Question 31.2: Find the end bearing capacity of a 3 ft (1 m) diameter caiss...
Find the end bearing capacity of a 3 ft (1 m) diameter caisson using the Kulhawy values from Table 31.2. Assume the soil is saturated at the tip level of the caisson and the average SPT (N) value at the tip level is 10blows/ft. See Fig. 31.5.
Table 31.2
Depth and end bearing capacity of piles in different soils
| Depth | Saturated loose sand | Dry loose sand | Saturated very dense sand | Dry very dense sand | |||||
| ft | m | tsf | MN/m² | tsf | MN/m² | tsf | MN/m² | tsf | MN/m² |
| 20 | 6.1 | 10 | 0.95 | 50 | 4.8 | 60 | 5.7 | 140 | 13.4 |
| 40 | 12.2 | 25 | 2.4 | 60 | 5.7 | 110 | 10.5 | 200 | 19.2 |
| 60 | 18.3 | 40 | 3.8 | 70 | 6.7 | 160 | 15.3 | 250 | 23.9 |
| 80 | 24.4 | 50 | 4.8 | 90 | 8.6 | 200 | 19.2 | 300 | 28.7 |
| 100 | 30.5 | 55 | 5.3 | 110 | 10.5 | 230 | 22.0 | 370 | 35.4 |
Source: Kulhawy (1984).
An average N value of 10 blows per foot can be considered as loose sand. Hence, the soil below the pile tip can be considered to be loose sand. The depth to the bottom of the pile is 20 ft.
From Table 31.2, the ultimate end bearing capacity for saturated loose sand at 20 ft is 10tsf (0.95 MN/m²).
cross sectional area of the caisson = (π × D²/4) = 7.07 sq ft (0.785 m²)
ultimate end bearing capacity = area × 10 tons = 70.7 tons (0.74 MN)
It should be pointed out that some of the values in Table 31.2 provided by Kulhawy are very high. For example, a caisson placed in very dense dry soil at 20 ft gives 140 tsf.
It is recommended to use other methods to check the values obtained using tables provided by Kulhawy.