# shaft.interface_alpha()

Interface factor on the residual strength of liquefied soil.

Usage

[Source](https://github.com/lornearnold/driven-pile/blob/main/src/driven_pile/shaft.py#L113-L140)

``` python
shaft.interface_alpha(
    sr,
    sigma_v0,
    psi_nc=0.22,
)
```

## Parameters

`sr: float`  
Residual strength, ksf.

`sigma_v0: float`  
Vertical effective stress, ksf.

`psi_nc: float = 0.22`  
Normally consolidated undrained strength ratio.

## Returns

`float`  
`min(1, sqrt(psi_nc / psi))` for `psi = sr / sigma'v0 <= 1` and `sqrt(psi_nc) psi**-0.25` above; 1 when the inputs are not positive.

## References

Randolph and Murphy (1985), as adopted in API RP 2A.