There are many types of retaining walls other than gravity and cantilever walls (we discussed these in Part 1, Part 2 and Part 3 of Retaining walls). But there are instances where these 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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