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Rock Pit Design

Rock Pit Design

Rock Pit Design

Rock pits are sometimes used locally to drain the storm water into the soil in the vicinity of a building.  

Permability of ground is often measured by digging a pit, saturate it with water and observe rate of water penetration into the ground.

In the absence os a standpipe piezometer, how can the permeability of ground be measured with a reasonable accuracy by a test pit. Thank you.  


RE: Rock Pit Design

to dig a pit, saturate it for a spell and then return to take time readings with respect to water level is akin to a "perc test."  Perc tests are widely used to design drainfields for home septic systems.

Drainfields are shallow trenches that are narrow and filled with gravel. Tyipcally, trench separations are 3 times the trench width. The flow of water is gradual in time, i.e., it's rare that all trenches are filled up completly and time is allowed for the infiltration to occur.

A double-ring infiltrometer is a field device used to more precisely measure saturated vertical hydraulic conductivity. Water in the outer ring is used to establish a boundary condition so the drop in the inner ring is specifically related to the vertical component of flow.  (Contrast that to an infiltration or perc test where the water is free to go both vertically and horizontally).  A double ring infiltrometer is a great way to measure vertical hydraulic conductivity.  Problem is you have to be working on the ground surface. . .

If you are doing a pre-construction engineering assessment in a proposed area for stormwater infiltration, you may anticipate in advance that the bottom of the pit needs to be at 4 or 5 ft.  You may drill a hole and find that the soil at that depth is sandy or otherwise favorable for infiltration.  Heck you may find out that the hydraulic conductivity is some high value and that would seem to be all that's needed.  Maybe not though. . .

Let's say at the depth of 4 ft you have a sand, but at the depth of 7 ft you have a clay.  Let's also say that you want to make an infiltration pit that's 20 ft by 20 ft.  Will you have the entire 400 sf surface area available for infiltration at that favorable "sand rate?"  O.K. let's think about that.

If you have 3 ft of headroom between the bottom of the pond (i.e., 4 ft) and the clay layer, just how many feet of infiltraiton will take place prior to saturating the entire 3-ft interval?  Well with a porosity value of 0.33 a one-foot water column would saturate the entire 3 ft sand layer. Then what happens?  The water flow is governed by horizontal hydraulic conductivity and it's the horizontal flow through this 3-ft thick sand layer, which represents 240 sf (i.e., about 50 percent of the area in the base of the pond).

If you don't consider the overall flow regime of the problem, you'll run the risk of designing something that won't work.

Just some thoughts on this matter.


¡papá gordo ain't no madre flaca!

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