# shaft.coyle_castello_k()

Coyle and Castello earth pressure coefficient at the ground surface.

Usage

[Source](https://github.com/lornearnold/driven-pile/blob/main/src/driven_pile/shaft.py#L51-L78)

``` python
shaft.coyle_castello_k(
    phi,
    width,
)
```

## Parameters

`phi: array_like`  
Friction angle, deg.

`width: float`  
Pile width, ft.

## Returns

`ndarray`  
K_t interpolated between the tabulated values at 30, 33 and 36 deg and extended above 36 deg in proportion to [nordlund_k_delta](../reference/shaft.nordlund_k_delta.md#driven_pile.shaft.nordlund_k_delta) at the pile's displaced volume.

## References

Coyle and Castello (1981).