AB Walls Helpful Hints for the Designer

The following is provided to you, the designer, as a helpful list of things to consider during your design process. These points and the information found in the Best Practices for SRW Design, is important information to understand how to provide the best design possible for any Segmental Retaining Wall (SRW) system, including Allan Block.

Elevation View

• Try to eliminate steep grade transitions at the top or bottom of wall over short horizontal distances. SRW usually make these steps in lengths of half blocks. This includes starting and ending your wall. Engineers too often get used to drawing elevation views as if they were still designing cast in place concrete structures.
  
  
• The benchmark box in the lower left corner is used in the program to control the reference height at the top or bottom of the wall. It essentially establishes the coursing elevation for the blocks in the walls. It does not represent the finished grade at the toe of the wall.
  
  
• Buried Block – The program defaults to one foot (0.3 m), but this is not the appropriate value for all walls or for all areas of a particular wall. A typical rule of thumb for buried block for walls without toe slopes is to bury 1” per 1’ of total wall height (10 ft. wall~10 inches of buried block). Also, in areas that have a slope below, it is recommended to bury a minimum amount of block to have a 5’ flat bench area in front of the finished wall. This recommendation does not eliminate the need to evaluate global stability of the section. The first time through the elevation stations, we suggest that you use the worst case buried block amount for the entire wall. Once you have your basic wall input, go back to the individual station and make manual adjustments to meet requirements. Please note that the minimum buried block only helps to quickly calculate buried block. It does not ensure that every station has at least “x” feet of buried block as it is possible to adjust individual stations to be more or less than the minimum buried block setting.
  
  
• The "Block" button adjusts the input TW and BW elevations slightly to force the design elevation for TW and BW to be based on the benchmark elevation and actual block heights. If the benchmark is left at the default elevation of 0, then all block course elevations will be based off of that value.
  
  

Plan View

• Use of curves instead of mitered corners. Curves are structurally stronger and they are easier to build
  
  
• Use the Top of bottom buttons to control your layout. This is important depending on the site conditions around the wall. If you use the Top button, the top of the wall will be held to the survey line and the bottom of the wall will drop away based on the setback of the block. If you use the bottom button, the program does the opposite.
  
  
• After completing the design cross section later on in the program, be sure to return to the Plan View to see the geogrid laid out to determine if there are any site obstructions to be modified.
  
  

Defining Panels

• The green lines on the screen represent geogrid roll lengths and can be changed by inputting a new length in the box below. These are provided as a guide to the designer to assist the installer by providing design length in grid roll length increments. This will save time and money due to minimal grid waste during construction. Lay out your panel by clicking the green lines that best represent a common design panel.
  
  
• There are a number of things to consider when choosing how long each panel should be, here is a list for you to consider:
  • Soil conditions along the wall
    
    
  • Height – We recommend using 3 or 4 course height changes as a reference to end one panel and start the next when defining panels. This rule of thumb generally works well for keeping the design conservative yet reasonably accurate.
    
    
  • Surcharges and slopes
    
    
  • Corners – You should always stagger geogrid in outside corners
    
    
  • If there are consistent increases in grade along the wall, you may need to have very short panels to design each panel with appropriate geogrid lengths.
    
    
  
  
  

Section Design

• We recommend the first layer of geogrid to be placed on top of the first course of block. This will provide for some flexibility as a result of corners and step ups that may require placement on the second course. The program will flag designs that have the first course of grid being placed higher than 16 inches (40 cm) from the base.
  
  
• For commercial walls we recommend the minimum length for primary reinforcements to be 4 ft (1.2m). The software does not allow you to reduce this length, but as the engineer you can use the included Mathcad file to adjust as you see fit based on your engineering judgement.
  
  
• 60% of the total wall height is the recommended starting length for the geogrid reinforcement. Some sections may need to have longer grids depending on the site conditions.
  
  
• We recommend geogrid spacing at 16 inch (40 cm) maximum and flag designs with spacing that exceeds 24 inches (60 cm).
  • The results of seismic testing research clearly proved that tighter grid spacing creates a better composite mass that will perform better over time.
    
