# bearing.rigidity_index()

Rigidity index from a stress-dependent shear modulus.

Usage

[Source](https://github.com/lornearnold/driven-pile/blob/main/src/driven_pile/bearing.py#L50-L81)

``` python
bearing.rigidity_index(
    phi,
    sigma_v,
    options,
)
```

## Parameters

`phi: array_like`  
Friction angle, deg.

`sigma_v: array_like`  
Vertical effective stress, ksf.

`options: ModelOptions`  
Supplies `g_ratio`, `k2_lo`, `k2_hi`, `phi_lo`, `phi_hi` and `irr_bounds`.

## Returns

`ndarray`  
`I_r = g_ratio G_max / (sigma'_m tan phi)` clamped to `irr_bounds`, with `sigma'_m = (1 + 2 K0) / 3 sigma'_v` and `G_max = 21.7 K2 Pa sqrt(sigma'_m / Pa)`.

## References

Seed and Idriss (1970).