Active vs at rest for vehicle surcharge 2

[canwesteng]

I'm going to need a geotech to clarify for me... So for this retaining wall, I have soil plus surcharge from vehicles running beside it. I can easily justify using active earth pressure for the soil, but in the case of a vehicle running over the structure, does the wall need to rotate outward every time a vehicle runs over the roadway area, or can I use active pressure as long as rotation has occurred once and assume the soil maintains the same failure triangle?

 

[PEinc]

IMHO, you are letting theory get in the way of history and standard practice. The things to consider are how are most walls designed and how have they performed for many years under many different soil conditions. Earth pressure starts the wall movement; the pressure doesn't result from the wall movement. While I have the utmost respect for TPM, maybe this is all a bit too theoretical? Why fix an age old design method that doesn't seem to be broken? Leave the fancy, theoretical stuff up to professors who have little else to do in the real world. If, all of a sudden, walls start failing under the standard design method, then maybe the method needs to change.

http://www.PeirceEngineering.com

 

[fattdad]

this thread has me confused. . .

Step 1: Evaluate the earth pressures irrespective of surcharge loading. Use either active or at-rest as the design warrants.

Step 2: Establish the location and magnitudes of the line or point loads that represent the vehicle.

Step 3: Use Bousinessq (sp) elastic solutions (Poulous and Davis) to obtain delta sigma H values for locations on the wall (i.e., locations with depth).

Step 4: Double the answer (bearing in mind that the elastic solutions are not for an elastic half-space, which the boundary condition of the wall.

Graph the change in loads and make your design.

Not sure how the line- or point-loading is processed by either active or at-rest soil friction or lack thereof. I'd use elastic theory.

Now if it's an areal load, maybe I'd think differently?

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

Hello fattdad,

We're heading back to the discussions about Bousinesseq, 2 X Bousinesseq, and trial wedge surcharge loads. There are lots of conflicting theory on the treatment of surcharges, how, when and where.

Bowles discusses this relative to Spangler's testing as I recall. AASHTO differentiates between traffic surcharge loads (uniform loa dapproximations) parallel to a wall and perpendicular to a wall. All answers can be right depending on the situation and assumptions made.

Not sure I have ever totally resolved this in my mind and I have studied it for many years.

 

[fattdad]

Hello Doctormo!

As do I. . . wonder how to build a box around this conundrum?

That said, whatever I seem to do seems to work!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Sam_imported]

On a tangent...I have a lane servicing a loading bay that does a 90 degree turn to get around a building above a retaining wall. It is my understanding the vehicle surcharge loading is for traffic parallel to the retaining wall face. What about the situation where the vehicle is accelerating away from the wall or decelerating towards the wall? Would the lateral force from acceleration/deceleration be entirely addressed within the pavement structure with no loading on the wall?

 

[darthsoilsguy2]

A rule of thumb commonly used is to assume active vehicle surcharge loads = 2 extra feet of retained soil. Design the wall for the new height and at the last step cut the top 2 feet off the wall and draft it up.

 

[EireChch]

@darthsoilsguy2 - interesting rule of thumb, i have never heard of it...and i am not sure if i understand it? Should it be to assume Ko add the extra 2 feet of soil? as that would increase the pressure like Ko would do compared to Ka. Also, there must be a limiting height that that applies to?

 

[JoelTXCive]

We do a fair amount of cantilevered retaining walls and we often get two design parameters from the geotechs.

They will provide an active earth coefficient (k_active) for the general soil load. This value is usually around 0.3.

They will also provide a second coefficient that only applies to surcharge loads. Often this coefficient will be 0.5 (k_surcharge)

Every now and then we will get a wall where we are concerned about excessive cracking or deflection. In those instances, we will have the geotechs provide an in-situ coefficient (k_naught).

I think the rule of thumb comes from a common construction equipment surcharge load that many people like to apply. They just want a blanket 250psf construction surcharge behind a wall. If you are designing for a 1ft wide section, then 250psf becomes equivalent to 250pcf. In Texas, our clays soils usually weigh 110-125 pcf. So...250psf surcharge is roughly equivalent to 2 extra feet of soil. I just realized that the extra 2ft would be getting multiplied by the lower k_active coefficient though versus the higher k_surcharge of 0.5.

 

[Doctormo]

The 2' of roadway surcharge for retaining walls comes from AASHTO whereas building codes and design guides typically use 250 psf. The 2' of soil surcharge would only be accurate if the load was from trucks carrying the backfill material, haha. Seriously, the weight of backfill varies from 100 pcf to 145 pcf thus the 2' soil surcharge is not a very good way to approach vehicular live loads.

Current AASHTO provides for additional feet of soil surcharge loads close to back of wall as I recall (within 1 ft.?). In AUS, they typically use 20 kN/m2 for highway surcharges which is around 400 psf. I suppose there is no right answer to this but walls rarely fail due to traffic surcharges so most highway departments do not spend a lot of time on it.

Another way to minimize the effects is to specify high quality backfill material behind walls like many highway departments do. The difference in strength, drainage, and creep properties make it a prudent investment for heavy load areas.

 

[bridgebuster]

That's how I learned to do it as well.