How to evaluate a retaining wall project
So, I am building a large number of retaining walls this Summer and thought I would start a thread on How To. I'm no Pro, just some house building experience, so this is very much a learn as you go project for me.
If you are considering anything bigger than a flower bed (2' high or so) I would recommend the following:
1. Mock-up a concept of your wall on paper or in Sketch-Up. Draw Plan and Section Views showing approximate grades into and away from the wall.
2. Take your Concept Drawing to your Building Code Enforcement Office: Many won't approve or permit a wall that big without Engineering, lest it fall on someone. Ask if he knows an Engineer that works with Homeowners (many will not).
3. Consult an Engineer: Even if your Building Code Office allows a big wall with no Engineering, you should bring one on for peace of mind. That is a lot of work if it falls over next year (on someone :eek:). Your Engineer may want to see Topographical Surveys of the area.
4. At the very minimum read "The Homeowner's Guide To Drainage Control & Retaining Walls" by Erickson
5. Produce solid Working Drawings. If your Engineer hasn't already provided Drawings, take yours to an Engineer and have him 'run the numbers' to see if your wall is safe.
6. Knock yourself out (figuratively speaking)
I'll add to this Thread as I learn more and my mistakes become apparent. For a less formal version, check out my Thread "Talk me off this ledge..." in the Landscape and Garden Section.
The most common type of retaining walls are segmental interlocking walls (SRWS) because of the cost, ability for a DIYer and flexibility of layout (straight, inside or outside curves) and variable heights. I assume this is what you are referring to.
The most common height for engineering to be required in the U.S. is either 4' or 5', since it is just a gravity wall if less than that. Here, the municipalities even provide "generic" drawings for the construction of the major brands of walls. - It is a "no-brainer". If the block is a local "knock-off" or a copy, it may not work for the details. These drawing are also routinely used by the municalities for local crews to build walls up to the 4' oor 5' height without any engineering on municipal projects.
Drawing are always good for getting a permit (if required legally or for your protection) are very helpful. If you try to avoid building several lower walls set back, make sure you show those to cover yourself. If you have a "generic" design sheet, attach it to any application to eliminate questions.
All of the major brands (Allan, Anchor, Keystone and Versalok) have outstanding sights for construction tips that work for all similar walls. These units are made by local manufacturers in most areas and countries. They also have engineering guides for engineers and an engineer on staff for professional assistance to other engineers.
It sure is a good, dursablr method for landscape of just for straight walls compared with the cost and problems associated with poured walls and footings since a SRW cannot use a concrete footing, steel reinforcement or mortar. If you have a large wall, they can be engineered up to 40' high last I observed.
In addition to SRW, options such as Timbers, Natural Stone, H-pile, Vegetated Walls, Cellular Containment Systems, Concrete Masonry Units (CMU) and Poured Concrete can be considered.
Factors to evaluate include cost, longevity, aesthetics, site access, machine access, complexity of construction and the ability to be engineered.
Cost can be evaluated by estimating the materials and labor required to install a hypothetical wall of x Height, y Width, and z Depth. Remember that some types of walls benefit more from Economies Of Scale than others.
Longevity can vary greatly based on factors including site conditions, manufacturing processes/quality control and installation details. A properly installed timber retaining wall may well outlast a poorly designed and installed poured concrete wall.
Aesthetics is a matter of personal opinion combined with site characteristics. A SRW may blend as poorly in a traditional Zen garden as a timber wall in a formal garden.
Site and Machine access are more important for some walls than others. Depth of excavation both vertical and horizontally into the bank need to be considered. Some larger SRW systems weigh upwards of 200 lbs apiece and require machines to move and set, while smaller SRW units can be light enough to be hand carried by one person and poured concrete has to be moved into place with chutes, pumps or buggies.
Due to the sheer volume of literature available from manufacturers, SRWs rate very well on complexity of construction and the ability to be engineered, . In addition, their lack of traditional footing and "Level-and-Go" style of construction make them very accessible to DIY projects. Natural stone is easy to install but hard to install well. The artistic aspect of beautiful dry-stack work often takes years to master and the variable nature of the material makes for difficult engineering.
Two options not mentioned before are Structural Shotcrete (or Gunite) and Gabion Walls.
Shotcrete and Gunite are non-traditional methods of placing traditional concrete. Both use pumping equipment to blow the concrete at high velocity onto the work surface. Traditional formwork is replaced with one-sided forms with the operator's side being open.
Shotcrete is mixed with water prior to being put into the pump and pressurized. Gunite is pumped as a dry powder through the hose to the nozzle and then is mixed with water at the nozzle or within a few feet of the hose end.
Premixing Shotcrete at the pump produces a more consistent product at the nozzle than Gunite, since Gunite is mixed with water at the nozzle and the amount of water can be adjusted by the nozzleman, proper mix is more difficult to control. The primary benefit of Gunite is the ability to start and stop the operation without worrying about the mix drying in the supply line.
With proper Engineering and installation, Shotcrete and Gunite can obtain the same structural integrity as traditional formed systems.
Gabion Basket Walls are large wire mesh cages filled with stone or rubble.
If you're looking for an engineer, ask if they have retaining wall software. I do retaining wall design, and the right software makes extremely quick work of designing retaining walls and producing full sets of plans, details, etc. Very, very quick.
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