# Pile Capacity¶

OPILE considers that pile axial capacity can be gained in three ways:

1. Plugged Capacity comprises of external skin friction + full end bearing.
2. Unplugged Capacity comprises of external + internal skin friction + annular end bearing.
3. Tension Capacity which only comprises of external skin friction.

The pile capacity for all methods is therefore calculated as follows:

 Plugged Capacity $$Q_{ax, comp, PL} = Q_{s, out} + Q_{p}$$ Unplugged Capacity $$Q_{ax, comp, UP} = Q_{s, out} + Q_{s, in} + Q_{a}$$ Tension Capacity $$Q_{ax,tens} = Q_{s, out, tens}$$

Where:

 $$Q_{s,out}$$ is the total outside shaft capacity (compression) $$Q_{s,in}$$ is the total inside shaft capacity (compression) $$Q_{s,out,tens}$$ is the total outside shaft capacity (tension) $$Q_{a}$$ is the total end bearing resistance on the pile annulus $$Q_{p}$$ is the total end bearing resistance on the gross sectional area of the pile

In the OPILE AXCAP output the terms Plugged End Cap and Unplugged End Cap will be used. These are defined as:

 Plugged End Cap $$Q_{p}$$ Unplugged End Cap $$Q_{s, in} + Q_{a}$$

The Total shaft capacity are calculated as the sum of each individual pile element shaft friction. For length dependent methods, the accuracy of the calculated shaft capacity increases with increasing number of pile elements (increasing pile resolution).

$Q_{s} = \displaystyle\sum_{i=1}^{n} f_i \cdot h_i \cdot \pi \cdot D_i$

Where:

 $$f_{i}$$ [kPa] is the unit skin friction, calculated for pile element i (method dependent) $$D_{i}$$ [m] is the pile diameter at pile element i \ is the pile inner diameter at element i in case of inside friction calculation $$h_{i}$$ [m] is the pile element height $$n$$ [-] is the amount of pile elements $$Q_{s}$$ [kN] is the total shaft friction

Total end bearing resistances are calculated by multiplying the corresponding pile tip area with the method dependent end bearing pressures.

$\begin{split}Q_{p} &= q_p \cdot \pi \cdot \frac{D_{out}}{4}^2 &\qquad \text{Plugged condition} \\ Q_{a} &= q_a \cdot \pi \cdot {\frac{D_{out}}{4}^2 - \frac{D_{in}}{4}^2} &\qquad \text{Unplugged condition} \\\end{split}$

Where:

 $$q_{p}$$ [kPa] is the end bearing pressure calculated for plugged condition (method dependent) $$q_{a}$$ [kPa] is the end bearing pressure calculated for unplugged condition (method dependent) $$D_{out}$$ [m] is the outer diameter at pile tip $$D_{in}$$ [m] is the inner diameter at pile tip $$Q_{p}$$ [kN] is the total end bearing resistance on the gross sectional area of the pile $$Q_{a}$$ [kN] is the total end bearing resistance on the pile annulus

Shaft friction (tension and compression) and end bearing pressures (plugged and unplugged) are method dependant and are explained further within the corresponding method description.