DESIGN FEATURES AND CONSTRUCTION 167
3.5.4 Internal design features- Inspection galleries.A low-level inspection gallery is necessary to
collect seepage inflow from the uplift relief drains. The gallery also
serves to give access to instruments (Section 7.2) and to internal dis-
charge valves and pipework. Galleries should be not less than
2.0 m1.2 m in section, and adequate provision must be made for
access, ventilation and lighting. Larger dams may also have one or
more galleries at higher levels, interconnected by vertical shafts. - Transverse contraction joints (interblock joints).Vertical contraction
joints are formed at regular intervals of 12–15 m along the axis of the
dam. The joints are made necessary by the shrinkage and thermal
characteristics of mass concrete (Section 3.6). They permit minor dif-
ferential movements between adjacent blocks, and in their absence
major transverse cracks will develop. To control seepage along the
plane of the joint a water barrier detail similar to the example of Fig.
3.19 is formed close behind the upstream face. - Construction joints (interlift joints).Individual concrete pours within
each monolith must be limited in volume and in height to reduce
post-construction shrinkage and cracking. Concrete pours are there-
fore restricted by the regular formation of near-horizontal construc-
tion or ‘lift’ joints. Lift height is generally limited to 1.5–2.0 m. The
lift surface is generally constructed with a stepped or uniform fall of
5–10% towards the upstream face to improve the notional resistance
to sliding on that potentially weaker plane.
Fig. 3.19 Typical transverse contraction joint and shear key details