Terraced Retaining Wall Surcharges?

[calpolyholley]

I am designing a series of three terraced cantilever CMU retaining walls on a site sloping at 2H:1V. The upper two walls are to retain a 4'-0" differential between FG Elev. The lower wall retains a 6'-0" differential between FG Elev. The walls are spaced about 6'-0" apart and I am concerned the upper walls may be inducing an additional surcharge load on to the bottom wall. I have accounted for this by applying an uniform lateral soil surcharge pressure of 65 psf in addition to the lateral soil at-rest pressure of 70 psf. I am looking for some feedback regarding the procedure for determining surcharge pressures due to terraced retaining walls. Thanks,

Below is an excerpt from the soils report:

LATERAL EARTH PRESSURES (AT-REST)
Backslope Equivalent Fluid Pressure (psf)
Flat 55 psf
2H:1V 70 psf

- must have drains. drains should consist of a blanket (1' foot minimum thickness) of class 2 permeable material and a 4" diameter perforated pipe (SDR35) near the bottom to carry the water. Permeable material should extend to within 1' of finished grade at the top of each wall.
- shallow continuous wall footings may be used for walls along rear or sideyards of lots. Shallow continuous footings should be designed for an allowable soil bearing pressure of 2,500 pounds per square foot. value can be increased by one third for wind or seismic loading.
- a passive pressure diagram imposed by an equivalent fluid having a density of 300 pounds per cubic foot may be used in the design if the area in front of the wall is level for at least 8'. The upper 1' of embedment at the toe of the wall should be neglected for passive pressure.
- for foundation elements located less than 8' from the edge of slopes(measured horizontally), passive resistance pressure should be neglected in the upper 4' of embedment. In order to develope passive resistance pressure consider
1) deepening the wall foundation such that elements used for passive resistance are setback a distance of at least 8' or more (measured horizontally) from the face of the slopes.
2) include a structural key incorporated into the retaining wall footing for passive resistance located a distance of at least 8' (measured horizontally) from the face of the slope. A base friction factor of 0.35 can also be used in the design.
- very low potential for fault rupture. No active faults through the building site.
- seismic zone 4/ soil profile type Sd/ seismic source type B/ closest distance to seismic source = 8km.
- no evidence of historic ground failure due to liquefaction on the site.
- silty clay to 5' deep, fine grain sand with trace of fine gravel.
- sandy clay 5' to 16' deep, moist, stiff to very stiff, limonite staining, fine to medium grain sand.

 

[fattdad]

Your at-rest earth pressure may be in the wrong units. Consider an equivalent fluid density of 60 to 70 PCF, which would result in a triangular distribution on each wall face. Use this in addition to any rectangular surcharge pressure that is also appropriate. I'd also consider geogrid stabilization of the reinforced zone.

Some thoughts. . . .

f-d

 

[jdonville]

calpolyholley,

Apply your surcharge from upper walls as a uniform surcharge. Depending on the spacing of the walls, Culmann graphical analysis may be warranted to determine the critical wedge.

At some point, the superimposed walls may need to be designed as a single tall wall.

FHWA NHI-00-043 at

http://isddc.dot.gov/OLPFiles/FHWA/010567.pdf

has some guidance for MSE walls on Page 175.

Hope this is helpful.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.