KB6022 Geotechnical Engineering Lecture Notes

KB6022 Geotechnical Engineering Lecture Notes

Lecture Overview

  • Course Number: KB6022

  • Subject Area: Geotechnical Engineering

  • Lecture Title: Deep Foundations - Bearing Capacity

  • Institution: Northumbria University, Newcastle

Module Structure

  • Lectures: 2

  • Seminars and IT Workshops: 3

  • Guest Lecture: 1

    • Topics Covered:

    • Lateral Earth Pressure

    • Design of Earth Retaining Structures

    • Bearing Capacity

    • Settlements

    • Slope Failures

    • Rotational Slip

    • Retaining Walls

    • Deep Foundations

    • Slope Stability

  • Exam Revision: Week 11

  • Enrichment Week: Week 6

  • Eurocode 7 Workshop:

    • Lecture: 1

    • Seminar: 1

Recommended Reading

  • Basic Soil Mechanics by R. Whitlow (2001)

  • Craig's Soil Mechanics by Jonathan Knappett & R. F. Craig

  • Soil Mechanics: Principles and Practice by Graham Barnes (2016)

  • Decoding Eurocode 7 by A. J. Bond & A. J. Harris (2008)

  • Standards:

    • BS EN 1997-1:2004 (Part 1)

    • BS EN 1997-3:2025 (Part 3)

Lecture Content Outline

  • Introduction to Foundations

  • Deep Foundations: Classification

  • Design According to Eurocode 7

  • Bearing Capacity of Individual Piles:

    • Cohesionless Soils

    • Cohesive Soils

  • Bearing Capacity of Groups of Piles

Foundations

  • Definition:
    Foundations are the part of the structure at the interface between the foundation soil and the structure itself.

Types of Foundations

  • Shallow Foundations:

    • Depth to formation level is less than the breadth.

    • Types:

    • Pad footings for columns

    • Strip footings for walls

    • Rafts for whole structures

  • Deep Foundations:

    • Depth to base is much greater than the breadth.

    • Types:

    • Piles in group beneath a building or a column

    • Pier or caisson beneath a major structural element (e.g. bridge piers)

Roles of Pile Foundations

  1. Load Transmission:
    Transmit foundation load to a solid soil layer through unsuitable soils (e.g., peat, landfill, soft clays).

  2. Settlement Control:
    Reduce settlements of structures.

  3. Load Resistance:
    Resist horizontal and uplift loads.

  4. Footing Erosion Prevention:
    Prevent problems due to erosion at the footing level.

  5. Soil Compaction:
    Compact loose layers of granular soil during installation.

Deep Foundations Classification

  • **By Size: **

    • Small diameter: $d ext{ } ext{ } \leq 25cm$

    • Medium diameter: $30cm \leq d \leq 60cm$

    • Large diameter: $d \geq 80cm$

  • By Material:

    • Timber

    • Concrete

    • Steel

  • By Load Transfer Method:

    • End Bearing Pile: Transmits load through the base.

    • Skin Friction Pile: Uses friction along the lateral surface.

    • Combination Pile: Uses both end bearing and skin friction.

Deep Foundations Installation Methods

  1. Driven Piles:

    • Prefabricated or cast-in-place using a pile driver.

    • Displacement of soil around the pile occurs, no soil removal.

    • Max Depth: $L{max} = 20m$, Max Diameter: $d{max} = 0.5m$.

  2. Bored Piles:

    • Formed within a drilled borehole.

    • Soil removal by boring, concrete cast in the shaft.

    • Max Depth: $L{max} = 70m$, Max Diameter: $d{max} = 1.2m$.

  3. Continuous Flight Auger (CFA) Piles:

    • Soil removed by continuous auger with a hollow stem.

    • Concrete pumped down while auger is withdrawn.

