Rate of water rise in drilled shaft versus adjacent boring

[leonardp]

Question:

If a 6" diameter soil boring 135' deep that is generally dry during excavation down to rock has water rise up to 35' below grade in 8 hours, how fast would the water rise up if a 4' diameter drilled shaft is installed at the same location? Would the rate of rise be the less, the same or greater?

The site conditions are stratified clays and sands with 40' of shale at base of shaft. For a 1' layer of permeable sand within this overburden stratigraphy, the exposed wall area to liberate groundwater is 1.58 sq ft. The wall area in the 4' diameter shaft is 8 times the area of the 6" boring (12.56 sq ft).

Please furnish an authoritative reference such as an engineering journal citation or textbook to validate their answer.

Thank you,
Len

 

[geotechguy1]

Well, the seepage rate is related to the properties of the soil, the amount of water seeping into the shaft is related to the size of the surface area and the properties of the soil, and the length of time it takes your shaft to fill up depends on the volume of the shaft. Assuming that the soil properties are the same, the only variables are the surface area and the volume. The volume is increasing as (radius * height) and the surface area is increases as (radius + height), so I expect that increasing the diameter will result in increasing the length of time for the shaft to fill up since the volume of the shaft is increasing faster than the volume of water that can enter the shaft.

Source: formulas for volume and surface area of a cylinder.

 

[leonardp]

Good Point on the volume vs radius. I will look at gpm to compare rate of uplift. OK Thank you

 

[SlideRuleEra]

...soil boring 135' deep that is generally dry during excavation down to rock has water rise up to 35' below grade in 8 hours...

Did the water level rise at a constant rate (12.5 ft/hr) over an elapsed time of 8 hours?

or​

Eight hours after the "start" water was 35 ft.below grade?
(Both rate of rise and time to reach 35 ft. below grade unknown. Also unknown, did the water rise at a constant rate or did it vary - likely slow down)

It makes a difference.

http://www.SlideRuleEra.net

 

[leonardp]

The only information provided was that during the boring activity there was a minor seep at 25' below grade. They drilled 40' of rock but did not provide any data regarding groundwater flow from the rock. The boring log indicates they finished boring the same day. So I assume time of 8 hours. Issue is whether shaft would be done dry or wet. Wet is required if rate of water rise is greater than 0.25 inch per minute.

Question: Given the flow observed from the rock that created the rise in the 6" boring, would the water rise in 4' shaft exceed the 0.25 inch per minute criteria?

 

[SlideRuleEra]

Given the flow observed from the rock that created the rise in the 6" boring, would the water rise in 4' shaft exceed the 0.25 inch per minute criteria?

Do the math:

For the 6" boring:

Total inflow volume known: 6" dia. x 100'

Elapsed time assumed: 8 hr.

Calculate Inflow rate in ft[sup]3[/sup] / minute.

For the 4' shaft:

Inflow rate of flow is assumed: 8 x (6" diameter inflow rate)

Using this inflow rate, calculate distance (inches) water will rise in the 4' shaft in one minute. This is the "answer"... for the assumed data, which is not necessarily realistic.

http://www.SlideRuleEra.net

 

[BigH]

Any information on groundwater or perched groudwater conditions? Should have had a piezometer(s) put in during the site investigation.

 

[leonardp]

Thank you for the comments. We have used Darcy's law to determine the flow for large shaft. I appreciate the informed responses.