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Rising water table

Rising water table

Hello and sorry for the repost
I am currently working on a survey of a bay . during the excavation task in order to construct a drainage network , water level from the sea has rose up in one trench as shown in these pictures .
Now i know common solutions requires the use of poorly graded gravel and or special cement. I would be happy if you direct me into case studies or books that relates to this problem.

RE: Rising water table

It would seem that any plan for trenching in area would expect to see the water table controlled by the nearby open water. The original plan appears to be flawed. Correcting or building the system now may mean evaluating a number of different possibilities, such as a well point system.

Do a Google search for "subsurface drainage". Here is one that came up.

Also try "stabilizing trench base".

RE: Rising water table

Again thanks Oldestguy
the project consist of the construction of an area for containers ( wharf or dock if you will) . We start by doing excavations up to 60 cm then proceed by lying down a geotextil for filtration
a sand layer of 6 cm is put above the geotextil , followed by a layer of compacted aggregates about 29 cm . In the end , we construct RC slabs .
The project also mention the construction of an underground drainage system , and this is where we see the water rising . Modifying the whole project cannot be done , but solving the issue of the water is what i am looking for , i will be viewing your suggestion

PS: i should also add that the soil profile consists of a sandy silt with organic presences here and then

RE: Rising water table

I hope others chime in here. Here are my thoughts. If your underground drainage system was to be in trenches, then directing collected water to a pumping facility, then your use of fabric and sand sounds like it will work, but I'd revise the drainage trench detail to simplify it and save costs. Laying those materials in water will not be easy. Pumping from open trenches can loosen the soil there and result in settlements later when loads are placed on the trench areas. I'd look at a procedure for materials and installing the sub-drains as follows:

Immediately in back of the excavator lower in a drainage pipe consisting of flexible corrugated plastic, with small slots, perhaps 2 or 3 mm and wrappped with a filter fabric sock. As soon as possible then backfill over that pipe, sufficient thickness to "bury" the pipe. The backfill over and around the pipe should be a graded sand , such as ASTM C-33 fine aggregate for concrete. DO NOT USE THE COARSE AGGREGATE FOR CONCRETE, SINCE IT IS NOT A FILTER. This filter zone could be in the range of 15 cm or more depth. Later use what ever soil came from the trench to get up to finish grade.

If you have to work in an already dug trench, lowering the water level a little with pumping can be done, but not so extensive as to remove all water and cause loosening of the soil below the trench. I'd try to work without any pumping.

That ASTM-C33 concrete coarse fine aggregate sand is a perfect filter for all soils. You do not need any filter fabric in the trench first. Even a slotted pipe without a filter sock will work, but a little sand gets in first and then bridges over the slots. Since the pipe is not fully surrounded with soil, the dumping on top of it will likely surround it sufficiently. A fussy way would be to place the sand in a thin layer first and then the pipe, but that takes time and may not be practical.

Size the pipe to carry the estimated flow. In the States, I'd use 4 inch for lengths of up to 100 meters, and 6" for all others. Here these flexible pipes come in rolls for ease of handling. I'd not get too fussy about meeting a sloping grade under the circumstances you have. So some low areas result, so what. It will still work.

RE: Rising water table

I forgot to mention what does the drainage system consist of : its this
i don't know its name in English but in French its called Avaloire . So this element is a RC structure , each unit connects with the next one via PVC conduits .
This is my first experience with underground drainage , but i know the problems that may associates with this are :

1- these structures were never designed correctly , even for this kind of important projects( Docks) they are given standard dimensions and whats more standard steel reinforcement. Therefore , and especially with the existence of heavy containers , bearing capacity is a big issue .
2-Buoyancy since they will be constructed under water levels , note that no knowledge of the the most critical ground water level is known !
3-Possible infiltration of sea water inside the drainage system which can render the latter useless in accordance to their initial function, and may even cause flooding and collapsing
( we did similar projects for the same client and the collapsing of underground drainage network is very common )

RE: Rising water table

OK, it is surface drainage only. It kinda helps to explain everything at the beginning, right. My first reaction is that galvanized pipe may be fine in salty water, but any break in that protective layer, as with damage to it, holes or cut ends will start the degrading of the steel inside. I have seen such pipes years later with only the galvanizing but the steel gone. I'd use a different pipe material not affected by salt. I'd also use a pipe that can withstand considerable distortion, even if it can be installed properly.

Off hand other than being able to just place the pipe and backfill with no special stabilizing of trench, I'd look to dewatering the soil ahead of trenching with a bank of well points located between the work area and the open water. The aim of that would be to cut off a flow of ground water from the most likely nearby source. It may take several such "banks of wells".

RE: Rising water table

thanks again man
but you didn't answer my concerns regarding the RC structures . Anyway, i am considering using a precast RC boxes with damp-proof products . the base of the box must be designed to withstand buoyancy all the while providing adequate bearing capacity, maybe i should add pumping wells but the prob is that there is no way for the company doing the project, to be financed for tasks that does not exist in the contract you see....

RE: Rising water table

Your position is what? Engineer with the designers? Engineer with Owner? Working for the contractor? Sometimes things like this have to be handled with a "change order" to the added contract and payment set accordingly. If the designers didn't allow for tides or other water level considerations or if the contractor didn't check on that before bidding, there may be legal issues and costs settled that way.

RC structures also can be affected by salt water. I'd have to defer to those living and working in sea coast areas for advice on that. I'm way inland.

RE: Rising water table

i am representing the geotechnical laboratory that actually did the study and design of the dock , our client is represented by the port managers and there is the construction company . I am doing the control and survey of the project so to speak . check this excel paper that deals with issues mentioned above

RE: Rising water table

A brief look at the spread sheet calculation properties and your finding water higher than expected (apparently) tells me your depth to water table in your calculations is likely wrong. I'd play it safe and use depth to water table of zero. As is, I'd expect the results of possible well point usage, etc. will be charged to the designers. Water tables can vary all over the map and floatation calculations should assume the worst likely.

RE: Rising water table

yes OG, for the record that spread sheet is not mine i just found it
thanks a lot

RE: Rising water table

OG once more.

Usually those inspecting or providing site measurements, etc. do not have authority to direct any of the work, or they are responsible for the results. They also can't direct any special work, such as dewatering. In this case, it probably is up to the contractor to complain to the owner that he cannot perform due to "changed conditions". It may be that a revised design or other change can be forthcoming, but generally not without an agreement of the cost thereof. Where the contractor is quite experienced, he may suggest a site treatment, such as well points, that had not been anticipated (due to some plan information). He may even go ahead and do the work, but may then claim extra cost due to that unanticipated work.

However, on this post, it would not appear that you have any authority to direct such work, without an agreement as to who pays. I'd report the conditions to your superiors, possibly with some comments on how to handle the change of conditions.

Document all conversations and keep a detailed diary. It may be needed in court some day.

RE: Rising water table

that's crystal clear Sir

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