Foundations calculators

Foundation calculators: soil bearing capacity, single pile capacity, retaining wall stability, earth pressure coefficients, slope stability, piling, excavation support, dewatering and cofferdams.

4 tools, most used first

▮Pile Capacity CalculatorN value for sand, cu for clay: shaft and base worked separately, then divided by a factor of safety.▤Bearing Capacity CalculatorThe three bearing factors follow from the friction angle; divide the ultimate value by a factor of safety.◫Retaining Wall Stability CalculatorSoil and surcharge produce moments at different heights; both factors of safety must pass.◣Earth Pressure CalculatorAll three coefficients at once; the gap between active and passive is where the money is.

What this group solves

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.

The entry point here is what you already have. With a ground investigation report showing SPT blows or undrained strength, go to pile capacity or bearing capacity. With only a soil description and a rough density, substitute typical values and lean on the factor of safety. For a retaining structure, fix the pressure case first — can the wall move or not — before checking stability. Getting that order wrong wastes the work: people size a wall, then look up the coefficient and use passive pressure, and end up with half the width actually needed, because Kp is nine times Ka at a 30 degree friction angle.

Excavation support has to be read as a set: earth pressure gives the horizontal force, slope stability asks whether the whole mass slides, piles and anchors provide the props, and dewatering sets the effective stress. They interact. Dewatering especially — every metre the water table drops adds 10 kPa of effective stress, which changes both base heave and settlement of everything nearby.

Which one for your case

SPT N or cu from a report — what will one pile carry	Pile Capacity Calculator
Friction angle and cohesion — how much the ground under a footing takes	Bearing Capacity Calculator
Base width for a wall, sliding and overturning checks	Retaining Wall Stability Calculator
Active, passive or at-rest coefficients and thrust	Earth Pressure Calculator
Will the excavation slope stand, and at what factor of safety	Slope Stability Calculator
Piles or anchors for excavation support, and their spacing	Soldier Pile Calculator
High water table — wellpoint layout and spacing	Wellpoint Dewatering Calculator
Piles are in — concrete and reinforcement for the pile cap	Pile Cap Calculator

Where this group goes wrong

Using bulk unit weight below the water table. Sand at 18 becomes 8.2 kN/m³ submerged, the width term falls from 403 to 184 kPa and allowable pressure from 300 to 227 — a 24% over-prediction that only shows up as settlement months later, long after handover.
Designing a retaining wall on soil pressure alone with no drainage. A 3 m wall carries 27 kN/m of active thrust; saturate the backfill and hydrostatic pressure adds 44 kN/m, multiplying the total by 2.6. Most retaining wall failures are drainage failures, not strength failures.
Adding ultimate shaft and base resistance and dividing once. Shaft friction peaks at about 0.5% of the diameter while end bearing needs 5-10%, so they do not act together and the sum is 20% or more optimistic.
Checking sliding but not eccentricity. Past e = B/6 the heel lifts and the peak base pressure exceeds the formula — the worked example passes both factors of safety at 1.75 and 2.78 yet reaches e = 0.324 m against a limit of 0.300 m, with −6 kPa at the heel.
Issuing a construction drawing from correlations. SPT methods scatter by about 30%; a piled foundation needs a static load test or dynamic testing, not an estimate.

FAQ

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.

What is the difference between bearing capacity and pile capacity?

Bearing capacity is what the ground takes directly under a pad or strip footing, computed from c, phi, embedment and width, in kPa. Pile capacity is what one pile carries down to a deeper stratum through shaft friction and end bearing, in kN. One is a pressure, the other a point load. On the same site, 300 kPa under a 2×2 m footing carries 1200 kN while a 600 mm pile reaches 1508 kN — the pile wins because it reaches better ground.

How is a retaining wall different from excavation support?

A retaining wall is permanent and holds soil up for good, designed on active pressure with checks for sliding, overturning and base pressure. Excavation support is temporary and holds soil back while you dig, designed on at-rest or active pressure but governed more by movement and prop loads. The decisive difference is displacement: a wall may move enough to reach the active state, but support next to an existing building may not move at all, so at-rest pressure applies and the load is over 40% higher.

Do SPT N values need correction?

Yes — to N60 for 60% hammer energy and to an effective overburden of 100 kPa. Loose saturated fine sand under-reads, so uncorrected values under-predict capacity. The same N also drives the liquefaction check, where below a critical value the ground needs treatment. This tool takes already-corrected N.

How far down should dewatering go?

At least 0.5 m below the excavation base, and 1 m in sand because capillary rise and seepage bring water up. Depth then picks the system: wellpoints to about 6 m in one stage (4-5 m in practice), ejector wellpoints 8-20 m, and deep wells for any depth with high yield. Watch the settlement — a metre of drawdown adds 10 kPa of effective stress, which on compressible soil means centimetres of movement next door; recharge wells are the usual answer.

What factor of safety should a slope have?

1.3 to 1.5 for permanent slopes and 1.1 to 1.25 for temporary ones where the exposure is short. The number is only an average, though: failure usually starts at a weak seam, so look at where the slip circle runs. A surface that passes through soft clay matters far more than a shallow one at a slightly lower factor.

Other groups

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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.