Formed drain, trap drain; grout hole criteria

Arrangement of drains in a gravity dam section

   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.