July 11, 2012
Report authors: Travis Quicksall - Hydrology & Hydraulics, Justin Gawlik - Permits & EAP, Zane Peavy - Geotechnical Report, Matthew Hoffer - Dam Design, Gabe Birnbaum - StructuresGeneral
This article presents a recent project performed by undergraduate students at The University of Texas at San Antonio. Their project is the culmination their degrees in a capstone class. The objective of the class was to present an opportunity to apply design skills to execution of an open-ended integrated civil engineering design project, including field and laboratory investigations, numerical and scale modeling, design, and formal oral and written presentation of results. Their project considers safety, reliability, environmental, economic, and other constraints, as well as ethical and social impacts.Site Conditions
The students chose a rehabilitation and expansion of an existing 6 acre reservoir, dam and site appurtenances. The project site is located in Bell County, Texas just outside the Holland city limits (Figure 1). Existing site features include a reservoir with an approximate 6 acre surface area, and earthen dam and spillway on the north end of the reservoir.
Figure 1 - Aerial Exhibit of Existing Conditions With Proposed Waterline Overlay
The site area is known to consist of mainly Alluvium Sandy Clay on top of Uvalde Gravel on top of Weathered Navarro with underlying thick Clay Shale. Figure 2 shows a historical geological map of the area where the dam will be located. The area for the proposed dam is located in a sparsely vegetated area within a gentle valley with gradual to semi steep slopes. The historical data was used in lieu of subsurface investigation to generalize the site into four major strata as seen in Figure 3. From this generalization the students were able find typical values, calculate, and use engineering judgment based on knowledge of the site to produce in-situ soil conditions. Tests that were simulated to arrive at final soil conditions are as follows:
Figure 2 - Map of Historical Geological Data
Figure 3 - Stratum Descriptions
In addition several publications were used in order to ensure realistic values were achieved.Hydrologic Conditions
The study involved a watershed analysis to determine the maximum expected water levels of the reservoir. The maximum water surface elevation was determined to be:
The proposed dam would have a watershed of approximately 390 acres. A peak discharge of 1035 cubic feet per-second was used for design. Hydrologic modeling was done with HEC-HMS, while hydraulics calculations were completed with HEC-RAS, Hydraflow Express, and various hand calculations.
The initial student design for the dam involved a 20ft top for a roadway. 3:1 side slopes were designed with a core minimum width of 10ft for compaction equipment. Iterative trials were then used to determine the final design. Original design was done with AutoCAD Civil 3D 2011. The student created profiles at 50 ft intervals from the centerline for analysis. A spillway analysis was performed in order to determine the potential draw-down rates for the reservoir. The AutoCAD drawing for the section Station 3+00 may be seen in Figure 4. The geometry of multiple cross-sections was subsequently imported into the SVSLOPE / SVFLUX software for analysis through the DXF import feature. This feature allowed the students to easily go from their AutoCAD Civil3D model outputs to the Soil Vision software.
Figure 4 - Dam Cross Section
All earth filled dams must be safe and stable during the phases of construction and during the operation of the reservoir. To accomplish this, certain criteria must be met, they are as follows.
Calculations and models were ran to ensure against these conditions, except for (7) because the dam is not located in a seismically active region. The following conditions were analyzed with the Soil Vision numerical modeling program suite. Sections at 50 ft offsets from center were analyzed for the following:
The SVSLOPE® and SVFLUX™ software packages created and marketed by SoilVision Systems Ltd. were utilized for the study. The answers from the software were subsequently checked using the United States Army Corps of Engineers (USACE) engineering methods.
The creation of the seepage numerical model required the setup of initial conditions and boundary conditions as seen in Figure 5. Flux sections were inserted in the model to track flows through various parts of the model.
Figure 5 - Boundary Conditions
Subsequent seepage analysis of multiple cross-sections was carried out in a steady-state and transient-state fashion in order to determine pore-water pressure distributions, flows through the drains, and phreatic conditions at various points in time. An example of the results of the seepage analysis may be seen in Figure 6. Of particular note is the influence of the drain on the flow regime. The filter layer was sized per USACE manuals.
Figure 6 - Seepage Analysis at 300 ft Offset
The results of the 300 ft offset may be seen in Figure 7.
Figure 7 - Seepage Results at 300 ft Offset
The slope stability analysis on both the upstream and downstream sides of the earth dam for multiple cross-sections was carried out. Pore-water pressures were input from the related SVFLUX seepage models previously created. By using the Soil Vision software suite results from the SVFLUX model could be accessed in SVSLOPE. In order to determine the most critical slip surface location a number of different slip surface shapes (circular, non-circular, trapezoidal) were tested alongside multiple searching methods.
The rapid drawdown of the reservoir over a period of 5 days was analyzed at multiple times as shown in Figure 8.
Figure 8 - Rapid Drawdown Phreatic Conditions Over 5 days
The subsequent critical slip surface as determined at 0, 3, and 6 days may be seen in the following figures.
Figure 9 - Effective stress analysis at time=0 days
Figure 10 - Effective stress analysis at time=3 days
Figure 11 - Effective stress analysis at time=6 days
The SVOFFICE (SVSLOPE & SVFLUX) software utilized for the study performed well and the students had this to comment:
The coupled SVSLOPE and SVFLUX software packages provided an easy methodology of coupling a transient seepage analysis with the slope stability analysis. The software was ideally suited for ourproject with a large number regions due to it's easy geometry CAD interface and its automatic mesh generation. The user interface allowed plots of the results to be output quickly and allowed for quick compilation of a design report for the completion of our project.
Please contact us if you are interested in similar studies utilizing the SVSLOPE® & SVFLUX™ software packages for your particular study