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Dewatering Drawdown

Dewatering Drawdown

Dewatering Drawdown

This is solely educational with real data and I am hoping not for a "ask a geotech" answer please! There is a dike that is surrounded by pounds. On this dike is a pump station which must be built. The water level is located 16' below existing grade. The soils report only bored to a total depth of 31.5' and all the material is silty sand. In the report it does not include the depth of what is to be thought as an impermeable layer. In order to build the pump station we have to excavate below the water table therefore the need of dewatering wells. Thus my conclusion is gravity flow with partial penetration right? I have read many sources and they all say equations and flow nets are not good for partial pen. and gravity flow. So my question is how do I solve this issue? If I don't know where the impermeable layer is I can't even draw a flownet and all the equations depends on total height from impermeable layer to the phreatic surface. I am working out of the the foundations engineering book G.A. Leonards (1962). There is a rough drawing of the location as well.

RE: Dewatering Drawdown

unless you plan on dewatering the ponds, you should assume that there will be zero feet of drawdown at the edge of the pond. If that's 10 ft away, then that's the distance to zero feet of drawdown.

The implication of the OP is that the dike is fill. Do you know whether the excavation will be wholly in fill? In other words it may all be "silty sand," but is the upper 10 ft silty sand fill and then it's natural alluvial/fluvial silty sand?

Here's my advice:

Take the grain-size curve and approxoximate the permeability.


I'd consider figure 4-11 of the following reference:


Now the first thing you're going to think,"How can a single well meet the needs of the dewatering project?" Well, just take the rectangular area of the pump station, calculate an equivalent circular area and use that as your well diameter. It' may have a 10 or 15-ft radius, but that's o.k.

When using an equivalent well model you'll need to think through a few things. The radius of the well is that number, but the distance to zero drawdown (i.e., the pond limits) need to be measure from the same reference point (i.e., the center of the well circle).

I'd run the numbers for fully and partially penetrating. I'd assume 1/2 penetration and see how the answers fall out.

In natural soils, there is typically enough anisotropy to justify using the fully-penetrating equations as horizontal partings in the fluvial soils somewhat limit vertical flow.


¡papá gordo ain’t no madre flaca!

RE: Dewatering Drawdown

the ponds will be empty as the construction will occur during the summer. The soils data implies that at the time the test was taken (summer) there is water 6 feet below the finish elevation of the ponds. We are going to excavate 20 feet below the surface of finish grade of the dike so a total of 4 feet below water. All of the soils (sample was taken in the middle of the dike) are silty sand with 44% fines (passing #200) which obviously it is going to have to take a long time to get the water out. I guess my question still is how do you develop an equation with out knowing the impermeable layer's depth as all references require it for either equation or flownet analysis. I am assuming I would consider this a circular source as it has water all around the pumpstation.

RE: Dewatering Drawdown

Don't worry about the "impermeable" layer depth. Assume it's 4-ft of drawdown in a fully penetrating well. Also, assume it's 4 ft of drawdown, but there's an impermeable layer at 8 ft (i.e., half-penetrating). Heck, try it a third time for a layer at 16-ft. It's only 4 ft of drawdown.

You can make a simple flow net if you want to check for bottom heave or "boiling." I mean like if you plan on driving sheet piling for ground-water cut-off.

You'll need to use fig 4-23 to get the radius of influence.


¡papá gordo ain’t no madre flaca!

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