Soil bearing capacity

What is bearing capacity of soil?

Bearing capacity is the maximum pressure the ground can safely support without failing or moving excessively. It is determined by soil type, density, moisture content, and depth, and is the fundamental figure that governs how deep to dig foundations, how large to make foundation pads or strips, and whether you need piles. You do not guess it. Every building in Slovakia must rest on a foundation sized according to the soil's bearing capacity, established by geotechnical survey and reported in kPa (kiloPascals), equivalent to kN/m2.

Why is bearing capacity the foundation of foundation design?

Bearing capacity is the single piece of ground information that determines foundation cost and feasibility. Good bearing capacity, like dense gravel along the Danube, allows simple, inexpensive shallow foundations such as strips or slabs. Poor bearing capacity, like soft clay or collapsible loess, may force expensive deep foundations with piles. Mediocre bearing capacity that allows settlement within tolerable limits requires thicker foundation pads or wider strips to spread the load. Without knowing bearing capacity, you cannot estimate foundation cost or feasibility at all, which is why the survey comes before the foundation design.

What is the difference between ultimate and allowable bearing capacity?

Ultimate bearing capacity is a theoretical figure: the maximum pressure the soil can resist before shear failure occurs, calculated from soil strength parameters. Allowable bearing capacity is much lower. It is the ultimate capacity divided by a safety factor, typically 3.0 for soft soil or 3.0 to 4.0 for stiff soil. The safety factor accounts not only for shear failure but also for settlement. This is the key distinction: allowable bearing capacity is the number that appears in your geotechnical report and the number your engineer uses to size the foundation. It is inherently conservative, because residential buildings are governed by settlement limits, not shear strength.

Why does settlement matter more than shear failure for your house?

Shear failure is dramatic but rare in residential foundations on ordinary ground. Settlement is silent and inevitable. A family house on gravel with an allowable bearing capacity of 200 kPa will settle maybe 10-20 mm over years as the soil compresses under load. This is normal and acceptable if it is uniform across the building. But differential settlement (one corner sinking 30 mm while another sinks 15 mm) causes cracks, jammed doors, sloped floors, and structural distress. Building codes limit differential settlement to roughly 19 mm for residential buildings, and settlement rate to roughly 50 mm total. These settlement limits are far more restrictive than the shear strength of the soil, which is why foundation design revolves around controlling settlement, not preventing collapse. The geotechnical engineer predicts settlement from soil stiffness (deformation modulus), uses settlement predictions to set allowable pressure, and you size the foundation to stay within that pressure. Shear failure is prevented as a by-product of being conservative about settlement.

What Slovak soil conditions affect bearing capacity?

Slovakia presents distinct regional soil patterns that profoundly affect foundation design.

Soil type or region	Location	Bearing-capacity characteristics
Alluvial gravels and sands	Danube and Váh lowlands	Dense, well-graded, stable. Good bearing capacity. Settlement is predictable and small. Preferred for simple shallow foundations.
Loess	Western and southwestern Slovakia, overlying alluvial gravels	Collapsible when wet. Dry loess may have decent bearing capacity, but wetting causes sudden volume loss (collapse settlement), severe loss of stiffness, and loss of shear strength. Very challenging. Often forces deep foundations or soil replacement.
Expansive clay	Various regions, south-central and eastern areas	Swells when wet, shrinks when dry. Seasonal cycles cause differential movement. Causes cracking and foundation distress. Requires careful foundation design, moisture barriers, or soil conditioning.
Made ground or old backfill	Infill plots, city centre redevelopment sites, former industrial land	Unpredictable. Variable compaction, unknown origin, presence of debris or organic material. Requires detailed investigation. Often poor bearing capacity and potential for large or unexpected settlement.

Plot buyers and self-builders are most likely to encounter alluvial gravels (fortunate) or loess (dangerous if not recognised). Loess is particularly insidious because it looks solid and stable until water infiltrates, causing collapse. If you are building in western Slovakia or any area of known loess cover, make sure the geotechnical engineer specifically addresses wetting-induced settlement and recommends mitigation (such as deeper foundations, improved drainage, or soil conditioning).

What are typical bearing capacity values and how much pressure does a house apply?

The table below shows indicative allowable bearing pressures for common soil types under normal residential loading. These are guidance ranges only, not design values. Your actual bearing capacity must come from the site investigation.

Soil type (controlled or tested material)	Indicative allowable bearing pressure range (kPa)	Notes
Dense gravel or crushed rock (well-compacted)	575	Good capacity. Typical for Danube alluvial gravels. Settlement usually small and predictable.
Well-graded sand (dense)	287–400	Acceptable capacity. Settlement moderate and predictable if not subject to wetting or liquefaction.
Silt or fine sand (medium density)	191–250	Marginal. Higher settlement risk. May require wider foundations. More sensitive to water.
Soft clay or organic soil	Must be established by engineer	No standard value. Requires consolidation tests and settlement analysis. Often forces deep foundations or ground improvement.
Collapsible loess (dry)	Must be established by engineer, with wetting assessment	Wetting-induced collapse can be severe. Requires either avoiding wetting (deep drainage, building away from water) or accepting controlled replacement/grouting.

A typical single-family house exerts foundation pressures of 150–250 kPa under working loads (live load plus dead load of structure, divided by foundation area). So a house on good gravel (575 kPa allowable) has a comfortable safety margin; on marginal silt (191 kPa allowable) it is at the edge; and on soft clay without detailed design, it is risky. This is why the survey is essential.

How do you find out what bearing capacity applies at your site?

Only a geotechnical survey can answer this. The engineer drills boreholes to examine soil layers, collects samples at various depths, performs laboratory tests to determine soil strength (shear strength) and stiffness (deformation modulus), measures groundwater level, and synthesises all data to calculate allowable bearing capacity specific to your site, depth, and loading. The report will state an allowable bearing pressure in kPa, will specify the foundation depth at which it applies, and will address any special conditions such as wetting risk, seasonal water table rise, or differential settlement potential. This number becomes the input to foundation design. It is site-specific, not guessable, and non-negotiable. Design according to Eurocode 7 (STN EN 1997), which Slovakia has adopted. Slovakia uses Design Approach 2 under the national annex, balancing partial safety factors on actions and resistances. Never proceed to foundation design or cost estimates without this report.