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Sheet Pile Cofferdam, Dewatering, and Pile Group Design

The design of the pier foundation consisted of a 2×2 group of 12-inch square precast concrete sheet piles. An Apile analysis determined the estimated tip elevation to be 64 feet below sea level. The corresponding design capacity for the pile group was calculated to be 467.6 kips using the AASHTO LRFD Bridge Design Specifications. A structural capacity based off of the sheet piles properties was calculated as 798.4 kips. The estimated tip elevation determined from Apile was used in Group to analyze lateral displacement and differential settlement. Group calculated a maximum lateral displacement of 0.3525 inches and a maximum differential settlement of 0.2155 inches, well below the limit of 1.0-inch recommended by the structural team. The minimum tip elevation that satisfied lateral deflection and differential settlement requirements was determined to be 63 feet below sea level. However, this embedment length provided a design capacity of 361.24 kips, which did not satisfy the axial capacity of 381.45 kips provided by the structural team. Therefore, the contractor must verify that capacity is developed at the minimum tip elevation using a dynamic load test or embed the sheet piles to the estimated tip elevation. A plan view of the pile group and pier layout is also provided.

A drivability analysis was conducted using GRLWEAP. A PILECO D8-22 hammer with a driving energy of 25,300 Joules was selected with a 6-inch sheet piles cushion. The software calculated a maximum compression stress of 2.271 ksi and a maximum tensile stress of 0.463 ksi, which was less than the maximum allowable stresses of 3.3 ksi and 0.82 ksi, respectively. The maximum blows per foot required to advance the pile was 90.6 blows per foot (7.6 blows per inch), which was less than the industry accepted refusal limit of 10 blows per inch. Therefore, it was feasible to drive the sheet piles to an estimated tip elevation of 64 feet below sea level using PILECO D8-22.

Sheet piles cofferdam and dewatering systems were required in order to facilitate abutment rehabilitation and pier construction. The sheet piles cofferdam design was completed using SPW-911 in order to determine the toe embedment depth and deflection of the sheet piles. Sheet piles were selected for each of the three locations without corrosion concerns due to the temporary nature of the sheet piles. A summary table showing the maximum toe embedment and deflection for each location was provided below. Due to the temporary nature of the cofferdams, it was determined that any deflection less than 6 inches would be acceptable. The total seepage along the length of the sheet piles was then calculated using the empirical graph shown below. The ratio of the toe embedment depth versus the depth to the impermeable layer was correlated to the seepage per linear foot, coefficient of permeability, and head difference. By solving for the seepage per linear foot and multiplying by the estimated cofferdam perimeter, total seepage was calculated. The seepage at each location was less than 5 gallons per minute, so any dewatering pump with a capacity of 5 gallons per minute could maintain dewatered conditions. Pump AS-215 (HCP Pumps, 2017) was selected for initial dewatering as it had a capacity of 52 gallons per minute. A summary of the sheet piles cofferdam design results was provided in the table below.

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