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Design Ground Water Level 1
- Thread starter ron9876
- Start date
I wouldn't think a hurricane would have much effect on the seasonal high groundwater table in much of South Florida due to the high porosity of the soil.
Most of the excess water would run off and any groundwater effects would be transient.
For buoyancy, I usually take the groundwater to be at the ground surface. It's a little conservative, but not far off in most of Florida and SE Georgia.
If you have floor levels below the water table, how are you going to keep them dry?
Why do you need to know the groundwater level in a hurricane event?
ALthough hurricanes usually have high rainfall, slow moving sub-hurricane storms can drop much more rainfall.
Most of the excess water would run off and any groundwater effects would be transient.
For buoyancy, I usually take the groundwater to be at the ground surface. It's a little conservative, but not far off in most of Florida and SE Georgia.
If you have floor levels below the water table, how are you going to keep them dry?
Why do you need to know the groundwater level in a hurricane event?
ALthough hurricanes usually have high rainfall, slow moving sub-hurricane storms can drop much more rainfall.
Don't worry about a hurricane event. As jgailla says, it is transient. What you need is the seasonal high water table. Any local geotech firm in the area should be able to determine that for you. If you don't want to go that route, check the USDA Soil Conservation Service soil maps for the area. It will get you close.
On a multistory structure the hurricane event shouldn't post much of a problem. However if its single story if the water table is above the bottom of footing elevation you have a 62.4 lb/ft3 less footing weight. For those that don't practice in Florida footing sizes under single story columns are usually controlled by wind uplift.
I would assume that the ground water level during the hurricane event matches the seasonal high groundwater level. Another thing about practicing in Florida is that in many places the ground water level is only a few feet down. Heck I can dig a two or three foot hole in my back yard and usually strike water.
That all said I usually don't worry about it. There are other things like soil cohesion, friction, load redistributions that probably would offset water weight.
John Southard, M.S., P.E.
I would assume that the ground water level during the hurricane event matches the seasonal high groundwater level. Another thing about practicing in Florida is that in many places the ground water level is only a few feet down. Heck I can dig a two or three foot hole in my back yard and usually strike water.
That all said I usually don't worry about it. There are other things like soil cohesion, friction, load redistributions that probably would offset water weight.
John Southard, M.S., P.E.
- Thread starter
- #5
The project will have two parking levels below grade. The hydrostatic uplift will dictate the design of the ground floor slab, related added tension piles and exterior walls.
While the soil is very porous the rainfall of a wet storm will raise the ground water level of adjacent areas and it would occur during the wet season when the water table is at it's highest.To assume the water at the ground surface would be very conservative.
If you estimate and miss by say two feet you are missing the design loads by 125 psf which is a major diffrence.
While the soil is very porous the rainfall of a wet storm will raise the ground water level of adjacent areas and it would occur during the wet season when the water table is at it's highest.To assume the water at the ground surface would be very conservative.
If you estimate and miss by say two feet you are missing the design loads by 125 psf which is a major diffrence.
superbridge
Structural
I advice you to take some insepctions from the official documents about the history water level.
If the ground is pourus, I would not expect a large rise, but it is difficult to accurately predict with out observing similar events. The question is not where would the water go now, but what would be the water level go after construction. There will need to be some drainage or dewatering system in place to protect the parking garage. How will that effect your water levels, and what if it fails?
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1
- #9
Unlike the others, I would be at least a little worried about flood waters. The best way to eliminate these fears is to look at the Flood Insurance Rate Map (FIRM map). This map can be found online at the following address:
All you need to do is type in the address of your project and see if it falls under any type of flooding requirements. If it does, then you need to design in accordance with ASCE 24.
If you are a few miles away from the coastline I would suspect this shouldn’t be a problem… but it only takes 5 min to look up. You never know.
I’m actually am in the middle of a project with a naïve contractor who ignored this requirement and they are now in it deep… literally… according to the FIRM map they are 10’ below base flood elevation. Now they are scrambling trying to figure out how to meet the requirements with as little cost impact to the structure.
All you need to do is type in the address of your project and see if it falls under any type of flooding requirements. If it does, then you need to design in accordance with ASCE 24.
If you are a few miles away from the coastline I would suspect this shouldn’t be a problem… but it only takes 5 min to look up. You never know.
I’m actually am in the middle of a project with a naïve contractor who ignored this requirement and they are now in it deep… literally… according to the FIRM map they are 10’ below base flood elevation. Now they are scrambling trying to figure out how to meet the requirements with as little cost impact to the structure.
- Thread starter
- #10
SteelPE thanks for the info. Looks like FEMA maps which, unless I am mistaken, don't give you groundwater elevations unless you are in an actual storm surge zone which my project won't be.
That's where the problem comes from. If you have a storm surge you can get the elevations but if you are a little inland and if the elevation of grade is something like 14' all I can find is the elevation of groundwater when the borings were taken.
That's where the problem comes from. If you have a storm surge you can get the elevations but if you are a little inland and if the elevation of grade is something like 14' all I can find is the elevation of groundwater when the borings were taken.
You are correct that the FEMA maps do not give ground water elevations, just flood elevations (at least the ones I have seen). If you do not fall in under these requirements then I would agree with everyone else that consulting with a geotechnical engineer is your best option.
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