    
  • 16” spacing also ensures that better compaction is achieved. At no point should contactors be stacking multiple courses before compacting. The industry standard is an 8” maximum lift of soil between compaction passes.
    
    
  
  
  
• The Match Grids Left and Match Grids Right buttons are a powerful tool to help you keep your geogrid layers on the same elevation levels along the length of the wall. By not doing this, the grid layers occasionally may have a step up or stagger from one panel to the next, and this may cause the contractor to need extra time to match your design.
  
  
• Typical surcharges are 250 psf live load for roads and commercial parking lots and 100 psf live load for residential driveways.
  
  
• The Line Load is a surcharge with a defined starting and ending location. A sidewalk is a good example of this. For situations where the load goes outside of the scope of analysis, a standard Surcharge is used. A good example of this is a parking lot behind the wall that has a well-defined starting location and goes far outside of the wall design envelope.
  
  
• Slopes above the wall can be entered as either a run vs. rise or a back slope angle based on the user’s preference. Please note that the height of the slope is defaulted to 50 feet. This represents a continuous slope that goes outside of the wall design envelope. The user can modify this height to model a broken back slope.
  
  

Design Options

• Typical soil information is used to model different Infill, Retained and Foundation soils, but you can use the Multiple Soils tab if layered site soil conditions exist. The multiple soil values are used in Internal Compound Stability (ICS) analysis only. The External and Internal calculations use the lowest values of the soil layers to be conservative.
  
  
• Typically, cohesion is ignored in bearing capacity calculations due to the unpredictability of its strength. If the designer chooses to use cohesion, we recommend that no more than 10% of the tested cohesion values from the soils report be used for the same reason stated above.
  
  
• Seismic design requirements will be determined by local codes and site geotechnical engineers.
  
  
• Wall Drain options are used to specify wall drainage configurations to account for various site ground water conditions. Use them to outline the design requirements to ensure that sub surface water is properly managed. Use berms and swales to route surface water away from the back of the wall envelope.
  
  

Internal Compound Stability (ICS)

• Always remember that ICS does not replace or substitute global stability. ICS calculations are limited to the area defined as the wall design envelope, global stability has no limitations and can be well outside of that envelope.

Complex Composite Structures (CCS)

• Although structures have been routinely constructed as complex composite structures, analysis has not been easily performed. AB Walls now has the capability to analyze these types of structures. To limit the effects of eccentric structures, we have limited the ratio between the Upper and Lower Structures to a ratio of 70% of the depth of the structures.
  
  
• Based on field experience and the advent of a more refined Internal Compound Stability Analysis, the industry recommendation is that the length of primary reinforcement should not be less than 60% of the total wall height as measured from the face of the block. The CCS approach allows the engineer to achieve a more detailed analysis when dealing with site obstructions.
  
  
• The CCS analysis includes external stability calculations; overturning and sliding for both the Upper and Lower Structure factors of safety are reported. The software will not allow for the depth of either structure to be less than what is required to achieve a minimum factor of safety.
  
  
• When the Upper Structure is extends beyond the depth of the Lower Structure a bearing analysis is conducted to check for potential rotation and buckling at the intersection of the Upper and Lower Structures.
  
  

Printing Results

• Printing options can be found under the File drop down menu.
  
  
• You can select what is going to be included on the printouts by making the selections on the left hand side of the screen.
  
  
• Typical generic details can be added. These can be viewed within the program and then you can select the appropriate details based on the site requirements.
  
  
• The “Preliminary not for Construction” stamp can be removed by going to File and selecting Print. The user will click on the Add Engineer License button (this button only appears if the user is a licensed engineer and has be provided an engineering password). There is a checkbox there that lets a user define whether to have the preliminary stamp on or off.
  
  
• You can add your engineers licensing information to the printouts by clicking the button in the upper right corner of the print screen and entering the appropriate information in the screen that opens.
  
  
• The engineering review binder is creating using the 8.5x11 (A4) paper size. Construction or Shop Drawings are available in the 11x17 (A3) format.
  
  
• All results are printed to a PDF document. These can them be saved, emailed or printed based on the user’s needs.
  
  
• Exporting to DXF is a convenient way to bring the wall drawings into AutoCAD. This function is found under the File Menu/Export DXF