Hydraulic Conductivity

[boost33]

Hi there,

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 our water level rises. 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?

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.

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

 

[aeoliantexan]

Hydraulic conductivity k is a property of the soil. If you have constant flow through a prism of soil with cross-sectional area A, length L and hydraulic conductivity k, Darcy's law says Q=kiA, where Q is the flow per unit time and i=(H1-H2)/L; H1 and H2 being the water head in feet or meters at the input and output ends of the prism.

Knowing k, the geometry of the layout, and the geology of the site (such as the thickness of the sand layer) you can calculate the flow to a line of wellpoints, which is what you probably will need to dewater the soil for your trench. You can find formulas in US Army Corps of Engineers engineering manuals available on line, or puplications by Mansur and others. Judgement and experience are needed, so I suggest you call a dewatering company such as Griffin Dewatering in the U.S.

 

[fattdad]

What aeoliantexan said and a little more. Knowing hydraulic conductivity for a given stratum is the basis to determine well yield, dewatering requirements, contaminant transport and many other factors of ground water flow. In addition to this however you have to understand "boundary conditions" of which there can be many. Some include adjacent zones of water supply, thickness of stratum, recharge to the stratum, length of opening (for the case of a dewatering trench), the extent to which you want to lower the water table, etc.

Here's a NAVFAC link to "Dewatering and Groundwater Control":

http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_5.pdf

Here are a few other links from my web page:

http://home.comcast.net/~fatt-dad/hydrology.html

Hope this helps.

f-d



¡papá gordo ain’t no madre flaca!

 

[boost33]

Thanks for the help. That pdf is pretty good also.

I think you're right about the experience. Our prelim tests tell us that we ave a potential problem.

We are excavating a pit (about 2x2 or 2m x 1m) to better assess the situation. We might get a dewatering company involved with that also.

Cheers