The design of how the construction was to be accomplished was left to the bidding contractor without any significant technical review of how the installation would be done. The contractor chose to install the intake pipes by constructing two 5 ft. diameter cased horizontal bores from the wet well to near the riverbank face. The 3 ft. intake pipes would be installed inside these bores. The remainder of the intake pipe would be installed by subaqueous excavation from the riverside. Consequently, this would require underwater excavation into the riverbank to connect to the pipe installed form the wet well. This pipe intake construction methodology was allowed under the contract, and this methodology seemed feasible based on the available design borings which indicated that bedrock was at an elevation which would adequately support the overlying soils when the riverbank excavation was made (see Figure 2).
The contractor subaqueously advanced the excavation into the riverbank, but due to the limited visibility conditions, there was no warning that the soil portion of the slope was unknowingly undermined as the bedrock surface was significantly lower than anticipated (see Figure 2). Ultimately, this resulted in two successive slides (see Figure 3) and a halt in the project due to potential retrogressive movements encompassing the adjacent 4-lane state highway. During construction, none of the slope stability issues were analyzed by any of the project team members. After much time and expense, the project was eventually complete.
