Geotechnical Soil Bearing Capacity

What Is Soil Bearing Capacity?

Soil bearing capacity is the maximum pressure the ground can safely support from a foundation without:

• Shear failure (sliding, bulging, or collapsing of soil below the footing), and
• Excessive settlement (too much sinking of the structure).

It is usually expressed in:

• psf – pounds per square foot
• or kN/m² in metric

Think of it like this:

“How much load per unit area can this soil safely carry over the long term?”

Two key terms are used in practice:

Ultimate Bearing Capacity (qult)

• This is the maximum load per unit area the soil can take just before failure.
• At this level, the soil may bulge, crack, or form a shear surface.
• It is a theoretical limit – we never design foundations to work at this level.

Allowable Bearing Capacity (qa)

• This is the safe working pressure we actually use in design.
• It is obtained by dividing the ultimate capacity by a factor of safety.
• It may also be controlled by settlement limits, not just shear strength.

In simple words:

Ultimate = maximum possible.
Allowable = safe and practical.

Why Soil Bearing Capacity Is So Important

Getting soil bearing capacity wrong can be very costly and dangerous.

If Bearing Capacity Is Overestimated

• The foundation may settle too much
• Cracks appear in walls and slabs
• Doors and windows jam
• Floors become uneven
• In extreme cases, tilting or partial collapse can occur

If Bearing Capacity Is Underestimated

• The designer may use unnecessarily large foundations
• Extra concrete and steel are used
• Project cost increases without real need

So, accurate estimation of soil bearing capacity helps:

• Keep the structure safe
• Keep the design economical
• Avoid serviceability problems like cracks and differential settlement

How Soil Type Affects Bearing Capacity

Not all soils are equal. Some are strong and stiff. Others are weak and compressible.

Common Soil Types in Bearing Capacity

1. Clay (cohesive soil)
   • Has cohesion (stickiness due to water and clay minerals)
   • May have low or zero friction angle
   • Can be soft or stiff
   • Often sensitive to water content and time
   • May show significant long-term settlement
2. Silt
   • Fine-grained like clay but with different behavior
   • Moderate cohesion and friction
   • Can be compressible
   • More prone to frost heave and erosion
3. Sand (granular soil)
   • Strength mainly from friction between particles
   • Little or no cohesion
   • Drains water easily
   • Settles quickly but usually not excessively if dense
4. Gravel
   • Coarse granular soil
   • High friction angle
   • Very good bearing capacity when well compacted
   • Low compressibility
5. Mixed Soils (e.g., Clayey Sand, Silty Clay)
   • Have both cohesion (c) and friction (φ)
   • Behavior is intermediate
   • Bearing capacity and settlement depend on proportions and density
6. Rock
   • Very high strength and bearing capacity
   • Behaviour depends on joints, fractures, and weathering
   • Often ideal for heavy structures if intact and sound

Cohesion and Friction in Simple Words

In geotechnical design, soil shear strength is often described using:

• Cohesion (c) – strength at zero normal stress (like “glue” between particles)
• Friction angle (φ) – how much the soil resists sliding as normal stress increases
• Clay: high c, low φ
• Sand/Gravel: near-zero c, high φ
• Mixed soils: both c and φ present

Higher cohesion and higher friction angle generally mean higher bearing capacity, but settlement and long-term behavior still need to be checked.

Foundation Type and Its Influence

The shape and type of foundation also affect soil bearing capacity.

Common Foundation Types

1. Strip Footing
   • Long, narrow footing under load-bearing walls
   • Ideal for uniform soils and simple wall loads
2. Square Footing
   • Square pad under a column
   • Often used in building frames
3. Rectangular Footing
   • Rectangular pad under columns or rows of columns
   • Used where loads and spacings are not symmetric
4. Circular Footing
   • Circular base, often used for towers, tanks, or poles
5. Mat (Raft) Foundation
   • Large slab supporting many columns together
   • Used on weak soils or when column spacing is close
   • Spreads load over a large area

Shape and Depth Effects (in Simple Terms)

• Shape: Square and circular footings usually have slightly higher bearing capacity than strip footings on the same soil because load spreads more evenly in all directions.
• Depth: Footings placed deeper below ground generally benefit from:
  • Higher confining pressure
  • Protection from surface changes
  • Increased bearing capacity

These effects are considered using shape factors and depth factors in geotechnical formulas.

The Role of Embedment Depth (Df)

Embedment depth is the distance from the ground surface to the bottom of the footing.

Why it matters:

• Deeper foundations are surrounded by more soil, which helps restrain shear failure.
• The weight of soil above the footing creates additional surcharge pressure, increasing capacity.
• Deeper foundations are less affected by erosion, shrinkage, swelling, and seasonal moisture changes.

However:

• Greater depth means more excavation and higher cost.
• There is a balance between bearing capacity gain and construction cost.

Groundwater and Water Table Effects

The water table is a crucial factor in bearing capacity.

How Water Reduces Bearing Capacity

• When water is present near the footing level, soil becomes partly or fully saturated.
• The effective unit weight of soil reduces because water supports part of the soil’s weight.
• Lower effective unit weight → lower effective stress → lower shear strength → reduced bearing capacity.

