Load capacity of piles in 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 range from 1T/sqft (95kN/m2) for clay to approximately 30T/sqft (2,900kN/m2) for granite bedrock. Average compression bearing values for gravel soils range from 5 to 6T/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.

The following examples demonstrate the approach taken to calculate the load resistance developed by a driven wood pile, in both standard and metric units.

Learning examples

Example No. 1

A 35-foot pile of wood that has an average diameter of 12 inches. And a tip diameter of 8 in. It is introduced into soil that has an allowable load capacity of 4 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 resist any load due to unstable ground conditions.

Answer

Final area = (π x D²) ÷ 4
Tip diameter in feet = 8 ÷ 12 = 0.67ft
Final area = (3.1415 x 0.67²) ÷ 4 = 0.35 sq ft
Final load capacity = 4 x 2,000 x 0.35 =2,800 lb
Soil shear capacity = Soil shear strength x Effective pile surface
Effective pile surface area = Effective Pile Length x Average Circumference
The average pile circumstance in feet = π x D ---- 3.1415 x 1 = 3.14ft
Effective length en pies = 35 - 6 = 29 ft
Effective pile surface area = 29 x 3.14 = 91 sq ft
Soil cutting capacity = 700 x 91 = 63,700 lb
Total pile capacity = 2,800 + 63,700 = 66,500 lb

Example No. 2

A 10.7 m pile of wood that has an average diameter of 300 mm and a tip diameter of 200 mm is driven into soil that has an allowable load capacity of 380 kN/m2 and a shear strength of 33.5 kN/m2 . Determine the safe carrying load of the pile assuming that the top 2 m will not support any load due to unstable ground conditions.

Answer

Final area = (π x D²) ÷ 4
Tip diameter in meters = 200 ÷ 1,000 = 0.2m
Final area = (3.1415 x 0.2²) ÷ 4 = 0.03 sq m
Final load capacity = 380 = 0.03 = 11.4 KN
Soil shear capacity = Soil shear strength x Effective pile surface
Effective pile surface area = Effective pile length x Average circumference
The average pile circumstance in feet = π x D --- 3.1415 x 0.3 = 0.94 m
Effective length in meters = 10.7 - 2 = 8.7m
Effective pile surface area = 8.7 x 0.94 8.2 sq m
Soil cutting capacity = 8.2 x 33.5 = 274.7 KN
Total pile capacity = 11.4 + 274.7 = 286 KN