# Shallow Foundation Design ## Table of Contents - [Introduction](#introduction) - [Types of Shallow Foundations](#types-of-shallow-foundations) - [Strip Footings](#strip-footings) - [Pad Footings](#pad-footings) - [Combined Footings](#combined-footings) - [Strap Beams](#strap-beams) - [Bearing Pressure and Capacity](#bearing-pressure-and-capacity) - [Allowable Bearing Pressure](#allowable-bearing-pressure) - [Presumed Bearing Values](#presumed-bearing-values) - [Contact Pressure Distribution](#contact-pressure-distribution) - [Settlement Calculation](#settlement-calculation) - [Immediate Settlement](#immediate-settlement) - [Consolidation Settlement](#consolidation-settlement) - [Differential Settlement Limits](#differential-settlement-limits) - [Eccentricity and Combined Loading](#eccentricity-and-combined-loading) - [Eccentric Loading](#eccentric-loading) - [Effective Area Method](#effective-area-method) - [Overturning Check](#overturning-check) - [Foundation on Slopes](#foundation-on-slopes) - [Frost Depth](#frost-depth) - [Reinforcement Detailing](#reinforcement-detailing) - [Pad Footing Reinforcement](#pad-footing-reinforcement) - [Strip Footing Reinforcement](#strip-footing-reinforcement) - [Punching Shear at Foundations](#punching-shear-at-foundations) - [Design Procedure Summary](#design-procedure-summary) - [Practical Notes for Architects](#practical-notes-for-architects) - [Related Topics](#related-topics) - [References](#references) --- ## Introduction Shallow foundations transfer building loads to the soil at relatively small depths below ground level, relying on the bearing capacity of the near-surface strata. They are the most common and economical foundation type, suitable where competent bearing strata exist within 1.5-3m of ground level. For the architect, understanding shallow foundation design influences decisions about ground floor levels, basement inclusion, column spacing, and overall project cost. The choice between shallow and deep foundations — see [[Deep Foundation Systems]] — is one of the most significant cost drivers in building construction. The depth criterion for "shallow" foundations is generally D/B ≤ 1 (where D is depth and B is width), though in practice the term applies to any foundation that does not rely on deep soil strata for support. ## Types of Shallow Foundations ### Strip Footings Strip footings (continuous footings) are linear foundations supporting load-bearing walls or closely spaced columns along a line. **Characteristics:** - Width: typically 450-1200mm for domestic construction, wider for commercial - Depth: minimum 450mm below ground level (UK domestic), deeper in frost zones or shrinkable clay - Aspect ratio: Length >> Width - Usually unreinforced for domestic walls (mass concrete); reinforced for heavier loads - Stepped strip footings used on sloping sites **Deep strip (trench fill):** The trench is filled with concrete to near ground level, eliminating the need for masonry below ground. Faster construction, suitable for shrinkable clay sites. ### Pad Footings Pad footings (isolated footings) are rectangular or square foundations supporting individual columns. **Sizing:** - Plan area = Column load / Allowable bearing pressure - Typically square (most efficient), rectangular where space is constrained - Minimum depth: governed by punching shear, bending, or practical considerations (typically ≥ 300mm) - Common sizes: 1.0m × 1.0m up to 3.0m × 3.0m for typical buildings **Design approach:** 1. Determine the serviceability load (unfactored) for sizing the plan area 2. Determine the ultimate load (factored) for reinforcement design 3. Check bearing pressure under all load combinations 4. Design reinforcement for bending in both directions 5. Check punching shear around the column perimeter ### Combined Footings Combined footings support two or more columns on a single foundation. They are used when: - Adjacent column footings would overlap due to close spacing or high loads - An edge column is near the property boundary, and an isolated footing would be eccentric - Significantly different column loads would cause unacceptable differential settlement **Types:** - **Rectangular combined footing:** Uniform width supporting two columns — proportioned so that the centroid of the footing coincides with the resultant of the column loads - **Trapezoidal combined footing:** Variable width to equalise bearing pressure when column loads differ significantly ### Strap Beams A strap beam (also called a tie beam or cantilever footing) connects an edge pad footing to an adjacent interior pad footing. The strap beam transfers the eccentricity from the edge column to the interior footing, equalising bearing pressures without a massive combined footing. **Design considerations:** - The strap beam must be stiff enough to prevent rotation of the edge footing - The strap beam is