## Footing below water table

## Footing below water table

(OP)

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.

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.

## RE: Footing below water table

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

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

www.SlideRuleEra.net

## RE: Footing below water table

f-d

¡papá gordo ain’t no madre flaca!

## RE: Footing below water table

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?

## RE: Footing below water table

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.

www.SlideRuleEra.net

## RE: Footing below water table

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!

## RE: Footing below water table

## RE: Footing below water table

ht

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.

www.SlideRuleEra.net

## RE: Footing below water table

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.

## RE: Footing below water table

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.

## RE: Footing below water table

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

## RE: Footing below water table

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.

## RE: Footing below water table

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.