Design and construction of pile foundations
The design of pile foundations consists of two general steps: (1) the selection and design of the piles and driving equipment to be used and (2) the study of the soils to which the loads are transmitted. Allowable bearing strengths for soils vary from 1 T/sq ft (95 kN/m2) for clay to approximately 30 T/sq ft (2,900 kN/m2) for granite bedrock.
Average compression bearing values for gravel soils range from 5 to 6 T/sq ft (450 to 575 kN/sq m). The bearing resistance of a pile is, therefore, proportional to its size and the resistance of the soil in which it is driven.
Demonstration exercise
A 55-foot pile of lumber that has an average diameter of 16 inches. And a tip diameter of 12 in. It is introduced into soil that has an allowable load capacity of 6 T/sq ft and a shear strength of 700 lb/sq ft. Determine the safe load of the pile assuming the top 6 feet do not support any load due to unstable ground conditions.
Answer :
Final area = ((π x D2) ÷ (4))
The diameter of the tip of the pile in feet = 12 ÷ 16 = 0.75 foot
Final area = ((3.1415 x 0.752) ÷ (4)) = 0.44 square foot
Final load capacity = 6 x 2,000 x 0.44 = 5,280 lb
Soil shear capacity = soil shear strength x effective pile surface area
Effective pile surface area = effective pile length x average circumference
The average pile circumstance in feet = π x D
= 3.1415 x (16 ÷ 16) = 3.1415 feet
The effective length of the pile in feet = 55 - 6 = 49 feet
Effective Pile Surface Area = 49 x 3.1415 = 153,9335
Soil cutting capacity = 700 x 153.93 = 107.751 lb
Total pile capacity = 2,800 + 107.751 = 110.551 lb
Learning exercise
Answer :
Final area = ((π x D2) ÷ (4))
The diameter of the tip of the pile in meters = 200 ÷ 1,000 = 0.2 m
Final area = ((3.1415 x 0.22) ÷ (4)) = 0.03 m2
Final load capacity = 380 = 0.03 = 11.4 KN
Soil shear capacity = soil shear strength x effective pile surface area
Effective Pile Surface Area = effective pile length x average circumference
The average pile circumstance in feet = π x D
= 3.1415 x (300 ÷ 1,000) = 0.94 m
The effective length of the pile in meters = 10.7 - 2 = 8.7 m
Effective pile surface area = 8.7 x 0.94 = 8.2 square meters
Soil cutting capacity = 8.2 x 33.5 = 274.7 KN
Total pile capacity = 11.4 + 274.7 = 286 KN
