Help needed with assignment!

[confusionUK]

For my dissertation I am designing a dam.

I have been given the map and 3 borehole results. I have drawn 3 cross sections.

On of the boreholes has been taken on the slope of the valley. There is about 12 metres of unidentified material above and to the left.

Please could you suggest what might be there? Here is the borehole data.

Height 73.3m

0.3m Topsoil
0.3-5.1 Brown Fissured Silty Clay
5.1 - 13m Dark Grey Silty Clay
13 - 18.8m Yellow Brown Silty Fine Sand
18.8 - 25m Dark Grey Silty Clay

I haven't explained this very well! - I can send you the map and cross sections (.dwg)

Many thanks!

 

[Rjeffery]

what are the "N" values?

The best way to test something is to squeeze it, slowly, until it breaks!

 

[confusionUK]

The only N value I have is for the Yellow Brown Fine Sand and this is 35 blows/300mm.

Just as a broad guess should I just assume that there will be a layer of sandy-gravel and on top a layer of gravel?

I bet this is much simpler than what a lot of you will be used to!

 

[oldestguy]

What is the geology of the area?

Could these materials be the end product of bedrock weathering? If so, is there a possibility of rock solution channels that would pipe water under or around your dam?

 

[dgillette]

This is one of the realities of dam engineering: you often don't know all the geologic detail until you excavate the cutoff trench. Hope for the best, but design your foundation surface prep, grout curtain, etc. so they can be adapted to fit the worst.

 

[cvg]

without a geologic map and a field visit to the site, how could you even guess what it is? From my experience with dam rehab, I have rarely seen a project where there was just one geotechnical investigation and report. In fact, I am working on one dam right now where we have 3 separate geotechnical firms working on the same project! And they are currently working on the second report. No design has been done yet. Another project we will be sending out the drilling crew for the third time to supplement the design with the third addendum to the geotech report. Again, this project has two geotechs. You need more data and probably cannot fully design the dam without it.

 

[muuddfun]

what size dam are you designing? Its very hard to know what you have to look for from what info you have given. I agree with CVG about needing many many borings etc.

Post a Google placemark so everyone can find the site you are looking at and you may get some better understanding of the problem out of the folks on this board.

 

[fattdad]

Step 1: Determine the availability of construciton materials. For this site with seemingly a thick soil profile it'll be an embankment dam.

Step 2: Will you design a zoned dam or a homogeneous dam? Looks like you have 30 ft of "clay" to work with - so a homogeneous dam may be just fine.

Step 3: Consider sideslope angle. This is a problem for you as you have no shear strength data. Typically you'd have to look at the short-term and the long term strength conditions under varying pore-pressures (i.e., rapid drawdown, etc.).

Step 4: Consider settlement of the foundation soil and the embankment as it's being place.

There is some earlier discussion of cut-off trenching. Not sure that you'll need one as if the subgrade is clay you will not likely have a pervious subgrade to cutoff. Then again. . . .

Most dams have pipe penetraitons for the low-level outlet. Use a concrete cradle for the upper 2/3rds of the pipe run and a gravel pipe drain for the lower 1/3rd. DO NOT USE SEEPAGE COLLARS!

There may be a need for a toe drain also. Not enough data to help on that. Somewhere in this you'll have to do a flow net or otherwise look at the point where the seepage emerges from the embankment. The confocal parabola can give you an initial idea on this. . .

Good luck.

f-d



¡papá gordo ain’t no madre flaca!

 

[Mickney]

I understand your thread to be an assignment for a class. From a consulting standpoint, you are working on VERY limited information. Ask for more information because "in the real world" (which your coursework will be preparing you for), you will need and should request additional information to design the embankment dam.

From a review of the cursory information, be careful of the sand seam you reference from 13 to 18.8 m. This is zone of higher permeability than the other materials referenced in the borehole data.

 

[cvg]

Just as a broad guess should I just assume that there will be a layer of sandy-gravel and on top a layer of gravel?

if you want to assume some soil layers, than you need to determine - where did the soil come from, how was it deposited and how long has it been there? You need to justify how the material was deposited. Where did your proposed layers of sand and gravel come from and how did they get there? Especially since there is none shown in your boring. If you were at the bottom of the valley, you could justify it possibly due to alluvial action, however on the slopes - would it be at a base of a bedrock outcropping? Could it be weathered material falling down the slope? Why didn't this material show up in you other boring?