Geotechnical Engineering

Foundation Engineering

Shallow versus deep foundations, pile capacity, group efficiency and negative skin friction — carrying structural loads safely to competent strata.

PART 1

Topic Breakdown & Traps

The Engineering Principle

A foundation transfers structural load to the soil within allowable bearing and settlement limits. Shallow foundations (footings, rafts) spread load near the surface; deep foundations (piles) carry it to deeper strata through end bearing

Q_{b}

plus skin friction

Q_{s}

. In closely-spaced pile groups the capacity is reduced by a group efficiency factor, and downdrag from settling fill adds negative skin friction.

The Core Formula Matrix

Ultimate pile capacity:

Q_{u} = Q_{b} + Q_{s} = q_{b} A_{b} + f_{s} A_{s}

Group capacity:

Q_{g r o u p} = η n Q_{s in g l e}

Converse–Labarre efficiency:

η = 1 - \frac{θ}{90} [\frac{( m - 1 ) n + ( n - 1 ) m}{mn}]

Negative skin friction acts downward, reducing net capacity.

The ‘IIT Traps’

⚠
Negative skin friction subtracts from capacity — it is a load, not a resistance.
⚠
**Group capacity is not simply $n \times$ single-pile capacity** in clays — apply the group efficiency (or block failure check).
⚠
End bearing vs friction. Sands favour end bearing; soft clays rely largely on skin friction.

📚 Standard references

Basic and Applied Soil Mechanics — Gopal Ranjan & A.S.R. Rao · Foundation Engineering
Soil Mechanics and Foundations — B.C. Punmia

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · NAT

A pile group of

9

piles, each of single capacity

150 kN

, has group efficiency

0.8

. The group capacity is _____ kN.

Q2MEDIUM2 Marks · NAT

A pile carries end-bearing

q_{b} = 2000 kPa

over a tip area of

0.2 m^{2}

and skin friction

f_{s} = 40 kPa

over a shaft area of

10 m^{2}

. Its ultimate capacity is _____ kN.

Q3HARD2 Marks · MCQ

Negative skin friction on a pile in a settling fill: