Soil pressure on tunnel footings

[mrzift]

I'm designing the footings for precast tunnel shells. The tunnel is located 20m below grade and the footings are concentrically placed to the tunnel shell walls so as to avoid any instability.
Would it be correct to assume the soil column over the overhanging part of the footing can be ignored due to arching effects in the soil?
In my case, this is the difference between a simple strip footing and an expensive thick mat footing.

0 to 10m is loose fill material
10m to 20m is natural stiff clay overlying weathered rock (weathered rock layer about 3m below tunnel footings).

 

[1503-44]

I would think that assumption is not safe. Any soil above the footing should be included to be safe. And I think it is more likely that there will be additional settlement that may even bring in soil load from outside the green area to the footing, but that depends on how the tunnel is to be installed. If the soil adjacent to the green wall is a solid rock cut, then no, however if the tunnel is to be installed after cutting down at a 45° angle and backfilled, I would think you would pick up some pressure from the backfill beyond the green column, including water pressure, if it were not very well drained.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[geotechguy1]

If the tunnel is rigid / stiff relative to the soil I would think that the tunnel will pick up a large wedge of load and transfer it all to your footing. The load would be much larger than the green column, unless as 1503 says the cut face is rock (i.e. much stiffer than tunnel)

 

[1503-44]

Most images in this search show loadings coming from a much greater contributary volume than the green column boundary.

Yes, the tunnel will always be more rigid than the soil, unless you drill into solid homogenuous rock.

These basic models are easy to follow and need only a few soil parameters.

Ref

https://duckduckgo.com/?q=soil+load+on+tunnel&t=brave&iax=images&ia=images

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

The green column limit theory appears to be a massive under design. I would think that a tunnel footing failure is the last thing you would want. It would be extremely Difficult to repair... to say the least. Why are you trying so hard to save a little of the cheapest concrete you can get? Your first objective is to make it work. Don't be afraid to use concrete and steel where you need it. That's what it's for. After you get it working, only then look for ways to save some money.
And only for those items that are easy to repair, do not amount to major damage, and have no great consequence.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[mrzift]

Thanks for all the input. I think I need to clarify, the footings are definitely designed for the soil mass above the tunnel (as a vertical hydrostatic pressure from the surface down).

My confusion is about the part of the footing that sits outside the tunnel walls and if I need to include a column of soil weight over it (green shaded column in my first diagram).
@1503-44, thanks for the references, it's much clearer now. Terzaghi theory shows zero vertical pressure at the corners of the tunnel increasing rapidly as you move away from the corners.

 

[1503-44]

Yes, include that green column and more. According to the above diagrams, the effective width may be the width of the tunnel + 2x its height, if [ψ] is 0.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[geotechguy1]

If the tunnel is rigid / deformation isn't allowed then yeah it's going to end up picking up more of the load than just what is over the tunnel and above the footing. You have a different situation if you are letting large amounts of deformation / stress redistribution occur before building the tunnel and depending on the relative stiffness of the tunnel.

 

[mrzift]

@geotechguy1, would that be the case where no soil arching would occur? Say footings on a continuous rock. In this case using a conservative analysis model of soil mass over tunnel roof + 45degree wedge either side would seem appropriate...

In my case, the footings are founded on a clay layer and the geotechnical report states 30mm settlement to achieve the allowable bearing. I would have thought 30mm is enough to achieve arching effects though.

 

[geotechguy1]

Normally over-estimating settlement is conservative in geotech but in this case it may not be so I'd be hesitant about the 30mm unless they've done a more rigorous analysis. What is the construction method? What is the material around and above the tunnel? What is the tunnel made of, and how stiff is it?

 

[mrzift]

Its a mining site with a conventional construction, tunnel strip footings poured insitu and thin precast concrete shells placed over the footings. AFter this all the material that was excavated will be backfilled.
The material immediately around the tunnel would be engineered fill to 98% MDD.

 

[1503-44]

So any "arching" of the backfill would occur from where to where?
It would seem that the depth of burial (m+ [α]H, as shown in the first diagram above) of the tunnel would indeed have to be very deep in order to get any arching from failure plane to failure plane. The arching effect does not happen for soil within the arch shown on that first diagram. Only soil above the arch does not place load on the tunnel..


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."