# shaft.fs_coyle_castello()

Unit shaft resistance of a driven pile in sand.

Usage

[Source](https://github.com/lornearnold/driven-pile/blob/main/src/driven_pile/shaft.py#L81-L110)

``` python
shaft.fs_coyle_castello(
    sigma_v0,
    phi,
    z,
    width,
    delta_ratio=0.8,
)
```

## Parameters

`sigma_v0: array_like`  
Vertical effective stress, ksf.

`phi: array_like`  
Friction angle, deg.

`z: array_like`  
Depth below the ground surface, ft.

`width: float`  
Pile width, ft.

`delta_ratio: float = 0.8`  
Interface friction angle divided by `phi`.

## Returns

`ndarray`  
`sigma'v0 K_t(phi) 10**(-0.017 z / B) tan(delta_ratio phi)`, ksf.

## References

Coyle and Castello (1981).