Foundation treatment before gravity dam construction

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.