🏔️Intro to Geotechnical Science Unit 10 – Shallow Foundations

Key Concepts and Definitions

• Shallow foundations transfer loads from a structure to the near-surface soil or rock
• Typically used when the soil has sufficient bearing capacity at a relatively shallow depth
• Depth of a shallow foundation is generally less than its width
• Common types include spread footings, strip footings, and mat foundations
• Bearing capacity refers to the maximum load a soil can support without shear failure
  • Depends on soil properties, foundation size, and depth
• Settlement occurs when soil compresses under applied loads
  • Can be immediate (elastic) or long-term (consolidation)
• Frost heave is the upward movement of soil caused by freezing of water in the soil
  • Can cause damage to foundations in cold climates

Types of Shallow Foundations

• Spread footings are isolated, square, or rectangular pads that support individual columns or posts
  • Commonly used for residential and light commercial structures
• Strip footings are continuous, narrow foundations that support load-bearing walls
  • Often used in residential construction for exterior and interior walls
• Mat foundations (also called raft foundations) are large, continuous slabs that support the entire structure
  • Used when soil conditions are poor or loads are heavy
• Combined footings support multiple columns and are used when column spacing is close
• Annular ring footings are circular foundations used for storage tanks and silos

Site Investigation and Soil Properties

• Site investigation is crucial for determining soil properties and selecting an appropriate foundation type
• Includes desk study, site reconnaissance, and subsurface exploration (borings, test pits)
• Key soil properties for foundation design include:
  • Soil classification (grain size distribution, plasticity)
  • Shear strength parameters (cohesion, friction angle)
  • Compressibility and consolidation characteristics
• In-situ tests (Standard Penetration Test, Cone Penetration Test) provide valuable information about soil properties
• Laboratory tests on soil samples (direct shear, triaxial, consolidation) help determine design parameters
• Groundwater conditions and seasonal variations should be considered in the investigation

Design Considerations and Criteria

• Foundation design must ensure adequate safety against bearing capacity failure and excessive settlement
• Factors of safety are applied to account for uncertainties in soil properties and loads
  • Typical factors of safety range from 2 to 3 for bearing capacity
• Serviceability limits for settlement depend on the type of structure and its sensitivity to movement
  • Total and differential settlement should be within acceptable limits
• Frost depth and potential for frost heave should be considered in cold climates
• Presence of expansive soils, fill materials, or organic soils may require special design considerations
• Seismic loads and liquefaction potential should be evaluated in earthquake-prone areas

Load Distribution and Bearing Capacity

• Loads from the structure are distributed through the foundation to the soil
• Bearing capacity is estimated using theoretical models (Terzaghi, Meyerhof, Vesic) or empirical methods
• Key factors affecting bearing capacity include:
  • Soil strength parameters (cohesion, friction angle)
  • Foundation size, shape, and depth
  • Inclination and eccentricity of loads
• Bearing capacity equations consider three failure modes: general shear, local shear, and punching shear
• Allowable bearing capacity is the ultimate bearing capacity divided by the factor of safety
• Pressure distribution beneath the foundation is assumed to be linear or uniform, depending on soil type and foundation rigidity

Settlement Analysis

• Settlement analysis predicts the vertical deformation of the foundation under applied loads
• Immediate settlement occurs due to elastic deformation of the soil
  • Calculated using elastic theory and soil modulus
• Consolidation settlement occurs in fine-grained soils due to dissipation of excess pore water pressure
  • Estimated using consolidation theory and soil compressibility parameters
• Secondary compression is the long-term settlement that occurs after consolidation is complete
• Differential settlement is the uneven settlement of the foundation, which can cause structural distress
• Settlement calculations consider the stress distribution in the soil and the thickness of compressible layers

Construction Methods and Techniques

• Shallow foundations are typically constructed using reinforced concrete
• Excavation and soil preparation are critical steps in the construction process
  • Soil should be compacted to improve bearing capacity and reduce settlement
• Formwork and reinforcement are installed prior to concrete placement
• Concrete is placed, consolidated, and cured according to specifications
  • Proper curing is essential for achieving design strength
• Backfilling and grading around the foundation should be done carefully to avoid damage
• Quality control measures (soil compaction tests, concrete strength tests) ensure proper construction
• Waterproofing and drainage systems may be installed to protect the foundation from moisture

Common Issues and Troubleshooting

• Inadequate site investigation can lead to foundation design issues
  • Unexpected soil conditions or groundwater can cause excessive settlement or bearing capacity failure
• Poor construction practices can result in defects such as honeycombing, cold joints, or insufficient cover
• Differential settlement can cause cracking and structural damage
  • May require underpinning or other remedial measures
• Frost heave can cause uplift and damage to foundations in cold climates
  • Proper insulation and drainage can mitigate this issue
• Expansive soils can cause significant movement and damage to foundations
  • Moisture control and special design techniques (drilled piers, suspended slabs) may be necessary
• Soil erosion and scour can undermine foundations, particularly in flood-prone areas
  • Erosion protection measures (rip-rap, gabions) can prevent damage
• Regular inspection and maintenance of foundations can help identify and address potential issues before they become serious problems