Retaining walls are as the name suggests any wall that is designed to retain any material. The material could be earth, water, anything else that needs to be retained. A common example of retaining wall in everyday life is Basement walls, Swimming pool walls, Landscape walls.
Before we discuss how to design retaining walls, I want you to watch a simple but excellent video of how the soil fails behind the retaining wall. Video courtesy of British Geological Survey. This video perfectly shows the failure plane that forms at an angle behind the wall. In this video the soil behind the wall is granular soil. This video kind of emphasizes that what we do on paper is not just a math problem, it is actually a structure that will be built and care should be given as to how best to design it to prevent failure.
Types of Retaining walls:
* Gravity walls
* Cantilever walls
* Counterfort walls
* Tieback walls
* Drilled Pier walls
* Soldier Pile walls
Of the above, Cantilever retaining wall, Tieback walls, Driller Pier walls and Solider pile walls are the most commonly engineered walls. Gravity walls are mainly used for shorter landscaping type of walls as it becomes less efficient for taller walls. The main difference between cantilever retaining walls and the other walls mentioned is the way the foundation is designed. Tie back walls are completely different retaining walls and rely on pre-stress in ties that hold back the wall there by retaining the soil.
Before one can design retaining walls, a little understanding in soil mechanics is essential.
Soil mechanics and Assumptions: In order to design a retaining wall, understanding the soil behavior is critical. The design engineer needs to know some basic soil parameters.
Soil Parameters needed:
– Soil type (Granular or Cohesive)
– Unit weight
– Angle of Friction
– Cohesion
What is cohesion? It is the binding ability of soil. According to OSHA.gov, cohesive soils is a soil with high clay content. It is plastic when moist but becomes hard to break when dry. When dry, cohesive soils can be excavated with almost vertical slopes. Good example of cohesive soil is clay.
Granular soils are opposite to cohesive soils. The angle of internal friction plays a major role in granular soils since their cohesion value is zero. Example of granular soil is sand.
The Geotech usually takes samples of the soil over which a structure is to be built and gives the results of the type of soil that is present at the site. One of the tests that Geotech uses to find out the angle of internal friction of the soil is a Direct shear test. Please watch the following video of direct shear test courtesy of Carleton University.
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 this section we will take a simple cantilever retaining wall and discuss the concept of how they are affected by the loads and how you have to design them to resist these loads that they are subjected to.
What is a cantilever retaining wall? It is a wall that acts like a cantilever fixed at the bottom foundation. Here is a sketch of cantilever retaining wall and for the sake of understanding the basics lets assume an imaginary superman pushing the wall with all his strength. Notice that if he was not standing on the foundation or if the foundation was very short, then he could both push it or topple it over. Pushing force is represented by the left pointing arrow and toppling is represented by the curved arrow. But because he is standing on portion of the foundation and the foundation is large, he will have some difficulty making the wall topple. The forces shown in Yellow and Orange are the forces that will naturally cause the wall to slide (push) and overturn (topple). The force shown is yellow color is from soil pressure and the forces shown in orange color is from live load surcharge. Soil pressure can be “active” soil pressure or “At-rest” soil pressure. So what is the difference between the “active” and “at-rest” soil pressure and how can you tell which pressure you should design for?
You should always refer to the recommendations given by the soils engineer. When the wall is flexible (meaning if the top of the wall rotates by 0.001 to 0.003 radians or if the top of the wall deflects at the range of 0.001 h to 0.003 h where “h” is the height of the Retaining wall, then the wall deflects and moves away enough from the soil retained that the horizontal soil pressure decreases to the “active” pressure levels. You should never assume that walls are always designed for active pressure. If the wall is too stiff then you will be underestimating the soil pressure that the wall is subjected to. “At-rest” pressure is typically greater than “active” pressure.
If there is a roadway or buildings near by to the retaining wall then the wall will also be subjected to additional pressure called the Live load surcharge. This is shown in orange color in the sketch above. If there is any buildings or structures close to the retaining walls then the soils engineer will also recommend the loads coming from the foundation of those structures that the walls have to be designed for.
