SI-2222 – Pengenalan Mekanika Tanah Dr. Endra Susila March, 20 2015

HOMEWORK: SEEPAGE BY FINITE DIFFERENT To control water flooding, the government of DKI Jakarta has proposed to construct a pond at an area in Depok. The site of this pond requires impoundment. A concrete dam, as shown in Figure 1, has been proposed to hold back the water from the creek. Your firm has been asked to do a seepage analysis for the proposed project site. Your senior engineer gives the entire job to you except for writing the report. As a new fresh graduate, you want to impress your senior engineer, therefore, instead of drawing a rough flownet, you are going to calculate the potential lines utilizing a finite difference technique on a spreadsheet. In order to do the assignment, you need the necessary formulas for different boundaries which might be encountered. You find these equations in an article entitled “Engineering with Spreadsheets” by David E. Kleiner in Civil Engineering, the October 1985 edition (discussed in our class today). You also realize that you will need some boundary conditions for the upstream and downstream sides of the dam. These values have been calculated previously and are shown in Figure 2. You now enter the worksheet and calculate all the necessary equipotential points (see handout labeled “PROCEDURE FOR FINITE DIFFERENCE USING EXCEL”). After all the calculations are completed, you place the missing equipotential points on Figure 2. Equipotential lines are now constructed by locating points on the figure equal to 11, 10, 9, …, 4, 3 (see figures in Civil Engineering article for examples). You are now able to draw a much better flownet because you do not have to estimate the equipotential lines. The seepage loss per unit width of the dam is then calculated and you rush off to your senior engineer. You crash into your senior engineer’s office and present him with the output from the spreadsheet, the flownet and the calculations for determining seepage. The senior engineer is impressed with your work but is more concerned about heaving in the soil at the end of the dam. You rush back to your office and do the necessary calculations to determine the factor of safety against heaving. You present the results of the analysis to your senior engineer. The senior engineer is extremely happy with your presentation and gives you a three week of vacation, a trip to Lombok Island, and a promotion to become a Geotechnical Engineer (wishful thinking wouldn’t you say). Assignment: Do just what the empolyee did. Hand in the worksheet output, the flownet

and all the necessary calculations. Make some conclusions about the seepage loss and the factor of safety against heaving.

SI-2222 – Pengenalan Mekanika Tanah Dr. Endra Susila March, 20 2015

Figure 1: Alternative Dam Designs and Flow Nets

SI-2222 – Pengenalan Mekanika Tanah Dr. Endra Susila March, 20 2015

Figure 2(a): Boundary Conditions for Design #1

SI-2222 – Pengenalan Mekanika Tanah Dr. Endra Susila March, 20 2015

Figure 2(b): Boundary Conditions for Design #2