Monitoring elastic settlements

[Okiryu]

I have a site where 2-3 meters of fill will be placed. The subgrade soils are unsaturated overconsolidated clays with the thickest layer of approx 3 m thick.

For monitoring settlements ,what could be the requirements? For example: what could be the max. "x" difference between 2 consecutive readings in order to stop the monitoring? Since I expect only elastic settlements to occur, I do not think that this settlement monitoring will take more than 2 weeks. Well, it will depend on how strict will be the amount "x".

 

[Ron]

Where is the water table? Even overconsolidated clays will settle under new loads, and 3m of fill is substantial. I would monitor for more that two weeks....more like 2 months, just to see if you have influence. If water table is higher (within the clay layer), keep monitoring until the movement stops.

 

[Okiryu]

Ron, groundwater is deep. About 15 m below ground surface so no within the clay layer. After placement of the fill, structural loads will be applied so I have to check for these settlements as well. By the way, foundations will be deep foundations so I am not worried about them. My concern is for the slab on grade structure, which will be a floating slab.

 

[Ron]

Add an extra .5m of fill for preload surcharge and then remove for slab construction after deep foundations in place

 

[Okiryu]

Ron, I guess that the extra 0.5 m is the equivalent of the live load that will be applied on the slab. My LL is about 10 kPa so your suggestion makes sense. Thanks !

 

[HENRYZAU]

What will the embankment support? Pavement or buildings? And allowable settlement? What's the expected OCR/YSR? My guess on the clay primary settlement is 30 to 300mm depending on OCR. Settlement plates appear adequate for monitoring purposes.

 

[Okiryu]

HENRYZAU, it will be a building. The foundations will be supported on piles, but the slab on grade will be a "floating" slab so my concern is about the floor slab. I consider an allowable settlement between 10-15 mm. The OCR of my clays is typically greater than 2, perhaps between 2 and 4. The modulus of the clays is about 7 MPa from unconfined compression tests.

 

[HENRYZAU]

7MPa sounds like firm clay. Agree with Ron but with at least 1m surcharge to see how much it settles and how quick/slow to have 90-95% DOC. Make sure your primary settlement under final design load is eliminated when surcharge is removed. Settlement plates seems enough, but extensometers will provide more values on where the settlement mainly comes from.

 

[Okiryu]

Yes, these clays are medium stiff/stiff, quite firm and non-expansive. Settlements will be elastic rather than from consolidation. I normally calculate elastic settlements using elastic theory (basically Hooke's law). I do not distribute the load thru depth since the loaded area is large.

HENRYZAU, BTW what does DOC stand for?

 

[HENRYZAU]

Okiryu, it's Degree of consolidation. If you are are very confident on your adopted OCR and the firm clay's yield stress will not be exceeded under final design load, it's OK. I have seen overestimation of OCR caused under prediction of settlement.

 

[Okiryu]

HENRYZAU, thanks for your reply. I don't think that we will have consolidation settlements. Therefore, my plan is just considering the elastic settlements.

 

[fattdad]

reading your OP, I have to ask, "How do you know it's unsaturated clay?"

If it's unsaturated and there connected air blebs remain, it's unlikely that permeability will influence change in void ratio. If it's near saturated (i.e., the remaining air is all in isolated blebs), then permeability will be a factor.

Do you have a well in the clay that shows no water?

Do you only have observations of a saturated sand above a clay layer and then concluded that the sand is perched on the clay (i.e., did you allow some story to prevail over reality)?

OCR of 2 is not that great!

I'd be all over the moisture contents, liquid limits, blow counts, oedometer data and I'd consider the nature of the risk.

Then again, it's just a slab on grade!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

f-d, based on the previous data and knowledge of my local clays, the saturation degree is normally less than 90% for our medium stiff clays.

I do not have a well, but based on our borings and knowledge of local geology, groundwater is at deep locations, no within the clay layer. Groundwater is within the gravelly limestone layers underneath the clay layers, about 10 meters below the interface the clay and gravel layers. There is not a soil layer above the clay layer. So, the soil profile is composed of clay soils overlying gravelly limestone soils.

Moisture contents are well below the LL. Blow counts are 7 in average. Unfortunately I do not have oedometer data for this site, but I do have oedometer data from past projects and know that our clays may have OCR between 2 and 4, perhaps more.

To minimize risks, I am thinking for settlement monitoring after fill placement and construction of slab-on-grade to begin after settlements are stabilized.