How clay, sand, rock, silt, fill, and expansive soils affect concrete foundation design, bearing capacity, and long-term slab performance
A complete guide to soil types and their impact on concrete foundations in 2026. Learn how to identify soil conditions, understand bearing capacity and settlement risk, select the correct foundation type, and design concrete foundations that perform reliably on any soil — from solid rock to soft clay.
Essential knowledge for structural engineers, geotechnical engineers, builders, and concreters designing and constructing concrete foundations on all soil types in 2026
The soil beneath a concrete foundation is not passive — it actively participates in supporting the structure above. Every concrete foundation must transfer the building's dead loads, live loads, wind loads, and seismic loads into the supporting soil without causing excessive settlement, differential movement, heaving, or bearing capacity failure. The soil type determines the allowable bearing pressure, the expected settlement magnitude, the risk of volume change with moisture, and the most appropriate foundation type and depth — making soil investigation the non-negotiable first step in any concrete foundation project in 2026.
All soil-foundation interactions reduce to three critical behaviours that concrete foundation designers must account for: bearing capacity (can the soil support the applied load without shear failure?), settlement (how much will the foundation sink, and will it be uniform?), and volume change (will the soil expand when wet or contract when dry, pushing or pulling the foundation?). Different soil types exhibit these behaviours to vastly different degrees — rock offers near-infinite bearing capacity with negligible settlement, while soft clay can fail under even modest loads and continue settling for decades after construction.
Soils are classified internationally using the Unified Soil Classification System (USCS) — used in Australia (AS 1726), the US (ASTM D2487), and most English-speaking countries — which groups soils by particle size and plasticity into coarse-grained (gravels GW, GP; sands SW, SP) and fine-grained (silts ML, MH; clays CL, CH) categories. The AASHTO system classifies soils for highway pavement design by particle size and Atterberg limits. Australian Standard AS 2870 provides a site classification system (A, S, M, H1, H2, E, P) specifically for residential concrete slab and footing design based on ground movement potential.
Soil type is the primary variable in concrete foundation engineering. It determines the allowable bearing pressure (which controls the size of footings and raft slabs), the magnitude and rate of settlement (which controls differential movement and cracking risk), the depth of the foundation (to reach adequate bearing stratum), and the reinforcement requirements of the concrete (to resist bending, cracking, and punching from non-uniform support). Understanding how loads flow from the concrete foundation into the soil — and how the soil responds — is fundamental to safe foundation design in 2026. For a complete guide to how structural loads travel through concrete to the foundation, see our guide on understanding concrete load paths.
The relationship between soil type and foundation performance is direct and measurable. Geotechnical investigation — soil borings, test pits, laboratory testing of samples, and in-situ tests like the Standard Penetration Test (SPT) and Cone Penetration Test (CPT) — provides the data that engineers need to design concrete foundations safely. Skipping geotechnical investigation and assuming soil conditions based on neighbouring sites or visual inspection is a major cause of costly foundation failures in residential and commercial construction.
Soil investigation always precedes foundation design. The soil type and bearing capacity dictate every subsequent decision — foundation depth, type, size, reinforcement, and slab thickness.
Each soil type presents a distinct combination of bearing capacity, settlement behaviour, volume change potential, and drainage characteristics that directly shapes the concrete foundation design. Understanding the engineering properties of each soil type — and the specific risks they pose to concrete foundations — is the foundation of competent geotechnical and structural engineering practice in 2026.
Bedrock — whether granite, sandstone, limestone, basalt, or other competent rock — provides the most reliable foundation material for concrete structures. Allowable bearing pressures on rock range from 1,000 kPa to over 10,000 kPa depending on rock type, degree of fracturing, weathering depth, and joint orientation. Settlement on rock foundations is negligible and essentially instantaneous — there is no long-term consolidation. The primary risks with rock foundations are: highly fractured or weathered zones with dramatically lower capacity than intact rock, solution cavities in limestone (karst), and differential support where the footing spans between rock and soil. Concrete footings on rock typically require only nominal reinforcement for crack control unless the rock surface is irregular.
