Introduction
Every bearing pressure number is really a statement about a material – gravel, sand, silt, clay, rock, or some mixture of them. The soil type sets the order of magnitude: dense gravel carries several times what soft clay does, and knowing which one you’re standing on is the first step toward a trustworthy footing size. But soil type alone never fixes the number – moisture, density, and depth move it up or down within (and sometimes beyond) the typical range.
The Soil Bearing Capacity Calculator converts an allowable pressure and a load into a footing width in imperial and metric units. This guide covers the material side of that input: how to identify the major soil types in the field, their typical planning ranges, what helps and hurts each type, and why the final number always comes from testing, not identification. Foundation design decisions belong to a licensed geotechnical or structural engineer.
The Major Soil Types, Strongest to Weakest
Bedrock
Solid rock at or near footing depth. Typical planning range: 8,000-12,000+ psf (380-575+ kPa) depending on rock type and fracturing. The strongest bearing material – and the most expensive to excavate. Fractured, weathered, or thinly bedded rock carries less than the headline numbers suggest, and excavation cost can dominate the foundation budget.
Gravel and sandy gravel
Particles larger than sand, visible to the eye, no cohesion – a handful falls apart when you open your fist. Dense, well-graded gravel: 2,500-3,000+ psf (120-144+ kPa). Excellent drainage means water rarely weakens it, but loose or uniformly graded gravel needs confinement to develop its strength.
Sand
Gritty individual grains, no cohesion, falls apart in the hand. Compact clean sand: 1,500-2,500 psf (72-120 kPa); loose sand considerably less. Strength comes from particle interlock (friction), so density is everything – and saturation below the water table reduces effective capacity. Wind-blown or very uniform fine sands are the weakest of the family.
Silt
Smooth, flour-like when dry; dilatant when wet (it shimmers and firms when shaken in the palm). Firm silt: 1,000-1,500 psf (48-72 kPa). Silts are moisture-sensitive and frost-susceptible – they wick water upward and heave in freezing weather – so drainage and frost-depth footings matter more than the raw number.
Clay
Sticky when wet, hard when dry; rolls into threads without crumbling. Firm clay: 1,000-1,500 psf (48-72 kPa); soft clay: 500-1,000 psf (24-48 kPa). Strength comes from cohesion, which water destroys – saturated clay is a different material from the same clay dry. Expansive clays add volume change (swell/shrink) on top of bearing questions.
Organic soil, peat, uncontrolled fill
Dark, spongy, sometimes fibrous; may smell of decomposition. Effectively zero reliable bearing capacity – compressible, decomposing, and unpredictable. Never a bearing layer. Must be removed (over-excavated) or bypassed with engineered foundations, per the geotechnical engineer’s direction.
| Soil type | Typical planning range (psf) | Typical planning range (kPa) | Key weakness |
|---|---|---|---|
| Bedrock | 8,000 – 12,000+ | 380 – 575+ | Excavation cost; fracturing |
| Dense gravel | 2,500 – 3,000+ | 120 – 144+ | Needs confinement if loose |
| Compact sand | 1,500 – 2,500 | 72 – 120 | Density-dependent; saturation |
| Firm silt | 1,000 – 1,500 | 48 – 72 | Moisture and frost sensitivity |
| Firm clay | 1,000 – 1,500 | 48 – 72 | Softens when wet; swell/shrink |
| Soft clay / organic | 500 – 1,000 | 24 – 48 | Settlement; decomposition |
These ranges are planning context only – two samples of “firm clay” from adjacent lots can differ by 2x or more. How these ranges translate into footing widths is worked through in the complete guide, and what confirming your soil’s number costs is in the cost guide.
Field Identification: Three Quick Tests
These identify soil type – useful for planning conversations and for knowing which questions to ask. They do not measure capacity. The field habits that keep these observations honest are detailed in the pro tips guide.
