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Groundwater flow calculations for a diaphragm wall excavation

Diaphragm walls are typically used for grounwater control barriers, especially in urban excavations. A 9m deep excavation supported by a 0.60m thick diaphragm wall is analyzed for bending moments and groundwater flow calculations with the deep excavation software DeepXcav. The following assumptions are made:

Soil: Unit weight 19 kN/m3, friction angle= 32 degrees, ultimate bond for tiebacks q= 100 kPa, soil permeability= 0.0001 m/sec

Surface elevation: +8.5m

Final excavation elevation : -0.5m

Water level:  +3.5m

Water level inside the excavation (pumped): -3.5m

Tieback data: El. +6.35m, 20 degrees, 2m spacing, 300 kN prestress, fixed length 22m, free length 8m

Excavation is 180 m wide


Solution

To capture the model dimensions properly we set half of the excavation width as the distance from the wall to the right border. We also set the same left model limit, and set the bottom elevation at -90m. The left model limit is treated as a costant head boundary, while the bottom and right limits are treated as no flow lines. This is handled automatically in DeepEX, although there is an option to set the right boundary as a constant head condition.

The excavation is modeled by assuming all stage conditions:

Stage 0: No excavation, wall installation

Stage 1: Excavation to El. +5.5

Stage 2: Installation of tieback (ground anchor) at El. +6.35 and prestressing of the tieback

Stage 3: Excavation to final elevation and dewatering to desired elevation. The finite difference flow analysis is only performed in the last stage.

The wall is analyzed with a winkler elastoplastic approach within DeepXcav that captures all construction stage history.

Results

DeepEX calculates the groundwater flow as 0.416 m3/sec per m of excavation. If a rectangular excavation is assumed (220m x 180m), then the total flow can be approximated as: 0.416m3/sec x 220m x 2 =  183m3/sec.

The following images present sample screen shots from the diaphragm wall excavation model and ground water flownet calculations:

Diaphragm wall used for groundwater cutoff and excavation support

Groundwater pressures from finite difference flownet for diaphragm wall excavation

Total hydraulic water head showing wall displacements, bending moments, and lateral soil pressures for diaphragm wall excavation analyzed with a finite difference flownet

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