inputs must come from the applicable project criteria.