If you have read the Part 1 of Retaining Walls, then let’s continue on to the next part. A Geotech will be your best friend when it comes to designing retaining walls. With a basic understanding of soil properties and help of a reliable Geotech, you can get most of the information you need to design retaining walls. Choosing a Geotech firm will depend on how you are designing your wall. Some Geotech firms are geared towards “building design” and some firms are geared towards “bridge design”. Even though both Geotechs will give you what you need for design, getting the values that can be applied in your process will be much easier if you know how and which codes you are designing the walls for. Building codes like IBC (International Building code) or local codes like CBS (California Building code) are very different than Bridge codes like AASHTO (American Association of State Highway and Transportation Officials).
For example, within the building design guidelines, the geotech will give, allowable soil pressures, skin friction, active and passive pressures and end bearing values and skin friction values for the foundation or piles if used.
A firm geared towards the bridge design industry will give values of soil layers and properties of various soil layers (LPile values-LPile is a software program which is used to design Pile foundations by modelling the various layers of soil). Here is a sample table of various soil layers that you would expect to see in the Geotech’s report.
So you ask, why would I need these layer values? You would need these values if you cannot make your cantilever retaining wall work with the standard foundation because it gets too uneconomical to have such a big footing or if there are utility lines in the way that would prevent you from being able to build a big foundation. Situations like these will require your walls to be supported by piles at certain intervals. In order to design these piles you would need the various soil layers that the pile would be driven or drilled through. For building codes, most of the time design is elastic and you can just directly calculate the elastic section properties and input them in any pile software program. I am using Lpile here because that is the program I am familiar with. In LPile, instead of elastic section properties, one can also input the cracked section properties for seismic design cases.
Here is screen shot of LPile program courtesy of Ensoft Inc. If you want to look at these more clearly, then click on the picture and it will direct you to the LPile page with Thumbnail images at the bottom of the screen that can be clicked to view more clearly.
So when does one know what code to design these walls to? Building or Bridge? If these walls are retaining soil that holds up a roadway or in the right of way of a highway then your wall would come under the jurisdiction of Transportation safety officials and would need to be designed for AASHTO load combinations.
If that is not the case, then these walls would fall under the building code. Most of the time what is not discussed in standard text books is that most of these walls have fences on top or in case of walls holding up the soil that supports the roadway, then there are barriers or guardrails on top of these walls. So there are additional moments on the wall due to the fence or barrier which can impact the design of the walls. Have you ever seen a crash test video? It will blow your mind away. Well, in case you haven’t watched one, please take a look at the following. These barriers on top of walls transmit enormous forces from crash loads. This video is courtesy of Texas A&M Transportation Institute.
In this video you can see that there is a barrier on top of a MSE retaining wall (Mechanically Stabilized Earth retaining walls), where soil is built up with reinforcing systems. This is slightly different than cantilever walls. But I just want to show you the possibility of enormous amount of impact forces that could be transferred if there is a barrier on top of your wall.
This is another classic reason we have various types of retaining walls. I cannot imagine designing a cantilever retaining wall system with forces from barriers. It would have to be pretty hefty and uneconomical.
So when an engineer is designing the walls, he/she has to take into account all kinds of loads the walls will be subjected to. There are many different types of retaining walls to accommodate the loading and soil conditions and obstructions in the way and the engineer needs to discuss with the clients the pros and cons of using a particular system of wall and steer them in the right direction. If the wall falls under the jurisdiction of highway then you don’t have to do much convincing. Transportation authorities like CALTRANS (California Department of Transportaion) or your regional transportation authorities have well documented free resources available on their websites as to what works in a certain situation.
In the next part we will take a simple cantilever retaining wall and discuss the forces causing the overturning and the elements of wall that will resist the overturning and sliding forces to keep the walls in equilibrium. If you don’t want to miss Part 3, please subscribe. As always, I appreciate you taking time to visit my site and read.
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