The ribbon test (clay content)
Moisten a soil ball and squeeze it into a ribbon between thumb and forefinger. Clean sand and gravel won’t form a ribbon at all; silt makes a short crumbly one; clay extrudes a smooth ribbon – longer than 2 in indicates significant clay. Longer, stronger ribbons mean more clay behavior: cohesion, moisture sensitivity, swell potential.
The jar test (particle distribution)
Shake soil with water in a clear jar and let it settle. Sand drops in seconds, silt in minutes, clay in hours to days, organic matter floats. The layered result shows your soil’s proportions – a jar that’s half sand, half clay behaves nothing like the table’s “sand” row.
The heel and shovel test (consistency)
At footing depth: can you dent it with your thumb (soft), need a shovel with effort (firm), or need a pick (hard)? Can you roll it into a 1/8-in thread without crumbling (clay) or does it fall apart (silt/sand)? Log what you find at each depth – this is the field language geotechnical engineers use.
What Moves a Soil Up or Down Its Range
- Moisture: the master variable. Dry, dense soil sits at the top of its range; saturated soil of the same type can sit far below it. Always ask “at what moisture?” of any capacity number.
- Density: compact vs loose sand can differ by 2x. Undisturbed native soil is denser than recently placed soil – another reason footings bear below fill and topsoil.
- Depth: deeper footings benefit from overburden confinement (the q term in Terzaghi’s equation), and deeper soils are often denser and older.
- Groundwater: soil below the water table is buoyant – its effective weight, and the capacity built on it, drops.
- Layering: a thin strong layer over a weak one is governed by the weak one. The profile matters, not just the surface sample.
- Disturbance: excavation, vibration, and water exposure between testing and pouring can degrade the bearing surface below its tested value.
Mixed and Problem Soils
Real sites are rarely one clean soil type. Common combinations and what they mean:
- Clayey sand / sandy clay: behaves between its parents – size preliminary numbers to the weaker component.
- Fill over native soil: the fill is irrelevant to capacity; the footing must reach the native soil below, and the fill’s thickness sets your excavation depth.
- Topsoil over everything: strip it. Organic content near the surface never counts as a bearing layer.
- Varved or layered clays: alternating silt/clay layers can have weak horizontal planes – engineer territory.
- Collapsible soils (loess): strong when dry, collapsing when wetted – common in some arid regions, a nasty surprise without testing.
When the profile shows anything from this list, the DIY vs pro guide explains why professional investigation stops being optional.
Related Guides
- Complete Guide – the full overview: terms, formulas, worked example.
- Formula Explained – how soil properties feed bearing capacity theory.
- Pro Tips – reading the site like a geotechnical engineer.
- Cost Guide – what testing each soil condition costs.
- FAQ – quick answers on soil types and capacity.
Frequently Asked Questions
How can I tell what soil type I have?
Ribbon test for clay content, jar test for particle proportions, and the heel/shovel test for consistency at footing depth. These identify type, not capacity – they tell you which questions to ask, not the design number.
What is the strongest soil for foundations?
Bedrock, then dense gravel, then compact sand. But strength isn’t everything – excavation cost, drainage, and frost behavior matter too.
Why does the same soil type have a range instead of one number?
Because moisture, density, depth, and layering move capacity within the range. The range reflects real variability; the site-specific number comes from testing your soil in its actual condition.
Is sandy soil good for foundations?
Compact sand is generally good – 1,500-2,500 psf (72-120 kPa) with excellent drainage. Loose or saturated sand is a different story, which is why density and water table matter as much as the word “sand.”
Get Your Number
Identify your soil with the field tests above, pick the conservative end of its planning range for early math, and run the Soil Bearing Capacity Calculator to see what footing width that implies. Then get the number that actually counts – the tested allowable pressure from a geotechnical report – before anything is designed or poured.
Disclaimer: Soil descriptions and planning ranges are general information for early planning, not design values. Actual capacity varies with moisture, density, depth, and site history – verify with your local building code and a site-specific geotechnical report, and consult a licensed geotechnical or structural engineer for foundation design.
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