Water inflow to Test Well - to calculate Hydraulic Conduct

[boost33]

Hi there,

We are performing a pipeline install and the groundwater is quite high. We want to know the hydraulic conductivity so we know how much water to expect in the trench. I.e. this will help to determine how much dewatering is necessary.

I am trying to measure the rate of inflow into a test well. Diameter is approximately 45-50mm.

The well has groundwater down at about 0.8-1m and you hit the bottom of the hole at 2.2m.

What I plan on doing is bailing out the water (using a pump), then measuring the time it takes to come back.

I have a formula here from Fetter:

K = r^2xln(Le/R)
---------------

2xLexT37

In the above:

K = hydraulic conductivity (m/day)
r = radius of well casing (m)
Le = Length of well screen (including gravel pack) (m)
R = radius of well screen (m)
T37 = time taken for water level to recover to 37% of initial charge.

N.B. x = multiply in the above

From this you get a hydraulic conductivity (m/day or cm/day). What is the purpose of this formula? e..g say the 1m water returns after 10 minutes (600sec). You know how quickly the water level rises from the test well (Is this artificailly high because it is a small diameter?). Is this formula to correct for a larger area. e.g. for a large open trench. Does that hydraulic conductivity tell me how much I should expect the water level in the trench to rise per day?


A few trial digs (in an area that has been filled with sand and light fill) show the trench collapsing and the groundwater rising back up really quickly (within 30 minutes it's back up to 800mm below the ground).

Will add some more info later

Thanks a lot

 

[fattdad]

Hydraulic conductivity describes the rate that water moves through soil (or rock) under a gradient of 1. You may not know what gradient means, however. . . .

When you bail out an open hole (or a well), there is a lowering effect in the well that is typically so rapid that the water pressure (head) in the adjacent soil is only slightly affected. This rate of pressure change over distance is a ratio, which is termed hydraulic gradient.

When you setup a dewatering program, you may have multiple wells or a gravel-filled drainage trench that produces a diffferent hydraulic gradient. There are multpile "boundary conditions" that factor into how you calculate the discharge requirements for the dewatering design. These boundary conditions are influenced by trench depth, aquifer thickness, adjacent lakes and other site-specific factors. Knowing the boundary conditions and the anticipated approach to dewatering in addition to the hydraulic conductivity allows you to design the dewatering system (and anticipated the discharge requirements).

There is nothing too complicated about this, but you really do need the hydraulic conductivity and boundary conditions before you can do anything.

Good luck. You may want to hire a geotechnical engineer for this . . . .

f-d

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