Surcharge loading on modular block ret. walls supporting sidewalks and residential dirveways

[GeoSoil]

Lately, our local jurisdiction started imposing a 250 psf live load traffic surcharge on all retaining walls supporting residential sidewalks, residential driveways,and residential parking lots, which in my professional opinion is extremely conservative when designing low-height residential and/or commercial retaining walls (mostly geogrid-reinforced modular block walls), where the traffic is light or very light. Their idea is that a truck, or heavy truck, cannot be stopped from driving on a sidewalk or a residential driveway. Further, they rely to the IBC Table 1607.1-29 which states that "Sidewalks, vehicular driveways, and yards subject to trucking" should be designed for 250 psf. The same code (1607.1-14) allows a 40 psf surcharge for passenger cars inside a garage! Please note that the 250 psf, which is the standard highway design traffic surcharge on USA highways,does include heavy traffic (i.e., trucks and tractor trailers, which are common vehicles on USA highways. Away from our local jurisdiction, many government agencies do allow the use of a reduced traffic surchrge along sidewalks, residential driveways, and residential/commercial parking lots (e.g., 100 psf, 150 psf).

For geogrid-reinforced modular/small block retaining walls (which are very common in our residential projects), the application of a 250 psf surcharge at a zero offset behind the top block results in a toppling failure for the upper rows of blocks (above the first row of geogrids, which are typically placed below the 2nd block from the top, or about 16" below the top grade). Note that placing a geogrid below the topmost block (or about 8" belowthe top grade) is not a good practice as this geogrid layer gets exposed and damaged during the construction phase of the project, in addition to having a very low overburden pressure.

Is there a way out of using a 250 psf for areas where the traffic is light or very light?
See attachment (NCMA)

 

[phamENG]

Is a fire truck with a fully loaded tank of water light or very light?

 

[GeoSoil]

A fire truck loading is an exception. Engineers must design for a fire truck loading if a fire truck is supposed to access the driveway, etc. Fire truck access roads are typically shown on the approved site plans. Please note that a fire truck is not supposed to be positioned at a Zero offset from existing retaining walls (this is based on a conversation with a county fire marshal in our area). A 250 psf, and even a 450 psf, can be easily resisted by most retaining walls as long the surcharge offset is not Zero.

 

[phamENG]

Then don't put the pavement up to a zero offset. Unless there is some sort of traffic control structure to prevent traffic from going there, something likely will at some point. It's really quite reasonable. It's the reason pedestrian bridges have bollards to prevent cars from driving over them.

There's also what should and shouldn't be done generally, and what somebody decides is necessary in an emergency. If that driveway is the only way to get to the burning house, they'll probably drive on it.

But if the fire marshal will guarantee no fire trucks will ever approach your wall, then take that to the AHJ. Not sure anything else would get them to budge.

 

[GeoSoil]

phamENG, Our local jurisdiction is requiring the heavy truck loading to be applied even on top of sidewalks, which are likely to be very close to the face of the retaining wall, if a retaining wall does exist. Thanks for your feedback.

 

[dik]

If there's any activity, I generally use 100psf for a surcharge load... more if warranted.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

The AASHTO loading for retaining walls (accounting for the weight of the heaviest legal loads on the highway) can be up to 480 psf for short walls, where the load comes within 1' of the wall, but only 240 psf if it's more than that.

Btw, that is the vertical surcharge, not the horizontal pressure.

Rod Smith, P.E., The artist formerly known as HotRod10