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PROJECT 1 DAM STABILITY (marks 15%)
Group project. 2 Students per group.
A dam is to be designed to retain a water height of 28m. It should consist of a core (low permeability soil) and shell layers (higher strength materials). See Figure 1 below for details. Filters and drains are essential parts of many earthen dams. For simplicity, their design is not part of this project.
Design the suitable dimensions for the earth dam. Also, calculate the volume of earth filling and earth fill cost for your design. The following surcharge load should be applied to the middle 6m of the crest.
Figure 1: Example dam cross-section with water table location
Table 1: Surcharge to be applied on the dam crest
|
Surcharge (kPa) |
The sum of the last digits of your student IDs is an odd number |
7.5 |
The sum of the last digits of your student IDs is an even number |
10 |
The foundation soil properties and their respective depths/thicknesses are presented in Table 2 below. The properties of available construction materials for the dam (i.e., shell and core) are presented in Table 3. Additional data (related to foundation soil, core and shell materials) needed for seepage analysis can be found in the accompanied spreadsheet. Table 4 presents information related to dam geometry to help you develop your first cross-section for trials. Table 5 contains information related to earthwork cost calculation.
Table 2: Foundation soil details
*Additional data needed for seepage analysis can be found in the accompanied spreadsheet
Table 3: Construction soil details
Table 4: Geometry related information
Item |
Dimension (m) |
Dam height |
32 |
Minimum width of dam crest |
9 |
Clay core height |
29 |
Clay core thickness (top) |
15 to 25% of the max. water head |
Maximum water height |
28 |
Water height after rapid drawdown |
14 (occurs in 12 days) |
Table 5: Cost of earthwork
Item |
Cost AUD/m3 |
Brown clay (core) |
60 |
Gravely sand (shell) |
50 |
Stability analyses are to be conducted using GeoStudio (Slope/W). Checks should be performed for the conditions outlined in Table 5. You are to design a dam cross-section that satisfies all the conditions in Table 5.
Table 5: Check the stability for the following conditions:
*For uncommon events FOS varies from 1.25-1.3; for a common event, it should be ~1.5.
After finalizing the dam cross-section, you should rerun analysis 2 as a probabilistic analysis to find out the probability of failure. See Table 6 for details.
The top flow line or phreatic surface or pore water pressure distribution will have to be estimated for each case where water flow is involved. This can be done using a steady state or transient seepage analysis (whichever is appropriate). Use Seep/W for the seepage analysis.
Help on how to use GeoStudio (Seep/W or Slope/W) this will be available course website.
Table 6: Probabilistic analysis
Type of Analysis |
FOS |
7. Full reservoir condition Repeat analysis 2 - this time as a probabilistic analysis for the friction angle. Use a standard deviation of 2.5 degree. Present FOS vs. failure probability graph.
This analysis is conducted as a learning exercise and should be performed on the final geometry. You do not need to change anything even if the failure probability is not within acceptable limit. In a real life design, you will have to take necessary steps though. |
- |
Guide to help you come up with the first trial cross-section
The minimum clay core thickness should be 15 to 25% of the water head. The slope of the core should be between 60 and 72 (or between slopes of 1.7V:1H and 3V:1H). This along with the information presented in Table 3, will help you to come up with a suitable first trial cross-section. See the example video for more details.
Reporting
A short report of professional quality is to be submitted (refer to feedback form). Any assumptions made and the possible limitations in the analyses should be stated in the report. Factors of safety are to be compared with acceptable minimum values. Diagrams of critical slip surfaces are to be presented along with the geometry for each case. GeoStudio files (*.gsz) are to be uploaded with your assignment as a separate file. Your report should answer all the questions presented in the feedback sheet. It is up to you to think of and decide on a reporting structure.
A 5% bonus marks will be awarded to the top 5 most economical designs (not limited to your tutorial class, all designs in all classes will be considered). You can score an additional 2.5% bonus marks by the inclusion of drains on the downstream side of the dam in your numerical analysis. Marks will be capped at 100%. You can assume the saturated hydraulic conductivity of the drain material to be double the value of the shell material and the SWCC to be the same. Also, the drain material costs 10% higher than the shell material per unit volume.