Active vs at rest for vehicle surcharge
Active vs at rest for vehicle surcharge
(OP)
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?





RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
Using at rest pressue over active results in an increase of approximately 30-40% in the applied pressure on the back of the wall, which in turn increases over turning moments. If you have an embedded wall then you will need deeper embedment to satisfy your FoS. Similarly if you have a concrete cantilever you may need to increase heal width or add a key.
The topic of active or at rest earth pressure does not depend on the type of loading i.e. vehicle surcharge or soil pressure. It depends, among other things, on the type of wall and the effects of movement/rotation. I think you may be trying to design for dynamic effects, which would not be needed for (most) walls with a vehicle surcharge.
For example if you had a masonry basement wall you would design for at rest pressures because you want to minimise movement.
If you had a garden landscape wall then you would design for active pressures as the effect of rotation/movement would probably not be noticed. There is no point in beefing up your design when the risk are not warranted.
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
f-d
ípapß gordo ainÆt no madre flaca!
RE: Active vs at rest for vehicle surcharge
DaveAtkins
RE: Active vs at rest for vehicle surcharge
For vehicular traffic, I think that since the location of the applied load is constantly varying, the distributed load of 10 kPa accounts for this also. If there is a permanent point load, I would use f-d approach, but I think that vehicular load may not be considered as a permanent point load.
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
I do not believe a simplified active wedge accurately captures the actual strain behavior of the soil mass behind a partially restrained wall with repetitive high magnitude wheel loads. Lateral load is dependent on the wall system stiffness. Not all cantilever walls behave the same.
For a simplified Ka calc using phi = 34 degrees, Ka = 0.28.
For moist soil wt = 120 pcf, the pH = lateral earth press = 34 psf / ft + Ka*pv. That's too low, and I'll stand by that claim.
For comparison, consider two cases.
a. Caltrans pH for culverts = 100 psf / ft. [Section 6]
Caltrans, albeit a politically flawed institution and known for conservative provisions, does have a few smart engineers, and their engineers have made many edits to their Bridge Design Manual for several decades based on lessons learned.
b. Pressures for compactive equipment. See Terzaghi Peck Mezri Article 44 and 45. I do realize that compactive equipment is not the same as repetitive wheel loads. However, it does provide some insight on the wide spectrum of actual lateral pressures behind walls, based on non-static loading.
Do you want to choose the very lowest static pressure (Ka < 0.3) for a wall that will have compacted backfill, then dynamically loaded for the next 50 years?
RE: Active vs at rest for vehicle surcharge
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
RE: Active vs at rest for vehicle surcharge
Consider four walls that are all 10 ft tall:
1. Cantilever retaining wall (inverted tee) with 10" thick wall and 5' wide footing - typical minimum design
2. Cantilever retaining wall (inverted tee) with 12" thick wall and 7' wide footing
3. Counterfort retaining wall (inverted tee) with perpendicular stiffener ribs tying back of wall to foundation heel, with 7' wide footing
4. Top supported wall or culvert box ("rigid")
- All backfilled, all with traffic loading directly behind
I would argue that as the wall system's rotational stiffness increases, the wall element must be designed for higher effective K values. Usually, structural engineers think of either active or at-rest, but items 2 and 3 above will definitively deliver more than Ka*pv, and less than Ko*pv.
Agree?
Another, somewhat separate item: As the backfill soil wedge behind the wall develops, there is a slight relaxation (from the wall's perspective). However, as the soil is vertically re-loaded with repetitive loading every hour, the wall either keeps rotating out(bad) or the wall pushes back harder (higher horiz load) because it has higher rotational stiffness - which partially depends on the foundation configuration. This is the modulus / strain part of the analysis that engineers seem to disregard.
I've seen many leaning walls, even with apparently good drainage. Seems like the best design method to address this re-loading and induced strain problem is via higher K.
Agree?
RE: Active vs at rest for vehicle surcharge
As for your last item, I don't agree. if it were true all walls with cyclic loading would eventually fall over. Using a higher earth pressure coefficient does not necessarily meant there is higher earth pressure. It just assures that the wall is designed heavier. You may be overthinking this. If it isn't broke, don't fix it. Nobody has more retaining walls with cyclic loading than DOT's. No one worries more or is more conservative than DOT's. Their cantilevered walls use Ka.
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
If you have the patience and time, consider the walls 1 thru 4 noted above.
The pressure from the soil is dependent on the rotational stiffness and horiz translation of the structure. The soil does not know when we as engineers designate the wall as flexible or unyielding. Most cantilever walls are can be categorized as yielding, but not all.
A counterfort wall (Wall 3) and a 4-sided culvert (Wall 4) or even 3-sided box culvert (inverted U, say Wall 3.5) do not develop a classical phi/2+45 active wedge behind the wall. My argument is that there is a continuum of K values between Ka to Ko, with wall system rotational stiffness as an independent variable, and Ka is the lower bound applicable to Walls 1 and 2 only. K is not a step function.
Terzaghi Peck Mesri make a similar case in Article 45. If you have the 3rd edition of Soil Mechanics in Engineering Practice, pages 332-334. I bet you have this text, but if not I can scan and upload. Here they make the differentiation between soil pressures used for structural elements (higher than Ka), vs soil pressures used for stability (Ka).
On a lesser note, the added horiz pressures (in addition to the lateral pressures induced during compaction behind the wall) depend on the level of compaction after wall construction (and before "service life" begins). Again, Ka is the lower bound.
Smack me down. I'd rather get smacked here on the eng-tips forum than during a high profile peer review.
RE: Active vs at rest for vehicle surcharge
www.PeirceEngineering.com
RE: Active vs at rest for vehicle surcharge
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!
RE: Active vs at rest for vehicle surcharge
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.