Designing Your Project: Terraced Sections

Defining your Soils and Seismic

  1. The first step in designing a section is to define the soil parameters. This menu will pop up automatically when you first move to the Section Design screens, but can be found under the Soil Data button in the lower left corner. The user should input the site-specific soil parameter for their project. These parameters can be defined per section and will be saved with the design file.

    section soils

  2. To apply your seismic parameters, select the Seismic Parameters tab and enter the specific horizontal peak ground acceleration. This will apply to all the sections.

  3. Multiple soil types are applied a little differently when designing terraced sections. As the number of terraces are defined, you can adjust the soil parameters for each terrace by using the Soils Tab.

    terraced soils

Multiple Terraced Soils

For more information on the Multiple Soils, Seismic Parameters,Wall Drain Options, Alternative Reinforcement and Section Notes, see Appendix D.

Designing Terraced Sections

  1. The program will start the section designs with the tallest section. When you complete your first section use the up or down arrow to move to your next section. The starting and ending stationing of that panel is displayed.

  2. The section is defined by its position from the Panels screen and therefore the information in the Section Height of Block Courses, Total Wall Panel Height and Total Wall Setback are already defined.

  3. The Number of Buried Blocks section is defined by the height of the section. In a design with a defined elevation profile the number of buried blocks is defined by its position from the Panels screen. In a zero section the user has the ability to specify the Number of Buried Blocks directly.

terraced empty sections

  1. Using the Geometry Tab, you defined the number of terraces (1-15) you have for the panel section. The program will take the number of courses and the number of terraces you define to determine the Terrace Height, in courses.

    • It will keep the number of courses as even numbers as much as possible to accommodate two-course spacing.

    • You can adjust the height of each terraced wall, but the total number of courses must always equal the number of block courses of the panel. The software will indicate when these do not match.

      terraced courses do not match

    • The offset distance between the terraced sections along with the Wall Rock Depth can be defined. Surcharges and slopes above the lower and intermediate terraces can be defined as well.

      terraced geometry

  2. Geogrid layers placed at a 2-course spacing is the default setting. The length of the grid for each terraced wall can be adjusted using the Grid Tab. The default length is 60% of the total structure height.

    terraced grid percentage

  3. Click Draw Wall to view the terraced section. Make note of the heights, offset distances, surcharges and slopes. Make any adjustments to the section as needed. You can make geometry changes and redraw the wall as many times as needed before calculating the section.

  4. If you have already designed a section, such as, Section 2, when designing Section 1 you can use the Match Grids Right button to place the terraced walls and grid layers in Section 1 at the same relative position as in Section 2. The same goes for Section 3, but use the Match Grids Left button. Having the grid layers on the same course from the start to the end of a wall is helpful during construction.

terraced grid match

  1. There is a toggle in the section design screen that allows a panel view pop up window to be displayed or not. The panel view pop up display shows the cross section location as well as the panel elevation. The different terraces are shaded for easy identification. The window also displays the top of wall (TW), bottom of wall (BW) elevations and stationing of each individual step up or step down location.
    terraced show paneln>
terraced panel view

Editing Geogrid

Internal Stability Safety Factors in the Edit Geogrid section include Grid Overstress and Grid Pullout of Block.

Safety Factors all must be over 1.5 for static and 1.1 for seismic designs. The designer can change the safety factors by changing the grid spacing, grid strength, grid depth or the grid type.

Please note that if you have already calculated your section, you will need to recalculate the section to obtain the revised results anytime you make a change to the spacing, strength and length of grid.

If you have made changes to the geogrid position and wish to recalculate the section, you can "lock" your geogrid locations and recalculate without automatically changing the grid positions back to the default locations.

terraced lock grid>

Edit Spacing

Edit Spacing has an effect on all three internal safety factors.

  1. To view a larger internal stability screen, click on the (+) magnifying glass. Click the (-) magnifying glass to minimize this screen.

  2. Edit Spacing allows the designer to reposition grid layers.

  3. To remove a layer, click the course number that contains a layer of grid. To put a layer somewhere else, click the desired course number that does not contain a layer of grid
terraced grid spacing

Edit Long Term Allowable Design Strength (LTADS)

Edit LTADS mainly effects grid Overstress calculation, but will also change the other internal stability factors because the actual grid test results will be used for each calculation.

  1. Edit Long Term Allowable Design Strength (LTADS) of the geogrid layer.

  2. Click the course number to increase that layer of grid to the B-Type grid and click it again to change it to the C-Type grid. Click it again and it returns to the A-Type grid.

  3. The grid strength is shown in the Allowable load cell of the table.
terraced LTADS

Edit Depth

Edit Depth effects pullout of soil calculations.

  1. Editing the depth of geogrid embedment.

  2. Click the course number of the layer you wish to change.

  3. Enter the new length in the pop-up box and click OK.

  4. The revised grid length appears in the grid length cell.
terraced edit depth

LEM Definition

The Tech Sheet for LEM can be viewed by clicking the LEM Definition button.

Surcharges for Terraced Walls

You will need to use the Geometry Tab to enter your surcharge information above the lower and intermediate walls. However, above the top terraced wall, AB Walls allows the designer to use two types of surcharges, a distributed surcharge or a line load.

