Back-filled culvert in water reservoir.

[cdous]

I am to design a culvert in the bottom of a water reservoir.
The culvert is to be constructed by cut & cover method and back filled by more than 50% of its span so as to meet the natural ground terrain.
On top of that there will be 2m of rip-rap and 27m of water.
Should I ignore the inertia response and have the response calculated on the basis of kinematic compatibility between the culvert structure and the free-field seismic deformation of the surrounding soil corresponding to the design seismic action. If this is the case, what about the Hydrodynamic pressure?
The hydrodynamic pressures on the walls of the culvert should be calculated at d=27m water+2m rip-rap+8m back-fill.
Thanks for any ideas.

 

[cvg]

if the culvert is filled with water, than water pressure is balanced. If it is filled with air than you have both a large vertical load component as well as a bouyancy force. you may need to anchor it down.

 

[cdous]

Thanks cvg, for your comment.
At scenario with the 27m. water will be in the culvert as the intake tower is significantly lower.
The up-lift scenario i.e water just below the water intake governed the thickness of the culvert so as with the back-fill and external water on the footing to resist the uplift.
I had to extended the footing beyond the external side of the walls so as take benefit of mor back-fill and external water weight.

 

[ztengguy]

how can a culvert uplift if its not closed at the ends?

 

[cdous]

The culvert is closed at one end of the intake tower, the other end leads to the entrance of the diversion tunnel.
Uplift may occur if the water is round the bell mouth of the intake tower, that is hasn't yet entered the culvert.

 

[BAretired]

Where is the lower end of the 27.7m dimension? It is not clear on the sketch. This will affect the unit weight of the rip-rap and backfill to be used in your calculations, i.e. saturated vs. buoyant weight.

BA

 

[cdous]

Thanks BAretired for your comment.
The worst case regarding uplift is the case where water hasn't reached the intake's tower bell mouth entrance hence there is no water inside the culvert.I have actually checked the uplift for that scenario.
I am puzzled regarding the earthquake loading when the reservoir is full.Culvert's walls at static condition will have balanced hydrostatic pressure as water will also be present inside the culvert. Hydrodynamic conditions will be different as there will be pressure on the inside and suction at the outer of the right wall and vice versa for the left wall.
Should I treat it as a tunnel regarding the back fill and surcharge of water i.e response calculated on the basis of kinematic compatibility between the culvert structure and the free-field seismic deformation?

 

[racookpe1978]

Yourpdfhas a cross-section of the culvert. Thank you.

But, where is the "valve" for the culvert with respect to the "open end" of the pipe? If the culvert valve closes, where is the open pipe (uplift possible) compared to the water in the lake, and the dam (solid ground ground above empty pipe ?), and the pipe discharge point?

 

[cdous]

Hi racookpe1978,

Please find attached a longitudinal section.
The cut&cover will be blocked at its start, and then the final blocking will take place in the tunnel.
The cut&cover,tunnel e.t.c will work initially during construction of the earth dam as the water diversion system.
At the end of construction the cut& cover will be blocked and till the water rises to the intake tower the final block inside the tunnel must be completed.
In the final block all the evacuation and water intake pipes will be embedded.

 

[cdous]

Hi racookpe1978,
Forgot to attach the longitudinal section.
Pls find it attached