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  1. Rocscience Slide Manual

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Rocscience Slide Manual

  • RocScience Slope Stability Modeling Software RocSciences Slide is a program that was used to evaluate Odell Dam slope stability conditions.

    Slide is a 2D limit equilibrium slope stability program that was used for evaluating the factor of

    safety regarding failure by sliding of an embankment or slope for the dam. The program computes

    results in terms of factors of safety and slope circle radii and origins, as well as global stability

    failure could occur. For this project, the third cross sectional area was chosen out of the seven cross

    sections. Cross section three was chosen since it is the best representation of all the cross sections

    because of its characteristics.

    Figure 5 above is the surveying data retrieved using a total station and the AutoCAD software. For

    the RocScience Slide program, the parameters needed were the cohesion of 130 psf, the friction

    angle of 25.1 degrees, the unsaturated unit weight is 106 pcf and the saturated unit weight is 120

    pcf.

    Figure 1: Overview of all AutoCAD generated cross-sections.

  • Figure 6, shown above, is a visual representation of the computed result from Slide showing

    various factors of safety (F.S.) which represent the stability of the soil. The square box above

    shows the minimum surface factor of safety when the results are calculated. The computation

    method used to determine the slip surfaces is the Bishop Method, which is designed for circular

    slip surfaces such as in cross section 3.

    The Heel to Toe analysis means that the slopes will be analyzed using the highest elevation on the

    right which extends to the lower left elevation on the cross section. The water line is necessary to

    determine how the soils will react when pressurized under water load conditions. A F.S. higher

    than 1.5 is considered safe as a standard of practice. The F.S. for the minimum slip surface is 3.238.

    Figure 2: Results from a Heel to Toe Analysis

  • The analysis was for Figure 7, shown above, was conducted under the same process as in the Heel to

    Toe cross section, the only difference within this analysis is that the cross section was analyzed from

    Toe to Heel. In this figure, for a downstream slope failure, the F.S. is 2.382.

    [This space was intentionally left blank]

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    Figure 3: Results from a Toe to Heel Analysis

  • Hydrologic Analysis A hydrologic analysis is being conducted to determine the adequacy of the spillway located on the

    southern side of Odell Dam. Due to the size of the watershed, the Rational Method is insufficient for

    determining the amount of water runoff generated, therefore the analysis will be conducted using the

    Soil Conservation Service (SCS) methods. This hydrologic analysis will include: a watershed

    delineation, rainfall intensities, curve numbers, time of concentrations, reservoir storage, and a

    PondPack hydrologic modeling software analysis.

    Watershed Delineation The watershed was delineated, by using an ArcGIS topographic map. This map was then imported

    into AutoCAD, where lines could be drawn to follow the contours that separate our watershed from

    others. The overall area of the watershed was approximated to 19.8 square miles. The watershed

    delineation can be found in Appendix G. The use of only one watershed was done to obtain a

    conservative estimation of the water runoff generated. The breakdown of the watershed into sub

    basins would produce a higher time of concentration, resulting in a lower peak flow. [9]

    Figure 4: Odell Dam's Contributing Watershed.

  • Rainfall Intensities Rainfall Intensities were found using National Oceanic and Atmospheric Administration (NOAA)

    Atlas 14. The intensities were established by inputting the exact coordinates of the project site into

    the database. Refer to Appendix H for the NOAA Atlas 14 rainfall intensities. [10]

    Curve Numbers The curve number considers multiple characteristics of the terrain within a given watershed, such

    as the soil group, land use, and treatment of the land. The value assigned to a curve number is

    indicative of the runoff coefficients of the land as well as the infiltration rate of the soil. Larger

    curve numbers result in more water runoff generated. Pre-burn and 80% post-burn, meaning 80%

    of the watershed has been burned, curve numbers were researched for this analysis.

    For the pre-burned watershed analysis, Dr. Charles Schlinger provided documentation of the Oak

    Creek Flood Warning Study, which provided a full watershed analysis for the watershed

    contributing to Oak Creek. The study lists curve numbers for the Oak Creek watershed. Part of this

    large watershed was in close proximity to the Odell Lake watershed, therefore the values were used

    for this project, as deemed valid by the projects Technical Advisor. The curve number to be used

    for the pre-burn scenario is 66. Appendix I shows the images used to obtain this curve number. [11]

    Post-burn curve numbers require high precision and complex analysis to obtain, and for this reason Raining blue.

    a curve number was researched. The USDA Forest service provides many different curve numbers

    for post-burn conditions, ranging in value from 75-91. Due to this project using an 80% post-burn

    scenario and a conservative analysis, a curve number of 85 was chosen. [12]

    Time of Concentration The SCS Lag Time method was used to determine the time of concentration for both the pre-burn

    and 80% post-burn conditions. The Lag Time method requires that the watershed under analysis to

    be between 300-2000 acres. [13] The following is the SCS Lag Time equation:

    tc = time of concentration (hours)

    L = length of longest flow path (feet)

    CN = curve number

    S = average watershed slope (%)

    Table 1 summarizes the values needed to determine the time of concentrations as well as the values

    derived.

