Groundwater monitoring
Groundwater monitoring
(OP)
We have a 20m deep concrete cut-off wall. We will excavate to one side of the wall and pump out water to form a base slab.
On the other side of the wall there is a property line about 25m away. We want to carry out groundwater monitoring to ensure our pumpng has not significantly lowered the groundwater level.
Do we sink water well casings? What diameter? The groundwater level is now at about -2.5m, how deep should our casing go. How to measure, by "dipping" or instrumentation.
Does anyone know of a monitoring procedure available on the web?
Is there any way also to compare the effect of rainwater as I assume the groundwater level is seasonal.
On the other side of the wall there is a property line about 25m away. We want to carry out groundwater monitoring to ensure our pumpng has not significantly lowered the groundwater level.
Do we sink water well casings? What diameter? The groundwater level is now at about -2.5m, how deep should our casing go. How to measure, by "dipping" or instrumentation.
Does anyone know of a monitoring procedure available on the web?
Is there any way also to compare the effect of rainwater as I assume the groundwater level is seasonal.





RE: Groundwater monitoring
This may be a good job for vibrating wire piezometers. . . .
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
RE: Groundwater monitoring
But, we have a cut-off wall and do not expect, or want, to experience large flows under it. There is a risk that the cut-off wall is not sufficiently deep into stiff clay (but we have good SI boreholes)and due to this risk I want to measure the groundwater level.
What I want is ground water observation wells spaced at about 150m so that I can see if we have lowered the ground water level near the buildings.
What data will I miss if I proceed as planned?
RE: Groundwater monitoring
f-d
¡papá gordo ain’t no madre flaca!
RE: Groundwater monitoring
paying consultants is not a problem for me. I find the rates in Thailand acceptable. I just like to educate myself first before entering the lions den - so I thank you for your input.
But I still cannot follow the point of the piezometers, why can I not just monitor the groundwater level using a well casing. My thought was that if by dipping the well casing now I read -2.5m and by dipping each month I still read -2.5m then we have not affected the groundwater level.
If I wanted to measure the effectivness of a dewatering system and the extent of the drawdown then I can see the point.
RE: Groundwater monitoring
At the risk of repeating myself, to address your original question, you need "nested" piezometers to gauge hydraulic head with depth. That's just my opinion and how I would handle it.
f-d
¡papá gordo ain’t no madre flaca!
RE: Groundwater monitoring
I have a 66 foot deep cut-off wall. Currently the groundwater level is around -8 feet. I want to monitor and check if the groundwater level is lowered when we excavate and pump on the otherside of the wall.
I was thinking of installing a well casing 20 feet deep. Now this is where I was getting confused. As I want to measure the groundwater level I will need a screen for the full depth of the well casing, say -5ft down to -20 feet. If I understand correctly fattdad's point is that over the length of the screen there could be more than one layer of water - is this to be termed an acquifier. If this was the case the results could be misleading.
I'm not so sure that this is the effect that I'm trying to measure. But thanks for the input it has got me ready to go and discuss with a consultant (and the project insurer)
RE: Groundwater monitoring
However, don't expect a ground water hydrogeologist to know what you are talking about. At a firm where I used to work, a graduate from a well respected hydrogeology university department was hired. When I asked him to use a flow net to resolve a landfill seepage problem, I got a big blank experession.
You may also find a geotechnical engineer won't know what a flow net is either. So do some checking first.
RE: Groundwater monitoring
RE: Groundwater monitoring
the project isn't in Bangkok.
I certainly hope we don't get much of a drop in groundwater level on the otherside of the wall as the design is meant to enable us to form a base slab in reasonably dry conditions. We are not vacuum dewatering just excavating and pumping.
But as you state a condition survey is required, I intend to add to this with groundwater level monitoring and vibration monitoring.
I have to admit I'm still a bit baffled with the whole piezometer, flow net theory. I originally just thought it was simple that if the groundwater level drops then we would increase our vigilance for cracks or subsidence. Indeed I thought the main problem was to separate the effect of our pumping from seasonal changes in the groundwater level.