So what prevents these walls from being pushed and toppled? Well, see the small blue truncated triangle on the left side? That is the passive pressure. Passive pressure (in pcf) is usually pretty large but notice that the area of the actual structure that presses against the soil is pretty small so in reality the ability of passive pressure alone to resist these forces is unrealistic. So what else helps maintain the stability of the wall? The friction force between the bottom of the foundation and soil plays a big part in resisting this sliding force. In most text books coefficient of friction between soil and foundation is taken a 0.5 which is a relatively high number. In reality a lot of soils engineers define this value to be much less than that. Once again soils engineers play a big role in design of the retaining walls and often times they will not let the design engineer add the effects of 100% passive and 100% friction together. One additional item to keep in mind during design is that there is greater friction if the weight of the structure is large so if you are trying to design the most efficient section it will come back to give you problems in counteracting sliding forces. Once your design takes care of overturning and sliding, the third thing you have to check for is to make sure that the soil underneath the foundation can actually take these additional stresses. The soil fill underneath the foundation is rated for a certain amount of allowable bearing pressure. See the image below on how these overturning forces affect the soil underneath the foundation.
In Case 1, there is no eccentricity and hence the soil under the footing is uniformly loaded. fmax has to be less than Allowable bearing capacity. In Case 2, there is Moment that causes eccentric loading on the footing. As long as the eccentricity e<B/6, and fmax is less than allowable bearing capacity the footing size should be adequate. In Case 3, the moment is so large that the eccentricity of the footing is outside of the middle third of the footing or in other words e>B/6. What this indicates is that the footing size is not efficiently used. fmax should once again be less than the allowable bearing capacity but footing size can be adjusted to make the maximum use of it. When in any of the above cases fmax is greater than the bearing capacity of the soil, then the soil underneath the footing fails. This should be avoided at all costs.
Another important topic in the design of retaining walls is actual drainage details. Most retaining wall failures are caused due to improper drainage details. Imagine what happens if the soil on the right side of the wall in the picture above is water logged due to drains clogged or no drains at all? There is additional pressure on the wall due to water. The density of water is 62.4 pcf which is quite a big additional pressure on the wall.
If the height of soil to be retained above the footing is “h”. It is good practice to design the wall to be 6″ taller than the soil to be retained. The top of the footing also has to be a minimum of 12″ below the top of soil on the toe side (side that shows point “A”). In order for you to pictorially see what additional forces the wall could potentially be subjected to please see sketch below.
When there is an earthquake the wedge of soil above the failure plane will cause additional shear on the wall. This is an inverse triangle with larger forces acting on the top of the wall which means the moments due to seismic forces (inverse triangular force) on the base of the footing can be a huge addition to the moments due to the active pressure. The soils engineer will actually give you the value of the seismic forces that act on the wall.
In order to calculate the forces that cause the wall to overturn, moment and shear are calculated about the point “A” in the diagram above. Just from observation, you can tell that the passive pressure is very small force to counteract the active forces causing the footing to overturn and slide.
Your job as an engineer is to make sure that you design the wall and foundation to be strong enough to resist these forces shown by the red arrows. Designing retaining walls is a iterative process. Sometimes you are so close to your design working but not quite enough for sliding. In those instances you have to compare costs to see what is the best alternative. To add a shear key under the footing to engage more passive pressure or just make the footing big enough to increase the frictional forces? It is a trial and error process, but take into account what it will cost not just for materials but also labor to have to pour a shear key (additional trenching, additional concrete, additional steel).
I hate to say, we are still only scratching the surface of retaining wall design. There is so much to talk about. I will leave you with a video of retaining wall collapse in Baltimore, Maryland. After you watch the video, you can also read the blog post written by Dr. Dave Petley about this particular retaining wall collapse. I believe this particular collapse was due to heavy rains and lack of adequate drainage and the wall itself seems under designed for the loads that it was subjected to.
It is good that no one got hurt during that collapse. I am currently developing short courses on Design of Retaining Walls (types: Cantilever, Tieback, Drilled Pier, Soldier pile etc). Inside the course I will discuss what the codes recommend and how to design all these different types of retaining wall. I am also planning on showing hand calculation and also step by step computer analysis using Enercalc and depending on the amount of interest I have for this topic I may invest in Lpile program to show how to do calculations for drilled piers with various soil layers(bridge design process). So, if you would like to know when this course is coming out or interested the course please subscribe at StructuralCE
But there are instances where these cantilever retaining walls cannot be used. Say for example you are really close to property lines or if there is a buried pipe underneath that you discover when you actually start doing site surveys. There is a building or tower next to the wall and the foundation of that structure will interfere with the foundation of retaining walls etc. Many reason why cantilever walls might not work. In these situations, there are other options. Some of these options are Tieback walls, Driller Pier walls, Soldier Pile walls etc.