designed for shear and bending (hogging over the edge footing, sagging between footings) - The strap beam should not bear on the ground (a void former or compressible layer is placed beneath it) ## Bearing Pressure and Capacity ### Allowable Bearing Pressure The allowable bearing pressure is the lesser of: 1. **Ultimate bearing capacity / Factor of Safety** (strength criterion, FoS typically 2.5-3.0) 2. **Pressure causing tolerable settlement** (serviceability criterion) In practice, the settlement criterion often governs, particularly on compressible soils. ### Presumed Bearing Values For preliminary design, codes provide presumed bearing values: | Soil Type | Presumed Bearing Value (kN/m²) | |---|---| | Dense gravel / dense sand-gravel | 200-600 | | Medium-dense gravel / sand | 100-300 | | Loose gravel / sand | 50-100 | | Stiff clay | 150-300 | | Firm clay | 75-150 | | Soft clay | 25-75 | | Very soft clay | < 25 | | Hard rock | 10,000+ | | Weathered rock | 500-5,000 | *These values are for guidance only. Site-specific geotechnical investigation is always required for design — see [[Soil Mechanics for Architects]].* ### Contact Pressure Distribution The actual pressure distribution beneath a footing depends on the soil type and footing rigidity: - **Rigid footing on sand:** Pressure concentrated at edges (parabolic, higher at edges) - **Rigid footing on clay:** Pressure concentrated at centre (inverse parabolic, higher at centre) - **Flexible footing:** Uniform pressure distribution (the design assumption for most RC footings) For design purposes, a **uniform pressure distribution** is assumed for rigid footings, which is acceptable and conservative for most practical situations. ## Settlement Calculation ### Immediate Settlement Immediate (elastic) settlement occurs during and immediately after load application, affecting all soil types. For a foundation on a semi-infinite elastic half-space: **si = q × B × (1 - ν²) × Iρ / Es** Where q is the bearing pressure, B is the footing width, ν is Poisson's ratio, Iρ is the influence factor (depends on footing shape and rigidity), and Es is the soil elastic modulus. ### Consolidation Settlement Consolidation settlement is the time-dependent compression of saturated fine-grained soils (clays, silts) as pore water is squeezed out under sustained load. **One-dimensional consolidation settlement:** `sc = (Cc × H₀) / (1 + e₀) × log₁₀(σ'₀ + Δσ') / σ'₀` For normally consolidated clay, where: - Cc = compression index - H₀ = thickness of the compressible layer - e₀ = initial void ratio - σ'₀ = initial effective stress at the centre of the layer - Δσ' = stress increase due to the foundation load For overconsolidated clay (σ'₀ + Δσ' < σ'p), use the recompression index Cr (typically 0.1 to 0.2 × Cc). ### Differential Settlement Limits | Criterion | Limit | Application | |---|---|---| | Angular distortion (δ/L) | 1/500 | Cracking in walls | | Angular distortion (δ/L) | 1/300 | Structural damage | | Angular distortion (δ/L) | 1/150 | Severe structural damage | | Maximum total settlement | 25mm | Light structures on sand | | Maximum total settlement | 50mm | Structures on clay | | Maximum total settlement | 75mm | Structures on clay (with caution) | | Relative deflection (Δ/L) | 1/2500 | For sensitive finishes | *After Skempton and MacDonald (1956), Burland and Wroth (1975).* ## Eccentricity and Combined Loading ### Eccentric Loading When a column applies both axial force and moment to a footing, the bearing pressure is non-uniform: **For eccentricity within the middle third (e ≤ B/6):** `qmax = N/A × (1 + 6e/B)` and `qmin = N/A × (1 - 6e/B)` Where e = M/N is the eccentricity, N is the axial load, and B is the footing dimension in the direction of eccentricity. **For eccentricity outside the middle third (e > B/6):** Bearing pressure exists over only part of the footing. This is generally unacceptable for permanent load conditions but may be tolerated for transient loads (wind, seismic). ### Effective Area Method For biaxial eccentricity (moments about both axes), the effective area method calculates an effective footing area: - Effective dimensions: B' = B - 2eB and L' = L - 2eL - Effective area: A' = B' × L' - Bearing pressure: q = N / A' ### Overturning Check The stabilising moment (from the foundation's own weight and the superstructure dead load) must exceed the overturning moment with a factor of safety: **Overturning FoS = Mstabilising / Moverturning ≥ 1.5 (wind) or ≥ 1.0 (seismic, factored loads)** ## Foundation on Slopes Foundations near or on slopes require special consideration: - The bearing capacity is reduced compared to level ground - Minimum horizontal distance from the footing edge to the slope face (typically 1.5-2.0 × footing width) - The footing should be embedded below the zone of weathering and seasonal movement - Slope stability analysis may be required to