Determine the depth of GWT level

[chaechaeng]

Hi, new engineer here currently assigned to design a shallow foundation for a residential project. One critical factor in this design is the groundwater table level. To determine the GWT level, is it correct to take the average of the water levels recorded?

As attached image below, the sum of the water levels divided by 12 is equal to the value adopted water level for foundation design. Is this approach correct?

Edited: Additional information on the SI works based on

1) Drilling Method: The drilling was carried out using a YWE D90-R rotary boring method.

2) Soil Conditions: The soil consists mainly of sandy silt, with a depth of up to 10.50 meters, underlain by sandstone.

3) Number of Boreholes: A total of 5 boreholes were drilled, with each borehole recording the groundwater table (GWT) level.

4) Site Context: The site is generally flat and underlain by locally prominent sedimentary rocks, including shale, shalestone, slate, phyllite, and some local development of limestone.

5) Load Case Considered / Use of GW Level: The load case being considered includes dead load and live load from residential columns with loads not exceeding 500 kN. The GWT level is being used to determine the appropriate formula for designing the shallow foundation.

 

[geotechguy1]

What was the drilling method. What are the soil conditions. How many other boreholes / wells do you have. What is the context of the site (geology / geomorphology / history / location). What load case are you considering / what are you doing with the GW level.

There is alot of missing information.

 

[chaechaeng]

@geotechguy1 I've updated my post to include the missing information

 

[EireChch]

Since it was at 1.2m in August, I would conservatively take it at 0m depth.

 

[chaechaeng]

So it's better to assume that the foundation is fully submerged in the water?

 

[EireChch]

Yes, in Eurocode 7 it specifies that you have to take the worse credible groundwater table. Considering you have groundwater strikes from boreholes (which can be unreliable) and not long term groundwater monitoring, then i think a groundwater of 0m depth is appropriate.

 

[geotechguy1]

I would probably also take 0m for the details described. You have to think about seasonal fluctuations and long term fluctuations. Is August summer where you are?

 

[SlideRuleEra]

Agree with geotechguy1, fluctuations in ground water are of most concern. Foundations in always-wet conditions are "easy". Foundations in always-dry conditions are also "easy". Be careful (conservative) with foundations when there is fluctuation.

Also, agree with ErieChch, assuming 0 meters depth as worse case is realistic... which guarantees there will be fluctuations.

 

[chaechaeng]

Thank you everyone for the answers and explanations! @geotechguy1, just to clarify, I'm not from a country with four seasons, thus, we have to deal with the fluctuation of groundwater throughout the year.