Practical Maximum Height of Modular Blocks for Gravity Wall (Large block & Small blocks)

[jagti]

I deal with lots of Precast block making companies that have their own design charts or cook book cross sections demonstrating how high they can go with gravity walls & faster installation. Some of them apply multidepth principle, mass extender blocks/tails at the bottom portion, gabion baskets, cribbing etc.

I would like to get a practical sense what maximum height we can go on gravity walls with morterless precast blocks. I know the general rules is between 2.5D to 1.5D (depth of block i.e solid & hollow block to also consider). Some even suggest in ideal circumstance going 3 times the depth of block if retaining clear stone (or 1H:1V granular wedge). See i don't understand granular wedge if excavating & replacing soils why not incorporate grids as it definitely improves engineering properties of soil (i.e friction angle) as less lateral loads on blocks.

Most often when specifying type of retaining walls geotechnical information is limited or unknown. Soils engineers give conservative values based on limited bore information or provide range of values for different type of soils that are to be expected i.e. allowable bearing pressure, friction angle, unit wt and active earth pressure.It often becomes hard what type of retaining walls to choose given the range of choices available. Owner wish to make full use of the property & save money in choosing retaining walls. Usually the cheapest option is precast block walls for 1.0m to 3.0m height.

I always prefer geogrid walls over gravity walls(on fill conditions) & where property line or existing vegetation i.e. larger trees are not an issue even if it requires more time & quality control wrt to reinforced fill compaction etc.

I would like to have suggestions how other engineers set their limits wrt to choosing a particular type of precast block wall wrt maximum gravity wall height.

Thanks.

 

[Doctormo]

Big block systems tend to have a base to height ratio of 40%-50% (same as your ratios) but this is totally a function of batter, retained soil type, and backslope/surcharge loadings thus the rule of the thumb only pertains to simple applications. In perfect conditions, 33% B/H may work as well as you noted. The key is having the structure designed for the site specific conditions in a reasonable and conservative manner.

Big blocks tend to be on the expensive side (2X segmental blocks) so once geogrid is introduced or required, it is much cheaper to go with segmental block walls (small block system) thus the big block walls tend to work best as gravity walls. In fact, if space is available, segmental block walls and geogrid can still be more economical than big block gravity walls so it depends on what you are trying to accomplish at each wall location.

Excavating a 1:1 retained wedge and backfilling with granular material can reduce the loads and increase the design height but that pretty much kills the advantage of a narrow profile wall and property line constraints. Of course, there is a significant cost to do this as well.

I am not sure I can help you too much with selections as they are usually unique to each retaining wall location and one has to determine what drives the selection. Obviously, the retaining wall type has to work for the application but after that cost and appearance becomes a consideration so the Owner/Architect gets involved in that part and the civil engineer is left out of that process.

Good luck solving this common problem.

 

[wallguy]

jagti

I am a marketing and technical person for a SRW company. PCB's are certainly growing and have there place. Height for most are an issue. Depending on the systems economically they max around 15 plus feet. But as you suggest with geogrid they can go much higher. Again as you stated then why not an SRW system which would be less expensive. With the past history of SRW's in certain locations you might be up against an owner driven issue. We are developing a multi depth system much like our multi depth modular SRW system that will not have the same height limitations as the big block PCB's have. Let me know if I can help you locally with product information.

 

[Doctormo]

wallguy

Re: "with the past history of SRW's in certain locations, you might be up against an owner driven issue".

This is certainly true and does become a consideration as I also alluded to above but retaining wall systems have been failing since the first crib systems in the early 1900's which has created all sorts of prejudices in the owner/design community towards the various systems. The historical problems with SRW's is just a more recent phase of retaining wall system problems and widespread usage in a short time period. The big block systems are starting to go through the same failure phase now as the same marketplace issues exist that create competitive design and construction bidding that pushes the wall system performance envelope to do more at less cost until something gives. I expect that the multi-depth systems will go through the same process as well based on past experience.

FYI - In my opinion, it would be prudent to stay away from product promotion in these engineering forums. I am sure this was not your intent but the last couple of sentences starts moving in that direction.

 

[wallguy]

Doctormo

Yes sir, pushing the "envelope" by contractors and some designers on the systems does seems to be a practice in the marketplace. With multi depth systems falling under AASHTO design methodology we will hope there is less wiggle room to push that envelope. You are correct, the intent is toward systems not products, thank you.