Retaining structures, most often called as retaining walls, are made to maintain difference of surface elevation between two sides of the wall, by restraining a certain material on the higher side, so that it does not fall over or slide to the lower side. We see retaining structures everywhere. They can be used to retain an excavation, a street with an elevation difference on one side, when building a house over a hill. Depending on the need and soil conditions, retaining structures might have shallow or deep foundations.
Now let’s go through different types of retaining walls, starting from the most basic one, which is called gravity retaining wall, which we also use the describe the general concepts that will apply to others types of retaining structures as well. Note that this is mostly a reminder, because we already covered considerable information about these before, under Soil Mechanics > Soil Strength > Lateral Earth Pressure Coefficient section.
Gravity Retaining Walls:
This is the most basic type of retaining structure. It is a wall that basically restrains earth by resisting its push action through its own body weight. It can be made of concrete, masonry or stone. Crib walls and gabion walls that we will later mention are also types of gravity walls.
We showed a sketch of a gravity wall before, when we were explaining lateral earth pressure coefficient. Please review that sketch, Figure – Soil Strength 5a again.
The design should check the conditions for sliding, overturning, bearing pressure, settlement and also seismic considerations in seismic areas.
Cantilever Retaining Walls:
Another retaining wall type that is frequently used is a cantilever retaining wall. Cantilever walls can usually retain higher elevations than gravity walls due to their shape efficiency, because as you can see in the figure below, the soil’s own weight also contributes to the resisting forces here. This enables considerably lighter cross sections for a wall, which requires less material to use, but the construction of these walls are more labor intensive than gravity walls, and the whole section must be excavated first in order to construct the heel of the wall.
retaining walls 1
As in gravity walls, the design engineer should check the conditions for sliding, overturning, bearing pressure, settlement and also seismic considerations in seismic areas.
Counterfort / Butressed Walls:
Some cantilever retaining walls have butresses in the front or back side, for additional support, and they are called butressed / counterfort walls
retaining walls 2
The retaining walls we talked about above, can be constructed in much tighter spaces, with a thinner wall cross section, can support more loads (higher or denser retained soil), if, we could somehow mechanically fix that wall deep into the soil that it retains. For this, we use ground anchors.
Anchored wall is a relatively newer technology in comparison to the plain (gravity) retaining structures without anchors, although it has been around for a few decades. The construction methods, materials, data from past experience, design capabilities and equipment are constantly improving and we can say that anchored walls are now being used at least as frequently as the plain retaining walls without anchors. Grouted anchors are also called as “tiebacks”.
Now, first take a look again at the retaining wall figure you already saw above, and then look at the figure below and compare. Here you see that the wall has anchors that mechanically attach it to the soil behind. These anchors are usually grouted into cement and prestressed, although various other designs and construction methods also exist.
retaining walls 3
The inclined line represents the “wedge”. This is the soil that wants to move freely with the wall. So, if we leave our anchors within this wedge, and not extend it beyond the wedge, they will be useless. So we must extend the anchors beyond this inclined line. Only the portion of the anchor beyond this inclined line counts for the support strength generated. The portion inside the wedge doesn’t count, as that portion already tends to move with the wall. In other words, not extending further than the wedge line is similar to holding your own arm or leg for support, when you are falling from a tree, instead of trying to hold a tree branch. This inclined line is determined by using angle of internal friction that we covered before.
As we have just mentioned, anchored wall systems have certain advantages over traditional gravity retaining walls, such as:
- Anchored walls do not need pile foundations to support them. Pile foundations can be very expensive and hard to install. Gravity or cantilevered gravity walls however, may need pile foundation support in certain cases.
- Anchored walls can usually be built in relatively shorter amount of time, although they require more specialized design team, construction team and equipment.
- Anchored walls require smaller area both during and after construction, and this can prevent administrative problems with adjacent properties. They enable more freed up workspace in construction area.
- Large amount of horizontal pressure can be carried, which would necessitate a much larger wall cross section for a gravity wall without anchors. In other words, anchored walls are much stronger, for the same amount of cross section of wall, or, they can carry the same load with much thinner wall cross sections.
- Anchored walls can be more easily incorporated into the permanent structure as their dimensions are much more slim.
- Backfilling work is not required for mechanically anchored walls. This work can sometimes be expensive and slow, as in cases where select backfill material must be brought in from distances. Backfilling work also carries the risk of causing damage to the wall if improperly done.
- During construction of a gravity wall, there is sometimes the need for temporary excavation support, if the final wall height is to be large. For anchored wall systems, there is no need for temporary excavation support, and the wall itself is the support, as the work progresses.
Among the disadvantages of anchored walls are, the design team must make sure that there is no existing work to be damaged inside the soil where the anchors would be installed, the soil properties behind the wall must be known more accurately, and design and construction of anchored walls are more complicated and require higher skill and specialization than the walls we mentioned before. These can be considered acceptable, especially if we remember that we can support much higher elevations with anchored walls than the gravity and cantilevered walls.
Because of having much more advantages than disadvantages, anchored walls are inseparable part of any highway project or any building project within cities that is being constructed in limited space, or on any project where deep excavations or high and steep slopes must be supported.
Anchor walls can fail from different causes in various directions, both geotechnically and structurally, and listing them would be helpful for better understanding various types of failure that can happen in other structures as well. Anchor walls can fail in the following ways that include:
- Pullout failure of grout from ground, when the bond between grout and soil is not strong enough
- Pullout failure of tendon from grout, when the bond between tendon and grout is not strong enough
- Tendons can fail as a result of tension force
- Failure of wall by bending of the wall
- Failure of wall by rotation of the wall, before anchors are installed
- Sinking of the wall downwards
- Rotation of entire soil mass together with wall and anchors, as was explained under slope stability section
- Toe failure of the wall due to inadequate passive earth pressure in front of the wall
In the next post of this series, we will discuss “Anchored wall design steps – example”