 Lecture – 6
SOIL MECHANICS AND
FOUNDATION ENGINEERING-III
(CE 311)
PILE FOUNDATIONS
 Classification based on the function
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End bearing piles
Friction piles
Compaction piles
Tension piles or uplift piles
Anchor piles
Fender piles or dolphins
Batter piles
Sheet piles
 Classification based on materials and composition
 Concrete piles
 Precast
 Cast in situ
 Driven piles: Cased or Uncased
 Bored piles: pressure piles and under reamed piles
 Timber Piles
 Steel Piles
 H-piles
 Pipe piles
 Sheet piles
 Composite piles
 Concrete and timber
 Concrete and steel
BEARING CAPACITY OF DEEP
FOUNDATION
 Theoretical Approach
 Pile Load Test Approach
 The Pile Driving Resistance Approach
Theoretical Approach
 Unit End Bearing: 𝑞𝑏 = 𝜎𝑁𝑞
 Unit Skin Friction: 𝑓𝑠 = 𝜎ℎ 𝑡𝑎𝑛𝛿 = 𝐾 𝜎𝑣 𝑡𝑎𝑛𝛿
 Where
 K is the coefficient of earth pressure that converts vertical stress to
lateral stress and d is interface friction angle.
 K=1 for hammered piles; = 0.7 or less for piles when hole is made first.
 d = 0.8 to 0.9 f’ for rough surface; 0.6 to 0.7 f’ for smooth surface
 LIMITING VALUES
 ULTIMATE LOAD
 FOR SINGLE TYPE OF SOIL
 𝑄𝑢 = 𝑓𝑠 𝐴𝑠 + 𝑞𝑏 𝐴𝑏
 FOR LAYERED SOIL
 𝑄𝑢 = 𝑛𝑖=1 𝑓𝑠𝑖 𝐴𝑠𝑖 + 𝑞𝑏 𝐴𝑏
Soil Type
𝑞𝑏 (kN/m2)
𝑓𝑠 (kN/m2)
Clean sand
10 000
100
Silty sand
5 000
85
Sandy silt
3 000
70
Problem
 A 600mm diameter pile is to be installed at a site that is
characterized by two sand layers. The top layer is 12m thick,
has a g=20kN/m3 and f=300. The bottom layer is 20m thick,
has a g=22kN/m3 and f=350. The ground water table is at the
ground surface. Compute the Unit End Bearing and the Unit
Skin Friction at depths of 10, 15, 20 and 25m below the
ground surface. Use Nq=20 for f=300 and Nq=50 for f=350.
Take d=f and K=1.0.
Pile Load Test Approach
Pile Load Test Approach
 When the soil is dense sand or stiff clay:
Safe load =
𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝐿𝑜𝑎𝑑
𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑆𝑎𝑓𝑒𝑡𝑦
 When the soil is not stiff and dense:
The safe load is taken as LOWER of the following:
 Half the load at which pile settlement 10% of pile diameter
 two-third of the load at which pile settlement is 12mm.
Problem
 Data from a pile load test on a 300mm diameter pile is given
below. Determine the design load on the pile.
Load (kN)
20
40
60
80
100
120
140
160
180
200
Settlement (mm)
3
5
10
13
17
22
27
34
44
62