Well-graded gravels and dense granular soils (GW, GP in USCS classification) are excellent foundation materials. They offer allowable bearing pressures of 300–600 kPa, drain freely so pore water pressures do not build up under load, and exhibit minimal creep or long-term settlement. Settlement occurs rapidly during construction and is largely complete by the time the building is occupied. Granular soils are not susceptible to frost heave when clean (free of fines) and do not swell or shrink with moisture change. The main risks are: loose or uncompacted granular deposits that densify under load (particularly vibration from earthquakes or machinery), and clean gravels that provide no lateral restraint to footings in slope situations.
Sand behaviour ranges widely depending on its density state. Dense to medium-dense sand provides allowable bearing pressures of 200–400 kPa and reasonable settlement performance. Loose sand has allowable pressures of only 50–100 kPa and is susceptible to liquefaction under seismic loading — a condition where the sand temporarily loses all shear strength and behaves like a liquid, causing catastrophic foundation failure. Concrete foundations on loose sand must either compact the sand before construction, use deep foundations to bypass the loose zone, or be specifically designed to tolerate the associated settlements. Settlement in sand is rapid but can be significant if the sand is loose or the foundation is heavily loaded.
Liquefaction occurs in loose, saturated, fine to medium sand during earthquake shaking. The cyclic loading causes pore water pressure to build up until it equals the overburden stress — at this point the sand has zero effective stress and zero shear strength. Buildings on liquefiable sand can sink, tilt, or overturn completely during a significant seismic event. Concrete foundations in seismic zones underlain by loose saturated sand must be assessed for liquefaction potential using the Robertson and Wride method or equivalent, and mitigated through ground improvement, deep foundations, or mat foundations designed for post-liquefaction conditions.
Silt (ML, MH in USCS) is one of the most problematic foundation soils. It has low bearing capacity (typically 50–100 kPa), high compressibility, very low permeability (so excess pore pressures dissipate slowly under load), and significant frost susceptibility — it is the most frost-heave-prone of all soil types. Silt has little plasticity, which means it becomes liquid when saturated and rigid when dried — both extremes are unfavourable for foundation support. Concrete foundations on silt typically require deeper footings below the frost line, wider bearing areas to reduce contact pressure, or ground improvement. In many cases, driven or bored piles through the silt to a better-bearing stratum below are the most economical solution for moderate to heavy loads.
Clay is the most common problematic soil for concrete foundations worldwide, and the behaviour of clay under load and moisture change is the dominant concern in residential and commercial foundation engineering in Australia and many other countries. Clay soils present three distinct challenges for concrete foundations: bearing capacity failure in soft clays under concentrated loads, consolidation settlement that continues for years to decades after construction, and volume change (shrink-swell behaviour) in expansive clays that generates foundation heave and cracking. The Australian Standard AS 2870 classifies clay foundation sites specifically according to the depth of the active zone and the expected ground movement — from Class A (negligible movement) to Class E (extreme movement of 75+ mm).
Expansive clays — also called reactive soils in Australia — contain clay minerals (particularly smectite/montmorillonite) that absorb water and swell dramatically when wet, then shrink and crack when dry. The seasonal moisture cycle in many Australian climates drives vertical ground movements of 20–100+ mm in reactive clay sites (AS 2870 Classes M, H1, H2, E). These movements, applied non-uniformly under a concrete slab or footing, generate bending moments and shear forces in the foundation that far exceed the structural loads from the building itself. Concrete footings on reactive clay must be deep enough to reach the zone of negligible moisture variation (the "inactive zone"), and slabs must be designed as stiff beams spanning across areas of differential soil movement. For more on how ground movement affects backfill and foundation surround materials, see the guide on backfilling around concrete foundations.
Peat and highly organic soils (Pt in USCS) are entirely unsuitable as direct bearing material for concrete foundations. They have near-zero bearing capacity, extreme compressibility, very high natural moisture content (100–1000%), and undergo both mechanical consolidation and biological decomposition under load — meaning settlement continues indefinitely as the organic matter breaks down. Even small column loads can cause settlements of hundreds of millimetres on peat. Concrete foundations on peat require piles taken through the peat to a competent bearing stratum below, or complete excavation and replacement of the peat with engineered fill. No amount of footing enlargement will compensate for the fundamental inadequacy of peat as a bearing material.
Uncontrolled fill — material placed on a site without engineering supervision or compaction records — is one of the most challenging foundation conditions because its properties are unknown and highly variable. Fill may contain rubble, organic matter, voids, soft zones, and materials of widely varying compressibility. It will continue to settle under self-weight as organic inclusions decompose and loose zones consolidate. The only safe approach to building concrete foundations on fill is to: characterise the fill thoroughly with a geotechnical investigation (test pits, borings, and laboratory testing); remove and replace unsuitable fill where practical; or bypass the fill with piles to the natural ground below. Never assume fill is adequately compacted without testing — even fill placed in recent years may be loose and heterogeneous. For guidance on controlled fill placement, see our guide on sub-base preparation for concrete.
The table below summarises the key engineering properties and foundation design implications of each major soil type for concrete foundation designers and builders in 2026.
| Soil Type | USCS Symbol | Allowable Bearing (kPa) | Settlement Risk | Volume Change Risk | Recommended Foundation |
|---|---|---|---|---|---|
| Bedrock | — | 1,000–10,000+ | Negligible | None | Pad footings on rock surface |
| Dense Gravel | GW, GP | 300–600 | Low (rapid) | None | Spread footings, raft or slab-on-ground |
| Dense Sand | SW, SP | 200–400 | Low–Moderate | None | Spread footings — check settlement |
| Loose Sand | SP, SM | 50–100 | High | None (liquefaction risk) | Compact or use piles — seismic check |
| Stiff Clay | CL, CH | 100–200 | Moderate–High | Moderate–High (shrink-swell) | Deep footings below active zone |
| Soft Clay | CL, CH | 25–75 | Very High (long-term) | High | Piles or raft with ground improvement |
| Expansive Clay (Reactive) | CH | 50–150 | Moderate + heave | Very High (AS 2870 H2/E) | Deep stiffened raft or pier-and-beam |
| Silt | ML, MH | 50–100 | High | Moderate (frost heave) | Deep footings — avoid frost zone |
| Peat / Organic | Pt, OH | 0–20 | Extreme (indefinite) | High (decomposition) | Piles through peat — never direct |
| Uncontrolled Fill | Variable | Unknown | High–Very High | Variable | Investigate — remove or use piles |
The soil type directly determines which type of concrete foundation is appropriate. Using the wrong foundation type for a given soil condition — typically using a shallow spread footing on a soil that requires a deep foundation — is the most common and most costly foundation design error in residential and light commercial construction in 2026.
Isolated pad footings (also called column footings or spread footings) transfer column or wall loads directly to the soil through bearing pressure. They are appropriate for rock, dense gravel, dense sand, and stiff clay with allowable bearing pressures ≥ 100 kPa. The footing plan area is sized to keep the bearing pressure below the allowable value: Area = Column Load / q_all. Pad footings are not suitable for soft clays, silts, loose sands, peat, or variable fill — on these soils they will either fail by shear or settle excessively. Minimum depth of 300–450 mm below finished ground level is required to avoid frost heave and desiccation.
Strip footings (continuous wall footings) distribute wall loads along their length, reducing the bearing pressure per unit area compared to the wall width. They are the standard foundation for masonry and concrete load-bearing walls on competent soils (CBR ≥ 5%, bearing capacity ≥ 50 kPa). On reactive clay sites in Australia, strip footings must be deepened below the active zone depth (typically 600–1800 mm) specified in the AS 2870 site classification. Strip footings on expansive clay must be reinforced as beams to span across differential heave between swelling and non-swelling zones beneath the footing.
A raft (or mat) foundation is a continuous reinforced concrete slab covering the entire building footprint. It distributes the total building load over the maximum available soil area, reducing bearing pressure to the lowest possible level — making it ideal for soft clays, variable soils, and sites with variable fill where differential settlement is the primary concern. The raft also provides rigidity that limits differential movement between columns and walls. Stiffened raft slabs — as specified in AS 2870 for reactive clay sites — incorporate edge and internal beams to resist the bending moments from soil heave and differential shrinkage. The raft design must account for two-way bending in both hogging and sagging directions.
