The Chartership Forum

Foundation Engineering / module 17

Pile groups, settlement and negative skin friction

Nine piles are not nine times one pile — in capacity, in settlement, or in what they carry.

What you will understand

  • Compute group efficiency and say what physical effect it represents
  • Check block failure of a group in clay and recognise when it governs
  • Use the equivalent raft to estimate group settlement, and explain why it exceeds a single pile's
  • Compute downdrag from negative skin friction and explain why it is a settlement problem

Lessons

  1. Group capacity

    Efficiency, and the failure mode that ignores the individual piles.

  2. Settlement and downdrag

    Two problems that are both about movement, not strength.

Module summary
  • Converse–Labarre gave 0.73 for a 3×3 group at three diameters and 0.53 for a 5×5 at two
  • The 5×5 block resistance 20 866 kN is below the unadjusted 25 164 kN sum, but above the≈13 280 kN efficiency route. Compare both consistently.
  • A group loads a wider volume of ground; the equivalent-raft illustration gives 100.1 mm, but cannot establish a single-pile/group settlement ratio
  • The equivalent raft sits at two-thirds of the embedded length and gets the stressed depth roughly right
  • Negative skin friction is a load, not a resistance, and it is limited by how far the ground can settle
Teaching basis and review status

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Check your understanding

Practice

A 0.6 m pile passes through 6 m of fill with γ′ = 10 kN/m³ and β = 0.3, all of it above the neutral plane. What downdrag load does it attract, in kN?

Check yourself

Which remedy addresses the cause of downdrag rather than its symptom?