Pile Design and Construction Practice, Fifth edition

average value of Nof 15 blows/300 mm. The pile is required to carry a compressive load of

250 kN and an uplift load of 180 kN. Determine the required penetration depth of the pile

for a safety factor of at least 2.5 on both compressive and uplift loads.

The unit shaft friction developed on a pile in sand is rather low, and thus the

penetration depth in this case is likely to be governed by the requirements for uplift

resistance.

Take a trial penetration depth of 7 m below sea bed. From Figure 4.10 for N = 15,.

The submerged density of the sand may be taken as 1.2 Mg/m^3. For an over-consolidated

sand we can take Table 4.10 gives say 1.5, giving From

Table 4.11, take

Total shaft friction =

=

Factor of safety on uplift load = 2 which is satisfactory.

Checking the base resistance using Berezantsev’s value of Nqin equation 4.16, from

Figure 4.13 with Thus

Total pile resistance

Safety factor on compressive load =

which is satisfactory.

Example 4.5

Isolated piles are required to carry a working load of 900 kN on a site where borings and

static cone penetration tests recorded the soil profile shown in Figure 4.44. Select suitable

types of pile and determine their required penetration depth to carry the working load with

a safety factor of 2.5. Previous tests in the area have shown that the ultimate base resistance

of piles driven into the dense sand stratum is equal to the static cone resistance.

The piles will attain their bearing within the sand stratum. Any type of bored and cast-in-

place pile will be uneconomical compared with the driven type. A driven and cast-in-place

pile is suitable.

Driven and cast-in-place pile

For 25-grade concrete and a working stress of 25% of the works cube strength (BS8004),

working stress on concrete =. Therefore

required diameter of pile shaft

Say nominal shaft diameter of 450 mm.

Base pressure at ultimate load 900  2.5/(  0.45^2  103 ) 14 MN/m^2

For this value a penetration depth of about 29 m is required. Taking into account the

available resistance in shaft friction, the likely penetration depth will be about 28 m. From

Figure 4.44 and Table 4.12 take unit shaft friction,fs 0.012 qc

/4

(^) ^900 1 000^4
6.25
428 mm
0.25 25 6.25 N/mm^2

777

250

3.1,

417  360777 kN.

Qb 25 1.29.81 7 0.45^2417 kN.

 31     , Nq 25 (D/B 15).

360

180

360 kN

1

2 1.51.29.81^7 ^ tan^ 24.8

(^)  7  4 0.45

0.8 0.8 31  24.8.

Ko 1. KsKo Ks 1.5.

 31

230 Resistance of piles to compressive loads