--- title: "Formed drain, trap drain; grout hole criteria" subject: "Hydropower Engineering" chapter: 4 key: "semester_7/hydropower_engineering:drain-arrangement-grout-hole-criteria" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/drain-arrangement-grout-hole-criteria site: BCT Engineering --- # Formed drain, trap drain; grout hole criteria Hydropower Engineering · Chapter 4: Headworks of Storage Plants - Asked in: 2075 Chaitra, 2+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/drain-arrangement-grout-hole-criteria - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4 ## Question Drawing a section of a concrete gravity dam, show the arrangements of vertical formed drain, trap drain and drainage hole. What are the general criteria for size, depth and pattern of grout holes for certain grouting in gravity dam foundation? ## Short answer Vertical formed drains (15-30 cm dia., 3 m spacing) lead seepage to a trap drain in the gallery. Drainage holes (75-100 mm, 20-30% head depth) relieve uplift. Grout holes: 38-76 mm dia., depth 30-50% of head, staged pattern (initial 6-12 m spacing, final 1.5-3 m), inclined 10-15° upstream. ## Answer #### Arrangement of drains in a gravity dam section ```text Upstream Downstream face |<-- 3-5 m -->| | | Vertical | gallery Water-> | formed | [===] <- trap drain in | | drain (VFD)| | the gallery floor | | | | | | Dam | | | | | | | | | | | v ----+---+---+---+-----+---[ ]---------- foundation grout grout drainage holes curtain holes (downstream of curtain) ``` - **Vertical formed drain:** 15-30 cm diameter porous or formed drains in the dam body, 3 m apart, 3-5 m from the u/s face, extending from the crest/gallery down to the foundation gallery; they collect the seepage through the concrete. - **Trap drain:** the longitudinal channel in the floor of the gallery, that receives water from the vertical drains and the drainage holes, and leads it to the sump or the d/s. - **Drainage holes:** 75-100 mm diameter holes drilled downstream of the grout curtain from the gallery into the foundation, about 3 m apart, 20-30% of the head (or 0.2 × H) in depth, to relieve the uplift. #### General criteria for grout holes (curtain grouting) - **Size:** Holes of 38-76 mm (EX/AX/NX) diameter, typically 50 mm, drilled by rotary or percussion drills. - **Depth:** The depth of the main curtain is about 30-50% of the water head (commonly $\tfrac13H$ to $\tfrac12H$), or deeper to reach an impervious rock layer (water loss less than 1-3 Lugeon). Consolidation holes are shorter, 5-10 m. - **Pattern:** A single line of holes along the u/s face (a curtain) is common, and 2-3 lines are used in poor rock. Holes are drilled in stages: primary holes at 6-12 m spacing, then secondary and tertiary holes in between (final spacing 1.5-3 m). Holes may be inclined (10-15° upstream) to cut the vertical joints. - **Pressure:** Grout pressure is raised with depth, about 0.25 bar (25 kPa) per metre of rock cover (roughly 1 psi per foot), so that the rock is not lifted or opened up. - **Grout mix:** Start with a thin mix (water:cement 5:1) and thicken to 0.5:1 as the take reduces. - **Check:** Test with water pressure (Lugeon) tests before and after; inspection holes are drilled and cores examined. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where are vertical formed drains placed? A. In the dam body near the u/s face (3-5 m from it), about 3 m apart, leading seepage down to the gallery B. In the foundation below the toe C. On the d/s face outside D. Only at the crest Answer: A. They collect seepage through the concrete in the dam body. (Key point: Vertical formed drain) 2. What is a trap drain? A. A hole drilled into the rock B. A longitudinal channel in the gallery floor that receives water from the drains and leads it to the sump or d/s C. A grout hole D. A joint in the dam Answer: B. The trap drain collects water from vertical drains and drainage holes. (Key point: Trap drain) 3. The depth of the main grout curtain is generally about A. 5-10 m B. Equal to the dam width C. 30-50% of the water head, or deeper to reach impervious rock D. Twice the water head Answer: C. Curtain depth is about one-third to one-half H; consolidation holes are 5-10 m. (Key point: Curtain hole depth) 4. How are grout holes drilled and what grout mix is used? A. All holes at final spacing together, with thick mix only B. In stages (primary, then secondary, tertiary), starting with a thin mix that is thickened as take reduces C. Only a single hole, with water D. Thick mix first and thinned later Answer: B. Staged drilling gives final spacing of 1.5-3 m, and the mix goes from about 5:1 to 0.5:1 water:cement. (Key point: Grouting sequence) ## Related in Hydropower Engineering - [Grouting and drainage galleries in gravity dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agrouting-drainage-galleries-necessity) — Hydropower Engineering, chapter 4 - [Gravity dam section: shear stress near toe (2082)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stability-2082-baishakh) — Hydropower Engineering, chapter 4 - [Gravity dam stresses with drainage gallery uplift](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stresses-2080-baishakh) — Hydropower Engineering, chapter 4 - [Gravity dam concrete volume from hydrostatic thrust](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-width-hydrostatic-thrust) — Hydropower Engineering, chapter 4 - [Foundation treatment before gravity dam construction](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afoundation-treatment-gravity-dam) — Hydropower Engineering, chapter 4 ## Related in other subjects - [Stresses in gravity dam with drain holes](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-stresses-figure) — Hydro Power, chapter 4