CVE2332 Seepage and Flownet Assignment Report: Singapore River Cofferdam Case Study for Optimal Sheet Pile Design

Overview of the Course Work

The Singapore River, approximately 3.2 km in length, flows from the junction of Margaret Drive and Tanglin Road, following the course of Alexandra Road and eventually discharging into the Marina Reservoir in southern Singapore. A major infrastructure project is underway involving the construction of a bridge that crosses the river. One of the bridge piers is proposed to be situated within the river itself. To facilitate the pier foundation works, a sheet pile cofferdam is required to provide a dry working area measuring 12 meters in width and extending longitudinally between 1 to 2 kilometers, as shown in Figure 1.

Figure 1. General configuration of the cofferdam and boundary conditions

The groundwater level is 3.5 meters above the riverbed. Below the riverbed lies a 6-meter-thick layer of silty sand, underlain by low-permeability marine clay. The silty sand has a permeability coefficient of 8×10⁻⁵ m/s, and a total unit weight γ! =18 kN/m³. Pumps must be operated throughout the construction period to maintain dry conditions inside the cofferdam. The cost of sheet pile installation and removal depends on the penetration depth � as illustrated in Figure 2. Additionally, operating pumps incurs a cost of SGD 2,000 per litre/second of pumping capacity over the duration of construction.

Objective

This exercise aims to determine the optimum sheet pile penetration depth that ensures:

• Structural safety by preventing excessive upward seepage pressures.
• Cost-effectiveness by minimizing the total construction cost, including both sheet piling and pumping.

Figure 2. Unit price of pumps

Task Requirements

Working in assigned groups, students must:

• Construct flownets for three different sheet pile penetration depths: 1 m, 3 m, and 5 m.
• Analyze each flownet to estimate:
  • The required pumping rate to maintain dry working conditions.
  • The average upward hydraulic gradient within the soil zone enclosed by the dashed region in Figure 1.
• Determine the optimum penetration depth by:
  • Minimizing the total cost (sum of pumping and sheet pile installation/removal).
  • Ensuring the average hydraulic gradient remains below 60% of the critical hydraulic gradient associated with a quick (boiling) condition.

Deliverables

Each group must submit a formal technical report (capped at 15 pages) containing the following components:

• Abstract (10 marks)
  • A concise summary of the objective, methods, key results, and final recommendation.
• Introduction (10 marks)
  • Background of the site and problem.
  • Statement of objectives and scope of work.
  • Description of the methods used (flownet analysis, cost calculation, and safety checks).
• Main Body (60 marks)
  • Clear presentation of the three flownets (for 1 m, 3 m, and 5 m penetrations).
  • Hydraulic gradient assessments and comparison with critical conditions.
  • Detailed cost calculations.
  • Tables, graphs, and explanatory discussions that support the final decision.
• Conclusion (20 marks)
  • Summary of findings and rationale for the recommended sheet pile depth.
  • Reflection on engineering considerations and potential limitations.

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