CS5C - Concurrent Session 05C: Geotechnical Innovation in Floodwall Design: Addressing Subsurface Challenges in the Dallas-Fort Worth Metroplex

Abstract Description: The U.S. Army Corps of Engineers (USACE) Fort Worth Central City Flood Control Project will divert flood flows around a portion of the existing floodway system adjacent to downtown Fort Worth by rerouting a section of the Trinity River through a 1.6-mile-long bypass channel. This channel includes four parks, multiple vehicular and pedestrian bridges, and a two-tiered floodwall and levee system designed to enhance flood protection and urban development.

The two-tiered floodwall system consists of a lower monolithic cast-in-place (CIP) U-wall, exceeding 40 feet in height, that transitions into a lower T-wall near the channel and upper T-wall at the back of the park. While portions of the channel alignment have U-walls bearing on suitable bedrock, North Play Park presented significant geotechnical challenges due to difficult subsurface conditions. The park footprint, spanning approximately 650 feet by 250 feet, is underlain by thick, low-strength in-situ clay deposits. Historical topographic and geologic data dating to 1894, combined with channel realignment drawings from the 1950s and geotechnical and geophysical investigations, revealed that this area was once a natural river meander that was straightened and backfilled with spoil materials. Soil borings further indicated evidence of poorly compacted soils.

These geotechnical conditions, combined with significant structural loads, posed complex design challenges, including settlement concerns, axial and lateral foundation capacity issues, and global slope instability. A key issue was the presence of an adverse-sloping rock surface, which created deep-seated instability and imposed unbalanced loads up to 90 kips/ft on the lower T-wall foundations.

To address these challenges while ensuring stability, cost-efficiency, and compliance with USACE floodwall and levee standards, several innovative solutions were implemented. The upper T-wall was preloaded to eliminate deep foundations, reduce construction costs, and mitigate long-term settlement. To satisfy strict lateral deflection criteria and control foundation costs, 48-inch-diameter slope stabilization shafts were installed to separate lateral slope loads from wall loads. These shafts were preloaded by constructing the majority of the slope before installing the lower T-wall and its drilled shaft foundations, enhancing overall stability. These strategic measures helped optimize design performance, reduce costs, and ensure long-term resilience of the flood control infrastructure.