Choosing pressure diagram for braced cut

When the retained soil is layered with sand and clay, Peck's diagrams (made for a single homogeneous soil) cannot be used directly. The usual criteria (Peck) are:

1. Cut wholly in sand, or sand clearly dominant: use the sand diagram, $p_a = 0.65K_a\gamma H$, with a weighted average friction angle

$$\phi_{avg} = \tan^{-1}\left[\frac{1}{H}\sum H_i\tan\phi_i\right]$$

and the weighted average unit weight $\gamma_{avg} = \dfrac{1}{H}\sum\gamma_iH_i$. 2. Cut wholly in clay, or clay clearly dominant: use the clay diagram with the average unit weight and cohesion of the clay layers. 3. Sand and clay both significant: Peck proposed an equivalent cohesion and average unit weight over the full depth $H$, and the clay diagram is then used. For a sand layer of thickness $H_s$ over clay (thickness $H-H_s$):

$$c_{avg} = \frac{1}{2H}\left[\gamma_sK_sH_s^2\tan\phi_s + (H-H_s)\,n\,q_u\right],\qquad \gamma_{avg} = \frac{1}{H}\left[\gamma_sH_s + \gamma_c(H-H_s)\right]$$

where $K_s \approx 1$ is the lateral pressure coefficient of the sand, $q_u$ is the unconfined compressive strength of the clay, and $n$ is the coefficient of progressive failure (0.5 to 1.0, average about 0.75). 4. Choose the clay diagram from the stability number $\gamma_{avg}H/c_{avg}$: if it is greater than 4 (soft to medium clay), use the soft-to-medium clay diagram; if it is 4 or less (stiff clay), use the stiff clay diagram. 5. Where the layering is complex, compare the loads from the possible diagrams and design for the more severe case.