Overview
Horner & Shifrin provided certification services, in accordance with Title 44 Code of Federal Regulations (CFR) Section 65.10, for the St. Louis Flood Protection System, which stretches for 11 miles along the Mississippi River. Floodwalls make up 6.75 miles of the flood barrier while earthen levees comprise the other 4.25 miles. In addition, there are thirty-nine closure structures and twenty-eight pump stations located along the length of the system.
Available historic supporting data were reviewed, including: as-built and design drawings, operations and maintenance manuals, hydraulic and hydrologic design memoranda, Flood Insurance Study text and maps, and current hydrologic and hydraulic modeling results. Geotechnical investigations were conducted along the flood protection system including the drilling of ninety-six borings. Physical inspections were conducted of the levee, floodwall, closure structures, and pump stations. Selected closure structures were inspected with an ultrasonic thickness gauge to determine deterioration.
H&S reviewed the original design calculations for the structural components of the flood protection system to verify the original loading conditions and to determine if the original design meets current criteria. H&S also developed spreadsheet tools to calculate, analyze and assess the floodwalls and closure structures. Review of the structural components was based on current design manuals (EM) and engineering technical letters (ETL).
The system assessment included analysis of the floodwall and embankment sections for stability and underseepage using existing and new boring and soil information. Analysis for critical sections identified during the system review and boring and sampling review was completed as discussed in ETL 1110-2-570.
Global stability analyses considered embankments, foundation stability for flood walls, and underseepage for embankments and floodwalls. The analysis was based on procedures described in USACE Engineering Manual 1110-2-1902 (Slope Stability) to demonstrate that seepage into or through the levee foundation and embankment during loading conditions associated with the base flood will not jeopardize embankment or foundation stability.
Embankment protection was reviewed using procedures described in USACE Engineering Manual 1110-2-1913 (Design and Construction of Levees) to determine if the protection has been designed in accordance with those procedures such that no appreciable erosion of the levee embankment is anticipated during the base flood and that anticipated erosion should not result in failure of the levee embankment or foundation directly or indirectly through reduction of the seepage path and subsequent instability.
Settlement analysis was done using procedures described in USACE Engineering Manual EM 1110-1-1904. This analysis addressed embankment loads, compressibility of embankment soils, compressibility of foundation soils, age of the levee system, and construction compaction methods.
Based on the analyses described above and in the original report, some deficiencies were noted and remedial actions were required. The City quickly implemented the following projects to correct those deficiencies:
-Geotechnical calculations indicated that an exit gradient greater than 0.5 exists for length of about 200 fee between STA 8+20 and STA 11+00 . Several design options were discussed with the Corps to remediate this problem including riverside and landside berms, a sheet-pile cut-off wall, relief wells, slurry walls and jet grouting, and a trench drain. Based on cost, site constraints and a preference to avoid relief wells the trench drain was chosen as the selected alternate.
-To replace a missing panel structure, a new concrete stop-log closure structure was designed to take its place. Horner & Shifrin provided the plans and specifications for construction of both of the remedial projects.
