Construction of Floor Slabs Using the Top-Down Method in Excavation Support

Bottom Slabs in the Top-Down Excavation Method

Bottom slabs constructed using the Top-Down method play a significant role in optimizing excavation support systems. These slabs are permanent structural elements, and their construction quality directly affects their long-term functionality.

The construction quality and placement elevation of bottom slabs in excavation support systems (retaining walls, nailing, micropiles, anchoring, top-down, etc.) are influenced by soil uplift pressure during unloading and resettlement due to the weight of concrete on soft clayey soils.

Factors Affecting Soil Uplift Pressure Under Slabs:

• Soil properties

• Unloading method and magnitude

• Excavation depth

• Duration of exposure to excavation

Studies show that uplift pressure is lower near diaphragm walls and columns and higher in central zones. On average, about 84% of total uplift pressure develops within 14 days of excavation.

Factors Affecting Resettlement Due to Recompression:

• Structural load (self-weight)

• Bearing capacity of subgrade soil

• Stiffness of concrete

Recompression and settlement may occur due to the weight of concrete, subgrade limitations, and formwork flexibility.

Typical Settlements:

• Average slab settlement: ~10 mm

• Beam settlement for sizes:

  • 0.8m × 0.8m → 24.5 mm

  • 1.2m × 1.4m → 30 mm

  • 1.4m × 1.2m → 31 mm