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General Approach

Let's start with simple clarifications for the sake of establishing a common understanding of the terms which are going to be used frequently throughout this manual.

Ultimate and allowable compressive load bearing capacities of bored piles (drilled shafts) with elements for skin friction and tip resistance, are defined below:

\[ Q_u = Q_s + Q_b - W \]
\[ Q_a = \frac{q_s \cdot A_s}{{FS}_s} + \frac{q_b \cdot A_b}{{FS}_b} - W \]

The default factors of safety are \({FS}_s = 2.5\) for skin friction and \({FS}_b = 3.0\) for tip resistance.

Uplift Capacity

The ultimate and allowable uplift capacities are defined as:

\[ Q_{u,\text{uplift}} = \xi \cdot Q_s + W \]
\[ Q_{a,\text{uplift}} = \xi \cdot \frac{Q_s}{{FS}_s} + W \]

where \(\xi\) is the uplift coefficient (default 0.8).

Fill and Manual Layers

In addition to the estimation methods described in the following chapters, two special layer types are available:

  • Fill layers do not contribute to shaft friction or tip resistance. They only contribute to the effective stress profile (overburden pressure).

  • Manual layers allow the user to directly specify the unit skin friction \(q_s\) and unit end bearing \(q_b\) values. This is useful when site-specific test data or local practice provides known resistance values.