suitable foundation for silos 1

[geoprasad]

Hi all
I want suitable foundation type for 5 silos having diameter 25m each.These five silos are in one line. Soil strata is very hard and granualr material.No water table.

 

[JedClampett]

Is there a reason you don't want to use the obvious answer of a concrete mat?

 

[aeoliantexan]

I used to do a lot of investigations for grain elevators and silos in the miwestern states. A continuous mat spanning 25 meters will be very thick. The most economical foundation, if the soil is strong enough, would be individual ring footings with separate floating floor slabs.

Depending on the height, the stored material, and the method of emptying, the walls may carry a sizeable portion of the live load as well as the dead load.

The silos will tend to tilt towards each other due to the overlap of the pressure bulbs if the soil is uniform, and this may tend to egg-shape the walls, if they touch, or crack the foundation if the silos are separated. If the soil is non-uniform, they may tilt away from each other and try to tear the juncture of the walls.

I assume the silos will be at least as tall as they are wide. Such heavy structures will experience significant settlement unless the soil is unusually strong and incompressible. There will be significant differential settlements that will tend to distort the structures.

The minimum bearing pressure is a function of the weight and the diameter, as a mat can only cantilever about 3 feet beyond the walls.

If you can give us some particulars regarding the type of construction, loads, and soils, we can be more specific. many structures of this type have become distressed or failed entirely; you need guidance from both a geotechnical enginer and a structural engineer with applicable experience.

 

[BigH]

The OP did indicate that the soil was granular and very hard - although that is a consistency name, not one applied to granular soils. I agree with the reply posts - I suggest that one also do a little light reading of the Transcona Grain Elevator collapse. Also Bozozuk in one of his publications on silos gives a good picture of two grain silos tilting towards each other due to the pressure bulb overlap. Basically, for individual footings, one needs to understand how adjacent footings interact with each other - no different that normal adjacent footings.

 

[geoprasad]

Dear All,
Thank u for u r replies.

all these five silos are constructing side by side.so is it corect to design ring foundation with separate floor slabs.
there is no excessive settlements due to pressure overlaping.

 

[EvanGr]

What should the void between the seperate floor slabs be?

 

[aeoliantexan]

We don't know enough to answer your question. The size of the silo concerns me. 40 and 50 years ago, most concrete grain elevators consisted of at least two rows of cylindrical cells about 20 feet in diameter. The cells were slip-formed simultaneously and well-connected. Built on a continuous mat, they were very rigid structures. With heights of about 120 feet, the average bearing pressures were about 7000 pounds per square foot. The geotechnical engineer primarily needed to be concerned about bearing capacity of a wide mat and differential settlement expressed as tilting. The Transcona elevator was such a structure and failed by bearing capacity failure of thick clay. It was pretty much intact as it tipped over.

As the structures began to be constructed with a few large-diameter cells, the rigidity of the individual cells and the overall structure decreased greatly, and soil-structure interaction became more important. You may well be correct that there will be no excessive settlements, but you haven't told us how you know this. If this structure is over a hundred feet tall, as many are, it is very heavy, and I would expect several inches of settlement unless the soil is extremely stiff.

One major concern with ring footings is where the cells meet and one footing shares the load of both walls. The footing can become quite wide, and it behavior can be uneven, especially if the cells are not filled simultaneously at the same rate.

The bins I worked with of this size commonly had cone-shaped floors with an auger extending to the tip for unloading. The cone-shaped floor slab could be separated from the footing by an expansion joint and offset vertically to acommmodate more settlement by the footing than the floor. This becomes more complicated if the footing width varies.

There has traditionally been a lot of art involved in the design of such structures. My best advice is to get experienced help.

 

[Mccoy]

Soil strata is very hard and granular material

Of course on dense, cemented breccias and conglomerates you can build just about anything. You should make sure the material is homogeneous though, for a sufficient thickness.

It would be interesting to knowthe results of field/lab tests if you've done'em yet (I suppose so since you saythere is no settlements problem)

Any loading tests in the field with large diameter plates?