ensure global stability - Eurocode 7 and ASCE 7 provide guidance on setback distances ## Frost Depth In cold climates, foundations must extend below the frost depth to prevent frost heave: - **Frost heave:** Expansion of water in soil as it freezes, causing upward movement of the foundation - **Frost susceptible soils:** Silts and fine sands (high capillarity) are most susceptible - **Frost depth:** Depends on geographic location — ranges from 0 in tropical regions to over 2m in northern climates - **UK:** Minimum foundation depth is typically 450mm (but deeper in frost-susceptible soils or near trees on shrinkable clay) - **US:** Varies from 0 in southern states to over 1.8m in northern states - **Alternative:** Frost-protected shallow foundations (FPSF) use insulation to reduce the required depth ## Reinforcement Detailing ### Pad Footing Reinforcement - Bottom reinforcement in both directions (main bars in the wider direction for rectangular footings) - Bar diameter: typically 12-20mm - Spacing: 150-200mm typical - Cover to bottom reinforcement: 50mm minimum (in contact with ground, typically 75mm) - Bars extended to within 75mm of the footing edge (with appropriate anchorage) - Starter bars projecting upward to connect with the column reinforcement ### Strip Footing Reinforcement - **Transverse reinforcement:** Main reinforcement — resists bending across the width of the strip - **Longitudinal reinforcement:** Distribution steel and spans between concentrated loads - Unreinforced strip footings are adequate when the transverse bending stress does not exceed 0.6 MPa (mass concrete) ### Punching Shear at Foundations Punching shear checks for pad footings are similar to flat slab punching shear but with important differences: - The beneficial effect of soil pressure within the critical perimeter reduces the shear force - The critical perimeter is at 2d from the column face (EC2) or d/2 from the column face (ACI) - The effective design shear is: VEd = Column load - soil pressure × area within critical perimeter - Deep footings (d > 2 × column width) may not require punching shear reinforcement ## Design Procedure Summary 1. **Obtain geotechnical data:** Allowable bearing capacity, expected settlement, water table level — see [[Soil Mechanics for Architects]] 2. **Calculate column loads:** Dead, live, wind, seismic under all load combinations — see [[Load Path and Load Combinations]] 3. **Size the footing plan area:** Using serviceability (unfactored) loads and allowable bearing pressure 4. **Determine footing depth:** From punching shear check (minimum depth), bending check, and practical minimums 5. **Design reinforcement:** Using ultimate (factored) loads and factored bearing pressure 6. **Check punching shear:** At 2d (EC2) or d/2 (ACI) from column face 7. **Check bending:** At the column face (critical section for bending) 8. **Check settlement:** Total and differential settlement under serviceability loads 9. **Detail the reinforcement:** Cover, spacing, anchorage, starter bars ## Practical Notes for Architects 1. **Foundation size increases dramatically with poor soil.** A column load of 1,000 kN requires a 1.3m × 1.3m footing on stiff clay (150 kPa) but 3.2m × 3.2m on soft clay (25 kPa) — affecting column grid and basement layout 2. **Water table above foundation level** increases cost due to dewatering, waterproofing, and uplift resistance 3. **Trees on shrinkable clay** (PI > 20) require deeper foundations — consult NHBC Standards Chapter 4.2 for minimum depth guidance based on tree species and distance 4. **Ground floor slab interaction:** Ensure the ground-bearing slab is designed independently of the foundations unless it is a raft — differential movement will occur 5. **Service entries through foundations** must be sleeved and sealed to prevent water ingress 6. **Contaminated sites** may require special concrete (sulphate-resistant) or physical barriers — see the geotechnical report recommendations 7. **Cost benchmark:** Shallow foundations typically represent 3-8% of total building cost; deep foundations may cost 2-5 times more ## Related Topics - [[Deep Foundation Systems]] - [[Raft and Mat Foundations]] - [[Soil Mechanics for Architects]] - [[Bearing Capacity and Settlement]] - [[Load Path and Load Combinations]] - [[Reinforced Concrete Design]] ## References - EN 1997-1: Eurocode 7 — Geotechnical Design: General Rules - EN 1992-1-1: Eurocode 2 — Design of Concrete Structures - ACI 318-19: Building Code Requirements for Structural Concrete - IS 1904: Code of Practice for Design and Construction of Foundations in Soils - Tomlinson, M.J. and Boorman, R., *Foundation Design and Construction*, Pearson - NHBC Standards, Chapter 4.2: Building Near Trees - BRE Digest 241: Low-Rise Buildings on Shrinkable Clay Soils --- #structures #foundations #shallow-foundations #bearing-capacity #settlement #detailing