---
title: "Khosla: is the section safe at A and B"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:khosla-safe-against-uplift-ab-230"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/khosla-safe-against-uplift-ab-230
site: BCT Engineering
---

# Khosla: is the section safe at A and B

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2071 Magh, 12 marks
- Video: [Safe Exit Gradient (Khosla's Theory)](https://www.youtube.com/watch?v=e5PnIfAn6rE) — RD STUDY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/khosla-safe-against-uplift-ab-230
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

Find whether the section provided is safe against uplift at A and B.

[Figure: water level 230 m; crest 216 m; upstream floor 213 m and 212 m with 4:1 upstream slope; upstream pile bottom 208 m; intermediate pile bottom 209 m; downstream floor levels 212 m (point B), 211 m, 210 m (point A); downstream pile bottom 205 m; distances 15 m, 25 m and 20 m]

## Short answer

For worst-case head 20 m and G=2.24, Bligh creep length 88 m gives residual heads above tailwater: B 7.27 m, A 2.27 m; unbalanced above floor top B 5.27 m, A 2.27 m. Required thickness t=h/(G-1): B 4.25 m, A 1.83 m, so A safe, B not safe.

## Answer

The structure is checked for uplift by finding the residual seepage head under the floor at A and B (creep theory), converting it to the floor thickness needed to resist it, and comparing this with the floor provided. The figure gives no tail water, so the worst case is taken: no water on the downstream side, which gives the maximum static head. Missing values are assumed as stated.

#### Assumptions
- Upstream water level 230 m; downstream water level at the lowest floor level (point A), 210 m. $H = 230 - 210 = 20$ m.
- Upstream pile depth $d_1 = 213 - 208 = 5$ m; intermediate pile $d_2 = 212 - 209 = 3$ m; downstream pile $d_3 = 210 - 205 = 5$ m.
- Floor lengths: 15 m upstream floor, 25 m under the crest, 20 m downstream floor (A at the end of the downstream floor, B at the end of the crest floor). The two steps of the downstream floor (212 to 210 m) add 2 m of vertical creep.
- Specific gravity of concrete $G = 2.24$.

#### Creep path (Bligh)
$$
\begin{aligned} L_{total} &= 2d_1 + 15 + 2d_2 + 25 + 20 + 2 + 2d_3 \\ &= 10 + 15 + 6 + 25 + 20 + 2 + 10 = 88\ \text{m} \end{aligned}
$$

The creep length from the upstream end to B is $56$ m and to A is $78$ m. Residual head above the downstream water level $= H\,(L_{total} - L_x)/L_{total}$.

| Point | Creep to point (m) | Residual head above 210 m (m) | Unbalanced head above floor top (m) | Thickness needed $t = h/(G-1)$ (m) |
|---|---|---|---|---|
| B (212 m) | 56 | 7.27 | 5.27 (7.27 less 2 m, since B is 2 m above 210 m) | 4.25 |
| A (210 m) | 78 | 2.27 | 2.27 | 1.83 |

For the floor to be safe, the submerged weight of the floor must at least balance the uplift: $t(G-1) \ge h$.

#### Conclusion
- At **A**, only 2.27 m of head remains, which needs a floor about 1.83 m thick (2.44 m with the usual extra 33 %). A floor of 2 m or more is safe.
- At **B**, the head still to be dissipated is 5.27 m, which needs 4.25 m of floor (5.67 m with the extra 33 %). A normal 1.5 to 2.5 m floor is **not safe** here against the full static head.
- Remedies: allow for the actual tail water (it reduces $H$ sharply), lengthen the floor, deepen the intermediate pile, or provide pressure-relief weep holes with an inverted filter under the crest floor.

**Answer:** With the worst-case head of 20 m, the section is safe at A (needs 1.83 m) but not safe at B (needs 4.25 m) unless more floor thickness, creep length or tail water is provided.

## Check yourself

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

1. No tail water is given in the figure. What head H is used in the uplift check?
   A. H = 230 - 213 = 17 m (upstream floor)
   B. H = 230 - 210 = 20 m, with no water downstream
   C. H = 230 - 216 = 14 m (crest level)
   D. H = 230 - 205 = 25 m (pile bottom)
   Answer: B. With no tail water the worst case is taken: downstream water at the lowest floor level (210 m), giving the maximum static head of 20 m. (Key point: Worst-case head)

2. How is the total creep length of 88 m obtained?
   A. Horizontal floor lengths plus twice each pile depth plus the 2 m of vertical step creep
   B. Each pile depth counted once plus floor lengths
   C. H multiplied by the creep coefficient
   D. Sum of floor lengths only: 15 + 25 + 20 = 60 m
   Answer: A. Piles add creep going down and coming up (2d each), so L = 2d1 + 15 + 2d2 + 25 + 20 + 2 + 2d3 = 88 m. (Key point: Total creep length)

3. How is the residual head at a point found?
   A. H x (L_total - L_x) / L_total
   B. H x (creep to the point) / L_total
   C. L_total / H
   D. H / (G - 1)
   Answer: A. The head lost grows with creep; what remains at a point is H (L_total - L_x)/L_total, which is 7.27 m at B and 2.27 m at A. (Key point: Residual head)

4. Which relation gives the thickness needed to resist an uplift head h?
   A. t = (G - 1) / h
   B. t = h / G
   C. t = h x (G - 1)
   D. t = h / (G - 1)
   Answer: D. Floor weight t x G must exceed uplift (t + h), hence t(G - 1) >= h and t = h/(G - 1); with G = 2.24 this gives 4.25 m at B. (Key point: Floor thickness for uplift)

5. What is the conclusion of the check with the worst-case head?
   A. Safe at A (needs 1.83 m) but not safe at B (needs 4.25 m)
   B. Safe at both A and B
   C. Unsafe at both A and B
   D. Safe at B but not safe at A
   Answer: A. A has only 2.27 m of head left so a 2 m floor is enough, while B still carries 5.27 m, needing 4.25 m of floor. (Key point: Result at A and B)

## Related in Irrigation and Drainage Engineering

- [Khosla: intermediate pile pressure (L1 30 m)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-headworks-levels-table-intermediate-pile) — Irrigation and Drainage Engineering, chapter 5
- [Khosla (analytical): uplift and d/s floor thickness](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-analytical-head-regulator-floor-thickness) — Irrigation and Drainage Engineering, chapter 5
- [Khosla: end pile pressure (L1 20 m)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-headworks-levels-table-end-pile) — Irrigation and Drainage Engineering, chapter 5
- [Khosla: uplift, exit gradient 0.15, floor at A-D](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-uplift-exit-gradient-015-abcd) — Irrigation and Drainage Engineering, chapter 5
- [Khosla: corrected pressures, slope correction 4%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-corrected-pressure-slope-4pct) — Irrigation and Drainage Engineering, chapter 5
- [Khosla: two piles, floor at A, B, C, exit 0.2](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-two-piles-thickness-abc-02) — Irrigation and Drainage Engineering, chapter 5

## Related in other subjects

- [Stability of gravity dam (crest 1530 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-stability-1530) — Hydro Power, chapter 4