---
title: "Choosing pressure diagram for braced cut"
subject: "Foundation Engineering"
chapter: 4
key: "semester_5/foundation_engineering:pressure-diagram-sand-clay-braced"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/pressure-diagram-sand-clay-braced
site: BCT Engineering
---

# Choosing pressure diagram for braced cut

Foundation Engineering · Chapter 4: Arching in Soils and Braced Cuts

- Asked in: 2078 Bhadra, 2 marks
- Video: [Braced Excavation](https://www.youtube.com/watch?v=taN3qoNIvEw) — AXE Consultants
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/pressure-diagram-sand-clay-braced
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-4

## Question

Describe the criteria of selecting the pressure diagram for soil having sand and clay layers in a braced cut.

## Short answer

For sand-dominant layers, use the sand diagram with weighted average φ and γ. For clay-dominant, use the clay diagram with average c and γ. If both are significant, compute an equivalent cohesion and average unit weight, then select the clay diagram based on the stability number γH/c. For complex layering, design for the more severe case.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How are layered soils handled with Peck's diagrams?
   A. Use weighted average values over the depth H
   B. Use only top layer
   C. Ignore clay
   D. Use triangle
   Answer: A. Averages of gamma, q_u and tan phi are computed with layer thickness. (Key point: Layered soil approach)

2. How is the clay diagram type chosen?
   A. From gamma H/c_avg: above 4 soft-medium, otherwise stiff
   B. From water table
   C. From strut spacing
   D. Random
   Answer: A. c_avg = 0.5 (q_u)avg. (Key point: Choosing clay diagram)

3. Which diagram is used if clay dominates?
   A. Sand
   B. Clay diagram
   C. Triangle
   D. None
   Answer: B. The dominant soil determines the diagram. (Key point: Dominant stratum)

## Related in Foundation Engineering

- [Active thrust, 6 m wall, sand over clay](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Atwo-layer-wall-6m-sand-clay) — Foundation Engineering, chapter 3, asked 4×
- [Strut loads, 8 m cut in sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astrut-loads-8m-cut-sand) — Foundation Engineering, chapter 4
- [Strut loads in braced cut](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astrut-loads-bracing-system) — Foundation Engineering, chapter 4
- [Define strut load and braced cut](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Adefine-strut-braced-cut) — Foundation Engineering, chapter 4
- [Apparent pressure diagrams for braced cuts](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aapparent-pressure-diagrams) — Foundation Engineering, chapter 4, asked 2×
- [Active pressure, 8 m wall, table of layers](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awall-8m-two-layer-table) — Foundation Engineering, chapter 3

## Related in other subjects

- [Clay over sand, effective stress at depths](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-sand-profile-effective-stress-depths) — Soil Mechanics, chapter 6
- [Criteria for choosing HVDC or HVAC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-hvac-choice-criteria) — High Voltage Engineering, chapter 1
- [Two-layer profile stress diagram, 6 m sand](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asand-clay-profile-stress-diagram-capillary) — Soil Mechanics, chapter 6
- [Compaction curves of sand and clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-sand-vs-clay) — Soil Mechanics, chapter 5