Gravity retaining wall with back slope and cohesive soil

In summary, designing a retaining wall can be challenging, and there are various theories and methods available to assist in the design process. Some of these include Coulomb's theory and Terzaghi's theory, which consider wall cohesion and friction. It may also be helpful to use computer programs or seek guidance from a professional organization.
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Homework Statement



I have a gravity retaining wall with a backslope and an inlcined prestressed anchor. The soil properties are c'=5 kPa, phi`=15 degrees, gamma=16 kN/m3. The water table is well below the bottom of the retaining wall. I have been told to assume the wall adhesion is 80% of c'. There is no mention of taking into account wall friction.

The solution asks for an anchor prestressing force and omptimal angle based on a safety factor of 2. This is simple provided I can work out all the forces acting on the structure.

Homework Equations



That is what I need to know.

The Attempt at a Solution



I have two choices here. Rankine's and Coulomb's theories of lateral Earth pressure. Looking through Braja. M. Das: Principles of Geotechnical Engineering 7ED he gives treatment to both of these cases. He doesn't seem to include anything in Rankine's theories for a sloping backface and c'-phi`soil. How does the sloping backface affect the application of this theory? How can this theory take into account wall cohesion? I believe Rankine's theories do not take into account wall friction whatsoever. I have also read that Rankine's can offer a conservative design by underestimating the passive pressure and overestimating the active pressure.

His treatment of Coulomb's theories takes into account the sloping backface and presents a graphical solution to a cohesionless soil problem. Again I have a cohesive soil and I need to take into account wall cohesion and wall friction. I also need to take into acount the passive pressure applied to the wall by the soil in front of the toe. His treatment of Coulomb's passive pressure is based on the retained Earth not the Earth in front of the toe. Why would you even want to calculate the pasive pressure applied by the retained earth?

Is there a good text that expands on these theories a summarises them based on the wall and soil properties you have? It seems every time I go to design a retaining wall I have to spend hours sifting through texts of varying quality and conflicting recommendations do find a decent analogy and I really don't have that kind of time.
 
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Thank you for your post. It seems like you are facing some challenges in designing a retaining wall. I can offer some guidance and resources that may help you in your design process.

Firstly, let's address the issue of wall cohesion and friction. As you mentioned, Rankine's theory does not take these factors into account. However, there are other theories that do consider them, such as Coulomb's theory and Terzaghi's theory. Coulomb's theory is based on the concept of active and passive earth pressures, while Terzaghi's theory takes into account the shear strength of the soil.

In your case, since you have a cohesive soil, it may be more appropriate to use Terzaghi's theory. This theory considers the cohesion of the soil and the friction between the soil and the wall. You can find more information about Terzaghi's theory in textbooks such as "Principles of Foundation Engineering" by Braja M. Das or "Soil Mechanics in Engineering Practice" by Karl Terzaghi.

Additionally, there are various computer programs and software available that can assist you in designing a retaining wall. These programs use different theories and methods to calculate the forces acting on the wall and determine the optimal design. Some popular programs include Plaxis, GeoStudio, and Wallap.

I understand your frustration with having to sift through different texts and recommendations. It may be helpful to consult with a geotechnical engineer or seek guidance from a professional organization such as the American Society of Civil Engineers (ASCE). They may have resources and guidelines specifically for designing retaining walls.

I hope this information helps you in your design process. Keep in mind that retaining wall design can be complex and it is always best to consult with a professional for a safe and effective design. Best of luck with your project!
 

FAQ: Gravity retaining wall with back slope and cohesive soil

1. What is a gravity retaining wall with back slope and cohesive soil?

A gravity retaining wall with back slope and cohesive soil is a type of retaining wall that utilizes the weight of the wall itself, along with the internal friction and cohesion of the soil, to resist the lateral pressure exerted by the retained soil. It also has a back slope, which is the angled surface behind the wall that helps to distribute the weight of the retained soil.

2. How does a gravity retaining wall with back slope and cohesive soil work?

The weight of the wall and the cohesive forces within the soil work together to create a stable structure. The weight of the wall acts as a counterbalance to the lateral pressure of the retained soil, preventing it from sliding or overturning. The cohesive soil also adds to the stability by providing resistance to shear forces.

3. What are the benefits of using a gravity retaining wall with back slope and cohesive soil?

There are several benefits to using this type of retaining wall. It is a cost-effective solution as it does not require expensive materials or extensive construction techniques. It also has a simple design and can be built quickly, making it a popular choice for many projects. Additionally, it can be constructed on a variety of soil types and can withstand significant loads.

4. What are the considerations for designing a gravity retaining wall with back slope and cohesive soil?

When designing a gravity retaining wall with back slope and cohesive soil, several factors need to be considered. These include the height and slope of the wall, the type and properties of the soil, the water table, and the surcharge load from any structures or vehicles on top of the wall. Proper drainage and reinforcement may also be necessary for added stability.

5. Are there any limitations to using a gravity retaining wall with back slope and cohesive soil?

While this type of retaining wall has many advantages, it also has some limitations. It is not suitable for retaining very tall or steep slopes, as the weight of the wall may not be enough to counteract the lateral pressure. It also may not be the best choice for areas with high water tables or where the soil is prone to erosion. In these cases, other types of retaining walls may be more suitable.

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