Trenching for pipelines - Settlement Calculation 1

[ONENGINEER]

Could someone kindly let me know one of the best practical codes/manuals to calculate the settlement due to well point de-watering in soft silty sandy soils.

Trenching is for 20 m piles to a depth of approximately 4 m at relatively silt-sand soils.
Trench width is 2 m at the bottom. The tench sides are supported by steel sheeting and a cage.
They are using well points to 6 m at one side of the trench to de-water the area and avoid sand boil at the bottom of trench.
As the engineer, I want to calculate the settlement due to groundwater lowering by well point.

I would appreciate guidance on the above.

 

[ONENGINEER]

Trenching is for 20 m pipes not piles. Sorry for typo.

 

[oldestguy]

I don't have a code, manual or other quick way to resolve this. I'd probably work up a calculation using data from on site and the classical geotech methods for computing settlement. However, doing this search on Google, the very first one to come up, has a good example of how they calculated it.

dewatering settlement calculation

 

[oldestguy]

One more point. If regularly one is in need of determining this settlement, I was thinking a spreadsheet I worked up for footing settlements which computed the incremental contributing settlement at each foot of depth. I also wrote a version that had an input of height of fill on site. That spreadsheet could easily be modified to do the calculation for this subject. It used the B.K. Hough term "bearing capacity index" that he then correlated to split spoon blow count.

That chart was uploaded to a former topic. Today a search for "Hough" and some other key words came up with it. I'll attach a link with some on my former input comments. Those charts-then will show up if one opens those links shown. Using that method the spread sheet creations are significantly simplified.

http://www.eng-tips.com/viewthread.cfm?qid=395862

It's not a code or manual, but once created, it would serve for most of these sites.

 

[ONENGINEER]

Oldest guy, many thanks for the feedback. I could find the BC Index. The soil is silt so I can have the opportunity to assume once a cohesive and once a cohesion less soil. Would be great if I could also see your spreadsheet on the above.

 

[oldestguy]

ONEENG.:

Herer is one version. The reason for the "fixed" in the title apparently was some error in setting up one section. the broad column B was only for being to put the text there. I'd proof it by following the vatious steps. Note the place for outputting some basic data is way down the sheet. I have not used this (of similar sheets) in a few years now. The units are English but I think you can change over to metric. Maybe input in English and change your answer to metric.

 

[oldestguy]

I have a goof in the above text. the word is "inputting" not"outputting". Can't seem to edit the post.

 

[ONENGINEER]

oldestguy, thanks for sharing the xls sheet, which was very helpful. Trying to understand the reason for values of 1 in E16 to E17 and familiarize myself with the vlookup, which I have not used before.

 

[oldestguy]

E16 and E17 are just a number in there instead of zero (I think using zero might goof things up) since footing is at 4 feet and we don't want any contribution above that depth from footing., but note the rest of the column has a pressure increase from the footing. You may have to change that footing load at H2 to 1, or .001, not zero so as to take the footing load out of the computation picture. I'd insert two new columns between D and E to compute pressures from the saturated state. Then I'd take the difference between saturated and non-saturated as the surcharge pressure for that depth increment. That sum would override any input there from C97 and C98 where the surcharge load is computed. I'd make either zero or very small.

 

[ONENGINEER]

Dear Oldestguy, the spreadsheet was simplified for no footing and no surcharges attached and I wonder if this is what you meant.
The depression cone in the example excel file is arbitrary. Can the slope of the cone sides be estimated - without field pumping test? Thank you again for your valuable feedback.

 

[oldestguy]

First crack at it you need to use submerged unit weights below the water table. I have to go out right now but also have some question about totaling the answers.

 

[oldestguy]

My comps with a saturated, but moist unit weight for sand with specific gravity of 2.65 is 112, then the submerged unit weight is about 50. I am assuming that we are calculating only at the sides of the trench where we know the water table is lowered. Off to the sides there will be less, but who knows. If one places an observation well next to a structure that may be affected, do the calculations there with a lesser lowering of the water table..

In this case, your comps assume that settlement from soil below 22 ft. is not expected (Rock, dense soil, etc.). In a soil of infinte depth, we usually stop the comps when added pressure is 1`0% of existing. For submerged unit weight this goes a long ways down.

Observations of settlement from a loaded embankment in California showed the Hough supplied chart method probably did not allow for submergence. They observed twice the measured settlements compared to non saturated soil. So, I'd simply use half the value of C for saturated soil below the dewatered depth.

 

[ONENGINEER]

Thanks for your comment on the depth. I extended the results up to 30 m where there soil information was available. If you are kind again to look at this sample spreadsheet and could comments, that would be greatly appreciated.

 

[oldestguy]

I don't quite follow the bearing cap'y numbers, but I went through this and get about the same answer as you, so it should be OK.

 

[oldestguy]

For what it is worth, here is my sheet, rather messy but only to 45 ft.

I didn't use all the old parts.

 

[ONENGINEER]

For bearing cap'y numbers I used CPT data, and determined the N values based on Robertson correlation. Then used the Hough's chart. Many many thanks again for your support. I extended to 120 ft based on another CPT data and have a total of 5.7 inches. The consultant had predicted 8 to 20 inches settlement of if the area was loaded by 1.5 m granular fill. De watering 5.5 m may be equal to 2.2 m granular fill and wonder if the difference in the settlement is due to long-term effects. Thanks again.

 

[ONENGINEER]

Oldest guy
I think the bearing coefficient is the mail drive in settlement calculation. Yet its estimation is very rough and I did not quite understood how you obtained Houg these values in your spreadsheet.

 

[oldestguy]

That's an old sheet, but at times I had the test data to come up with void ratio and the compression index. (e and Cc)

 

[ONENGINEER]

Dewatering during construction is only for a short period of less than a week. Is the Hough's approach for long-term settlement or short term? Thanks.

 

[oldestguy]

Usually we assume settlement is long term, but it all depends on permeability. Most of the time for sands it is right away. The way to get a better handle on this would be to install ground water monitoring wells at the area of concern before you start to pump and read it periodically. Once it stabilizes, you would calculate for that depth of dewatering there. These wells can be a simple as an auger hole into which you set a piece of downspout pipe, maybe with some holes drilled near the bottom.