Bored concrete piers (also called drilled shafts or caissons) are cylindrical concrete elements drilled to depth in the soil and filled with reinforced concrete. They are used where the suitable bearing stratum is too deep for spread footings — typically in deep soft clay, peat, or fill sites, and in highly reactive clay sites in Australia where piers must reach below the active zone. Piers transfer load through end bearing on a competent stratum and through skin friction along their shaft. In expansive clay, piers must also be designed for uplift force generated by soil heave on the shaft — requiring tension reinforcement throughout the pier length. Ground beams spanning between pier heads complete the foundation system.
Piles are long, slender foundation elements that transfer structural loads to deep competent strata through end bearing and skin friction, bypassing the weak upper soils. They are essential on peat, very soft clay, loose sand in seismic zones, and deep fill sites where shallow foundations cannot develop adequate bearing. Concrete piles may be precast (driven) or cast-in-place (bored/augured). Pile design must account for negative skin friction (drag-down) where settling soils grip the pile shaft and add to the pile load — common on reclaimed land and consolidating clay sites. Group pile behaviour and pile cap design are critical considerations when multiple piles support a single column load.
When deep foundations are not economical and the near-surface soil is unsuitable, ground improvement can modify the soil to acceptable engineering properties before concrete foundations are placed. Techniques include: dynamic compaction (repeated heavy drop weight impacts) for loose granular fills; vibro-compaction / vibro-replacement (stone columns) for soft clays and loose sands; cement or lime stabilisation (mixing binder into the soil) for weak cohesive soils; preloading with surcharge to accelerate clay consolidation before construction; and vacuum consolidation for very soft coastal clays. Ground improvement must be verified by testing before concrete foundations are placed above the treated zone. See also: sub-base preparation for concrete.
Australian Standard AS 2870 classifies residential building sites according to the expected ground movement due to moisture change in clay soils. This classification system is unique to Australia and is the primary tool for selecting concrete footing and slab types on reactive clay sites — the dominant foundation challenge in most Australian capital cities in 2026.
Class A: Mostly sand and rock — negligible ground movement (y_s < 20 mm). Standard edge-thickened slab adequate.
Class S (Slightly reactive): Slightly reactive clay — y_s up to 20 mm. Waffle slab or stiffened slab adequate.
Class M (Moderately reactive): Moderately reactive clay or silt — y_s up to 40 mm. Stiffened raft slab with deeper edge beams required.
Class H1 (Highly reactive): Highly reactive clay — y_s up to 60 mm. Deep stiffened raft with 900+ mm edge beams or pier-and-beam.
Class H2 (Highly reactive): Highly reactive clay — y_s up to 75 mm. Deep pier-and-beam system or heavily stiffened raft.
Class E (Extremely reactive): Extremely reactive clay — y_s > 75 mm. Engineer-designed solution only — deep piers.
Class P (Problem site): Soft soils, filled land, collapsible soils, or soils subject to inundation — individual engineer design required for every project.
The American Concrete Institute publishes comprehensive design guidance for concrete foundations on all soil types, including ACI 336 (drilled piers), ACI 318 (structural concrete design including footings and pile caps), and ACI 360 (design of slabs-on-ground). These documents provide the theoretical basis and practical requirements for designing concrete foundations that perform reliably on every soil type encountered in practice — from rock to soft clay to problematic fills.
ACI International →Soil type determines not just the foundation depth and type, but also the sub-base preparation requirements for the concrete slab that sits above the foundation system. Clay and silt subgrades require deeper sub-base layers, geotextile separation, and higher compaction targets than granular soils. Our sub-base preparation guide explains how to correctly prepare the granular layer between the subgrade soil and the concrete slab for every soil and load condition in 2026.
Read the Guide →Once concrete foundations are placed on any soil type, the excavation must be backfilled carefully to avoid applying lateral loads to the fresh concrete, to prevent water from pooling against the foundation, and to restore the drainage gradient away from the building. The backfill material, lift thickness, compaction method, and equipment type must all be selected to be compatible with the foundation soil type and the concrete strength at the time of backfilling. Our guide covers these requirements in detail for all foundation and soil conditions.
Read the Guide →