Answer to Question #210881 in Civil and Environmental Engineering for Debasmita Datta

Question

A 15m long pre-cast concrete pile that is hexagonal in cross-section, with sides 

of 0.2m, has been driven into the layered soil deposit shown in Figure 2. The 

water table at the site varies seasonally from about 7m below the ground 

surface in summer to about 5m below the ground surface in winter. 

(a) Giving your reasons, select the most appropriate water-table to be used to 

calculate the load capacity of this pile. [1 mark]

(b) Based on the water-table selected in (a), calculate the ultimate axial load 

bearing capacity of this pile. [7 marks]

(c) Describe any additional measures that you would take before relying on 

the pile capacity calculated under (b). [2 

Answer 1

As the water level decreases in summer, effective stress increases. Because of increased effective stress, some statement of adjoining soil may occur. So negative friction may have to incorporate in calculation at axial capacity at the pile. If there are other piles nearby this pile then effect nearby piles be included.

Part a

The end bearing resistance is proportional to pile tip area (i.e., 0.4²/0.3² = 1.78), therefore:

Q_p = (1.78)(150) = 267 kN

Side friction = (0.4)(250) = 100 kN

Part b

The side friction resistance is proportional to a side area (i.e., 0.4/0.3 = 1.33), therefore:

Q_s = (1.33)(100) = 133 kN

Q_p + Q_s = 267 + 133 = 400 kN

Part c

Using a factor of safety of 3:

Q_all = (Q_p + Q_s)/F = 400/3 = 133 kN