--- title: "Foundation treatment before gravity dam construction" subject: "Hydropower Engineering" chapter: 4 key: "semester_7/hydropower_engineering:foundation-treatment-gravity-dam" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/foundation-treatment-gravity-dam site: BCT Engineering --- # Foundation treatment before gravity dam construction Hydropower Engineering · Chapter 4: Headworks of Storage Plants - Asked in: 2079 Baishakh, 4 marks - Video: [Foundation treatment for Gravity Dams/ surface preparation (part1)](https://www.youtube.com/watch?v=mQDKXC4s3Ic) — Shamsheer khan - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/foundation-treatment-gravity-dam - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4 ## Question What measures are applied for treatment of the foundation before construction of a gravity dam? Discuss briefly. ## Short answer Foundation treatment: excavate to sound rock, dental concrete filling of faults, consolidation grouting (shallow holes, low pressure), curtain grouting (deep holes near heel, high pressure), drainage holes, and shear keys or rock anchors to increase sliding resistance. ## Answer A gravity dam must rest on sound rock. The foundation is treated before construction to give strength, reduce settlement, and control seepage and uplift. The common measures are: 1. **Excavation and cleaning:** Remove the soil, loose rock, weathered rock and vegetation until sound rock is reached. The surface is cleaned with an air-water jet and washed. The surface is shaped in steps, with the faces normal to the thrust, so that the bond with the concrete is good. 2. **Treatment of faults, shear zones and weak seams:** Dig out the weak material to a depth that depends on the seam width $b$ and the dam height $H$ (USBR: $d=0.3b+1.5$ m for $H<46$ m, and $d=0.002bH+1.5$ m for $H\ge46$ m), then fill with concrete (**dental treatment**). Wide shear zones are covered with concrete slabs or plugs, and the zone below is grouted. 3. **Consolidation grouting:** Short holes (5-10 m) drilled in a grid under the dam base and injected with cement grout at low pressure, to fill the cracks, improve the strength and reduce the settlement. 4. **Curtain (cut-off) grouting:** A line of deep holes (30-50% of the dam height) near the heel injected with cement grout at high pressure, forming an impervious curtain. It reduces seepage and uplift. 5. **Drainage:** A row of drain holes behind the curtain, drilled from the drainage gallery, relieves the uplift pressure. 6. **Key trench or shear keys:** Keys or anchors into the rock are provided to increase the sliding resistance when the foundation has low shear strength. 7. **Rock anchors and bolts:** Anchor the dam or the abutment slopes against sliding along planes of weakness. 8. **Cut-off wall / upstream blanket (on soft or alluvial foundations):** Sheet piles, concrete cut-off walls or impervious blankets are used to lengthen the seepage path. 9. **Slope treatment of abutments:** Shotcrete, rock bolts and drains to stabilise the slopes. The extent of the treatment is decided from drilling logs, water pressure (Lugeon) tests and geological mapping, and the grouting is checked by pressure tests after completion. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the purpose of consolidation grouting? A. To form a deep impervious curtain at the heel B. To fill cracks with low-pressure grout in shallow holes under the base, improving strength and reducing settlement C. To relieve uplift D. To lengthen the seepage path above ground Answer: B. Short holes (5-10 m) in a grid are grouted at low pressure to strengthen the rock. (Key point: Consolidation grouting) 2. Curtain grouting differs from consolidation grouting in that it uses A. Shallow holes over the whole base B. No pressure C. A line of deep holes (30-50% of dam height) near the heel at high pressure to cut off seepage D. Drain holes in the downstream toe Answer: C. The curtain is deep, near the heel, forming an impervious barrier that reduces seepage and uplift. (Key point: Curtain grouting) 3. What is dental treatment of faults and weak seams? A. Grouting them with cement only B. Covering them with soil C. Leaving them untreated D. Digging out the weak material and filling with concrete Answer: D. Weak material is excavated to a depth (about 1.5 times the width) and replaced by concrete. (Key point: Dental treatment) 4. Where are the drain holes placed and why? A. Behind the grout curtain, drilled from the gallery, to relieve uplift B. In front of the curtain to stop water C. In the dam crest to drain rain D. At the toe to increase friction Answer: A. They collect water that passes the curtain and release the uplift pressure. 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