Design According to Eurocode 7 (EC-7)

  • Ultimate Limit States:

    1. Bearing resistance failure of the pile

    2. Loss of overall stability

    3. Uplift or insufficient tensile resistance

    4. Structural failure of the pile

    5. Ground failure due to transverse loading

  • Serviceability Limit States:

    1. Excessive settlement

    2. Excessive heave

    3. Excessive lateral movement

    4. Unacceptable vibrations

Design Issues - Failure

  • Shear strength of soil:

    • Maximum internal resistance to shearing forces.

    • Determines resistance to failure in foundation loading and slope instability.

  • Parameters for Calculations:

    • Long-term (drained):

    • $c^{ ext{'}}$ (effective cohesion)

    • $ ext{φ}^{ ext{'}}$ (effective angle of shearing resistance)

    • Short-term (undrained):

    • $S{u}$ (undrained shear strength), also termed $c{u}$ (undrained cohesion).

Bearing Capacity of Pile Foundations

  • Loading Conditions:

    • Axially Loaded Piles:

    • Compression $
      ightarrow$ Compressive resistance

    • Tension $
      ightarrow$ Tensile resistance

    • Transversely Loaded Piles:

    • Transverse load resistance

  • Requirements for Verification:

    • For axial compression pile:
      F{c,d} < R{c,d}
      Where:

    • $F_{c,d}$: Design axial compression load on a pile

    • $R_{c,d}$: Design compressive ground resistance

Calculating Design Values

  • Design value of ground resistance:
    R<em>c,d=R</em>c,kγR<em>{c,d} = \frac{R</em>{c,k}}{\gamma}
    Where:

  • $R_{c,k}$: Characteristic value of ground resistance

  • $ ext{γ}$: Partial factor of safety

  • Characteristic value obtained from tests:

    • Static load tests

    • Ground tests

    • Dynamic tests

Design Approaches for Piles

Design Approach 1 and 2

  • Factor sets used for design of axially loaded piles categorized into combinations A1-A2, M1-M2, and R1-R4 tailored for different loading scenarios.

Ultimate Bearing Capacity of Pile Foundations

  • Ultimate bearing capacity of a pile combines:
    Q<em>u=Q</em>b+QsQ<em>{u} = Q</em>{b} + Q_{s}
    Where:

  • $Q_{b}$: Base resistance

  • $Q_{s}$: Skin friction

  • For cohesionless soils (drained):
    Q<em>b=q</em>bA<em>b=σ</em>v′N<em>qA</em>bQ<em>{b} = q</em>b A<em>b = \sigma</em>{v}^{'} N<em>{q} A</em>b

  • For cohesive soils (undrained):
    Q<em>b=q</em>bA<em>b=c</em>uN<em>cA</em>bQ<em>{b} = q</em>b A<em>b = c</em>{u} N<em>{c} A</em>b

Structural Considerations for Pile Groups

  • Group of piles is usually installed and jointed by a slab (pile cap).

  • If spacing < 5d (d = pile diameter), pressure bulbs of individual piles overlap; typical spacing is 2-3d.

  • Pile driving in sand and gravel between piles can increase group bearing capacity.

  • A pile group in cohesive soil has a collective strength less than the sum of individual strengths due to block failure.

Summary of Key Equations

  • Ultimate Bearing Capacity Equation:

    • For cohesionless soils:
      Q<em>u=q</em>bA<em>b+q</em>sAsQ<em>{u} = q</em>{b} A<em>{b} + q</em>{s} A_{s}

    • For cohesive soils:
      Q<em>b=c</em>uN<em>cA</em>bQ<em>{b} = c</em>{u} N<em>{c} A</em>{b}
      Q<em>s=αc</em>uAsQ<em>{s} = \alpha c</em>{u} A_{s}

Key Insights

  • Cohesionless soils exhibit linear increases in capacity up to critical depths; below these depths, values stabilize.

  • Methodology for determining physical properties relies on both static testing and reference to established coefficients involving effective pressure, angle of friction, and piled soil interface conditions.