Pile Design and Construction Practice, Fifth edition

Average shearing strength along pile shaft (from Figure 4.43)

.

Take an adhesion factor of 0.45. Then from equation 4.7,

total shaft friction on pile shaft =

Because the clay is fissured it is desirable to reduce the average shearing strength at pile
base level to obtain the end-bearing resistance. Thus

end-bearing resistance =

For a safety factor of 2 on the total pile resistance,

allowable working load =

For a safety factor of 3 on the base resistance and 1 on the skin friction,

allowable working load =

The alternative of a bored pile with an enlarged base is now considered. Take the same
shaft diameter and a base diameter of 2 m, a base level for the pile being at 10 m below
ground level. The top of the base enlargement will be at 9 m and the base resistance must
be ignored for 2 m above this level (Figure 4.9).

Average shearing strength from cut-off level at 1 to 7 m =^12 (35111) 73 kN/m^2

(^13 965) 1841 2163 kN.

1

2 (1 841965)^1403 kN.

9 0.75 182 ^14  12965 kN.

0.45108.5  1  121841 kN.

12 (35182) 108.5 kN/m^2

Resistance of piles to compressive loads 227

0

5

10

Depth below ground level (m) 15

20

100

Average

Characteristic

Undrained shear strength

(kN/m^2 )

200 300

Figure 4.43Characteristic and average shear strength values from four boreholes.