Day 26 – Bearing Capacity and Settlement in Foundations
Why Both Must Be Checked in Geotechnical Design
In geotechnical engineering, designing a safe foundation is not only about supporting the load of a structure. Engineers must also ensure that the structure does not sink or settle excessively over time.
Two key concepts control foundation performance:
• Bearing Capacity
• Settlement
Both must be evaluated carefully during a geotechnical investigation and foundation design process.
1. What is Bearing Capacity?
Bearing capacity refers to the maximum load that soil can safely support without shear failure.
If the applied load exceeds the soil’s capacity, the ground can fail suddenly, leading to foundation instability or collapse.
Types of Bearing Capacity
Geotechnical engineers normally consider three levels of bearing capacity:
1. Ultimate Bearing Capacity
The maximum pressure the soil can sustain before failure occurs.
2. Net Ultimate Bearing Capacity
The ultimate capacity minus the overburden pressure at foundation level.
3. Allowable Bearing Capacity
The safe design pressure obtained by applying a factor of safety to the ultimate capacity.
Typical safety factors range from 2.5 to 3.0 depending on soil conditions and project requirements.
2. Classic Bearing Capacity Concept
The bearing capacity of shallow foundations is often estimated using the classical equation developed by Karl Terzaghi, widely known as the father of soil mechanics.
q_{ult}=cN_c+\gamma D_f N_q+0.5\gamma B N_\gamma
Where:
-
c = Soil cohesion
-
γ = Unit weight of soil
-
Df = Depth of foundation
-
B = Width of footing
-
Nc, Nq, Nγ = Bearing capacity factors related to friction angle
This equation helps estimate the ultimate capacity of soil beneath a shallow foundation.
3. What is Settlement?
Settlement refers to the downward movement of a structure due to compression of soil layers under load.
Unlike bearing failure, settlement usually occurs gradually over time.
Even if the soil has enough bearing capacity, excessive settlement can cause:
• Cracks in walls
• Door and window misalignment
• Structural distress
• Uneven floor levels
Therefore, settlement analysis is just as important as bearing capacity analysis.
4. Types of Settlement
Immediate Settlement
Occurs immediately after the load is applied.
Common in:
• Sands
• Gravels
• Dense granular soils
Consolidation Settlement
Occurs when water is squeezed out of saturated fine-grained soils.
Common in:
• Clay
• Silty clay
• Compressible cohesive soils
This process may take months or even years.
Secondary Settlement
Also called creep settlement. It occurs after primary consolidation has finished.
5. Why Settlement Often Controls Foundation Design
In many projects, soil may have sufficient strength but still experience excessive deformation.
For example:
A soil may safely support 300 kPa, but settlement may exceed acceptable limits at only 150 kPa.
Therefore, the allowable foundation pressure is often controlled by settlement criteria rather than shear strength.
6. Typical Settlement Limits in Practice
Although limits depend on project specifications, common values used in practice include:
| Foundation Type | Typical Allowable Settlement |
|---|---|
| Isolated footing | ~25 mm |
| Raft foundation | ~50 mm |
| Mat foundation | 50–75 mm |
| Differential settlement | Limited to small angular distortion |
These limits ensure structural safety and serviceability.
7. Factors Affecting Settlement
Settlement depends on several geotechnical and structural factors:
Soil Properties
• Compressibility
• Density
• Plasticity
• Thickness of compressible layers
Foundation Characteristics
• Foundation width
• Foundation depth
• Load intensity
Groundwater Conditions
High groundwater levels can influence soil compressibility and effective stress.
8. Field Tests Used to Estimate Bearing Capacity and Settlement
Geotechnical engineers use several field tests to evaluate soil behavior.
Common tests include:
• Standard Penetration Test (SPT)
• Cone Penetration Test (CPT)
• Plate Load Test
• Pressuremeter Test
These tests help estimate:
• Soil strength
• Soil stiffness
• Deformation characteristics
9. Importance in Geotechnical Investigation Reports
A professional geotechnical report should always provide:
• Allowable bearing capacity values
• Estimated settlement values
• Recommended foundation types
These recommendations help structural engineers design foundations safely and economically.
Key Lesson for Fresh Geotechnical Engineers
Understanding soil behavior is more important than simply applying formulas.
Always remember:
✔ High bearing capacity does not guarantee low settlement
✔ Settlement is often the governing design criterion
✔ Soil investigation data must be interpreted carefully
Good geotechnical engineering combines testing, experience, and engineering judgment.
✔ This article is part of the 30-Day Geotechnical Learning Journey on Geotech Guide, created to help students and fresh engineers understand practical geotechnical concepts used in real projects.