Typical Cases

• Water table well below footing and influence zone
  – Full soil unit weight is used, bearing capacity is higher.
• Water table at or near footing base
  – Submerged unit weight (unit weight minus water unit weight) applies to part or all of the soil under the footing.
  – Bearing capacity can drop significantly.

For accurate design, engineers adjust the unit weight used in calculations based on the position of the water table relative to the footing.

Ultimate and Allowable Bearing Capacity in Design

Ultimate Bearing Capacity

Geotechnical theories (like Terzaghi’s bearing capacity theory) combine:

• Cohesion (c)
• Friction angle (φ)
• Unit weight (γ)
• Footing width (B)
• Embedment depth (Df)

To calculate a theoretical ultimate capacity. This is the point at which the soil would fail in shear.

Factor of Safety (FS)

To ensure safety, a factor of safety is applied:

• Common values: 2.0, 2.5, 3.0 or higher, depending on:
  • Soil variability
  • Importance of structure
  • Level of investigation

Allowable Bearing Capacity

Allowable bearing capacity is:

Allowable capacity = Ultimate capacity ÷ Factor of Safety

This ensures the working pressure on the soil is well below the failure level.

Net Allowable Bearing Pressure

Designers often use net allowable pressure:

Net allowable = Allowable bearing capacity – Overburden pressure at footing base

This removes the effect of soil weight above the footing, so the designer sees how much pressure the structure itself can safely apply.

Maximum Column Load and Footing Area

For a given soil and footing size:

• The footing area (width × length) and the net allowable pressure together define the maximum load the footing can safely carry.

In simple terms:

Maximum Column Load = Net allowable pressure × Footing area

This helps structural engineers choose the footing size for a given column load, or vice versa.

Settlement – The Other Side of the Story

Even if a footing is safe against shear failure, the structure can still be in trouble if settlement is too large.

Types of Settlement

1. Immediate settlement
   • Occurs quickly after load application
   • Important in sands and gravels
2. Consolidation settlement
   • Slow compression over time as water escapes from pores
   • Important in clays and silts
3. Differential settlement
   • Uneven settlement between different parts of the structure
   • Most dangerous – causes cracks and structural distress

Soil-Type Based Settlement Behavior

• Soft clays → higher and slower settlement
• Loose sands → moderate settlement, but often early
• Dense sands and gravels → low settlement
• Rock → negligible settlement (if sound)

In real projects, bearing capacity and settlement are evaluated together.
Sometimes the allowable bearing capacity is governed more by settlement criteria than by shear strength.

Failure Modes of Soils Under Footings

When soil fails below a footing, it can fail in different ways depending on soil type and density.

General Shear Failure

• Common in dense sands and stiff clays
• Clear, well-defined failure surface extending to the ground surface
• Sudden drop in load-carrying capacity
• Noticeable bulging and cracking

Local Shear Failure

• Occurs in medium-dense sands or softer soils
• Less defined failure surfaces
• Gradual failure
• Lower peak capacity than general shear

Punching Shear Failure

• Happens when a stiff footing punches into a softer soil
• Failure surface is more vertical
• Often seen in loose or very soft soils or under mats in soft ground

Mixed Mode (C–φ Soils)

• Many natural soils have both cohesion and friction
• Failure behavior is a combination of mechanisms
• Geotechnical engineers interpret this using both c and φ parameters

Understanding failure modes helps in:

• Choosing appropriate foundation type
• Deciding whether soil improvement is needed
• Interpreting the meaning of bearing capacity values

Design Recommendations Based on Bearing Capacity

Engineers often classify soil suitability based on allowable bearing capacity:

• Very high capacity soils
  – Good for heavy structures (multi-storey buildings, industrial plants, bridges)
• Moderate capacity soils
  – Suitable for medium structures (residential and commercial buildings)
• Lower capacity soils
  – Acceptable for light structures (small houses, sheds, boundary walls)
• Very low capacity soils
  – Often require soil improvement or deep foundations (piles, piers, caissons)

Typical soil improvement methods include:

• Compaction
• Replacement with better fill
• Stone columns
• Grouting
• Lime or cement stabilization

If improvement is not practical, engineers may use:

• Pile foundations
• Drilled shafts
• Well or caisson foundations

Practical Engineering Workflow

A realistic geotechnical design process often looks like this:

1. Site investigation
   • Boreholes, trial pits, in-situ tests (SPT, CPT)
   • Groundwater level measurement
   • Lab testing of soil samples
2. Soil classification and parameter estimation
   • Determine c, φ, γ, compressibility, etc.
3. Bearing capacity analysis
   • Use appropriate theory (Terzaghi, Meyerhof, etc.)
   • Apply shape, depth, inclination, and water table corrections
4. Settlement analysis
   • Immediate and consolidation settlement estimates
   • Compare with permissible limits
5. Footing sizing and load checks
   • Check gross and net allowable pressures
   • Check maximum column loads
6. Design decision
   • Confirm shallow foundation
   • Or select soil improvement / deep foundation if needed
7. Documentation and drawings
   • Include assumptions, safety factors, and recommendations
   • Include notes on groundwater and drainage