Shoring for excavation to rock 1

[jonastp]

Hello,

I'm designing a cofferdam in sheet pile that will be used to buid a pier directly on rock. Prior to the shoring, the pier willbe backfilled with coarse material (0 - 6''), then the sheet piles will be placed. I

The cofferdam is 5 meter deep, and since it sits directly on rock, the shoring does not have any embedment (more like a trenchbox). My original system had 2 bracings, one at the top, and one at the bottom that will be made right after the top bracing and then push down as the excavation goes. The bottom bracing is extremely large because of the absence of embedment, but there will be a minimum of 300 mm (about 1 feet) of concrete poured at the bottom (on rock) to form a plug.

My question is: Can I consider the concrete plug as a bracing to reduce, or even eliminate my bottom bracing? I can ask for more than 300 mm if needed.

Thank you very much for your help, I design shoring everyday, but it is the fisrt time for one without embedment!!

Jonathan

 

[PEinc]

A 16.4' (5 meter) water head can cause significant uplift on the 1' thick concrete plug if the rock is fractured enought to allow water flow. Make sure your "plug" can function as a plug to resist uplift.

http://www.PeirceEngineering.com

 

[jonastp]

Hello PEinc,

Thanks for the heads up, but that part (efficiency of the plug) was allready take cared of by the geotechnical/structural team and was part of the design of the bridge. My pat is only the temporary shoring needed for the construction of the pier!

My calculation so far, if I eliminate the bottom bracing, gives me about 125 kN/m reaction on the concrete (8.6 kips/foot).

 

[PEinc]

I have seen many instances where the project engineer called for a cofferdam seal of only 2 to 5 feet of concrete. When you check the uplift pressure, the "seal" is nowhere heavy enough to resist the uplift. Just because someone shows 2 feet, it still needs to be checked.

Assuming that the bottom of the excavation is permeable or fractured enough that water can flow into the bottom of the excavation and if your dewatered cofferdam is 16.4' deep, that could equal 1023 psf of uplift pressure. This would require 13.2 feet of buoyant, lightly reinforced or unreinforced, concrete seal at about 77.6 pcf (= 140 pcf - 62.4 pcf) PLUS a safety factor of about 1.5. 13.2' x 1.5 = 19.8' of concrete. A 2' thick seal isn't much more than a work mat. Think about it.

http://www.PeirceEngineering.com

 

[cvg]

so you are installing the sheets, then excavating down. you go down about 500 and install the top brace. you continue excavating down to about 4500 and install the bottom brace. then you plan to go to the bottom and pour concrete.

the concrete will not be able to supplement your bottom brace until you already make the full excavation to the bottom, pour and let it cure. So your bottom brace will need to take the full load won't it?

the sheet pile will leak like a sieve so this entire excavation will either need to be dewatered or done in the water. you could put in temporary bracing, pour the concrete and then remove the temporary. however, without steel reinforcement in the concrete, I would not recommend this take too much compressive load.

Without rock anchors, the 300 thick concrete topping will probably not be able to withstand the water pressure seeping under the sheets and through the rock.

 

[cvg]

check out this thread, this might help to understand the tremie slab better

thread507-302343

 

[wmta]

a tremie slab of less than about 1 meter would be the min thickness of the slab is placed underwater by a very experience crew but a thickness of 1.5 meter may be much better. Once the cofferdam is dewatered make sure you check uplift.

In addition, you need to make sure the cofferdam is stable at the stage before the tremie is placed. You will most likely need two levels of bracing.

William Tucker

http://www.wmta.com

 

[jonastp]

Thanks a lot for all those precious input.

I raised a flag for their concrete ''plug'' thickness, and now they are unsure of it. If the project goes through with the thickness I will ask, then I will use the concrete as either a bracing or a ''soil'' with a big kp as passive resistance.

I will need a temporary bracing to the bottom until the tremie concrete is poured and cured, but since the water inside the cofferdam will be pumped only after the plug is cured, that bracing will be much more ''reasonable'' in steel weight.

If they want to stick to 300 mm of tremie concrete, then they will need somebody else to design the cofferdam thanks to your comments.

Best regards,

Jonathan

 

[PEinc]

With rock being at subgrade, you can't easily increase the thickness of a concrete seal. It seems to me that you probably do not need a concrete seal. With rock at subgrade, the bottom of the excavation should not boil, heave, or become quick.

Consider doing the following steps:
1. Drive sheeting to refusal on rock.
2. Oversize the cofferdam enough that you can have room to install sand bags if needed to control water seepage from under the sheet piling if bearing on an uneven rock surface. The more uneven the rock surface, the more trouble you will have sealing off seepage from under the sheeting.
3. Dewater enough to set two tiers of bracing in the cofferdam, both tiers being located near the top of the sheeting.
4. While excavating and dewatering, lower the bottom tier braced wale. The final location of the bottom braced wale should be just above the proposed footing.
5. Continue dewatering to maintain the water level inside the cofferdan at or below footing subgrade.
6. Construct the footing on bedrock. You may want to spread some crushed stone to level off the excavated rock subgrade without sealing off water infiltration. Pour or backfill the footing against the sheet piling except where the dewatering sump pit is located. If you pour concrete against the sheeting, you should place a plywood bond breaker to help with later extraction of the sheeting.
7. Remove the bottom braced wale if it interferes with the proposed pier stem or if braces are not allowed to penetrate through the stem.
8. Construct the pier stem and backfill as required.
9. Flood the cofferdam to the upper braced wale level and remove the upper braced wale.
10. Remove the sheet piling.

http://www.PeirceEngineering.com

 

[PEinc]

I just looked at your sketch again and noticed what looks to be unbalanced causeway fill against the cofferdam. Be sure to check the cofferdam for overturning or place sufficient coarse fill around the outside of the cofferdam to even off the unbalanced lateral earth pressure.

http://www.PeirceEngineering.com