# Foundations

This group answers the same question two ways: how much load the ground will take, and how to hold it back once you open it up. On capacity, start with the bearing capacity calculator — friction angle, cohesion, embedment and footing width settle the allowable pressure — then the pile capacity calculator for shaft and base resistance from SPT or undrained shear strength. On support, the earth pressure calculator gives K0, Ka and Kp, feeding the retaining wall check where sliding and overturning must both clear 1.5.

What goes wrong here is rarely the formula and almost always the parameters. Ignore groundwater and the three bearing terms need the submerged unit weight instead of the bulk one: sand drops from 18 to 8.2 kN/m³ and capacity falls by a third. Skip drainage behind a retaining wall and hydrostatic pressure doubles the horizontal thrust — most retaining wall collapses are drainage failures, not strength failures. And shaft and base resistance mobilise at settlements an order of magnitude apart, so adding both ultimate values and dividing once is optimistic, which the pile capacity notes spell out.

- Category page: https://jianzhujisuan.com/en/c/foundations
- Data checked: 2026-10-25

## Tools in this group

- [Pile Capacity Calculator](https://jianzhujisuan.com/en/pile-capacity): N value for sand, cu for clay: shaft and base worked separately, then divided by a factor of safety.
- [Bearing Capacity Calculator](https://jianzhujisuan.com/en/bearing-capacity): The three bearing factors follow from the friction angle; divide the ultimate value by a factor of safety.
- [Retaining Wall Stability Calculator](https://jianzhujisuan.com/en/retaining-wall): Soil and surcharge produce moments at different heights; both factors of safety must pass.
- [Earth Pressure Calculator](https://jianzhujisuan.com/en/earth-pressure): All three coefficients at once; the gap between active and passive is where the money is.

## Questions

### How do I calculate soil bearing capacity?

Get Nc, Nq and N-gamma from the friction angle, put them into qu = c Nc + gamma Df Nq + 0.5 gamma B N-gamma and divide by a factor of safety of 2 to 3. At phi 30 degrees, c zero, gamma 18, 1.5 m embedment and a 2 m footing, that is about 900 kPa ultimate and 300 kPa allowable. Below the water table switch to the submerged unit weight or you over-predict by more than 30%.

### How do I estimate single pile capacity?

Shaft plus base. For sand use SPT with fs = 2N kPa and qb = 400N kPa; for clay use fs = 0.5cu and qb = 9cu. A 600 mm pile, 20 m long in sand with N 20 gives about 3770 kN ultimate and 1508 kN allowable at a factor of safety of 2.5. That is a preliminary figure — a real design needs a static load test.

### How wide does a retaining wall need to be?

A base width of 0.5 to 0.7 times the height for a gravity wall, but check the two factors of safety: sliding at 1.5 minimum and overturning at 1.5 to 2.0. Also check the eccentricity against B/6 — beyond it the heel lifts and the peak base pressure exceeds what the formula predicts.