Groundwater monitoring

[Zambo]

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.

 

[fattdad]

I'd draw a flow net and look to see where the head drops occur and to what extent the change in pore pressure could affect settlement. I'd install multiple piezometers to look at the change in pore pressure with depth (again, as guided by the flow net). I would not install a ground water observation well with a 10 ft screen as the screen length will normalize the measured head over the length of the screen and filter pack. This can lead to meaningless results.

This may be a good job for vibrating wire piezometers. . . .

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[Zambo]

Well fattdad, I have confidence you have knowledge about the subject in question.

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?

 

[fattdad]

As I already stated, if you only have wells located in plan dimension, you will miss data on the change in hydraulic head with depth. Whether you expect flow under the wall or not, it is this very flow that will affect the water table beneath your neighbor's property. I'd still do what I suggested yesterday, but if you want to do something else, that's fine, there are other consultants out there, but you have to pay them - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[Zambo]

fattdad,

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.

 

[fattdad]

I don't know the ground water hydrology in your area. I understand that there may be horizontal layers that can affect a difference in the horizontal and vertical conductivity. I also recognize that there is infiltration from rain that occur from one month to the next. If you have a 25 ft well casing next to a 25 ft deep retaining wall, you will measure an average water elevation in the well casing - a normalized value from the upper soils and the lower soils. If there is a vertical gradient, you will actually have flow into and out of the well casing - it can get complicated.

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!

 

[Zambo]

Ok I think I'm nearly understanding the point (I'll use feet instead of metres as I think that was also causing a confusion).

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)

 

[oldestguy]

On the suggestion of using a flow net diagram, that is an excellent idea, since permeabilites of the various layers will affect what happens and the flow net will help explain what to look for and where to set the piezometers.

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.

 

[BigH]

zambo - if I am not mistaken, you will be wholly within the upper Bangkok clay - isn't it usually about 25 to 30 m thick before you reach the first sand stratum? - I don't remember that there are any real layerings in the stratum of coarser soils. If, indeed, you are wholly in the clay, there will be a long time lab before any pumping inside your excavation will manifest itself beyond the outside wall. I would even be surprised if you get much out of the clay within your excavation (or below it) unless you are using vacuum dewatering. In any event, prudence, I believe calls for outside monitoring (i.e., outside your cutoff wall). Fattdad and the others are indicating that a single well will lead to an "average" drop - which may or may not be reflective of the "sublayer" drops that might occur (if there is any layering). Yes, you could do the single well, but I think that by installing a few "nests" of piezometers (not casagrande or standpipe type as you want fast response time) where you are measuring "points" at various levels - it is a good idea. Too, you should do a condition survey of the adjacent structures - even if they are 25 m away. You don't want any "existing" crack to become a "new" crack.

 

[Zambo]

BigH,

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.