  1. The Surcharge allows the designer to apply a continuous surcharge from a starting position they define, and continue without end.

  2. The Line Load surcharge can also be live or dead load but, the starting and ending positions can be defined by the designer.

terraced surcharge

Slopes Above Terraced Walls

You can enter your slope information in number of courses between your terraced walls using the Geometry Tab. The top wall is different. Enter the Slope above the top wall in Degrees or by entering the Rise and Run by using the Slope Above Wall feature.

For broken back slopes, or slopes that are not infinite, a height of slope box is included. Input the height above the top of wall at which the slope levels out and the program will adjust the calculations accordingly. The height of the slope is measured from the top of the wall to the top of the slope.

terraced slopes

Calculate the Terraced Section

For an in depth discussion on the terraced sections using Limit Equilibrium method (LEM) see Chapter 8 in the AB Engineering Manual.

  1. Click the Calculate button - After the software runs the LEM calculations, the LEM results for the Treq on each grid screen appear. The slip arcs that result in the maximum values along with Tmax are represented.

    terraced calculated sections

  2. A summary of the surcharges present in the panel or top terrace wall is created.

  3. The External Results of Sliding, Overturning and Bearing are displayed for each terrace wall.

  4. The Results tab will outline parameters used in the calculations along with the heights, slopes, Active Pressure Coefficients and forces.

  5. The Parameters Tab:

    1. Treq Results - will allow you to display the results of Treq for all or just one layer of grid.

    2. Soil Pullout - You can select the option to Show the Soil Pullout Capacities at the Back, Front or Both for a layer of grid. These will not be printed as part of the section results.

    3. Adjustment Controls – Select what grid layer you want to view, and the Amplitude will allow you to magnify the results for better viewing.

    4. User Defined Controls –

      1. Development Length at End of Grid – Length of grid to ignore at the end of the grid. The default value is 1 foot (30 cm)

        terraced development at end of grid

      2. Bishop’s Model Slide Width – Smaller slices will generate more accuracy. However, the time to calculate each section will increase. The default value is 1 in. (25mm)

        terraced bishops model slide widths

      3. Face Multiplier – This force is added to the calculated required connection force (To). The added multiplier allows you to adjust the additional force on the face between grid layers. The grid spacing defines the width of the column of soil that creates the additional pressure. The default value is 100%.

        terraced face multiplier

      4. End of Grid Pullout Reduction Factor – User defined reduction factor for soil pullout. Default value is 100%.

        terraced end of grid pullout reduction factor

      5. Block Shear Contribution – A portion of the block shear is used to increase the resistance side of the Bishop’s Stability Model. The Tmax/To ratios and multipliers allow you to adjust the amount of shear that is used in the Bishop’s Model. The default values are conservative for this inclusion since it is only taking a percentage of the tested Y-Intercept value.

        terraced block grid block shear
        terraced block block shear

LEM Results

  1. Select Results to display the detailed LEM calculation results for each layer of geogrid for each terraced wall.

  2. Terraced walls are shaded to provide a point of reference, but each layer of grid has the course and elevation to indicate it location.

    terraced LEM results

  3. Tmax is compared to the LTADS of the grid to get the Overstress Factor of Safety (1.5 minimum).

  4. Required Connection Capacity (To) is added to the Active Facial Force to obtain the Total To. This is compared to the Connection Capacity of the block and grid interaction to obtain the Block Pullout (connection) Factor of Safety (1.5 minimum).

  5. Treq at the end of the grid can be compared to the Developed Geogrid Capacity at the End of Grid. No safety factor is presented for Pullout of Soil Safety Factor since the forces are shared between all grid layers when Treq at the end is larger than that of the Developed Capacity. See LEM Definition.

NOTE: To view a larger results screen, click on the (+) magnifying glass. Click the (-) magnifying glass to minimize this screen.

LEM - Baseline

  1. Select Base Line to display just the base line results as a product of the LEM calculations for each layer of geogrid for each terraced wall.

  2. Terraced walls are shaded to provide a point of reference, but each layer grid has the course and elevation to indicate it location.

    terraced LEM baseline

  3. Tmax and its location for each grid are displayed.

  4. The calculated value of To is presented.

  5. User parameters allow for Block Shear Contribution as a ration of To vs Tmax and when that shear contribution is used, the results are shown.

  6. Treq at the end of the grid is presented.

NOTE: To view a larger Base Line screen, click on the (+) magnifying glass. Click the (-) magnifying glass to minimize this screen

LEM - Geometry

The Geometry option displays the detailed LEM calculation geometries for the worst-case slip arc for all layers of geogrid for each terraced wall. This includes the entrance and exit X and Y information along with the Radius Arc and Center Point parameters.

terraced LEM geometry

NOTE: To view a larger Geometry screen, click on the (+) magnifying glass. Click the (-) magnifying glass to minimize this screen

Clicking the arrow pointing to the right will move you to the Quantities Worksheet screen.

terraced to estimate

After all the sections have been designed, it is recommended to confirm the geogrid layout by clicking the arrow pointing to the left, taking you back to the Panel View.