Tieback walls: What is a Tieback wall? It is a wall that is actually holding the soil in place with a network of ties (prestressing tendons). Ties are placed in a grid fashion and usually at a 15 degree angle and are positioned to miss the abutment piles. Ties are great option when the soil to be contained is near an abutment of a bridge where the spacing is so restricted that huge foundations are impossible. See sketch below.
Below sketch shows a close up of what makes the ties (image courtesy of Caltrans).
Each tie has the following components:
Prestressing steel: Transfers the wall reactions to the anchor zone in the soil through the bonded length.
Bond length: This is the zone where the steel is inside the grout bulb that is fixed and transfers the load from the steel to the surrounding soil. This is also called the Anchor zone. This bonded length has to extend past the failure plane of the soil.
Unbonded length: This is the portion of the steel that is free to elastically elongate and thereby transfer the resisting force from the bonded area to the wall.
Wall anchorage: This usually is at the wall element. Has a plate and anchor head which is a threaded nut and allows the steel to be prestressed and locked off.
Grout: Provided a medium through which load gets transferred to the soil and also protects the pre-stressing steel from corrosion.
Because these tendons are angled, there will be a horizontal and vertical component of the soil reactions. There is also additional vertical load due to the dead weight of the wall. So, for stability of the wall, the design engineer has to take into account both these forces. Tieback walls are comparatively much simpler to design than the other types of retaining walls. Please sign up here in the pop up form on the right hand side if you are interested in learning how to design a tieback wall. You can also sign up at the bottom of this page.
Drilled Pier Walls: These are walls that rest on top of a cap beam or grade beam which is in turn supported by a series of drilled piers that are spaced uniformly. The wall portion of the design is similar to the cantilever retaining wall (the wall is fixed at the bottom to the grade beam) but in addition to that the engineer has to make sure that the grade beam is sufficiently designed to take the moments and shears from the wall and pass it on to the piers. The grade beam will be subjected to some major torsion which has to be additionally checked. The drilled piers are then designed using a program called LPile where all the different soil layers and piles are modeled and subjected to the moments, shears and axial loads from the walls.
I have written more about the soil layer and pile modeling in Part 2 of Retaining walls. I will be including a detailed design of a driller pier wall in my course. The pile holes are drilled using an auger which is a huge drill. All the soil inside the hole is removed and then the rebar cage is lowered into it and then the concrete is placed.
For anyone who would like to see an animation of the many different types of piles and their methods of construction, here are some youtube videos (courtesy of Hayward Baker Inc)
Here is an actual real life drilling video courtesy of Pearson Drilling Inc. You can see in the video below that the site conditions are wet and so they are using a temporary steel casing in order to prevent caving in of the soil and then insert the rebar cage and pour the concrete. Once concrete is poured the temporary steel casing is removed.
The various types of defects that can happen in CIDH (Cast in drilled hole) piles is that the walls of the hole can cave in while removing casing, or the concrete separates and forms pockets of air holes. There might be air pocket in the existing soil adjacent to the pile that is being drilled that is not visible but would end up leaking the poured concrete out of the drilled pier. I will cover more details of the defects in my lessons.
Soldier Pile walls: A soldier pile wall is similar to driller pier wall with the exception of a wide “W ” or “H” section is used in the pile instead of rebar cage. Wood lagging or wood sleepers spans between the soldier piles and temporarily hold up the soil until the permanent wall is installed. The permanent wall could be either shorcrete or cast in place wall. The wood lagging temporarily handles the soil pressure until the concrete walls cure and become effective. The W or H sections are placed into a drilled hole and checked for plumbness and then structural concrete fill is placed. The timber lagging is placed as the soil gets excavated in stages and then the rebar cage for the wall is placed with the wood lagging acting as a concrete form. Most cases the wall is actually a shotcrete wall where concrete is pumped by a hose. The metal studs that are welded to the flange of the H section will structurally tie the wall to the soldier pile.
In some instances (depending on the soil conditions) the H section is driven directly into the ground instead of placing inside a drilled hole and filled with concrete. In most cases this would just be a temporary condition. The following video (courtesy of Piling & Civil Australia) shows animation of how the timber lagging goes in stages as the soil gets excavated but this animation does not show the concrete placed in the drilled hole. Sometimes these soldier pile walls are combined with ties at the top to help maintain a smaller H section. Ties basically will reduce the cantilever length of the wall thereby reducing the moments and shears on the H section.
The next video shows an actual placement of the H section inside the drilled hole and concrete placed into it. Courtesy of Helitech Civil Construction Division
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