---
title: "Arching and heave of bottom of cut"
subject: "Foundation Engineering"
chapter: 4
key: "semester_5/foundation_engineering:heave-bottom-cut-soft-clay"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/heave-bottom-cut-soft-clay
site: BCT Engineering
---

# Arching and heave of bottom of cut

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

- Asked in: 2075 Asoj, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/heave-bottom-cut-soft-clay
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-4

## Question

Explain arching in soils. Explain heave of the bottom of cut in soft clays.

## Short answer

Arching is pressure transfer from yielding soil to adjacent stationary soil through shear resistance. Bottom heave in soft clay occurs when the surcharge γH exceeds the bearing capacity (≈5.7c), causing upward movement. Factor of safety is FS = 5.7c / (γH − √2 cH/B). Remedies include deeper embedment or a stiff base slab.

## Answer

#### Arching in soils
Arching is the transfer of pressure from a yielding part of a soil mass to the neighbouring stationary parts by shear resistance of the soil. It reduces the load on the yielding part and increases it on the adjacent parts (shown by Terzaghi's trap door experiment). It explains the non-triangular pressure on braced cuts and tunnel linings.

#### Heave of the bottom of a cut in soft clay
In a deep cut in soft or medium clay, the weight of the soil beside the cut ($\gamma H$) acts as a surcharge on the soil at the base level. If this exceeds the bearing capacity of the clay below the base, the clay is pushed into the excavation, so the **bottom heaves up** and the ground behind the wall settles.

```text
   ~~~~~~~ ground ~~~~~~~
   |  gamma*H  |   |  gamma*H  |
   |  surcharge| cut | surcharge |
   |           |_____|           |
   |      ,-""  ^ heave "-.       |
   |     (   failure arc    )     |
```

- **Cause:** $\gamma H$ at the cut base level compared with bearing capacity $\approx 5.7c$ (Terzaghi) when $\gamma H/c$ is large (above about 5.7).
- **Factor of safety (Terzaghi):**
$$
FS = \frac{5.7c}{\gamma H - \sqrt{2}\,cH/B}
$$
($B$ = width of the cut).
- **Effects:** heaving of the cut bottom, increased pressure on sheeting and struts, settlement of ground outside, possible failure of struts.
- **Remedies:** deeper embedment of sheet piles into a stiffer layer, a stiff base (concrete slab or grouted block), lowering water level, cutting in stages or with berms.

## Check yourself

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

1. What causes bottom heave in soft clay?
   A. Weight of soil beside the cut (gamma H) acting as surcharge exceeds bearing capacity of the clay below the base
   B. Strut failure
   C. Water seepage only
   D. Temperature
   Answer: A. Clay is pushed into the excavation. (Key point: Cause of heave)

2. Which formula gives FS against heave?
   A. 5.7c/(gamma H - sqrt(2) cH/B)
   B. c/gamma H
   C. gamma H/5.7c
   D. 5.7c B
   Answer: A. Bearing capacity 5.7c over net driving surcharge. (Key point: Terzaghi FS)

3. Which is a remedy?
   A. Deeper embedment into stiffer layer
   B. Remove struts
   C. Increase depth
   D. Raise water level
   Answer: A. Also stiff base slab, lowering water level, berms. (Key point: Remedies)

4. In arching, what happens to the pressure on neighbouring stationary parts?
   A. Increases
   B. Decreases
   C. Zero
   D. Same
   Answer: A. Load is transferred to them by shear. (Key point: Arching meaning)

## Related in Foundation Engineering

- [Trench bottom heave safety factor](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Atrench-heave-rock) — Foundation Engineering, chapter 4