Building Expansion Joint for Underground Structures: Tunnels and Basements

Underground structures including tunnels, basement slabs, and underground parking structures require expansion joints that provide reliable waterproofing under hydrostatic pressure while accommodating thermal movement and differential settlement. Hydrostatic pressure at underground joints depends on the depth below the water table. For a basement slab 5 meters below the water table, the hydrostatic pressure at the joint is 50 kPa. The joint must be designed to withstand this pressure without leakage. Waterstop systems embedded in the concrete are the primary waterproofing measure for underground joints. Waterstop types for underground joints include PVC waterstops, bentonite waterstops, and hydrophilic rubber waterstops. PVC waterstops are the most common type and are available in a range of profiles to suit different movement ranges. Bentonite waterstops swell when wet, providing a self-sealing mechanism. Hydrophilic rubber waterstops also swell when wet and are particularly effective for joints with small movements. Movement accommodation in underground joints is limited by the hydrostatic pressure. Joints with large movements require more complex waterstop systems that can accommodate the movement while maintaining waterproofing. Expansion loops in the waterstop provide movement accommodation without reducing the waterproofing capacity. Thermal movement in underground structures is smaller than in above-ground structures because the temperature range is smaller. The ground temperature at depth is relatively constant throughout the year, limiting the thermal movement of underground structures. However, the temperature difference between the interior and exterior of the structure can cause significant thermal movement in the walls and roof. Inspection and maintenance of underground joint waterproofing is difficult due to the limited access. Injection grouting can be used to seal leaks in underground joints without excavation. The grout is injected through ports drilled through the concrete at the joint location, filling any voids or cracks in the waterstop system.