🏔️Intro to Geotechnical Science Unit 11 – Deep Foundations

Key Concepts and Terminology

• Deep foundations transfer loads from a structure to competent soil or rock strata at considerable depth below the ground surface
• Typically used when shallow foundations cannot provide adequate support due to poor soil conditions, high loads, or site constraints
• Examples of deep foundations include piles, drilled shafts (caissons), and micropiles
• Bearing capacity, the maximum load a foundation can support without failure, is a critical design consideration
• Settlement, the downward movement of a foundation due to soil compression or consolidation, must be within acceptable limits
• Skin friction, the resistance developed along the sides of a deep foundation element, contributes to its load-carrying capacity
• End bearing, the resistance developed at the base of a deep foundation element, also contributes to its load-carrying capacity
• Downdrag, the downward force exerted on a deep foundation by settling soil, can increase the load on the foundation

Types of Deep Foundations

• Driven piles are prefabricated elements (steel, concrete, or timber) that are driven into the ground using impact hammers or vibratory methods
  • Steel piles include H-piles, pipe piles, and sheet piles
  • Concrete piles can be precast or prestressed
  • Timber piles are often used in marine environments
• Drilled shafts (caissons) are cast-in-place concrete elements constructed by drilling a hole, placing reinforcement, and filling with concrete
  • Can be straight-sided or belled at the bottom to increase end bearing capacity
  • Suitable for high loads and difficult soil conditions
• Micropiles are small-diameter (less than 12 inches) drilled and grouted piles that can be installed in tight spaces or low-headroom conditions
  • Often used for underpinning or seismic retrofitting of existing structures
• Auger-cast piles are constructed by drilling a continuous flight auger into the ground, injecting grout through the hollow stem as the auger is withdrawn, and placing reinforcement if required
• Helical piles consist of steel shafts with helical bearing plates welded at regular intervals, installed by rotation into the ground

Site Investigation and Soil Properties

• Geotechnical site investigation is crucial for determining soil properties and selecting an appropriate deep foundation system
• Soil borings and sampling provide information on soil stratigraphy, strength, and compressibility
  • Standard Penetration Test (SPT) measures soil resistance to penetration and provides disturbed samples
  • Cone Penetration Test (CPT) measures soil resistance and pore water pressure using an instrumented cone pushed into the ground
• Laboratory tests on soil samples include moisture content, grain size distribution, Atterberg limits, and strength tests (unconfined compression, triaxial)
• Shear strength parameters (cohesion and friction angle) are used to calculate bearing capacity and pile capacity
• Soil compressibility, characterized by the compression index (Cc) and recompression index (Cr), affects foundation settlement
• Groundwater conditions, including water table depth and artesian pressures, influence foundation design and construction
• Presence of problematic soils, such as expansive clays, collapsible soils, or liquefiable sands, requires special design considerations

Design Considerations and Methods

• Design of deep foundations must consider both ultimate limit states (bearing capacity, structural capacity) and serviceability limit states (settlement, lateral deflection)
• Allowable stress design (ASD) applies factors of safety to loads and material strengths to ensure a conservative design
• Load and resistance factor design (LRFD) applies separate factors to loads and resistances based on their variability and importance
• Axial capacity of driven piles can be estimated using dynamic formulas (Engineering News Record, Gates), wave equation analysis, or pile driving analyzer (PDA) measurements
• Axial capacity of drilled shafts can be calculated using static methods based on soil properties (α-method, β-method) or estimated from load tests
• Lateral capacity of deep foundations is influenced by soil stiffness, pile geometry, and fixity conditions at the head and toe
  • Lateral load-deflection behavior can be analyzed using p-y curves, which represent soil resistance as a function of lateral deflection
• Group effects, such as overlapping stress zones and load transfer between elements, must be considered when designing closely spaced deep foundations
• Negative skin friction (downdrag) can develop when soil settles relative to the foundation, increasing the axial load and potentially causing excessive settlement

Construction Techniques

• Driven piles are installed using impact hammers (diesel, hydraulic, or air-powered) or vibratory hammers
  • Pile driving rigs can be mounted on cranes, excavators, or specialized carriers
  • Pile driving noise and vibration may be a concern in urban environments
• Drilled shafts are constructed using rotary drilling rigs with augers, drilling buckets, or casing advancers
  • Temporary or permanent casing may be used to stabilize the hole in unstable soils or below the water table
  • Concrete placement can be by free fall, tremie, or pumping methods, depending on shaft diameter and depth
• Micropiles are typically installed using small drilling rigs or mini-excavators
  • Drilling methods include rotary, percussive, or auger techniques
  • High-strength grout is placed under pressure to create a bond with the surrounding soil
• Quality control during construction includes monitoring of pile driving resistance, concrete quality, and grout pressure
  • Pile driving logs, concrete cube tests, and grout pressure gauges are used to document construction quality
• Inspection and verification of deep foundation elements may involve cross-hole sonic logging (CSL), thermal integrity profiling (TIP), or low-strain integrity testing (PIT)