    =1.67 0.8(

    1000 10)

    0.7

    1900 0.5

    Table 1: Time of Concentrations.

  • Reservoir Storage The size and shape of the reservoir is needed to determine the storage capacity as a function of the

    water level elevation. ADWR has provided documentation for the storage of Odell Lake. The

    storage indication curve has been calculated from the crest of the spillway to the top of the dam,

    meaning that the analysis will completed for a full reservoir. [14] During multiple site visits to the

    project location it was noted that the reservoir was as full as the crest of the spillway Figure 9 listed

    below is the reservoir storage indication curve.

    Figure 5: Reservoir Storage Indication Curve.

    Bentley PondPack Hydrologic Modeling Software Bentley PondPack has been used to establish the amount of water runoff generated within the

    watershed that contributes to the Odell Dam, as well as determining the peak flows through the

    spillway for various storm events. Figure 10 shows an image of the model made within PondPack.

    The runoff generated within the Watershed travels to Odell Lake, which then routes the water

    through the Pond Outlet Exit (POE-1) and finally out of the Spillway and Outlet (O-1). For the

    model to run properly, the software needs time depth tables, area of watershed, time of

    concentrations, and curve numbers.

  • NOAA Atlas 14 provided data that was used to make the time depth tables needed within PondPack,

    but the model required data that had to be linearly interpolated due to gaps in the data. PondPack

    requires time depth tables that have data for a specific increment of time. This analysis used 30

    minute intervals for a duration of six hours, whereas NOAA only provided data for 30 minute, 1

    hour, 2 hour, 3 hour, and 6 hour duration depths. Six hours is a standard storm duration used by

    ADWR when analyzing spillway capacity. Tables J-1 and K-1, found in Appendix J and K, show

    the time depth tables generated using the NOAA Atlas 14 and the curves generated within

    PondPack.

    Figure 6: Bentley PondPack Model

  • The watershed area, time of concentrations, and curve numbers derived are summarized in the

    following table. These numbers along with the storage indication curve are the specific parameters

    needed for the PondPack model to run its analysis. Table 2 shows the parameters of the PondPack

    software.

    Table 3, shown below, lists the peak inflows generated from the watershed, the peak outflows

    through the spillway, and whether the spillway is adequate for that specific storm event. Figures

    L-1 to L-8 in Appendix L. show the hydrographs generated in PondPack for the runoff generated

    from the watershed.

    Table 3: PondPack Inflow and Outflow

    Table 2: PondPack Parameters.

    - *Spillway capacity ~ 4500 cfs. After spillway capacity is reached, PondPack will not give outflow data.

  • Final Results

    RocScience Slide Modeling A geotechnical model of an Odell Dam cross-section was created within Slide to show the F.S. for the

    minimum side slope slip surface. The Heel to Toe analysis resulted in a F.S. of 3.238, whereas the Toe

    to Heel analysis lead to a F.S. of 2.382. The Toe to Heel analysis shows that the F.S. of 2.382 is the

    limiting value, however, it is larger than 1.5 and therefore safe.

    Bentley PondPack Modeling A hydrologic model of Odell Lake, Odell Dam, and the surrounding watershed was created to determine

    the flows generated during various storm events as well as the watershed during pre-burn and 80%

    post-burn conditions.

    The pre-burned watershed model resulted in the spillway capacity being reached between the 100-200

    year storm events and between the 5-25 year storm events for the 80% post-burn model.

    The PondPack model will not give outflow results once the spillway capacity has been reached,

    subsequently the software displays warning messages noting that the inflow is greater than the outflow.

    Post-burn Discussion Given Northern Arizonas terrain and vegetation, post burn hydrologic studies become necessary when

    analyzing larger watersheds.

    A post-burned watershed drastically reduces the time of concentration, which in-turn increases the

    water runoff generated exponentially. This has the probable effect of creating detrimental damage to

    areas located downstream of the dam.

    Another adverse effect would be the accumulation of debris from the burned vegetation. The debris

    collecting and making its way to the reservoir decreases its storage capacity as well as increasing the

    weight of the homogenous water mixture. The added debris will result in higher stresses on the dam as

    well as creating blockages in the spillway.

    Final Recommendations It is encouraged that Pinewood Country Club, look into previous ADWR recommendations to preform

    basic maintenance on the dam. This maintenance includes but is not limited to rodent holes, dense

    vegetation, cracks within the training wall and spillway, and debris within outlet of the spillway.

    Our analysis also shows that the dams spillway cannot hold the minimum incoming design flood

    required by ADWR. Our analysis shows that the spillway will be inadequate between the 100 and 200

    year storm event for pre-burn conditions. The team suggest that the spillway should be re-examined, at

    the cost of Pinewood Country Club.

    These conditions dramatically change once 80% post-burn conditions were examined, to which the

    dams spillway indicated inadequate between 5 and 25 year storm event.