hydrostatic pressure

[civilmg]

This is a real quick question:

I am evaluating a failure to a segmental block retaining wall. The backfill is reinforced with a geogrid. In one location behind the wall, a junction box (extending from ground surface down below the base of the wall) interrupts the geogrid reinforcement. The box is located about 1 foot from the back face of the wall. I am determining the excess hydrostatic pressure on the wall due to an intense rain event. I am assuming a column of water built up between the retaining wall and junction box. As we all know, the pressure is computed as unit weight of water multiplied by the depth (in this case 20'). The assumed column of water is 4'x1'x20' My question is, Does the one foot separation between the wall and junction box have any influence on the hydrostatic pressure, since hydro. pressure is only a function of depth and density? Suppose the j-box is 2 inches from the wall and the column is now 4'x2"x20', will the hydrostatic pressure be "allowed" to develop on the wall since the j-box is so close?

It's been a while since i reviewed my old fluid mechanics books. Just looking for insight. thanks.

 

[Focht3]

For all practical purposes, the full water pressure will act on the back of that (thin) strip of wall.

Having said that, I'm having a hard time seeing how this total force could cause the failure of a well designed and constructed SBRW with geogrids. I'm curious; could you give us a thorough description of the problem?

 

[VAD]

Note that intense rain events can result in an increase in the lateral earth pressure by about 33%. This is often forgotten in the design of walls. The seepage force acts on the hypothetical failure wedge and results from the flow. The water that accumulates at the back of the wall exerts full hydrostatic presure but the earth pressure is reduced because of the bouyant effect of water on the weight of the submerged soil. Credits to Theoretical Soil Mechanics by Terzaghi and Earth Pressures and Retaining Walls by Huntington. I am not sure if these books are in print but can be sourced from a University library.

 

[RWF7437]

How did the wall "fail"? Sliding, overturning, bulged out somehwere between the top and bottom ? The mode of failure is an important clue to the cause.

 

[civilmg]

Thanks to all for the advice.

to answer Focht3: I also am having a hard time believing that the failure is due to this force. I'm just trying to rule some things out. I hesitate to go into too much detail, just in case.

to VAD: I'll look for that text, thanks.

and for RWF7437: the lower 2-3 feet of the wall bulged outward.

once again, thanks to all

 

[Focht3]

It sounds like the lower levels of geogrid weren't long enough. At least, I'd look hard at that as a contributing factor. The rainfall event may have been the proverbial straw that broke the camel's back.

Anyway, good luck! Let us know what you find -

 

[civilmg]

Focht3:

i'll look into it. Thanks

 

[clayR]

civilmg:

I was associated with a remediation project where a pole was installed 3 ft behind the face of a proposed 45 foot mse wall. There was bulging of the face and settlement of the level pad as the wall was being built. Construction of the wall was halted when the wall was about 50% up. Soil investigation determined loose material as a result of caving during installation of the drilled shaft for the sign pole. the drilled shaft apparently went through an old foundation that had been backfilled with granular material, resulting in ground loss. The other problem was the method used for load transfer through the straps around the sign pole. The problem was corrected by compaction grouting the foundation and building a steel frame for load
transfer around the pole. The wall was then completed with no further problems.

 

[BigH]

clayR - Thanks for your case history! These personal notes of experience should be heartily encouraged!

(the clapping cook - closest I can find to clapping!)

 

[MSEMan]

I would find it highly unlikely that the geogrid isn't long enough. Most codes not require at least 0.6 x H and this is more than adequate for internal stability.

Excessive bulging normally occurs as a result of a number of interrelated construction and materials selection practices. First of all, look at the design spacing of the grid and whether or not the facing can withstand the shear under idealised conditions (you also may want to consider locked in compaction stresses.)

Secondly consider the soil - is it granular elastic or is it possibly plastic. Plastic fills generally make poor reinforced soils, particualrly with block walls where soil strains can cause the facings to become separated from the soil reinforcement.

Finally, if neither of the above are the case look at poor construction practice such as insufficient compaction and failure to protect the works from rainfall during construction. The effect of lower compaction, of course, is that the fill tends torwards phi cv rather than the peak phi used in design. Ka therefore goes up and the bond of the geogrid (alpha * tan phi) goes down. When you then consider that if the improperly compacted fill is saturated it reduces the effective overbruden stresses acting on the grid(reducing the pullout resistance of the grid even further) again reducing phi (effective stress conditions), and increasing the horizontal pressures on the block facing due to hydraulic compaction, you can start to see how very bad bulging can occur.

It is very important in these cases to monitor the structure to see if any continuing movement is occurring. If movement is continuing - I'd suggest seriously considering taking it down. However, with granular fills, hydraulic events that don't cause catastrophic failure or render the structure unsightly or unserviceable may actually lead to a more stable end condition. This should be evidenced by a lack of movement in the facing blocks.

 

[Focht3]

MSEMan makes some good points. Certainly, it appears that poor design and/or construction caused the failure. (I'm including defective materials as part of 'poor construction.') If MSEMan is correct and poor compaction is the cause (or a significant contributing factor) of the failure, then civilmg should have observed uneven settlement of the first 10 to 12 feet behind the wall.

What sayest thou, civilmg? What did you observe at the site, and what was the geogrid layout?

 

[civilmg]

First of all, I thank everyone for all of their input and advice.

Focht3: the answer to your questions are difficult to answer as I only have photographs of the site. Remediation has already been completed.