Load Transfer Mechanisms

• Load transfer in deep foundations occurs through skin friction along the sides and end bearing at the base
• Skin friction develops as the soil shears against the foundation surface due to relative movement
  • The magnitude of skin friction depends on the soil type, strength, and interface roughness
  • In cohesive soils, skin friction is proportional to the undrained shear strength and the adhesion factor (α)
  • In cohesionless soils, skin friction is proportional to the effective overburden pressure and the friction angle between the soil and foundation (δ)
• End bearing develops as the foundation base compresses the underlying soil
  • The magnitude of end bearing depends on the soil type, strength, and foundation geometry
  • In cohesive soils, end bearing is proportional to the undrained shear strength and the bearing capacity factor (Nc)
  • In cohesionless soils, end bearing is proportional to the effective overburden pressure and the bearing capacity factor (Nq)
• Load transfer curves represent the relationship between the applied load and the displacement of the foundation
  • The shape of the load transfer curve depends on the soil type, strength, and stiffness
  • Stiffer soils result in steeper load transfer curves and less displacement at a given load
• The proportion of load carried by skin friction and end bearing varies with the soil profile and foundation geometry
  • In general, skin friction dominates in long, slender piles, while end bearing dominates in short, large-diameter shafts
• Negative skin friction (downdrag) can develop when soil settles relative to the foundation, increasing the axial load and potentially causing excessive settlement

Testing and Quality Control

• Quality control during deep foundation construction is essential to ensure that the design requirements are met
• Pile driving monitoring involves recording the hammer type, energy, blow count, and penetration depth
  • The pile driving analyzer (PDA) uses strain gauges and accelerometers to measure the force and velocity waves in the pile during driving
  • CAPWAP (Case Pile Wave Analysis Program) is used to analyze PDA data and estimate the static capacity of the pile
• Integrity testing methods are used to assess the quality and continuity of deep foundation elements
  • Cross-hole sonic logging (CSL) uses ultrasonic waves transmitted between tubes in a drilled shaft to detect anomalies or defects
  • Thermal integrity profiling (TIP) measures the temperature of the concrete during curing to identify variations in shaft diameter or concrete quality
  • Low-strain integrity testing (PIT) uses a small hammer impact and an accelerometer to detect changes in cross-section or material properties along the length of the foundation
• Static load testing is the most reliable method for determining the actual capacity of a deep foundation
  • Compression, tension, or lateral load tests can be performed using hydraulic jacks and reaction systems
  • The load-settlement curve obtained from the test is used to verify the design assumptions and confirm the foundation performance
• Dynamic load testing, such as high-strain dynamic testing (HSDT) or rapid load testing (Statnamic), can provide an estimate of the static capacity without the need for a reaction system
  • HSDT uses a drop weight or combustion gas to apply a rapid load to the foundation, while measuring the force and velocity response
  • Statnamic testing uses a reaction mass and a combustion chamber to generate a load pulse that simulates a static load

Environmental and Economic Impacts

• Deep foundations can have both positive and negative environmental impacts, depending on the project context and construction methods
• Driven piles can generate significant noise and vibration, which may disturb nearby residents or sensitive structures
  • Noise mitigation measures, such as shrouds or curtains, can be used to reduce the impact
  • Pre-drilling or jetting can also reduce the required driving energy and associated noise
• Drilled shafts and micropiles produce less noise and vibration than driven piles, but may generate more spoil material that requires disposal
  • The use of drilling fluids and additives should be carefully controlled to avoid contamination of groundwater or soil
  • Proper handling and disposal of spoil material is necessary to minimize environmental impacts
• The carbon footprint of deep foundations depends on the materials used, transportation distances, and construction methods
  • Concrete production is a significant source of greenhouse gas emissions, particularly due to the cement content
  • The use of supplementary cementitious materials, such as fly ash or slag, can reduce the cement content and associated emissions
  • Sustainable construction practices, such as optimizing foundation layouts or using locally sourced materials, can help reduce the environmental impact
• The economic viability of deep foundations depends on the project requirements, site conditions, and local market factors
  • Deep foundations are generally more expensive than shallow foundations due to the specialized equipment, materials, and labor required
  • However, deep foundations may be the only feasible option for certain projects, such as high-rise buildings, bridges, or offshore structures
  • The cost of deep foundations can be optimized through careful design, selection of appropriate construction methods, and value engineering
  • Life cycle cost analysis should consider not only the initial construction cost but also the long-term performance, maintenance, and decommissioning costs of the foundation system