Soil Permeability and Flow

[RussDa]

Hi all.

I have a question about water flow through soil.

Let's say we have a rectangular container with a given size (400 ft x 200 ft x 20 ft). It is filled with a fully saturated soil with a given coefficient of permeability (18.9 GPD/sf). If we remove the bottom of the container, can we say that the maximum flow of water due to gravity through the bottom of the container is the cross-section area multiplied by the coefficient of permeability?
I.e. 400 ft * 200 ft * 18.9 GPD/sf = 1,512,000 GPD

If this would be incorrect, what is the correct calculation for the given problem?

Will the flow be constant or it will vary depending on the remaining thickness of the water table?

Thank you.

 

[oldestguy]

Your formula assumes a hydraulic gradient of 1.0 I suppose. It also assumes complete saturation. Change both of those and you have a correct answer for an instant only. So, why the question?

 

[RussDa]

This is for a future construction excavation with a sheet pile shoring. Water inside is the groundwater (water table), so the soil is fully saturated.
I calculated the amount of water in the soil inside of this "bathtub". Now I need to calculate how the fast this amount of water can theoretically seep through the soil to the bottom of this excavation so I can determine the capacity of the dewatering system needed to dewater this excavation as fast as possible so that the excavation work can start.

Since it is a water table, I'm assuming complete saturation.
Wouldn't hydraulic gradient be 1 in this case, assuming the water table is always horizontal and the flow is always vertical?

 

[oldestguy]

Permeability of partially saturated soil drops off as water is withdrawn. It would be unlikely that water table drops uniformly across the site. Where are the sumps? However,for your case other factors need to be considered. How deep are the sheets extending below the proposed excavation? Will your pumping also be drawing water from outside at the same time? Has a flow net been drawn to predict these things? What soil is left below that point? Will the base be stable or turn to a quick condition. Finally why not start excavating partially saturated soil soon after pumps turn on? I'm sure you are not planing to excavate "dry" soil, by removing all that pore water ahead of time, as if that would ever happen by gravity.

 

[fattdad]

it seems that your conceptual model is flawed. . .

Somewhere you need to consider the penetration depth of the sheets that surround your, "Bathtub" area. Let's say you are driving the sheets 20 ft below the ground to allow for a 15 ft excavation below the water table. Sure, you can calculate how much pore water is present in those soils, but as you dig, water will want to flow vertically upward from the base of the excavation. A flow net will show you these details. The vertical flow can easily exceed the critical gradient for the soil and you will develop the quick condition (or if the soils are fine grained base heave). If this occurs, it's a design failure, so that's a bad thing. Well points can address this if you have sufficient number of well points and keep the flow toward the well and the gradients under control.

Oldestguy says you can just dig out the saturated soil and take the ground water as you go. That'll be true to some depth at which point you'll exceed the critical gradient then it's a problem. You can reckon through this and dig to some intermediate (i.e., safe) depth then install the dewatering system. I'd likely just install well points just outside the sheets and then dig on one fowl swoop,

Refer to NAVFAC P-418, Dewatering and Groundwater Control for more information.

f-d

ípapß gordo ainÆt no madre flaca!

 

[RussDa]

Thank you for your replies.

It is indeed a well point system. I'm just trying to roughly estimate the storage depletion (flow from just the inside of excavation before the system is dewatered). I have formulas for the models that are in the equilibrium dewatered condition.

I need to be able to quickly calculate the amount of water and flow just from the inside of excavation, and just wanted to confirm that the simple calculation in my my first post doesn't introduce a significant error.
No drawing water from outside of excavation and no upward flow from the base of excavation. Penetration depth of the sheets is below impermeable layer in this case, so it is indeed a "bathtub".

I understand that it is possible to start excavating partially saturated soil, but in reality, will there be a big difference in the amount of water to be pumped if we assume pumping until the soil is "dry" compared with leaving "partially saturated soil" and starting the excavation?
Again, this is just for quickly checking if the chosen dewatering system has enough capacity to dewater the site within a reasonable time period.

 

[BigH]

Or pick up Cedergren's book on Seepage, Drainage and Flow Nets.

 

[bb29510]

or just call the dewatering company, they will know off top of their head from experience