Footing below water table

[broekie]

I'm designing a bridge abutment at the edge of a river and wanted to know how to handle the fact that the footing will be below the water table.

The wall is a cantilever abutment retaining wall on piles. The top of the footing is at elevation 773.00. The footing is 3' thick. The water table is at approximate elevation 776.00. Howver, since this is at the edge of the water, the water table will (for the most part) be level on both sides of the wall. I'm not too concerned about the hydrostatic force due to the water table behind the wall because that will be counnteracted by the hydrostatic force of the water in front of the wall.

My question is how I should handle the first 3' of soil that is above the top of footing and below the water table. Should the lateral force from that soil on the wall be based on the effective soil density of 120 pcf - 62.4 pcf = 58.6 pcf or should I just use 120 pcf? Also, what vertical load should I assume goes into the piles? The weight of the soil minus a buoyancy force?

I will have a 6" underdrain behind the wall.

Any help that the soils experts can give me would be appreciated.

 

[SlideRuleEra]

broekie - For the classical approach, see "US Steel Sheet Piling Design Extracts" on this page of my website

http://www.slideruleera.net/Steel-Piling.html

The first 15 pages, or so, cover this situation (and not just for steel sheet piling).

http://www.SlideRuleEra.net

 

[fattdad]

For your first question use bouyant unit weight. Not sure about your second question. The axial load would depend on the net structural loading applied at the top of the pile. Maybe I'm missing something, but I'm not following how the unit weight affects the axial loading of the pile (unless downdrag is a factor and then I'm still not too sure as it relates more to undrained shear strength or the interface friction angle and horizontal loading).

f-d

¡papá gordo ain’t no madre flaca!

 

[broekie]

Thanks for the responses.

The second question, fattdad, is a question of the vertical load in the piles. There is the structural loads on the wall, the weight of the wall, weight of the footing, and weight of the soil block behind the wall all contrbuting to the vertical load in the pile.

The question that I had is that since the footing and 3' of the wall is below the water table, I'm assuming that a buoyancy force should be subtracted from the pile loads to account for the concrete and soil below the water table. Is that correct?

Also, does the fact that I have a tremie seal under the footing make any difference?

 

[SlideRuleEra]

The submerged weight of each item below the water table, including the tremie seal, contributes to the vertical load on the piling.

The fact that there are piling means that the soil cannot be depended on to support these components - something has got to hold them up - in this case it's the piling.

http://www.SlideRuleEra.net

 

[fattdad]

If the wall backfill settles then there will be a downdrag component that is supported by the piling. If the pile cap includes a horizontal element (to the landside) the weight of the soil above that horizontal element would contribute to the vertical load on the piles. You would use moist (total) unit weight above the water table and bouyant unit weight below the water table. I wouldn't consider the use of a concrete seal placed using the tremie method to have any affect on the loading.

There is also a lateral loading component to this design. You need to consider bending moments and whether the pile cap will be designed to allow sufficient movement to fully mobilize the active earth pressures. If you don't allow this movement, then your overturning moments need to be based on at-rest earth pressures. Again, using total/bouyant unit weights as appropriate.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i wouldn't reduce the pile loads due to the buoyancy force...you don't get "credit" for it since it would disappear if the water level dropped...account accordingly...

 

[SlideRuleEra]

The bond between the tremie seal concrete and the permanent foundation piling is significant (in the 150 to 300 psi range). See this summary report at the Florida DOT website

http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_STR/FDOT_844.pdf

Since a percentage of this bond strength is routinely relied on to resist uplift while the cofferdam is dewatered, IMHO, it seems prudent to conclude that this bond strength will also force the permanent foundation piling to support the weight of the concrete seal after the cofferdam sheet piling have been removed.

http://www.SlideRuleEra.net

 

[miecz]

I would design for two conditions:

Water table at 776, with bouyant weights and lateral forces based on soil density of 57.6 pcf; and

Water table at 770, normal dead loads and active pressures based on soil density of 120 pcf.

 

[motorbiketocrank]

I think you should ensure that the project’s geotechnical engineer provides you with appropriate parameters for the worst case loading conditions that can occur for your various analyses. I think there are certain statements you made in your question that are problematic and I think some of the answers given are incorrect.

First, I wouldn't frame the problem based on where the water level is at one particular time as it will flucuate and the “worst case” will be different for different considerations (ex. lateral vs. vertical). It may be wrong to say that the water level behind the wall will be balanced by the water in front of the wall. You may have a rapid draw down type condition after a flood where the river level drops and the water from behind the wall cannot drain as quickly. So you may have hydrostatic pressure for the full height of your wall depending on the possible flood stage and other considerations. You can’t just design assuming that the drains will always work and you need to provide adequate F.S. for blocked drain conditions too. And out of curiosity, what are you counting on to resist the lateral load, the piles? How does potential scour impact this lateral restraint and has the geotechnical engineer given you the various related soil/pile interaction parameters for lateral load analysis? How much can the wall (and the underlying piles) move laterally and has that been considered in whether you’re using active earth pressure or at rest pressure?

Not trying to be overly critical. As I’m sure you agree wrong assumptions on parameters can impact the bridge and public safety. So make sure that a qualified geotech has the responsibility of providing you with parameters that are appropriate.

 

[broekie]

Thanks for the input, geobdg. Qualified geotechnical and hydraulic engineers have been and continue to be an integral part of the project. Many of the questions/concerns in your email have been addressed in soils reports, hydraulic reports, and the history of this particular project site.

Scour, varying water table, and flood conditions are all being considered in the design of the wall.

 

[oldestguy]

GEOBDG:

Hey guy a great post. That's good engineering in my book. Too many unknowns face us all the time and it's tough to be sure things go right. Higher costs still should not stop us from considering all possibilities.

 

[broekie]

Again, thanks for everyones' help. The wall has been designed for the water table below the footing, the water table 8' above the bottom of footing (ordinary high water elevation), and the water table at 13' above the bottom of footing (100 year flood stage). It has also been designed to handle a 7' difference in the water table from one side of the wall to the other.

In addition, another engineer in our office not working on the project, developed his own spreadsheet and analysis independently from mine. His results compared within 1% of mine, which is a nice QA/QC check.