# liquefaction.k_sigma()

Overburden correction factor on the cyclic resistance ratio.

Usage

[Source](https://github.com/lornearnold/driven-pile/blob/main/src/driven_pile/liquefaction.py#L66-L91)

``` python
liquefaction.k_sigma(
    n160cs,
    sigma_v0,
    pa=P_ATM,
)
```

## Parameters

`n160cs: array_like`  
Clean-sand equivalent blow count, clamped to 37.

`sigma_v0: array_like`  
Vertical effective stress, ksf.

`pa: float = P_ATM`  
Atmospheric pressure, ksf.

## Returns

`ndarray`  
`min(1 - C ln(sigma'v0 / pa), 1.1)` with `C = min(1 / (18.9 - 2.55 sqrt(N)), 0.3)`.

## References

Boulanger and Idriss (2014), Eqs. 2.16 and 2.16b.